Reactor aspects of counterstreaming-ion tokamak plasmas

Toroidal DT plasmas in which the D and T ions make up two distinct, quasi-thermal velocity distributions, oppositely displaced in velocity along the magnetic axis, can be set up by introducing all ions by tangential injection of neutral beams, and by removing ions from the plasma shortly after they have decelerated to an energy <̃2T_e by Coulomb drag on the plasma electrons. A simple physical model for this counterstreaming-ion operation allows one to deduce the ion velocity distributions and required energy and particle confinement times that are in good agreement with the results of previous Fokker-Planck calculations. These distributions are used to calculate fusion reactivities, from which the variation of fusion power gain and power density with injection energy and T_e are determined. The practical problems of implementing counterstreaming operation in a tokamak include charge-exchange losses, impurities, and the need to minimize plasma re-cycling.

Several properties of the counterstreaming system that make it advantageous for near-term reactor applications are (1) optimal performance at relatively low injection energy (40–60 keV), because of head-on nuclear collisions, (2) small electron energy confinement time for Q ∼ 1 (e. g. n_eτ_E ≈ 2 × 10¹² cm⁻³·s at T_e ≈ 3.5 keV), (3) fuelling solely by the injected beams, and (4) possible maintenance of the plasma current by the beams. For realistic tokamak parameters, neutron wall loading is limited to 0.5 MW/m² or less. Plasma parameters appropriate to radiation test reactors and fusion-fission hybrid reactors are discussed.