(TH/P8-4) Plasma Shaping Effects on Temperature Gradient-Driven Instabilities and Geodesic Acoustic Modes

Zhe Gao1), Lili Peng1), Ping Wang1), Jiaqi Dong2)3), H. Sanuki4)
 
1) Department of Engineering Physics, Tsinghua University, Beijing 100084, China
2) Southwestern Institute of Physics, Chengdu 610041, China
3) Institute for Fusion Theory and Simulation, Zhejiang University, Hangzhou, China
4) National Institute for Fusion Science, Toki, Gifu 509-5292, Japan

Abstract.  A gyrokinetic theory is established in noncircular toroidal plasmas by employing the local MHD equilibrium model. The temperature gradient driven instability and the GAM are investigated as two limits of this problem. The GAM is close related to the poloidal dependence, or its poloidal average, of the curvature drift and the transit motion, while the temperature gradient driven instability is mainly decided by the local behavior around θ≈ 0. In specific, we focus on the effect of the elongation κ , including its radial derivative sκ = (r/κ)(∂κ/∂r) , in the large aspect ratio limit.An analytical formula of the dependence of the GAM frequency on the elongation is given. It is found that the GAM frequency sharply decreases with an increasing elongation by dependence of [(2 - sκ)/(κ2 + 1)]1/2, which comes from the modification of classical ion polarization balanced by that of curvature drift polarization. However, for temperature gradient driven instability, as κ increases, the kθ-spectrum of growth rate is greatly shifted towards larger values of kθ, while the maximum of the growth rate only slightly decreases at sκ = 0 . However, the radial deviation of elongation sκ can significantly influence the stability property of temperature gradient instability by modifying the parallel wave number. Dependence of the critical gradient on the elongation deformation is numerical studied and a semi-analytical formula is given as (R0/LTec)/(R0/LTec)κ=1 = (1 + 0.36sκ)[1 + 0.11(κ - 1)].

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