research briefing on contemporary. problems in plasma science

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Research Briefing on Contemporary. Problemsin Plasma sCience

Plasma .Science Committee

Board on Physics and AstronomyCommission on Physical Sciences, Mathematics, and Applications

National Research Council

National Academy Press

Washington, D.C. 1991

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NOTICE: The project that is the subject of this report was approved by the Governing Board of theNational Research Council, whose members are drawn from the councils of the National Academy ofSciences, the National Academy of Engineering, and the Institute of Medicine. The members of the

committee responsible for the report were chosen for their special competences and with regard forappropriate balance.

This report has been reviewed by a group other than the authors according to proceduresapproved by a Report Review Committee consisting of members of the National Academy of Sciences,the National Academy of Engineering, and the Institute of Medicine.

The National Academy of Sciences is a private, nonprofit, self-perpetuating society of dis-

tinguished scholars engaged in scientific and engineering research, dedicated to the furtherance of scienceand technology and to their use for the general welfare. Upon the authority of the charter granted toit by the Congress in 1863, the Academy has a mandate, that requires !t to adbise the federal governmenton scientific and technical matters. Dr. Frank Press is p.resident of the National AcAdemy of Sciences.

• .The.National A_:ademy of Engiqeen'ng was. e_tablished in 1_64, under the charter of the National.. "Acade/nY of Sciences_,_. a pa/'allel organizati0n .of: outstanding engineers. It'is autmi0mous in tts

administration and in the selection of its members, sharing with .the National Academy of Sciences the

responsibility for advising the federal government. The National Academy of Engineering also sponsorsengineering p'rograms aimed at meeting national needs, encourages education and research, and recog-nizes the superior achievements of engineers. Dr. Robert M. White is president of the National Academyof Engineering.

The Institute of Medicine was established in 1970 by the National Academy of Sciences to securethe services of eminent members of appropriate professions in the examination of policy matters

pertaining to the health of the public. The Institute acts under the responsibility given to the National•Academy of.Sciences by its congressional charter to be anadviser to the federal government.and, uponits own initiative, to identify issues of medical care_ research, and education. Dr. Samuel O..Thier ispresident of the Institute of Medicine.

The National Research Council was established by the National Academy of Sciences in 1916 toassociate the broad community of science and technology with the Academy's purposes of furtheringknowledge and of advising the federal government. Functioning in accordance with general policiesdetermined by the Academy, the Council has become the principal operating agency of both the NationalAcademy of Sdermes and the National Academy of Engineering in providing services to the government,the public, and the scientific and engineering communities. The Council is administered jointly by bothAcademies and the Institute of Medicine. Dr. Frank Press and Dr. Robert M. White are chairman and

vice chairman, respectively, of the National Research Council.This project was supported by the Department of Energy under Grant No. DE-FG05-88ER53279,

the National Aeronautics and Space Administration and the National Science Foundation under GrantNo. PHY-8814509, and the Office of Naval Research under Contract No. N00014-89-J-1728.

Additional copies of this document are available from:

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Washington, DC 20418

Printed in the United States of America

Plasma Science Committee

.. .

CHARLES F. KENNEL, University of California, Los Angeles, Oua'rFRANCIS PERKINS, Princeton University, Vice ChairJONATHAN ARONS, University of C.alifornia, BerkeleyDAVID E. BALDWIN, University of Texas

IRA B. BERNSTEIN, Yale UniversityJOHN M. DAWSON, University of California, Los AngelesRICHARD A. GOTI'SCH0; AT&T Bell Laboratories .. • . . . ..JOHN H. MALMBERG, Oniversity of C.aliforni_/; Sa_ DiegoROBERT L. McCRORY, University o_ RochesterJOSEPH PROUD, GTE Laboratories IncorporatedRAVI SUDAN, Cornell University

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Ronald D. Taylor, Senior Program Officer

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PRECEDIMG PAGE" _' r_K

Board on Physics and Astronomy

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FRANK D. DRAKE, University of California, Santa Cruz, Chair

LLOYD ARMSTRONG, Johns Hopkins University•W. DAVID ARNE'IT, University of ArizonaHOWARD C. BERG, Harvard University

RICHARD S. BERRY, University of ChicagoWILLIAM F. BRINKMAN, AT&T Bell Laboratories ..GEORGE W. CLARK_ MassachusettsInstituteofTech.n_51ogy' ..i!-• , , . . . • .

HAROLD P. FURTH, PrincetonUniversity " "

MARTHA P. HAYNES, CornellUniversityCHARLES F.KENNEL, Universityof California,Los Angeles

WALTER KOHN, Universityof California,San DiegoSTEVEN E.KOONIN, CaliforniaInstituteofTechnology

LEON LEDERMAN, Fermi NationalAcceleratorLaboratory

VERA RUBIN, CarnegieInstitutionofWashington

DAVID N. SCHRAMM, Universityof.ChicagoDANIEL TSUI, PrincetonUniversity

STEVEN WEINBERG, UniversityofTexas

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Donald C. Shapero, Staff DirectorRobert L. Riemer, Associate Staff Director

Ronald D. Taylor, Senior Program OfficerSusan M. Wyatt, Administrative AssociateMary Riendeau, Administrative SecretaryAnne K. Simmons, Secretary

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Commission on Physical Sciences, Mathematics, and Applications

NORMAN HACKERMAN, Robert A. Welch Foundation, Chm'rman

PETER J. BICKEL, Univers'.tty of California, BerkeleyGEORGE F. CARRIER, Harvard UniversityHERBERT D. DOAN, The Dow. Chemical Company (retired)DEAN E. EASTMAN, IBM T.J. Watson Research Center

•MARYE ANNE FOX, University of Texas .PHILLIP A_ GRI!_IT!'HS, Duke Uni.versit.y •NEAL F.' LANE, Rice Univers!ty: : " "ROBERT W. LUCKY, AT&T Bell Laboratories "CHRISTOPHER F. McKEE, University of California, BerkeleyRICHARD S. NICHOLSON, American Association for the Advancement of ,Science

JEREMIAH P. OSTRIKER, Princeton University ObservatoryALAN SCHRIESHEIM, Argonne Nationa[ LaboratoryROY F. SCHWITITr.aRS, Superconducting Super Collider LaboratoryKENNETH G. WILSON, Ohio State University

• . . . .

NORMAN METZGER, Executive Dired0r

O_!C_N,'I,L P_.'_E IS

OF PC)OR QUALITY

Preface

The Plasma Science Committee of the National Research Council has prepared this research briefing,

at the request of the director of the Office of Energy Research of the Department of Energy (DOE), toprovide DOE, other federal agencies, and the plasma science community at large with a rapid assessmentof plasma science. This briefing may be the first unified assessment of plasma science in its entirety.

In addition to giving a glimpse of forefront research in plasma science, this research briefing identifiesand discusses selected opportunities for new i'esearch and applications, and identifies some problemsassociated withthese areas of opportunity.. Although the areas of research discussed in this briefingwere not selected on the basis of any.. perceived relation to missioris of the DOE, there is a dearconnection with a number of DOE programs, including, for example, the magnetic fusion energy

program. No attempt has been made . to conduct a detailed assessment of pl_ma science or to developreff)mmendati.6ns or set .p_ogrammatic'.priorities. As.:e.xplal.ned further below, the committee has

•recommended that such a .detailedev'aluation 6f plasma sden_e be carried "out." In the meantime,.:th'ecommittee hopes that this briefing will be of use in _ssisting federal agendes and oth,_rs with near-term

program planning.The Plasma Science Committee was created by the National Research Council in 1989, upon the

recommendation of the executive committee of the Division of Plasma Physics of the American PhysicalSociety. Currently funded by the National Sdence Foundation, the Department of Energy, the NationalAeronautics and Space Administration, and the Office of Naval Research, the Plasma Science Committeeis now a permanent standing committee that reports to the Board on Physics and Astronomy. Its terms

of reference are to appraise.the development of plasma science as a whole, to foster a sense of unity and/.-ommonality in the field, to promote the teaching of plasma science, to assess the need for new facilities,to encourage interagency cooperation, and to oversee the interfaces of plasma science with othersciences.

The Plasma Science Committee has undertaken three major projects since its inception. First, it

cosponsored, with the Office of Naval Research, a workshop on nonneutral plasmas that broughttogether many of the hitherto loosely affiliated practitioners of this rapidly growing new field of plasmascience. Next, it sponsored a study on plasma processing of materials that is now nearing completion.Finally, the committee has developed a proposal for a detailed study to assess the health of basic plasmascience in the United States, to highlight new opportunities for research and applications, to assess the

quality and size of the educational programs in plasma science, to characterize basic experimentalfacilities needed, and to identify changes in institutional infrastructure that could improve research andeducation. The present briefing is based on the work of the Plasma ,Science Committee done inpreparation for this broad assessment of the field, which will f_-'us on new opportunities in plasmasdence and technology. The Plasma Science Committee, the only comniittee on the i_ational level with

responsibiliLy for the health of basic plasma science, regards this kind of periodicassessment of the fieldas central to its role.

