Transport in gyrokinetic tokamaks

A comprehensive study of transport in full-volume gyrokinetic (GK) simulations of ion temperature gradient driven turbulence in core tokamak plasmas is presented. Though this ''gyrokinetic tokamak'' is much simpler than experimental tokamaks, such simplicity is an asset, because a dependable nonlinear transport theory for such systems should be more attainable. Toward this end, two related lines of inquiry are pursued. (1) The scalings of GK tokamaks with respect to important system parameters are studied. In contrast to real machines, the scalings of larger GK systems (a/ρs approx-gt 64) with minor radius, with current, and with a/ρs are roughly consistent with the approximate theoretical expectations for electrostatic turbulent transport which exist as yet. Smaller systems manifest quite different scalings. (2) With the goal of developing a first-principles theory of GK transport, the GK data are used to infer the underlying transport physics. The data indicate that, of the many modes k present in the simulation, only a modest number (Nk∼10) of k dominate the transport, and for each, only a handful (Np∼5) of couplings to other modes p appear to be significant, implying that the essential transport physics may be described by a far simpler system than would have been expected on the basis of earlier nonlinear theory alone. Part of this analysis is the inference of the coupling coefficients Mkpq governing the nonlinear mode interactions, whose measurement from tokamak simulation data are presented here for the first time