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.

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Prebreakdown field emission

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