Effects of bias potential upon H− density near a plasma grid of a negative ion source

A molybdenum plasma grid was installed in a 30-cm-long 16-cm-diam 16-pole magnetic multicusp ion source to simulate the effect of a plasma electrode bias of a negative ion source. Effects caused by the bias voltage applied to the electrode upon the plasma parameters, the density of negative hydrogen ions (H−), and the drift velocity of plasma perturbed by the photodetachment of H− were investigated with the direct current laser photodetachment method. The electron density at the distance of 0.5 cm from the plasma grid decreased from 8×1010 to 4×1010cm−3 by increasing the grid bias from 0 to +4V, while the H− density increased from 8×108 to 4.5×109cm−3. The drift velocity of plasma perturbation was changed by a factor of 3 corresponding with the gradient of the plasma potential near the electrode.