Using corrugated fins to decrease the beamwidth of the radiation pattern from a Vlasov launcher

Summary form only given. Some gyrotrons which are under development for ECRH generate power in a rotating TE/sub mn/ mode in circular cylindrical cavities. These modes are not suitable to use directly for plasma heating because their radiation patterns have off axis peaks and are not linearly polarized. Mode conversion to a more suitable mode using perturbed-wall mode converters is not always practical. This problem can be overcome by using a Vlasov-type launcher feeding a series of two or more reflectors. The Vlasov launcher is a member of the slot radiator family. The fields inside the waveguide can be represented by rays which fill out the region between a caustic circle inside the guide and the waveguide walls. In this geometrical optics model, all of these rays will exit the waveguide through a slot of minimum width /spl phi//sub c/ and length L. The length of the slot is determined by the mode, frequency and waveguide size. The minimum azimuthal extent of the slot and hence the azimuthal character of the radiation pattern are determined only by the mode. For example the radiation pattern from a TE/sub 22,6/ mode Vlasov launcher will have approximately the same azimuthal beamwidth regardless of the frequency or waveguide size. The radiation from the slot can be shaped and directed by means of suitably designed reflectors so that it has the desired characteristics for transmission through the gyrotron window and then for plasma heating. We have attached corrugated flares to the aperture to decrease the azimuthal beamwidth and found that a narrower beamwidth with lower side lobes can be achieved. Experimental results are presented.