ION-SOURCE ELECTRODE BIASING TECHNIQUE FOR MICROSECOND BEAM PULSE RISE TIMES

Heavy ion fusion (HIF) induction accelerators require ion sources that can deliver intense heavy ion beams with low emittance. The typical p ulse length is 20 mu s with a rise time less than 1 mu s and a repetit ion rate of 10 Hz. So far, the surface ionization sources have been us ed in most HIF induction linac designs. However, there are other ions of interest to HIF (e.g., Hg, Xe, Rb, Ar, and Ne) which cannot be prod uced by the surface ionization sources, but rather by volume ion sourc es. In this paper, we describe an experiment that uses a multicusp sou rce with a magnetic filter to produce beam pulses that have a rise tim e in the order of 1 mu s. By applying a positive biasing pulse on the plasma electrode with respect to the source body, the positive plasma ions can be temporarily repelled from the neighborhood of the extracti on aperture, leading to a suppression of the ion beam. As the bias is removed, positive ions flow to the extraction region, enabling a fast- rising beam pulse. The beam current pulses show that there are two dis tinct groups of ions. An initial fast current rise time (< 2 mu s) cor responding to ions originating from within the magnetic filter region followed by a second group of ions with a longer rise time (10-20 mu s ) originating from the plasma bulk region. Proper positioning of the f ilament cathode and the magnetic filter field relative to the extracti on aperture was found to be critical. (C) 1998 American Institute of P hysics.