Session 6E - High Intensity Laser Plasma Interactions.
ORAL session, Wednesday afternoon, November 13
Majestic Ballroom, Adam's Mark

Results of 3D and 2D fully explicit electromagnetic relativistic particle-in-cell (PIC) simulations are presented (also as video) for ultra-short laser pulses with intensities of I\lambda^2 = 10^18 - 10^20 Wcm^-2\mum^2, propagating in near-critical plasma. Strong currents of relativistic electrons are obtained in the direction of laser propagation. They generate quasistatic magnetic fields as high as 100 MG which influence the relativistic filamentation/self-focusing and lead to coalescence of laser filaments. A single magnetized superchannel is formed containing a major part of the laser power. 3D PIC simulations show that a laser pulse incident with 10^19 W/cm^2 produces intensities as high as 2\times 10^20 W/cm^2 on the channel axis. Plasma cavitation and ion channel formation are observed. Being generated in overdense plasma regions behind the critical surface, the magnetic field collimates the fast electrons. Efficiency of conversion into fast electrons and the solid angle into which they are emitted are investigated; both are critical issues for the fast ignition of precompressed fusion cores.

[6E.02] Experimental Studies of the Propagation of Ultrashort, Intense Laser Pulses in Underdense Plasmas.

The propagation of ultrashort (270 fs), ultraviolet (248 nm), high power (1 TW) laser pulses focused to relativistic intensities (10^19W/cm^2) in underdense plasmas has been examined. The propagation path of the laser pulse was imaged using two methods. In one case a lens system was used to image the Thomson scattered light from the plasmas. These results include a direct measurement of the width of the channel formed by the relativistic and charge-displacement process. In the second case, a CCD/pinhole camera was used to obtain single-shot, spatially-resolved images of the X-ray emitting regions of the plasmas. By comparing the spatial intensity distribution of the X-ray signals in the plasmas with the predicted laser pulse intensity distribution obtained with a numerical simulation of the propagation, it is concluded that relativistic and charge-displacement-induced self-channeling of the laser pulses has been observed. The observed channel lengths were \sim25 times the Rayleigh range of the optical system used in the experiment and the peak intensity in the channel was estimated to be \sim10^20 W/cm^2.

[6E.03] Experimental Evidence of Photon Acceleration of Ultra-Short Laser Pulses in Relativistic Ionization Fronts

We present the results of the collision of a weak ultra short probe pulse (620 nm, 65fs, \approx \muJ) with a relativistic ionization

front (interface gas-plasma) created by an ultra intense laser pulse (620 nm, 65 fs, 5 mJ). Frequency up-shifts of the probe pulse are observed in co (20 degrees) and counter-propagation (160 degrees). The comparison between the shifts in the two directions allowed us to estimate the velocity of the front and the maximum electron density. These results are confirmed by measurements of Moir=E9 interferometry. The measured up-shifts are in good agreement with 2-D ray tracing simulations.

[6E.04] Brillouin Scattering of Picosecond Laser Pulses in Preformed, Short-Scale-Length Plasmas

Brillouin scattering (BS) has been studied in short-scale-length, preformed plasmas. The backscattered and specularly reflected light resulting from the interaction of high-power picosecond pulses with preformed silicon plasmas has been measured. A first laser pulse forms a short-scale-length plasma -- without significant BS -- while a second delayed pulse interacts with an expanded, drifting underdense region of the plasma with density scale length (0 \leq L_n \leq 600 \lambda _L). The pulses are generated at \lambda _L = 1054 nm, with intensities up to 10^16 W/cm^2. The backscattered light spectra, threshold intensities, and enhanced reflectivities have been determined for different plasma-density scale lengths and are compared to Liu, Rosenbluth, and White's(C. S. Liu, M. N. Rosenbluth, and R. B. White, Phys. Fluids 17, 1211 (1974).) WKB treatment of stimulated Brillouin scattering in inhomogeneous drifting plasmas. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC03-92SF19460.

