A study of unipolar arcs in a low-pressure mercury discharge

Abstract

An experimental study of unipolar arcs was conducted in a low-pressure mercury rf-generated plasma. The results were found to be consistent with the concept of a sheath mechanism for driving the unipolar arcs. Floating double-probe measurements yielded electron temperatures of ∼2 eV and electron-number densities of ∼1×1011 cm−3. The variation of the unipolar arc current with (1) the electron temperature and density and (2) the metal surface area exposed to the plasma verified the predicted dependence of the arc current on the plasma parameters and the metal surface area. Finally, alternative mechanisms for sustaining the observed arcs by high-frequency rectification were ruled out on the basis of the recorded current waveforms of the unipolar arcs.

A simplified analytical model for an anchored cathode root in a DC mercury vapour arc

“…An increase of I, results in increase of a , T , and W-,eva except when I , is close to the maximum current through a single root. Although the number of cathode spots has not been described, Johnson and Lee (1981) measured a maximum current of 1.2-2.1 A in the unipolar arc mode. This value was the current needed to sustain a cathode spot on the mercury cathode under a particular experimental condition.…”