A Microwave Reflectometry Based On Amplitude Modulation onHT-7 Tokamak

B.L.Ling,Institute of Plasma Physics, Chinese Academy of Sciences, P.O.Box 1

126, Hefei, P.R.China, 230031ling@ipp.ac.cn

Abstract A broadband reflectometer operating in the frequency range 48-72GHz has bee

n designed on HT-7 tokamak (R0=1.22m, a-0.27m, Bt:1.8-2.0Tesla) and is able to measure electron density profiles at the edge from the scrape-off layer up to typical r/a=0.5. The system uses extraordinary mode polarization and will perform routine measurement with heterodyne detection. The determination of the time delay for each frequency is achieved by measuring the phase delay of a 200MHz amplitude modulation envelope superimposed on the millimiter wave probing signal. The electron density profile is able to obtain edge electron density profile during a 1ms sweep of the microwave source. A 2MHz reference signal and the probing signal are obtained by mixing a 198MHz local oscillator with the demodulated microwave signals. Phase signals are generated at 2MHz and amplitude information is obtained by the ratio of 2MHz probing signal before and after limited amplification. The amplitude information is of great importance for determination of the profile initialization. The goal of the instrument is to investigate the lower hybrid wave coupling into plasmas for current drive and the radio frequency wave coupling problems for heating on HT-7.

Cut-off frequency profile for an assumed parabolic density profile and possible central density ne0 on HT-7 tokamak

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Structure of the AM reflectometer

Microwave transmission of the reflectometry

The output of the microwave source vs. frequency

Phase measurement in lab test

Summary1) A reflectometry base on amplitude modulation has been d

esigned in hope to obtain electron density profile on HT-7 tokamak.

2) In order to cover the wide radius range under the different discharge conditions, frequency range is chosen to be 48-72GHz.

3) The microwave source of the reflectometer is a hyperabrupt varactor-tuned oscillator, which generates frequencies between 12 and 18 GHz, and with an active multiplier chain ,the required frequency range can be fulfilled.

4) After the necessary test and calibration , and combining these diagnostics, the density profile can be obtained by thi

s reflectometry system.