TRAPPING BACKSTREAMING IONS FROM AN X-RAY CONVERTER USING AN INDUCTIVE CELL

High current electron beams have been used as x-ray drivers for x-ray radiography. Typically, several thousand amperes of electron beam current at 20 MeV is focused to a millimeter spot size on a x-ray converter. Within a single pulse, the heating of the target by the electron beam will lead to rapid desorption of surface contaminants. The space charge potential of the electron beam will pull ions out of this plasma layer upstream into the beam. These backstreaming ions can act as a focusing lens which cause the beam to be overfocused at a waist upstream. The final beam spot size on the target would then be larger than intended, and the x-ray radiography resolution is reduced. We have designed a self- biased ion trap for the Experimental Test Accelerator (ETA-II) beam by using an Advanced Test Accelerator (ATA) inductive cell to prevent the backstreaming ions from moving upstream and forming a long ion focusing channel. We have studied the effects of this type of ion trap on the final focusing of the electron beam with the ETA-II beam parameters. Simulation results will be presented.