Turbulent Equipartition Theory of Toroidal Momentum Pinch Page: 4 of 26
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Understanding momentum transport, which influences plasma rotation, is very important
since it can play a crucial role in reducing turbulence and transport as well as in stabilizing
MHD instabilities including the Resistive Wall Mode (RWM). In most cases, the toroidal
momentum transport from experiments is observed to be anomalous, i.e., higher than neo-
classical theory predictions, and therefore, believed to be caused by low frequency, ion gyro-
radius scale, electrostatic drift wave turbulence, including ion temperature gradient (ITG)
mode turbulence and trapped electron mode (TEM) turbulence. For instance, the toroidal
momentum diffusivity x$, was comparable to the ion thermal diffusivity xi,1 in TFTR ex-
periments, in rough agreement with theoretical predictions based on ITG turbulence.2
While the toroidal momentum transport is often described by a diffusion coefficient xqeff
alone, there's accumulating evidence that a variety of rotation phenomena of great poten-
tial importance cannot be properly characterized by the diffusion coefficient only. This
includes the observation of spontaneous toroidal rotation of plasmas in the absence of ap-
parent external torque input-". Some prefer to call it an "intrinsic rotation."8 In many
cases, rotation profiles are peaked near the axis, even for off-axis deposition, zero torque, or
no neutral beam injection (NBI), suggesting the existence of a nondiffusive inward flux of
toroidal angular momentum.12 In addition, recent perturbation experiments on JT60-U" 4
and NSTX'5 neutral beam heated plasmas showed the need for an "inward pinch term" of
angular momentum to match the measured centrally peaked rotation profiles.
Theoretically, one can write an expression for the radial flux of the toroidal momentum
II= -Xq U + VpinnU + S.
Here, the non-diffusive component of the turbulence driven radial transport of toroidal
momentum'6 includes not only the turbulent convective (TurCo) pinch (Vinch), but also the
residual stress (S), which does not depend on the flow explicitly. It should be emphasized
that a non-diffusive flux of momentum can be obtained from various physics mechanisms.17-2
Depending on plasma parameters and configurations, a specific mechanism can be more
relevant than others, and sometimes a combination of two or more mechanisms is necessary
to reproduce basic features of experiments. For instance, a commonality of spontaneous
rotation of plasmas26 in NBI-free H-mode plasmas is the empirical "Rice" scaling5 which
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Hahm, T. S.; Diamond, P. H.; Gurcan, O. D. & Rewaldt, G. Turbulent Equipartition Theory of Toroidal Momentum Pinch, article, January 31, 2008; Princeton, New Jersey. (https://digital.library.unt.edu/ark:/67531/metadc929711/m1/4/: accessed April 24, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.