Observations of Alfvenic MHD Activity in the H-1 Heliac

: The H-1 heliac is a medium sized helical axis stellarator (R = 1 m, a = 0.15–0.20 m). Its flexible
heliac coil set permits access to a wide range of magnetic configurations, achieved by precise control of the helical winding current, providing rotational transform (ι) in the range 0.9 < ι < 1.5 for B0 < 1 T. Poloidal Mirnov arrays of 20 coils are located in two of the 3 toroidal periods. Observations of MHD activity during RF plasma production are presented for magnetic configurations of both positive (stellarator-like) and negative (tokamak-like) shear, including configurations where the sign of the shear reverses. Data mining techniques, SVD, wavelet and Fourier analysis are applied. Signals range from highly coherent, often multi- frequency, to near broad band (δf/f ∼ 0.02–0.5), and in many cases, observed frequencies exhibit Alfenic scaling with electron density variation. Clear structure is found near resonant transforms,
and evidence is given for a relationship of the observed frequencies to the lowest stationary point of the Alfv´en resonant frequency for low order resonances such as ι ∼ 4/3 and 5/4 . Density fluctuation profiles, measured by a fast sweeping interferometer (2 ms, 200 GHz) indicate that these modes are large amplitude (δne/ne < 0.05) and may extend beyond radii at which the Alfv´en resonance condition is met. Mode structure and possible localisation are investigated, and mode numbers up to m ∼ 6 are found, including values consistent with the above resonant transforms (m = 3, 4). Fast particle driving sources are under investigation, and include both fast electrons and minority heated H ions. The complex plasma shape creates problems in analysis, possibly broadening the poloidal mode spectrum beyond expected toroidal coupling effects, and the variable plasma – probe distance makes mode localisation difficult to interpret. This work demonstrates that the well diagnosed and finely controlled H-1 plasmas, when coupled with selective ion and electron rf heating, provides a productive environment in which to develop integrated models of Alfv´en eigenmodes, to contribute to assessment of Alfenic activity in modern-day and planned extremely energetic fusion plasmas.