James E. Murray

Z-Beamlet: a multikilojoule, terawatt-class laser system

A large-aperture (30-cm) kilojoule-class Nd:glass laser system known as Z-Beamlet has been constructed to perform x-ray radiography of high-energy-density science experiments conducted on the Z facility at Sandia National Laboratories, Albuquerque, New Mexico. The laser, operating with typical pulse durations from 0.3 to 1.5 ns, employs a sequence of successively larger multipass amplifiers to achieve up to 3-kJ energy at 1054 nm. Large-aperture frequency conversion and long-distance beam transport can provide on-target energies of up to 1.5 kJ at 527 nm.

Spatial filter pinhole development for the National Ignition Facility

We present results from a major experimental effort to understand the behavior of spatial filter pinholes and to identify and demonstrate a pinhole that will meet the requirements of the National Ignition Facility (NIF). We find that pinhole performance depends significantly on geometry and material. Cone pinholes are found to stay open longer and to cause less backreflection than pinholes of more conventional geometry. We show that a +/-150-microrad stainless-steel cone pinhole will pass a full-energy NIF ignition pulse with required margins for misalignment and for smoothing by spectral dispersion. On the basis of a model fitted to experimental results, a +/-125-microrad stainless-steel cone pinhole is also projected to meet these requirements.

High performance avalanche transistor switchout for external pulse selection at 1.06 μm

The design and performance of an avalanche transistor switchout are described. The device selects a single pulse from a train of cw or Q-switched mode-locked pulses and introduces less than +/-1% amplitude variation in the selected pulse. The prepulse rejection ratio exceeds 10(7), and a lifetime of greater than 10(7) shots has been achieved.

Spatial filter pinhole for high-energy pulsed lasers

Spatial filters are essential components for maintaining high beam quality in high-energy pulsed laser systems. The long-duration (21 ns) high-energy pulses envisioned for future inertial-confinement fusion drive systems, such as the U.S. National Ignition Facility (NIF), are likely to lead to increased plasma generation and closure effects within the pinholes in the spatial filters. The design goal for the pinhole spatial filter for the NIF design is to remove small-angle scatter in the beam to as little as a ?100-murad divergence. It is uncertain whether this design requirement can be met with a conventional pinhole design. We propose a new pinhole architecture that addresses these issues by incorporating features intended to reduce the rate of plasma generation. Initial experiments with this design have verified its performance improvement relative to a conventional pinhole design.

National Ignition Facility commissioning and performance

The National Ignition Facility at LLNL recently commissioned the first set of four beam lines into the target chamber. This effort, called NIF Early Light, demonstrated the entire laser system architecture from master oscillator through the laser amplifiers and final optics to target and initial X-ray diagnostics. This paper describes the major installation and commissioning steps for one of NIFs 48 beam quads. Using a dedicated single beam line Precision Diagnostic System, performance was explored over the entire power versus energy space up to 6.4 TW/beam for sub-nanosecond pulses and 25 kJ/beam for 23 ns pulses at 1w. NEL also demonstrated frequency converted Nd:Glass laser energies from a single beamline of 11.3 kJ at 2w and 10.4 kJ at 3w.

Off-axis multipass amplifier as a large aperture driver stage for fusion lasers

A multipass amplifier configuration is described which has potential as a large aperture, high gain driver stage for fusion laser systems. We avoid the present limitations of large aperture switches by using an off-angle geometry that does not require an optical switch. The saturated gain characteristics of this multipass amplifier are optimized numerically. Three potential problems are investigated experimentally, self-lasing, output beam quality, and amplified spontaneous emission output. The results indicate comparable cost for comparable performance to a linear chain, with some operational advantage for the multipass driver stage.

Flashlamp pumping of Nd:glass disk amplifiers

We present experimental results and a model of Nd:glass disk amplifiers which are used in inertial confinement fusion research. We first review our previous measurements on pulsed xenon flashlamps. We then discuss out measurements on the enhancement of the Nd fluorescence decay rate in laser disks by amplified spontaneous emission. Using these data, we have constructed a model of flashlamp pumping which treats the transfer efficiency of pump light from the flashlamps to the disks as an empirical function. We have found a simple description of this cavity transfer function which provides an excellent fit to the amplifier results for various pump pulselengths. We discuss the concept of the pump area ratio for describing the flashlamp packing density and show that amplifier performance is optimized for values of this parameter near unity. We finally present results for both a singlesegment and a multisegment disk amplifier. We have used these devices to investigate new amplifier designs for a large scale fusion driver.

Estimating Wind Velocities in Mountain Lee Waves Using Sailplane Flight Data

Abstract Mountain lee waves are a form of atmospheric gravity wave that is generated by flow over mountain topography. Mountain lee waves are of considerable interest, because they can produce drag that affects the general circulation, windstorms, and clear-air turbulence that can be an aviation hazard, and they can affect ozone abundance through mixing and inducing polar stratospheric clouds. There are difficulties, however, in measuring the three-dimensional wind velocities in high-altitude mountain waves. Mountain waves are routinely used by sailplane pilots to gain altitude. Methods are described for estimating three-dimensional wind velocities in mountain waves using data collected during sailplane flights. The data used are the logged sailplane position and airspeed (sailplane speed relative to the local air mass). An algorithm is described to postprocess this data to estimate the three-dimensional wind velocity along the flight path, based on an assumption of a slowly varying horizontal wind veloci...

Spatial filter issues

Beamlet measurements show that cone pinholes outperform other types tested with regard to both closure and back reflections. A +/- 150 (mu) rad stainless steel cone remained open for a 15.5 kJ, 10:1 contrast shaped pulse with +/- 7.5 (mu) rad of SSD divergence, which more than meets the requirements for a NIF ignition pulse. Measurements also showed the maximum tolerable pressures in the NIF spatial filters to be a few milliTorr, leading to recommendations of 0.1 nd 0.6 mTorr for the NIF transport and cavity spatial filters, respectively.

Laser chain alignment with low-power local light sources

Timely and repeatable alignment of the 192 beam National Ignition Facility (NIF) laser will require an automatic system. Demanding accuracy requirements must be met with high reliability at low cost while minimizing the turn-around time between shots. We describe an approach for internally self-consistent alignment of the mirrors in the laser chains using a network of local light sources that serve as near field and far field alignment references. It incorporates a minimum number of alignment lasers, handles many beams in parallel, and utilizes simple control algorithms.