Neutral-beam energy and power requirements for expanding-radius and full-bore start-up of tokamak reactors

Neutral-beam power arid energy requirements are compared for full-density, full-bore and expanding-radius start-up scenarios in an elongated plasma, The Next Step (TNS), as a function of beam pulse time and plasma density. Because of the similarity of parameters, the results should also be applicable to Engineering Test Facility (ETF) and International Tokamak Reactor (INTOR) studies. A transport model consisting of neoclassical ion conduction and anomalous electron conduction and diffusion based on 'ALCATOR scaling' leads to average densities in the range ⟨n⟩ ∼ 0.8–1.2 × 10 14 cm −3 being sufficient for ignition. Neutral-deuterium-beam energies in the range 120–150 keV are adequate for penetration, with the required power injected into the plasma decreasing with increasing beam energy. The neutral-beam power decreases strongly with increasing beam pulse length t b until t b exceeds a few total energy confinement times, yielding t b ∼ 4–6 s for the TNS plasma. In addition to avoiding skin current effects and possibly allowing for a more impurity-free plasma initiation, the expanding-radius scenario has slightly reduced beam energy and/or power requirements. When the expanding-radius scenario is extended to even larger power reactors, a neutral-deuterium-beam energy of 150 keV remains sufficient for penetration.