Regimes of ignited operation

Regimes of ignited operation are explored in terms of a general four-dimensional requirement on nτe, nτi, Ti and Te. The conditions under which ignited operation occurs with Te > Ti and Ti > Te are found. The amount of electron-ion de-coupling and the required value of nτe at ignition are determined as functions of the ion temperature and the ratio of the electron energy confinement time to the ion energy confinement time. Effects of anomalous transfer of alpha-particle energy to the ions are considered. Thermal-stability characteristics are determined in the context of the four-dimensional ignition requirement. An empirical scaling law for the electron energy confinement time and the neoclassical transport description of the ion energy confinement time are used to project specific features of ignited operation in recent next-step tokamak reactor designs. It is found that the ignition requirement on nτe can be significantly reduced by operating in the Ti > Te regime at high ion temperatures. This reduction in nτe leads to a considerable reduction in the neutral beam energy required for adequate penetration in a full-density start-up scenario. The ion temperature dependence of characteristic thermal runaway times has also been determined for the next-step reactor designs. The thermal runaway times are very short (on the order of τe) until ion temperatures approaching 50 keV are reached.