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Plasma Physics

Plasma jets and narrow bandwidth ion spectrafrom thin foils irradiated at high laser intensity

Plasma jet evolution and ion accelerationfrom micron to sub-micron thickness foilsfollowing the interaction of ultra intense(up to 3 ×1020 W/cm2) laser pulses has beeninvestigated using the Vulcan Petawattlaser. With the maximum achievableintensity on target, jets with Mach numberup to 10 have been observed from fewmicrons thick Cu foils expandinglongitudinally with a velocity of 2×106 m/s.Using circularly polarized pulses, theuniformity of the jet density profile isimproved significantly as compared to thejets observed from the interaction of alinearly polarized laser. While moderateenergy (KeV) overdense plasma jets areformed efficiently for micron thicknesstarget irradiated at 1020 W/cm2 intensity,decreasing the target areal density or

increasing irradiance on target leads to anacceleration regime in which Carbon ionsand protons are accelerated, in narrowband peaks, to tens of MeV/nucleonenergies.

Experimental data showing emergence of adense plasma jet at the rear side of theinteraction target, probed transversely at600 ps after the interaction with the highpower laser(above).

Proton andcarbon spectraobtained fromthe interactionof high powerlaser with 100nm thick target.

K. Kakolee, S. Kar, D Doria, B. Ramakrishna,G. Sarri, K.E. Quinn, M. Borghesi(Queen’s University, Belfast, UK),

J. Osterholz, M. Cerchez, O. Willi(Heinrich-Heine Universität, Düsseldorf,Germany),

X. Yuan, P. McKenna(University of Strathclyde, UK)

F.F. Kakolee, [email protected]

Influence of overlapping high-intensity laser beamson electron and ion generation and transport

M. Nakatsutsumi, S. Buffechoux,H-P. Schlenvoigt, P. Audebert, J. Fuchs(LULI, École Polytechnique, Palaiseau, France),

G. Sarri, L. Romagnani, M. Borghesi(Queen’s University Belfast, UK),

L. Vassura (Università La Sapienza, RomaItaly),

L. Ellison (Princeton University, USA),

M. Cerchez, T. Toncian, O. Willi(Heinrich-Heine Universität, Düsseldorf,Germany),

M. Quinn, O. Tresca, P. McKenna(University of Strathclyde, Glasgow, UK),

R. Heathcote, R.J. Clark (STFC, Rutherford Appleton Laboratory, UK)

J. Fuchs,[email protected]

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We studied the influence of overlappinghigh-intensity Vulcan TAW CPA laser beamswhen interacting with solid metal targets.We performed an experiment using twodifferent setups as shown in figure (a) and(b). A number of measurement techniqueswere used to diagnose the electrontransport and proton generation, includingproton radiography (see figure (c)), stackedcharged particle detectors, andshadowgraphy. We clearly observedstriking differences in phenomena whenusing, for the configurations shown in (a),two beams separated by < 3 ps, comparedto having them synchronized. When theywere slightly delayed, we indeed observedlongitudinal parallel jet-like protondeflection patterns, likely due to transversestrong field gradients at the target rear(non-irradiated side). In the case of dualoverlapping synchronized beams, as shownin (b), we observed a strongly collimatedaccelerated proton beam, likely to beattributed to the overlapping of bell-shaped electrostatic sheaths at the targetrear surface.

Two different setups of our experiment, usingeither (a) two temporally separated collinearlaser beams, or (b) two spatially separated,synchronised, quasi-orthogonal beams. (c)Typical proton probing data obtained in setup(a), recorded ~29 ps after the laser interaction.The bar corresponds to 500 µm in the targetplane.

HIGH POWER LASER SCIENCE Plasma Physics

CENTRAL LASER FACILITY Annual Report 2009 - 2010 7

Characterization of a line focus back-lighter

Laser produced plasmas are often used asback-lighters to other plasmas. In order tomake high quality measurements oftransmission, the EUV/x-ray back-lightersource must be well characterized.Accurate information regarding theemission from the back-lighter and itstransmission through the opaqueplasma allows for the opacity to beeffectively determined. This workdemonstrates how a time andspectrally integrated image of plasmaemission from a crossed-slit camerahas been used to infer a spatiallydependant profile of electrontemperature in a line-focus EUVback-lighter.

A.K. Rossall, E. Wagenaars,L.M.R. Gartside, N. Booth, G.J. Tallents(University of York, UK),

S. White, C.L.S. Lewis(Queens University Belfast, UK),

M.M. Notley, R. Heathcote(CLF, STFC Rutherford Appleton Laboratory, UK)

A.K. Rossall,[email protected]

Creation of persistent, straight, 2mm long laser-driven channelsin underdense plasmas for fast ignition applications

There are a number of applications ofintense laser pulses that require theformation of a long-lived, long-scale,density-depleted channel in an underdenseplasma. These include quality control of100s MeV electron bunches generated inlaser wakefield accelerators, highbrightness X-ray generation in thesechannels by betatron oscillations and FastIgnition.

Here we report the experimental detectionof a smooth, persistent density depletedchannel in a Deuterium plasma of initialelectron density ne � 2 x 1018cm-3. Thechannel has been detected to extend forthe entire plasma length (of the order of2mm) and to persist, almost unperturbed,for at least 100ps after the laser has started

to propagate through the plasma. Dataanalysis indicate that the channel preservesharp walls with a density accumulation ofthe order of 2-3 ne and an inner density of0.2-0.3 ne. The experimental results havebeen confirmed by matching 2-dimensionalParticle-In-Cell simulations.

These characteristics of the channel havebeen made possible by the particular set oflaser parameters employed(PL = 6TW, τL = 30ps); the laser lays in factbelow the threshold for relativisticself-focussing, thus hampering the onset ofdetrimental relativistic instabilities, and it islong enough to provide sustainedponderomotive push of the channel walls.

