Collisional Losses of Fast Particles in Low-Current Axisymmetric Tokamaks

Three-dimensional Fokker-Planck simulation of collisional losses of mega-electron-volt fusion products in axisymmetric tokamaks with plasma currents I < 2 MA is carried out. The calculations take into account both loss due to radial diffusion and loss caused by pitch-angle scattering in the first-orbit loss region in velocity space. Collisional losses of deuterium-deuterium (D-D) fusion products in the energy range 0.5 ≤ ε/ε₀ ≤ 1 (where ε₀ is the birth energy) are found to be increased with plasma current and comparable to a first-orbit loss at I > 1.5 MA. The loss mechanism considered may be responsible for the observed experimentally delayed losses of D-D fusion products in the Tokamak Fusion Test Reactor (TFTR). The dependencies of collisional losses on plasma current, effective charge number of the plasma (Z_eff), and aspect ratio are investigated. The distributions of escaped ions over pitch angles, energies, and poloidal angles are evaluated. The fraction of collisionally lost fast fusion products is shown to scale like (ν_⊥/ν_s)^0.6 or (here ν_⊥ and ν_s are characteristic collision rates of pitch-angle scattering and slowing down, respectively). The approach used may be considered as an alternative to the approach based on Monte Carlo modeling of scattering and can serve as a validity check of the latter.