John A. Oertel

Gated x-ray detector for the National Ignition Facility

Two new gated x-ray imaging cameras have recently been designed, constructed, and delivered to the National Ignition Facility in Livermore, CA. These gated x-Ray detectors are each designed to fit within an aluminum airbox with a large capacity cooling plane and are fitted with an array of environmental housekeeping sensors. These instruments are significantly different from earlier generations of gated x-ray images due, in part, to an innovative impedance matching scheme, advanced phosphor screens, pulsed phosphor circuits, precision assembly fixturing, unique system monitoring, and complete remote computer control. Preliminary characterization has shown repeatable uniformity between imaging strips, improved spatial resolution, and no detectable impedance reflections.

Streaked optical pyrometer system for laser-driven shock-wave experiments on OMEGA

The temperature of laser-driven shock waves is of interest to inertial confinement fusion and high-energy-density physics. We report on a streaked optical pyrometer that measures the self-emission of laser-driven shocks simultaneously with a velocity interferometer system for any reflector (VISAR). Together these diagnostics are used to obtain the temporally and spatially resolved temperatures of approximately megabar shocks driven by the OMEGA laser. We provide a brief description of the diagnostic and how it is used with VISAR. Key spectral calibration results are discussed and important characteristics of the recording system are presented.

Development and characterization of a pair of 30–40 ps x‐ray framing cameras

We have constructed two slightly different high‐speed framing cameras for use on NOVA and the OMEGA Upgrade. Both units are based on the gating of a microchannel plate, with one detector having a pore length to diameter ratio half that of the other. We will discuss the factors limiting the temporal resolution of each detector and will compare the results of modeling with gate width measurements taken using a short‐pulse laser. We will also compare time‐resolved x‐ray images recorded using one of these devices with data from an older (∼90 ps resolution) detector.

A framed monochromatic x-ray microscope for ICF (invited)

The Laser Fusion Experiments Groups from the Laboratory for Laser Energetics (LLE) and the Los Alamos National Laboratory (LANL) have jointly developed an instrument capable of simultaneously space-, time-, and spectrally resolving x-ray emission from inertial confinement fusion (ICF) targets. Uses of the instrument include framed imaging of line emission from fuel or shell dopants and monochromatic backlighting. The x-ray imaging is accomplished with a Kirkpatrick-Baez (KB)-type four-image microscope, which has a best spatial resolution of ∼5 μm and a sensitive energy range of ∼2–8 keV. Time-resolved x-ray images are obtained with a pair of custom framing cameras, each of which records two of the four images in two independent 80-ps time intervals. In addition, the energy range of the images can be restricted to a narrow (monochromatic) spectral range (∼10–100 eV) by the introduction of diffracting crystals. This technique has been demonstrated with an e-beam-generated dc x-ray source, and at the LANL Tr...

TRIDENT high-energy-density facility experimental capabilities and diagnostics

The newly upgraded TRIDENT high-energy-density (HED) facility provides high-energy short-pulse laser-matter interactions with powers in excess of 200 TW and energies greater than 120 J. In addition, TRIDENT retains two long-pulse (nanoseconds to microseconds) beams that are available for simultaneous use in either the same experiment or a separate one. The facilitys flexibility is enhanced by the presence of two separate target chambers with a third undergoing commissioning. This capability allows the experimental configuration to be optimized by choosing the chamber with the most advantageous geometry and features. The TRIDENT facility also provides a wide range of standard instruments including optical, x-ray, and particle diagnostics. In addition, one chamber has a 10 in. manipulator allowing OMEGA and National Ignition Facility (NIF) diagnostics to be prototyped and calibrated.

