EXPERIMENTAL INVESTIGATION OF DIFFERENT CONFIGURATIONS IN A FLEXIBLE HELIAC

The effect of varying the magnetic field configuration by adding an L-SCRIPT equals 1 helical winding to the standard heliac has been studied experimentally. Equilibrium plasma configuration in the range 0. 7 less than equivalent to //t(0) less than equivalent to 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 //t equals 3/2 and the m equals 2, n equals 3 vacuum field harmonics inherent in the geometry, are identified with features observed in both the plasma pressure and the floating potential files.