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Contents

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III

INTRODUCTION .................................................... 1

Perspective and Outlook ............................................. 1

PLASMA SCIENCE ................................................... 4

Low-Temperature P.lasmas ............................................ 4The Technology ....................... : ........................ 4The Science ........... ...................... •.............. "..... 5Key.lssues ..... . .............. ............. , .............. .- .... " 6

Basic Plasma .Science Opportunitiesin Fusion Energy .......................... 6g ibfi " 'Ma netic.Fus , ... ........ . .; .... .. ...... .., .. " 6

:hiertialFusion .:. _. . .'. . .'. _ _ . . .. . ..... ..... ...:.,... ..... .... .... ;,.. 7Ulti'afast Plasma Science .... ...... ................... 8Nonneutral Hasmas ................................................ 10

Diagnostics and Basic Plasma Experiments ................................. 12Analytical Theory and Computation ..................................... 14

Analytical Theory ............................................... 14Computational Plasma Science ...................................... 15Summary .................................................... 16

Space and Astrophysical Plasmas ....................................... 17

CONCLUDING REMARKS 19

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CONTEMPORARY PROBLEMS IN PLASMA SCIENCE

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IIntroduction

This research briefing presents the PlasmaScience Committee's rationale for the need for

a comprehensive study of plasma science.Plasma science often seems to be a collection of

independent applications. "I_heprograms that

provide support for plasma science general{yhave as their primary ;objective developing

• Wirious applications. This central.fact'has madeit difficult for.committees and decisionmakers to)

take a comprehensive view of the whole subject,even though the vitality of one branch of plasmascience can be seriously affected by events

occurring in neighboring branches. For exam-ple, the steady decline in the funding for fusionresearch in the past decade has indirectly hurtall of plasma science. It has proven difficult to

organize and support basic research that has .notyet led to an identifiabJe application. Educa-tional programs in plasma science may notreflect the breadth and depth of the subject.

Nearly all the problems and areas of concernidentified in this briefing are related to thecommittee's perception that the present organi-zation of plasma science around its applicationsmay lead to missed opportunities, either forbasic research or for new applications. There

are some areas in which the opportunities seemclear cut, and the question is whether and how

to capitalize on them. There are also areas ofplasma science where structuial impedimentshave prevented.asking whether new opportuni-ties exist. This research briefing touches on

situations of both types.This briefing starts with an overview, the goal

of which is to provide a broad perspective on allof plasma science. The next section containsdetailed discussions of scientific opportunities in

various subdisciplines of plasma science. Thefirst subdiscipline to be discussed is the areawhere the contemporary applications of plasmascience are the most widespread--low-tempera-

ture plasma science. Opportunities for newresearch and technology development that haveemerged as byproducts of research in magnetic

and inertial fusion are then highlighted. There

follows a discussion of new opportunities inultrafast plasma science opened up by recentdevelopments in laser and particle-beam tech-nology. The briefing then turns to laboratoryresearch that uses smaller-scale facilities, takingup first a success story, the new field of non-

neutral plasmas, and then the area of greatestconcern to the committee, basic plasma experi-ments. Discussions of analytic theory andcomputational plasma physics and of space and_sttophysicalplasma. physics complete, tl_epresentations in this sei2tion'. Concluding re-

marks are presented in the final section.

PERSPECTIVE AND OUTLOOKWith the rise of electrical science in the nine-

teenth century came intimations of plasmaeffects. In the 1830s, Michael Faraday createdelectrical discharges to study the chemical trans-

formations induced by electrical currents. Thesedischarges exhibited unusual structured glowsthat were manifestations of a new state of

matter--plasma. The study of low-temperature,partially ionized plasmas, initiated then, contin-ues today to be a productive enterprise, richnow in useful and widespread applications, asdetailed below. With the discovery of theelectron in 1895 and of the atomic theory ofmatter shortly thereafter, plasma science was

poised for takeoff in the twentieth century. Thedisciplines of electromagnetism, fluid mechan-ics, statistical mechanics, and atomic physics

• were eventually assembled" into a unified

methodology for the "study of the nonlinearcollective interactions of electrically charged

particles with one another and with electric andmagnetic fields, i.e., plasma physics.

In the 1920s, Irving Langmuir discoveredcollective plasma oscillations in laboratory gasdischarges and radio waves were first reflectedfrom the earth's ionosphere, the very edge of

space. Until the late 1950s, plasma sciencedeveloped along the two distinct lines exempli-

fied above. One line was a byproduct of iono-spheric, solar-terrestrial, and astrophysicalresearch, motivated by such things as the desireto understand how radio waves propagate, how


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solar activity leads to auroral displays andmagnetic storms on the earth, and how magnet-ic fields influence the behavior of stars, galaxies,and the interstellar medium. In the laboratory,the study of low-temperature collisional plasmaswas pursued vigorously, focusing on electronconduction and breakdown in gases, electronemission and other cathode phenomena, andcollisional excitation, of atoms and molecules.

These studies shaped what is tod.ay called the.subje_tof.gasedu,_.e![ectroniCs .... • . "" .

In 1946, the existence., of a collective intera¢tion between waves and resonant particles in aplasma without collisions was proposed theoreti-cally. By the 1950s, it was clear that collective

interactions are an essential property that high-lights the physics of hot, fully ionized, collision-free plasmas.

The present phase in the development ofplasma science began in the la.te 1950s. Twoevents symbolizing the deeper intellectual

currents of those years were the first successfullaunch of an artificial earth-orbiting satellite bythe Soviet Union in 1957 and the revelation in

1958, through dedassification, that both theUnited States and the Soviet Union had been

trying to use plasmas to harness the energysource of the sun--thermonuclear fusion--for

•peaceful purposes. The discoveries of theearth's radiation belts and the solar wind

showed that our exploration and future under-standing of the earth and the sun's space envi-ronment would also be couched in terms of

plasma science. Thus two powedtd motivationsawakened interest in the physics of hot plasmas.Fusion research seeks a source of clean energyfor human use that will last for a time compara-ble with the present age of the earth. Spaceresearch seeks useful comprehension of nature'sprocesses on a global and, indeed, solar-systemscale, reflecting mankind's dependence on theterrestrial environment and curiosity about thecosmos.

The international effort to achieve controlled

thermonuclear fusion has been the primarystimulus to the development of laboratoryexperiments on collisiordess plasmas. As earlyas 1958, the theta-pinch configuration produced

fusion temperatures at high plasma densities.However, the energy confinement time wasorders of magnitude lower than that required fornet energy production. The simultaneousachievement of high temperatures, densities,and confinement times, similar to the plasmaconditions at the centers of stars, requiredsignificant improvements in forming and under-standing plasmas. The technology needed tocreate fusion plasma conditions in the lab0rato-_TC-high-field, large-volu .n_ suPerconductingmagnets, intense energetic 'nedtral beams,powerfullasers, vacuum'and sudace techniques,and high-power radio-frequency sources span-ning a wide range of frequencies--was systemat-ically assembled. As fusion-quality plasmaswere diagnosed with increasing precision,theory was stimulated to explain observationsmade possible by more detailed and completemeasurements.

The"scientific feasibility Of t'ontrolled.•fusionwill likely be demonstrated in the coming de-cade. Already, non-burning e_)eriments indeuterium have achieved densities, confinement

times, and temperatures essentially consistentwith _break even," where the energy produced

by the deuterium-tritium reaction would equalthat required to heat the plasma. The keytokamak fusion power parameter has increasedby 10,000 in the past 15 years, and only anadditional factor of seven is needed to achieve

ignition. The U.S. Department of Energy hasproposed construction of a burning plasma

_xperiment (BIDQ to study the physical process-.es occurring in a tokamak fusion reactor tliatwill produce several hundred megawatts offusion power. The United States, the SovietUnion, Japan, and the European Communityhave arrived jointly at a conceptual design forthe International Toroidal Experimental Reactor0TER), which will address the network of

interacting physics and engineering issuesassociated with practical tokamak reactors. TheITER project is moving into the engineeringdesign phase.

In inertial confinement fusion, pellet compres-sions of several hundred times liquid density attemperatures of several KeV have been attained.

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While DOE funding of research in inertial con-finement has been provided almost entirely onthe basis of defense applications, the energyapplication has played an important philosophi-cal role as a long-term goal. A proposed up-grade of the present NOVA facility will producefusion yields of interest for defense applications.The. laboratory microfusion facility currently'being planned will be designed to demonstrateignition, of importance tOthe energy application..

....In_..September 1990, DOE.'s.:Fus!on.P.ol!cy .Advisory'Committee .ire_m.mended, to the-

Secretaryof .Energythat the magnetic _usion

program change itsemphasis from researchto

development and that a program in inertia[

fusionenergy development be initiated.The

goalofeach willbe an operatingfusionpower

plantby the year 2040; a choicebetween the

two approaches willbe .made at a laterdate.

The change inemphasis from researchtodevel-

opment, and thenece.ssarynarrowing of f.o_us

accompanying it,willinevitablyaffectallparts

ofplasma science.

Itissignificantthatone discipline,plasma

science,definesthe basiclanguage now used

both in fusion and space plasma research.

Space plasma research is concerned with plas-mas in the solar system, those occurring at thesurface of the sun, in the solar wind, and in the

magnetospheres and upper atmospheres of theplanets and comets. Space plasma physicistsare beginning to understand quantitatively theinterconnected chain of plasma processes thatlink solar flares and Corol_l mass ejections at :the sun to magnetic storms and auroral displays •on the e.arth. The same types of plasma pro-cesses are found to be at work in the magneto-spheres of the various planets, a graphic illustra-tion of the generality of the principles of plasmaphysics.

