Ultrashort-Pulse Reflectometer(USPR) is considered by way of having a potential for determining the electron density profile in a large magnetic fusion device. USPR experiments were initially carried out on CCT tokamak[1] subsequently on GAMMA 10 tandem mirror[2], also on SSPX[3].
We have been developing a new microwave reflectometer using an ultrashort-pulse to measure the electron density profile and fluctuation for a Large Helical Device(LHD)[4]plasma. The reflectometer has higher resolution for time and space than other conventional methods. This diagnostic is based on time-of-flight measurement and has not only the possibility of measurement for electron density profile but also for fast phenomena as a MHD oscillation, an electron density fluctuation and a pellet injection. An ultrashort-pulse has broad band frequency components in a Fourier - space. We use -2.2 V, 23 ps full-width half-maximum (FWHM) impulse and have succeeded to extract the desired range of the millimeter wave from it for a plasma measurement. When they are launched into the plasma, they reflect from the corresponding each cut-off layers. These reflected waves are detected by a heterodyne system. When the time-of-flight measurement of each frequency component pulse is done, the electron density profile can be determined. In our USPR system the electron density profile is estimated by measurement of six cut-off layers. Space resolution of 6 mm has been obtained by the calibration experiment using a mirror. The electron density fluctuations at six cut-off layers are simultaneously measured. Our USPR system is now installed on the HYPER-I device. In HYPER-I device several structures in rotating plasma have been observed. They are hole[5], vortex[6], spiral[7] structures and so on. We apply our USPR to such plasmas and research effects on the reflectmetric measurement of the electron density profile and fluctuation that is measured.

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