Breakdown Conditions of Local Sheath Discharge in Front of Positively Biased Electrode Immersed in Inductively Coupled Plasma

Summary form only given. Increasing ion beam current is one of the most challenging issues in plasma ion source. It was reported that ion beam current was drastically enhanced by generating additional plasma, called local sheath discharge, near the extraction hole located at center of the electrode by biasing positively.The breakdown condition and discharge mechanism, however, were not elucidated sufficiently. In this study, breakdown conditions for the local discharges have been experimentally investigated by varying the discharge conditions such as gas pressure, power for generating background plasma, and geometry of the electrode. An electrode screened by dielectric material except small circular area, is immersed in inductively coupled plasma (ICP) and biased with positive voltage with respect to the space potential of the plasma. A bright plasma bubble is observed locally just above the exposed part of the biased electrode with abrupt increase of current to the electrode when the bias voltage exceeds a certain threshold, breakdown voltage. The breakdown voltage decreases as the gas pressure is raised. Radio frequency power for generating ICP as well as the electrode geometry influences on the breakdown voltage too. These experimental results indicate that ambient plasma properties such as density, temperature, and space potential play an important role in generating local sheath discharges.