Polarized fusion and potential in situ tests of fuel polarization survival in a tokamak plasma

Abstract

The use of spin-polarized fusion fuels would provide a significant boost towards the ignition of a burning plasma. The cross section for D+T→ α+n, would be increased by 1.5 if the fuels were injected with parallel polarization. Furthermore, our simulations demonstrate additional non-linear power gains in large-scale machines such as ITER, due to increased alpha heating. Such benefits require the survival of spin polarizations for periods comparable to the particle confinement time. During the 1980s, calculations predicted that polarizations could survive a plasma environment, although concerns persisted regarding the cumulative impacts of wall recycling. In that era, technical challenges prevented direct tests and left the large scale fueling of a power reactor beyond reach. Over the last decades, this situation has dramatically changed. Detailed simulations of ITER have predicted negligible wall recycling in a high-power reactor, and recent advances in laser-driven sources project the capability of producing large quantities of ~100% polarized D and T. The remaining crucial step is an in-situ demonstration of polarization survival in a plasma. For this, we outline a measurement strategy using the isospin-mirror reaction, D+3He→ α+p. Polarized 3He avoids the complexities of handling tritium, while encompassing the same spin-physics. We evaluate two methods of delivering deuterium, using dynamically polarized Lithium-Deuteride (with vector polarization PV D of 70%) or frozen-spin Hydrogen-Deuteride (with PV D of 40%), together with a method of injecting optically-pumped 3He (with 65% polarization). Pellets of these materials all have long polarization decay times (~6 minutes for LiD at 2K, ~2 months for HD at 2K, and ~3 days for 3He at 77K), all far greater than a plasma shot in a research tokamak such as DIII-D. Both species can be propelled from a single cryogenic injection gun. We review plasma requirements and strategies for detecting polarization survival. Polarization alters both fusion yields and the angular distribution of fusion products, and each of these provides a potential signal. In this paper we simulate a selection of shots with similar characteristics in a future high-Tion H plasma, and find ratios of yields from shots with fuel spins parallel and antiparallel reaching 1.3 (HD+3He) to 1.6 (LiD+3He) over a wide range of poloidal angles. (A companion paper finds sensitivity to fusion product angular distributions as reflected in the pitch angles of protons and alphas reaching the plasma facing wall.)

Conceptual design of DIII-D experiments to diagnose the lifetime of spin polarized fuel

“…For deuterium, one approach uses an H-D capsule and another approach uses a 7 Li-D capsule. The expected values for the H-D capsule are 0.4 for the vector polarization and 0.12 for the tensor polarization [10]. The 7 Li-D capsule can potentially deliver larger values of 0.70 and 0.41 for the vector and tensor polarizations, respectively.…”

“…The first practical complication is that current technology cannot produce fuel pellets with 100% polarization. A reasonable value for vector polarization of 3 He is 0.65 [10]. For deuterium, one approach uses an H-D capsule and another approach uses a 7 Li-D capsule.…”

“…(Significant dσ − /dΩ tensor polarization variation is not presently available.) Although these are likely values using existing technology, new schemes discussed in [10] should be able to produce P V D = 1 and P T D = 1 and possibly even P T D = −2 but require extensive research and investment [10]. Figures 12(a A second practical complication is detector count rates.…”

“…For a poloidal array of three 3.6 MeV alpha detectors, the uncertainty associated with counting statistics in inferring either P V D P He in a thermonuclear experiment or P T D in a beamplasma experiment is very small (<0.01), implying that systematic errors will determine the ultimate resolution. D P He coefficient is inferred from data similar to figure 13, assuming that P T D is given by equation (10). Each symbol and error bar is from an ensemble of trials with randomly generated counting statistics.…”

“…A companion paper [10] considers many important issues with minimal coverage here, including a historical overview of research on spin polarized fusion in tokamaks, details of the relevant nuclear physics, depolarization mechanisms, and benefits for a fusion reactor. The preparation and delivery of polarized fuel pellets is discussed in considerable detail.…”