Beam driven tokamak fusion reactors

The recent successes in neutral beam current drive experiments on large tokamaks prompt us to consider the prospect of a beam driven steady state tokamak fusion reactor. A neoclassical theory on the beam driven current, which fully includes the toroidal effects on the induced ion and electron currents and is the most reliable to date, is reviewed in this article. The prospect of steady state tokamaks predicted by the theory may be somewhat pessimistic if the plasma current is sustained only by the beam driven current, because too much beam power is necessary for the current drive. However, thanks to the bootstrap current which was discovered in many tokamaks in recent years, the feasibility of a continuous tokamak operation will probably be highly improved because several tens of percent of the current will be produced by the bootstrap current. The energy gain Q would attain 30 to 50 in the power reactor and about 10 even for next generation (demonstration size) tokamaks.