Diffusion of thermonuclear alpha particles under the influence of intense LH waves

Abstract

Recently, a new mechanism was proposed by Fisch and Rax which should lead to amplification of lower hybrid (LH) waves at the expense of alpha particles energy, improving thus the LH current drive efficiency. The present contribution investigates the possibility of achieving the conditions on which this mechanism — based on the spatial diffusion of fusion alpha particles — could become operative. The analysis performed shows that in reactor grade tokamaks, the penetration of LH waves into the plasma column is very poor due to the intense electron Landau damping and the complicated geometry of the equilibrium magnetic fields. Consequently, at acceptable LHW input power levels, the waves do not reach the regions where fusion alpha particle power could reasonably compensate the losses of LHW energy due to the damping by electrons. The model of Fisch and Rax is based on special features of the induced diffusion of alpha particles in the energy-configuration space. Using suitable Hamiltonian formalism, the LHW induced radial diffusion of alpha particles and the energy transport between LHW and alpha particles in the frame of generalized quasilinear lines of diffusion constraint is also investigated. A rather strong change ofk‖ of LHW along the ray trace can contribute to the change of energy transport between LHW and alpha particles as well.