4 - The Interelectrode Plasma

Abstract

This chapter presents an experimental and theoretical characterization of the interelectrode plasma of the cathodic arc, in the absence of an anode spot. The cathodic arc is discussed in terms of the number of cathode spots. The single cathode spot arc (SCS) is defined, which is an arc at relatively low current, depending on the cathode material, and also the multi-cathode spot arc (MCS). The discussion focuses mostly on the MCS arc, which has a well defined and developed interelectrode plasma region. The interelectrode plasma of the vacuum arc discharge is a passive conductor that serves to conduct the arc current from the cathode to the anode. It is also a medium that interacts strongly with the electrodes, and the physical processes occurring in it affect significantly the nature of the discharge. The interelectrode plasma consists mainly of charged atomic particles emitted by the cathode spots, which are located on the cathode. The plasma is emitted from the cathode in the form of a number of discrete jets, which then merge to form a beam, which flows from the cathode. The ionization distribution, and the spatial distribution of the various atomic species, are to large extent determined by the physical properties of the cathode. The fraction of the ion current in relation to the arc current is approximately constant for most metals