Experimental investigation of different configurations in a flexible heliac

The effect of varying the magnetic field configuration by adding an ℓ = 1 helical winding to the standard heliac has been studied experimentally. Equilibrium plasma configurations in the range 0.7 ≤ t(0) ≤ 1.86 have been obtained. Analyses of the plasma pressure profiles measured by Langmuir probes in this range show good agreement between the plasma isobars and the computed vacuum magnetic surfaces; for configurations with a rotational transform t(0) close to unity it is necessary to take known error fields into account. When low-order rational surfaces are present, a deterioration of the plasma confinement is clearly observed. Magnetic islands, resulting from the resonance between the low-order rational surface ι = 3/2 and the m = 2, n = 3 vacuum field harmonics inherent in the geometry, are identified with features observed in both the plasma pressure and the floating potential profiles.