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 rho(*) which are typical of current experiments. Here, rho(*)=rho(s)/a is the ratio of gyroradius, rho(s), 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 rho(*)< 0.012. This result is consistent with scaling arguments, which suggest that the parallel nonlinearity should be O(rho(*)) smaller than the ExB nonlinearity. Indeed, for the plasma parameters under consideration, the magnitude of the parallel nonlinearity is a factor of 8 rho(*) smaller (for 0.000 75 <rho(*)< 0.012) than the other retained terms in the nonlinear gyrokinetic equation. (c) 2006 American Institute of Physics.