Model for relaxation oscillations in a helicon discharge

Relaxation oscillations observed in the large-volume, helicon plasma experi ment WOMBAT (Waves on Magnetized Beams and Turbulence) [R. W. Boswell and R . K. Porteous, Appl. Phys. Lett. 50, 1130 (1987)] are modeled. These oscill ations have a period of several milliseconds and have been identified as tr ansitions between a low-density, inductive discharge and a high-density, he licon-wave discharge. In the model, it is assumed that the mode transitions are triggered by variations in the neutral density in the source region. T he neutral density decreases due to ionization augmented by ion pumping and increases due to refilling of the source chamber from the much larger diff usion chamber. The system is modeled using two, coupled, nonlinear, ordinar y differential equations that describe the neutral and plasma densities in the source chamber. Ionization by inductively-coupled fields and ionization due to electrons accelerated by helicon waves with phase velocities near t he threshold electron velocity for ionization are considered. The model is found to reproduce experimentally measured variations of the plasma density and helicon wave phase velocity with rf power, neutral pressure and magnet ic field. The negative impedance needed for the existence of a relaxation o scillation is provided by the helicon-wave coupling mechanism. (C) 1999 Ame rican Institute of Physics. [S1070-664X(99)03405-9].