The dynamics of arc cathode spots in vacuum: New measurements

Are 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 1000 m s(-1). 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 1-3 mu s, had a most probable duration (width of the light pulse) of 200-300 mu s. The spatial resolution, 1.22 Irm 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 (R(2)) was measured to be (R(2))/tau = (2.3 +/- 0.6) x 10(-3) m(2) s(-1 ) for observation times tau 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.