[en] Detailed knowledge of the experimental plasma equilibrium is becoming as important for stellarators as it is for tokamaks, but reconstruction of magnetic flux surfaces from magnetic measurements is more difficult in the three-dimensional geometry of stellarators than in axisymmetric configurations. We report the first reconstructions of experimental stellarator equilibria with the V3FIT[1] code. We also introduce the signal effectiveness, and describe initial studies of density-driven disruption in current-carrying stellarator plasmas. At low vacuum transform, disruptions can be triggered at a plasma density comparable to the Greenwald limit, but disruption signatures disappear as the vacuum rotational transform is raised above 0.1 (copyright 2010 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

[en] The Compact Toroidal Hybrid (CTH) is a low-aspect ratio torsatron with adjustable vacuum rotational transform and shear that operates with ohmic plasma current to investigate the equilibrium and stability of current-carrying helical plasmas. Equilibrium reconstruction of experimental stellarator plasmas for optimization of stability and transport has become more important in recent years because of the finite levels of pressure-driven currents now achieved in high β helical plasmas, and also because the NCSX and QPS stellarators will make use of bootstrap and/or driven plasma current to generate a non-negligible fraction of the plasma rotational transform. A new computational scheme to reconstruct equilibrium flux surfaces in stellarators (V3FIT) is under development, and will be tested in CTH with external and internal magnetic diagnostics in strongly current-driven plasmas. Moreover, issues of current-driven MHD instability and the potential for disruptions in stellarators are also of paramount interest in such hybrid configurations. In this regard, CTH will investigate the onset of resistive and ideal current-driven instabilities in magnetic equilibria in which key parameters (global shear, vertical elongation, vacuum rotational transform, fraction of current-driven rotational transform) are varied in order to assess the potential for passive disruption control in current-carrying stellarators. The CTH device is a five-field period, L = 2 torsatron (R₀ = 0.75 m; a_Vessel = 0.3 m) with toroidal field coils for control of the vacuum rotational transform, and three independent sets of poloidal field coils for equilibrium control, and an ohmic transformer to produce an expected maximum current of 50 kA. In addition, a set of 15 error-correction coils has been installed to minimize static islands in vacuum and plasma configurations, and may also be used to influence rotating islands. The minimum plasma aspect ratio in vacuum magnetic configurations is A_p = 4. The maximum magnetic field on axis is B_o ≤ 0.7 T with zero supplementary toroidal field (ι(a) ≅018), though most initial work is carried out at B_o = 0.33 T to make use of 2nd harmonic ECH at 18 GHz. ECH plasmas in CTH were first attained in Feb. 2005. Current efforts center on field-mapping and precision alignment of the multiple coil sets to achieve low aspect ratio vacuum configurations with minimal islands over a range of vacuum rotational transforms. Results of the field-mapping and the use of the flexible error-correction coil set will be presented. (author)

No abstract available

[en] Axisymmetric equilibrium reconstruction using magnetohydrodynamic equilibrium solutions to the Grad–Shafranov equation has long been an important tool for interpreting tokamak experiments. This paper describes recent results in non-axisymmetric (three-dimensional) equilibrium reconstruction of nominally axisymmetric plasmas (tokamaks and reversed field pinches (RFPs)), and fully non-axisymmetric plasmas (stellarators). Results from applying the V3FIT code to CTH and HSX stellarator plasmas, RFX-mod RFP plasmas and the DIII-D tokamak are presented. (paper)