The consequences of small scale-length precursor plasmas on high-intensity laser-driven relativistic electrons are studied via experiments and simulations. Longer scale-length plasmas are shown to dramatically increase the efficiency of electron acceleration, yet, if too long, they reduce the coupling of these electrons into the solid target. Evidence for the existence of an optimal plasma scale-length is presented and estimated to be from 1 to 5μm. Experiments on the Trident laser (I=5×10(19)W/cm(2)) diagnosed via Kα emission from Cu wires attached to Au cones are quantitively reproduced using 2D particle-in-cell simulations that capture the full temporal and spatial scale of the nonlinear laser interaction and electron transport. The simulations indicate that 32%±8%(6.5%±2%) of the laser energy is coupled into electrons of all energies (1-3 MeV) reaching the inner cone tip and that, with an optimized scale-length, this could increase to 35% (9%).

Experiments on the Titan laser (∼150 J, 0.7 ps, 2 × 1020 W cm−2) at the Lawrence Livermore National Laboratory were carried out in order to study the properties of fast electrons produced by high-intensity, short pulse laser interacting with matter under conditions relevant to Fast Ignition. Bremsstrahlung x-rays produced by these fast electrons were measured by a set of compact filter-stack based x-ray detectors placed at three angles with respect to the target. The measured bremsstrahlung signal allows a characterization of the fast electron beam spectrum, conversion efficiency of laser energy into fast electron kinetic energy and angular distribution. A Monte Carlo code Integrated Tiger Series was used to model the bremsstrahlung signal and infer a laser to fast electron conversion efficiency of 30%, an electron slope temperature of about 2.2 MeV, and a mean divergence angle of 39°. Simulations were also performed with the hybrid transport code ZUMA which includes fields in the target. In this case, a conversion efficiency of laser energy to fast electron energy of 34% and a slope temperature between 1.5 MeV and 4 MeV depending on the angle between the target normal direction and the measuring spectrometer are found. The observed temperature of the bremsstrahlung spectrum, and therefore the inferred electron spectrum are found to be angle dependent.

A 2-D multi-stage simulation model incorporating realistic laser conditions and a fully resolved electron distribution handoff has been developed and compared to angularly and spectrally resolved Bremsstrahlung measurements from high-Z planar targets. For near-normal incidence and 0.5-1 × 1020 W/cm2 intensity, particle-in-cell (PIC) simulations predict the existence of a high energy electron component consistently directed away from the laser axis, in contrast with previous expectations for oblique irradiation. Measurements of the angular distribution are consistent with a high energy component when directed along the PIC predicted direction, as opposed to between the target normal and laser axis as previously measured.

We show that the interaction of relativistic-intensity, picosecond laser pulses with solid targets is affected by the reflected light through the strong currents and 10(4) T magnetic fields it produces. Three-dimensional particle-in-cell simulations, with the axisymmetry broken by a small angle of incidence, show that these magnetic fields deflect the laser-accelerated electrons away from the incident laser axis. This directly impacts the interpretation of electron divergence and directionality in applications such as laser-driven ion acceleration or fast-ignition inertial fusion.

The effect of target material on fast-electron transport is investigated using a high-intensity (0.7 ps, 10(20)  W/cm2) laser pulse irradiated on multilayered solid Al targets with embedded transport (Au, Mo, Al) and tracer (Cu) layers, backed with millimeter-thick carbon foils to minimize refluxing. We consistently observed a more collimated electron beam (36% average reduction in fast-electron induced Cu Kα spot size) using a high- or mid-Z (Au or Mo) layer compared to Al. All targets showed a similar electron flux level in the central spot of the beam. Two-dimensional collisional particle-in-cell simulations showed formation of strong self-generated resistive magnetic fields in targets with a high-Z transport layer that suppressed the fast-electron beam divergence; the consequent magnetic channels guided the fast electrons to a smaller spot, in good agreement with experiments. These findings indicate that fast-electron transport can be controlled by self-generated resistive magnetic fields and may have important implications to fast ignition.

Submitted for the DPP09 Meeting of The American Physical Society Simulations of relativistic positron creation using ultra-intense, short pulse lasers1 S.C. WILKS, H. CHEN, C.D. CHEN, S.N. CHEN, J. GRONBERG, LLNL, J. MYATT, D.D. MEYERHOFER, LLE, G. GREGORI, C.D. MURPHY, J. MITHEN, CCLR RAL and University of Oxford, D. WELCH, Voss Sci. — The recent generation of positrons using ultra-intense lasers will be discussed in detail [1]. Although good agreement between theory and experiment for the number of positrons created is obtained [2], the positron peak appears to be shifted in energy by several MeV, depending on the number of electrons that are heated by the laser. We attribute this shift to the TNSA mechanism [3], and will present simulation results consistent with this hypothesis. Indications are that even with targets a few millimeters thick, the electric field on the rear of the target can be ∼10 MeV/micron. Particle-in-cell and Monte Carlo simulations of the process have been performed in an attempt to maximize the positron production, and these results will be presented. [1] H. Chen, et al., PRL 102, 105001 (2009). [2] J. Myatt, et al., PRE 79, 066409 (2009). [3] S.C. Wilks, et. al., Phys. Plasmas 8, 542 (2001). 1Prepared by LLNL under Contract DE-AC52-07NA27344. LLNL-ABS-414748.

