Session 9Q - Laser-Plasma Coupling, and Beat Wave.
POSTER session, Friday morning, November 15
Exhibit Hall - Concourse Level, Adam's Mark

The realistic simulation of ICF target performance presents several challenges. Laser-plasma interactions (LPI) are particularly difficult to simulate because the evolution is over disparate length and time scales, is strongly nonlinear, turbulent, and because kinetic effects enter importantly. We are developing a fluid-particle hybrid simulation code in which the bulk of the distribution function is treated in a fluid approximation, and the suprathermal particles are treated explicitly in a way similar to standard particle-in-cell (PIC) simulations. The ultimate aim is to develop algorithms which will realistically model the essential physical phenomena described above and which are sufficiently efficient so that they may be incorporated directly into hydro simulation codes. Initial, one-dimensional, results will be reported.

[9Q.02] Theory and Simulation of Parametric Instabilities in Turbulent Plasmas Illuminated by Lasers with Spatial Structure

SOFTSTEP simulations of high frequency parametric instabilities in spatially nonuniform plasmas illuminated by multiple RPP beams is reported. Particular attention is given to the reflectivity of stimulated Raman scattering (SRS) and its saturation. The study of parametric interactions in multidimensional plasmas, illuminated by multiple laser beams are the goals of SOFTSTEP codes, especially as they reflect on conditions that will exist in NIF gas-filled hohlraums.

SOFTSTEP codes are pseudo-spectral and rely on operator splitting techniques and exact propagators for each split step. A filamenting pump model including propagation up to and reflection from a critical layer as well as pump depletion are easily included in the SOFTSTEP algorithm.

[9Q.03] Stimulated Raman scattering from a multi-beam pump

The NIKE KrF laser facility at NRL consists of an array of multiple beams which focus onto a planer target. The finite angle between the beams and the specific geometry gives rise to SRS with one plasma wave and multiple back-scattered waves or a single back-scattered wave with multiple plasma waves. We have developed a 2D code for SRS which has a realistic geometry for the multi-mode pump wave. The code computes the threshold for the onset of SRS averaged over many realizations. The symmetry of the beam geometry with respect to the axis, normal to the planar target, is very important in determining the threshold for SRS. The spectral characteristics of the scattered waves as well the plasma waves for intensities beyond the threshold value is also obtained. We have made a systematic study of the threshold as a function of the number of beams as well as the angle between the beams. The code has been developed to study SRS in homogeneous as well as inhomogeneous plasmas.

[9Q.04] Multi-dimensional Evolution of Stimulated Scattering and Filamentation.

We have constructed a three-dimensional code (F3D) to study the interaction of stimulated back scattering and filamentation instabilities driven by laser beams that have large but statistically well-understood nonuniformity, e.g. at the focal plane of a laser with random phase plates (RPP). In support of gasbag experiments at LLNL with the Nova laser (and reported at this conference) in which the electron density is .05 - .15 n_c (n_c \sim 9 \ \times \ 10^21 cm^-3), the electron temperature is T_e \sim 2-4 keV, and nearly constant over 1 - 2 mm, we have studied the nonlinear behavior of this competition and collaboration between instabilities as a function of laser intensity, laser f-number, ion acoustic damping rate, and electron density. The effects of laser beam smoothing produced with SSD are also examined. Simulations in plasmas with strong flow and density gradients that limit SBS and SRS growth but not filamentation will be compared to the uniform plasma simulations.

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[9Q.05] Backscatter, filamentation and laser light smoothing in flowing plasmas.

The three-dimensional (3-D) code F3D with nonlinear hydrodynamics (C. H. Still et al., BAPS 40), 1823 (1995). is used to examine filamentation and backscatter driven by non-uniform laser beams. Previous work with linearized hydrodynamics demonstrated that both supersonic and subsonic transverse flow deflects the laser beam in the flow direction. (D. E. Hinkel et al., accepted, PRL, June, 1996.) In agreement with analytic estimates, (D. E. Hinkel and E. A. Williams, BAPS 37), 1376 (1992). 3-D simulations of uniform, initially stationary plasma show that laser beam RPP hotspots move, yielding a time averaged laser intensity smoother than the instantaneous pattern. This naturally occurring smoothing will be compared to that in two-dimensional simulations. (A. J. Schmitt and B. B. Afeyan, BAPS 40). 1824 (1995). The influence of 3-D beam structure on filamentation, Brillouin backscatter and beam smoothing will be examined with and without axial and transverse flow.