M. Borghesi , G. Sarri (Queen's UniversityBelfast, UK),

K.L. Lancaster (STFC Rutherford AppletonLaboratory, UK),

R. Trines (STFC Rutherford Appleton Laboratory,UK and University of Lancaster, UK)

E.L. Clark, S. Hassan, M. Tatarakis(Technological Educational Institute of Crete)

J.R. Davies, N. Lopes, C. Russo, J. Jiang (GoLP,Instituto de Plasmas e Fusao Nuclear, Portugal)

N. Kageiwa, K.A. Tanaka T. Tanimoto(Graduate School of Engineering, Japan)

R. Ramis, M. Temporal (ETSI Industriales,Universidad Politecnica de Madrid, Spain)

Z. Najmudin, A. Rehman (Imperial CollegeLondon, UK)

P. A. Norreys , R.H.H. Scott (STFC RutherfordAppleton Laboratory, UK and Imperial CollegeLondon, UK)

G. Sarri,[email protected]

CENTRAL LASER FACILITY Annual Report 2009 - 20108


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Spatially dependant profile of electrontemperature in a line-focus EUV back-lighter,inferred from a time and spectrallyintegrated emission profile of a crossed-slitimage.

a) Top view of the experimental arrangement.b), c) Proton radiographs of the channel 100psand 150ps after the beginning of theinteraction. d) Channel creation as simulatedby the 2D PIC code. e) Simulated ion densityprofile across the channel.

Observation of post-soliton expansion followinglaser propagation through an underdense plasma

The late-time stage of the interactionbetween a relativistically intense laserpulse (EL = 200J, τL = 30ps,IL � 3 x 1018Wcm-2, Int laser in Figure) and adeuterium plasma with initial electrondensity of the order of a fraction of thecritical density has been experimentallystudied via the proton radiographytechnique. The data indicate the presenceof several density depletions, locatedaround the main laser-driven channel andat the end of the laser filaments, that areinterpreted to be the late-time remnants ofelectromagnetic solitons (namely post-solitons). Electromagnetic solitons arisefrom the trapping of the laser radiationinside the plasma and, on a time scale longenough to allow ion motion, are predicted

to expand as a consequence of Coulombexplosion of the positively chargedsoliton core. These entities are of centralrelevance to this class of laser-plasmainteractions, being one of the mainvectors of laser energy dissipation duringits propagation through the plasma.

For the frst time, the temporal evolution ofthese structures has been experimentallymeasured. The data indicate deviationsfrom a pure spherical expansion, unveilinglaser polarisation effects on the postsoliton dynamics. Experimental data havebeen found to be in good agreement withad-hoc 3-dimensional Particle-In-Cellsimulations and published analyticalmodels.

M. Borghesi, G. Sarri (Queen's UniversityBelfast, UK),

K.L. Lancaster (STFC Rutherford AppletonLaboratory, UK)

E.L. Clark, S. Hassan, M. Tatarakis(Technological Educational Institute of Crete)

J.R. Davies, N. Lopes, C. Russo, D.K. Singh,J. Jiang (GoLP, Instituto de Plasmas e FusaoNuclear, Portugal)

N. Kageiwa, K.A. Tanaka T. Tanimoto(Graduate School of Engineering, Japan)

Z. Najmudin, A. Rehman (Imperial CollegeLondon, UK)

P.A. Norreys , R.H.H. Scott (STFC RutherfordAppleton Laboratory and Imperial CollegeLondon, UK)

G. Sarri,[email protected]

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a) Top view of the experimental arrangement.b) Typical proton radiograph of the interaction:the dark circle outlines a density bubbleinterpreted to be a post-soliton. c) 3D PICsimulated ion density of the interaction.

Perspectives on radiative blast waves in laser-heated clustered gases

D.R. Symes (CLF, STFC Rutherford AppletonLaboratory, UK)

M. Hohenberger, H.W. Doyle, S. OlssonRobbie and R.A. Smith (Imperial CollegeLondon, UK)

A.S. Moore, E. T. Gumbrell(Plasma Physics Division, AWE plc, Reading)

R. Rodríguez, J.M. Gil (University of LasPalmas de Gran Canaria, Las Palmas de GranCanaria, Spain and Nuclear FusionInstitute-Denim, Polytechnic University ofMadrid, Spain)

D.R. Symes,[email protected]

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Gases composed of clusters absorb intenselaser light extremely efficiently (>90%) toform a high energy density environment(up to 109 Jg-1). The subsequent explosionof the heated gas launches fast cylindricalblast waves (up to Mach 50). In highZ materials, energy losses throughionization and radiation are highlysignificant to the blast wave behavior.These can be categorized as optically thinradiative shocks, a regime of particularinterest for laboratory astrophysicsexperiments. We compare the properties ofradiative shocks launched in clusters tothose created by other techniques whichemploy kJ-class laser systems. Themorphology and evolution of the blastwaves are sensitive to gas density becauseof the change in optical depth of themedium. We explore this by applying theRAPCAL code to calculate ionization levels

and radiative loss rates. We discuss thenecessary conditions for radiativeinstabilities in the shocks and directions forfuture experiments.

Blast waves launched in clustered gases with atable-top laser system. These shocks exhibitturbulent structure in hydrogen (a) and largeionization precursors indicating strong radiativeenergy losses in xenon (b).



FemtosecondPulse PhysicsRadiation pressure effects on ionacceleration on the Gemini laser

In most laser-driven ion accelerationstudies carried out to date, ions areaccelerated by sheath fields established byrelativistic electrons at target surfaces, viathe so-called Target Normal SheathAcceleration (TNSA). A separatemechanism, Radiation PressureAcceleration (RPA), has attracted extensivetheoretical attention in recent years.Radiation pressure is exerted at the laserreflection point on a foil surface, resultingin local electron-ion displacement, and ionacceleration via the ensuing space-chargefield. Cyclical reacceleration of the targetions in the Light Sail RPA mode accessiblewith ultrathin foils is predicted to lead tohigh acceleration efficiencies, and produce

energetic, narrow band ion beams. In anexperiment carried out on theAstra-Gemini laser we detected featuresconsistent with the onset of this newacceleration mechanism. Spectral peaks asshown in figure were observed for verythin foils and for circularly polarizedpulses, and simulations indicate their originfrom RPA.