A novel time-resolved diagnostic is used to record the critical surface motion during picosecond-scale relativistic laser interaction with a solid target. Single-shot measurements of the specular light show a redshift decreasing with time during the interaction, corresponding to a slowing-down of the hole boring process into overdense plasma. On-shot full characterization of the laser pulse enables simulations of the experiment without any free parameters. Two-dimensional particle-in-cell simulations yield redshifts that agree with the data, and support a simple explanation of the slowing-down of the critical surface based on momentum conservation between ions and reflected laser light.

We report on a numerical study of the effects of preplasma scale length and laser intensity on the hot-electron (≥1 MeV) divergence angle using full-scale 2D3V (two dimensional in space, three dimensional in velocity) simulations including a self-consistent laser-plasma interaction and photoionization using the particle-in-cell code LSP. Our simulations show that the fast-electron divergence angle increases approximately linearly with the preplasma scale length for a fixed laser intensity. On the other hand, for a fixed preplasma scale length, the laser intensity has little effect on the divergence angle in the range between 10(18) and 10(21) W/cm(2). These findings have important implications for the interpretation of experimental results.

The effect of increasing prepulse energy levels on the energy spectrum and coupling into forward-going electrons is evaluated in a cone-guided fast-ignition relevant geometry using cone-wire targets irradiated with a high intensity (10(20) W/cm(2)) laser pulse. Hot electron temperature and flux are inferred from Kα images and yields using hybrid particle-in-cell simulations. A two-temperature distribution of hot electrons was required to fit the full profile, with the ratio of energy in a higher energy (MeV) component increasing with a larger prepulse. As prepulse energies were increased from 8 mJ to 1 J, overall coupling from laser to all hot electrons entering the wire was found to fall from 8.4% to 2.5% while coupling into only the 1-3 MeV electrons dropped from 0.57% to 0.03%.

Author(s): Jarrott, Leonard Charles | Abstract: Fast Ignition (FI) is a form of Inertial Confinement Fusion where the compression phase and the ignition phase are separated. In this scheme, a radially symmetric configuration of driver beams composed of either direct laser illumination or laser produced x-ray radiation are used to isochorically compress the fuel shell to 300g/cc. Once the fuel is assembled, a high-intensity ignition beam (short pulse'') is used to generate relativistic electrons which then transport to the assembled fuel and deposit their energy which then causes it to ignite, initiating a thermonuclear burn wave to propagate throughout the rest of the assembled fuel. Understanding relativistic electron generation and transport is extremely important for the development and success of the FI scheme. Previous integrated FI experiments measured neutron yield enhancements to infer increased energy delivered to the compressed core by relativistic electrons generated from the short-pulse laser. While this method has demonstrated enhancements in core heating, the exact location of neutron generation continued to be model dependent and the trajectory of these relativistic electrons was not investigated, leaving questions about the processes that influence energy coupling. In this work, first-ever experimental observations of the spatial energy deposition of cone-guided relativistic electrons into an imploded FI plasma core are reported. Utilizing a new experimental platform that has been developed on the OMEGA Laser Facility, the spatial energy deposition of these relativistic electrons was characterized via relativistic electron induced K-alpha fluorescence from a copper tracer added to a deuterated plastic FI shell. Two-dimensional images of the copper K-alpha fluorescence were obtained using a spherically bent Bragg crystal. The data show Cu K -alpha emission from a 300 micron region surrounding the cone tip, correlating well with the predicted core size. Also, copper K-alpha emission was seen to be produced away from the cone tip, along the cone walls, indicative of a large pre-formed plasma within the cone as a result of the OMEGA-EP pedestal pulse. To validate experimental findings, relativistic transport simulations were carried out utilizing a retrograde analysis of the relativistic electron conversion efficiency, divergence, generation position and temperature. The simulated copper K-alpha spatial distribution was then compared with experimental findings to examine the sensitivity each of these parameters on the short pulse energy coupling efficiency to the assembled core. These findings helped facilitate new target designs and implosion dynamics which resulted in a factor of four improvement to the energy coupling and also defines a clear path towards high coupling efficiencies on large-scale laser facilities

Measurements of fast electron temperature and conversion efficiencies using bremsstrahlung x-rays emitted from laser-produced, fast electrons are presented. Experiments were carried out using the Titan laser (150 J, 1.5 ps) at the Lawrence Livermore National Laboratory. The maximum intensity was 2 × 1020 W/cm2 on 250 μm thick silver foil targets. The emission of bremsstrahlung x-rays from the fast electrons in the target was measured using a filter-stack based detector. The conversion efficiency of laser energy into fast electrons and the electron temperature were studied as a function of incident laser energy. Several models of the electron divergence angle were investigated, and the effect of the assumed divergence angle on the inferred conversion efficiency was quantified. This allows for upper and lower bounds on the conversion efficiency to be established for a range of possible divergence angles. The value for upper bound is 60% (from a 75° divergence angle model) and for the lower bound is 25% (from a 0° divergence angle model).