Arc spot ignition caused by sheath instability

The transition from the stationary and homogeneous state of the collision-free space charge sheath between plasma and wall to a strongly inhomogeneous and instationary state with are spots is initiated by local enhancement of power supply from the plasma, connected with increasing electron emission. The general relations between surface emissivity, net current density, sheath potential, and power density in such "active" boundary layers are derived, taking the feedback of emitted electrons on the plasma parameters into account, as far as necessary. Joule heat generation is taken into account. The decisive effect of the transition is the rapidly increasing yield of thermo-field electron emission. Conditions and possible paths to sheath instability and arc spot formation are discussed.