LASER FUSIONon aTABLE TOP

Todd Ditmire

For many years,laser-driven fusion has been the realm of large-scale laser science.A number of recent experimants have demonstrated that compact,femtosecond lasers of modest size can also drive nuclear fusion.

T he development of ultrahigh-in-tensity, ultrashort-pulse lasers inthe late 1980s and 1990s has led to

a revolution in the study of laser interac-tions with matter. Lasers of this class,which use chirped-pulse amplification(CPA)1 and now routinely produce laserpulses with peak powers of many terawatts(1012 W) in pulse durations of a few tensof femtoseconds, yield unprecedentedlight intensity. When focused to spot sizesof a few micrometers, these lasers achieveintensity ranging from 1018 W/cm2 to over1020 W/cm2. Even more remarkable is thefact that the latest generation of lasers thatcan produce such extreme brightness aretabletop in scale.

Only in recent years have researchersbegun to explore the wealth of science ren-dered accessible with this ultraintenselight. Under irradiation at such extremeintensities, exotic forms of matter result.These states include plasmas with pres-sures exceeding 1 Gbar and electronbeams accelerated to highly relativistic en-ergies of many MeV. Another fascinatingfinding that has come to light in a numberof recent experiments is the fact that table-top lasers can drive nuclear fusion.

Nuclear fusion in laser-produced plasmasFor many years, laser-driven fusion hasbeen the realm of large-scale laser science.In particular, fusion research with intenselasers has centered on the study of thetechnique known as inertial confinementfusion (ICF).2 In this approach, a releaseof nuclear energy is sought from nuclearreactions between deuterium and/or tri-tium ions through the nuclear reactions D + D → He3 + n (+ 3.3 MeV of excess en-ergy), D + D → T + p (+ 4.0 MeV), or D + T → He4 + n (+17.6 MeV). These nu-clear reactions will only occur with anysignificant probability if the plasma isheated to high temperatures (>107 K ormany keV). In the ICF experiments, alarge, multibeam laser irradiates a pelletcontaining deuterium or a deuterium/tri-tium mixture. The pellet implodes underpressure from the ablation of material bythe laser. The ablation pressure of the lasercompresses and heats the fuel to a condi-tion in which nuclear fusion occurs. Inthese experiments, nanosecond-durationlasers with ten or more beams are used,and pulse energy of many kilojoules isusually necessary. Such lasers—like the re-

cently deactivated Nova laser at LawrenceLivermore National Laboratory, or the op-erating Omega laser at the University ofRochester—are quite large in scale.

This approach holds the greatest prom-ise of attaining ignition, the state at whichthe nuclear fuel burns on its own and, ulti-mately, releases more energy than was in-jected to implode and heat it in the firstplace. Any real hope of using laser-drivenfusion to drive a power plant still restswith the ICF approach, which is why ICFresearch is being pursued in a number oflaboratories around the world. The con-struction at Lawrence Livermore of theenormous National Ignition Facility laser(which, when it is completed, will deliver1.8 megajoules of ultraviolet laser energy)represents the culmination of many yearsof successful ICF research in the U.S.

Nuclear fusion with large lasers hasbeen achieved in a number of other waysas well. Although the experiments differfrom one another, generally they all relyon colliding plasmas produced from theexplosion of a nanosecond-laser-heatedtarget containing deuterium.3 A large laserheats a target with nuclear fuel and the ex-plosion of the hot plasma gas generates thehot ions necessary to drive the fusion. Theprincipal product of such experiments isthe release of neutrons from the nuclearfusion reactions (through the D + D →

He3 + n reaction). Such experiments oftenproduce quite a lot of neutrons, althoughachieving this result still requires facility-scale, ~kilojoule-class lasers that fire insingle-shot mode.

