Session S9: Inertial Confinement and High Energy Density Plasma

Hydrodynamic Jet Experiments at LLE

Observed jet and jet-like morphologies range from highly collimated flows associated with young stellar objects and active galactic nuclei to less-collimated flows associated with planetary nebulae. A technique, where seven beams from the OMEGA laser are incident onto a mid-$Z$ plug embedded in a tungsten washer and two beams are used to generate x rays for radiography, is used to study jet outflows. An adiabatic model$^{1}$ best describes jet propagation. Episodic flows are created using double-pulse laser irradiation and show a different jet structure with more material along the jet stem. Episodic experiments have been designed for the OMEGA EP Laser System where the time between outflows can be made comparable to the hydrodynamic evolution time. 2-D Eulerian hydrodynamic simulations both model OMEGA and design OMEGA EP experiments. $^{1}$E. C. Ostriker \textit{et al.}, ApJ \textbf{557}, 443 (2001). This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302.   

Isochoric heating of matter by laser-accelerated high-energy protons

Producing matter at a high temperature (1-25 eV) and solid density is of prime interest for fundamental plasma physics or ICF. The use of laser-based high energy proton beams to achieve such state of matter is interesting since they are short ($<$ 1 ps) and they deposit their energy volumetrically; thus can heat, before they expand, much thicker samples than allowed using laser-heating. We performed, using two intense short pulses of the LULI 100 TW facility, experiments to characterize the achieved state of matter, coupled to a detailed hydro-modeling. A laser-generated proton beam irradiated and heated a secondary target positioned after a vacuum gap. Three diagnostics were used: (i) 1D time-resolved optical self-emission of the heated target rear-surface at two wavelengths, (ii) time-resolved interferometry of a chirped probe beam reflecting off the heated target rear-surface, (iii) x-ray absorption spectroscopy through the heated target using a laser-produced backlighter detecting its K$\alpha $-edge softening.   

Breakeven Fusion in a Staged Z-Pinch

We are studying a dense-plasma, Z-pinch configuration, where a cylindrical, Xe shell implodes onto a co-axial, deuterium-tritium target. The configuration is modeled using MACH2. During implosion current amplification occurs at the outer surface of the DT target, leading to a shorter and more energetic implosion (Ref. 1). Shocks preheat and preaccelerate the DT without Rayleigh-Taylor (RT) instability (Ref. 2), even as the Xe liner becomes RT unstable. Proper choice of the initial radius, density, and driver parameters provides a fusion-energy yield larger than the stored (capacitor-bank) energy. A specific example is presented, involving a 2 MJ, 100 ns system that produces a 5 MJ fusion yield. These studies are of interest, since fusion breakeven has yet to be demonstrated in any laboratory experiment. \newline \newline [1] H. U. Rahman, F. J. Wessel, N. Rostoker, ``Staged Z Pinch'', Phys. Rev. Lett \underline {74}, p. 714(1995). \newline [2] N. Rostoker and H. Tashiri, A Perspective of Physics Vol. 2, Gordon and Breech, ``RT Instability of Impulsively Accelerated Shells'', p. 217(1978).   

Hot Electron and X-ray Production from Intense Laser Irradiation of Wavelength-scale Polystyrene Spheres

In an attempt to control the electric fields at the surface of a high intensity solid target we have studied hot electron generation and x-ray production from targets coated with microspheres. This work is motivated by the possibility that spheres with size comparable to the wavelength of the incident laser radiation can result in electric field enhancements through well know Mie resonances. This local field enhancement can then lead to more efficient electron generation. We investigated hard x-ray (above 100 keV) generation from copper and fused silica targets coated with a monolayer covering of polystyrene microspheres. We performed the experiment using the 20 TW THOR laser system at the University of Texas. We frequency doubled the laser to improve temporal contrast and irradiated the spheres with 400 nm pulses at an intensity of 2 x 1017 W/cm2. Hard X-ray emission from the plasma was observed using filtered NaI scintillation detectors and K-alpha emission was measured with a Von Hamos spectrometer. We illuminated polystyrene spheres of diameters 0.1 -2.9 microns on a glass substrate, with the 400 nm 100fs pulse, and find that there is a clear Mie enhancement in the field and hot electron generation for a specific range of sphere sizes.   

Intense laser-driven electrostatic shocks and its acceleration of ions in overdense plasmas

The formation and propagation of the electrostatic shocks in overdense plasmas irradiated by intense ultrashort laser pulses is studied with particle-in-cell simulations. The dependence of the initial shock speeds on the parameters of the plasma and the laser is explained by invoking the modified momentum conservation model with 2D effects taken into account. The details of the acceleration process of ions by the shocks are also investigated. Such topic is believed as an important issue in the fast ignition scheme of the inertial confinement fusion researches.   

Measuring Ultrafast Dynamics in a Dense Plasma

Transient effects were measured in free standing metal film (200-400 nm) following direct femtosecond laser heating at 1-10x10$^{14}$W/cm$^{2}$. Ultrafast electron-ion equilibration dynamics in the dense plasma were studied with a single-shot measurement of the time-dependent reflectivity and phase shift at the rear surface. The measurement revealed the dynamics of heat and shock waves on the picosecond time scale with sub-picosecond resolution. The experimental results were compared to the calculated reflectivity and phase shift derived from the output parameters (electron density and dc electron conductivity) of hydrodynamic simulations. We simulated our experiments using different equation of states and ionization models in the HYADES hydrodynamic code. Our experimental results allowed us to test the currently available models.   

On the Baronova-Stepanenko X-ray crystal spectropolarimeter.

A few years ago Baronova and Stepanenko realized that the two orthogonal X-ray polarizations can reflect in mutually perpendicular direction from hexagonal crystals, and other crystals with the necessary symmetry. This paper will discuss their geometry, and the advantages it may have for diagnosing anisotropic effects in high energy density plasmas, e.g., non-Maxwellian electron beams or electromagnetic fields. Any results obtained with a prototype crystal will be presented.   

Transient Formation of keV Super-Explosives under High Pressure for Thermonuclear Ignition.

At pressures of the order 100 Mb, chemical reactions at keV energies can take place, leading to molecular configurations through the binding of the inner electron shells. For this reason, matter suddenly put under high pressure can form a super-explosive, which releases intense bursts of keV-X ray photons, powerful enough for the ignition of thermonuclear reactions.