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Configurational Effects on Stability and Confinement in the H-1NF Heliac

H-1 is a helical axis stellarator of major radius R=1m, and average minor radius ~ 0.150.2 m. Its flexible heliac coil set permits access to a wide range of magnetic configurations. RFgenerated plasma shows a very complex dependence on configuration of both the electron density and the nature of fluctuations. Magnetic fluctuations range from highly coherent, often multi-frequency in sequence or simultaneously, to approaching broad-band (df/f ~ 0.02-0.5), in the range 1-200 kHz. Application of datamining to a wide range of configurations classifies these fluctuations and extracts poloidal and toroidal mode numbers, revealing that a significant class of fluctuations exhibit scaling which is Alfvenic with electron density (within a constant factor) and shear Alfvenic in rotational transform. An array of optical and interferometric diagnostics is combined with the magnetic probe arrays to provide information on internal structure of the MHD modes, and associated 3D effects. The configurational dependence is closely related to the presence of low order rational surfaces. The plasma density falls to very low values at these rationals prompting the question: how much is this due to the effect of magnetic islands? Magnetic islands are expected to be relatively less important in a moderate shear device such as the heliac, than in conventional stellarators. Results from a uniquely accurate magnetic field mapping system, combined with a comprehensive model of the vacuum magnetic field in H-1 show that magnetic islands do not dominate the configuration for the low order resonances in the configuration scan. Studies of plasma in configurations with magnetic islands show some surprising albeit small improvements in confinement and rapid changes in radial electric field. The results demonstrate how H-1’s unique combination of flexibility and variety of advanced diagnostics can contribute to two key issues for future fusion devices: Alfvenic activity, and the effects of magnetic islands.