EDGE DENSITY PROFILE MEASUREMENTS BY AMPLITUDE-MODULATION REFLECTOMETRY ON PBX-M TOKAMAK

A reflectometer, based on the amplitude modulation technique, has bean developed and operated on a PBX-M for the measurement of electron density profile, The system, operating with the extraordinary mode in the range of 32-50 GHz, is able to measure the density profile in the plasma edge from the scrape-off layer up to typically r/a = 0.7. The determination of the time delay for each frequency is achieved by measuring the phase delay of a 200 MHz amplitude modulation envelope superimposed on the millimetre wave probing signal. The system has a final bandwidth of 40 kHz and is able to obtain the edge profile during a 1 ms sweep of the microwave source. High quality profiles are obtained in systematic good agreement with Thomson scattering measurements. The profile reconstruction from the raw data is direct, with only a need for minimal data processing. Profiles have been measured for ohmic, RF and NBI heated discharges. Features of the profile changes in the L-H transition are shown. One of the goals of the instrument has been the measurement of the slight modifications to the edge density profile produced by the injection of ion Bernstein waves. These changes have been clearly observed and are in agreement with theoretical expectations.