THE DEPENDENCE OF PHOTODETACHMENT CURRENT ON DISCHARGE PARAMETERS IN A PULSED LOW-PRESSURE PLASMA

Photodetachment measurements were carried out in a pulsed low-pressure, high-power volume ion source. The photodetachment current can be time-resolved throughout the discharge and post-discharge periods. Here we present results of the dependence of the photodetachment current both on the discharge current and on the source filling pressure. An increase in the measured photodetachment current in the post-discharge of the plasma occurs at low operating pressures, 0.1 Pa, but this increase is shown to be a strong function of the source pressure. At pressures above 0.67 Pa no increase in the photodetachment current is observed after the source has been switched off and as the source pressure is decreased the post-discharge enhancement in the photodetachment current increases. This is consistent with the higher electron temperatures and fast electron densities expected at low operating pressures. The ratio of the maximum photodetachment current in the post-discharge to the photodetachment current just before the source is switched off is shown to increase with power input up to discharge currents of over 100 A. These results show that, for low-pressure (< 0.1 Pa), high-power (> 100 A) discharges, large enhancements in the negative ion density can be expected after switching the discharge current off. The risetime of the photodetachment current in the post-discharge is also shown to have a strong dependence upon the source pressure. The results presented indicate that the increases in the negative ion density and the risetime of the photodetachment current in the post-discharge period are dependent upon the power density of the plasma.