Channeling of Fusion Alpha-Particle Power Using Minority Ion Catalysis

Abstract

Maintaining fuel ions hotter than electrons would greatly facilitate controlled nuclear fusion. The parameter range for achieving this temperature disparity is shown here to be enhanced by catalyzing the α-channeling effect (wave-induced simultaneous expulsion and cooling of α particles) through minority ion heating. Specifically, a wave can extract energy from hot α particles and transfer it to colder minority ions, which act as a catalyst, eventually forwarding the energy to still colder fuel ions through collisions. In comparison with the traditional α-channeling mechanism, the requirements are thereby relaxed on the waves that accomplish the α-channeling, which no longer have to interact simultaneously with α particles and fuel ions. Numerical simulations illustrate how the new scheme may increase, for example the effective fusion reactivity of mirror-confined plasmas.

The alpha channeling effect

“…It remains, of course, to find suitable waves. Certain contained modes can be utilized [67,68,69], and the parameter space of possibly useful waves can be extended by using minority ions to catalyze the channeling effect [70].…”

Elementary processes underlying alpha channeling in tokamaks

“…Alpha channeling can be practiced in particular in mirror machines, where the concept of plasma periphery now includes the loss cone in velocity space [18]. In a mirror machine, contained modes can be utilized [19,20], and the parameter space of possible waves can be extended by using minority ions to catalyze the channeling effect [21].…”

Applying Alpha-Channeling to Mirror Machines

“…To ensure that the wave is in resonance with deeply trapped α particles, but is not Landau damped on electrons, heavy minority ions such as 22 Ne or 21 Ne are indicated [45]. The amplitude of the employed waves must be sufficiently large to cause fast α particle extraction, but small enough that the bulk plasma is not perturbed significantly.…”

“…In Ref. 45, the possibility to satisfy all these conditions simultaneously was demonstrated through Fokker-Planck simulations of both the α-particle and ion dynamics for the system with the following parameters: B 0 ≈ 0.9 T, n e = 3 × 10 13 cm −3 , n D = 0.9n e , n T = 0.1n e and T e = T D = T T = 10 keV. The minority ions were prevented from overheating because the polarization of the fast Alfvén waves used for α channeling is opposite to the direction of ion gyration.…”

“…Thus, to remove the restrictions on the waves in the straightforward α-channeling technique, the technique can be modified using a "catalytic" effect of minority ions [45]. Specifically, minority ions injected into plasma can act as mediators, extracting the energy resonantly from the operating wave ( Fig.…”