Operating modes of a magnetized cold cathode plasma in helium 50–6400 mtorr

We characterize the operating modes of a magnetized cold cathode plasma in helium at pressures 50–6400 mtorr (6.7–850 Pa). We find at least three distinct modes: (i) a diffuse magnetron plasma with representative plasma voltage 250 V; (ii) a constricted mode with a bright spot at the cathode surface that moves slowly, with representative plasma voltage 50 V; and, (iii) a constricted mode with a bright spot at the cathode surface that orbits rapidly around the racetrack region defined by the magnetic field, with representative plasma voltage 80 V. The constricted-orbiting mode is of interest because it appears to be a cold-cathode plasma operating at unusually low voltage, and it is associated with a low rate of cathode mass-loss. We find that it appears only if the initial rate of current-rise is high, and if the cathode surface is smooth and featureless. It also appears to be correlated with cathode materials that form strong oxide films, that can dramatically increase the field emission of electrons from the cathode surface.