The dynamics of arc cathode spots in vacuum: new measurements

Abstract

Arc cathode spots in vacuum with Ti- and Cu-cathodes have been studied by using the high-speed framing camera IMACON 468 in the time range down to 10 ns. The spots exhibited an inner structure that indicates the existence of fragments. At currents of 70 A their number was 5 - 7. These structures showed high dynamics, attracting and repelling one another with speeds up to . This was associated with quasi-periodic fluctuations of the spot brightness, yielding a new explanation for the brightness fluctuations reported in a preceding paper. One of the fluctuations, occurring every , had a most probable duration (width of the light pulse) of 200 - 300 ns. The spatial resolution, per pixel, was sufficient to allow observation of surface craters in situ. So it turned out that the fragments are located at crater rims and that the crater structures may change in times <100 ns. As a consequence of periodic attracting and repelling of the fragments, the spot displaces at random. The mean square displacement was measured to be for observation times of 100, 200 and 500 ns. From the time scale of surface changes it is concluded that the main energy source for crater formation is surface bombardment by the plasma ions rather than Joule heating of the crater volume.

Characterization of worn Ti–Si cathodes used for reactive cathodic arc evaporation

“…As illustrated in figure 9(a), the cathode spots are in a molten state during its lifetime due to the extremely high temperature 30 . The dense plasma in front of the cathode spots generates a high pressure, which exceeds atmospheric pressure by orders of magnitude (0.5 GPa) 31 .…”

The fractal nature of vacuum arc cathode spots

“…The diffusion constant is material dependent, for example Jüttner found experimentally for copper [41] and for molybdenum [42]. The mean value for displacement is ( )…”

Cathode potential drop at a transient cathode spot on a microprotrusion

“…At the first cycle the time τ can be related to the time of the initial triggering plasma life, which depends on the kind of arc triggering [5]. At the stage of transient spot development the plasma brightness fluctuations in vacuum arc can be considered as a cyclic life time [6]. The observed mentioned times of τ are in range of 10-100ns [1,6] and also ~10 µs [3].…”

“…At the stage of transient spot development the plasma brightness fluctuations in vacuum arc can be considered as a cyclic life time [6]. The observed mentioned times of τ are in range of 10-100ns [1,6] and also ~10 µs [3]. A uniform temperature distribution in the protrusion volume was assumed.…”

“…It is expected that the CPD should be strongly decrease during the first 1µs, similar to that for a spot on a flat bulk cathode surface [2,4]. The CPD dependence on plasma life time is important for understanding the mechanism of arc ignition which occurs in 10-100 ns [6].…”