Downstream physics of the helicon discharge

Measurements of the radial and axial profiles of both the plasma parameters and the wave properties in a long, thin helicon discharge show that most of the RF power is deposited near the antenna and that a dense, cool ( eV) plasma can be obtained in the downstream region. The density n and electron temperature profiles in that region can be explained quantitatively with classical collisional theory, and factor-of-two agreement can be obtained on total particle and energy balance. Spatial modulation of the helicon wave amplitude can be explained by the beating of two different radial modes launched simultaneously by the antenna. Though the helicon wave can be shown to be essential to the production of high densities, it plays little role in the downstream evolution of the plasma. These results indicate that helicon discharges can produce the cool plasmas normally associated with afterglows without the attendant loss of density.