Relevance of the parallel nonlinearity in gyrokinetic simulations of tokamak plasmas

The influence of the parallel nonlinearity on transport in gyrokinetic simulations is assessed for values of ρ* which are typical of current experiments. Here, ρ*=ρs/a is the ratio of gyroradius, ρs, to plasma minor radius, a. The conclusion, derived from simulations with both GYRO [J. Candy and R. E. Waltz, J. Comput. Phys., 186, 585 (2003)] and GEM [Y. Chen and S. E. Parker J. Comput. Phys., 189, 463 (2003)] is that no measurable effect of the parallel nonlinearity is apparent for ρ*<0.012. This result is consistent with scaling arguments, which suggest that the parallel nonlinearity should be O(ρ*) smaller than the ExB nonlinearity. Indeed, for the plasma parameters under consideration, the magnitude of the parallel nonlinearity is a factor of 8ρ* smaller (for 0.000 75<ρ*<0.012) than the other retained terms in the nonlinear gyrokinetic equation