Extraction of H− and D− ions from a large magnetic multipole source

Results are presented on the production of H− ions in a large (55×31×20 cm3) magnetic multipole discharge, which is sufficiently large to act as the plasma generator for a neutral beam injector. H− ion current densities up to 57 mA/cm2 have been extracted in pulses up to 3 s in length at low energies. When the discharge is operated in deuterium, a maximum D− current density of 27 mA/cm2 is obtained. The scaling laws of the H− production are consistent with production by dissociative attachment to rovibrationally excited molecules and loss by mutual recombination.