Dense Metal Plasma in a Solenoid for Ion Beam Neutralization

Space-charge neutralization is required to compress and focus a pulsed high-current ion beam on a target for warm dense matter physics or heavy ion fusion experiments. We describe an approach to produce dense plasma in and near the final focusing solenoid through which the ion beam travels, thereby providing an opportunity for the beam to acquire the necessary space-charge compensating electrons. We use a high-current (>;4 kA) pulsed vacuum arc plasma from a ring cathode near the edge of the solenoid. The plasma distribution is characterized by photographic means, by an array of movable Langmuir probes, by a small single probe, and by evaluating Stark broadening of the Balmer H_β spectral line. The plasma is produced at several cathode spots distributed azimuthally on the ring cathode. It is shown that the plasma is essentially hollow, as determined by the structure of the magnetic field, although the plasma density exceeds almost everywhere in the volume defined by the solenoid. The plasma is nonuniform and fluctuating. However, since its density exceeds the ion beam density, it is believed that this approach could provide a practical solution to the space-charge neutralization challenge.