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The IFS/PPPL Transport Models

implemented by
Aaron J. Redd
Lehigh University

The 1995 IFS/PPPL model[] is an anomalous thermal transport model based directly upon the predictions of nonlinear gyrofluid (GF) turbulence simulations, in flux-tube geometry[]. The turbulent thermal diffusivities from this nonlinear GF code were compared with the predictions of a gyrokinetic (GK) linear stability code[]. It was found that the ratio of the nonlinear tex2html_wrap_inline32 to a mixing-length estimate tex2html_wrap_inline34 from the linear code (tex2html_wrap_inline36) was a slowly varying function. This observation encouraged Kotschenreuther, et al, to map out the plasma parameter space utilizing the linear GK code. Each linear result could then be multiplied by the nearly-constant ratio tex2html_wrap_inline38, giving a value for tex2html_wrap_inline40 that would be a good estimate of the nonlinear code result.

This technique represents a significant savings in human effort and computational time, as the linear GK code can be run much faster than the nonlinear GF code.

Once this method was established, ``many hundreds'' of linear GK runs were conducted, mapping out the parametric dependence of the drift instability. Then, interpolation formulas were devised, giving tex2html_wrap_inline42 and tex2html_wrap_inline44 as functions of various plasma parameters. The resulting turbulence model was published in 1995 as the IFS/PPPL model.

It is important to note, however, that the nonlinear GF runs that produced the 1995 IFS/PPPL model utilized certain simplifying assumptions:

There is evidence suggesting that both the MHD equilibrium (analytic vs numerically calculated) and the exact gyrofluid closure have significant effects on the tex2html_wrap_inline46-mode linear threshold; these issues are being addressed by the Cyclone Project (1997-8). Recent refinements of the IFS/PPPL model take account of low or reversed magnetic shear and more realistic MHD equilibria.

This coding of the 1995 IFS/PPPL model also includes a plasma elongation stabilization factor, as suggested at the 1996 Varenna Meeting[], a preliminary model of rotational stabilization[], and several corrections and refinements to the model that was reported in the 1995 paper. These modifications were suggested by Bill Dorland[].

Currently, the GF turbulence code, called Gryffin, is maintained and operated by Mike Beer (PPPL). The linear GK stability code is maintained by Mike Kotschenreuther (IFS), and has been benchmarked against an eigenvalue kinetic stability code (the FULL code[, , ], written and maintained by Gregory Rewoldt of PPPL) for a series of simple cases[].

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Wed Jul 8 11:54:05 EDT 1998