Hydrodynamic Instabilities in Laser Plasmas

1. Hydrodynamic Instabilitiesin Laser Plasmas Cris W. Barnes P-24 July 3, 2002

2. DEMONSTRATIONS • Why does water fall out of jar? • Instability and stability

3. Plasmas as Fluids • This talk is about “fluid dynamics” • But I’m a plasma physicist…. • Plasma fluids: • Unmagnetized • No E or B fields • Highly collisional • Can be treated as single species (“no electron physics”) • Generally caused by application of high energy density to near-solid material (such as by shock) • (Can be) Compressible

4. Fundamental Equations • Conservation of mass: particle balance • Conservation of momentum: force balance (pressure balance) • Conservation of energy: energy balance Ignoring time derivatives can define “hydrostatic equilibrium”. Putting in time dependence and perturbing that equilibrium can analyze for stability.

5. Examples of Rayleigh-Taylor • ICF • Geophysics (material strength)

6. Nonlinear behavior • What happens when ball hits floor? • When mode amplitude is “significant fraction” of mode wavelength, the system become “nonlinear”. • These instabilities generate “turbulence” and cause “mix” • ICF example

7. Nano-technology (both foam production and precision machining) is an enabling technology for cylindrical implosion hydrodynamics Outer Ablator Direct-drive cylinder target for OMEGA “Mix” Mix Region Foam Core “Features”, both single and periodic Increasingly Nonlinear Hydrodynamics

8. Time resolved X-ray images provide substantial information from a single shot • 1.5 m initial amplitude; see trajectories, perturbation modal amplitudes, shock arrival, illumination asymmetry C8H7Br CH2foam phase reversal C8H6Cl2 polystyrene

9. Ambiguous image center allows choice that reveals Fourier spectrum with fundamental (m=10), illumination imprint (m=4), and weak nonlinear coupling (m=6 & 14) + * Choose image center to reduce ±1 sidebands around fundamental Plotting Imaginary vs Real part of FFT for each mode for each frame: see well-defined phases Varying phase between m=10 and 4 and applying mode-coupling simulates “odd-shaped” data

10. More Stuff • Peter Wilson / Malcom Andrews Texas A&M water tank experiment This may be a “classic” field (Lords Kelvin and Rayleigh from the Nineteenth Century) but cool stuff still possible today! • Kelvin-Helmholtz • Richtmyer-Meshkov (impulsive) • Bell-Plesset (convergence matters) CYLMIX and DNS calculations, and CYLART results from our own team

11. We’ve established a useful, laser-based test bed for compressible convergent mix experiments • Implode cylinder with direct laser irradiation • Hydrodynamically unstable at plastic/Au and Au/foam interfaces (now using epoxy/Al) • Shine x rays through cylinder • Measure radial extent of “mix layer” of Au or Al into adjacent materials • 1D experiment with Mach number ≈ 10, convergence ≈ 3, Pressure > 45 Mbars

12. Experimental Results are compared to DNS (Direct Numerical Simulations) for different initial conditions • Mix with rough initial conditions overwhelms end effects • Differences exist between codes • Both “bowing” and “filigree” exist PETRA r-Z calculations RAGE r-Z calculations

13. We have made measurements of mix growth in time with rough surface finish 22602 3.2 ns Frames 3c 18687 4.7ns Frames 2c • Done on different shots (OMEGA backlighter limitations) but with the same type of target • Growth consistent with linear after shock breakout, and with DNS simulations Preliminary!

14. Hydrodynamic Instability and Turbulent Mix • Classic field of fluid dynamics studied world-wide at many institutions 8th International Workshop on the Physics of Compressible Turbulent Mixing was held in Pasedena last December. • High Atwood number (“variable density, inhomogeneous turbulence”), compressible (high energy density), convergent systems are at the forefront of challenging science at National Labs with new experimental and theoretical tools becoming available to study them.