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[9Q.06] Stimulated Brillouin Scatter in Pic-Fluid Simulations

BZOHAR studies of Stimulated Brillouin Scatter (SBS) in plasma parameter regimes appropriate to NOVA and planned NIF experiments are reported. We compare results from electromagnetic simulations to those with an imposed ponderomotive driver. In the latter simulations we more readily isolate and diagnose those effects associated with nonlinearities in the ion waves which contribute to the saturation of SBS and the resulting SBS reflectivity in the electromagnetic cases.

[9Q.07] Laser hot-spot interaction taking into account the effects of plasma dynamics

We present results of our code KOLIBRI which models the interaction of intense electromagnetic beams with the low-frequency dynamics of the plasma fluid in two or three spatial dimensions without

the restriction of paraxial optics approximation. Two neighboring laser hot

spots modelled by two focused Gaussian beams in an underdense homogeneous

plasma. This can be considered as a reduced model for multiple hot spot

interaction in optically smoothed laser beams. A strong interaction occurs

if the distance d between the beams is smaller than the diameter D of a

single one in the focal region. The merging of the beams occurs even

though the incident power is below the self-focusing critical power. If

the distance between the beams exceeds D, interaction becomes very weak

and both beams behave like two independent ones. The SBS reflectivity is

highly sensitive to the inter-beam distance and their relative phase,

\psi. SBS remains unaffected by the long-wavelength density

modifications caused by the beam merging. However in the case d < D and

\psi < \pi/2, strong gradients in the light intensity caused by

SBS near the side where the laser beam enters enforces the formation of a

single channel.

[9Q.08] Observations of laser beam deflection in transverse flow.*

The formation of density channels in the presence of transverse flow in low-Z and high-Z underdense plasmas has been studied using the Janus laser at LLNL. A 100-ps, 1.06 \mum interaction pulse with a peak intensity of 5\times 10^16 W/cm^2 interacts with plasmas preformed using tilted targets to introduce a transverse flow. The background density profile and the channel formed by the interaction pulse are measured using interferometry. Side-scattered laser light is also imaged to trace the propagation path of the interaction beam. We can systematically vary the laser intensity I_L and the peak electron density n_e to compare experiment to predicted beam deflection scaling. (D.E. Hinkel and E.A. Williams, in press, Phys. Rev. Lett.(1995); W.L. Kruer, BAPS 40), 1824 (1995). The experimental results are modeled using both the linear and nonlinear version of the F3D code (R. Berger et al., Phys. Fluids B 5), 2243 (1993).. \vskip .1in \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.

[9Q.09] Filamentation Dynamics: an Improved Description

We have previously investigated the interaction of lasers and plasmas in the context of axial coupling and flow effects(A.J. Schmitt and B.B. Afeyan, Bull. Am. Phys. Soc. 40), 1824 (1995).. However, because of the paraxial wave approximation used to model the laser, the effects of larger off-axis propagation angles and critical surface interactions could not be properly calculated. We have eliminated these limitations by replacing the laser propagation model with a slow temporal envelope parabolic electromagnetic wave solver based upon the SOFTSTEP algorithm(B.B. Afeyan et al., p2.25, High Temp. Plasma Diag. Conf., Monterey, CA, May 1996.). With this improved description of the light propagation, we study the interaction of filamentation and axial coupling in more realistic plasmas, properly including laser off-axis propagation and reflection. The laser imprinting problem in direct drive laser fusion and the filamentation of crossing beams in high temperature hohlraums are both addressed by these simulations.

