The Kelvin-Helmholtz instability in National Ignition Facility hohlraums as a source of gold-gas mixing

Abstract

Highly resolved radiation-hydrodynamics FCI2 simulations have been performed to model laser experiments on the National Ignition Facility. In these experiments, cylindrical gas-filled hohlraums with gold walls are driven by a 20 ns laser pulse. For the first time, simulations show the appearance of Kelvin-Helmholtz (KH) vortices at the interface between the expanding wall material and the gas fill. In this paper, we determine the mechanisms which generate this instability: the increase of the gas pressure around the expanding gold plasma leads to the aggregation of an over-dense gold layer simultaneously with shear flows. At the surface of this layer, all the conditions are met for a KH instability to grow. Later on, as the interface decelerates, the Rayleigh-Taylor instability also comes into play. A potential scenario for the generation of a mixing zone at the gold-gas interface due to the KH instability is presented. Our estimates of the Reynolds number and the plasma diffusion width at the interface support the possibility of such a mix. The key role of the first nanosecond of the laser pulse in the instability occurrence is also underlined.

Nonequilibrium and morphological characterizations of Kelvin–Helmholtz instability in compressible flows

“…The Kelvin-Helmholtz instability (KHI) occurs at a perturbed interface between two fluids or two parts of the same fluid with different tangential velocities [1]. As an efficient and important initiating mechanism of turbulence and mixing of fluids [2][3][4][5][6][7], it plays crucial roles in various fields, ranging from high-energy-density physics [8], geophysics and astrophysics [9][10][11][12][13][14], inertial confinement fusion (ICF) [15][16][17], combustion [18][19][20], to Bose-Einstein condensate [21,22] and graphene [23], etc. Concretely, in geophysical and astrophysical situations, on the one hand, the fully developed KH billows are responsible for the formation of large-scale vortical structures in systems such as hurricane [9], galaxy spiral arms [10], heliopause [11,12], and solar wind interaction with the Earth's magnetosphere [13,14], leading to violent intermixing across shear layers; on the other hand, the significantly suppressed KH roll-ups contribute to the sufficiently long, stable and highly collimated supersonic astrophysical jets [16,[24][25][26] with length-to-width ratios as high as 100 or more, emanated from young stellar objects or active galactic nuclei [27], and jet-like long spikes observed in the high-energy-density laboratory astrophysics experiments [28].…”

Recent progress in quantifying hydrodynamics instabilities and turbulence in inertial confinement fusion and high-energy-density experiments

“…KHI arises from adjacent fluids streaming at different velocities, and could also be observed as a side effect on RTI spikes and bubbles at the highly nonlinear stage. KHI was recently suggested to develop at the gas/gold interface of ICF hohlraum [32]. In ICF implosions, RTI develops first at the ablation front [33] where it is seeded by capsule surface roughness and, in direct drive, the imprinted speckles of the laser beams [34].…”