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Author(s):
H. Naitou, T. Kuramoto, T. Kobayashi, M. Yagi, S. Tokuda and T. Matsumoto
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Title:
Stabilization of Kinetic Internal Kink Mode by Ion Diamagnetic Effects
Date of publication:
Apr. 2000
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Key words:
tokamak, internal kink mode, gyrokinetic theory, kinetic effect, diamagnetic effect, gyro-fluid model, gyro-reduced-MHD model
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Abstract:
Ion diamagnetic effects on the m=1(poloidal mode number) and n=1(toroidal mode number) kinetic internal kink mode are studied numerically by the three-field gyro-reduced-MHD code in the cylindrical coordinates, GRM3F-CY. In the derivation of the gyro-reduced-MHD model including the ion diamagnetic effects, finite gyroradius effects of ions are added to the gyrokintic Poisson equation (quasi-neutral condition) and the convection term of the conservation law of the ion density. It is found that the long wavelength approximation, k_perp rho_ti ll 1, where k_perp is the wavenumber perpendicular to the magnetic field and rho_ti is the thermal ion gyroradius, fails to reproduce the correct dispersion relation; the formulation valid even for k_perp rho_ti gg 1 is necessary. The results of numerical calculation coincide with the theory for |omega_ast e| + |omega_ast i| > 2 gamma_0, where the growth rate reduces as the density gradient increases. Here omega_ast e and omega_ast i are electron and ion diamagnetic angular frequencies estimated at the rational surface of q=1(q is a safety factor), respectively, and gamma_0 is the growth rate for the uniform density. Very weak instability, however, is observed for |omega_ast e| + |omega_ast i| > 2 gamma_0, where the theory predicts the complete stabilization. This residual instability appears since the region with the density gradient is limited in the radial direction and the stabilization by the outgoing drift-wave like mode becomes incomplete.
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