Ferroelectric plasma source for heavy ion beam space charge neutralization

摘要

Plasmas are a source of unbound electrons for charge neutralizing intense heavy ion beams to allow them to focus to a small spot size and compress their axial pulse length. The plasma source should be able to operate at low neutral pressures and without strong externally applied electric or magnetic fields. To produce 1 m-long plasma columns, sources based upon ferroelectric ceramics with large dielectric coefficients are being developed. The sources utilize the ferroelectric ceramic BaTiO_3 to form metal plasma. The drift tube inner surface of the Neutralized Drift Compression Experiment (NDCX) will be covered with ceramic material, and high voltage (～7 kV) will be applied between the drift tube and the front surface of the ceramics. A prototype ferroelectric source, 20cm in length, has produced plasma densities of 5x10~(11)cm~(-3). It was integrated into the Neutralized Transport Experiment (NTX), and successfully charge neutralized the K~+ ion beam. A 1 m-long source comprised of five 20-cm-long sources has been tested. Simply connecting the five sources in parallel to a single pulse forming network power supply yielded non-uniform performance due to the time-dependent nature of the load that each of the five plasma sources experiences. Other circuit combinations have been considered, including powering each source by its own supply. The 1-m-long source has now been successfully characterized, producing relatively uniform plasma over the 1 m length of the source in the mid-10~(10) cm~(-3) density range. This source will be integrated into the NDCX device for charge neutralization and beam compression experiments.