Modeling ionization by helicon waves

The response of the electron distribution function in one dimension to a traveling wave electric field is modeled for parameters relevant to a low-pressure helicon wave plasma source, and the resulting change in the ionization rate calculated. This is done by calculating the trajectories of individual electrons in a given wave field and assuming no collisions to build up the distribution function as the distance from the antenna is increased. The ionization rate is calculated for argon by considering the ionization cross section and electron flux at a specified position and time relative to the left-hand boundary, where the distribution function is assumed to be Maxwellian and the wave travels to the right. The simulation shows pulses in the ionization rate that move away from the antenna at the phase velocity of the wave, demonstrating the effect of resonant electrons trapped in the wave's frame of reference. It is found that the ionization rate is highest when the phase velocity of the wave is between 2 and 3 x 10(6) m/s, where the electrons interacting strongly with the wave (i.e., electrons with velocities inside the wave's ''trapping width'') have initial energies just below the ionization threshold. Results from the model are compared with experimental data and show reasonable qualitative agreement. (C) 1997 American Institute of Physics.