[6E.05] Spectroscopic analysis of thin foils heated with ultra short laser pulses^+

In the present set of experiments, we examine the competition between cooling due to expansion versus thermal conduction and the effect of the target thickness on line shape. The experiment is done using thin aluminum foils of successive thickness ranging from 250 Å\, to 1250 Å\,. The foils were heated with a 400 nm,

150 fs (FWHM) ultra short pulse laser. The laser energy was approximately 200 mJ and was focused to a spot size of 3 \mu\,m, resulting in a peak intensity of \,1.9\,\times\,10^19\,W/cm^2\,. The prepulse to main pulse contrast was determined to be better than \,10^-7\,. The \,1s^2-1s2p\,, \,1s^2-1s3p\, transitions in He-like aluminum and the \,1s-2p\, transitions in H-like aluminum were temporally resolved using a 900 fs x-ray streak camera. These spectral lines are used to determine the effects of target thickness, and hence thermal conduction, on the rate of cooling in the plasma. A simple model is used to interpret the experiment. We present the data and findings from this study. \hrule width4cm \vskip .1in + Work performed under the auspices of the U.S. Dept. of Energy by Lawrence Livermore National Laboratory under contract W-7405-ENG-48.

[6E.06] Picosecond time-resolved XUV absorption spectroscopy of solid target plasmas created by 100-fs laser pulses

We present measurements of plasma dynamics initiated by 100-fs laser-solid interactions using broadband XUV absorption spectroscopy with picosecond time resolution. A 10-Hz, 100 mJ Ti:Sapphire laser system is used to create a continuum of XUV radiation from a gold plasma (30 Å\lambda < 300 Åto act as a probe source for the characterization of solid-target sample plasmas produced by the same laser. The ultrashort XUV probe pulse is 5-20 ps long, and its duration can be controlled by adjusting the laser intensity on the gold target (J. Workman et al, Phys. Rev. Lett. 75, 2324 (1995)). The samples are thin films of aluminum or boron (1000 Åand we use L or K edge-shift absorption spectroscopy in a pump-probe geometry to infer the dynamics of the compression and relaxation waves as they move through the sample. These measurements are made for several different laser intensities. Corresponding hydrodynamic simulations will also be presented.

[6E.07] Simulations of K-shell Photoabsorption in Solid Density Plasmas

\hyphenationLASNEX

Only sparse experimental data currently exists for the equation of state of high-density materials heated to temperatures of 10's of eVs. In particular are questions of the ionization balance, line shapes, and spectral shifts of absorption edges and line positions.

Using the 100-fs timescale of the laser heating pulse, ultrashort-pulse lasers heat material inside layered targets to 10's of eVs .

Targets are thin foils of Be embedded in plastic. Plama conditions in the Be depend on the thickness of the layers and choice of laser parameters.

We use the hydrodynamics code LASNEX to simulate the heating, energy conduction, and shock formation in targets heated by a 100-fs laser. Using the LTE opacity code OPAL, we calculate the absorption spectra for Be K-shell. Together, the OPAL and LASNEX codes predict changes in the Be absorption spectra over several picoseconds as the Be layer heats and then cools.

\indent *Work performed under the auspices of the U.S. Dept. of Energy by the Lawrence Livermore National Laboratory under Contract No. W-7405-Eng-48.

[6E.08] Characterization of a self-modulated laser wakefield using time resolved forward Thomson scattering

Because the electrostatic fields present in large amplitude Langmuir plasma waves can approach atomic scale values and exceed those achievable in conventional accelerators, plasma based accelerators have received considerable attention as compact sources of high energy electron pulses. In this paper, we demonstrate that the temporal envelope of plasma density oscillations in the wake of an intense (I \sim 4 \times 10^18 W/cm^2 , \lambda=1 \mum) laser pulse (400 fs) can be measured using forward Thomson scattering from a copropagating, frequency doubled probe pulse. From a measurement of the Thomson scattering efficiency of the probe light as a function of its delay with respect to the pump pulse, the plasma density oscillations in a fully ionized helium plasma (n_e=3\times 10^19cm^-3) are observed to have maximum amplitude (\deltan_e/n_e \sim 0.1) after the peak of the pump pulse and extend for \sim 1.5 ps after the peak of the pump pulse. The onset of electron acceleration (2 MeV) coincides with the appearance of spectral modulation on both the first and second order Thomson scattered satellites of the probe pulse and the forward scattered Raman shifted light from the pump pulse.

