Current Disruptions in a Plasma Driven Probe Circuit

Abstract: Measurements are made to investigate the nature of a fast current disruptionappearing in one of the experimental devices at the Alfven Laboratory. Thedisruption appears when we let the induced electric field ( -vxB) in a transverselymagnetized flowing plasma drive a current between two short circuited plasmaprobes. It is the current measured in the circuit connecting the two probes thatshow the disruptive behaviour. The parameters of the typical disruption are a 5 Adrop in 25 ns.The results of the measurements indicate that the disruption is a relatively generalprocess. Instead of being a consequence of some special detail in the probeconfiguration it turns out to be a process appearing whenever we draw a currentbetween two probes in the plasma.The origin of the disruption is located and it turns out to be at, or at least near,the negatively biased probe.Simultaneous current and plasma density measurements indicate that the currentin the probe circuit generally exceeds the theoretical ion saturation current. Forthis to be possible there has to be some sort of electron emission process present atthe surface of the negatively biased probe. The conclusion is that it is this processthat fails when the current disrupts. Cathode spots are sometimes observed on thenegative probe, and they are believed to be the emission process in question.An analysis of the probe circuit gives that the measured current-voltagebehaviour during a disruption in a good way agrees with the assumption that acathode spot is extinguished at the negatively biased probe. When combining themodel of the circuit with a negative current-voltage characteristic for the cathodespot we get an unstable system. The disruption is believed to reflect this instability.The negative current-voltage characteristic needed to make this descriptionsuccessful is well described by curves found in literature on cathode spots.