Fluctuation measurements in the plasma interior on TFTR

Abstract

New beam emission spectroscopy and correlation reflectometry density fluctuation diagnostics on TFTR allow examination of the crucial long wavelength (kperpendicular to rho i<0.1) region, where microwave scattering has indicated where the bulk of the power density lies. The turbulent spectrum for low k is broadband and dominated by low frequencies (<200 KHz), and the exact nature of the spectrum is dominated by Doppler effects. These core diagnostics show n/n rising as tau E decreases. Radial and poloidal correlation lengths from BES are comparable and are roughly 2-3 cm near the plasma edge and gradually drop to 1-2 cm in the region of r/a=0.5-0.7. Values <1.5 cm are indicated deeper in the core region, but precise values await further analysis which takes into account the effects of finite spatial resolution of the BES measurements. A strong anisotropy in the radial and poloidal wavenumber spectra is indicated from the shape of the correlation functions. The poloidal wavenumber spectrum does not increase indefinitely as k goes to 0 cm-1, but has a peak in the range of 1-2 cm-1.