Recent experiments using compact,femtosecond lasers have now demonstrat-ed laser-driven fusion on a much moremodest scale. In these experiments, scien-tists have observed nuclear fusion, and theresultant release of energetic neutrons,from lasers with pulse energy of less than 1 J and repetition rate of 10 Hz. Such ex-periments have leveraged the advantage ofCPA lasers, which allow high pulse intensi-ty to be achieved with modest pulse ener-gy. Deuterium fusion (DD fusion) bymeans of compact, tabletop-scale lasersrepresents a different class of experimentsfrom the large-scale fusion research de-scribed above: the physical mechanismsfor creating the hot ions to drive the fusionare different. Furthermore, the high-repe-tition-rate capabilities of modern, fem-tosecond multiterawatt lasers enablesstudy on a level of detail not possible withlarge-scale systems. Experiments with ter-awatt, femtosecond lasers are interestingnot only because of the insights they yieldinto the study of high-intensity laser inter-actions, but also because they offer thepromise of a unique, high-repetition-ratesource of (pulsed) high-energy neutrons.

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Figure 1. Events that lead to DD fusion in a solid target irradiated by an intense short-pulse laser. First,the laser accelerates electrons in plasma at the surface.These electrons then race into the target, settingup an electrostatic potential which propels the deuterium ions behind them.The accelerated ions thenfuse with cold ions in the bulk material.

Fusion in solid targets:the laser ion accelerator The first class of experiments in which DDfusion has been observed using tabletoplasers is in interactions of terawatt-laserpulses with solid targets. A number ofgroups have observed DD fusion fromplasmas created when these pulses are fo-cused onto the surface of a deuterated tar-get, most often a deuterated plastic target(i.e., [CD2]n).

For example, Pretzler et al. observed fu-sion neutrons when a 200 mJ, 160 fsTi:sapphire laser was focused on the sur-face of a slab composed of deuteratedpolyethylene powder.4 The laser was fo-cused to an intensity of about 1018 W/cm2

on the surface of these targets at 10 Hz, re-sulting in roughly 102 n/shot being emit-ted in all directions. Similar experimentswere performed by Hilsher et al., who fo-cused 300 mJ, 50 fs pulses on deuteratedpolyethelene, also at an intensity of ~ 1018

W/cm2[Ref 5]. In these experiments, thescientists observed around 104 neutronsper shot. The neutrons, which arise fromthe D + D → He3 + n reaction, have well-defined energy near 2.45 MeV.

Although a number of explanationshave been offered for the appearance inthese interactions of fusion neutrons, it isgenerally accepted that the neutrons arisefrom the acceleration of deuterons by thelaser into the target, a physical mechanism

illustrated in Fig. 1. The accelerateddeuterons then collide with other (cold)deuterons in the target, and fuse.

As shown in Fig. 1, when the laser pulseimpinges on the target at high intensity,the light ionizes the solid and creates aplasma. Some expansion of this plasmaduring the pulse gives rise to the forma-tion of a cloud of plasma in front of thetarget (although this cloud may only be afraction of a laser wavelength thick, i.e.,<< 1 �m). As the intense laser pulse prop-agates into the plasma, it accelerates elec-trons. The complex mechanisms leadingto this acceleration are an active topic ofstudy. However, one simple mechanismfor this acceleration, the mechanismwhich dominates for very-short-laserpulses, can be understood by examiningthe trajectories of electrons of the plasmain the laser field. The oscillating electricfield of the laser accelerates electrons ofthe plasma back and forth, in and out ofthe interface between plasma and vacuum.During this field-driven motion, some ofthe electrons get launched into the solidduring a phase of the laser-field oscillationin such a way that the electron escapes theforces of the field before the laser can re-verse the direction of the electron. In thisway, electrons with energies of well over 1 MeV are possible (when the laser inten-sity is >1018 W/cm2, as it was in the exper-iments described in Refs. 4 and 5.)

These accelerated electrons set up anelectric field in the target, which then ac-celerates the ions behind them. In a sense,

the electrons pull the ions along withthem. If the target contains deuterium, thedeuterons get accelerated into the targetwith energies ranging from a few keV upto over 1 MeV. These energetic deuteronswill travel into the bulk of the target andfuse with other deuterons deep inside thematerial. One signature of this process is ashift in the neutron energy. If the deuterontravels into the target with high energy,then the energy of the neutrons that getemitted along its path will be shifted up toa higher level. (The shift occurs when theenergy of the incoming deuteron becomessignificant when compared to the total en-ergy release from the fusion event. In asense, the energetic deuteron gives an ad-ditional “kick” to the outgoing neutronalong the direction of the incomingdeuteron.)

