Beam-bunching in an ECR ion source by the pulsed gating-potential method

A pulsed gating-potential method for millisecond beam-bunching has successfully been applied for the first time to a single-stage 6.4 GHz ECR ion source. Clear bunching effects were observed when a pulsed alternating potential of a few hundred volts was applied between the ECR chamber of the source and an electrode placed near its exit hole. The bunched current observed at a sufficiently low repetition rate of the bunching potential shows a sharp peak in the beginning of the duty-on period and decreases exponentially with time, approaching a value expected without the bunching potential. From this time structure of the current, we could deduce mean residence time of ions in the plasma zone. When the repetition rate of the potential is sufficiently high, the bunched current is observed to be almost constant, its magnitude being determined by the duty factor of the bunching potential and the ionization efficiency of the source. We also describe theoretical formulation of the bunching effect in the above two cases, from which the ionization efficiency can be extracted easily.