Investigation of microwave plasmas produced in a mirror machine using ordinary-mode polarization

Abstract

This paper presents results of plasma production in a mirror machine with the microwaves being coupled from a side port, in the ordinary-mode polarization. In this method of coupling, contamination due to the launching antenna is avoided; also, there is very little window degradation since plasma bombardment on the side port is negligible. The coupling of the microwaves to the plasma in this method relies on the existence of certain surface waves that exist at low values of the magnetic field (below that for electron cyclotron resonance) and have a dominant E_z component. The existence of such surface waves is predicted by a recent theory (Ganguli A et al 1998 Phys. Plasmas 5 1178). The plasma characterization at microwave power levels of about 200 W, indicates moderately overdense plasmas (density 10¹¹ cm^-3), with fairly uniform radial and axial profiles. From the data, the mechanism of plasma production and confinement in the mirror has been analysed. To gain insight into the nature of the waves in the plasma-loaded waveguide, the wavelengths of the waves within the plasma and the radial profiles of the wave electric field were measured. The results are compared with the theory of Ganguli et al for wave identification.