Generation of triggerless silicon shunting plasma and ion extraction

Abstract

The electrical characteristics of a pulsed silicon ‘shunting plasma’ are described and compared with carbon and niobium shunting plasmas. The plasma development occurs in three phases. Initially the solid material is heated by the discharge current, followed by a surface discharge during a glow-like phase, and finally the main plasma is formed as an arc by joule heating of the chosen solid material. The time at which the surface discharge bridges over the silicon plate corresponds to an abrupt decrease in voltage across the plate. Both streamer plasma bridging and an abrupt voltage decrease initiate the arc discharges as the final phase of the plasma formation process. Observations of optical emission spectra indicate that the plasma is composed of the original chosen solid material. Ion current is extracted from the plasma to a nearby target by application of a pulsed voltage to the target. The extracted current depends on the ambient gas pressure in the target region. For a pressure of 2.7 Pa a sharp current peak occurs upon application of voltage, while for a pressure of 133 Pa a rectangular current pulse shape is formed, similar to the shape of the applied voltage pulse. This may be a result of the formation of a matrix sheath surrounding the target that is immersed in the shunting plasma.