Microwave Reflectometry in the GAMMA 10 Device

Abstract

Two types of microwave reflectometries are installed in the GAMMA 10 device for electron density profile and fluctuation measurement. Microwave is injected into the plasma in the O-inode or in the X-mode and is reflected at the cutoff layer where the electron plasma frequency corresponding to the electron density is equal to the frequency of incident wave.An ultra short pulse method uses pulse train having duration of 65 ps and repetition rale of 250 kHz generated by an impulse generator. This signal has a broad frequency spectrum in microwave region. So simple transmitting system is realized. Reflected wave is divided into five frequency channels. Their center frequencies are 7, 8, 9, 10 and 11 GHz. Time of flight of each frequency component is measured using a time to amplitude converter. Location of reflected point is calculated from the time of flight. An electron density profile is reconstructed with one-shot data.Fast frequency-modulation method is also used. The microwave source is swept from 11.5 GHz to 17.5 GHz: The signal is injected in the X-mode. Phase difference between reference wave and reflected wave is observed. Density profile is reconstructed from the phase difference data. There is low frequency fluctuation in the plasma. When sweep time is faster than 20 μ s, reliable data are obtained. This system has a rather simple receiving system.Though reflected wave has an information of fluctuation, several methods of frequency analysis are tried and distribution of fluctuation is observed simultaneously.Magnetic fluctuation is also observed by cross-polarization scattering method.