Michael H. Key

Electron, Photon, and Ion Beams from the Relativistic Interaction of Petawatt Laser Pulses with Solid Targets

In recent Petawatt laser experiments at Lawrence Livermore National Laboratory, several hundred joules of 1 μm laser light in 0.5–5.0-ps pulses with intensities up to 3×1020 W cm−2 were incident on solid targets and produced a strongly relativistic interaction. The energy content, spectra, and angular patterns of the photon, electron, and ion radiations have all been diagnosed in a number of ways, including several novel (to laser physics) nuclear activation techniques. About 40%–50% of the laser energy is converted to broadly beamed hot electrons. Their beam centroid direction varies from shot to shot, but the resulting bremsstrahlung beam has a consistent width. Extraordinarily luminous ion beams (primarily protons) almost precisely normal to the rear of various targets are seen—up to 3×1013 protons with kTion∼several MeV representing ∼6% of the laser energy. Ion energies up to at least 55 MeV are observed. The ions appear to originate from the rear target surfaces. The edge of the ion beam is very shar...

High energy electrons, nuclear phenomena and heating in petawatt laser-solid experiments

The Petawatt laser at LLNL has opened a new regime of laser-matter interactions in which the quiver motion of plasma electrons is fully relativistic with energies extending well above the threshold for nuclear processes. In addition to -few MeV ponderomotive electrons produced in ultra-intense laser-solid interactions, we have found a high energy component of electrons extending to -100 MeV apparently from relativistic self-focusing and plasma acceleration in the underdense pre-formed plasma. The generation of hard bremsstrahlung, photo-nuclear reactions, and preliminary evidence for positron-electron pair production will be discussed.

Particle-in-cell simulations of ultra intense laser pulses propagating through overdense plasma for fast-ignitor and radiography applications

Zohar (two-dimensions, particle-in-cell) [C. K. Birdsall and A. B. Langdon, Plasma Physics via Computer Simulation (McGraw–Hill, New York, 1985)] simulations of ultra intense laser beams boring into overdense plasmas whose parameters are guided by the fast-ignitor concept and radiography applications are presented. Complex low frequency magnetic field structures, narrow channel formation, and beam deflection are all evident. Particle tracking diagnostics elucidate the nature of the currents that produce and interact with these static magnetic fields which are larger than 109 G for simulations at 1021 W/cm2 in a 50nc plasma. Tracking electron orbits provides a more complete understanding of the hot electron generation as the short pulse, high intensity laser penetrates overdense plasma. Particles which constitute the current in the narrow channel are partially confined by the low frequency magnetic field. In contrast, the return current particles on the outside of the channel are defocused by the high magn...

Fast track to fusion energy.

Nuclear fusion could solve the worlds energy problems, but its potential remains untapped. Can a new way to ignite a tiny ball of high-density fuel make the dream come true?

Beam-Weibel filamentation instability in near-term and fast-ignition experiments

High intensity laser-plasma interactions accelerate electrons to suprathermal velocities. Their current is neutralized by an induced cold electron return current. These inter-penetrating and anti-parallel currents are subject to electrostatic and electromagnetic instability. Two analytical models for electron transport are used to predict the growth rates of the linear electromagnetic beam-Weibel filamentation instability in both near-term laser-solid experiments as well as in future fast-ignition experiments. Specifications and calculations of the relevant physical parameters are made. Both models predict that instability growth is significant for the fast-ignition case. Instability development in near-term experiments is also significant, but with a greater difference between the models’ predictions at low densities.

Methods for mitigating surface damage growth in NIF final optics

We report a summary of the surface damage, growth mitigation effort at 3(omega) for fused silica optics at LLNL. The objective was to experimentally validate selected methods that could be applied to pre-initiated or retrieved-from- service optics, to stop further damage growth. A specific goal was to obtain sufficient data and information of successful methods for fused silica optics to select a single approach for processing NIF optics. This paper includes the test results and the evaluation thereof, for several mitigation methods for fused silica. The mitigation methods tested in this study are wet chemical etching, cold plasma etching, CO2 laser processing, and micro-flame torch processing. We found that CO2 laser processing produces the most significant and consistent results to halt laser-induced surface damage growth on fused silica. We recorded successful mitigation of the growth of laser-induced surface damage sites as large as 0.5-mm diameter, for 1000 shots at fluences in the range of 8 to 13 J/cm2. We obtained sufficient data for elimination of damage growth using CO2 laser processing on sub-aperture representative optics, to proceed with application to full- scale NIF optics.

