H- Ion Source 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 work at UCI and more recent work at the Lebedev Institute have shown that pulsed magnetically insulated ion diodes can produce current densities larger by factors of the order 102 - 104. We 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. We have developed a TiH2 plasma source that consists of 120 discharges on a flashboard. The resultant cathode plasma is quite homogeneous compared to typical polyethylene passive cathodes. H- ions with a current density of 6 A/cm2 and a divergence of 10 milliradians were observed with this source. The total current of H- ions was about 1 kA. The current density is many orders of magnitude larger than would be expected from a thermal plasma source. A non-equilibrium model is advanced to explain this fact.