ECR Plasma Source for Heavy Ion Beam Charge Neutralization*

Philip C. Efthimion, Erik Gilson, Larry Grisham, Pavel Kolchin and Ronald C. Davidson Plasma Physics Laboratory Princeton University, Princeton, New Jersey, 08543, USA Simon Yu and B. Grant Logan Lawrence Berkeley National Laboratory University of California, Berkeley, California, 94720, USA Highly ionized plasmas are being considered as a medium for charge neutralizing heavy ion beams in order to focus beyond the space-charge limit. Calculations suggest that plasma at a density of 1 100 times the ion beam density and at a length ~ 0.1-2 m would be suitable for achieving a high level of charge neutralization. An ECR source has been built at the Princeton Plasma Physics Laboratory (PPPL) to support a joint Neutralized Transport Experiment (NTX) at the Lawrence Berkeley National Laboratory (LBNL) to study ion beam neutralization with plasma. The ECR source operates at 13.6 MHz and with solenoid magnetic fields of 1-10 gauss. The goal is to operate the source at pressures ~ 10 Torr at full ionization. The initial operation of the source has been at pressures of 10 10 Torr. Electron densities in the range of 10 10 cm have been achieved. Low-pressure operation is important to reduce ion beam ionization. A cusp magnetic field has been installed to improve radial confinement and reduce the field strength on the beam axis. In addition, axial confinement is believed to be important to achieve lower-pressure operation. To further improve breakdown at low pressure, a weak electron source will be placed near the end of the ECR source.