Improving 351-nm damage performance of large-aperture fused silica and DKDP optics

A program to identify and eliminate the causes of UV laser- induced damage and growth in fused silica and DKDP has developed methods to extend optics lifetimes for large- aperture, high-peak-power, UV lasers such as the National Ignition Facility (NIF). Issues included polish-related surface damage initiation and growth on fused silica and DKDP, bulk inclusions in fused silica, pinpoint bulk damage in DKDP, and UV-induced surface degradation in fused silica and DKDP in a vacuum. Approaches included an understanding of the mechanism of the damage, incremental improvements to existing fabrication technology, and feasibility studies of non-traditional fabrication technologies. Status and success of these various approaches are reviewed. Improvements were made in reducing surface damage initiation and eliminating growth for fused silica by improved polishing and post- processing steps, and improved analytical techniques are providing insights into mechanisms of DKDP damage. The NIF final optics hardware has been designed to enable easy retrieval, surface-damage mitigation, and recycling of optics.

Measurements of direct drive laser imprint in thin foils by radiography using an x-ray laser backlighter

In direct drive inertial confinement fusion, the residual speckle pattern remaining after beam smoothing plays an important role in the seeding of instabilities at the ablation front. We have used an x-ray laser as an XUV backlighter to characterize the imprinted modulation in thin foils for smoothing by random phase plate and spectral dispersion at both 0.35 pm and 0.53 pm irradiation, and induced spatial incoherence at 0.53 pm irradiation. We also demonstrate measurements of the modulation due to a single mode optical imprint generated by a narrow slit interference pattern, and modification of the imprint with a superposed smooth irradiation to study time dependence of the imprinting process. 8 refs., 10 figs.

Using low and high prepulses to enhance the J=0−1 transition at 19.6 nm in the Ne-like germanium XUV laser

Abstract We report a study of the effect of prepulses on XUV lasing of Ne-like germanium for an irradiation geometry where ≈20 mm long germanium slab targets were irradiated at ≈1.6×10 13 W cm −2 using ≈0.7 ns (1.06 μm) pulses from the VULCAN glass laser. Prepulses were generated at fractional power levels of ≈2×10 −4 (low) and ≈2×10 −2 (high) and arrived on target 5 and 3.2 ns respectively in advance of the main heating pulse. For both the low and high prepulses the output of the 3p-3s, J =0–1, line at 19.6 nm was enhanced such that the peak radiant density (J/st) for this line became greater than that for the normally stronger J =2−1 lines at 23.2 and 23.6 nm. The J =0–1 line, whose FWHM duration was reduced from ≈450 ps to ≈100 ps, delivered ≈6× more power (W) than the average for the combinned J =2−1 lines, whose FWHM duration was ≈500 ps for both levels of prepulse. The higher prepulse was more effective, yielding ≈2× more radiant density and ≈7× more power on both the J =0–1 and J =2−1 transitions compared to the low prepulse case. The most dramatic observation overall was the ≈40× increase of power in the J =0–1 line for the high prepulse (≈2%) case compared with the zero prepulse case. These observations, coupled with measurements of beam divergence and beam deviation through refractive bending, as well as general agreement with modelling, lead us to conclude that, for germanium, the main influence of the prepulse is (a) to increase the energy absorbed from the main pulse, (b) to increase the volume of the gain zone and (c) to relax the plasma density gradients, particularly in the J =0–1 gain zone.

Development of Compton radiography using high‐Z backlighters produced by ultra‐intense lasers

High‐energy x‐ray backlighters will be valuable for radiography experiments at the National Ignition Facility (NIF), and for radiography of imploded inertial confinement fusion cores using Compton scattering to observe cold, dense plasma. Key considerations are the available backlight brightness, and the backlight size. To quantify these parameters we have characterized the emission from low‐ and high‐Z planar foils irradiated by intense picosecond and femtosecond laser pulses from the TITAN laser facility at Lawrence Livermore National Laboratory. Spectra generated by a sequence of elements from Mo to Pb, spanning the x‐ray energy range from 17 keV to 75 keV, have been recorded using a Charged Coupled Device (CCD) in single hit regime and a Dual Crystal Spectrometer (DCS). High‐resolution point‐projection 2D radiographs have also been recorded on Fuji BaFBr:Eu2 image plates using calibrated resolution grids. We discuss the results in light of the requirements for applications at NIF.