Spontaneous current disruptions in a plasma-driven double probe

Summary form only given. Measurements are made to investigate the nature of fast current disruptions appearing in one of the experimental devices in the Alfven Laboratory. The disruptions appear when we, in a transversely magnetized plasma flow, let the induced electric field (E=-v/spl times/B) drive a current between two externally short-circuited, floating, probes. In this configuration, the effective probe bias is E/spl middot/d, where .d is the distance across the magnetic field between the probes. One of the two probes collects ions (the ICP), and the other collects electrons (the CP). Simultaneous current and plasma density measurements show that the current during a disruption typically drops from a value 1-2 times larger than, and down to, the local ion saturation current at the ICP. We believe that cathode spots are repeatedly ignited at the surface of the ICP, and that the observed disruptions reflect that these cathode spots are extinguished 10-100 ns after ignition, during a phase when the current is still rising. The roles of the external circuit, the shapes of the probes, the probe bias, and the plasma stream parameters are investigated in detail.