Relevance of plasma-induced arcs for divertor tokamaks

Abstract

Plasma-induced arcs and their ignition conditions have been studied in the divertor chamber of the ASDEX tokamak by introducing arc probes and Langmuir probes in front of the divertor plates. Arcs are rare events and are always correlated with unstable phases of the plasma, such as at disruptions or during neutral-beam injection. Arcs require high ignition potentials (100-500 V) and burning voltages of the order of 100 V. In addition, an internal limitation of the rate of current rise to values of about lO'A'S^has been observed. The probability of arcing during short transient voltages is thus reduced. Arcs are generally preceded by diffuse discharges carrying currents of a few amperes. Since diffuse discharges occur more frequently than arcing events, a significant contribution of diffuse discharges to surface erosion is suspected.

Plasma-facing materials for fusion devices

“…However, it may also cause the ignition and burn of electrical arcs between the solid, acting as the cathode and the plasma representing the anode. Traces of the cathode spots of these electrical arcs are seen at several vessel wall areas [56][57][58][59][60][61][62][63]. On the vessel wall, which acts as the cathode, material is eroded by the electrical arcs due to evaporation and emission of droplets (Fig.…”

Properties of Arc Cathode Spots

“…In this phase, a current up to some 100 mA is flowing that exceeds the ion saturation current from the plasma. (Such precurrents have also been found at negatively biased electrodes in a tokamak [26,27]). After some time the spots start to operate as bright localized structures.…”

Dust observation with a visible fast camera in the TJ-II stellarator

“…A possible explanation of the observed dust ejection with the negative limiter biasing is an increased Lorentz force that accelerates the charged dust particles towards the plasma. Dust (at least in rest) is negatively charged due to the dominating effect of the ambipolar diffusion of the electrons onto a floating surface in contact with a plasma (see, e.g., [10]). If we apply a negative voltage to the limiter, the electric field will be changed in front of the limiter so as to increase the Lorentz force and the acceleration of the dust particles towards the plasma.…”

“…Another completely different but also possible explanation of the effect of surface biasing on dust generation is that biasing can induce unipolar arcing, which was thought in the past to be a possible mechanism for dust formation e.g., [10][11][12][13]). It has already been observed with visible cameras in the past that arcs can be accompanied by the eruption of 'incandescent particulates' or UFOs [5], probably dust particles emitting thermal and atomic radiation when observed with cameras.…”

“…It has already been observed with visible cameras in the past that arcs can be accompanied by the eruption of 'incandescent particulates' or UFOs [5], probably dust particles emitting thermal and atomic radiation when observed with cameras. Moreover, it has been reported that arcs can be ignited at probes in limiters and divertors when applying relatively low negative voltages of the order of −100 V, the value being strongly dependent on the surface condition and geometry [10]. Therefore, it is also possible that the negative biasing can enhance the dust formation itself through induced arcing.…”