Prototype of a Quasi-Optical Launcher System of a 4 mm Round-Trip Interferometer for the QUEST Spherical Tokamak Experiments

A launcher system for a round-trip interferometer, with a corrugated horn antenna and two quasi-optical mirrors, has been developed for the QUEST spherical tokamak experiment. The corrugated horn antenna has been designed using a three-dimensional (3D) electromagnetic simulator. At low power bench test, intensity and phase profiles at 0.1 m from antenna aperture show coaxial patterns as predicted by the 3D simulator, in agreement with the calculation results using an in-house developed Kirchhoff integral code. The designed waist sizes of the focused beam at a target position are 22 mm in the horizontal x direction and 43 mm in the vertical y direction. The beam reaches the target plate at QUEST's center-post and is reflected back along the line-of-sight round-trip. A well-focused beam of 32 and 36 mm in size (x, y) has been obtained at the target position. The HE11 mode excitation, beam transmission, and focusing properties are discussed for the prototype launcher system.