(TH/P8-37) Limitations, Insights and Improvements to Gyrokinetics

P.J. Catto1), F.I. Parra1), G. Kagan1), A.N. Simakov2)
1) MIT Plasma Science and Fusion Center, Cambridge, MA, United States of America
2) Los Alamos National Laboratory, Los Alamos NM, United States of America

Abstract.  We focus on (i) the limitations of gyrokinetic quasineutrality to evaluate the axisymmetric radial electric field; (ii) a gyrokinetic entropy production restriction on the ITER ion temperature pedestal; and (iii) a hybrid gyrokinetic-fluid treatment valid on slowly evolving time scales. (i) Standard gyrokinetics incorrectly determines the axisymmetric, long wavelength electrostatic potential to leading order in gyroradius over major radius as demonstrated by considering a steady-state theta pinch with a distribution function correct to second order. We argue gyrokinetic quasineutrality often improperly determines the potential in the long wavelength, axisymmetric limit for a distribution function evaluated to within numerical resolution [1]. (ii) Using canonical angular momentum as the radial variable allows strong gradients to be treated gyrokinetically. Entropy production then requires a physical lowest order banana regime ion distribution function to be nearly an isothermal Maxwellian [2] with the ion temperature scale much greater than the poloidal ion gyroradius. Thus, the background ion temperature profile in ITER cannot have a pedestal like that of the density. Weak ion temperature variation with subsonic pedestal flow requires electrostatically restrained ions and magnetically confined electrons. (iii) Simulating tokamaks on transport time scales requires evolving drift turbulence with axisymmetric neoclassical and zonal flow radial electric field effects retained. Coupled flux tube gyrokinetic simulations evolving density and temperature are becoming available [3]. However, full electric field effects are more difficult to keep since they require evaluating the ion distribution function to higher order than standard gyrokinetics - well beyond forseeable numerical resolution1. An electrostatic hybrid gyrokinetic-fluid treatment using moments of the full Fokker-Planck equation removes the need to go to higher order. This hybrid description evolves potential as well as density, temperatures and flows, and models all electrostatic turbulence effects with wavelengths much longer than an electron gyroradius.

Supported by US DoE at MIT PSFC, CMPD at U. Md & LANL.

[1] M. Barnes and W. Dorland, Intl. Sherwood Fusion Theory Conf. 2D2 (2007). [2] P. J. Catto and R. D. Hazeltine, Phys. Plasmas 13, 122508 (2006). [3] M. Barnes and W. Dorland, Bull. Amer. Phys. Soc. 52, 90 (2007).

Full paper available (PDF)