Measurements of long-wavelength density fluctuations in TFTR

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Abstract
Several experiments have been devised to measure plasma fluctuations in an effort to help elucidate a possible connection between plasma microturbulence and anomalous transport. Results from microwave scattering on the Tokamak Fusion Test Reactor (TFTR) [Nucl. Fusion 18, 1089 (1978)] show that the level of plasma fluctuations increases toward the long‐wavelength region (k⊥ρs≤0.2), at which point the fluctuations cannot be spatially resolved. The desire to measure long‐wavelength fluctuations has motivated the development of two fluctuation diagnostics, beam emission spectroscopy (BES), and microwave reflectometry on TFTR. BES measures long‐wavelength density fluctuations (k⊥≤2 cm−1) by observing the fluorescence emitted from collisionally excited atoms in a TFTR heating beam. In L‐mode discharges with relatively flat density profiles, the fluctuations measured with BES are concentrated in the low‐frequency region (≤30 kHz). In the laboratory frame, the fluctuations have a poloidal propagation velocity that is approximately equal to that of the plasma rotation, and the frequency spectra are broadened by Doppler effects. Measured fluctuation levels are 5%–10% at the edge of the plasma. In the core, the level falls to less than 1%, which is comparable to observations made with microwave reflectometry and scattering. The fluctuation level in the core (r/a=0.7) is seen to increase with injected beam power, as is observed with microwave scattering at shorter wavelengths (k⊥≊2–10 cm−1). In contrast, the fluctuation level in the edge region does not change significantly with neutral beam power.

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