24 simulated the redistribution of alpha particles in a JET-like configuration, showing that the energy threshold for decoupling can be expected to lie at a few 100 keV for trapped, and above 1 MeV for passing alpha orbits when ∣1 − q ∣ is sufficiently small. Using a heuristic model of a sawtooth crash, Jaulmes et al. It turns out, however, that the q profile can have a significant influence on the transport of alpha particles. In a 2014 review 22, it was concluded that “the effects of kink modes on fast ions seem to be understood.” This was followed by the construction of computationally efficient reduced models, which are used in integrated transport simulations to make quantitative predictions for experiments such as ITER, assuming that all sawtooth crashes have the same effect 23. Nevertheless, until recently the existing evidence indicated that the majority of particles, including fast alphas, undergoes mixing during a sawtooth crash, so that their density profiles are flattened in the relaxation domain (unless, of course, the profile had already been broad before the crash 21). The energy threshold above which ions decouple from the internal kink mode has been estimated theoretically by Kolesnichenko and Yakovenko 19 and confirmed experimentally by Muscatello et al. A driving force behind these studies was the insight that sufficiently energetic charged particles can decouple from the dynamics of the bulk of a magnetized plasma, which has been known since the early years of MCF research (see ref. In contrast, resonant interactions with the internal kink and other low-frequency magnetohydrodynamic (MHD) modes can cause rapid ballistic transport before the alphas have transferred their energy to the bulk 15.Īfter decades of research - especially during the 1990s when the Tokamak Fusion Test Reactor (TFTR) and JET operated with D-T plasmas 16, 17 - this conundrum of needing to confine fast alphas while expelling slowed-down helium ash is still being actively explored. A recent study shows, however, that the RF-cooled alphas may remain in the plasma core 14. In the case of so-called alpha channeling via radio-frequency (RF) waves 12, 13, it is envisioned that the alpha particles gradually diffuse outward while transferring their energy to thermal ions via damped plasma waves. For a toroidal surface with long circumference 2 π R and short circumference \(2\pi \overline\) ions (‘fast alphas’) should ideally be left unperturbed, since they provide the heating power in a self-sustained burning fusion plasma. The helically wound B field consists of a dominant toroidal component B tor that is provided by external coils and a weaker poloidal component B pol that is induced by electric currents carried by the plasma itself. Tokamaks use a strong magnetic field to confine hydrogen isotope plasmas with high temperatures (~10 keV) in a toroidal volume as sketched in Fig. While their use as a power plant still awaits breakthroughs, the accumulated scientific evidence suggests that tokamaks can produce a burning plasma, where fusion reactions are self-sustained for times much longer than the confinement times of thermal energy and charged particles. Spearheaded by ITER 1, the tokamak concept is the present mainstream approach to magnetically confined fusion (MCF). Laboratories around the world have intensified R&D activities for experimental reactors that should demonstrate the practical feasibility of extracting useful energy from controlled nuclear fusion. Besides causing asymmetry between co- and counter-going particle populations, magnetic drifts determine the size of the confinement window by dictating where and how much reconnection occurs in particle orbit topology. The physical picture - a synergy between magnetic geometry, optimal crash duration and rapid particle motion - is completed by clarifying the role of magnetic drifts. Here, we report results of kinetic-magnetohydrodynamic hybrid simulations of a large tokamak plasma that confirm the existence of a parameter window where such energy-selective confinement can be accomplished by exploiting internal relaxation events known as sawtooth crashes. On the other hand, decelerated helium ash must be expelled before diluting the fusion fuel. On the one hand, MeV-class alpha particles must stay confined to heat the plasma. Long-pulse operation of a self-sustained fusion reactor using toroidal magnetic containment requires control over the content of alpha particles produced by D-T fusion reactions.
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