2-D FLUID CALCULATIONS OF ANOMALOUS TRAPPED ION-TRANSPORT IN TOKAMAKS

The collisional, nonlinear trapped-fluid equations of Kadomtsev and Pogutse (K.P.) are used as a description of the dissipative trapped-ion modes. The equations are solved numerically in two spatial dimensions as an initial-value problem. Numerical results are given for the resulting anomalous diffusion coefficient D at late times, when the dissipative trapped-ion instability saturates. Examples of the time development D(t) are given. The numerical values of D (at late times) are generally larger than according to the K.P. formula, and the scaling of D with the equilibrium parameters resembles Bohm scaling. The dependence of the results on the numerical grid and on the initial conditions was also studied. Several analytical results are presented, some of which have been successfully used for testing the numerical code.