Turbulence in High-Energy-Density Plasma

Hydrodynamic instabilities are understood to pose a serious challenge to achieving inertial confinement fusion ignition. During implosion, the injection of cold, inert materials into what should be a hot, burning region quenches energy production. In supernovae undergoing the reverse process of explosion, instabilities are favored to explain the observed transport of material from stellar depths to the outer debris. Yet, questions have remained concerning the true mechanism by which instabilities in such high-energy-density (HED) environments achieve these effects.

While in classical fluids, instabilities typically pass through nonlinearity into the disordered phase known as “turbulence,” it has long been questioned whether in a dense plasma the new degrees of freedom (ionization, plasma waves, radiation transport, etc.) might modify or even prohibit that path.

With a new generation of HED experiments, the field can at last answer this question in the affirmative, that HED turbulence can develop analogously to classical fluids [Doss et al. Phys. Plasmas 27 032701 (2020)]. Building on the efforts of many in validating early-time behavior, a four-year campaign using LLNL’s National Ignition Facility successfully measured the deeply nonlinear regimes and confirmed that turbulence emerges as the instabilities develop, even in timescales as short as 10s of nanoseconds.

Following background and an overview of other families of experiments [Casner et al. Nucl. Fusion 59 032002 (2019)], we review the Shock/Shear campaign which conclusively demonstrated HED turbulence by studying shear flow subject to the Kelvin-Helmholtz instability, the most well-understood classical route to turbulence. A comprehensive scaling analysis unifies data from over 50 distinct NIF experiments, themselves scaled ~10 orders of magnitude from classical fluid shear experiments.