Such a shift of neutron energy from adirected beam of deuterons into the solidtarget was observed recently in the experi-ments of Hilscher et al.5 The effect isshown in the data of Fig. 2 [from Ref. 5],in which the observed spectrum of neu-trons emitted into the target was centeredat 2.59 MeV, shifted up from an energy of2.45 MeV, the energy expected from fusionof near-stationary deuterons. The neu-trons observed in the backward direction(i.e., up from the surface of the target,back toward the incoming laser beam) ex-hibit an energy which is shifted down, to2.29 MeV. (This occurs because the neu-tron receives a momentum kick in a direc-tion away from the observer.)

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Figure 3. Illustration of how an intense laser pulse creates fast ions from a cluster.The short pulse firstionizes the atoms in the cluster (on the rising edge of the pulse).The intense part of the pulse quicklystrips the free electrons from the cluster, the ions of which then repel each other, leading to an energeticCoulomb explosion.

Figure 2. Neutron-energy spectrum data record-ed from fusion in a solid-target interaction. Theneutrons emitted forward are shifted to slightlyhigher energy while neutrons emitted backward(i.e., back toward the laser pulse) are shifted to aslightly lower energy. Reproduced from Ref. 5 withpermission.

Though the experiments are largerthan traditional tabletop experiments, thisapproach to driving DD fusion with short-pulse lasers has been scaled to much high-er neutron yields, using much higherlaser-pulse energy. For example, Disdier etal. observed as many as 107 neutrons froma solid target irradiated with a much high-er energy laser that emitted a 20-J, 400-fslaser pulse.6 Even higher yields have beenobserved at Lawrence Livermore NationalLab by users of the 500-J, 500-fs Petawattlaser.7 These experiments, performed onlarge-scale, “single-shot” machines, sug-gest the tantalizing possibility that muchhigher neutron yields may be achievedfrom solid-target experiments.

Fusion from clusters: a laserdriven “nanoexplosion”Another class of tabletop laser-fusion ex-periments revolve around the use of a verydifferent kind of target. In these experi-ments, the laser is not focused onto a solid;instead, the pulse is focused into a gas tar-get composed of small clusters of atoms.The clusters, which can range in size froma few atoms to many thousands of atomseach (i.e., a few nanometers in size), are awidely studied form of matter. Interest inthe interaction of high-intensity-laserlight with clusters has blossomed in recentyears because clusters are seen as a bridgebetween molecules and solids.8

Although clusters themselves can beproduced in a number of ways, the exper-iments conducted to observe DD fusionuse van der Waals bonded clusters of deu-terium or deuterated methane. When a gasof atoms or molecules is allowed to ex-pand into vacuum through a gas jet, thegas cools and clusters form; these clustersare, in a sense, nanometer ice particles in acold gas. This process is a relativelystraightforward method for making clus-ters of a variety of species, including hy-drogen and deuterium.

DD fusion has been observed in exper-iments in which an intense laser pulse isfocused into a gas containing these clus-ters. For example, Ditmire et al. observedDD-fusion neutrons when a gas of pureD2 clusters, with average sizes around 5 nm, was irradiated with 35 fs, 150 mJlaser pulses focused to an intensity of upto 5 x 1017 W/cm2.[Ref. 9] In this experi-ment, approximately 104 neutrons pershot were observed. More recently, Balcouet al. observed DD fusion from a gas of