M. Borghesi, R.Prasad, S. Ter-Avetisyan,D. Doria, K.E. Quinn, L. Romagnani, M. Zepf(Queen's University Belfast, UK),

P.S. Foster, C.M. Brenner, P.L. Gallegos,J.S. Green, A.P.L. Robinson, D. Neely(CLF, STFC Rutherford Appleton Laboratory,UK),

N. Dover, C.A.J. Palmer, M.J.V. Streeter,J. Schreiber, Z. Najmudin(Imperial College, London, UK)

D.C. Carroll, O. Tresca, P. McKenna(University of Strathclyde, Glasgow, UK)

M. Borghesi,[email protected]

Ion acceleration from foil targets in the ultraintenseultrahigh contrast regime

An experiment was conducted on theAstra-Gemini laser with one of the mainobjectives being to investigate theacceleration of carbon ions at the highestlaser intensity achievable today. The Astra-Gemini laser delivers pulses of duration~50fs with energies up to 12J at awavelength of 800nm, reaching intensities~1021 Wcm-2. We explore the TNSAacceleration mechanism using a range oftarget materials and thicknesses as well asinvestigating the effect of the laser

incidence angle. We found that the TNSAmechanism gives maximum ion energy fora target thickness around 100nmindependent of its composition. Thehighest carbon ion energies were producedfrom pure carbon targets. Our mostinteresting result is the first observation ofan increase in ion energy when theincidence angle of the laser is changedfrom 0o to 35o, in contrast to previouswork.

O. Tresca, D.C. Carroll, M.N. Quinn, X.H. Yuan, P McKenna (University ofStrathclyde, UK),

R. Prasad, L. Romagnani,S. Ter-Avetisyan, K.E. Quinn, M. Zepf,M. Borghesi (Queen’s University Belfast, UK),

P.S. Foster (CLF, STFC Rutherford AppletonLaboratory, UK and Queen’s University Belfast,UK),

P. Gallegos, D. Neely, C.M. Brenner(University of Strathclyde, UK and CLF, STFCRutherford Appleton Laboratory, UK)

J.S. Green (CLF, STFC Rutherford AppletonLaboratory, UK)

M.J.V. Streeter, F.H. Cameron,A.P.L. Robinson, T. Baeva (CLF, STFCRutherford Appleton Laboratory, UK andImperial College London, UK),

N.P. Dover, C.A.J. Palmer, J. Schreiber, Z. Najmudin (Imperial College London, UK)

O. Tresca,[email protected]

Spectrum of C6+ and H+ ions, showing aproton peak at the high energy end.

Average of the maximum C6+ ionenergy over multiple shots as afunction of target thickness fordifferent materials. The errors barsrepresent the standard deviation ofthe maximum energy.

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10 CENTRAL LASER FACILITY Annual Report 2009 - 2010

HIGH POWER LASER SCIENCE Femtosecond Pulse Physics



Characterisation of debris emissionfrom thick targets on Astra-Gemini

Debris characterisationmeasurements were made duringradiation commissioning of Astra-Gemini. During the commissioningthe distribution of debris from thicktargets was studied. The targetsused were 0.5 mm Cu, CH and Taand 50 µm Au. The diagnostics usedto measure the debris distributionwere glass witness plates positionedaround the target. For the0.5 mm thick targets it was foundthat the debris was mainlydistributed along the rear surfacetarget normal and laser directionswith particulates typically of around400 µm. For the 50 µm thick targetthe distribution was mainly alongthe front and rear target normal directionswith particulate sizes of less than 100 µm.Damage to the witness plate surface wasalso observed for 50 µm thick targets along

the rear target normal direction. Thisindicates that target thickness has asignificant effect on debris production anddistribution.

D.C. Carroll, O. Tresca, P. McKenna(University of Strathclyde, UK),

R.J. Clarke, J.S. Green, C. Spindloe (CLF, STFCRutherford Appleton Laboratory, UK),

P.S. Foster (CLF, STFC Rutherford AppletonLaboratory, UK and Queen’s UniversityBelfast),

M.J.V. Streeter (CLF, STFC RutherfordAppleton Laboratory, UK and Imperial CollegeLondon, UK ),

D. Neely (CLF, STFC Rutherford AppletonLaboratory and University of Strathclyde, UK),

D.C. Carroll,[email protected]

High rep rate Thomson parabola-MCP assemblyfor multi-MeV ion spectroscopy

The absolute calibration of a micro channelplate (MCP) detector, installed as detectorin a Thomson parabola spectrometer hasbeen carried out. The calibration deliversthe relation between a registered countnumbers in the CCD camera (on which theMCP phosphor screen is imaged) as a resultof the impact of an ion beam onto the MCP.The particle response of the wholedetection system was evaluated by usinglaser accelerated ions with proton energiesup to 3 MeV and carbon ion energies up to16 MeV. In order to obtain an absolutemeasurement of the number of ionsincident on the MCP detector, slottedCR-39 track detector was installed in frontof the MCP. The signal registered on theMCP due to ions propagating through theCR-39 slots is compared to the number ofparticles counted on the adjacent CR-39stripes after the etching. Carefulconsideration is required in evaluating theresponse of the MCP, since in the

arrangement employed different energyions are incident on the MCP at differentangles (unlike , for example, ionsaccelerated by linear accelerators) , andthis may affect the gain and secondaryelectron yield. For this reason, an in-situcalibration is important for a correct dataanalysis.

Correlation ofintegrated countsin dE energyinterval due to MCPsignal with numberof particles onCR-39 in dE energyinterval plottedwith respect toenergy for Protonsand C6+.

R. Prasad, D. Doria, S. Ter-Avetisyan,K.E. Quinn, L. Romagnani, M. Zepf,M. Borghesi, (Queen's University Belfast,Belfast, UK),


C.M. Brenner, P.S. Foster, P.L. Gallegos,J.S. Green, A.P.L. Robinson, D. Neely(CLF, STFC Rutherford Appleton Laboratory,UK)

N.P. Dover, C.A.J. Palmer, M.J.V. Streeter,J. Schreiber, Z. Najmudin(Imperial College London, UK)

R. Prasad,[email protected]

Debris damage to glass witness plate at thetarget normal rear surface position. Theimages at the top are scans of the damage tothe plate surface and the bottom diagram isa schematic of features found in the damagedistribution.

