Circular TE11-based waveguide polarizer for efficient coupling to electron Bernstein wave

Abstract

A circular waveguide polarizer for 2.45 GHz microwaves has been designed and produced for electron Bernstein (EB) wave heating and current drive experiments on the low aspect ratio Torus experiment (LATE) device. In the experiments the microwaves are injected obliquely to the toroidal field to drive current carrying fast electrons. In the LATE case of the oblique injection of long free space wavelength waves onto a small plasma the coupling to the EB waves from the injected electromagnetic waves is improved when the injected waves have appropriate elliptical polarizations depending on the injection angle and the density gradient at the upper hybrid resonance layer (Igami et al 2006 Plasma Phys. Control. Fusion 48 573). Any elliptically polarized mode can be generated from a circular TE11 mode by the combination of a half wave section and a quarter wave section of circular waveguide. The transformation of polarization arises from the difference in axial wave numbers between an orthogonal pair of modified circular TE11 modes, which is realized by the insertion of a Teflon plate in a circular waveguide made of aluminum. Such a polarized wave is launched from the open end of a circular waveguide toward the plasma. An elliptically polarized mode adjusted to a steep density gradient and injected obliquely to the toroidal field has been found to be effective to sustain a highly over-dense plasma.