J. Peebles

Reflective multilayer optic as hard X-ray diagnostic on laser-plasma experiment

A multilayer-based optic was tested for use as an X-ray diagnostic on a laser-plasma experiment. The multilayer optic was employed to selectively pass X-rays between 55 and 100 keV. An order of magnitude improvement in signal-to-noise ratio is achieved compared to a transmission crystal spectrometer. A multilayer response model, taking into account the source size and spectral content, is constructed and the outlook for application above 500 keV is briefly discussed. LLNL-JRNL-664311.

Investigation of the hard x-ray background in backlit pinhole imagers.

Hard x-rays from laser-produced hot electrons (>10 keV) in backlit pinhole imagers can give rise to a background signal that decreases signal dynamic range in radiographs. Consequently, significant uncertainties are introduced to the measured optical depth of imaged plasmas. Past experiments have demonstrated that hard x-rays are produced when hot electrons interact with the high-Z pinhole substrate used to collimate the softer He-α x-ray source. Results are presented from recent experiments performed on the OMEGA-60 laser to further study the production of hard x-rays in the pinhole substrate and how these x-rays contribute to the background signal in radiographs. Radiographic image plates measured hard x-rays from pinhole imagers with Mo, Sn, and Ta pinhole substrates. The variation in background signal between pinhole substrates provides evidence that much of this background comes from x-rays produced in the pinhole substrate itself. A Monte Carlo electron transport code was used to model x-ray production from hot electrons interacting in the pinhole substrate, as well as to model measurements of x-rays from the irradiated side of the targets, recorded by a bremsstrahlung x-ray spectrometer. Inconsistencies in inferred hot electron distributions between the different pinhole substrate materials demonstrate that additional sources of hot electrons beyond those modeled may produce hard x-rays in the pinhole substrate.

Experimental Analysis of the Acceleration Region in Tungsten Wire Arrays

We present the first analysis of the ablated plasma flow acceleration region in tungsten cylindrical wire arrays within 1 mm of the wire core. We apply a recently developed modification to the Lebedev rocket model to infer the 2-D distribution of effective velocities which redistribute the array mass as a function of time. From these data, it is possible to directly observe the acceleration region in a wire array. Analysis of radiography data from the 1-MA Cornell Beam Research Accelerator machine suggests a region of rapid acceleration extending up to 300 μm from the wire core in 16 wire tungsten arrays.

Target material dependence of positron generation from high intensity laser-matter interactions

The effective scaling of positron-electron pair production by direct, ultraintense laser-matter interaction is investigated for a range of target materials and thicknesses. An axial magnetic field, acting as a focusing lens, was employed to measure positron signals for targets with atomic numbers as low as copper (Z = 29). The pair production yield was found to be consistent with the Bethe-Heitler mechanism, where the number of positrons emitted into a 1 steradian cone angle from the target rear was found to be proportional to Z2. The unexpectedly low scaling results from Coulomb collisions that act to stop or scatter positrons into high angles. Monte Carlo simulations support the experimental results, providing a comprehensive power-law scaling relationship for all elemental materials and densities.

Impact of pre-plasma on fast electron generation and transport from short pulse, high intensity lasers

Previous experiments and modeling examining the impact of an underdense, pre-formed plasma in laser-plasma interactions have shown that the fast electrons are generated with energies higher than predicted by ponderomotive scaling [4, 3–14]. We report on experiments using the Texas Petawatt high intensity (150 fs, 1.5 × 1020 W cm−2) laser pulse, which were conducted to examine the mechanism for accelerating these high energy electrons. These experiments gauge the impact a controlled low density pre-formed plasma has on electron generation with a shorter time scale than previous experiments, 150–180 fs. Electron temperatures measured via magnetic spectrometer on experiment were found to be independent of preformed plasma. Supplemental computational results using 1D PIC simulations predict that super-ponderomotive electrons are generated inside a potential well in the pre-plasma [1]. However, while the potential well is established around 150 fs, the electrons require at least an additional 50 fs to be trapped and heated inside it.

