Dynamics of the Electromagnetic Fields Induced by Fast Electron Propagation in Near-Solid-Density Media.

The propagation of fast electron currents in near solid-density media was investigated via proton probing. Fast currents were generated inside dielectric foams via irradiation with a short (∼0.6 ps) laser pulse focused at relativistic intensities (Iλ^{2}∼4×10^{19} W cm^{-2} µm^{2}). Proton probing provided a spatially and temporally resolved characterization of the evolution of the electromagnetic fields and of the associated net currents directly inside the target. The progressive growth of beam filamentation was temporally resolved and information on the divergence of the fast electron beam was obtained. Hybrid simulations of electron propagation in dense media indicate that resistive effects provide a major contribution to field generation and explain well the topology, magnitude, and temporal growth of the fields observed in the experiment. Estimations of the growth rates for different types of instabilities pinpoints the resistive instability as the most likely dominant mechanism of beam filamentation.

Density and temperature characterization of long-scale length, near-critical density controlled plasma produced from ultra-low density plastic foam.

The ability to produce long-scale length (i.e. millimeter scale-length), homogeneous plasmas is of interest in studying a wide range of fundamental plasma processes. We present here a validated experimental platform to create and diagnose uniform plasmas with a density close or above the critical density. The target consists of a polyimide tube filled with an ultra low-density plastic foam where it was heated by x-rays, produced by a long pulse laser irradiating a copper foil placed at one end of the tube. The density and temperature of the ionized foam was retrieved by using x-ray radiography and proton radiography was used to verify the uniformity of the plasma. Plasma temperatures of 5-10 eV and densities around 10(21) cm(-3) are measured. This well-characterized platform of uniform density and temperature plasma is of interest for experiments using large-scale laser platforms conducting High Energy Density Physics investigations.

Magnetically guided fast electrons in cylindrically compressed matter.

Fast electrons produced by a 10 ps, 160 J laser pulse through laser-compressed plastic cylinders are studied experimentally and numerically in the context of fast ignition. K(α)-emission images reveal a collimated or scattered electron beam depending on the initial density and the compression timing. A numerical transport model shows that implosion-driven electrical resistivity gradients induce strong magnetic fields able to guide the electrons. The good agreement with measured beam sizes provides the first experimental evidence for fast-electron magnetic collimation in laser-compressed matter.

Laser smoothing and imprint reduction with a foam layer in the multikilojoule regime.

This Letter presents first experimental results of the laser imprint reduction in fusion scale plasmas using a low-density foam layer. The experiments were conducted on the LIL facility at the energy level of 12 kJ with millimeter-size plasmas, reproducing the conditions of the initial interaction phase in the direct-drive scheme. The results include the generation of a supersonic ionization wave in the foam and the reduction of the initial laser fluctuations after propagation through 500 mum of foam with limited levels of stimulated Brillouin and Raman scattering. The smoothing mechanisms are analyzed and explained.

Characterization of high-intensity laser propagation in the relativistic transparent regime through measurements of energetic proton beams.

Experiments were performed to investigate the propagation of a high intensity (I approximately 10(21) W cm(-2)) laser in foam targets with densities ranging from 0.9n(c) to 30n(c). Proton acceleration was used to diagnose the interaction. An improvement in proton beam energy and efficiency is observed for the lowest density foam (n(e)=0.9n(c)), compared to higher density foams. Simulations show that the laser beam penetrates deeper into the target due to its relativistic propagation and results in greater collimation of the ensuing hot electrons. This results in the rear surface accelerating electric field being larger, increasing the efficiency of the acceleration. Enhanced collimation of the ions is seen to be due to the self-generated azimuthal magnetic and electric fields at the rear of the target.

Supersonic-jet experiments using a high-energy laser.

In this Letter, laboratory astrophysical jet experiments performed with the LULI2000 laser facility are presented. High speed plasma jets (150 km.s(-1)) are generated using foam-filled cone targets. Accurate experimental characterization of the plasma jet is performed by measuring its time evolution and exploring various target parameters. Key jet parameters such as propagation and radial velocities, temperature, and density are obtained. For the first time, the required dimensionless quantities are experimentally determined on a single-shot basis. Although the jets evolve in vacuum, most of the scaling parameters are relevant to astrophysical conditions.

Study of electron-beam propagation through preionized dense foam plasmas.

The transport of an intense electron-beam produced by the Vulcan petawatt laser through dense plasmas has been studied by imaging with high resolution the optical emission due to electron transit through the rear side of coated foam targets. It is observed that the MeV-electron beam undergoes strong filamentation and the filaments organize themselves in a ringlike structure. This behavior has been modeled using particle-in-cell simulations of the laser-plasma interaction as well as of the transport of the electron beam through the preionized plasma. In the simulations the filamentary structures are reproduced and attributed to the Weibel instability.

Interaction of soft-x-ray thermal radiation with foam-layered targets.

We have studied the interaction of soft-x-ray thermal radiation with foam-layered metal targets. X-ray radiation was produced by focusing a high-energy laser inside a small size hohlraum. An increment in shock pressure, up to a factor of approximately 4 for 50 mg/cm(3) foam density, was observed with the foam layer as compared to bare metal targets. This follows from the propagation of radiation-driven shock wave in the foam and the impedance mismatch at the foam-payload interface.

Inhibition in the propagation of fast electrons in plastic foams by resistive electric fields.

The propagation of relativistic electrons in foam and solid density targets has been studied by means of K-alpha spectroscopy. Experimental results point out the role of self-generated electric fields in propagation and the role of heating of matter induced by the passage of fast electrons. A simple analytical formulation has been given and Spitzer conductivity has been shown to be fairly compatible with experimental results.

Use of low-density foams as pressure amplifiers in equation-of-state experiments with laser-driven shock waves.

The applicability of foams to equation of state experiments with laser-produced shocks has been studied. The pressure increase due to impedance mismatch at the payload-foam interface was measured experimentally using sub-ns laser pulses smoothed with phase zone plates. Foams of density in the range 5-900 mg/cm(3) and of thicknesses of 50-150 microm were used. A model has been developed to study pressure amplification and the conditions under which the shock is stationary. Two-step two-material targets, allowing simultaneous measurements of the shock velocities in the two materials, were then used to obtain relative equation of state data. Pressures higher than 100 Mbar were achieved in gold.

Foam-induced smoothing studied through laser-driven shock waves

The influence of foams on the uniformity of laser energy deposition has been studied by measuring laser-driven shock waves breakout from foam-aluminum layered targets. Well characterized laser nonuniformities have been produced first by using phase zone plates to get a smooth beam and then by inserting different opaque grids before the foam. Smoothing has been studied as a function of foam density and grid materials (producing different radiative effects).