The American programs of research on fusionand space plasmas are of comparable scale. Theearth's magnetosphere proves to be as challeng-ing to understand as a fusion tokamak, and inthe 1990s an international flotilla of spacecraftwill be sent on a coordinated study of theearth's magnetosphere as part of the Interna-tional Solar-Terrestrial Physics Program--the

space physics analog of the ITER fusion pro-gram. The Galileo spacecraft will study plasmaprocesses in Jupiter's magnetosphere. NASA isplanning a mission to study the plasma in thesolar corona in situ. These are the largest ele-ments of a broad U.S. program of research on

solar system plasmas in the 1990s.Space plasma research is now an important

source of ideas for other branches of plasmascience. The experimental diagriosis and theo-

retical _interpretatio.n of many space pla_napr0ces_.s now match'in precision theb_. { ofcurrent laboratory practice. As a result, spacehas become one of the primary experimental

arenasforbasicplasma research.The present

understandingof collisionlessshocks ismainly

aresultofsolar-systemplasma research.Final-

ly,theplasma phenomena in the solarsystem

have proven to be examples of generalastro-

physicalprocesses. The solarsystem has be-

come a laboratory in which :.astrophysical pro-cesses of great generality can be studied in situ.Observations of particles accelerated by solar-system shocks are providing a basis for under-standing the origin of galactic cosmic rays.Research on astrophysical plasmas will continuelong after the goal of fusion energy has beenachieved.

As the science and related experimentaltechniques have developed, other applications ofplasma science have come into view. One suchapplication is the free-electron laser, which cangenerate coherent radiation from microwaves tooptical frequencies and perhaps even into the x-ray range. The free-electron laser will findapplications in fusion research, many otherbranches of physics, other sciences, indush'y,

and medicine. New methods of particle acceler-ation using collective plasma effects are a secondapplication. Plasma scientists are working withaccelerator designers to create a new generationof devices, such as the beat-wave accelerator,

which will operate at the frontiers of high-ener-gy particle physics. Intense, pulsed, relativisticelectrons have been used to generate intense

high-powersources of microwaves, for collectiveaccelerators, in high-current betatrons. Theseand other recent developments in laser and

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particle-beam technology have opened up a newfield, ultrafast plasma science, for exploitation.

Microwave tubes, free-electron lasers, and

electron and ion traps all involve nonneutral

plasmas, in other words, those in which theelectron and ion densities are unequal. Thissubfield, which has had notable and visible

scientific, successes in r_2ent years," in.volves

elegant experiments that are w.eU suited touniversities...Tw.o-dimens.ional flu'id turbulencecanbe Studied at higher Reynoldsnumbers in a

nonneutral plasma than it can in an ordinaryfluid. Recent experiments in ion traps havesucceeded in creating a two-dimensional Cou-lomb lattice for the first time. The area of

nonneutral plasmas is the most recent exampleof a successful investment in basic plasmascience.

The scope of gaseous electronics and iLsapplications multiplied after World Wa/II. Loiv-temperature plasma science now supports majorindustries and is an indispensable part of many

military technologies. The newest, and perhapsthe largest, business employing plasma scienceis the $50.billion-per-year electronics industry.Plasma p_ng is now used in about 30

percent of the steps in manufacturing semicon-ductor chips, and there is no known alternativeto these plasma techniques on the technologicalhorizon.

Numerical modeling of many-particlekinetic_ms.has been advanced very significanflybyplasma ,science. While modeling does notreplace discovery, it has made quantitative thestudy of complex plasma systems such as .toka-maks and magnetospheres, and it has clarifiedthe nonlinear collective processes that regulateplasma transport in such systems. The micro-

scopic plasma processes in distant astrophysical_j_tems cannot be observed directly, as they can

in space and in the laboratory. Now, however,modem computational techniques have opened

the door to modeling of the plasmas in theexotic environments of astrophysics, rangingfrom stellar atmospheres to quasars. The sametechniques are being applied to down-to-earthproblems in the area of plasma processing ofmaterials.

• . •

IIPlasma Science

LOW-TEMPERATURE PLASMAS

The TechnologyAlthough ionospheric and some space and

astrophysical plasmas are dassified as:low-

temperature plasmas, the bulk Ofplasma science•currently practiced in this domain is gaseous-elec-trotiics b_cl, In its ofigirial form,the g_•discharge branch of Langmuir's plasma pl:tysics,

founded in the early part of this century, fo-cused on electron conduction and breakdown in

gases; electron emission and other cathodephenomena; and excitation of atomic and molec-ular species by electron collisions. The carbon-arc light source invented in the 1880s wasperhaps the first application of plasma science.It was followed .by gas-discharge rectification,high-power switch gear, and welding arcs. Theubiquitous mercury arc lamp and the more

sophisticated fluorescent discharge !amp weredeveloped somewhat later.

These and related applications have drawnheavily on plasma science in their evolution

since World War II and have enlarged the scopeof the science to include plasma chemistry,atomic and molecular physics, surface chemistryand physics, optics, high-temperature physicsand chemistry, electrical engineering, and com-puter science. . This new science supports manyof the world's major industries and is an indis-pensable part of man3t military technologies.For example:

• The use of plasma-based switch gear per-sists in the now huge power industry,while pulse power switches have beendeveloped for use in high-power radars,discharge laser systems, and, indeed, In theswitching of stored electrical power forhigh-temperature plasma experimentation.

• The early work on discharge light sources

has evolved as the core technology in thelighting industry, now a $10-billion busi-

ness worldwide, with these efficient plas-mas consuming 1011 watts of electrical

• -: •


power. Here, the earlier carbon and mer-cury arcs have been supplanted with arcscontaining sodium-mercury mixtures aswell as rare-earth additives to improvecolor and efficiency.

• New lasing species, including excimers,

have been discovered and exploitedthrough the efforts of plasma science inves-

tigators with the result that pl_ma-based,high-power lasers have formed a new and

robust .technology. . "• Thermal ar_ for wdldinghave evolved it_to

high-power devices with the ability to meltand spray refractory coatings for wear andcorrosion resistance and to form new alloysand composite materials for many specialpurposes in the aerospace and automotiveindustries, as well as in subordinate indus-

tries such as the cutting-tool industry.M.odern jet engines depend critically.on

plasma coating techniques. More recently,thermal plasmas are finding growing appli-cation in the destruction and disposal ofbiological and other hazardous wastes.

• The newest, and perhaps the largest, tech-nology employing plasma science is theelectronics industry. Low-pressure plasmasare vital in many plasma etching, deposition,

and surface modification steps in the manu-facture of nearly all integrated circuits.

• New technologies such as plasma-aidedchemical vapor deposition (CVD) and plas-ma spray preparation of diamond as wellas superconducting films c'omprise a newwave of applications.

The pervasive use of low-temperature plas-mas in modem technology occurs because theyprovide physical conditions that are inaccessibleby any other means. No other medium canprovide gas temperaturesor current densities as

high as those achievable with plasmas, no othermedium can excite atomic and molecular speciesto radiate as efficiently, and no other mediumcan be arranged to provide similar transient andnonequilibrium conditions. Plasma technologyprovides a unique capability to obtain hightemperatures in a chemically controlled environ-

ment. In materials processing, plasma condi-tions provide pathways for chemical reactionsthat cannot be realized by any other means.

The Science

Low-temperature plasmas occupy a largeportion of the density-temperature plane, withelectron densities ranging from 105 to 1017/cm 3and electron temperatures (kT) ranging from

0.01 to 10 eleqtron volts (eV). These. plasmasa.re classical i_ the serise that the thermal kineticenergy is. large, in comparison to the average :Coulomb interaction energy. Thus, the chargedparfides interact weakly with each other, andmost electron collisions are with neutral atoms

and molecules. The degree of ionization mayvary from close to 100 percent to well below onepart per billion.

Low-temperature plasmas are governed bythe laws of collective phenomena and atomicphysics. They exhibit nonequilibrium conditio.nswith wide differences in electron, neutral, ion,

vibrational, and rotational temperatures. More-over, within each plasma species there aredepartures from equilibrium, and Maxwellianenergy distributions are seldom observed. Thishighly nonequilibrium character and the myriadinteractions that can occur in the volume and at

surfaces present both challenges and opportuni-ties. The fact that a highly excited medium can

be obtained that has no chemical or physicalcounterpart in thermodynamic equilibriumprovides the opportunity.

The diversity of applications of plasma-basedsystems used to pr6cess materials is matChed bythe diversity of plasma conditions, geometries,and excitation methods. In just one segment ofthe applications--plasma-enhanced CVD andetching--the gas pressures vary from atmo.spheric to millitorr, and the gas compositionincludes rare gases, highly reactive gases, com-plex molecules, and the products of reaction.Indeed, the complexity of the gas itself is ageneral condition found in most of the newplasma science applications and has prompteda more descriptive label: "reactive plasmas."To achieve progress, the practitioners of this

composite science must call upon a number of

L


science resoun_ that include plasma electrody-namics and kinetics, plasma and surface chemis-try, basic data for cross-sections and rates,diagnostics of discharge and surface processes,

theory and predictive modeling, and scaling andprocess control.

Key IssuesThelcey issues affecting the progress of low-

.temperature plasma:science are-related to itsdiversity and 'complexity. The 'present strong

technology pull now far outstrips its scientificunderpinning. Rapid growth and numerousdemands to use plasmas immediately in a vastarray of applications have led to a system-specif-ic approach that renders haphazard--and inhib-its--not only scientific understanding but also

technology transfer and process scaling. Manymanufacturers of .semiconductor fabrication

•equipment are" small fi/'ms fina'ncially, ill-equipped to conLribute to research and develop-ment. Many practitioners of plasma technologyare not trained in the appropriate science disci-plines. 1

4 '

BASIC PLASMA SCIENCE OPPORTUNITIESIN FUSION ENERGY

The Department of Energy has investedheavily to develop outstanding plasma physics,

laser, and particle beam facilities in its magneticfusion and inertial fusion energy programs. Inthis section, the Plasma Science Committee

touches briefly upon two questions. Howimportant is research in basic plasma science tothe fusion program? Could fusion energyfacilities also be used for basic plasma scienceresearch? Definitive answers to these and

related questions await the conclusion of theproposed "New Opportunities"study. Present-

ed hereareexamples ofissuesthatthecommit-

teehopes willbe examined inthatstudy.

