On the Retrograde Motion of Arcs in Magnetic Fields

Experiments are reported which allow the determination of retrograde velocity of individual cathode spots and of the plasma flow in a pulsed discharge using various metals and carbon as cathode materials. For discharge currents from 10 to 40 amps, pressures of 3 mmHg and magnetic field strengths of 6.103G retrograde spot velocities from 30 to approximately 300 m/sec are observed and the corresponding plasma flow velocities are in the range from 4,400 to 8,600 m/sec. On cathode materials with low melting points, the splitting rate of spots and the motion of individual spots is small, whereas under identical conditions the spots on refractory materials are highly mobile, the splitting rate is large, and the lifetime of individual spots is short.