Session CO6: Short Pulse Laser-Plasma Interactions I

Design of a Positron--Electron Pair-Plasma Production Experiment on OMEGA EP

It is shown that an $e^{+}e^{-}$ pair-plasma can be created on OMEGA EP, a feat yet to be achieved in the laboratory. Monte Carlo calculations show that a yield of between 10$^{11}$ and 10$^{12}$ positrons can be produced on OMEGA EP by a combination of the Bethe--Heitler conversion of hard-x-ray bremsstrahlung\footnote{ J. D. Bjorken and S. D. Drell, \textit{Relativistic Quantum Mechanics}, International Series in Pure and Applied Physics (McGraw-Hill, New York, 1964); D. A. Gryaznykh, Ya. Z. Kandiev, and V. A. Lykov, JETP Lett. \textbf{67}, 257 (1998); K. Nakashima and H. Takabe, Phys. Plasmas \textbf{9}, 1505 (2002).} and the trident process,\footnote{ E. P. Liang \textit{et al}., Phys. Rev. Lett. \textbf{81}, 4887 (1998).} assuming a total laser energy of 5 kJ. For the expanding $e^{+}e^{-}$ cloud to be a plasma, there must be many particles in a Debye sphere, and the cloud must be many Debye lengths in size. Particle-in-cell calculations are used to demonstrate that a megagauss DC magnetic field, produced by a second OMEGA EP beam, can be used to provide the necessary confinement and, therefore, density of the cloud. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement DE-FC52-92SF19460.   

High-Brightness $\sim $keV Source and Diagnostic Development

High-energy-petawatt (HEPW), laser-driven backlighter sources with photon energies from $\sim $1 to $\sim $ 3 keV have a broad range of applications in high-energy-density physics and inertial confinement fusion. Backlighter source development studies have been performed on the VULCAN petawatt laser at RAL and the Multi-Terawatt laser at LLE and will be continued on OMEGA EP. The x-ray emission was measured on aluminum flat-foil targets. A conversion efficiency of up to 1 $\times $ 10$^{-5}$ 1/eV/sr from laser energy into the Al He$_{\alpha }$-line energy was observed. Assuming a circular emission region with a FWHM of $\sim $25 \textit{$\mu $}m and an emission time of 30 ps a brilliance of $\sim $15 J/eV/ps/sr/cm$^{2}$ can be inferred. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement DE-FC52-92SF19460.   

The Trident 250TW Short-Pulse Laser Upgrade at LANL: System and Initial Results

The Trident laser-facility at Los Alamos has served for more than 20 years as an important tool in ICF and Material Dynamics research [1,2,3] An energy / power upgrade of the short pulse beam line to 150J / 250 TW and a new short pulse front end has been installed. Moreover, a third target area dedicated to combined short pulse / long pulse experiments is being built. The combination of this powerful new short-pulse beam line with the two flexible long pulse beams and a total of three different target areas, makes Trident a highly flexible and versatile research tool for high energy density laboratory plasma research. In this presentation, the upgraded capabilities are described, and results from initial operation are summarized. \newline [1] N. K. Moncur et al., Appl. Opt. \textbf{23}, 4274 (1995) \newline [2] D. S. Montgomery et al., PRL \textbf{87}, 155001 (2001) \newline [3] Swift, Damian C., et al., Phys Rev E \textbf{69}, 036406 (2004)   

Short-pulse laser K-alpha conversion efficiency in gas jet targets

We have measured the absolute conversion efficiency of K$_{\alpha}$ X-rays from short pulse laser irradiation of chlorine and argon gas jet targets, and performed a direct comparison of Cl K$_{\alpha}$ yield from both gaseous and solid chlorine-containing targets. The K$_{\alpha}$ conversion efficiencies in a 3.5\% Cl gas jet target and a 100\% Ar target ($n \approx 10^{19}$ $\textrm{cm}^{-3}$) are comparable to the conversion efficiency obtained for 33\% Cl solid saran targets ($n \approx 10^{23}$ $\textrm{cm}^{-3}$). The conversion efficiency integrated from K$_{\alpha}$ to K$_{\beta}$ is an order of magnitude higher in gas jet targets than in solid targets. This work was performed under the auspices of the U.S. Department of Energy by the University of California Lawrence Livermore National Laboratory, through the Institute for Laser Science and Applications, under contract No. W-7405-Eng-48. This work was also supported by the Lawrence Livermore National Laboratory Student Employee Graduate Research Fellowship program.   

