A. Takayama

H. Sugama

M. Wakatani

National Institute for Fusion Science (NIFS-Repository)

§22. Nonlinear Interchange Mode Including Reynolds Stress Takayama, A., Wakatani, M. (Kyoto Univ. Plasma Physics Lab.) Sugama, H. L to H transition1) is the recent topic in the magnetic fusion research, since confine-ment improvement of about a factor of two is crucial to realize ignition. It is also recognized that ELM (Edge Localized Mode) is essential to control H-mode plasmas.2) Therefore, it is important to understand mechanism of the 1-H transition and ELM. We have studied nonlinear evolution of the two dimensional interchange mode in the edge plasma with the poloidal shear flow .3) We ob-tain a sawtooth like relaxation oscillation in the nonlinear stage for Rayleigh number (Ra) of the order of 104 • This oscillation is caused by the change of poloidal velocity shear due to Reynolds stress. When the poloidal veloc-ity shear becomes weak, the fluctuation level increases suddenly and the Reynolds stress enhances the velocity shear to stabilize the nonlinear interchange. In this case the shear flow does not overcome the vortex flow, and the decrease of fluctuation level weakens the Reynolds stress gradually. Thus the sawtooth like relaxation oscillation of fluctuation energy appears. Here the Rayleigh number is increased up to the order of 105 . Then a new oscillatory behavior of nonlinear interchange mode is ob-tained in the presence of appropriate velocity shear as shown in Fig. 1. Here Prandtl num-ber is Pr = 1.0. When the shear flow produced by Reynolds stress increases sharply at t ::: 60 (see the m = 0 mode in Fig. 1 ), the fluctuation level dominated by the m = 1 mode decreases accordingly. The relaxation oscillation contin-ues as long as Ra and P" are fixed. This be-havior of the nonlinear interchange mode gives clear effects on the radial transport measured with Nusselt number as shown in Fig. 2. e; 1 o-3 Q) c: w -~ 10""" Qj c: s::: 10-5 10-6 100 300 Time Fig. 1. Time evolution of kinetic energy of dominant Fourier modes ( m = 0 rv 3) for Ra = 5.03 x 105 and P" = 1.0. To-tal kinetic energy is also shown. Time is normalized by the linear growth rate of interchange mode. Nu Evolution (Ra•5. 03Xl0s) - Pr-1.0 15 - Pr-1.5 ~ 10 5 TIME Fig. 2. Time evolution of N usselt number for the case shown in Fig. 1, and P" = 1.5 case. References 1) Wagner, F. et al., Phys. Rev. Lett. 49 (1982) 1408. 2) Burrell, K. H. et al., Plasma Phys. Con-trolled Fusion 31 (1989) 1649. 3) Takayama, A., Wakatani, M., and Sugan1a, H., J. Phys. Soc. Jpn 64 {1995) 791. 151 

Nonlinear Interchange Mode Including Reynolds Stress

Abstract

Similar works

Full text

Available Versions

National Institute for Fusion Science (NIFS-Repository)