Prospects for Direct In Situ Tests of Polarization Survival in a Tokamak

Abstract

The cross section for the primary fusion fuel in a tokamak reactor, $\mathrm{D}\,{+}\,\mathrm{T}\rightarrow \alpha + \mathrm{n}$, would be increased by a factor of 1.5, if the fuels were spin polarized parallel to the local field. The potential realization of such benefits rests on the crucial question of the survival of spin polarization for periods comparable to the energy containment time. While calculations from the 1980s predicted that polarizations could in fact survive a plasma environment, concerns were raised regarding the impacts of wall recycling. In addition, the technical challenges in preparing and handling polarized materials had long prevented any direct tests. Over the last several decades, this situation has dramatically changed. Detailed simulations of the ITER plasma have projected negligible wall recycling in a high power reactor. In addition, a combination of advances in three areas—polarized material technologies developed for nuclear and particle physics as well as medical imaging, polymer pellets developed for Inertial Confinement, and cryogenic injection guns developed for fueling tokamaks—have matured to the point where a direct in situ measurement is possible, using the mirror reaction $\mathrm{D} + {}^{3}\mathrm{He}\rightarrow \alpha + \mathrm{p}$. Designs for a proof-of-principle experiment at a research tokamak, such as the DIII-D facility in San Diego, are discussed.