Examination of Bow-Shock Formation in Supersonic Radiatively Cooled Plasma Flows

Radiative high-Z plasma bow shocks driven by a 200-kA current are investigated by using high-resolution dark-field laser Schlieren imaging and a Mach-Zehnder interferometer. Results demonstrate stationary high-density compressible bow shocks and provide data on the plasma Mach number and electron density.

Study of hot electron generation using kilojoule-scale high power lasers in shock ignition relevant conditions

Summary form only given. Shock ignition<;sup>1<;/sup> (SI), an alternative high gain laser fusion scheme, achieves ignition by launching a spherically converging ignitor shock in the latest stage of the compression phase using a spike pulse with absorbed intensity of ~5×10<;sup>15<;/sup> Wcm<;sup>-2<;/sup>. Nonlinear laser plasma instabilities (LPI) and hot electrons are critical issues for SI. Spike pulse generated hot electrons with moderate energies (<;150 keV) could potentially augment ablation pressure leading to gigabar shocks as demonstrated in recent OMEGA experiments<;sup>2<;/sup>.To investigate this potential benefit at SI-relevant high intensities, we have conducted experiments utilizing the high energy OMEGA EP laser to examine the effect of laser wavelength, intensity and plasma conditions on hot electron generation and energy coupling using multilayered planar foil targets. The target was first irradiated by multi-kJ UV beams at relatively low intensity to produce a long scalelength hot (~keV) plasma. The main interaction pulse, either a kJ 1-ns UV pulse with intensity ~1.6×1016 Wcm-2 or a kJ 0.1-ns IR pulse with intensity up to 2×1017 Wcm-2 was injected at varied timing delays. We have successfully demonstrated coupling of IR beam energy to the target after propagating over ~0.5 mm long scalelength plasma. The IR beam was found to break into many filaments near the quarter critical density region followed by propagation of those non-merging filaments to critical density, producing hot electrons with temperature of ~70 keV in a well-contained beam, suitable for electron-assisted SI. Details of the experiments and particle-in-cell simulations will be presented.

Analytical analysis of the ablation phase of low number wire arrays

Summary form only given. Whilst the dynamical evolution of wire arrays is well understood, and multi-dimensional Magneto-Hydrodynamic (MHD) modeling has demonstrated significant progress, a comprehensive predictive capability has not been realized to date. Recent experimental investigations have highlighted the need to more closely examine the ablation structure and its dependence on the initial parameters of the array. In particular, the range over which the ablated plasma is accelerated, and hence extent to which magnetic flux is convected into the array, is often a disputed point in the comparison simulation and analytical work. Recent work at UC San Diego [ 1 ] uses a modification of the Lebedev rocket model of wire ablation to fit the range of ablation velocities which are observed in experiments. This analysis can be extended to infer the 2D distribution of effective velocities which redistribute the mass as a function of time. From this data it may be possible to direct observe the acceleration region in a wire array. An analysis of the effect of array geometry on the determined acceleration region will be attempted using interferographic and radiographic data from experiments at 200 kA to 1 MA. Conclusions and possible extensions to this work will be presented and discussed.

Proton probing of magnetic fields in exploding wire experiments

Determination of B-field structures in pulsed power driven exploding wire experiments is vital to recover detailed information about the evolution, driving mechanisms of ablation, and subsequent instability development, but is complicated by the presence of large volumes of hot, dense plasma. Optical and electrical probe diagnostics typically fail early in the experiment. We present progress on a new project, which examines the use of proton deflectometry to measure magnetic fields in pulsed power plasmas.

Study of plasma diffusion across magnetic fields using double planar wire arrays

Accurately determining magnetic diffusion parameters in plasmas is important for benchmarking 3D MHD codes used for the design and interpretation of Z pinch and other plasma experiments. In this work, two planar arrays consisting of four wires each are at a fixed inter-plane spacing. The inter-wire spacing is then varied to alter the ratio of local to global magnetic field. This in turn determines the location of local precursor column structures, the current carrying regions, and the rate at which plasma may travel across the magnetic field toward the axis.