Drifting localization of ionization runaway: Unraveling the nature of anomalous transport in high power impulse magnetron sputtering

The plasma over a magnetron's erosion "racetrack" is not azimuthally uniform but concentrated in distinct dense ionization zones which move in the E×B direction with about 10% of the electron E×B/B2 drift velocity. The ionization zones are investigated with a gated camera working in concert with a streak camera for Al, Nb, Cu, and W targets in Ar or Kr background gas. It is found that each ionization zone has a high plasma density edge, which is the origin of a plasma-generating electron jet leaving the target zone. Each region of strong azimuthal plasma density gradient generates an azimuthal electric field, which promotes the escape of magnetized electrons and the formation of electron jets and plasma flares. The phenomena are proposed to be caused by an ionization instability where each dense plasma zone exhibits a high stopping power for drifting high energy electrons, thereby enhancing itself.