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 reversal (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 phase velocity is greater than a few times the thermal speed. This enhanced rate of work near the antenna can significantly exceed the far-field value due to Landau damping. For the conditions of typical low-field (<0.01 T) and low-density (<10¹⁸ 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 demonstrated that this spatial transient overshoot in the rate of work done by the 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 the wave and low velocity electrons as a result of this process. An important practical consequence is that the ionization rate of neutral gas can be significantly enhanced above the Maxwellian rate. The phenomenon is not restricted to helicon sources. This process may also explain the production of high-energy electrons in the near fields of antennas used in fusion plasma heating by radiofrequency waves.