Magnetic Fusion

The committee cites the specific example oftoroidal eigenmodes not to advocate one re-search program over another but to illustratemore general issues.

Until this past year, experimental studies of

stable magnetohydrodynamic (MHD) modes intokamaks have focused on a single applica-

tion--Alfv_n wave heating--and neglected possi-ble modes of much lower frequency, gap modes,which are now considered by theorists to beleading candidates for destabilization by alphaparticles in the proposed Burning Plasma Experi-ment. In other words, the theoretical frame-

work of MHD, developed in the late 1950s, has

"not b_.n subject to a C0mp_hensive experimen-:talexamination. BecauseAlfv_n wasteheating

appeared unattrhctis/erelative'toion cycJotron

heating,theUnitedStatesdidnotundertakethe

necessaryexperiments,and the researcheffort

became centeredin Europe. Recent observa-tionson thelargeU.S.fusiondevices,TFTR and

DIII-D,have discoveredmagnetic oscillations

thataredrivenby energeticparticlesand appear

tobe consistentwithgap modes. The threshold

forthesemodes remainsunexplained,however,becauseOfour lackofdetailedunderstandingof

/he underlyingstableMHD gap mode, whose

damping decrement issensitiveto the plasma

profile.What isneeded experimentallyisfor

low-power Alf_n wave antennas to excite

stableMIID modes in tokamaks and explore

their frequency, damping decrements, and

spatialeigenfunctionsover as wide a range of

plasma parametersas ispracticable,including

parameterregimes thatare not on the path tofusion energy.

Several object lessons can be drawn from thisillustrative example. Fun't, fusion energy re-search itself appears to have suffered fromearlier neglect of/k rather basic investigation ofrelatively modest scale. Second, the problem ofstable MHD modes illustrates the commonalityof plasma principles.The conoeptsappliedto

Alfv_n wave heating--wave absorption viageometric resonances--have also been used and

testedin the earth'smagnetosphere. More

generallyspeaking, much of the physics of high-temperature plasmas confined in toroidal mag-netic field experiments has application to the hotplasma contained in the dosed loops of the solarmagnetic field. Finally, there is a related, buteven more general, point. Although magneto-hydrodynamic theory is the fundamental de-

6


scription of plasma dynamics on scales largerthan the ion [,armor radius and is used to

describe nearly all magnetic fusion, space, and

astrophysical plasma configurations, very few ofthe predictions of magnetohydrodynamic theoryhave been reduced to laboratory practice. This

is particularly true in the case of magnetohydro-dynamic flows, which have no relevance tofusion research.

Part of the difficulty appears to be that, on the

one hand, any" apparatus dealing with plasmahot enough 'to exhibit C01|isionless phenom(_na

and large enough to support magnetohydro-dynamic phenomena is a costly facility by pre-vailing basic research standards. On the otherhand, those who allocate research time on theeven more cosily and complex devices designedfor fusion research are naturally reluctant to

spend resources on activities that do not un-questionablycontribute to the fusion energy goal.

There are also suggestions for utilizing exist-

ing magnetic fusion devices in different waysthat will benefit plasma science. For example,

magnetic mirror confinement of. collisionlesscharged particles is one of the fundamentaltopics of plasma physics. Of its many applica-tions, that to Earth's and Jupiter's radiation belts

may be the best lmown. Although magneticmirrors no longer are part of the U.S. fusion

program, magnetic mirror research could beresurrected by operating shaped tokamaks withand without toroidal magnetic field and plasma

current. From the point of view of gainingfundame, ntal new insights into the physics ofmirror confinement, the ability to produce

toroidal as well as poloidal magnetic field com-

ponents of variable relative strength could wellbe one of the more attractive aspects of tokamakdevices. In addition, such fusion devices asreversed field pinches and spheromaks generateforce-free plasmas whose behavior is interestingin its own right and, with proper interpretation,can be used to gain some understanding of the

quiet solar corona and the fundamental process-es governing rapid energy release in solar flares.

In sum, basic results obtained in fusion plas-

mas are important not only to fusion researchbut also to many other parts of plasma science.

Existing constraints on resource allocation oftenpreclude using general importance to plasmascienceas a criterion in the allocation of research

time on facilities originally designed for fusionresearch. The decoupling of hot plasma re-search from the constraints imposed by fusionreactor prototyping could open the way toimportant experimental efficiencies. There is aneed to reduce the predictions of magneto-hydrodynamic, theory to laboratory practice. Tothe Plasma Science Committ .ee's knowledge,.

'these_and similar suggestions .have notbeefi the "

subjectofformal evalu=Rion.,and so it is difficultto determine whether important opportunitiesare being overlooked.

Inertial Fusion

Inertial fusion research has led to majoradvances in high-peak-power laser and particlebeam systems that provide several new research

•opportunities at reasonable incremental, cost.Lasers with intensities in the range of 1019 to1021 W/cm 2 will soon be available in the labora-

tory. Such lasers can produce electric fields inexcess of 1014 V/m, energy densities greaterthan 3 x 1010 Jlcm 3, and ponderomotive poten-

tials exceeding 100 MV.Ionization Physics. The ionizationof atoms and

ions in a high-intensity laser field allows thestudy of atomic structure under extreme condi-tions. The electric field of the laser can ap-proach or exceed the electric field binding anelectron to a n_Jcleus. At even higher intensi-ties, electron, motion in the laser field can be-

come relativistic. Short-wavelength lasers (0.35to 1.05 am) have recently been used to studyionization for values of the Keldysh parameterthat are less than unity.

Coher_ XUV/X-ray C,enerat/on. The devel-opment of x-ray lasers requires controllingpopulations of two excited levels in a hot plasmacontaining many charge states and thousands of

excited levels. Therefore, x-ray laser researchposes an interesting and difficult combination ofatomic physics and plasma physics problemsthat test present capabilities to model and char-acterize plasmas that are not in local thermody-namic equilibrium. X-ray laser research benefits

7d


from synergism between two branches of plas-ma science: nonneutral plasma traps formed byrelativistic electron beams are used to measure

the complex spectra of highly stripped ions.Laser exploding foil concepts have been used

to demonstrate x-ray lasing in the laboratory(e.g., 206.8-)_ iasing of No-like Se has been

amplified to 17 gainlengths). Other techniques

have been used to demonstrate lasing in col-[isional[y pumped.Ne-[ike Ge (at 196 to 287 .A),Li-like A_. (at 105 to'154 A), _ind in LiAike Ti at "47 A. The development of high-power, short-'

wavelength sources is important for a variety ofapplications such as (1) holography of livingcells, (2) x-ray microscopy, (3) x-ray laser-pro-duced plasmas, (4) nonlinear optics and quan-tum electronics at XUV and x-ray wavelengths,

and (5) phase-sensitive detection and measure-ment techniques at XUV and x-ray wavelengths.

X-Ray Lithography. The use of intense lasersto produce a point source of nearly moi_hro-matic x rays may make possible the mass pro-duction of submicron-resolution, very large scaleintegrated circuits. For resolution below 0.1/tm,basic collective phenomena occur in circuits thatwill he a new challenge to solid-state physicistsin the electronics industry. X-ray lithographic

techniques are also spawning a new generationof x-ray optics and advanced diagnostic mea-surement techniques.

Strongly C_. pled Plasnuts. High-density com-pressions of inertial fusion targets provide anopportunity to study strongly coupled plasmas,in which the potential energy of the Coulombelectrostatic interaction between particles farexceeds the energy in thermal motion. Experi-ments with argon (Ar)-filled capsules haveshown evidence of band structure in Ar; newsatellite lines have been seen on the short-

wavelength side of the resonance line of Arionized 16 times, for example.

Intense Charged Part/de Beams. Beams of ionsand electrons with powers exceeding 1013 watts

with 50-nanosecond pulse durations are avail-able for various applications such as (1) thegeneration of internal currents in a plasma tocreate particular magnetic field configurations,(2) beam-plasma interaction, (3) beam-con-

densed matter interaction, and (4) intenseradiation and x-ray sources. The focusing of

such high-power beams by magnetic lenses is animportant issue.

In summary, research in inertial fusion hasstimulated technological developments that inturn create opportunities for research in otherfields. It is one objective of the proposed "NewOpportunities" study to evaluate such opportu-nities as those described above and to suggest

..wa..ys:in which they. may be exploited. -.

X . As'r SCIENCEThe research topics in this section resemble

those in the previous section in the sense ofrelating to very high plasma temperatures andfast pulses of power, but they differ in the senseof being oriented to the acceleration of chargedparticles, rather than toward the release ofnuclear energy. As in the preceding case offusion energy research, there is an inspiringlong-term goal, namely, the use of intensecollective plasma phenomena for the advance-ment of high-energy physics into otherwiseunaffordable ranges of energy, and there is aselection of near-term objectives as well.