[9Q.10] Hybrid PIC Simulations of Stimulated Brillouin Scattering Including Ion-Ion Collisions

We investigate the role of Coulomb collisions in non-linear saturation and heating due to stimulated Brillouin scattering (SBS) in laser heated plasmas. Ion-ion collisions are particularly relevant to SBS from high-Z plasmas, where the collision rate of heated ions can be appreciable compared to the acoustic frequency. Our kinetic modeling makes use of particle-in-cell (PIC) techniques with binary Monte Carlo (MC) collisions. For plasmas composed of high and low-Z ion mixtures (e.g. Au-Be), simulations show that collisions can maintain near-linear damping for finite-amplitude waves by reducing the population of trapped ions. Inclusion of collisions reduces SBS compared to collisionless simulations due to modifications to the non-linearly heated distribution function. For single species plasmas (e.g. Au), collisions reduce the heat flow compared to free-streaming conduction, locally decreasing Z T_e / T_i which in turn reduces SBS. We also present results on numerical heating in hybrid simulations, which is particularly severe for plasmas with Z T_e / T_i >> 1.

[9Q.11] An Analytical Model Of Whole Beam Bending

We propose a simple model for the density response for a laser beam in a plasma that includes the effects of flow and damping. The resulting nonlinear equation for the beam intensity is solved in the limit in which the Random Phase Approximation (RPA) is applicable. The solutions provide an explicit expression for the radius of curvature of the bending of the beam. The differences between the 1-D and the 2-D case are discussed.

\parindent 0pt \vskip 3pt Work supported by the USDOE.

[9Q.12] Laser beam deflection by filamentation in a transversely flowing plasma.

A laser pointing problem discovered in gas-filled hohlraums at NOVA may have its root in the deflection of laser light by filamentation in a transversely flowing plasma. To investigate this phenomenon, a 1.5-times-diffraction-limited, 1.053\,\mu m, f/7 interaction laser beam was focused, at normal incidence, at the density maximum of a preformed, 0.1n_c, 1.0\,keV, quasi-planar, 400\,\mu m-scale-length, exploding-foil CH plasma. The transverse plasma flow relative to the laser beam was precisely set by sweeping the interaction beam across the stationary plasma. The laser spot motion on a gold witness foil was observed with a gated x-ray imager (GXI), both with and without prior beam propagation through the plasma. The interaction laser profile as the beam went through best focus was measured on each shot with equivalent plane imaging. The plasma density and electron temperature were monitored by collecting SRS from one of the plasma-producing beams. The laser beam deflection was thereby measured as a function of plasma transverse flow, laser intensity, and plasma density.

[9Q.13] Hybrid Guiding Center/Paraxial Model For Laser Propagation in Prescribed Density Profiles.

In radiation-hydrodynamics simulations of Inertial Confinement Fusion (ICF) targets, the laser is universally modeled by ray-tracing. For most ICF applications, the ray-trace model is adequate for describing the laser's propagation through, and interaction with, plasmas. However, for ICF applications in which a small perturbation in the laser deposition (due to statistical noise inherent to ray-tracing) can affect the outcome of the simulation significantly, it is desirable to have an alternative model that permits smoother laser deposition. In this paper, an ongoing investigation of such an alternative model is reported. Our approach here to to construct a hybrid model that combines features of both the ray-trace and the paraxial models. The details of this model, and test simulations of a gaussian beam propagating through: (a) a uniform background density, (b) a cylindrically symmetric hollow density profile, and (c) a hollow density profile with an axial dependence such that the laser beam is deflected, will be presented. The goal of this paper is to assess the feasibility of implementing this hybrid guiding center/paraxial model into a radiation-hydrodynamics simulation code. \vskip 0.1truein Work performed under the auspices of the US Department of Energy.

