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Electrons surf to high energies on plasma wavesAlan Cairns

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Physics WorldFebruary 1996

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2 2 Physics World February 1996

Electrons surf to high energieson plasma wavesFrom Alan Cairns in the School of Mathematicaland Computational Sciences, University of StAndrews, UK

THE accelerators used by particle physi-cists need to be many kilometres long toreach the energies at which new particlessuch as the Higgs boson are expected to befound. The particles are accelerated byelectric fields, and accelerator designersare always looking for ways to increase theelectric field strength since this will reducethe length of the machine. One possibilitythat has been investigated over thepast decade is the use of plasmawaves in laser-produced plasmas.The electric field in a large-amplitude plasma wave can beorders of magnitude strongerthan the fields produced inconventional accelerators. Thedownside is that the plasmamedium cannot be controlled asprecisely as metal conductors, soreplacing the large machinesrequired for high-energy physicswith plasma-based accelerators isstill a distant dream.

However, an important advancein particle acceleration to moremodest energies has recently beendemonstrated at the RutherfordAppleton Laboratory (RAL) inthe UK (A Modena et al. 1995Nature 377 606). A team fromImperial College in London, theUniversity of California at LosAngeles, the Ecole Polytech-nique in Palaiseau, France, theLawrence Livermore NationalLaboratory in California andRAL used the VULCAN laser atRutherford to accelerate electronsup to energies of 44 MeV in acollimated beam. The electrons wereaccelerated by the plasma wave producedwhen a short, high-intensity laser pulsefrom VULCAN was focused onto a gastarget.

The electric field in the plasma wave isproduced by differences in the electron andion charge densities in the plasma. Thefield is both periodic and parallel to thewave propagation direction (i.e. the wave islongitudinal). The plasma wave producedby a laser can have both a large amplitudeand a phase velocity close to the speed oflight, and a particle moving with the wave isaccelerated by this field, rather like a surferon a water wave.

So far, most discussion of plasmaaccelerators has focused on either "beatwave" or "wake field" schemes. In the beatwave scheme, two laser pulses that differ infrequency by the plasma frequency - thecharacteristic frequency of longitudinal

oscillations in the plasma - interact asthey propagate through the plasma. Thisgenerates a driving force at the beatfrequency, which, being in resonance withthe plasma wave, drives it up to a highamplitude. In the wake field scheme, asingle laser pulse leaves a wake, consistingof a plasma wave, behind it. The laser pulsemust be shorter than one wavelength of theplasma wave.

However, the present experiments usedsingle-frequency pulses that were too longto generate a wake. Therefore the large-

Preparing a short-pulse electron acceleration experiment using theVULCAN laser at the Rutherford Appleton Laboratory.

well amplitude plasma wave producing theacceleration was attributed to a thirdmechanism: forward Raman scattering.This process involves the resonant excita-tion of a second or "decay" electro-magnetic wave and a longitudinal plasmawave by an electromagnetic pump wave(such as a laser). Backward Ramanscattering, in which the decay electro-magnetic wave travels in the oppositedirection to the incident wave, has beeninvestigated in detail because it is thoughtto prevent effective energy absorption bythe solid targets used in laser-fusionexperiments.

In the less dense gas targets of thepresent experiments, forward Raman scat-tering occurs with the incident and decaywaves parallel. The generated plasma wavehas a high phase velocity, which can beclose to that of light under suitableconditions. In essence, the process issimilar to the beat wave scheme, with two

electromagnetic waves interacting to driveup the plasma wave. The difference is thatthe second wave is driven up from noiserather than being generated externally.This has the advantage that the frequencydifference of the waves automaticallymatches the plasma frequency, and thusthe careful control of the plasma densitythat is needed for a successful beat waveexperiment is not necessary.

The experiments used 25 TW, 0.8 pspulses focused to a 20 urn spot on the edgeof a 4 mm diameter plume of helium gas

from a pulsed supersonic gas jet.The most important diagnosticsmeasured the scattered wavespectrum and the electronenergy spectrum in the forwarddirection. The best results wereobtained at the highest gas densityused, 1.5 x 1019 cm"3, giving anelectron energy spectrum essen-tially flat up to 30 MeV, falling offslowly up to the detector limit of44 MeV. Although the highervalue is the greatest energyclaimed by the team, it seemsreasonable to suppose that thespectrum continues up to sub-stantially higher energies. Thisacceleration is accompanied by atremendous broadening of thescattered wave spectrum.

The acceleration is likely to bethe result of wave breaking. Thewave grows until the potentialwells associated with the long-itudinal wave field become deepenough to trap particles in themain part of the thermal distribu-tion of electrons. Substantialnumbers of particles are thenaccelerated up to relativistic ener-

gies. At the same time the coherence of thewave is destroyed. The process is analogousto the familiar breaking of a water wavewhere liquid at the crest of the wave isthrown forward and the coherent wavebreaks up.

Compared with a conventional particleaccelerator, the system produces beamswith a transverse eminence that matchesthose of photoinjector-based linacs, butwith a current that is between 10 and 100times less. However, the accelerationlength is estimated to be only a fewhundred micrometres, and the inferredpeak electric field is of the order of100 GVm"1, the highest collective fieldproduced to date in a laboratory. AlthoughVULCAN is a large system, advances inlaser technology make it possible toenvisage compact laser-based acceleratorsproducing electrons in the range from a fewMeV to a few hundred MeV in the not toodistant future. •

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