Applications of plasma involving beams andaccelerators have led to research opportunities

in the emerging fields of uitrafast and relativisticplasma phenomena. "Ultrafast _ means plasmadynamics on time scales shorter than an ionplasma period, and "relativistic" means wave or

plasma electron velocities approaching the speedof light. Ultrafast plasma science emphas'w.estransient rather San steady behavior on time

scales for which many competing plasma insta-bilities can be avoided. The plasma dynamicsare nevertheless rich in nonlinear phenomena,

including period doubling routes to chao6,relativistic self-focusing of light, excitation ofplasma wakes by short pulses of light or patti-

des, optical guiding of light by electron beams,particle beam focusing, relativistic particle accel-eration or bunching in plasma waves, photon

acceleration in plasma waves, FEL/CARM-typeradiation sources, and frequency shifting ofelectromagnetic waves in temporal densitygradients created by fast ionization of a plasma.


Materials Research. Inertial fusion research has

spawned new development to investigate theinteraction of short optical pulses (picosecondand femtosecond) with matter. Of particular

significance is the fact that such intense, ultra-fast laser facilities are suitable for university-scale research. These sources have been used to

characterize high-temperature, superconductingtransmission lines, biological processes involvingphotosynthesis and the properties of DNA,_. brace heating and melting as measured by:'picosecondelectron_ diffraction, and tunnelingtimes for electrons in coupled quantum wells,among others.

Relativistic Self-Focusing and Guiding of Light.Self-focusing of electromagnetic waves in aplasma is important to plasma accelerators, laserfusion, and ionospheric interactions. One

mechanism yet to be observed experimentally is

the focusing in a laser channel due to the refrac-tive index increase that follows from the relativ-

istic mass increase of the oscillating electron_.

High-brightness lasers (e.g., lO-terawatt power,1-picosecond pulse duration) now being devel-oped will be short enough and powerful enoughto isolate this basic effect from other self-focus-

ing mechanisms (e.g., ponderomotive andthermal) that operate on longer time scales.

Plasma Wake Excitation. The plasma response

to short pulses, either electrons or intense light(time scales on the order of the plasma period)is dominated by the excitation of wake fields.The phase velocity of the wake fields is tied tothe velocity of the disturbance and so can easilybe relativistic. The peak fields associated with

such waves are extremely large and can ap-proach wavebreaking for recently developedintense e-beams and laser sources.

The study of wake-field excitationand short-

pulse beam propagation in plasmas is basic toseveral plasma accelerator and lens concepts, tostable transmission of beams through the iono-sphere, and possibly to the evolution of energy

bursts from pulsars. Important research topicsinclude wake enhancement by shaped belfrns

and wake steepening and radial shock genera-tion by narrow beams.

Particle Beam Focusing in Plasmas and in OtherBeams. When a particle beam enters a plasma,the plasma can cause its strong focusing by

shielding the beam's space charge and allowingthe beam's azimuthal magnetic field to self-pinch it. The application of this well-knownmechanism to high-energy colliders has openedbasic questions in regimes previously not con-sidered. Examples are the focusing of positronbeams in the nonlinear regime, where thepositron ldensity exceeds the electron densRy,and the adiabatic ."squeezing_'..of .a beam.in Oslowly ramped plasma, de/',sity. Finally;'collider.beams are now becoming so dense that theinteraction of colliding electron-positron beamswill be dominated by collective (i.e., plasma)behavior--important questions such as the equi-librium radius and stability of the overlappingbeams are yet to be answered.

Acr.eleration of Relativistic Particles and Photonsin Plasma Waves. The acceleration of relativistic

particles in plasma" waves is en_rglng as anexciting area of plasma science research. Be-cause of their large amplitudes (on the order of10 GeV/m), such waves offer the potential tominiaturize accelerators. While test particleacceleration has been recently demonstrated,critical issues such as beam quality and maxi-mum current are yet to be investigated experi-mentally.

Other interesting topics include the bunchingof long copropagated beams and the wiggling(as in a free-electron laser) of counterpropagatedbeams. Long electronbeams (several wave-

"lengthslong)could be tightlybunched by a

plasma wave fieldinto bunchletsshorterin

lengththan 1/4of a plasma wavelength and intime less than100 fsec. The use of such bunch-es in free-electron lasers or other radiation

sources can produce new sources of femto-second radiation.

The study of acceleration in plasma waves canbe extended to photons as well. A short lightpulse loaded in the proper phase with respect toa co-moving plasma wave can be continuouslyupshifted in frequency, thereby increasing boththe energy and the group velocity of the photonpacket.


." - -°

Wave Propagation in Temporal Density Gradients.The advent of intense short-pulse lasers allowsrapid ionization of gases. The study of wavepropagation in temporally inhomogeneousplasmas opens a rich new area of investigationin basic physics. It also promises new applica-tions, such as tuneable radiation sources.

The emergence of ultrafast relativistic plasmaphenomena as a topic of basic research coincides

•with the development of short-pulse laser andbeam technologies capable of generating r_lativ-istic and._onlineareffects in plasmas. However,the opportunity to explore this new realm ofplasma physics has not been exploited. Thetechnologies and facilities required are new.Furthermore, many of these topics can only beexplored experimentally with a test-bed facilitybringing together a state-of-the-art relativisticpositron beam and short-pulse high-power laserbeams. These two major items do not currently

• exist at a single place.

NONNEUTRAL PLASMAS

Nonneutral plasmas have recently become anestablished subfield of basic plasma science thatpresents an important opportunity for researchand training. Applications of normeutral plas-mas are growing steadily in number and impor-tance as the base of fundamental understandingincreases.

The term "nonneutral plasmas" refers here tocollections of particles all having the same signof charge, such as pure electron plasmas, or

pure ion plasmas. Such plasmas are u.nusuallysimple botl_ experimentally and theoretically;

nevertheless, they exhibit a wide range of collec-tive and statistical mechanics effects. The plas-

mas are typically confined in cylindrical geome-try in a uniform magnetic field. They have beenconfined for much longer times than ordinaryplasmas. Except for a slow rotation, nonneutralplasmas can be formed at rest in the laboratory,enablingsubstantially different experiments thancan be performed on moving particle beams.Wall and impurity interactions can often be

made negligible, and so the plasma evolutioncan be governed by plasma physics uncompli-cated by atomic and surface physics.

Nonneutral plasmas are simple enough thatdirect comparisons between theory and experi-ment can be made. Much of this simplicitystems from the fact that nonneutral plasmas canrelax to a confined thermal equilibrium state.For theory, this means that the powerful tech-niques of equilibrium statistical mechanics canbe applied; for experiments, it means that onemay create quiescent, long-lived plasmas. Bothwaves and transport are readily studied innonneutrai plasmas, and. it seems likely thattr_insport can .fie fully understood, in at least

"some regimes. This will not in iLself explaintransport in neutral plasmas, but it would go along way toward establishing a set of transportparadigms that can then be applied to morecomplicated systems.

Much of the early research was directedtoward magnetrons, gyrotrons, and other radia-tion-producing devices with a high degree ofsuccess. The Hi-Pad experiment at the Avco-Everett Research Laboratory in the 1960s gener-ated a seminal body of nonneutral plasmatheory before the project terminated. Experi-ments and theory at the University of Marylandin the 1970s developed the field of nonneutralplasmas, albeit mostly in the direction of ener-getic beam dynamics. Other groups havecontinued developing nonneutral plasmas in theenergetic regime. More recently, low-energynonneutral plasma experiments and theory havebeen pursued at the Universities of California atSan Diego and Los Angeles.

Recent experiments on thermal relaxation and

particle transport illustrate how nonneutralplasma experiments and theory can develop"textbook" plasma physics. One set of experi-ments measured the relaxation that occurs after

the parallel and perpendicular temperatures aremade unequal. These experiments cover a verywide range of temperatures (5 x 10-3 eV < T <200 eV) and densities. The first experimentsgave the first quantitative verification of thisfundamental collisional relaxation process in theweakly magnetized regime. Recently, thetheory was extended into the highly magnetizedregime, resulting in a new theory of collisionaldynamics and an unusual many-particle adiabat-

10


ic invariant. Experiments in this regime thenobserved a dramatic five-decade drop in thecollisionality as the magnetization was in-creased, giving detailed verification of the newtheory.

Nonneutral theory and experiments on parti-

cle transport across magnetic fields have givensubstantial insight into this fundamental processbut are far from complete. From a theory

perspective, the trapped, quiescent nonneutral•plasma is ideal ila that all plasma parameters

and boundary conditions can be determined.Recent theory work hag attempted to system-atize the long-range interactions between parti-cles that arise due to shielded static fields,

dynamical waves, and discrete partide fluctua-tions. Experiments have measured enhancedtransport consistent with the new theories, butdetailed comparisons have not yet been made.When understood, these nonlocal =viscosity"processes may be found to be important inneutral plasmas also.A recent series of experiments of_nonlinear

vortex dynamics further illustrates how non-

neutral plasmas can elucidate _fundamentalphysics. It turns out that magnetized electronplasmas areone of the best experimental mani-festationsof two-dimensional inviscid fluids:

the equations governing drift motions of theelectrons across the magnetic field are isomor-phic to the two-dimensional Euler equations fora constant-density fluid such as water. Withproper imaging techniques, the flow of the

electrons in the plane transverse to the magneticfield can be measured accurately, for detailedcomparison to theory. Furthermore, the plasmacan be formed so as to have no turbulent boun-

dary layer at the walls, making the fluid dynam-ics substantially simpler. Recent electron experi-ments have illustrated the nonlinear interactions

of vortices, extending results from computersimulations and water tanks. More generally,the plasma experiments can test fluid instabili-ties such as the Kelvin-Helmholtz shear instabili-

ty in realistic systems, and some stimulatingdiscrepancies between the discrete particleexperiments and fluid theory have been found.

