Nonlinear Gyrokinetic Theory of Toroidal Momentum Pinch
Authors: T.S. Hahm, P.H. Diamond, O.D. Gurcan, and G. Rewoldt
Abstract:
The turbulent convective flux of the toroidal angular momentum density is derived using the nonlinear toroidal gyrokinetic equation which conserves phase space density and energy [T.S. Hahm, Phys Fluids, 31, 2670 (1998)]. We identify a novel pinch mechanism which originates from the symmetry breaking due to the magnetic field curvature. A net parallel momentum transfer from the waves to the ion guiding centers is possible when the fluctuation intensity varies on the flux surface, resulting in imperfect cancellation of the curvature drift contribution to the parallel acceleration. This mechnaism is inherently a toroidal effect, and complements the κ|| symmetry breaking mechanism due to the mean E x Β shear [O. Gurcan et al., in press Phys. Plasmas (2007)] which exists in a similar geometry. In the absence of ion thermal effects, this pinch velocity of the angular momentum density can also be understood as a manifestation of a tendency to homogenize the profile of "magnetically weighted angular momentum density," nmiR2ω||/Β2. This part of the pinch flux is mode-independent (whether it's TEM driven or ITG driven), and radially inward for fluctuations peaked at the low-Β-field side, with a pinch velocity typically, VTEP(over)Ang ~ –2XΦ/R0. Ion thermal effects introduce an additional radial pinch flux from the coupling with the curvature and grad-Β drifts. This curvature driven thermal pinch can be inward or outward, depending on the mode-propagation direction. Explicit formulas in general toroidal geometry are presented.
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Physics of Plasmas 14, 072302 (June 2007) 22 pp
doi: 10.1063/1.2743642
© (2007) American Institute of Physics.
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