Report of the Study Group GK2 on Momentum Transport in Gyrokinetics

Authors: J.A. Krommes and G.W. Hammett

This is the report of a study group that was asked to review recent work on the issue of turbulent momentum transport in gyrokinetic theory, forms of gyrokinetics that lead to a momentum conservation law, the order of accuracy needed to adequately predict momentum transport, and related topics. Very subtle issues are involved. Agreement is found with recent work that has argued that a very high order of accuracy (beyond what has been calculated or implemented) would be required in the low-flow gyro–Bohm ordering regime if one were to try a standard gyrokinetic approach using the gyrokinetic Poisson equation to determine the long-wavelength potential on the transport time scale. That work presents an alternate approach that directly employs a momentum conservation law in order to reduce the order of accuracy required. Another method is to couple a gyrokinetic code on the short turbulence time scale with a transport code for the longer transport time scale. Other regimes are also of interest and lead to somewhat simpler, though still challenging, equations. While progress has been made and there are some definite conclusions that can be stated at the present time, there are still important questions that are unsolved and momentum transport remains an area of ongoing research. The report includes extensive tutorial material that provides some of the background needed for an understanding of recent papers on gyrokinetics.

Published as a PPPL Report (October 2013)


Download PPPL-4945 (pdf 3.3 MB 147 pp)