MEASUREMENT OF NEGATIVE-IONS USING IMPROVED PROBE FOR DETECTION OF PHOTODETACHED ELECTRONS

Negative ion measurement applying an electrostatic probe to detect photodetached electrons in noisy plasma has been performed using a compensation circuit with an additional probe. The detected photodetachment current is dependent on the probe surface material. A carbon-coated probe yields the best results in reproducibility and sensitivity in our experiments. The time response of photodetached electron current caused by laser irradiation has also been monitored. The transient current is two to three orders of magnitude larger than the steadystate current at the instant the laser is turned on. Therefore, the absolute determination of steadystate photodetachment current using laser light chopping and the lock-in amplifier is unreliable.