Gyrokinetic analysis of tearing instabilities in a collisionless plasma

Sundaram, A. K.; Sen, A.
March 2011
Physics of Plasmas;Mar2011, Vol. 18 Issue 3, p032112
Academic Journal
Using a gyrokinetic description, an analytic investigation of tearing instabilities is carried out for a collisionless tokamak plasma, with particular emphasis on delineating the effects associated with Landau and ∇B resonances. The linear characteristics of Δ′-driven tearing modes are studied by including short wavelength variations across the confining magnetic field and long wavelength variations along the field. For the case when electrons are adiabatic and ions are fluidlike, the dispersion relation is solved analytically for mode widths lying between electron and ion excursion lengths. It is shown that electron Landau damping effect can significantly influence the tearing mode growth rate by making it proportional to (Δ′)1/2 in contrast to earlier kinetic results, which show a linear dependence on Δ′. The growth rate can further slow down when compressional mode coupling effects are taken into account. Likewise, analytic conditions for the growth of the gyrokinetic tearing mode in the presence of electron ∇B resonance effect are obtained for both the Δ′ driven global mode as well as the large Δ′ branch of this instability and expressions for the real frequency and growth rate of the modes are given. Our analytic results, besides providing physical insights into the influence of these 'resonance' effects, can also serve as useful benchmark signatures to look for in large scale numerical gyrokinetic simulations.


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