MHD Stability Analysis of Axisymmetric Surface Current Model Tokamaks Close to the Spheromak Regime

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  • Honma Toshihisa
    Department of Electrical Engineering, Faculty of Engineering, Hokkaido University
  • Kito Masafumi
    Electrotechnical Laboratory
  • Kaji Ikuo
    Department of Electrical Engineering, Faculty of Engineering, Hokkaido University
  • Fukai Ichiro
    Department of Electrical Engineering, Faculty of Engineering, Hokkaido University

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  • MHD Stability Analysis of Axisymmetric

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Abstract

In the toroidal coordinates, a stability analysis is presented for very low-aspect-ratio tokamaks with circular cross section which is described by a surface current model (SCM) of axisymmetric equilibria. The energy principle determining the stability of plasma is treated without any expansion of aspect ratio. Numerical results show that, owing to the occurrence of the non-axisymmetric (n=1) unstable modes, there exists no MHD-stable ideal SCM spheromak characterized by zero external toroidal vacuum field. Instead, a stable spheromak-type plasma which comes to the ideal SCM spheromak is provided by the configuration with a very weak external toroidal field. Close to the spheromak regime (1.0<aspect ratio\lesssim1.1), the minimum safety factor and the critical β-values increase monotonically with aspect ratio decreasing from a large value, and curves of βp versus β in the marginal stability approach to an ideal SCM spheromak line βp=β.

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