John K. Crane

An overview of LLNL high-energy short-pulse technology for advanced radiography of laser fusion experiments

The technical challenges and motivations for high-energy, short-pulse generation with the National Ignition Facility (NIF) and possibly other large-scale Nd : glass lasers are reviewed. High-energy short-pulse generation (multi-kilojoule, picosecond pulses) will be possible via the adaptation of chirped pulse amplification laser techniques on NIF. Development of metre-scale, high-efficiency, high-damage-threshold final optics is a key technical challenge. In addition, deployment of high energy petawatt (HEPW) pulses on NIF is constrained by existing laser infrastructure and requires new, compact compressor designs and short-pulse, fibre-based, seed-laser systems. The key motivations for HEPW pulses on NIF is briefly outlined and includes high-energy, x-ray radiography, proton beam radiography, proton isochoric heating and tests of the fast ignitor concept for inertial confinement fusion.

Detailed study of nuclear fusion from femtosecond laser-driven explosions of deuterium clusters

Recent experiments on the interaction of intense, ultrafast pulses with large van der Waals bonded clusters have shown that these clusters can explode with sufficient kinetic energy to drive nuclear fusion. Irradiating deuterium clusters with a 35 fs laser pulse, it is found that the fusion neutron yield is strongly dependent on such factors as cluster size, laser focal geometry, and deuterium gas jet parameters. Neutron yield is shown to be limited by laser propagation effects as the pulse traverses the gas plume. From the experiments it is possible to get a detailed understanding of how the laser deposits its energy and heats the deuterium cluster plasma. The experiments are compared with simulations.

PLEIADES: A picosecond Compton scattering x-ray source for advanced backlighting and time-resolved material studies

The PLEIADES (Picosecond Laser-Electron Inter-Action for the Dynamical Evaluation of Structures) facility has produced first light at 70 keV. This milestone offers a new opportunity to develop laser-driven, compact, tunable x-ray sources for critical applications such as diagnostics for the National Ignition Facility and time-resolved material studies. The electron beam was focused to 50 μm rms, at 57 MeV, with 260 pC of charge, a relative energy spread of 0.2%, and a normalized emittance of 5 mm mrad horizontally and 13 mm mrad vertically. The scattered 820 nm laser pulse had an energy of 180 mJ and a duration of 54 fs. Initial x rays were captured with a cooled charge-coupled device using a cesium iodide scintillator; the peak photon energy was approximately 78 keV, with a total x-ray flux of 1.3×106 photons/shot, and the observed angular distribution found to agree very well with three-dimensional codes. Simple K-edge radiography of a tantalum foil showed good agreement with the theoretical divergence-...

High-field harmonic generation in helium.

We observe harmonics of 526-nm laser light up to the 45th order, 11.7 nm, in helium. We discuss the extension of the harmonic plateau with increasing laser intensity. The data suggest that the highest harmonic order produced depends on the highest intensity seen by the atom before photoionization. Harmonics are generated predominantly from neutrals. Harmonic generation from ions is weak owing to poor phase matching between the fundamental and harmonic fields at high electron densities.

Progress on converting a NIF quad to eight, petawatt beams for advanced radiography

We are converting a quad of NIF beamlines into eight, short-pulse (1–50 ps), petawatt-class beams for advanced radiography and fast ignition experiments. This paper describes progress toward completing this project.

Measurement of the local electron density by means of stimulated Raman scattering in a laser-produced gas jet plasma.

By measuring the spectrum of the backscattered light from a short-pulse laser-produced plasma in a gas jet, a direct and highly accurate measurement of the local electron density can be obtained. The measurement is based on the density-dependent spectral shift of the backscattered Raman wave.