deuterated methane (CD4) clusters.10

They observed about 103 neutrons pershot using a 35-fs laser focused to > 1017

W/cm2.To understand the origin of fusion in

these experiments, it is necessary to lookin more detail at the dynamics of laser in-teraction with individual clusters on a mi-croscopic scale. Although the specific na-ture of the laser-cluster interaction de-pends to a large degree on the size of thecluster and on the species, (we know thatclusters of atoms with lots of electrons,like Xe, behave quite differently from clus-ters of small atoms, like deuterium), it issimple to summarize the nature of the in-teraction with deuterium clusters. Thesteps of the interaction are illustratedschematically in Fig. 3. When the laser ir-radiates the cluster, it optically ionizes theconstituent atoms. If the laser pulse isshort (a few tens of femtoseconds in thecase of deuterium clusters) the ions pro-duced in the cluster are largely stationary:they cannot move on the fast time scale ofthe laser pulse. The electric field thendrives oscillations of the liberated elec-trons in the cluster. If the laser field isstrong enough, it can extract some (or all)of the electrons from the cluster, leaving asphere of positively charged ions. The elec-trostatic repulsive forces between the ionslead to a “Coulomb explosion” of the cluster.

The powerful electrostatic forces in thecluster will eject ions with substantial ki-netic energy. For example, energy of up to1 MeV has been observed for ions fromthe explosion of Xe clusters.11 In the fusionexperiments described above, the kinetic

energy release of these explosions in deu-terium-bearing clusters is exploited. Aschematic of the fusion process is illustrat-ed in Fig. 4. In these experiments, a gas ofclusters is formed by a puff of deuterium(or deuterated methane) gas into vacuumvia a pulsed gas jet, backed with high pres-sure gas. In the case of the deuterium ex-periments, the gas is also cryogenicallycooled. The laser pulse is focused to a spotin the middle of the gas among the clus-ters. In the focal volume where the laser isintense, clusters explode from irradiationby the passing laser pulse. This leads to theformation of a cigar-shaped filament ofplasma. After passage of the laser pulsealong the “cigar,” energetic deuterons cancollide with other fast deuterons in the ci-gar itself or with deuterium atoms in thesurrounding gas. The collisions give rise tofusion events. Figure 5 shows a picture ofthis fusion plasma from the experiment ofRef. 9. Although neutron yield from thesecluster-fusion plasmas is similar to thoseseen in the solid-target experiments (104 - 105 n/shot with 100 mJ laser pulses),the neutron energy spectrum (shown inFig. 6 [from Ref. 12]) is sharper in energyand centered at 2.45 MeV. These data indi-cate that the fusion observed in this exper-iment arises from a slightly cooler plasmawith an isotropic velocity distribution.

This laser irradiation of a cluster gasleads, in a sense, to a plasma with very hotions, since each of the clusters will tend toexplode isotropically. The result is a short-lived, high ion temperature of sorts. Oneparticularly intriguing aspect of this inter-action is that virtually all the laser light isabsorbed by the clustering gas, a remark-

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Figure 4. Schematic of the fusion process in a gas of clusters.

able occurrence given that the averagedensity of the gas itself is only equal toabout that of one atmosphere (~1019 atoms/cm3) and is, of course, com-pletely transparent to low-intensity light.Most of the absorbed laser light is convert-ed to fast-ion kinetic energy in the explod-ing clusters. Another interesting feature ofthis phenomenon is that the emission timeof neutrons from these plasmas is fast. Theneutron pulse duration will be determinedby the time it takes the hot plasma to dis-assemble. The disassembly time has beenmeasured, and it appears that the emissiontime is on the order of 100 ps or less.12

Future prospects and conclusionWhere these tabletop fusion experimentswill lead is an open question. The princi-pal line of research today involves increas-ing fusion yield. As mentioned, neither ofthese approaches to short-pulse laser fu-sion will likely lead to a viable means ofenergy production: achievement of such agoal will still rely on the ICF approach.However, because of the high repetitionrate of these fusion experiments, theprospect of using them as a compactsource of neutrons is being investigated.

