Nonlinear theory of collisionless trapped ion modes

A simplified two-field nonlinear model for collisionless trapped-ion-mode turbulence has been derived from nonlinear bounce-averaged drift kinetic equations. The renormalized thermal diffusivity obtained from this analysis exhibits a Bohm-like scaling. A new nonlinearity associated with the neoclassical polarization density is found to introduce an isotope-dependent modification to this Bohm-like diffusivity. The asymptotic balance between the equilibrium variation and the finite banana width-induced reduction of the fluctuation potential leads to the result that the radial correlation length decreases with increasing plasma current. Other important conclusions from the present analysis include the predictions that (i) the relative density fluctuation level delta n/n(0), is lower than the conventional mixing length estimate, Delta r/L(n),; (ii) the ion temperature fluctuation level delta T-i/T-i significantly exceeds the density fluctuation level delta n/n(0); and (iii) the parallel ion velocity fluctuation level delta upsilon(il)/upsilon(Ti) is expected to be negligible. (C) 1996 American Institute of Physics.