[9Q.14] Strongly-Driven Laser Plasmas with Self-Consistent Electron Distributions

In high temperature hohlraums and many other applications,\footnote For example, see talks by T. Orzechowski and R. Kirkwood (Anomalous Absorption Conference 1996). the laser heated electrons have a zeroth-order distribution function which is quite different \footnote A.B. Langdon, Phys. Rev. Lett 44, 575 (1980); R. Jones and K. Lee, Phys. Fluids 25, 2307 (1992). \footnote For general distribution, see J.P. Matte et al., Plasma Phys. Conference Fusion 30, 1665 (1988). from Maxwellian. The numerous consequences include changes in the Landau damping and instability thresholds, reductions in the inverse bremsstrahlung coefficient, as well as changes in the heat transport, density profiles and atomic physics. In addition, unexpected absorption processes can be introduced. These absorption mechanisms are discussed and illustrated in fluid and PIC simulations.

[9Q.15] Characterization of electron temperature in hohlraum targets by x-ray spectroscopy

Current designs of targets for achieving fusion by indirect drive inertial confinement use gold cavities (hohlraums) that are filled with gas. To test our understanding of the bulk energetics in such designs we have measured the electron temperature at different positions within gas-filled Au hohlraums that are 2.75 mm long and 1.6 mm in diameter. Hydrodynamic simulations indicate that the electron temperature and density profiles can have significant differences. Data include x-ray spectra ( 2 - 4 Åfrom mid-Z dopants as well as x-ray pinhole images of the targets. The analysis reveals a difference of up to 1.5 keV when comparing the temperatures between targets having different density gas fills. These results will be compared with calculations.

[9Q.16] Experimental Investigation of the Electromagnetic Decay Instability*

We report on experiments which measure electromagnetic emission near the plasma frequency from laser produced plasmas at the Nova laser facility. The measurement is motivated by earlier studies\footnote R.K. Kirkwood et. al., submitted to Phys. Rev. Lett. and these proceedings \footnote J.C. Fernandez, et. al., submitted to Phys. Rev. Lett. and these proceedings which indicate that the SRS generated electron plasma wave is stimulating a secondary decay involving an ion wave and a third wave. The Electromagnetic Decay Instability (EDI) is a secondary decay process in which the electron plasma wave decays into both an ion wave and a light wave near ømega_p. Because this instability inhibits the growth of SRS it may affect the fraction of scattered light in a wide variety of laser-plasma experiments. Experiments to measure both SRS and EDI spectra in both thin foils and gas-filled targets will be discussed. \scriptsize *Work performed under the auspices of the U.S. Department of Energy by the Lawrence Livermore National Laboratory under Contract W-7405-ENG-48

[9Q.17] Modeling of Transient X-ray Diffraction from X-ray and Laser Shocked Crystals

The diagnosis of shocked crystals by transient x-ray diffraction is of interest as it allows observation of material response on the lattice level. Recent experiments at LLNL utilising the Nova laser have generated shock waves of approximately 300-kbar in single crystal silicon by using x-rays from a hohlraum target as the drive. X-rays from a separate Ti target were used to temporally resolve the lattice spacing. This type of experiment has the potential to significantly increase our understanding of several issues in fundamental shock wave physics, such as shock front thickness, elastic-plastic response, and polymorphic phase transitions. In addition, in-situ x-ray diffraction from crystals irradiated by spatially modulated laser beams has the potential to yield unique information on laser-imprint and thermal smoothing: such experiments have recently been performed on the Trident laser at LANL. We present here the basic x-ray diffraction models used to model these experiments, along with the results of preliminary simulations.

[9Q.18] The Use of Transient X-ray Diffraction in the Study of Low to Moderate Pressure Shock and Stress Wave Propagation in Solids

Transient x-ray diffraction (TXD) utilising laser plasma (LP) x-ray sources offers many advantages in the study of moderate strength shock and stress waves in solids. One of the chief advantages is the ability to very accurately synchronize the bright, short pulse, LP x-ray source with laser driven shock and stress waves. As a stress wave propagates into a crystalline material the compression changes the lattice spacing thus altering the Bragg diffraction angle. By directly monitoring the diffraction a direct measurement can be made of the dynamic strain. As stronger shocks are propagated in a crystal the transition to plastic flow and eventually melting can be observed. TXD is thus a potentially powerful method for the study of phase transitions and low to moderate pressure properties of materials. TXD measurements are relevant to both direct and indirect drive fusion. For direct drive, TXD may provide a sensitive method for the study of imprinting. For indirect drive, TXD may provide a good method for the measurement of low radiation temperature drive. In the present experiment we discuss the use of TXD in the study of the propagation of a spatially modulated shock wave in a silicon crystal especially as applied to the problem of direct drive imprint.

