Observations of Alfvénic MHD Activity in the H-1 Heliac

Magnetic configuration scans in the range 1.1 < ι0 < 1.5 in the H-1 flexible heliac have shown a detailed rotational transform dependence of plasma density and fluctuation spectra. Poloidal Mirnov arrays reveal magnetic fluctuations in the range 1-200kHz in plasma produced by RF heating in H, D and He mixtures ranging from highly coherent, often multi-frequency, to broad band. Both positive (stellarator-like) and negative (tokamak-like) shear are examined, including configurations where the sign of the shear reverses. Data mining techniques, SVD, wavelet and Fourier analysis are applied, typically by SVD spatial decomposition of modes on all coils, then a grouping of SVD eigenfunctions, based on spectral similarity, into fluctuation structures. The thousands of structures found are grouped into a small number (~10) of clusters of similar mode structure. Alfvénic scaling in both density and k|| are confirmed, although a scale factor ~3 in frequency remains unexplained. The k|| dependence provides a precise experimental determination of the position of rational surfaces (if the shear is low). Several features including observation of correlated density fluctuations indicate large amplitudes. The density fluctuations also provide information about the radial location of the modes. Excitation by fast ions is unlikely in H-1, but can not be ruled out, and excitation mechanisms involving fast electrons or steep thermal gradients are being considered. A significant fraction of clearly non-Alfvénic fluctuations indicate that other instabilities are present, such as interchange or ballooning modes, although the amplitude and mode number evidence available is not conclusive. These and other recent observations of Alfvén activity in a low temperature, basically thermal plasma suggests these phenomena may be more widely present-and thus fundamental to toroidal confinement--than previously thought.