Transition from unstable electrostatic confinement to stable magnetic confinement in a helicon reactor operating with Ar/SF6 gas mixtures

Two types of instabilities were previously identified in inductive discharges having an expanding chamber when negative ions are present: i the source instability, occurring in the neighborhood of the capacitive-to-inductive E to H transition, and ii the downstream instability, which was shown to be the periodic formation and propagation of double layers. These unstable double layers were found over the entire parameter space pressure/power of interest, and they were born at the interface of the source and diffusion chambers. They acted as an internal electrostatic barrier separating a low-electronegativity, high-electron-density plasma upstream in the source and a high-electronegativity, low-electron-density plasma downstream. In this paper we have investigated the effect of adding a static axial magnetic field, classically used to increase the confinement and the plasma heating via helicon wave propagation. This had the following consequences: i the unstable double layers, and therefore the axial electrostatic confinement, were suppressed in a large part of the parameter space, and ii the magnetic confinement leads to a radially stratified plasma, the center being a low-electronegativity, high-density plasma and the edges being essentially an ion-ion plasma. © 2006 American Institute of Physics. DOI: 10.1063/1.2191431