[9Q.19] Direct-Drive Hugoniot and Simultaneous Off-Hugoniot EOS Measurements*

A program was recently initiated to measure the equation of state (EOS) of deuterium using shocks driven by the NIKE KrF laser at the Naval Research Laboratory. The DT EOS is important, for example, in ignition capsule design; and significant changes in laser pulse shape are required depending on which EOS model is used. Detailed feasibility studies using 1- amp; 2-D hydro codes have been conducted. It is found that the off-Hugoniot EOS of likely launcher materials, such as Al, is not known with sufficient accuracy for an impedance match measurement of D_2; therefore direct measurements of particle speed (u_p) and shock speed (u_s) are required. Better than 2 \mum spatial resolution is needed for transverse x-radiographic measurement of u_p, and to this end a broadband (1--1.5 keV) 1-D x-ray microscope, with \simeq 1 \mum resolution has been designed for coupling to an x-ray streak camera. In principle, transverse radiography will yield measurements of u_p, u_s, and mass density using a single streak camera. The addition of a face-on visible streak camera will allow simultaneous measurement of off-Hugoniot EOS points in the Al launcher. Such information is crucial to the validity of the impedance match method. *Work supported by the US DOE.

[9Q.20] Design and Simulation of 100 MeV - 1 GeV Plasma Accelerator Experiments*

Recently acceleration of order 109 electrons up to energies as high as 100 MeV in 2 mm of plasma was demonstrated in a laser-driven experiment at Rutherford Laboratory in the UK. These results lay a foundation for proposing experiments in the 100 MeV to 1 GeV energy range. We use PIC simulations to model the physics of near term experiments in this range based on three different driving schemes. These are (i) single frequency lasers, (ii) two-frequency laser beat waves and (iii) particle beams. The single-frequency laser drivers evolve in a complicated way via relativistic self-focusing and Raman forward and sidescatter instabilities. The beat wave scenario produces very clean accelerating structures. To reach 3 100 MeV energies, the beating lasers must propagate over distances on the order of a Rayleigh length. On this scale, the laser evolves significantly and we model the effect of laser focusing and pump depletion on the phase of the bucket and on test particles' energy gain and quality. Finally, we consider acceleration driven by particle beams. The possibility of 1 GeV acceleration experiment over 1 meter of low density plasma (no = 2.5 x 1014 cm-3, lp = 2mm) using the Stanford Linear Collider (SLC) beam as a driver is explored. The advantage of the SLC beam is that at such high energy (50 GeV) it is very stiff and impervious to instabilities, phase slippage, etc. A fairly simple and clean acceleration test based on measuring the energy gain of the tail of the SLC beam is proposed and modeled.

*Work supported by AFOSR Grant #F4 96200-95-0248 and DOE Grant # DE-FG03-92ER40745.

[9Q.21] Laser Acceleration of Electrons in Vacuum, Gases, and Plasmas.

The general features of electron acceleration by lasers will be discussed.(E. Esarey et al., Phys.\ Fluids B \underbar 5), 2690 (1993); Phys.\ Rev.\ E \underbar52, 5443 (1995); P. Sprangle, E. Esarey, J. Krall, Phys.\ Plasmas \underbar 3, 2183 (1996). In vacuum, laser acceleration can be divided into two classes: (i) direct, in which the accelerating gradient E_acc is linearly proportional to the laser field, and (ii) ponderomotive, in which E_acc is proportional to the square of the laser field (e.g., the vacuum beat wave accelerator). Vacuum acceleration is limited by phase slippage, since the phase velocity v_p>c for a focused laser beam, and by laser diffraction. In the optically-guided inverse Cherenkov accelerator, a background gas is introduced to adjust v_p\leq c and to guide the laser beam. At higher intensities, the gas starts to ionize, and the guided laser is subject to an ionization-modulation instability. This limits both E_acc (<1 GeV/m) and the interaction distance. Plasmas can support ultra-intense laser fields and large amplitude plasma waves with v_p10 GeV/m, as in the laser wakefield accelerator. In addition, the laser pulse can be guided using plasma channels and relativistic effects.

