Neutral-beam-injected tokamak fusion reactors: a review

Abstract

The ion velocity distributions of tokamak plasmas sustained by injected neutral-atom beams fall into three categories, according to the partial pressure of the energetic-ion population, which in turn is determined by the plasma nTE' the injection energy, and the fraction of plasma fueling performed by the beams.There are correspondingly three regimes of beam-driven reactor operation in D-T or D-D: thermonuclear; beamtarget or TCT; ion pressure/electron pressure>> 1.This review treats the physical basis and plasma engineering parameters of beam-driven tokamak reactors.The theories of energetic-ion velocity distributions, stability, injection, and orbits are summarized.The many-faceted role of the energetic ions in plasma heating, fueling, and current maintenance, as well as in the direct enhancement of fusion power multiplication and power density, is discussed in detail for the three reactor types. The relevant implications of recent experimental results on several beam-injected tokamaks are examined. The behavior of energetic ions is found to be in accordance with classical theory,-iilarge total ion energy densities are readily achieved, and plasma equilibrium and stability are maintained.The status of neutral-beam injectors and of conceptual :design studies of beam-driven reactors are briefly'reviewed. The principal plasma-engineering problems are those associateci directly with achieving quasi-stationary operation. An especially attractive feature of beam-driven tokama~ plasmas is their capability 'for useful applicat~~P in devices of relatively sma l.l size.