ARC-SPOT IGNITION CAUSED BY SHEATH INSTABILITY

The transition from the stationary and homogeneous state of the collis ion-free space charge sheath between plasma and wall to a strongly inh omogeneous and instationary state with are spots is initiated by local enhancement of power supply from the plasma, connected with increasin g electron emission. The general relations between surface emissivity, net current density, sheath potential, and power density in such ''ac tive'' boundary layers are derived, taking the feedback of emitted ele ctrons on the plasma parameters into account, as far as necessary, Jou le heat generation is taken into account, The decisive effect of the t ransition is the rapidly increasing yield of thermo-field electron emi ssion, Conditions and possible paths to sheath instability and are spo t formation are discussed.