Clearly, if these approaches are to be

useful as sources of neutrons, the totalneutron yield per shot will have to be in-creased dramatically. It is not clear at pres-ent whether the yields necessary for realis-tic applications are achievable with com-pact lasers. Yet these fusion experimentsdo offer some rather unique features as apotential neutron source. For example, inthe case of both solid and cluster targets,the source of the neutrons is small (muchsmaller than 1 mm). No equivalent point-like source of neutrons can be generatedthrough conventional means, such as ac-celerator-based sources or nuclear reac-tors. This characteristic may yield advan-tages in imaging or in generating very highfluxes over small regions.

The energy spectrum of the neutrons isunique: it is largely a pure “fusion” spec-trum peaked at 2.45 MeV (unlike thebroad neutron spectrum from a fission re-actor). This may be of some interest in thestudy of neutron damage of materials des-tined to be used in a future fusion reactor.Materials damage by neutron irradiationis a very active area of study because of theneed to develop materials that will be ableto withstand the insult of high neutron ir-radiation in future fusion machines. Todaythere is no way to exactly replicate theneutron spectrum produced from a reac-tor fusion plasma at the high fluxes ex-pected. A high-repetition-rate laser fusionsource may permit this in the future.

Finally, these laser-driven fusion neu-

tron pulses have another unique facet:they are emitted in a short-duration pulse.This has been confirmed in the case of thecluster targets. Although yet to be meas-ured in the solid targets, a short neutronburst is likely there as well. The short-pulse nature suggests some interesting fu-ture applications inaccessible by use ofconventional neutron sources. For exam-ple, these neutrons might be used in fast-neutron radiography combined with gatedimaging. Even more exciting, they mightbe used as a “pump” pulse in time-resolvedpump-probe experiments. Neutron-radi-ation-damage dynamics in materials arepredicted, through various simulations, tooccur on time scales of a few tens to a fewhundreds of picoseconds. These neutronpulses might be fast enough to allow us,for the first time, to create and watch neu-tron-damage dynamics in a material inreal time. Estimates of the neutron yieldsnecessary to perform such an experimentsuggest that improvements of three to fourorders of magnitude will be necessary.Though no small task, improvements ofour understanding of these laser-inducedfusion processes coupled with advances inhigh-energy compact laser developmentmight make such experiments possible inthe near future.

Todd Ditmire is an associate professor in the Depart-ment of Physics at the University of Texas, Austin,Texas. His e-mail address is tditmire@physics.utexas.edu.

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Figure 6. Neutron energy spectrum data recorded from fusion in a deuterium cluster jet.These data arefrom the experiments reported in Ref. 12. Unlike the data of Fig. 2, this spectrum is centered at 2.45 MeVand exhibits a relatively narrow energy spread.

Figure 5. Picture of fusion plasma from the ex-periment of Ref. 9.The laser enters the gas fromthe right. The stainless steel gas jet pulses gasdown from the top into the path of the laser.Clus-ters form in this gas jet, scattering the near IR laserlight as it enters the plume.A white fusion plasmaspark is created directly underneath the nozzle ofthe gas jet.

References1. See, for example, G. Mourou,App. Phys B 65, 205-

211 (1997) for a discussion of how CPA works.2. J. Lindl, Phys. Plas. 2, 3933-4024 (1995).3. A.W. Obst, R. E. Chrien, and M. D.Wilke, Rev. Sci. In-

str. 68, 618 (1997).4. G. Pretzler, et al., Phys. Rev. E 58, 1165 (1998).5. D. Hilscher, et al., Phys. Rev. E 64, 016414 (2001).6. L. Disdier, J.-P. Garçonnet, G. Malka, and J.-L. Miquel,

Phys. Rev. Lett. 82, 1454 (1999).7. M. H. Key, et al., J. Fusion Energy 17, 231-236 (1998).8. A.W. Castleman and R. G. Keesee, Science 241, 36

(1988).9. T. Ditmire, et al., Nature (London) 398, 489 (1999).10. P. Balcou, et al. talk ITh21 at the Conference on Su-

perstrong Fields in Plasma,Varenna Italy 2001.11. T. Ditmire, et al., Nature (London) 386, 54 (1997).12. J. Zweiback, et al., Phys. Rev. Lett. 85, 3640 (2000).

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