Low-density ignition scenarios in injection-heated tokamaks

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Abstract
Plasma heating and ignition by neutral injection have been studied using a Monte-Carlo neutral-injection computer code coupled to a single-fluid, one-dimensional (1-D) transport code and a two-dimensional (2-D) flux-conserving equilibrium code. It is shown that, by taking advantage of central α-heating, profile effects, and flux surface shifts in elongated plasmas, it is possible to ignite a modelled, prototypical reactor plasma using 45–30 MW of 100–150 keV (D+) neutral beams. To do this, the plasma is started at full bore but with a density below that needed for ignition. The density is then increased by peripheral fuelling so that the central core begins to ignite at the time when the neutral beams no longer penetrate to this region. The fusion α-particles take over the heating requirements in the core region. Because of the decreasing beam line efficiency with increasing energy, it is found that a nearly constant extracted power of about 85–95 MW is needed for ignition in the range studied. There is thus little economic difference in this energy range. However, higher-energy beams around 150 keV imply fewer injectors and perhaps lower impurity production rates during heating to ignition.
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