Alan Wootton

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-...

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.

National Ignition Facility core x-ray streak camera

The National Ignition Facility (NIF) core x-ray streak camera will be used for laser performance verification experiments as well as a wide range of physics experiments in the areas of high-energy-density science, inertial confinement fusion, and basic science. The x-ray streak camera system is being designed to record time-dependent x-ray emission from NIF targets using an interchangeable family of snouts for measurements such as one-dimensional (1D) spatial imaging or spectroscopy. the NIF core x-ray streak camera will consist of an x-ray-sensitive photocathode that detects x rays with 1D spatial resolution coupled to an electron streak tube to detect a continuous time history of the x rays incident on the photocathode over selected time periods. A charge-coupled-device (CCD) readout will record the signal from the streak tube. The streak tube, CCD, and associated electronics will reside in an electromagnetic interference, and electromagnetic pulse protected, hermetically sealed, temperature-controlled ...

Research and development for X-ray optics and diagnostics on the linac coherent light source (LCLS)

The Linac Coherent Light Source (LCLS) is a 1.5 to 15 {angstrom} wavelength Free-Electron Laser (PEL), under development at the Stanford Linear Accelerator Center (SLAC). The photon output consists of high brightness, transversely coherent pulses with duration < 300 fs, together with a broad spontaneous spectrum. The output energy density per unit area, pulse duration, repetition rate, and small FEL spot size pose special challenges for optical components and diagnostics downstream of the undulator. Planning for the photon beam transport, manipulation and diagnostics downstream of the undulator has begun.

Isochoric heating of solid aluminium with picosecond X-ray pulses

Abstract High-energy-density matter in quite unique parameter regimes can be studied using an intense laser pulse to heat isochorically an initially cold solid density target. Such isochoric heating experiments permit study of the properties, such as the equation of state, of heated matter. One of the principal challenges of these experiments is to heat sufficiently thick layers so that they will be inertially confined over times scales sufficient for equilibration, times that are often many picoseconds, even at these high densities. One approach to this problem is to heat a solid target not with the laser pulse directly, which deposits its energy only over a few nanometres, but to heat with penetrating X-rays. In this paper, we present preliminary results where such ultrafast X-ray heating is demonstrated using a short-pulse laser-driven silicon Kα source to heat a layer of solid density aluminium.

Target area and diagnostic interface issues on the National Ignition Facility (invited)

The National Ignition Facility (NIF) is under construction at Lawrence Livermore National Laboratory for the DOE Stockpile Stewardship Program. It will be used for experiments for inertial confinement fusion ignition, high energy density science, and basic science. Many interface issues confront the experimentalist who wishes to design, fabricate, and install diagnostics, and to help this process, a set of standards and guideline documents is being prepared. Compliance with these will be part of a formal diagnostic design review process. In this article we provide a short description of each, with reference to more complete documentation. The complete documentation will also be available through the NIF Diagnostics web page. Target area interface issues are grouped into three categories. First are the layout and utility interface issues which include the safety analysis report, target area facility layout; target chamber port locations; diagnostic interferences and envelopes; utilities and cable tray dist...

X-ray optics and diagnostics for first experiments on the Linac Coherent Light Source (LCLS)

The Linac Coherent Light Source (LCLS) is a 1.5 to 15 A- wavelength free-electron laser (FEL), currently proposed for the Stanford Linear Accelerator Center (SLAC). The photon output consists of high brightness, transversely coherent pulses with duration <300 fs, together with a broad spontaneous spectrum with total power comparable to the coherent output. The output fluence, and pulse duration, pose special challenges for optical component and diagnostic designs. We discuss some of the proposed solutions, and give specific examples related to the planned initial experiments.

High-Energy Scaling of Compton Scattering Light Sources

Summary form only given. No monochromatic (Deltaomega/omega), high-brightness [>1020 photons/(mm2timesmrad2timesstimes0.1% bandwidth)], tunable light sources currently exist above 100 keV. Important applications that would benefit from such new hard X-ray and g-ray sources include: nuclear resonance fluorescence spectroscopy, time-resolved positron annihilation spectroscopy, and MeV flash radiography. In this paper, the peak brightness of Compton scattering light sources is derived for head-on collisions and found to scale inversely with the electron beam duration, Deltatau, and the square of its physical emittance, epsiv/gamma, and linearly with the bunch charge and the number of photons in the laser pulse. This gamma2 -scaling shows that for low emittance electron beams (1 nC, 1 mmmiddotmrad, 100 MeV), and tabletop laser systems (1-10 J, 5 ps) the X-ray peak brightness can exceed 1023 photons/(mm2timesmrad2timesstimes0.1% bandwidth) near 1 MeV; this is confirmed by three-dimensional codes that have been benchmarked against Compton scattering experiments performed at Lawrence Livermore National Laboratory. The interaction geometry under consideration is head-on collisions, where the X-ray flash duration is shown to be equal to that of the electron bunch, and which produce the highest peak brightness for compressed electron beams. Important nonlinear effects, including spectral broadening, are also taken into account in our analysis; they show that there is an optimum laser pulse duration in this geometry, of the order of a few picoseconds, in sharp contrast with the initial approach to laser-driven Compton scattering sources where femtosecond laser systems were thought to be mandatory. The analytical expression for the peak on-axis brightness derived here is a powerful tool to efficiently explore the 12-dimensional parameter space corresponding to the phase spaces of both the electron and incident laser beams and to determine optimum conditions for producing high brightness X-rays

X-ray optics research for free electron lasers: study of material damage under extreme fluxes

Free electron lasers operating in the 0.1–1.5 nm wavelength range have been proposed for the Stanford Linear Accelerator Center (USA) and DESY (Germany). The unprecedented brightness and associated fluence (up to 30 J cm−2) predicted for pulses < 300 fs pose new challenges for optical components. A criterion for optical component design is required, implying an understanding of X-ray—material interactions at these extreme conditions. In our experimental effort, the extreme conditions are simulated by the currently available sources ranging from optical lasers, through X-ray lasers (XRLs) at 14.7 nm down to K-alpha sources (∼0.15 nm). In this paper, we present an overview of our research project on X-ray—matter interaction, including both computer modeling and preliminary results from optical laser experiments, the COMET tabletop high brightness ps XRL and a K-alpha experimental campaign carried out at the JanUSP laser facility at the Lawrence Livermore National Laboratory.© 2003 Elsevier Science B.V. All rights reserved.PACS: 41.50; 42.70; 41.60. Cr; 42.55. Vc; 07.85. F

Compton scattering and photoluminescence for x-ray imaging

Experiments with high-intensity, submillimeter diameter, pulsed x-ray beams that will be generated by the planned Linac Coherent Light Source will require nonperturbing imaging of such beams. Two approaches to solving this problem are proposed: Compton scattering off a thin solid foil and photoluminescence induced in a thin gas jet. The first would be efficient for x-ray energies above a few keV, whereas the second can be used to detect lower-energy beams, below ∼1 keV. Spatial resolution of the time-integrated images would be ∼10 μm for the first technique and ∼60 μm for the second technique. The minimum number of x-ray quanta needed for reaching this spatial resolution is ∼10.11 The imaging does not introduce significant perturbations to the beam. A set of design equations and constraints is provided.