Neutral-beam-driven tokamak fusion reactors

The ion velocity distributions of tokamak plasmas sustained by injected fast atomic beams fall into three categories, according to the partial pressure of the energetic-ion population, which in turn is determined by the plasma ntausub(E), the injection energy, and the fraction of plasma fuelling performed by the beams. There are correspondingly three regimes of beam-driven reactor operation in D-T or D-D: thermonuclear; beam-target or TCT; and plasmas with ion pressure/electron pressure much >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, fuelling, 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 from several beam-injected tokamaks are examined. The behaviour of energetic ions is found to be in accordance with classical theory, large total ion energy densities are readily achieved, and plasma equilibrium and gross stability are preserved. The status of neutral-beam injectors and of conceptual design studies of beam-driven reactors is reviewed briefly. The principal plasma-engineering problems are those associated directly with achieving quasi-stationary operation, particularly the problem of high-throughput particle exhaust. An especially attractive feature of beam-driven tokamak plasmas is their capability for useful application in devices of relatively small size. (author)