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Proton/ion energy scaling and laser conversion efficiencyusing 50fs, 1020-1021 W/cm2 Astra-Gemini pulses

Experimental investigations of ionacceleration from thin foil targetsirradiated with ultra-short (~ 50 fs), highcontrast (~1010) and ultra-intense (up to1021 W/cm2) laser pulses on Astra-Geminilaser has been performed. Thesemeasurements provided for the first timethe opportunity to extend the scaling lawsfor the acceleration process in the ultra-short (tens of fs) regime beyond the1020 W/cm2 threshold. The scaling ofaccelerated ion energies was investigatedby varying a number of parameters such astarget thickness (down to 10 nm), laserlight polarization (circular and linear), angleof laser incidence (oblique-350, normal)and laser energy. Maximum protonenergy>20 MeV and C6+ energy~235 MeVhas been observed. The effect of targetthickness on the ion flux produced wasalso investigated at 350 and normal laserincidence on target.

R. Prasad, D. Doria, S. Ter-Avetisyan,K.E. Quinn, L. Romagnani, M. Zepf,M. Borghesi, (Queen's University Belfast,Belfast, UK),


C.M. Brenner, P.S. Foster, P. Gallegos,J.S. Green, A.P.L. Robinson, D. Neely(CLF, STFC Rutherford Appleton Laboratory,UK)

N.P. Dover, C.A.J. Palmer, M.J.V. Streeter,J. Schreiber, Z. Najmudin(Imperial College London, UK)

R. Prasad,[email protected]

Investigation of contrast of Astra-Gemini

The temporal contrast of the laser pulse ofextreme power up to 1022W/cm2 plays acrucial role in the high-field laser-matterinteraction experiments as the excessiveprepulse intensity can significantly affectexperiment conditions due to preplasmaformation. To meet an increasing demandfor higher temporal quality of laser pulse,the temporal profile of Astra-Gemini wasextensively investigated under variousconditions, using a 3rd-order auto-correlator. The effect of various techniqueshas been investigated, including the spatial

filter, aperture and minimising thescattering and loss etc. To minimise theoverall ASE noise level, the front-end wasupgraded and optimized to suppress theintracavity ASE noise, providing a cleanerseed pulse by a factor of ~10. As a result,the overall incoherent ASE and coherentcontrast was improved by one order ofmagnitude as seen below. Some of thereplica pre-pulses was reduced by a factorof ~100 or eliminated by usingcomponents with higher qualityantireflection coatings.

Y. Tang, C.J. Hooker, O.V. Chekhlov,S. Hawkes, K. Ertel and R. Pattathil(CLF, STFC Rutherford Appleton Laboratory,UK)

Y. Tang,[email protected]

Maximum proton/ion energy dependence ontarget thickness at normal incidence onaluminium target for linear and circularpolarisations

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Overall contrastbefore and afterfront-end upgrading.



HIGH POWER LASER SCIENCE Theory and Computation


Theoryand ComputationElectron transport during shock ignition

In the 'shock ignition' approach to InertialFusion Energy (IFE), the fusion target iscompressed at a relatively low temperatureand then ignited by a high-pressure shockdriven into the target by a short high-intensity laser pulse. Because of steeptemperature gradients, electrons whichtransport energy into the target to drivethe shock must be modeled kinetically bythe Vlasov-Fokker-Planck equation. Thelaser-target interaction also does not have

time to relax hydrodynamically to thesteady ablation profiles characteristic ofthe compression phase. Instead, shockignition lies at the boundary between the‘ablation’ regime and a ‘supersonic heatfront’ regime in which high energyelectrons stream freely into the target.High energy electrons transport energydeep into the compressed target wherethey beneficially generate high pressure athigh density.

A.R. Bell & M. Tzoufras(University of Oxford, UK),

T. Bell, [email protected]

New perspectives on the electrodynamicsof intense laser-plasmas

A summary is given of work undertakenover the past twelve months by the Lancaster Mathematical Physics Groupand Cockcroft Institute that focusses on new avenues for theoreticalexploration of the behaviour of intense laser-plasmas. Items coveredinclude an examination of Born-Infeld electrodynamics using plasmas,

new geometrical analyses of relativistic collisional and collisionlessplasmas including a new method for analysing Landau damping innon-stationary and inhomogeneous relativistic plasmas, and a newinvestigation of the interaction of electromagnetic fields with acceleratingmatter.

D.A. Burton, R.W. TuckerLancaster University, UK & The CockcroftInstitute, UK)

D.A. Burton,[email protected]

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Cavitation and shock waveformation in dense plasmas byrelativistic electron beams

The propagation of a high currentrelativistic electron beam throughdense plasmas, for example in fast-ignition inertial confinement fusion,produces strong heating and magneticfield generation. The j × B force andthermal pressure gradient that thereturn current creates may in factcavitate and cause shock waves in theplasma around the electron beam.

Work has been undertaken to investigatethis effect in different regimes of plasmadensity and hot electron current. Ananalytic model has been developed thatgives good estimates of the density,pressure, magnetic field and velocityobtained in the plasma. This model iscompared against the results from an MHDcode that includes the effects of resistivefield growth, Ohmic heating and the j × Bforce. The strength of the cavitation is

found to be dependent upon the ratiobetween j2 and the initial mass density. Itwas found that cavitation is indeedrelevant to fast-ignition, and is strongenough to launch shocks in certaincircumstances.

I. Bush, J. Pasley(University of York, UK),

A.P. L. Robinson(CLF, STFC Rutherford Appleton Laboratory, UK),

R.J. Kingham(Imperial College London, UK)

I. Bush,[email protected]

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A parameter scan across a range of hotelectron current density and initial plasmamass density. The colour scale shows thefractional amount of cavitation expected fromthe analytic model.

Alpha-particle stopping power fordense hydrogen plasmas

For beam energies from 0–20 MeV, agood agreement is obtained betweencalculated ion ranges for α-particlesstopping in dense hydrogen for bothhighly fluctuating electron densitiesfrom DFT-MD and uniform averageddensity profiles. In addition, the rangecalculations demonstrate goodagreement with analytical rangecalculations using stopping powerapproximations in the limit of both thehigh and low kinetic energy as well.This agreement implies that forcalculations of large-scale effects, such asion range or averaged energy deposition,the increased accuracy provided byapplying a DFT-MD density profile has littleeffect on the final result. It is also observedthat the general functional form of theBragg curves calculated for the variable

and uniform density cases is similar.However, in variable density DFT-MD cases,fluctuations in density cause dramaticchanges in the calculated stopping powerover small spatial scales. This hugestraggling must be considered in situationswith small fluxes where heating cannot beaveraged over many particles.

