Journal article

Role of nonadiabatic untrapped electrons in global electrostatic ion temperature gradient driven modes in a tokamak

In this work, role of nonadiabatic untrapped electrons in the context of a global ion temperature gradient driven mode has been investigated. In past studies, untrapped electrons have been assumed to be able to respond "instantaneously" to a disturbance. It is proposed that such adiabatic electron models should be reexamined for two important reasons: (i) It is known that global modes with n in the range of 3 <= n <= 15 (n is the toroidal mode number) have eigenmode widths spanning several mode-rational surfaces. It is being argued that close to these mode-rational surfaces, adiabatic electron models fail and a consistent treatment of nonadiabatic electrons is crucial for global modes. (ii) Electromagnetic effects depend on passing nonadiabatic electron dynamics. A minimal nontrivial model for the benchmarking of global linear and nonlinear gyrokinetic codes in the future becomes necessary, which can treat both passing ions and electrons on the same physics footing. As a first step, a study of the effect of nonadiabatic passing electrons in global electrostatic ion temperature gradients is presented. Interesting results include a demonstration of multiscale structure, downshift in critical eta(i) with increasing eta(e), and a reduction in mixing-length based transport. (C) 2008 American Institute of Physics.

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