Fluctuations and stability of plasmas in the H-1NF heliac

The H-1NF heliac is a medium-sized heliac stellarator experiment with major radius R = 1m, and average plasma minor radius a = 0.15-0.2m. Its 'flexible-heliac' coil set permits precise variation in the value and shape of the rotational transform (τ) profile, with regions of both positive and negative shear. Operation at low fields (B<0.2T) with argon plasmas heated by helicon waves produces plasmas that have large ion Larmor radii (ρ _i/a ∼0.4) and show confinement transitions at low power like those in the edge of large devices, yielding fundamental measurements concerning electric fields and zonal flows. At a higher field (0.5 T), precise rotational transform scans with H - He plasmas heated by ICRF show resonant equilibrium and stability phenomena which depend on the value of the rotational transform at the radius of zero shear.