Abstract
In order to clarify the instability phenomena of a low current vacuum arc, characterized by noise on the current trace prior to the actual current chopping, an analysis of silver cathode spot has been made. For the analysis, cathode spot region is recognized as the collisionless space charge sheath connected with singly ionized collisional plasmas.It is proposed that a current level below that no real solution exists is unstable current and this current corresponds to the point at which a vacuum arc turns into unstable. It is found that there is no real solution when the current is below 19.4A. The physical explanation is considered that the electrons returning to the sheath region from the plasma one dominate over positive ions and, consequently, the electric field at the cathode surface is F02<=0 below 19.4A. Similar results have already been obtained for copper cathode. In order to examine the analytical result for its validity, the probe measurements of electron temperature in the diffused arc plasma has been performed. The electron temperature-arc current characteristics were found to be similar between experimental and analytical results.
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