Probe measurement of residual plasma of a magnetically confined high-current vacuum arc

The election temperature and ion density of the residual plasma of a magnetically confined high-current vacuum arc were measured using the electrostatic (Langmuir) probe method. Measurements were performed in a triggered-vacuum gap using a 40-mm-diameter oxygen-free copper (OFCu) cathode and an anode of 100 mm diameter separated by a gap of 30 mm. The arc current is a sinusoidal wave having a half-period of approximately 1 ms and a peak value ranging from 1 to 5 kA. An axial magnetic field (AMF) was supplied by either a dc field coil or AMF-type electrodes, which generate an ac magnetic field in close proximity that has coincident phase with arc current. It was found that the electron temperature of the residual plasma did not depend on current peak,. and current gradient and space remained in a constant range of 2.2-3.0 eV. The radial distribution of ion density was diffusive, in contrast to the parabolic confined distribution of the steady-state are. The characteristics obtained for the dc magnetic field and those obtained for the AMF electrode were identical except for the ion density at current zero.