Thomas R. Boehly

Experimental investigation of smoothing by spectral dispersion

Measurements of smoothing rates for smoothing by spectral dispersion (SSD) of high-power, solid-state laser beams used for inertial confinement fusion (ICF) research are reported. Smoothing rates were obtained from the intensity distributions of equivalent target plane images for laser pulses of varying duration. Simulations of the experimental data with the known properties of the phase plates and the frequency modulators are in good agreement with the experimental data. These results inspire confidence in extrapolating to higher bandwidths and other SSD configurations that may be suitable for ICF experiments and ultimately for direct-drive laser-fusion ignition.

Characteristics of target interaction with high power UV laser radiation

Abstract Thermal conduction, mass ablation rate, pressure and preheat were investigated in the interaction of a frequency tripled Nd: glass laser of power 0.-0.2 TW with flat targets. In the range 10 13 −10 15 W cm 2 for 400 ps pulses we find: (a) thermal conduction may be described by a flux limiter f = 0.04 ± 0.01; (b) the mass abaltion rate depends on the incident laser irradiance as m = 4.4 × 10 5 (I/10 14 ) 0.53 g cm -2 s -1 ; (c) the pressure near the ablation surface increases approximately linearly with irradiance and is about 70 Mbar at 1015 W/cm2, and (d) preheat as evidenced by Kα X-ray line emission is significantly lower than in λ = 1.05 μm irradiation.

Cylindrical implosion experiments using laser direct drive

Direct-drive cylindrical-implosion experiments are performed to study perturbed hydrodynamic flows in convergent geometry. Two experimental campaigns have been conducted, to demonstrate the advantages of direct over indirect drive and to validate numerical simulations of zeroth-order hydrodynamics and single-mode perturbation growth. Results and analysis of three unperturbed-target shots and two perturbed-target shots are discussed in detail. For unperturbed-target implosions, positions of inner and outer shell edges agree between simulation and experiment during the laser pulse. However, observed shell thickness is greater than simulated in unperturbed targets during deceleration and rebound; the effect appears only at the shell’s exterior edge. For perturbed-target implosions, growth factors ∼10–14 are observed, whereas growth factors near 30 are expected from simulation. Rayleigh–Taylor growth appears to differ between simulation and experiment. Observed zeroth-order flow at the exterior edge of implod...

Thermal transport measurements in 1.05 μm laser irradiation of spherical targets

Transport and implosion experiments have been conducted on the OMEGA 24‐beam, uniform‐irradiation facility. Thermal transport in spherical irradiation was found to be different than in comparable, single‐beam target irradiation and could not be described in terms of a flux‐inhibited model. Deep energy deposition in spherical irradiation (by electrons on the tail of the thermal velocity distribution) was found to lead to a temperature profile which is not as steep as predicted by a flux‐inhibited model. This apparently leads to more explosive implosion (i.e., higher core temperature) than predicted by using such a model.

Ramp compression of iron to 273 GPa

Multiple thickness Fe foils were ramp compressed over several nanoseconds to pressure conditions relevant to the Earths core. Using wave-profile analysis, the sound speed and the stress-density response were determined to a peak longitudinal stress of 273 GPa. The measured stress-density states lie between shock compression and 300-K static data, and are consistent with relatively low temperatures being achieved in these experiments. Phase transitions generally display time-dependent material response and generate a growing shock. We demonstrate for the first time that a low-pressure phase transformation (α-Fe to e-Fe) can be overdriven by an initial steady shock to avoid both the time-dependent response and the growing shock that has previously limited ramp-wave-loading experiments. In addition, the initial steady shock pre-compresses the Fe and allows different thermodynamic compression paths to be explored.

Multiple film plane diagnostic for shocked lattice measurements (invited)

Laser-based shock experiments have been conducted in thin Si and Cu crystals at pressures above the Hugoniot elastic limit. In these experiments, static film and x-ray streak cameras recorded x rays diffracted from lattice planes both parallel and perpendicular to the shock direction. These data showed uniaxial compression of Si(100) along the shock direction and three-dimensional compression of Cu(100). In the case of the Si diffraction, there was a multiple wave structure observed, which may be due to a one-dimensional phase transition or a time variation in the shock pressure. A new film-based detector has been developed for these in situ dynamic diffraction experiments. This large-angle detector consists of three film cassettes that are positioned to record x rays diffracted from a shocked crystal anywhere within a full π steradian. It records x rays that are diffracted from multiple lattice planes both parallel and at oblique angles with respect to the shock direction. It is a time-integrating measur...

Nonlinear evolution of broad-bandwidth, laser-imprinted nonuniformities in planar targets accelerated by 351-nm laser light

Planar, 20 and 40 μm thick CH targets have been accelerated by 351 nm laser beams of the OMEGA laser system [Opt. Commun. 133, 495 (1997)]. Different beam-smoothing techniques were employed including distributed phase plates, smoothing by spectral dispersion, and distributed polarization rotators. The Rayleigh–Taylor evolution of three-dimensional (3D) broadband planar-target perturbations seeded by laser nonuniformities was measured using x-ray radiography at ∼1.3 keV. Fourier analysis shows that the perturbations evolve to longer wavelengths and the shorter wavelengths saturate. The saturation amplitudes and rates of growth of these features are consistent with the predictions of Haan [Phys. Rev. A 39, 5812 (1989)].

OMEGA Upgrade laser for direct-drive target experiments

Validation of the direct-drive approach to inertial confinement fusion requires the development of a 351-nm wavelength, 30-kJ, 50-TW laser system with flexible pulse shaping and irradiation uniformity approaching 1%. An upgrade of the existing OMEGA direct-drive facility at Rochester is planned to meet these objectives. In this article, we review the design rationale and specifications of the OMEGA Upgrade laser with particular emphasis on techniques planned to achieve the required degree of beam smoothing, temporal pulse shape, and beam-to-beam power balance.

Experimental configuration of direct drive cylindrical implosions on the OMEGA laser

Details about the cylindrical implosions using direct-drive irradiation on the OMEGA Laser facility are provided. The experimental configuration, including orientation, construction, and mounting of the targets is described. An attempt to characterize the modulation transfer function of the primary x-ray framing camera diagnostic results in insufficient exposure contrast but relative agreement with other determinations. The x-ray intensity of the titanium backlighter driven by the 2.5-nsec linear ramp of the laser beams is described, and the relative intensity on film is compared to similar Nova experiments. The parallax effects of different length marker layers of high-opacity dichloropolystyrene is measured, resulting in the conclusion that the marker layer length should be matched to the laser drive illumination profile.

Plastic Deformation in Laser‐Induced Shock Compression of Monocrystalline Copper

Copper monocrystals were subjected to shock compression at pressures of 10–60 GPa by a short (3 ns initial) duration laser pulse. Transmission electron microscopy revealed features consistent with previous observations of shock‐compressed copper, albeit at pulse durations in the μs regime. The results suggest that the defect structure is generated at the shock front. A mechanism for dislocation generation is presented, providing a realistic prediction of dislocation density as a function of pressure. The threshold stress for deformation twinning in shock compression is calculated from the constitutive equations for slip, twinning, and the Swegle‐Grady relationship.