Collective phenomena at the metal-plasma interface

Summary form only given. At a metal-plasma interface, besides classical elementary interactions of ions, electrons, neutrals, photons, and the like with the metal surface, collective phenomena associated with imperfection of the surface occur. First, this is charging of dielectric films and inclusions by plasma ions followed by breakdown of the dielectric. Second, when a plasma flows around the surface of a cathode, the current density is enhanced in metal microprotrusions, resulting in their Joule heating and electrical explosion. In both cases, a portion of /spl sim/10/sup 11/-10/sup 12/ electrons, termed an ecton, is ejected from the metal. For an ecton to appear, >= 10/sup -8/ J of energy is necessary. The criterion for the first phenomenon is t/sub s/n/sub i/v/sub i/>=10/sup 14/ cm/sup -2/, where n/sub i/ and v/sub i/ are the ion density and velocity, respectively, and t/sub s/ is the plasma-surface interaction time. For t/sub s//spl sim/10/sup -9/ s and v/sub i/=10/sup 6/ cm/s we have n/sub l//spl sim/10/sup 17/ cm/sup -3/. The criterion for the second phenomenon is /spl beta//sub j/n/sub l/v/sub i/>=10/sup 28/ cm/sup -2/ s/sup -1/, where /spl beta//sub j/ is the current density enhancement factor. For the plasma density in the second case is greater than in the first one by three orders of magnitude. Both phenomena result in the appearance of inhomogeneous plasmas whose density decreases from 10/sup 22/ cm/sup -3/ to the surrounding plasma density within several micrometers. Besides, a liquid metal pool appears on the metal surface. Splashing of the liquid metal under the pressure of dense plasma (10/sup 8/ Pa) given rise to droplets /spl sim/0. 1 /spl mu/m in size outflying with a velocity of 10/sup 4/ cm/s. These droplets may be responsible for dust plasma effects. Microplasmas favor the occurrence of new microexplosions at the metal surface, initiating, for instance, unipolar arcs.