Nonneutral antimatter plasmas are a field

where fundamental physics experiments andplasma applications overlap. Positron plasmasare now routinelycontained and manipulated inthe laboratory, and unexpected physics isemerging. A serendipitous study of positron-molecule resonances led to closer scrutiny of thestandard molecular excitation model. One

intended application of the positron plasma is toinject positronium into tokamak fusion plasmasand measure the transport by observing the

positron-electron annihilation radiation. A new• _ . ¢' . • " ..,

posttron-trappmg experiment LSbeing developedat the Lawrence Livermore National Laboratoi'yto Study positron-electron scattering, motivatedby the possibility of discovering a new particle ata few MeV.

Nonneutral plasmas have a major applicationto basic research on fundamental constants and

time standards. Here, one or more ions areco.ntained at low temperature in magnetostaticor shaped RF fields for detailed study. Contain-ment of many ions, desirable from signal-to-noise considerations, is often made undesirable

because of many-particle plasma physics phe-nomena; some of the most elegant experimentshave been carried out on single partides. Thisline of research has recently been extended tothe fundamental propertiesof antimatter. Forexample,researchers are currently attempting tomeasure the inertial and gravitational masses ofantiprotons, and these techniques should beapplicable to a variety of fundamental constants.

Recently, theory and experiments have beenprobing the correlated many-particle regime ofcrystalline ion dusters. Here, the whole rangeof plasma physics, many-body physics, statisticalphysics, and thermodynamics can be studied inan exceedinglysimple experimental system.

There is a direct correspondence between thethermal equilibrium properties of these trappedions and the much-studied "one-componentplasma" that occurs when one charge species isquantum-mechanically degenerate. However, industers of a few thousand ions, surface effects

are also important, and the phase transitionsfrom uncorrelated gas to liquid to solid becomemore subtle. Using sophisticated laser tech-

niques, experimentalists are able to directly

11


measuremany properties of the correlatedplasmas for comparison to theory.

Much of the research on low-energy non-

neutral plasmas began as initiatives by individu-al principal investigators with long-term supportfrom the National Science Foundation. Support

from the Department of Energy for nonneutralplasma research has been focused on that whichhas direct relevance to the fusion program, suchas tests .of resonant particletransport theory or

development of a positron diagnostic for toka-maks. The recent cutback by the Air Force.Office of Scientific Research for work on ion

traps and fundamental constants leaves theNational Science Foundation as the mainstay of

university research in the field. Recently, theOffice of Naval Research sponsored a researchinitiative in nonneutral plasmas that has helpedestablish a number of new experiments and

theory programs. New nonneutra.I plasmaexperiments are planned or urider way at fouruniversities and a national laboratory. In short,

the.subject of nonneutral plasmas is an exampleof a field whose potential cannot be realized bysingle-agencyprograms. Multiple-agencysup-

port is necessary to sustain the growth of thisnew area of basic plasma science.

DIAGNOSTICS AND BASIC PLASMAEXPERIMENTS

Measurement is the basis of physical knowl-

edge, and the techniques for measuring the

propertiesof plasmas, generallyreferredto as

plasma diagno_ics,playacriticalroleinadvanc-

ingplasma science.

Even when thetemperatureislow enough to

permitmeasurements insidetheplasma,deter-

mining,themany significantplasma parameters

that vary in complex spatial and temporal pat-terns is a formidable challenge. Measurements

of the temperatures of electrons and ions, ofions other than hydrogen, of the currents andelectric and magnetic fields within the plasma,and of the flow, drift, or rotation velocities are

needed. Understanding instabilities, fluctua-

tions, and turbulencerequiresthemeasurementofvariationsindensity,temperature,and fields

over a broad range of spatialand temporal

scales. The volume of data needed to character-

ize turbulent transport is impressive and re-quires the advanced data analysis techniquescharacteristic of numerical computations and of

space research.In no part of plasma science has the effort to

improve diagnostics been more focused than infusion research. In the case of fusion plasmas,

the extreme temperatures constrain one toobserve from the. outside the hot .gas itself.

Improved tech.njques for each moasurable quan-•tity have been combined .in a panoply of dia'g-nostics to give a comprehensive, compositepicture of the plasmas under study. Foremostamong diagnostic developments over the past 25years has been the establishment of Thomsonscattering as a universal standard for determin-ing electron temperature. The demonstrationthat many plasmas behave as black bodies at theelectron cyclotron frequency has made possibleanother simple measurement of electron temper-ature. Measurements of density have also

improved steadily with the development ofshorter-wavelength-microwave and far-infraredinterferometers. Spectroscopy has long beenwell established for cooler plasmas (those withtemperatures lower than a million degrees), butthere has been a great improvement in spectro-scopic measurements of hot plasma, lon tem-peratures can be measured by spectroscopictechniques using Doppler broadening or by thetraditional method of charge exchange, whichhas been refined and improved over the pastdecade. Measurements of the electric potentialor electric field within the pla._-ma using energet-

ic heavy-ion beams have been developed. Tech-niques from surface physics have been adaptedto study the interaction of plasma with the wallsof the vacuum vessel and the introduction of

impurities into the plasma. Crucial to the use ofall of these diagnostics has been the rapid andunceasing improvement of advanced computer-ized data-acquisition and data-processing sys-tems. Online data processing has enabled farmore intelligent and productive operation of

very expensive experiments.The committee has dwelled atlengthon the

development of fusiondiagnosticsin order to

12


make three pints. First, plasma science is aninformation-hungry subject. Next, only by theearly 1990s has it been possible to measure all ofthe quantifies needed to characterize plasmadynamics and transport in the laboratory. Thus,for the first time, theory can be confronted bymeasurements of the comprehensiveness re-quired for a meaningful test. This fact is implic-

itly recognized in the so-called "transport initia-five" recently und0rtaken by the U.S. fusionresearch program. The final point is critica! tothe/'esponsibilities of this committee: the use ofcomprehensive diagnostic techniques and otherstate-of-the-art experimentaltechniquesisvery

unevenly spread within plasma science.The uneven application of advanced tech-

niques implies an uneven experimental under-standing of the different types of plasmas understudy, which complicates the task of identifyingpromising new opportunities. Nowhere is thistruer than with the typically small-scale experi-ments whose purpose is to test our understand-ing of the principles of plasma physics. Here,the commitment to such desiderata as compre-

hensive diagnostics is an individual one, and itis often difficult to assemble the necessary

capability from fractionated funding sources.Because the number of basic laboratory investi-

gators is small, and not all of these have accessto state-of-the-art techniques, it is difficult toforecast what could be done with a state, f-the-

art approach to basic issues.The situation of basic laboratory plasma

experimentation in the United States has evokedexpressions of concern in the past; v/z., theremarks of the Plasmas and Fluids Panel of the

National Research Council (NRQ study, Physics

Through the 1990s (National Academy Press,Washington, D.C., 1986): "Direct support forbasic laboratory plasma physics research has

practically vanished in the United States. Thenumber of fundamental investigations.., issmall, and only a handful of universities receivesupport for basic research in plasma

physics .... "The Plasma Science Committee has found no

evidence of any significant change in this stateof affairs in the five years since the above state-

ments were written, despite generalized goodwill. But the field has not been completelystatic--several extremely important basic plasma

experiments have been completed in the recentpast. Of the recent research carried out on smalllaboratory plasma devices, three efforts stand outfor their general significance. A basic theory ofmodulational plasma turbulen_:e was tested in theiaboratoryforthe first time. (Similarexperimentswere subsequently carried out using high-powerRF irradiation of the ionosphere.) This theory,developed in 1972, involves the most m_lernideas in nonlinear science. It had been proposedin the 1940s that current-carrying plasmas would

developcollecfive strudures called double layers,and indirect evidence from space indicated that

double layers might play a role in accelerating theelectrons responsible for the aurora. However,there was no firm reason to believe in their

existence until they were created and studied inthe laboratory. Experiments on magnetic fieldreconnecti0n, a process thought involved in suchspectacular events as solar flares, magnetosphericsubstorms, and tokamak disruptions, werecarried out with comprehensive diagnostics forthe first time, but the magnetohydrodynamic

range of scale lengths was not achieved.The Plasma Science Committee is convinced

that the issue of basic collisionless plasma exper-iments needs urgent attention. An appropriatelevel of basic research is necessary for the vitali-

ty of any scientific undertaking and is necessaryto ensure that related applied research flourish-es. Basic research may even be more importantfor the discovery of new applications, for it is

only as understanding is distilled into funda-mental form that it becomes portable, to be

passed between research communities andscientific generations. From this perspective,the virtualdisappearance of basic experimentalresearch on neutral plasmas, particularly inuniversities, is of serious concern. Not only is

experimentation the ultimate test of understand-ing, but no application is possible unless therehas been laboratory experience with the relevantprocesses. The committee is made uneasy bythe prospect of continued neglect of basic labo-

ratory research in plasma science.

13


ANALYTICAL THEORY

AND COMPUTATION

Analytical TheoryThe high-temperature laboratory plasma state

is characterized by an enormous range of bothtemporal and spatial scales: from 10-_1 sec, theperiod of a plasma oscillation, to approximately1 sec for the energy transport time, and from10.3 cm for the Debye length to 102 cm for the

density-gradient length in a large tokamak. For•astrophysical and "space plasma, these rangesspan even more orders of magnitude. In addi-tion, there is a large array of dimensionlessnumbers that are needed to define the state of

plasma. By way of contrast, relatively fewquantities, like the Reynolds number, are need-ed in hydrodynamics. The complexity andrichness of the plasma state are both dauntingand challenging.

