The optimization of neutral beams for ignition and burn control on next-step reactors

The purpose of this paper is to identify the neutral-beam injection (NBI) parameters for a system on a next-step reactor, optimized for ignition and burn control, rather than current drive. As a main model we use the International Thermonuclear Experimental Reactor (ITER) engineering design activity (EDA) concept in its 1993 version (R0 = 7.75 m, Ip = 25 MA, BT = 6 T, a = 2.8 m, k = 1.6). The dependence of the ‘minimum power to ignite ITER EDA’ on beam energy, beam geometry and beam isotope species is investigated for a plasma which conforms to Rebut-Lallia-Watkins transport in time-dependent simulations. It is found that deuterium beams with an energy of 400 keV or above in a geometry with tangency radius RT approximately half the major radius are sufficient for efficient ignition. The beam simulation results are compared with simulations using an idealized heating scheme. Sensitivity of the results to variations in plasma behaviour are investigated. Impurities, particle confinement, energy confinement and the high-confinement-mode threshold are factors which influence the power required for ignition significantly, but have little effect on the choice of beam energy and geometry. Considerations with respect to loss of fast particles led us to adopt a beam geometry with RT ∼ 12R0 (very similar to that of the Joint European Torus), rather than perpendicular injection. Finally it will be shown that burn control using NBI is possible on ITER EDA in subignited regimes.