Retrograde motion of cathode spots

Although several different physical models were published in the past by different authors, as of today none of these models quantitatively explains the motion of cathode spot roots and their plasma jet against the direction of the magnetic (Lorentz) force. In this contribution, a new approach is presented which explains this “retrograde” motion in terms of ignition probability of new cathode spots in the vicinity of the initial spot root. The commonly accepted model of cathode spot “motion”, i.e. stepwise propagation, is extended by an analysis of the initiation probability of new spots. The power flux to the cathode surface, which is the major cause of spot ignition in the vicinity of a cathode spot, is analysed and is found to be non-symmetrical in the presence of a transverse magnetic field. The reason of this broken symmetry is the deflection of ions in the cathode fall by the superposition of the (symmetric) spot Eigenfeld and the non-symmetric transverse magnetic field. It is found that the probability of ignition of cathode spots is randomly symmetric in the close vicinity of the spot, but increases in the “retrograde” direction for distances of the order of several tens of micrometers.