The plasma state is the preeminent candidatefor the study of nonequilibrium statistical me-chanics, a field of great importance and of whichour present understanding is rudimentary. Thepath to equilibrium is determined by processesinvolving the huge number of collective degreesof freedom of the system. This aspect is truenot only of multicomponent plasmas but also forbeams of charged particles in high-energy accel-erators, pulsed power machines, and so on,when the number density exceeds a criticalthreshold, and even for single-component plas-ma.

The fundamental kinetic equations for theplasma have been available for some time, andthe connection between collisionless drift orbits

of particles and the fluid equations has beenestablished. The transport of heat and particlesby collisional processes in complex magneticgeometry has been _liy attacked. Thesituation is far less tractable when collisions are

rare and collective processes intercede. A briefoutline of what has been atx'omplished analyti-cally so far in this direction follows:

• The linear theory of perturbations, whichidentifies the various collective modes of a

plasma in inhomogeneous geometry, isalmost a closed topic.

• The linear stability of plasma to low-fre-

quency perturbations has been analyzedthrough a hierarchy of "energy principles*or variational principles of increasing so-

phistication.• A quasilinear theory of perturbations has

been developed to establish the lowest-order interaction between the particles andcollective modes.

• The weak interaction amongst particles anddispersive collective modes is described interms of a kinetic equation for the occupa-tional number of the collective modes. The

phases of these modes are assumed to berandom with a Gaussian distribution.

• The strong interaction between particlesand almost-nondispersive waves has been

approached in terms of field theoreticmethods that involve mass and chargerenormalization and are known as the

Direct Interaction Approximation (DIA).The distribution departs from the Gaussian.In this approach the principal effect of thequadratic nonlinearity is to cause the non-linear damping of the collective mode.

Coupled equations for the turbulent spec-trum and nonlinear damping emerge fromthe analysis. Turbulent transport of heat,particles, and so on can be calculatedthrough this technique, and it is found thatOnsager's principle may be violated be-cause the conditions are not those of equi-librium statistical mechanics.

• The renormalization group analysis (RNG)developed originally in the theory of phasetransitions has been applied to plasmaequations in those regimes where scaleinvariance holds. This approach is particu-larly useful for obtaining renormalizedtransport coefficients in the infrared limitresulting from short-wavelength fluctua-tions. Thus, it may be useful for develop-ing subgrid models for computational solu-tion of plasma equations.

• The current widespread interest in solitarywaves or solitons was stimulated in signifi-cant part by problems in plasma physics.It was plasma physicists--aware of the

14


physics of nonlinear dispersive waves inplasmas and shallow water--who formulat-ed the so-called inverse scattering theory 25

years ago. A large class of nonlinear dis-persive systems has been shown to evolveinto a sequence of robust localized struc-tunes called solitons.. In multidimensional

space the soliton may turn out to be unsta-ble and lead to a collapsing soliton or cavi-ton. Theories of high-frequency Langmuirturbulence based on solitons and cavitons

as the elements of a dynamical systemhave been verified in experiments in thelaboratory and the ionosphere. The solitonconcept proved to be so general that itsubsequently found fruitful application infields ranging from polymer chemistry toquantum field theory.

• Modern developments in the theory ofchaos in HamUtonian systems have alsobeen triggered by the problems of plasmaphysics; indeed the dynamics of trappedpartides in waves led to the well-knownChirikov-Taylor map. The stochasticity ofmagnetic field lines under certain condi-tions and the transport of particle and wavepropagation in such stochastic magneticfields are under active discussion. The

analysis of chaos in dissipative systems oflow order has many applications in plasmaphysics.

s An example of a plasma physics problemwith strong nonlinearity that has beensuccessfully solved is the collisionless shockin which dissipation is due to collectivemodes. The future holds the promise of

many such problems, especially in theinteraction of very high power short-pulsedlasers with plasma, where an electroncould reach relativistic energy in one cycle

of the laser frequency, and in pulsedpower applications to intense-charge-par-tide-beam physics and technology.

• There has been a very successful attempt atthe application of boundary layer theory toan almost dissipationless plasma. Butmuch more needs to be done, especially inspace and astrophysical plasmas, where

events take place on short time and spatialscales that cannot be understood in termsof the normal constraints on the motion on

a highly conducting plasma.

Since, in the main, linear theory is well

established, the major challenges lie in thenonlinear regime. The grand challenge inplasma theory is a compete understanding ofplasma transport and its relation to plasmaturbulence. However, many of the properties oftransport and turbulence in fusion plasmas arespecific to device geometry, and it is not yetpossible to arrive at a detailed understandingthat separates devioe-speciflc from fundamentalphenomena. What is needed is an equivalent ofthe well-known lsing model in ferromagnetism,in other words, a dear theoretical paradigm,and experiments that test the paradigm.

Theoretical plasma science is now at a stageof development analogous to that of hydrody-namics in the last quarter of the nineteenthcentury: the fundamental equations were wellunderstood and there was much mathematical

development. However, theory could notalways predict what the engineers obtained inpractice. It took Prandtl with his boundary layerto bridge the gap between ideal theory andexperiments. This development brought fluidmechanics to the point where today manypractical engineering problems have been re-duced to a matter of computation.

Today, plasma scientists face a similar situa-tion with respect to the issue of predictinganomalous transport. It will be necessary toforge a strong link between theory and appro-priately designed experiments to obtain theanswers to key questions before anomaloustransport is reduced to engineering practice.Forging such a link should lead, at least, to anunderstanding of the principles of plasma be-havior and, optimistically, to stronger founda-tions for the theory of nonequilibrium statisticalmechanics.

Computational Plasma Science

Computational plasma science has developedinto a powerful tool for understanding and

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II II


predicting the behavior of plasmas. The powerof computers has increased by six orders ofmagnitude over the last 30 years and showsevery indication of continuing this level ofgrowth over the foreseeable future. During thesame time, the methods of modeling plasmashave also improved greatly. Techniques arenow available for looking at plasmas on thefastest time scales (particle models) or on inter-mediate time scales (gyrokinetic, implicit mod-

els) and on long time scales (fluid models).Supercomputem and computational tech-

niques have advanced to the point that it nowseems possible to model tokamak behaviorstarting from physics-based models. This couldhave an important impact on the fusion pro-gram, and, in particular, such models mighteventually become useful as tools to designfusion reactors,

In the realm of space plasma, computer

modeling has brought new insight into therichness of the physics occurring in magnetized

plasma shock waves. High-Mach-numbershocks are proving to be recurrent and un-steady: they steepen, break, and steepen again.This physics was much too difficult for analyticphysics to describe, and it was almost impossi-ble to discern with clarity in space measure-ments. Plasma simulations are also sheddinglight on how particles can be accelerated inspace, in the aurora, in solar flares, and incollisionless shock waves. The next generationof space plasma experiments will use computersimulations as a regular tool in data analysis:the computer will simulate what each detectorought to observe as the various spacecraft of theInternational Solar-Terrestrial Physics Programmove across the different spatial regions of themagnetosphere. Global modeling will enablespace experimentalists to visualize the time-dependent behavior of the earth's magneto-sphere in three dimensions, thereby supple-menting the single-point measurements ob-tained by spacecraft.

Computer modeling has led the way in recentstudies of applications of plasma to high-energyphysics. It has also shown how plasmas can beused to make new sources of light with unique

properties (short pulse, chirped, up shifts offrequencies, and more). While experimentshave lagged far behind the computer modeling,a number of experiments have verified thecomputer results. Furthermore, computermodeling has proven to be an invaluable tool ininterpreting those experiments that have beencarried out.

.Computer models let us explore nonlinearplasma phenomenon in ways that we could nototherwise do. It seems clear that computermodeling will play a very important role in

understanding plasma turbulence. Computermodeling allows us to develop our intuition and

insight into what processes are really important.In the related area of the nonequilibrium statis-tical mechanics of plasma, modeling lets us peermuch deeper into the fundamental processes atwork.

Computer modeling promises to have a largeimpact on the teaching of plasma science. Anumerical plasma laboratory would allow stu-dents to carry out fundamental experiments inplasma science. The student could try out his orher own ideas, undoubtedly the most effet-tiveway to learn about plasma behavior.

Finally, building new and better numericalmodels of plasmas is full of challenges. Forexample, how does one best model a systemwith an enormous number of degrees of free-dom on a computer that is finite, though verylarge? This question strikes to the heart ofstatistical mechanics. • Also, how does oneextract useful results from the enormous mass

of data provided by modern computations?

SummaryThe analytic theory and computation carried

out in plasma science participate with greatvigor, and in several cases lead, in the practicaldevelopment of new concepts in nonlinearscience and nonequilibrium statistical mechan-ics. Theory and computation are proving to befertile generators of new ideas in plasma sci-ence. Plasma computation is still a new andactive field, and new methods are continuallybeing developed to take advantage of the relent-less advance of computing technology.

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The challenge for the near future is to find the

level at which the integration between theory,computation, and experimentation can be mosteffective. Since theory and computation cannotyet deal with the enormous complexity of manyplasma configurations of contemporary interest,many plasma scientists argue that integrationcan best be achieved with carefully tailoredexperimental and computational configurationschosen for their theoretical richness and signifi-cance.

