IONACCELERATION BYTHERELATIVISTIC TWO-STREAMINSTABILITY

Intenserelativistic electron beamspassing throughlowdensityplasmas are oftenoksSrved to accelerate plasmaionsto MeVenergies. Although a numberofplausible acceleration modelshavebeen proposed,46 thecomplexity oftherelatedphysical phenomena together withthelackofsufficiently detailedexperimental datahavethusfarprecluded any firmdecisionon theactualmechanism responsible. Indeed,collective ionacceleration occursundersuch widelydiffering experimental conditions thatperhaps severaldifferent processesare involved.7 Oneobviouscandidate isa relativistic two-stream instability betweentheinterpenetrating beamand plasma.Unfortunately, plasmaelectrons effectively shieldtheionsfromdirecttwo-streaminteraction withtheelectron beam.Indirect coupling byion trappingduringthenon-linear evolutionofan electron-electron two streaminstability isineffective becauseofveryhighphasevelocities intheunstable waves.Whilereturncurrentheatingof ionsviathe ion-acoustic instability isknowntooccur,iontemperaturesachieved are,atmost,a fewkeV.