Magnetically insulated H− diodes

At the University of California, Irvine, we have been studying the production of intense H⁻ beams using pulse power techniques for the past 7 years. Previously, current densities of H⁻ ions for various diode designs at UCI have been a few A/cm². Recently, we have developed diodes similar to the coaxial design of the Lebedev Physical Institute, Moscow, U.S.S.R., where current densities of up to 200 A/cm² were reported using nuclear activation of a carbon target. In experiments at UCI employing the coaxial diode, current densities of up to 35 A/cm from a passive polyethylene cathode loaded with TiH2 have been measured using a pinhole camera and CR-39 track recording plastic. We have also been working on a self-insulating, annular diode which can generate a directed beam of H⁻ ions. In the annular diode experiments a plasma opening switch was used to provide a prepulse and a current path which self-insulated the diode. These experiments were done on the machine APEX, a 1 MV, 50 ns, 7 Ω pulseline with a unipolar negative prepulse of ∼ 100 kV and 400 ns duration. Currently we are modifying the pulseline to include an external LC circuit which can generate a bipolar, 150 kV, 1 μs duration prepulse (similar prepulse characteristic as in the Lebedev Institute experiments cited above).