Persistence of Deuterium and Tritium Nuclear Spin-Polarization in Presence of High-Frequency Plasma Waves

TL;DR

This work assesses the viability of maintaining spin polarization in D–T fuel in SPF-enabled tokamaks by quantifying wave-induced depolarization across SPARC- and ITER-scale plasmas. It employs a self-consistent framework combining spin dynamics with full-orbit particle tracing, linear Maxwell–Vlasov analyses, and nonlinear PIC simulations to evaluate depolarization rates from resonant plasma waves. The key finding is that alpha-driven Alfvénic modes contribute negligibly to depolarization, while obliquely propagating Alfvén waves on the fast Alfvén branch present the main depolarization risk; overall depolarization remains modest (a few percent in SPARC-like cores) and is mitigated by higher magnetic field strength. These results, reinforced by ITER results, support the potential for SPF to boost fusion reactivity in future high-field magnetic confinement devices, while highlighting the need for long-time and multi-mode studies to fully confirm robustness.

Abstract

We present first-principles numerical calculations of the depolarization rate of spin-polarized deuterium and tritium nuclei in realistic tokamak plasmas, driven by resonant interactions with plasma waves. Backed up by first-of-a-kind linear and nonlinear simulations, we find that alpha particle-driven Alfvénic modes cause only negligible depolarization, which is contrary to expectations in prior literature. Other Alfvénic instabilities can in principle degrade polarization, but only under conditions unlikely to be realized on transport timescales. By combining full-orbit particle tracing with a dedicated depolarization solver, we demonstrate that wave-driven depolarization is surprisingly weak in SPARC and ITER-scale devices. These results provide strong evidence that spin-polarized fuel can maintain its polarization long enough to boost fusion reactivity, opening a viable path toward substantially enhanced performance in magnetic confinement fusion power plants.

Figures (22)

• Figure 1: Schematic for how a circularly polarized wave with field $\delta \mathbf{B}$ exerts torque $\tau = \mathbf{\mu} \times \delta \mathbf{B}$ on a magnetic moment $\mathbf{\mu}$. If the wave and magnetic moment precession frequencies are close, there will be a torque for sufficiently long to change the component of $\mathbf{m}$ along the background field $\mathbf{B}_0$. Dashed arrow indicates the change of $\mathbf{\mu}$ over time.
• Figure 2: Relative density of neutron emission $F (\vartheta$) (see \ref{['eq:Fvartheta']}) parallel and perpendicular to the magnetic field $\mathbf{B}_0$ for three spin-polarization schemes. In (a) we indicate the pitch angle $\vartheta$.
• Figure 3: Time evolution of the probability density $\mathbf{\rho}$ (see \ref{['eq:rho_eq']}) of deuteron spin states over the course of $10,000$ cyclotron periods in the presence of a left handed magnetic wave field with a frequency equal to the deuteron precession frequency and an amplitude $1/1000$ that of the bulk field. The curved traces represent the time evolution of the state probability and the straight lines show the temporal mean probability, values of which are listed in the legend.
• Figure 4: As in Fig. \ref{['fig:temporalprobabilityresonance']} except the wave field has a frequency $0.995$ that of the the deuteron precession frequency.
• Figure 5: Shading indicates the probability of finding a deuteron in a particular spin state after $10,000$ cyclotron periods in the presence of a left-handed magnetic field wave with varying relative amplitudes and frequencies. The y-axis indicates the scan in relative amplitudes of wave field to bulk magnetic field on a $log_{10}$ scale. The x-axis shows the frequencies in units of the deuteron Larmor precession frequencies. The deuteron is initially in $m_I=0$. A probability of 1/3 for $m_I$ indicates complete depolarization.