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 den sity profile, The system, operating with the extraordinary mode in the range of 32-50 GHz, is able to measure the density profile in the pla sma edge from the scrape-off layer up to typically r/a = 0.7. The dete rmination of the time delay for each frequency is achieved by measurin g the phase delay of a 200 MHz amplitude modulation envelope superimpo sed on the millimetre wave probing signal. The system has a final band width of 40 kHz and is able to obtain the edge profile during a 1 ms s weep of the microwave source. High quality profiles are obtained in sy stematic good agreement with Thomson scattering measurements. The prof ile reconstruction from the raw data is direct, with only a need for m inimal data processing. Profiles have been measured for ohmic, RF and NBI heated discharges. Features of the profile changes in the L-H tran sition are shown. One of the goals of the instrument has been the meas urement of the slight modifications to the edge density profile produc ed by the injection of ion Bernstein waves. These changes have been cl early observed and are in agreement with theoretical expectations.