1-millimeter wave interferometer for the measurement of line integral electron density on TFTR

Abstract

A two‐pass interferometer at 285 GHz has been developed to measure the line‐integrated electron density on the horizontal midplane of the toroidal fusion test reactor (TFTR). Presently, the interferometer employs a 2‐mW solid‐state source to supply the launch wave and a 2‐mm klystron oscillator and, a harmonic mixer to provide a superheterodyne front end. The transmission system consists of 25 m of C‐band rectangular waveguide, adjustable miter bends, and a spherical mirror in the vacuum vessel with a total round trip transmission loss of 21 dB. The interferometer signal to noise is ≳50 dB. Utilization of a feed forward tracking system provides long‐term stable operation. The interferometer routinely provides real‐time feedback control for the gas injection system and a permissive for neutral beam operation.