Laser scattering measurement of the electron density fluctuations in CHS

For measurement of electron density fluctuations (up to 1 MHz), an HCN laser scattering measurement system with a super rotating grating was developed on CHS. When we measured electron density fluctuations with kr=5.4 cm-1 in a plasma edge region at the outer side of torus, fluctuation spectra which have harmonic components up to the fifth were observed. The fundamental frequency was less than about 60 kHz, which changed during the discharge. The change in frequency correlated best with the plasma stored energy divided by the line averaged density. An edge transport barrier (ETB), which can improve particle transport in the edge region, has been observed in CHS and the measured fluctuation amplitude also decreases. The fluctuations decreased in about 5 ms, and the decrease of the fluctuations preceded by 3 ms to the drop of Hα emission signal. (author)