Characterization of a bright, tunable, ultrafast Compton scattering X-ray source

The Compton scattering of a terawatt-class, femtosecond laser pulse by a high-brightness, relativistic electron beam has been demonstrated as a viable approach toward compact, tunable sources of bright, femtosecond, hard X-ray flashes. The main focus of this article is a detailed description of such a novel X-ray source, namely the PLEIADES (Picosecond Laser–Electron Inter-Action for the Dynamical Evaluation of Structures) facility at Lawrence Livermore National Laboratory. PLEIADES has produced first light at 70 keV, thus enabling critical applications, such as advanced backlighting for the National Ignition Facility and in situ time-resolved studies of high- Z materials. To date, the electron beam has been focused down to σ x = σ y = 27 μm rms, at 57 MeV, with 266 pC of charge, a relative energy spread of 0.2%, a normalized horizontal emittance of 3.5 mm·mrad, a normalized vertical emittance of 11 mm·mrad, and a duration of 3 ps rms. The compressed laser pulse energy at focus is 480 mJ, the pulse duration 54 fs Intensity Full Width at Half-Maximum (IFWHM), and the 1/ e 2 radius 36 μm. Initial X rays produced by head-on collisions between the laser and electron beams at a repetition rate of 10 Hz were captured with a cooled CCD using a CsI scintillator; the peak photon energy was approximately 78 keV, and the observed angular distribution was found to agree very well with three-dimensional codes. The current X-ray dose is 3 × 10 6 photons per pulse, and the inferred peak brightness exceeds 10 15 photons/(mm 2 × mrad 2 × s × 0.1% bandwidth). Spectral measurements using calibrated foils of variable thickness are consistent with theory. Measurements of the X-ray dose as a function of the delay between the laser and electron beams show a 24-ps full width at half maximum (FWHM) window, as predicted by theory, in contrast with a measured timing jitter of 1.2 ps, which contributes to the stability of the source. In addition, K -edge radiographs of a Ta foil obtained at different electron beam energies clearly demonstrate the γ 2 -tunability of the source and show very good agreement with the theoretical divergence-angle dependence of the X-ray spectrum. Finally, electron bunch shortening experiments using velocity compression have also been performed and durations as short as 300 fs rms have been observed using coherent transition radiation; the corresponding inferred peak X-ray flux approaches 10 19 photons/s.

The commissioning of the advanced radiographic capability laser system: experimental and modeling results at the main laser output

The National Ignition Facility (NIF) at Lawrence Livermore National Laboratory is the first of a kind megajoule-class laser with 192 beams capable of delivering over 1.8 MJ and 500TW of 351nm light [1], [2]. It has been commissioned and operated since 2009 to support a wide range of missions including the study of inertial confinement fusion, high energy density physics, material science, and laboratory astrophysics. In order to advance our understanding, and enable short-pulse multi-frame radiographic experiments of dense cores of cold material, the generation of very hard x-rays above 50 keV is necessary. X-rays with such characteristics can be efficiently generated with high intensity laser pulses above 1017 W/cm² [3]. The Advanced Radiographic Capability (ARC) [4] which is currently being commissioned on the NIF will provide eight, 1 ps to 50 ps, adjustable pulses with up to 1.7 kJ each to create x-ray point sources enabling dynamic, multi-frame x-ray backlighting. This paper will provide an overview of the ARC system and report on the laser performance tests conducted with a stretched-pulse up to the main laser output and their comparison with the results of our laser propagation codes.

Phasing beams with different dispersions and application to the petawatt-class beamline at the National Ignition Facility.

In order to achieve the highest intensities possible with the short-pulse Advanced Radiographic Capability beamline at the National Ignition Facility (NIF), it will be necessary to phase the individual ARC apertures. This is made especially challenging because the design of ARC results in two laser beams with different dispersions sharing the same NIF aperture. The extent to which two beams with different dispersions can be phased with each other has been an open question. This paper presents results of an analysis showing that the different dispersion values that will be encountered by the shared-aperture beams will not preclude the phasing of the two beams. We also highlight a situation in which dispersion mismatch will prevent good phasing between apertures, and discuss the limits to which higher-order dispersion values may differ before the beams begin to dephase.

High-Energy, Short-Pulse Fiber Injection Lasers at Lawrence Livermore National Laboratory

A short-pulse fiber injection laser for the advanced radiographic capability on the National Ignition Facility has been developed at Lawrence Livermore National Laboratory. This system produces 100 ¿J pulses with 5 nm of bandwidth centered at 1053 nm. The pulses are stretched to 2.5 ns, and have been recompressed to subpicoseconds pulsewidths. A key feature of the system is that the prepulse power contrast ratio exceeds 80 dB. The system can also precisely adjust the final recompressed pulsewidth and timing, and has been designed for reliable, hands-free operation. The key challenges in constructing this system were control of the SNR, dispersion management, and managing the impact of self-phase modulation on the chirped pulse.