Experiments with the first complete high-6 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 Tl). Toroidal equilibrium was achieved by superposing helical fi = 1 and & = 2 (partly also £ = 0) fields on the slender (A = R T /r 0 » 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.
The spectral distribution of light scattered by a plasma has been treated theoretically by a number of authors. 1 It turns out that the type of the spectrum depends essentially on the value of a parameter a = (\k\D ;AuD sin^6 eHere k is the scattering vector, X 0 is the wavelength of the incident light, D e the electron Debye length, and 6 the scattering angle. When a > 1, collective effects play an important part. Consequently, the spectrum shows a central line at X 0 and two narrow lines located symmetrically about X 0 at a distance given roughly by the electron-plasma frequency. The half-width of the central line is determined essentially by the ion temperature, while its shape depends on the ratio of the electron to the ion temperature. Therefore, a measurement of the shape and width of this central "ion line" opens, on the basis of certain assumptions, the possibility for a very direct determination of the ion temperature. This technique is particularly well suited for the investigation of hot deuterium plasmas, for which conventional spectroscopic methods cannot be used directly. For such hot plasmas at electron densities of the order of 10 16 cm"" 3 , the condition a > 1 can be satisfied only by using appropriate small values of the scattering angle 6. This procedure is generally referred to as the method of forward scattering. 2 " 6 The existence of the central ion line in the scattering spectrum has been proved already by a number of authors. 2 > 3 > 7 > 8
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