Ion acceleration in the radially expanding plasma of the hot refractory anode vacuum arc

The ion energy and plasma potential as a function of radial distance were measured in a vacuum arc mode—the hot refractory anode vacuum arc. A 175 A arc was sustained between a 30-mm-diam water-cooled Cu cathode and a thermally isolated graphite anode. The plasma potential and electron temperature were determined using a triple probe, and the ion energy distribution was measured with a retarding field analyzer. It was found that the electron temperature decreased from 1.2 to 0.6 eV with increasing radial distance from 3 to 18 cm. In the same distance range, the directed ion energy per unit charge increased from 8 to 20 eV. The 12 eV ion energy enhancement significantly exceeded the measured drop of the plasma potential of about 2 V, indicating that the ions were not accelerated electrostatically.