High-Beta (ℓ = 1)-Stellarator Experiments

Recently, toroidal high-beta equilibria with rotational transform, but essentially without net toroidal current (high-beta stellarator) were proposed and subsequently refined by several authors. The experimental investigations reported here were concentrated on the (ℓ = 1)-stellarator because of its theoretically favourable stability properties. The experiments were done in straight as well as in toroidal geometry. In all cases the plasma was heated by shock waves (theta-pinch). In straight helical geometry a 5.4 m long coil in essentially four different versions was used. In all cases the typical plasma helix was formed after the fast compression. Oscillations correlated with the helix formation could be explained by MHD-theory. At high temperatures (Te + Ti ≈ 0.5 . . . 2.1 keV) ( m = 1, k ≈ 0) -instabilities are observed. A reduction in β led to lower growth. rates in agreement with the theory. From internal magnetic probe measurements magnetic field profiles and beta-values are derived. The field profiles indicated that longitudinal or helical currents also flow in the thin plasma (''halo'') outside the dense plasma column. Such currents may inftuence the plasma stability. In a small toroidal pre-experiment with low aspect ratio A = R/rp (bank energy 10 kj, major radius R = 26 cm, plasma radius rp ≈ 1 cm), effects which might not be seen in straight geometry were sought. The helical field was generated by a shaped coil. As in the linear experiment, a helix was formed and oscillations of the helix radius were observed. A net toroidal current, induced by the (ℓ = 1)-field, could be explained by a simple model. The shape of the magnetic field was varied, and additional toroidal plasma currents were induced by external conductors. The toroidal drift motoin of the plasma helix was a function of the various parameters. With a toroidal current well above the Kruskal limit, a highly unstable, screw-pinch-like equilibrium was found. These experiments will be continued with our device ISAR-I (bank energy 2.6 MJ) with a totus of 135 cm major radius and relatively large aspect ratio. (author)