SPACE AND ASTROPHYSICAL PLASMAS

More than 99 percent of the baryonic matterin the universe is in the plasma state. Spaceand astrophysical plasma physicists study theseplasmas. The distinction between the fields ismainly operational; space plasma physicistsperform in situ experiments on solar systemplasmas, while their astrophysical cousins relyentirely on remote sensing of more distantsystems. Yet these two fields share a number ofunifying themes, because the physical processesobserved in space plasmas appear to be analo-gous to those inferred in remote astrophysicalsystems. Among the most important of theseare the transport of heat and of angular momen-tum by collective processes, the acceleration ofultrahigh-energy particles in flowing plasmas,

the generation of high-brightness temperatureradiation by collective processes, and the gener-ation of magnetic fields in magnetohydro-dynamic dynamos.

Several examples illustrate the close relation-ship between space and astrophysical plasmaresearch. Spacecraft experiments have foundthat heat conduction in the low-density solarwind plasma is controlled not by electron-ioncollisions, but rather by the collective processesdriven by rapidly streaming electrons, togetherwith the natural limit to heat conduction set bythe fact that the heat-conducting electronscannot travel much faster than the electron

thermal speed. The problem of electron heattransport appears in many astrophysical con-texts, from galaxy dusters to the solar corona.The genetic issue is a cool cloud immersed in asurrounding hot gas; for "cloud," one can

substitute "star" or "galaxy. _ As heat is con-ducted into the doud, mass is evaporated ("ab-lated _) and blows away in a "wind." In therecently discovered "black widow" pulsars, theenergy lost from a neutron star in a binary starsystem causes mass to be lost from the "nor-mal" star; the rate of mass loss is controlled byheat conduction, just as is the case for the solarwind.

There has been significant progress in under-

standing the acceleration of high-energy particlesby shock waves. The original ideas were invent-ed mostly by theoretical astrophysicists in orderto understand the origin of cosmic rays. Inter-stellar shocks are created by stellar ex-plosions-supernovae; observations of sy_hro-tron radiation from supernova remnants revealthe presence of shock-accelerated relativisticelectrons, and it is natural to assume that ions

are accelerated also. Using these ideas, spacephysicists interpreted early spacecraft observa-tions of high-energy particles associated withshock waves in the interplanetary medium. Inthe late 1970s, efficient conceptual models of ionacceleration were devised, which were shown

later to explain observations of high-energy ionsassociated with interplanetary shocks. Theseconceptual models can now be applied withincreased confidence to the acceleration of

cosmic-ray ions in the astrophysical setting.

Nonlinear simulations and theory are now beingused to try to understand why the particleacceleration processes are so efficient.

The understanding of ion acceleration bycoUisionless shocks will result in increasingquantitative confidence and predictive power inthe 1990s. However, understanding of shockacceleration of electrons is much less secure.

This issue is essential to astrophysics, sinceremote sensing picks up photons emitted byaccelerated electrons.

The transfer of angular momentum in rotatinggaseous systems is an issue that appears both inthe formation of stars and in the acx:retion of

plasma onto compact objects. In both cases, theangular momentum of the accreting gas mustbe almost totally removed, if the gas is to un-dergo the observed infall, to form the final star

]7


or fuel the x-ray emission from the neutronstar or black hole. Theorists suspect that mag-netic stresses are the origin of the torques thatforce the gas to aocrete. Understanding angu-lar momentum transport requires the applica-tion of dissipative magnetohydrodynamics to a

differentially rotating system with ill-understoodsources of turbulence in the flow. Recent work

has revealed a mechanism by which a magneticfield might cause an instability in a disk thatcauses the gas to accrete. However, real prog-ress will come when computation and/or experi-ment can address the behavior of the finite-

amplitude fields in the accreting matter. Here,experiments might be the more informative,since many aspects of the three-dimensionalmagnetic stress are reproduced in laboratorydevices such as the spheromak, and rotation canbe produced by application of an electric field.

The dynamo generation of magnetic fields isa problem that arises in planetary, stellar, andgalactic contexts and that raises fundamentalissues of how large-scale, ordered magneticfields can be created by conducting, rotatingbodies. The basic issues are not yet re-solved--for example, is dynamo activity distrib-uted throughout the medium, or is it spatiallyintermittent with localized regions of intensefield growth on the boundaries of rotating fluidcells? Recent progress has been slow, becausethe simple phenomenological things have beendone, and theory and numerical simulation needbetter computational tools. No one has consid-ered formally whether an investment in basicexperimental research on dynamo activity inhighly conducting plasmas would be well spent.

Thus far, the committee has discussed the

fruitful relationship between space and astro-physical plasma physics. It will not reviewspace plasma research per se here, because it hasfrequently been reviewed elsewhere. Nonethe-less, the committee has a responsibility to com-ment on the relation between space plasmaresearch and laboratory plasma science. Thetimeless part of the relationship was eloquentlyaddressed in the NRC study Space Plasma Phys-ics: The Study of Solar System Plasmas (NationalAcademy of Sciences, Washington, D.C., 1978):

Space and laboratory experiments are com-plementary. They explore different ranges ofdimensionless parameters. Space plasma con-figurations usually contain a much largernumber of gyroradii and Coulomb mean free

paths than achieved in laboratory plasmaconfigurations. In the laboratory, spfeialplasma configurations are set up intentionidly,whereas space plasma configurations assumespontaneous forms that are recognized only asa result of many single-point measurements.Space plasmas are free of boundary effects;laboratory plasmas are not, and often sufferseverely from surface contamination. Becauseof the differenc_ in scale, probing a laboratoryplasma disturbs it; diagnosing a spa_ plasmausually does not. The pursuit of static equilib-ria is tx,ntral to high-temperature laboratoryplasma physics, whereas space plasma physicsis concerned with large-scale time-dependentflows.

Certain problems are best studied in space... certain problems could be more conve-niently addressed in the laboratory... Thea-ry should make the results of either laboratoryor space _riments m_dld_ for the benefitof the whole field of plasma physics.

To these remarks, the Panel on Physics andFluids added, in Physics Through the I990s (Na-tional Academy Press, Washington, D.C.,1986):

The recent strengthem'ng of theoretical spacephysics, together with the increasing cala_'li-ty of qxtce plasma instrumentation and thesuperioffty of the space plasma environmentfor certaintypesofmeasurements, means thatthe experimental diagu_'s and theoreticalinterpretation of certain space-plasma processesnow matches the best of current laboratory

mrti .

In summary, the solar system is an experi-mental test-bed for plasma concepts that areimportant to astrophysics and new to the labo-ratory. Indeed, questions first raised in spaceresearch have stimulated experiments in the

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laboratory devoted to problems such as magnet-ic field reconnection and double layers. None-theless, the Plasma Science Committee seessome evidence of structural problems that limitthe contributions space and laboratory plasmaresearch can make to each other. Laboratoryinvestigations of plasma processes discoveredfirst in space are often considered basic plasmascience by the space community and spacescience by the laboratory plasma science com-munity. And rarely does anyone ask whetherlaboratory mastery of a plasma process thathappens to have been uncovered first in spacemight lead to useful applications.

The so-called "critical ionization" problem isone pertinent example. Several decades ago, itwas proposed on dimensional grounds that aneutral gas in a magnetic field can suddenly bebrought to full ionization when a plasmastreams over it with an energy equal to theionization energy of the gas. The original appli-cation of this idea was to solar system andplanetary formation. Recently, space plasmaphysicists, stimulated by observations of theinteractions between neutral and ionized gases

in the Io torus in Jupiter's magnetosphere, inartifidal chemical injections in Earth's iono-sphere, and in cometary atmospheres, havearrived at a firmer understanding of the micro-scopic plasma processes that lead to criticalionization. Without a laboratory research pro-g_m to explore the usefulness of the criticalionization phenomenon, it will be difficult todetermine whether the process has potentialtechnological value. Neither space measure-ments nor computational modeling can answerthis question.

III

Concluding Remarks

The Plasma Science Committee hopes that

this research briefing, which discusses plasmascience in its entirety (we believe for the firsttime), will not only communicate the breadthand depth of the discipline, but will also lay thefoundations fora broad conceptual framework inwhich derisions concerning plasma science canbe considered.

Plasma science is particularly rich in the areaof nonequilibrium and nonlinear processes. It

was one of the first disciplines to grapple withthe relationship between nonlinearity and com-plexity, a problem that is recognized today to bea hallmark of late-twentieth-century .science.Nothing clarifies better the potential impact ofachieving a broad and fundamental understand-

ing of plasmas than a survey of the diversity ofthe applications of plasma science. However,the communities, committees, and agenciesconcerned with the applications of plasmascience--fusion, space and astrophysics, plasmaprocessing, energy conversion, military-directedenergy research, and others---naturally concen-trate on those efforts that have a direct andimmediate connection with their missions. In

the committee's view, decisions that are overly

constrained by considerations of relevance toone application may lead to missed opportu-nities, not only in basic research, but also fornew and unexpected applications in other areasof plasma science. It is the committee's convic-tion that such opportunities for research anddevelopment in plasma science are abundant.

1. Missed opportunities may well be the most serious consequence of the present approach. Many ofthese issues are being addressed by the Panel on Plasma Processing of Materials commissioned by theNational Research Coundl. The panel will complete its review and assessment during 1991 and willformulate specific recommendations designed to improve the organization and strength of the plasma

science that supports the burgeoning applications in this field.

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research briefing on contemporary. problems in plasma science

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