Observation of a Kelvin-Helmholtz Instability in a High-Energy-Density Plasma on the Omega Laser
A laser initiated experiment is described in which an unstable plasma shear layer is produced by driving a blast wave along a plastic surface with sinusoidal perturbations. In response to the vorticity deposited and the shear flow established by the blast wave, the interface rolls up into large vortices characteristic of the Kelvin-Helmholtz (KH) instability. The experiment used x ray radiography to capture the first well-resolved images of KH vortices in a high-energy-density plasma, and possibly the first images of transonic shocks generated by large-scale structures in a shear layer. The physical processes governing the evolution of a stratified fluid flow with a large velocity gradient (i.e., a shear flow) are of fundamental interest to a wide range of research areas including combustion, inertial confinement fusion (ICF), stellar supernovae, and geophysical fluid dynamics. Traditional experiments have used inclined tanks of fluid to initiate a flow, generally at low Reynolds numbers, or wind tunnels that combine two parallel gas flows at the end of a thin wedge, known as a splitter plate. The splitter plate experiments have explored flows with maximum shear velocities on the order of 10{sup 3} m/s and Reynolds numbers up to 10{sup 6}. Here we report the creation of a novel type of shear flow, achieved by confining a laser driven blast wave in a millimeter-sized shock tube, which produced shear velocities on the order of 10{sup 4} m/s and Reynolds numbers of 10{sup 6} in a plasma. This system enabled the first apparent observation of transonic shocklets, which are small, localized shocks believed to develop in response to a local supersonic flow occurring over a growing perturbation. These shocklets have been predicted previously in simulations, but have never to our knowledge been observed. These experiments are also the first to observe the growth of perturbations by the Kelvin-Helmholtz (KH) instability under high-energy-density (HED) conditions. In all flows having steep enough shear layers, small perturbations that initially develop on an interface are amplified by KH, driven by lift forces that result from differential flow across the perturbation. As the KH instability enters its non-linear regime the growth of the perturbation begins to saturate, at which point the interaction of secondary instabilities with the primary perturbation causes the flow to transition to a fully turbulent state. HED plasmas are created when an energy source, a multi-kilojoule laser in this case, creates pressures of order one Mbar or more. Such plasmas are compressible, actively ionizing, often involve strong shock waves, and have complex material properties. The one previous attempt to produce a shear flow under HED conditions was inconclusive and did not observe KH growth. The KH instability and shear flow effects in general are also of practical importance in a number of HED systems. They should be considered in multi-shock implosion schemes for direct drive capsules for inertial confinement fusion (ICF), since the KH instability may accelerate the growth of a turbulent mixing layer at the interface between the ablator and solid deuterium-tritium nuclear fuel. Some approaches to ICF (e.g., fast ignition) produce shear flows qualitatively similar to those discussed here. Some supernova explosion models also find that KH plays an important role. In addition, the experiments and simulations of HED and astrophysical systems have shown that structures driven by shear flow appear on the high-density spikes produced by the Rayleigh-Taylor (RT) and Richtmyer-Meshkov (RM) instabilities. Both RT and RM have important consequences for the evolution of ionized, compressible flows, including those found in ICF and astrophysical systems.
- Research Organization:
- Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- W-7405-ENG-48
- OSTI ID:
- 965959
- Report Number(s):
- LLNL-JRNL-410680; PRLTAO; TRN: US0904002
- Journal Information:
- Physical Review Letters, Vol. 103; ISSN 0031-9007
- Country of Publication:
- United States
- Language:
- English
Similar Records
A high energy density shock driven Kelvin-Helmholtz shear layer experiment
Vortex-sheet modeling of hydrodynamic instabilities produced by an oblique shock interacting with a perturbed interface in the HED regime