D.J. Edie, J. Vorberger, D.O. Gericke(University of Warwick, Coventry, UK)

D.J. Edie,[email protected]

Comparison of α-particle stopping in hydrogenusing a density profile obtained by DFT-MD anda constant mean density. The beam energy isE = 3.5MeV. The plasma has a temperature ofT=20000K and a mean electron density ofne = 1.661x1024 cm-3.

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An object-oriented 3D view-factorcode for hohlraum modelling

An easy to use 3D view-factor codehas been developed for use in highpower laser experiment design.View-factor codes use simplearguments to determine thequantities of radiation that can flowthrough a hohlraum geometry, andthen solve a power balanceequation to calculate the spatialdistribution of radiation throughouta target. Because of the relative speedinvolved in setting up and runningsimulations, 3D view-factor codes areespecially suited to prototypingexperimental target designs.The code allows the user to build up a widerange of hohlraum designs by combining anumber of geometric primitives. Drive laserconfigurations can be specified in terms ofindividual beams or in rings; the beamsthemselves are implemented via a self-adjusting ray-tracing algorithm.The code has been tested against resultspreviously published and obtained by otherexperimental groups, and has been shownto reproduce experimental results withreasonable accuracy.

M. J. Fisher(University of York, UK)

J. Pasley(University of York, UK and CLF, STFCRutherford Appleton Laboratory, UK)

J. Pasley,[email protected]

A cut-away rendering of a Nova hohlraumgeometry created by the code.

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Time-dependent temperature measurementsfrom HEP-1/Precision Nova shots (continuousline) compare well with numericalcalculations produced by the code.

Calculation of Siegert states in electric field:from atoms to molecules

We propose a numerical method tocalculate the Siegert states of molecules ina static electric field which are solutions ofthe stationary Schrödinger equationsatisfying the regularity and outgoing-waveSiegert boundary conditions. This workfollows the one for atomic Siegert states inthe reference, where they calculatecomplex eigenenergies and eigenfunctionsfor one-electron atomic potential. Here wedescribe the modifications necessary toobtain such quantities in complex

molecular cases. We discuss the reducednumber of symmetries in the molecularproblem and the inclusion of m-coupling.The two figures below present the resultobtained for the lowest sigma state of H2

+

with the soft Coulomb approximation forthe electric field of 0.5 a.u.. The moleculeaxis is parallel to the laser field.

P. A. Batishchev, O.I. Tolstikhinand T. Morishita, submitted.

L. Hamonou, T. Morishita, S. Watanabe (The Universityof Electro-communications,Tokyo, Japan),

O.I. Tolstikhin(Russian Research Center “KurchatovInstitute,”, Moscow, Russia)

L. Hamonou,[email protected]

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Finite size effects in high-intensity QED

With laser fields approaching magnitudesclose to the Sauter-Schwinger limit the useof strong-field quantum electrodynamicsbecomes mandatory. The latter is based onelectrons ‘dressed’ by plane wave fields,solely dependent on an invariant phase.The theory developed in the sixtiesassumed infinite spatio-temporal extentwhich at that time seemed a goodapproximation as the number of cycles perpulse was large compared to unity.Nowadays, however, at ultra-highintensities, pulses are ultra-short, and

effects of finite pulse duration must be taken into account.

We find that for both nonlinear Comptonscattering and stimulated pair productionfinite size effects tend to ‘wash out’ theclear spectral signals caused by long-timeaveraging which is no longer possible forshort pulses. Hence, observation ofintensity effects on emission andproduction spectra clearly will requireexperimental fine-tuning.

T. Heinzl(University of Plymouth, UK)

A. Ilderton, M. Marklund(Department of Physics, Umeå University,Sweden)

T. Heinzl,[email protected]

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Nonlinear Compton scattering or photonemission (left) and stimulated pair production(right). The processes are related by crossingsymmetry. Both the photon emission spectrumand pair production rate are rather sensitive tothe details of the laser beam, in particular itspulse duration.

Multi-electron dynamics on the femtosecond time-scale

The recent emergence of attosecond lightsources promises to provide deeper insightinto ultrafast correlated dynamics betweenelectrons in complex matter. Usingtime-dependent R-matrix theory we havebeen able to investigate such dynamicswithin an atomic system.

We consider C+ in its ground state withmagnetic quantum number M=0. The ion isexcited by a linearly polarized XUV pulseinto a superposition of the 2s2p2 2S and 2D states. The repulsion between the two2p electrons results in interferencebetween the two states which occurs on atime-scale of 1-2 femtoseconds. Bysubsequently ionizing C+ with a delayedultrashort pulse and analyzing theproperties of the ejected electron we areable to probe the interference andinvestigate the role of electron-electroninteractions in the transition fromultrashort pulse excitation to long-pulseexcitation.

M.A. Lysaght, S. Hutchinson,H.W. van der Hart(University Belfast, UK)

M.A. Lysaght,[email protected]

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2-dimensional momentum distributions of an electronejected from C+ obtained for three different excitationpulse durations: n = 6, 12 and 18 cycles. The plot showsa breathing motion for both 2p electrons for excitationpulse lengths of 6 and 18 cycles. ϕ corresponds to thephase of the breathing motion.





Hydrodynamics of the Yukawa onecomponent plasma

The hydrodynamic description is a verycommon starting point for investigating awide range of basic and complex physicalphenomena in fluids. We investigate usingMolecular Dynamics simulations when thismacroscopic description (rather than amore detailed microscopic description) isapplicable for a physical system. To achieve this we consider themicroscopic dynamics of the so-called Yukawa One Component Plasmamodel. In particular, we determine howthe domain of validity of the hydrodynamicdescription varies as the level of manybody correlations (i.e. non-ideality) isvaried.

Our results elucidate the nature of thehydrodynamic limit when many bodycorrelations are present, particularly forsystems with long ranged interactions.They can be used to model ion dynamics indense plamas (e.g. 'Warm Dense Matter').

