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 ${\mathrm{D}}^{\mathrm{-}}$ current from a 15-A discharge at a filling pressure of 2.4 mTorr is increased from 0.5 to 2.0 mA/${\mathrm{cm}}^2$ by discharge-current modulation at 10 kHz and 23% duty cycle. The ${\mathrm{D}}^{\mathrm{-}}$ 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 ${\mathrm{D}}^{\mathrm{-}}$ beams for neutral-beam injectors.