Beam-target Interaction Experiments for Bremsstrahlung Converter Applications

The DARHT II accelerator uses a pulsed high current electron beam and Bremsstrahlung converter target to generate an intense x-ray source for radiography. For the past several years, we have been performing an investigation of the possible adverse effects of (1) backstreaming ion emission from the Bremsstrahlung converter target and (2) the interaction of the resultant plasma with the electron beam during subsequent pulses. These effects would manifest themselves in a static focusing system as a rapidly varying x-ray spot. To study these effects, we are conducting beam-target interaction experiments on the ETA-II accelerator (a 6.0 MeV, 2.5 kA, 70 ns FWHM pulsed induction LINAC). We have determined spot dynamics and characterized the resultant plasma for various configurations. Our experiments show that the first effect is not strongly present when the beam initially interacts with the target. Electron beam pulses delivered to the target after formation of a plasma are strongly affected, however. We have also performed initial experiments to determine the effect of the beam propagating through the plasma. This data shows that the head of the beam is relatively robust, but that backstreaming ions from the plasma can induce a dynamic focus toward the tail of the beam. We survey the results of our experiments and attempts to suppress the adverse effects we have observed.