J.P. Mithen, G. Gregori(University of Oxford, UK),

J. Daligault(Theoretical Division, Los Alamos NationalLaboratory, Los Alamos, USA),

J.P. Mithen,[email protected]

The accuracy of rear-surface measurementsof the angular-divergence of a laser-generated fast-electron beam

The interaction between high-intensitylasers and solid targets is of keyimportance in fast-ignition inertialconfinement fusion, which may allowfusion ignition with less laser driver energy;and ion-acceleration, which has potentialapplications to cancer therapy.Understanding the properties of therelativisitic electron beam generated inthese interactions is crucial. In particular,its angular divergence has ramifications forthe aforementioned practical applications.This is often diagnosed using thetemperature profile or the size of theregion over which ion are emitted on thetarget’s rear-surface.

Simulations performed with the hybridVlasov code FIDO demonstrate thatre-circulating currents inside the targetlead to a broadening of the temperaturehot-spot on the rear, thus over-estimatingthe inferred angular divergence of thefast-electron beam. The sheath potentialwas observed to be weakly dependent onthe beam density and so the ion-spot size isa poor measure of the angular divergence.

C.P. Ridgers, M. Sherlock, R.G. Evans,R.J. Kingham(Imperial College of Science Technology andMedicine, London, UK)

C. Ridgers,[email protected]

Brillouin peak dispersion relation forinteraction potentials of various rangesagainst the hydrodynamic (linear)predictions.

21

22

Fast-electron number density with refluxing(top) and without refluxing (middle) after1035fs. Background temperature at therear-surface (bottom).

Fast-electron numberdensity, peak sheath fieldand sheath potential alongthe target’s rear-surfaceafter 100fs, and Ey

peak

(analytical) ∝ nf1/2.

High absorption of ultra-intense laser pulsesin near critical plasmas

Of all the problems that ultra-intenselaser-plasma physics is concerned with, thecoupling of energy from the laser pulseinto the kinetic energy of the plasmaparticles is perhaps the most important.Attaining efficient coupling is highlyimportant and even critical to a number ofprospective applications. Fast Igntion ICFand laser-driven ion acceleration areexamples of such applications wherecoupling efficiency is either a veryimportant or critical issue.

In the work reported here, we analyzed theabsorption of 30-100fs, λ ~ 1µm, a0 >> 1laser pulses in near-critical plasmas. Bythis we mean plasmas are relativisticallytransparent and span the density range0.1nc < ne < a0nc, where nc is the non-relativistic critical density.

From energy considerations we havederived new analytic formulae for the twomechanisms of absorption in near-criticalplasmas : Leading Edge Depletion (LED) andTransverse Ponderomotive Acceleration

(TPA). For LED, for example, we obtainedthe following new formulae for theleading-edge propagation velocity:

Our new formulae have been rigorouslycompared to a large number of 1D and 2Dfully electromagnetic Particle-in-Cellsimulations.

This study has thus shown that the twoabsorption mechanisms that have beensuggested for interactions with near-critical plasmas can both occur separatelyin different regions of parameter space,and, where they do, their gross propertiesare well described by the new analyticformulae derived here. Since LED is asignificantly faster absorption mechanismthan TPA this is an important step towardsoptimizing absorption in the near-criticalregime.

A.P.L.Robinson, R.M.G.M.Trines(CLF, STFC Rutherford Appleton Laboratory, UK)

A. Robinson,[email protected]

23

Modelling Spitzer transport in laser produced plasmas bydirect Vlasov methods with a BGK collision operator

In short-pulse laser matter interaction, theabsorption of incident laser energy isdominated by collisionless mechanisms butthe onward transport of these fast movingelectrons, and the resultant heating of thetarget, is dependent on the response of thecold, resistive, background plasma.

Here we briefly outline a simple approachto including collisional physics in the directVlasov solver VALIS, based on the BGK

(Bhatnagar, Gross and Krook) collisionoperator. We are able to demonstrate theefficacy of this approach in two keytransport problems in laser plasmainteraction: thermal conduction andelectrical resistivity.

N.J. Sircombe

AWE plc. Reading, Berkshire, University ofWarwick, Coventry, UK and CLF, STFC,Rutherford Appleton Laboratory, UK)

T.D. Arber

(University of Warwick, Coventry, UK)

N. Sircombe,[email protected]

24

Comparison of Spitzer currents(dashed lines) with those calculatedusing VALIS with a BGK collisionoperator (solid lines) for externalfields of: 107Vm-1 (blue); 5x107Vm-1

(cyan); 108Vm-1 (green); 5x108Vm-1

(yellow); and 109Vm-1 (red), for thecase of both electron-electron andelectron-ion collisions.





A Vlasov-Fokker-Planck code f or shock ignition

A 2D3P parallel object-orientedVlasov-Fokker-Planck code that relies onthe expansion of the electron distributionfunction to spherical harmonics has beendeveloped, in order to study non-localelectron transport for Shock Ignition.

The code makes use of a rigorousformalism for the collisions betweenelectrons, which derives from theRosenbluth potentials and conservesenergy and number. This code makes itpossible to accurately model the kinetic aswell as the hydrodynamic behaviour of theplasma and is particularly efficient forcollisional plasmas. For Shock Ignition theelectron temperatures range from morethan 100keV to 10eV while densities rangefrom less than critical to greater than solid.Shock Ignition is therefore an excellentcandidate for this VFP code, because thetarget is sufficiently collisional to allow forextremely efficient modelling.

M. Tzoufras, A.R. Bell(University of Oxford, UK)

M. Tzoufras,[email protected]

26

R.M.G.M. Trines and P.A. Norreys(CLF, STFC Rutherford Appleton Laboratory, UK)

R.A. Fonseca and L.O. Silva(GoLP/Instituto de Plasmas e Fusão NuclearInstituto Superior Técnico, Lisbon, Portugal)

C. Kamperidis, K. Krushelnick, Z. Najmudin(Blackett Laboratory, Imperial College, UK)

R. Trines,[email protected]

25Effect of channel profile evolution on laser-drivenelectron acceleration in plasma channels

Electron trapping in a laser-driven wakefield, versus laser intensity. Laseramplitude a0 is 1.19 (top left), 1.68 (top right), 2.0 (bottom left), 2.5 (bottomright), all other parameters equal. Laser spot diameter is 10 micron, wavelength is 800 nm, ω0/ωp = 13.2, pulse duration is 50 fs. An increased laserintensity leads to enhanced electron trapping and acceleration.

