HYBRID TRANSMISSION LINES FOR ECRH IN THE HSX STELLARATOR

A new transmission line has been designed for the planned upgrades for the 28GHz ECRH system on the HSX stellarator. In order to create a reliable and cost effective system, the new line utilizes quasioptical design techniques and oversized straight waveguide sections similar to those used in the existing transmission line. The first stage of the line converts the native TE₀₂ gyrotron output mode to a free space Gaussian beam. The two methods considered are a Vlasov mode converter and a TE₀₂–to–TE₀₁–to–TE₁₁–to–HE₁₁ conversion sequence using waveguide mode converters retained from a previous ECRH system. A matching optics unit couples the beam to a smooth, circular ₁₁ + TM₁₁ dual-mode waveguide sections for traversing the experimental hall, and a grooved polarizer rotates the beam polarization for O–mode or X–mode ECRH. Transmission line bends are implemented with ellipsoidal mirrors, with the final bend acting as a rotatable switch to select between HSX and a dummy load. A steerable mirror in boxport B allows independent selection of the heating surface in the plane of the boxport.