Bifurcated Helical Core Equilibrium States in Tokamaks

Tokamaks with weak to moderate reversed central shear in which the minimum rotational transform $q_{min}$ is in the neighbourhood of unity can trigger bifurcated MagnetoHydrodynamic (MHD) equilibrium states, one of which is similar to a saturated ideal internak kink mode. Peaked prescribed pressure profiles reproduce the ``snake'' structures observed in many tokamaks which has led to a novel explanation of the snake as a bifurcated equilibrium state.Snake equilibrium structures are computed in simulations of the TCV, DIII-D and MAST tokamaks. The internal helical deformations only weakly modulate the plasma-vacuum interface which is more sensitive to ripple and resonant magnetic perturbations. On the other hand, the external perturbations do not alter the helical core deformation in a significant manner. The confinement of fast particles in MAST simulations deteriorate with the amplitude of the helical core distortion. These three-dimensional bifurcated solutions constitute a paradigm shift that motivates the applications of tools developed for stellarator research in tokamak physics investigations.

Published in:
Proceedings of the 24th IAEA Fusion Energy Conference
Presented at:
24th FEC - 24th IAEA Fusion Energy Conference, San Diego CA (USA), 8-13 October 2012

 Record created 2012-11-15, last modified 2018-01-28

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