RF absorption and ion heating in helicon sources

Abstract

Summary form only given, as follows. Initial ion temperature measurements in The Hot hELIcon eXperiment (HELIX) helicon source showed that the ion temperature is non-negligible and can be as high as 1 eV. The measurements also showed that the ion temperature scales with magnetic field strength and that the ion temperature is anisotropic with respect to the magnetic field. The perpendicular ion temperature increases with increasing magnetic field strength while the parallel ion temperatures remain the same. Further investigation by Balkey et al. [Plasma Sources Sci. and Tech., 10, 284(2001)] showed that the increase in ion temperature and the electron density are correlated with the lower hybrid frequency. Recent experimental data will be presented that are consistent with hypothesis that the anomalous rf absorption in helicon sources is due to electron scattering arising from parametrically driven ion-acoustic waves downstream from the antenna. Theory predicts that the scattering should be largest near the lower hybrid frequency. We will also present ion temperature measurements that demonstrate anisotropic heating (T/sub /spl perp//>T/sub /spl par//) at the edge of the discharge when the rf frequency equals the lower hybrid frequency at the edge of the plasma. The most likely explanation is ion Landau damping of electrostatic slow waves at a local lower hybrid frequency resonance.