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Author(s):
N. Nakajima, K. Ichiguchi, K. Watanabe, H. Sugama, M. Okamoto, M. Wakatani, Y. Nakamura and C. Z. Cheng
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Title:
Neoclassical Current and Related MHD Stability, Gap Modes, and Radial Electric Field Effects in Heliotron and Torsatron Plasmas
Date of publication:
Sep. 1992
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Key words:
Neoclassical current, radial electric field, net toroidal current, Mercier criterion, TAE, HAE, poloidal shear flow, resistive and ideal interchangte modes
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Abstract:
By developing the neoclassical theory for parallel flow it is found out that if nu_e^* neq nu_i^* a parallel current directly generated by the radial electric field, which does not exist in axisymmetric systems, exists in the non-axisymmetric systems. This newly found current has a possibility to reduce the bootstrap current under a suitable condition even in the opposite direction in Heliotron/Torsatron. The differences between the rotations of bulk ions and impurities are discussed in the case where impurities are in the Pfirsh-Schluter collisionality regime. In connection with the bootstrap current the effects of the net toroidal current on the ideal interchange instability are numerically studied taking the Large Helical Device (LHD) as an example. The direction of the current is crucial to the Mercier criterion. Helicity-induced shear Alfven eigenmodes (HAE) attributed to the helicity of helical coils are considered for the first time in Heliotron/Torsatron and compared with TAE modes. Spectral gaps and eigenmodes significantly depend on the finite-beta effects. Radial electric field producing poloidal shear flow also plays a role for the edge anomalous transport in Heliotron/Torsatron. The electrostatic resistive interchange mode which is considered origin of the anomalous transport can be stabilized linearly by the poloidal shear flow. However, its effects on the turbulent state practically disappears by the nonlinear simulation for the resistive drift wave and interchange turbulence. It is pointed out that the poloidal shear flow destabilizes the electromagnetic ideal interchange mode.
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