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Author(s):
S.-I. Itoh, K. Itoh and M. Yagi
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Title:
Turbulence Trigger for Neoclassical Tearing Modes in Tokamaks
Date of publication:
Sep. 2003
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Key words:
neoclassical tearing mode, stochastic equation, turbulent noise, stochastic trigger, transition rate, probability density function, statistically-averaged amplitude, phase boundary, nonlinear MHD stabilization effect
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Abstract:
Stochastic trigger by microturbulence for neoclassical tearing mode (NTM) is studied. NTM induces topological change of magnetic structure and has subcritical nature. It is found that the NTM can be excited regardless of the presence of seed islands. Transition rate of, probability density function for and statistically-averaged amplitude of NTM are obtained. Boundary in the phase diagram is determined as the statistical long time average of the transition conditions. NTM can be excited by crossing this boundary even in the absence of other global instabilities. The boundary is expressed in terms of the poloidal beta value. Its dependence on the ratio between the ion banana width and radius of the rational surface, rho_b/r_s, is found to vary in the range of rho_b/r_s and (rho_b/r_s)^2 depending on the linear stability of the tearing mode.
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