DENSITY-FLUCTUATIONS ASSOCIATED WITH LIMITER H-MODES ON TFTR

Edge density fluctuations have been observed with microwave scattering in limiter H-mode plasmas on TFTR. The primary feature of these fluctuations is a strong average frequency shift in the electron diamagnetic drift direction corresponding to poloidal velocities of 1-3x10(6) cm/s and an inward radial electric field of 60 kV/m. The rotation begins to increase at the Halpha drop and rises to a steady state in 5 to 50 ms. well before ELMs begin. ELMS modulate the rotation feature, eliminating it for the duration of the ELM. After the ELM, the rotation feature resumes its pre-ELM value in about 0.1 ms. Short duration (0.01 ms) precursor bursts propagating in the ion diamagnetic direction are observed to precede the ELM event by approximately 0.5 ms.