High-beta stellarator experiments on ISAR T1

Experiments with the first complete high-β stellarator torus with helical magnetic axis are reported. The Garching 2.6-MJ capacitor bank was used to feed a toroidal theta coil with a major.diameter of 2.7 m (ISAR T1), Toroidal equilibrium was achieved by superposing helical ℓ = 1 and ℓ = 2 (partly also ℓ = 0) fields on the slender (A = RT/r0 ≈ 150) toroidal pinch plasma such that the plasma surface facing the torus centre was more corrugated than the outer side (M-and-S effect). The toroidal equilibrium condition and the corresponding plasma distortions were consistent with sharp-boundary-model predictions. Effects in connection with initial dynamics, toroidal plasma currents and transverse magnetic fields could be explained by simple models. In agreement with sharp-boundary theory, short wavelength m = 1 and m = 2 modes were found to be stable and long wavelength m = 1 modes were unstable, limiting the plasma life-time by wall contact. Long-wavelength m ≥ 2 instabilities were not observed in contrast to sharp-boundary theory, i.e. this model is much too pessimistic for m ≥ 2 modes, even if the finite gyroradius is included. No significant difference in the stability behaviour was found, compared with previous linear and toroidal sector experiments.