The National Ignition Facility Neutron Imaging System

The National Ignition Facility (NIF) is scheduled to begin deuterium-tritium (DT) shots possibly in the next several years. One of the important diagnostics in understanding capsule behavior and to guide changes in Hohlraum illumination, capsule design, and geometry will be neutron imaging of both the primary 14 MeV neutrons and the lower-energy downscattered neutrons in the 6-13 MeV range. The neutron imaging system (NIS) described here, which we are currently building for use on NIF, uses a precisely aligned set of apertures near the target to form the neutron images on a segmented scintillator. The images are recorded on a gated, intensified charge coupled device. Although the aperture set may be as close as 20 cm to the target, the imaging camera system will be located at a distance of 28 m from the target. At 28 m the camera system is outside the NIF building. Because of the distance and shielding, the imager will be able to obtain images with little background noise. The imager will be capable of imaging downscattered neutrons from failed capsules with yields Y(n)>10(14) neutrons. The shielding will also permit the NIS to function at neutron yields >10(18), which is in contrast to most other diagnostics that may not work at high neutron yields. The following describes the current NIF NIS design and compares the predicted performance with the NIF specifications that must be satisfied to generate images that can be interpreted to understand results of a particular shot. The current design, including the aperture, scintillator, camera system, and reconstruction methods, is briefly described. System modeling of the existing Omega NIS and comparison with the Omega data that guided the NIF design based on our Omega results is described. We will show NIS model calculations of the expected NIF images based on component evaluations at Omega. We will also compare the calculated NIF input images with those unfolded from the NIS images generated from our NIS numerical modeling code.

Target diagnostic system for the national ignition facility (invited)

A review of recent progress on the design of a diagnostic system proposed for ignition target experiments on the National Ignition Facility (NIF) will be presented. This diagnostic package contains an extensive suite of optical, x ray, gamma ray, and neutron diagnostics that enable measurements of the performance of both direct and indirect driven NIF targets. The philosophy used in designing all of the diagnostics in the set has emphasized redundant and independent measurement of fundamental physical quantities relevant to the operation of the NIF target. A unique feature of these diagnostics is that they are being designed to be capable of operating in the high radiation, electromagnetic pulse, and debris backgrounds expected on the NIF facility. The diagnostic system proposed can be categorized into three broad areas: laser characterization, hohlraum characterization, and capsule performance diagnostics. The operating principles of a representative instrument from each class of diagnostic employed in t...

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...

A high-resolution x-ray microscope for laser-driven planar-foil experiments

A soft x-ray microscope (E≲3 keV) with high spatial resolution (∼3 μm) has been characterized at the University of Rochester’s Laboratory for Laser Energetics and used for initial experiments on the Omega laser system [Boehly et al., Opt. Commun. 133, 495 (1997)] to study the hydrodynamic stability of directly driven planar foils. The microscope, which is an optimized Kirkpatrick–Baez-type design, is used to obtain four x-ray radiographs of laser-driven foils. Time-resolved images are obtained with either custom-built framing cameras (time resolution ∼80 ps) or by using short-pulse backlighter beams (Δt≲200 ps). In the former case, a spatial resolution of ∼7 μm was obtained (limited by the framing camera), while in the latter case a resolution of ∼3 μm was obtained. This paper details the testing, calibration, and initial use of this microscope in the laboratory and on Omega.

Shock propagation, preheat, and x-ray burnthrough in indirect-drive inertial confinement fusion ablator materials

The velocities and temperatures of shock waves generated by laser-driven hohlraum radiation fields have been measured in indirect-drive inertial confinement fusion (ICF) capsule ablator materials. Time-resolved measurements of the preheat temperature ahead of the shock front have been performed and included in the analysis. Measurements of the x-ray burnthrough of the ablation front and the ablator x-ray re-emission have also been made in the Cu-doped beryllium, polyimide, and Ge-doped CH ablator samples. The experiments utilize 15 beams of the University of Rochester Omega Laser [Soures et al., Phys. Plasmas 3, 2108 (1996)] to heat hohlraums to radiation temperatures of ∼120–200 eV. In the experiments, planar samples of ablator material are exposed to the hohlraum radiation field, generating shocks in the range of 10–50 Mbars. The experimental results are compared to integrated two-dimensional Lasnex [G. B. Zimmerman and W. L. Kruer, Comments Plasma Phys. Control. Fusion 2, 51 (1975)] calculations, in wh...