AND SIMULATION OF CONTROLLED BEAMFRONT MOTION IN THE LASER CONTROLLED COLLECTIVE ACCELERATOR

In th? Laser Controlled Collective Accelerator, an intense electron beam is injected at a current, above the vacuum space charge limit into an initiaIly evacuated drift tube. A plasma channel. produced by time-sequenced, multiple laser beam ionization of a solid target on the drift tube wall, provides the nccessary neutralization to allow for effective beam propagation. By controlling the rate of production of the plasma channel as a function of time down the drift tube, control of the electron hcamfront can be achieved. Recent experimental measurements of controlled beamfront motion in this configuration are prcsentcd, along with results of ion acceleration experiments conducted using two different accflrrating gradients. These results arc compared with numerical simulations of the system in nhich both controlled beamfront mot,ion and ion acceleration is observed consistent with both design expectations and experimental results. I. Introcluctiou The Laser Controlled Collect,ive Accelerator concept’-3 reprcscnts an attempt to extend the promising results from “naturitlly occurring” collective ion acceleration experiments to practical accelerators in which the accelerating gradient and distance can be systematically cont,rolled. The concept is similar to that employed in the IFA- and IFA- experiments of Olson4’“, although the actual experimental configurat,ion is q”it,c different. The basic concept behind the experiment is shown in Fig. 1. An intense relativistic electron beam is injectcd through a localized gas cloud int,o an evacuated drift tubes at, a current well above the vaculim space charge limit. A virtlml cathode then forms immediately downstream of the injection point and ions produced within the localized gas cloud are accelerated to modest energies in a manner similar to more conventional collective accelerators. At this point, a channel of plasma is produced in a time sequenced manner down the drift tube by laser ionization of a CH2 target strip located on the drift tube wall. The time sequencing of the plasma channel is achieved by dividing a Q-switched ruby laser pulse into ten approximately equal energy beams and using optical delays to ionize sequentially ten target spots equally spaced down the drift tube. In this manner, the virtual cathode at the beamfront can be carefully accelerated down the drift tube and ions trapped by the strong electric fields at the virtual cathode can be accelerated to high energies in a controlled manner. In this paper we present in section II results of experiments in which controlled beamfront motion has been confirmed for two different accelerating gradients. Results of ion acceleration experiments are also presented. Numerical simulations of the experiments presented in section III confirm both controlled beamfront motion and the controlled acceleration of ions by the moving virtual cathode over significant distances. Conclusions are drawn in section IV.