H - ion sources and high flux neutral beams

Conventional dc sources of H- are limited to current densities of the order of 50 mA/cm2 for sources with area larger than a few cm2. Early more recent work have shown that pulsed magnetically insulated ion diodes can produce current densities larger by factors of the order 102--104. The authors studied the production of negative ion beams in the coaxial, racetrack and annular diode geometries. The experiments showed that when using a passive dielectric cathode negative ions are emitted mainly from a few hot spots located on the cathode surface. The angular divergence of the beam was about 300 milliradians. The authors have developed a TiH2 plasma source that consists of 120 discharges on a flashboard. The resultant cathode plasma is quite homogeneous compared to divergence of 10 milliradians were observed with this source. The total current of H- ions was about 1 kA. The current density is many order of magnitude larger than would be expected from a thermal plasma source. A non-equilibrium model is advanced to explain this fact