A Real Time 155 GHz Microwave Interferometer for Remote Electron Density Measurements in Large Plasma Devices

A frequency modulated 155 GHz millimeter wave interferometer was built to perform real time electron density measurements on a Tokamak plasma machine with a plasma diameter of 1 meter. An intermediate frequency IF = 312 kHz was selected at a diode detector by a clocked sawtooth modulation of a reflex klystron oscillator. The zero crossing instants of this frequency were recorded. Since phase shifts of 30×2¿ and more are expected in the measurement branch of the interferometer a digital fringe counting system with analog conversion for real time applications was set up. A Camac interface allows a local data storage in a memory, data acquisition by a diagnostic computer and off-line display on a terminal. The accuracy and stability of the electronics were tested in a periodic experiment. The time resolution was tested on a pulsed toroidal plasma experiment. Density changes corresponding to 1 fringe in a 40 ¿sec were registered. The accuracy limit of the registration system is a phase shift of ¿¿=¿/8.