The first tensphericalharmonics.

The trapping and accelerationof electrons by a laser pulsepropagating through a plasmachannel has been investigatednumerically. The influence ofthe laser intensity, backgroundplasma density, channeltransverse profile and degreeof ionisation of the plasmawas studied. It was found thatthe laser intensity andbackground density have themost significant impact onelectron trapping, while thedegree of ionisation has hardlyany influence at all.

Multi-component effects on the x-rayscattering signal from warm dense matter

In this contribution, we investigate theeffects of multiple ion species on the x-rayscattering process from warm densematter. In particular, we discuss elasticscattering, that is, the weight of theRayleigh peak. Based on partial structurefactors from hypernetted chain solutions, ageneralised approach of the theoreticalmodel is applied to account for multi-component effects. We demonstrate thatmutual correlations significantly influencethe partial structure factors due to the factthat the ions with the highest chargeimprint their structure on the othercomponents. The weight of the Rayleighpeak that is directly related to the staticstructure factors is thus also sensitive tothe interplay between the differentcorrelated ions in the systems. Theseeffects are especially pronounced in thecase of forward scattering, i.e. for small kvalues. Furthermore, we demonstrate thatthe full multi-component description isalso necessary for cases where x-ray

scattering is dominated by one species.This effect is related to the differences inthe statistical weight of the differentcontributions to the Rayleigh peak.

K. Wünsch, J. Vorberger, D.O. Gericke(University of Warwick, Coventry),

G. Gregori(University of Oxford)

K. Wünsch,[email protected]

27

Comparison of the weight of the Rayleigh peakfor warm dense carbon, CH and CH2. Thecarbon density of nC = 5·1022 cm-3, thetemperature of T = 8eV and a charge state ofZC = 2 are fixed for all systems. For thecalculation of CH and CH2, fully ionisedhydrogen, i.e. ZH=1, with densities ofnH = 5·1022 cm-3 and nH = 1023 cm-3 is applied,respectively.



HIGH POWER LASER SCIENCE Ultrafast and XUV Science


Ultrafast and XUV Science

Quasi-classical model of non-destructivewavepacket manipulation by intensefew-cycle nonresonant laser pulses

Matter exposed to strong-field few-cyclelaser pulses is violently distorted on a timescale comparable to vibrational andelectronic motions. In molecules, theresulting electric field polarizes electronorbitals to the extent that ionization andCoulomb explosion are likely, without therequirement of photon energy resonance.By pumping D2 with an intense few-cycle(~10 fs) pulse, ionization launches avibrational wavepacket in D2

+ which maybe imaged some time later by a similarprobe pulse; such wavepackets have beenexperimentally observed.

A few-cycle “control” pulse arrivingbetween the pump and probe applies adipole force while the wavepacket ispropagating, transferring energy into or outof the system, causing a redistribution ofvibrational population. This quasi-classical

model (QCM) describes the modification ofthe wavepacket by simulating an ensembleof classical trajectories evolving on thedynamically-distorted potential energy ofthe system. Manipulating the internalstates of molecules has implications forquantum computation, attosecondelectronic state observation and theformation of a molecular quantum gas.

W.A. Bryan and G.R.A.J.Nemeth(Swansea University, UK),

C.R. Calvert, R.B. King, J.B. Greenwood,I.D. Williams (Queen’s University Belfast, UK),

W.R. Newell (University College London, UK)

W.A. Bryan,[email protected]

28

Final vibrational state populationdistributions for three different controlpulse intensities as the temporal separationbetween the pump and control pulse isvaried. The results of Niederhausen andThumm, Phys. Rev. A 77 013407 (2008) arereproduced for comparison with the QCM,which reproduces the periodicity andrelative shift of vibrational population asthe delay between the pump (6 fs, 1014 Wcm−2) and the control is varied.

Monochromatic XUV-photon + strong-field NIRcross-correlation by atomic excitation and ionization

State-of-the-art x-ray free electron lasersand attosecond high harmonic generation(HHG) sources are being used to probeelectronic dynamics under on ultrashorttime-scales and high intensities. In a novelcross-correlation experiment, wedemonstrate the capabilities of therecently commissioned Artemis facilitywhich neatly combines the advantageouscharacteristics of XFELs and attosecond

sources – energy tunability, sub-cyclesynchronization to an optical laser andultrashort pulse duration.

An intense 30 femtosecond near-infrared(NIR) laser pulse is split 3:1. The low energypulse produces a broad spectrum of XUVphotons through HHG in a gas jet, whichare monochromated allowing a single35.9 eV harmonic to be focused into akrypton target, generating highly excitedKr+ ions through single-photon absorption.The high energy NIR pulse is also focusedinto the krypton to a high intensity, and thedelay between NIR and XUV can becontrolled with sub-cycle resolution.Depending on the arrival times of the XUVand NIR, enhancement of the Kr2

+ ion yieldis used to measure the XUV pulse duration,performing an atomic cross-correlation.

W.A. Bryan, G.R.A.J.Nemeth(Swansea University, UK),

F. Frassetti, P. Villoresi, L. Poletto (Laboratoryfor Ultraviolet and X-rays Optical Research,Department of Information Engineering,Italy),

R.B. King, C. R. Calvert(Queen’s University Belfast, UK),

S.G. Hook, C.A. Froud, I.C.E. Turcu,E. Springate (CLF, STFC, Rutherford AppletonLaboratory, UK)

W.A. Bryan,[email protected]

29

Measured and predicted Kr2+ yieldas a function of NIR-XUV delay forharmonic 23 (photon energy 35.9eV, wavelength 34.6 nm). The bestleast-squares fit reveals the rise ofthe yield is the result of H23 havinga duration of 24 ± 2 fs.

Multi-pulse scheme for controlling electronlocalisation upon molecular dissociation

Quantum control of chemical reactions inmolecules requires the precisemanipulation of electronic wavepackets,which typically evolve on attosecondtimescales. A powerful technique forcontrolling the electronic dynamics duringmolecular dissociation is to use few-cyclefemtosecond laser pulses of infra-redradiation.