High resolution 17 to 68 keV K-alpha backlighter for high-energy density experiments .

Backlighters of energy $>$17 keV are needed for many high energy density experiments on new facilities such as NIF and Omega-EP. Such high energy source can be created from hot electron interactions with target materials when illuminated by high intensity short pulse lasers. We carried out experiments to demonstrate that high energy 1-D and 2-D radiography is possible using micro-foils and small micro-wires (10 x 10 x 300 microns long) targets attached to low-Z substrates. We have tested Mo (17 keV), Ag (22 keV), Sm (40 keV) and Au (69 keV) micro-foil and micro-wire targets using the Titan laser at LLNL and utilized them to radiograph laser driven samples. We measure spatial resolutions better than 10 $\mu $m. We also measured K-alpha yields using a single hit CCD camera and a crystal spectrometer up to Pb (78 keV). This paper will present our radiography results and K-alpha source characteristics comparing them with the required signal level for NIF experiments.   

Polarized K-shell radiation due to anisotropic fast electron distribution in ultra-intense laser produced plasmas

In fast ignition research, the transport of fast electrons generated by ultra-intense laser pulses is one of critical issues. To gain insight into the fast electron transport, polarized x-ray spectroscopy has been proposed. At a laser intensity of about $10^{17} W/cm^{2}$, the polarized K-shell radiation was observed [1]. A new time-dependent atomic population kinetics code has been developed [2], and the numerical predictions show that the polarization arises in corona region, and no polarization is found in dense region. An aspect ratio of anisotropic fast electron temperatures associated with longitudinal and transverse directions is correlated with the polarization. In the presentation, the prospect of fast electron transport will be discussed for fast ignition research. [1] H. Nishimura et al., PPCF, 47, B823 (2005). [2] T. Kai et al., PRA, 75, 012703 (2007), T.Kawamura et al., submitted (2007).   

Laser Generated Ion Beams in the Context of Sandia's HEDP Mission

First experiments were performed at the 100\,TW target area of the Z-Petawatt laser characterizing ion beam emission from laser irradiated solid density targets. These experiments addressed radiography and fast ignition concepts to be applied at the Z- Accelerator facility at Sandia National Laboratories. Cu, Al and Au targets were used for Target-Normal-Sheath- Acceleration of protons and heavier ions. Proton energies up to 30\,MeV were measured, and the dependence of ion spectra on varying laser parameters was studied. An imaging proton spectrometer was used to investigate the acceleration due to field-enhancement effects at the target edge (edge emission). The usage of imaging plates was successfully introduced for charged particle beam analysis and compared to CR39 detector results.   

Ion acceleration due to explosions of nanoparticles driven by hot electrons

Plasma expansion into vacuum and resultant ion acceleration is studied both analytically and numerically, where the expansion of an initially uniform nanoparticle with radius $R_{f0}$ and electron density $n_{e0}$ is driven by explosion of thrmal electrons with initial temperature $T_{e0}$. Such key outputs as the energy spectrum, maximum ion kinetic energy, and electrons-to-ions energy transfer efficiency are explicitly given as a function of $R_{f0}$, $n_{e0}$, and $T_{e0}$. The simulation results turn out to be well reproduced by a self-similar solution [Phys. Plasmas Vol.13, 012105 (2006)], which describes an expansion of a finite-size non-quasi-neutral plasma mass into vacuum with a full account of charge separation effects.   

