COLLISIONLESS ENERGY COUPLING TO HIGH-VELOCITY ELECTRONS IN THE NEAR-FIELD OF AN ANTENNA - NEUTRAL GAS IONIZATION BY HELICON WAVES

We propose a new kind of transit-time interaction in which the reversa l (or any localized transition) in the phase velocity of a wave within a wavelength of an antenna results in a high rate of work done by the wave (1/2 Re(j.E)) near the antenna for conditions where the wave ph ase velocity is greater than a few times the thermal speed. This enhan ced rate of work near the antenna can significantly exceed the far-fie ld value due to Landau damping. For the conditions of typical low-fiel d (<0.01 T) and low-density (<10(18) m(-3)) helicon wave-driven plasma sources, where the phase velocity parallel to the magnetic field can be a few times the thermal speed of electrons, it has been demonstrate d that this spatial transient overshoot in the rate of work done by th e wave is the dominant kinetic energy coupling process to electrons). In this paper it is demonstrated that, within a half wavelength of the antenna, it is the high-energy electrons that gain energy from both t he wave and low velocity electrons as a result of this process. An imp ortant practical consequence is that the ionization rate of neutral ga s can be significantly enhanced above the Maxwellian rate. The phenome non is not restricted to helicon sources. This process may also explai n the production of high-energy electrons in the near fields of antenn as used in fusion plasma heating by radiofrequency waves.