[9Q.22] Vacuum Beat Wave Acceleration

A vacuum beat wave accelerator (VBWA), in which two focused laser beams of differing wavelengths generate a beat wave that can impart a net acceleration to particles, is simulated and analyzed. The mechanism relies on the v \times B force, thus circumventing the Lawson-Woodward theorem. It is shown that the single-stage energy gain of the VBWA is limited by diffraction of the laser beams, particle slippage and radial walk-off. In the simulations the particles are synchronous with the beat wave for a short interval of time and the energy gain has the nature of an impulse delivered near the focal point. Simulations show that the problem of walk-off may be ameliorated by using a converging beam of particles, as naturally occurs for injection of a finite-emittance beam. For a 4.5 MeV beam and terawatt-level laser the energy can be increased to 12.5 MeV over a distance of under 4 mm, with a peak acceleration gradient > 15 GeV/m and an estimated trapping fraction of 1%. Scaling is illustrated by increasing the injection energy to 50 MeV.

[9Q.23] Wake Fields in Plasma Channels with Arbitrary Transverse Density Profiles

We examine wake fields produced by an intense, short laser pulse propagating in a plasma channel which has an arbitrary (continuous) density profile. Previous theoretical studies of plasma wakes in channels have considered either step-function density profiles, for which there is an exact expression for the wake, or, alternatively, parabolic profiles for which the wake is only computed approximately. Here we solve for the exact wake eigenmode taking into full account the channel profile. A consequence of a general channel profile is a spatially dependent plasma frequency; thus in a temporal Fourier decomposition, there exists the possibility of a resonance between the mode frequency and the plasma frequency. This resonance is manifest in the presence of (typically regular) singular points in the differential equation for the wake field amplitude. To obtain an accurate solution for wake eigenmode, such singular points must be handled with care. We present detailed analysis of the transverse structure of the wake for a wide range of experimentally accessible channel profiles.

[9Q.24] Laser Driven Electron Acceleration in Vacuum and Neutral Gases*

This presentation will cover the important principles and issues pertaining to laser acceleration of electrons in vacuum and neutral gases [1]. The effects of electron slippage, driffraction, instabilities, ionization and material damage will be discussed. For the case of acceleration in vacuum, a crossed laser beam accelerator and vacuum beat wave acelerator configuration are analyzed. For acceleration in neutral gas, a self-guided inverse Cherenkov accelerator (ICA) is analyzed. Finally, it is shown that for a fixed total laser power the energy gain in the conventional ICA can be significantly increased by using a nonideal Bessel (axicon) laser beam instead of a conventional Gaussian beam. * Work supported by Department of Energy and Office of Naval Research. [1] "Laser Driven Electron Acceleration In Vacuum, Gases, and Plasmas", P. Sprangle, E. Esarey, and J. Krall, Phys. Plasmas 3, 2183 (1996); "Vacuum Laser Accelerqation', P. Sprangle, E. Esarey, J. Krall, and A. Ting, Optics Communication 124, 69 (1996); "Laser Acceleration of Electrons in Vacuum", E. Esarey, P. Sprangle, and J. Krall, Phys. Rev. E 52, 5443, (1995).

[9Q.25] Electron Acceleration by a Laser Pulse in a Plasma

The motion of a charged particle in an electromagnetic field is a well-known paradigm of physics. Suppose that the field is associated with a laser pulse of finite extent propagating in a vacuum. As the pulse overtakes the particle, the particle gains energy and momentum. However, the oscillatory energy of the particle is wasted, and it is difficult to extract the particle from the pulse. We have found an exact analytic solution for the motion of an electron under the influence of a circularly polarized laser pulse in a plasma. This solution shows that a pre-accelerated electron can be accelerated efficiently and extracted easily. Although the pulse tends to generate a plasma wake, to which it loses energy, one can eliminate the wake by choosing the duration of the pulse judiciously. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC03-92SF19460.

