Faraday Cup Measurements of Ions Backstreaming into an Electron Beam Impinging on a Plasma Plume

The next generation of radiographic machines based on induction accelerators is expected to generate multiple, small diameter x-ray spots of high intensity. Experiments to study the interaction of the electron beam with plasmas generated at the x-ray converter and at beamline septa are being performed at the Lawrence Livermore National Laboratory (LLNL) using the 6-MeV, 2-kA Experimental Test Accelerator (ETA) electron beam. The physics issues of concern can be separated into two categories. The interaction of subsequent beam pulses with the expanding plasma plume generated by earlier pulses striking the xray converter or a septum, and the more subtle effect involving the extraction of light ions from a plasma by the head of the beam pulse. These light ions may be due to contaminants on the surface of the beam pipe or converter, or, for subsequent pulses, in the material of the converter. The space charge depression of the beam could accelerate the light ions to velocities of several mm/ns. As the ions moved through the body of the incoming pulse, the beam would be pinched resulting in a moving focus prior to the converter and a time varying x-ray spot. Studies of the beam-generated plasma at the x-ray converter have been previously reported. In this paper we describe Faraday cup measurements performed to detect and quantify the flow of backstreaming ions as the ETA beam pulse impinges on preformed plasma.