Generalized high-speed subdomain analysis method for quasi-optical antennae

The development of gyrotrons for application in the area of ECRH (electron cyclotron resonance heating) of magnetically confined plasmas has seen a corresponding development in the gyrotron output quasi-optical antenna systems. The whispering gallery mode (WGM) has been employed for this purpose. The WGM has smaller problems of mode competition and through its use higher output powers and frequencies (up to 1 MW at about 140 GHz) can be achieved from the gyrotron. The WGM is a helically winding mode propagating along a cylindrical waveguide. For ECRH, a uniformly polarized Gaussian beam is required for efficient heating. Therefore some modification of the output is required. This can be accomplished by use of the WGM quasi-optical reflector antennae. This is composed of a helically-cut output waveguide and a cylindrical-parabolic reflector. In the Department of Electronics at Kyoto University up until recently the design and performance of these antennas systems has been made using purely ray optical methods i.e. geometrical optics (GO). To improve the accuracy of the present design, diffraction at the edges of the antenna aperture had to be accounted for