Energy of ions in vacuum arcs between axial and radial magnetic field contacts

In vacuum circuit breakers two different contact types are used to overcome the consequences of arc constriction, which sets in when currents of several kiloamperes are exceeded. Radial magnetic fields (RMF) force the constricted arc to rotate and distribute its power more evenly on the contact surface. Axial magnetic field (AMF) contacts prevent the arc from becoming constricted up to higher thresholds. To improve the interruption capability of vacuum circuit breakers of both types it is essential to know about the processes and properties of the vacuum plasma ("vacuum arc") around current zero, such as plasma density and its decay and the energy of the plasma species. In this work the energy distribution of ions in the vacuum arc plasma during the last 3 ms before current zero has been investigated by means of a retarding field analyzer up to arc currents of 7.5 kA RMS. Significant differences could be observed between RMF and AMF contacts. For currents above 5 kA the distribution in both cases resembles a Maxwellian distribution, characteristic for a collision-determined plasma. On lower currents, i.e. when current zero is approached, RMF arrangements show ions with strongly directed motion, while the energy distribution for AMF contacts is much more influenced by collisions.