Enhanced production of negative ions in low-pressure hydrogen and deuterium discharges

A new approach to enhance the production of negative ions in low-pressure hydrogen and deuterium discharges is described. The procedure, which requires on-off modulation of the discharge current, is applied to a conventional magnetic multicusp or ``bucket'' volume negative-ion source. The extracted D^- current from a 15-A discharge at a filling pressure of 2.4 mTorr is increased from 0.5 to 2.0 mA/cm² by discharge-current modulation at 10 kHz and 23% duty cycle. The D^- beam current is not significantly modulated (5%). The extracted electron current density and average discharge current are reduced by over a factor of 4. The work is relevant to production of D^- beams for neutral-beam injectors.