Doppler reflectometry studies of plasma gradient instabilities in L-2M stellarator

Paper reports on a broadband spectrum analysis of density fluctuations in the edge plasma of the L-2M stellarator, measured using the Doppler reflectometry diagnostics. The evolution of the electron temperature-gradient (ETG) and ion temperature-gradient (ITG) instabilities in the edge plasma of the stellarator is analyzed. Two additional harmonics in the spectrum are found apart from those determined by the Doppler shift caused by poloidal rotation of plasma. The Doppler shifts of these harmonics are determined from the phase velocities of fluctuations generated by the ETG and ITG instabilities. Thus, local measurements of spectra using the Doppler reflectometry enabled us both to evaluate the poloidal rotational velocity of the plasma and describe the evolution of low-frequency plasma instabilities.