Angular distribution of fast electrons and protons in short pulse laser target interaction

Ultra intense short pulse laser pulses incident on solid targets can generate relativistic electrons either from direct laser electric field acceleration or the ponderomotive force associated with the gradient of the field. These electrons can then generate energetic protons due to the target normal sheath acceleration mechanism. These fast electrons and protons can effectively heat the solid target beyond the region of direct laser interaction and are important aspects of the fast ignition concept. One of the outstanding questions is the directionality of these fast particles, as the physics governing this will ultimately affect the energy transfer efficiency from laser to the compressed core. To this end, we have carried out experiments to systematically measure the angular distributions of fast electrons and protons for laser intensity from 10$^{18}$ to 10$^{20}$ Wcm$^2$ on the Callisto laser at LLNL for a variety of target materials and thickness, using multiple diagnostics. We found highly anisotropic distributions of particles for a variety of experimental conditions.   

Influence of sheath fields on hot electron emission from small foils irradiated by intense laser pulses

Strong sheath fields are excited around a rear surface of foils because hot electrons depart from foils irradiated by intense laser pulses. The field strength can be weakened because of a spreading of field-excited region caused by the charge flow on the foil surface. The field can inhibit the hot electron emission from the foils, therefore, the field strength has an influence on the emission number of hot electrons. The effect of field spreading on the electron emission is studied using foils with different areas. The signal intensity of hot electrons is reduced by 70\% in our experiments when the inscribed radius of foils is reduced from 870 ${\mu}$m to 87 ${\mu}$m, which is much shorter than the laser pulse length (${\sim}$ 200 ${\mu}$m). PIC simulations indicate that the number reduction is caused by a higher sheath potential in a small foil case. In that case, the sheath potential grows quickly and is high for a long period because the field spreading is restricted within the foil area.   

CPA backlighting using 1$\omega $ and 2$\omega $ light

A number of materials modelling and radiation hydrodynamics experiments proposed for high power laser systems have requirements for high-energy ($>$20keV), high resolution ($\sim $10$\mu $m), short duration ($>$100ps) backlighters. One method of producing such a radiation source is the use of CPA lasers to produce fast-electron induced K$\alpha $ fluorescence. The work outlined here covers the use of foils and fibres to produce radiographs of test resolution grids. Ag and Pd targets were illuminated with both 1$\omega $ and 2$\omega $ light from the HELEN CPA laser. Image plates were used to record an image of the test grid, which was used to determine source size and signal to background. An absolutely calibrated transmission crystal spectrograph was used to record the strength of the K$\alpha $ line, and the conversion efficiency calculated. A hard X-ray spectrometer was used to measure the electron temperature. Conversion efficiencies of 10$^{-4}$ were measured for 5$\mu $m thick foil targets, which produced $<$10$\mu $m resolution in one dimension. Slightly lower efficiencies were measured for 10$\mu $m fibre targets, $\sim $10$\mu $m resolution was measured parallel and perpendicular to the laser axis. No measurable effect of pre-pulse on the resolution was measured. The conversion efficiency with S-pol. 2$\omega $ laser light was significantly less than with P, as was the hot-electron fraction. P-pol. 2$\omega $ light produced lower K-alpha conversion efficiency than P-pol. 1$\omega $, but with the advantage of reduced background.   

Cone surface roughness and angle dependence in high intensity laser-plasma interactions

Cones targets of different surface roughness and angle were irradiated with the Thor laser (0.5J, 40fs, 800nm, 7micron focal spot, 3.10$^{19}$W/cm$^2$) at UT Austin. Smooth cones show surface structures of the order of 1-5 microns, while rough cones show surface structures of the order of 15 to 20 microns. A coordinated approach between the precision of laser parameters, available imaging diagnostics and target alignment, as well as optimizing signals on flat targets of similar material and thickness allowed us to systematically align and shoot these conical targets. Optical emission images, x-rays spectra, optical spectra of the emitted light from the tip were recorded. Preliminary data seem to suggest that a rough surface finish is more efficient. As an exploratory approach a few long nose targets were also shot. Experimental data and supporting simulations will be presented.