Through controlling the relative phaserelationship, ϕ, between the carrier fieldand pulse envelope, and using a sequenceof such pulses, a new scheme has beenidentified for directing the localisation ofthe electron upon molecular break-up, i.e.whether the electron goes to the ‘left’ or‘right ’nucleus.

This ultrafast scheme has been identifiedusing quantum simulations of thedeuterium molecular ion (D2

+) exposed to asequence of 7 fs pulses. With carefullychosen pulse parameters it is possible tooptimally control the vibrationalwavepacket dynamics and the finaldissociation event, resulting in a final‘left:right’ ratio of the electron localizationwhich exceeds 10:1.

R.B. King, C.R. Calvert, J.D. Alexander,L. Belshaw, J.F. McCann, J.B. Greenwood,I.D. Williams(Queen’s University Belfast, UK),

W.R. Newell (University College London, UK),

W.A. Bryan (Swansea University, UK)

C.R. [email protected]

30

Intensity-resolved ionization processes in few-cyclestrong-field laser pulses

Few-cycle strong-field laser pulses causetunnel ionization of atoms and moleculesby violently distorting the electron orbitals.It is commonly assumed that tunnellingleaves the ion in the ground-state.However, enhanced ionization yieldsindicate excitation processes whichpopulate lower-lying energy levels.Alternatively, these levels could bepopulated directly by tunnelling. Removalof electrons from lower-lying orbitals bytunnelling is also possible; there is recentexperimental evidence of this in molecules.

Tunnelling from multiple atomic energylevels is modelled and a subtle shift inionization probability is expected as afunction of intensity. We have measuredfocal-volume-dependent ionization ofxenon using NIR ultrashort pulsesgenerated at Artemis. As the experiment isable to resolve ionization as a function ofintensity, the ionization probabilities can beextracted and compared directly to theory.

G.R.A.J. Nemeth, W.A. Bryan(Swansea University, UK),

R.B. King, J.D. Alexander, C.R. Calvert,J.B. Greenwood, I.D. Williams(Queen’s University Belfast, UK),

W.R. Newell (University College London, UK),

S.G. Hook, C.A. Froud, I.C.E. Turcu,E. Springate (CLF, STFC Rutherford AppletonLaboratory, UK)

G.R.A.J. Nemeth,[email protected]

31

Predicted ionization probabilities ofxenon exposed to a few-cycle laserpulse. Ground-state only ionization(dashed line) and ionization from all5p and 5s states (solid line) arecompared; we anticipate theintensity offset will beexperimentally observable.

Proposed multi-pulse scheme: Pulse1 ionises a D2 target launching acoherent D2

+ vibrationalwavepacket. Pulse 2 non-destructively manipulates thebound wavepacket into a newvibrational distribution. Pulse 3 thendissociates the molecular ion, wherecareful choice of delay time allowsthe dissociation to be maximisedand tuning of ϕ leads to stronglocalization of the electron to aparticular nucleus.





Ultrafast dynamics of electronic structurein complex materials

We have used the new materials-sciencebeamline at Artemis to carry out time-resolved photoemission studies of thelayered Charge-Density-Wave andMott-insulating compound 1T-TaS2. Wedrove the system into a transient metallicphase using ultrashort pulses of laser light,and used high-harmonic UV pulses togenerate photoelectrons, revealing thechanges in electronic structure. Afterphotoexcitation there is a collapse inintensity of the Lower Hubbard Band,transfer of spectral weight towards theFermi level, and subsequent oscillation ofthe position of the band edge at the CDWamplitude mode frequency. These featuresagree with those seen at k=0 by Perfetti etal. [PRL 97 067402 (2006)]. Ourangle-integrated measurementsadditionally show an oscillation in the LHBpeak intensity, a new effect likely due tothe influence of the CDW amplitude modeon the band structure near k=π/a. Furtherexperiments will study the detaileddispersion of the dynamical response.

J.C. Petersen, (Oxford University and MPSD,CFEL, Hamburg)

N. Dean (Oxford University, UK),

C.M. Cacho, E. Springate, I.C.E. Turcu(CLF, STFC Rutherford Appleton Laboratory, UK),

S. Dhesi (Diamond Light Source, UK),

H. Berger (EPFL, Lausanne),

S. Kaiser, (University of Hamburg, Germany)

A. Cavalieri, (University of Hamburg,Germany and Oxford University, UK)

J.C. [email protected]

32

!0.5 0.0 0.5 1.017.0

17.2

17.4

17.6

17.8

18.0

18.2

Time, ps

Energy,eV

I"Imax

I"0

Photoelectron intensity map ofresponse to photoexcitation.

!0.5 0.0 0.5 1.0 1.50

2

4

6

8

10

12

14

Time, ps

LHBpeakreduction,"

Reduction in LHB intensity. Solid line: Full fit.Dashed line: Exponential component only.

Ultrafast time-resolved photoelectronimaging of excited state moleculardynamics: S2-S1 internal conversion inthe DABCO molecule

Femtosecond time-resolved photoelectronimaging was applied to the ultrafastinternal conversion dynamics inphotoexcited 1,4 diazabicyclo[2.2.2]octane.A systematic investigation of the electronicrelaxation dynamics upon the vibrationalenergy of the molecule was undertaken byscanning the pump wavelength between236.1-251.1 nm. The temporal evolution ofthe photoelectron spectrum and angulardistribution was measured. Analysis of thedata is ongoing.

R. Spesyvtsev, R. Minns(University College London, UK),

M. Siano, J. Marangos(Imperial College London, UK),

R. Livingstone, D. Townsend(Heriot-Watt University, Edinburgh, UK),

E. Springate, I.C.E. Turcu,(CLF, STFC Rutherford Appleton Laboratory,UK),

D.M.P. Holland(STFC Daresbury Laboratory, UK),

J.G. Underwood(University College London, UK)

J.G. Underwood,[email protected]

Pump pulse at 236.1 nm and probe pulse at390 nm. The raw experimental data is shown atthe top, and the Abel inverted data is shownbelow. The outer ring corresponds tophotoelectrons from ionization of the S2electronic state, and the inner ring correspondsto ionization of the S1 electronic state.

33

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