FROM FERMI ACCELERATION TO COLLISIONLESS DISCHARGE HEATING

The heating of electrons by time-varying fields is fundamental to the operation of radio frequency (RF) and microwave discharges. Ohmic heat ing, in which the phase of the electron oscillation motion in the fiel d is randomized locally by interparticle collisions, can dominate at h igh pressures. Phase randomization can also occur due to electron ther mal motion in spatially inhomogeneous RF fields, even in the absence o f collisions, leading to collisionless or stochastic heating, which ca n dominate at low pressures. Generally, electrons are heated collision lessly by repeated interaction with fields that are localized within a sheath, skin depth layer, or resonance layer inside the discharge. Th is suggests the simple heating model of a ball bouncing elastically ba ck and forth between a fixed and an oscillating wall, Such a model was proposed originally by Fermi to explain the origin of cosmic rays. In this review, Fermi acceleration is used as a paradigm to describe col lisionless heating and phase randomization in capacitive, inductive, a nd electron cyclotron resonance (ECR) discharges. Mapping models for F ermi acceleration are introduced, and the Fokker-Planck description of the heating and the effects of phase correlations are described, The collisionless heating rates are determined in capacitive and inductive discharges and compared with self-consistent (kinetic) calculations w here available. Experimental measurements and computer simulations are reviewed and compared to theoretical calculations. Recent measurement s and calculations of nonlocal heating effects, such as negative elect ron power absorption, are described. Incomplete phase randomization an d adiabatic barriers are shown to modify the heating in low pressure E CR discharges.