Description
Turbulence increases the reactivity of fusion plasma, allowing ignition at lower temperature. This “shear flow reactivity enhancement” [1] occurs because in plasmas, as opposed to other fluids, particles’ collisional mean free paths increase rapidly with their velocity. In sheared flows, and in solenoidal turbulent flows [2], energetic particles can travel long distances and pass between regions with large differences in flow, ending up at higher velocities relative to the local fluid. Such a velocity boost greatly increases the number of tail particles available to participate in energy-sensitive processes, such as fusion reactions. Furthermore, because the mean free paths of thermal ions and tail ions are widely separated, the effect can appear even in highly collisional plasmas; in fact, because thermal ions determine viscosity, their short mean free paths help to support strong flow gradients.
A central paradigm in inertial confinement fusion (ICF) – that imploding kinetic energy should thermalize to maximize fusion reactivity – must therefore be revised. In some cases, driving micron-scale turbulence inside ICF hot spots allows ignition at lower energy density [3]. Colder fuel improves the confinement of alpha particles, allowing for a smaller hot spot and offering particularly large energy savings in fast-ignition and shock-ignition configurations [1]. The shear flow reactivity enhancement effect raises the prospect of a new scheme for ICF in which targets are seeded with perturbations to generate turbulence on optimal scales during compression, reducing thermal losses and potentially enabling higher gain. Beyond the implications for high-yield experiments, consequences of the effect with broader scientific relevance will be discussed, including modifications to yield ratios and fusion-product spectra (affecting, for example, diagnostics based on these quantities).
[1] Fetsch, H. & Fisch, N. J. Enhancement to Fusion Reactivity in Sheared Flows. Phys. Rev. Lett. 135, 155101 (2025).
[2] Fetsch, H. & Fisch, N. J. Analytical models for the enhancement of fusion reactivity by turbulence. Phys. Plasmas 32, 112703 (2025).
[3] Fetsch, H. & Fisch, N. J. An ignition criterion for inertial fusion boosted by microturbulence. Phys. Plasmas 33, 020703 (2026).
This work was supported by the NNSA SSAP under DOE Cooperative Agreement DE-NA0004148 and by the NSF under Grant PHY-2308829.