This work is supported by the US DOE and the NSF.

[6E.09] High Intensity Light Absorption and Picosecond X-ray Radiation from Microstructured Surfaces

We have characterized absorption and x-ray conversion efficiency in a novel type of nanostructured metallic surface used as a target for high-intensity laser interaction. The infrared to x-ray conversion efficiencies from the interaction of an intense 1-ps laser pulse with this nanowire structure are factors of 4 and 5 times greater than for comparable flat targets in the spectral ranges around 125 eV and above 1 keV respectively. A comparison of x-ray yield from this new nanostructure and more conventional smoke and grating surfaces was made: streak camera measurements in the 125 eV range show highest intensity emission from the nanowire target; the duration (approx. 25ps) is one third of that measured from a comparable smoke target. The duration of keV emission was less than our resolution of 8 ps. We will describe the spatial distribution of the x-ray emission, the yield dependence on the laser angle of incidence and polarization, and recent laser-light absorption measurements.

[6E.10] Imaging of Supersonic Radiation Transport Driven Ionization Waves in Solid Targets

We have used ultra-short optical probing to time and space resolve the propagation of laser driven ionization waves into solid fused silica targets with picosecond resolution. We generate an ionization wave with a 2 ps pulse from an Nd:Glass CPA laser system focused onto a solid at an intensity of 10 ^17 Wcm ^2. We find that its velocity within the target (10 ^9 cm/s) exceeds that expected by simple electron thermal conduction by over an order of magnitude. We see penetration of the ionization wave 100 \mum into the cold solid in less than 20 picoseconds, and find that this velocity is consistent with radiation driven transport rather than electron thermal transport.

[6E.11] Sideward Stimulated Raman Scattering of a Short Laser Pulse in a Leaky Waveguide

A significant amount of sideward SRS was observed in recent PIC simulations of the propagation of a short laser pulse in a plasma.(K-C. Tzeng, W. B. Mori, and C. D. Decker, Phys. Rev. Lett. 76, 3332 (1996).) When the pulse propagates in a homogeneous plasma, the rate at which sideward SRS grows is limited by the convection of the Stokes wave through the side boundary of the pulse.(C. J. McKinstrie et al., Phys. Rev. E 51, 3752 (1995).) When the laser pulse propagates in a plasma waveguide, the outgoing Stokes wave is partially reflected. These reflections allow sideward SRS to become an absolute instability. The spatiotemporal evolution of sideward SRS is described in detail for both cases. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC03-92SF19460.

[6E.12] Vacuum Interaction of Relativistic Electrons with Ultrahigh Intensity Laser Fields*

The covariant dynamics of a point electron interacting with a classical electromagnetic field is studied theoretically, at arbitrary field strengths, within the context of the Dirac-Lorentz equation. The radiation damping term is identified with the energy-momentum scattered by the electron, while the Schott term corresponds to pump field depletion. The electron recoil due to radiative corrections is derived, as well as the nonlinear Doppler shift due to radiation pressure. Consequences are investigated for nonlinear Compton scattering experiments and the gamma-gamma collider. The equation of motion of a single electron interacting with an ultrahigh intensity laser pulse in vacuum are integrated both in the case of the Lorentz force, and for the Dirac-Lorentz equation. Stable trajectories are obtained in the latter case by integrating the system backwards in time. Connections to laser acceleration, time-symmetrical QED and nonlocal effects will also be discussed.

*Work supported in part by DoD/AFOSR (MURI) F49620-95-1-0253, AFOSR (ATRI) F30602-94-2-001, ARO DAAHO4-95-1-0336 and LLNL/LDRD DoE W-7405-ENG-48

Part 6 of program listing