[9Q.26] Acceleration of Background Plasma Electrons in the Self-Modulated Laser Wakefield Accelerator

The beating beween the primary laser pulse and a backward going wave in the laser wakefield accelerator (LWFA) can accelerate a small fraction of the plasma electrons to moderate energies where they can be trapped and further accelerated by the large amplitude wakefield. This process has been studied numerically by following the motion of a large number of test particles in analytically-prescribed fields. In practice, the backward wave could arise naturally from Raman backscatter, or it could be injected externally using a second laser pulse at or near the pump frequency. Threshold values of the primary laser, wakefield, and backward wave amplitudes required for significant electron acceleration have been determined, and implications for the NRL LWFA experiment will be discussed.(Supported by the Dept. of Energy and the Office of Naval Research)

[9Q.27] Simultaneous measurement of high energy electrons and plasma wave characteristics in the Self Modulated Laser Wakefield Accelerator (LWFA)

Multiple diagnostics for simultaneous plasma wave characterization and high energy electron measurement have been implemented in the LWFA at the NRL. These diagnostics allow determination of the optimal plasma wave characteristics for acceleration of background electrons. The plasma wave diagnostics include: Coherent Thomson scattering of a sub-picosecond variable-delay probe laser, forward and backward Raman scattering of the pump laser, and 90 degree imaging of Thomson scattered light from the plasma. Coincident with the plasma wave diagnostics, the acceleration of background electrons is investigated with a magnetic electron spectrometer. The results of these experiments will be presented. The incorporation of a 4.5 MeV RF electron gun (currently under construction) for injection into the wakefields will also be discussed.

[9Q.28] Relativistic laser pulse self-focusing*

The relativistic self-focusing behavior of laser radiation with power much larger than the critical power (1.6 1010 (nc/n)1/2 Watts in underdense plasmas) was investigated. Tight stable channels which could confine an arbitrarily large power were found to form. The characteristic diameter of these channels was on the order of a few wavelengthes. The dynamics of radiation filled channels including the effects of "electron cavitation" was investigated. Effects of plasma inhomogeneity, laser beam profile and prefocusing were evaluated. The stability of axially symmetric relativistic self-channeling was confirmed by 3D numerical simulations.

* Work at LLNL performed under the auspices of the U.S. Department of Energy under contract No. W-7405-ENG-48.

[9Q.29] Microwave Reflections from a VUV Laser Produced Plasma Sheet

A Vacuum Ultra-Violet (VUV) Laser is utilized for creation of a plasma sheet in a low-ionization-energy organic gas. Microwaves from an X-band horn antenna impinge on the sheet where they are reflected. A bi-static antenna system is used for detecting the microwave radiation. Heterodyne and homodyne detection systems are investigated. Reflected signals are measured for amplitude and phase analysis. Comparable amplitude and phase shifts are noted when compared to an aluminum conducting sheet placed in the same position as the plasma. The working gas is tetrakis(dimethylamino)ethylene (TMAE) with an inonization energy of 5.36 eV. The ionizing source is a VUV excimer laser (W_max = 20 mJ, \tau = 17 ns) operating at 193 nanometers (6.4 eV). A plasma sheet having a peak density 2.5 \times 10^13 cm^-3 and T_e = 0.8 eV is formed by passing the laser beam through a series of VUV coated Suprasil lenses. The dimensions of the plasma are 0.2 - 2.0 cm \times 8.5 cm \times 30 cm. Comparison between the experimental results and computer models show that this system is attractive for use as a microwave reflector. We also present recent research utilizing a new, higher power (230 mJ/pulse) laser and explore operation at a lower TMAE neutral pressure which will allow more axially uniform plasmas with longer lifetimes.

Part 9 of program listing