Residual zonal flows in tokamaks in the presence of energetic ions

We investigate the residual zonal flow (Rosenbluth and Hinton 1998 Phys. Rev. Lett. 80 724) in the presence of fusion product alpha-particles in tokamak plasmas using the modern gyrokinetic approach in the electrostatic limit. The residual zonal flow is predicted to be enhanced considerably for ITER plasmas in the radial wave-number regime of k(r)rho(i,eff) similar to 10(-1), where rho(i,eff) is the Larmor radius of a thermal ion of background plasma consisting of Deuterium and Tritium. This is a consequence of the fact that larger Larmor radius (banana orbit width, rho(i,b)) of an energetic alpha-particle leads to an enhancement (a reduction) of the classical (neoclassical) polarizability at that wave-number regime compared to the case without alpha-particles. This enhancement is slightly more pronounced for the slowing-down distribution compared to the Maxwellian if T-e greater than or similar to 20 keV for E-alpha = 3.5 MeV. In addition, we find that the Rosenbluth-Hinton formula for the residual zonal flow level that has been derived for the Maxwellian equilibrium ion distribution remains valid in the long wavelength (k(r)rho(i,b) << 1) and high aspect ratio limit for any well-behaved ion distribution function which is isotropic in velocity space.