Jerome M. Auerbach

National Ignition Facility laser performance status

The National Ignition Facility (NIF) is the worlds largest laser system. It contains a 192 beam neodymium glass laser that is designed to deliver 1.8 MJ at 500 TW at 351 nm in order to achieve energy gain (ignition) in a deuterium-tritium nuclear fusion target. To meet this goal, laser design criteria include the ability to generate pulses of up to 1.8 MJ total energy, with peak power of 500 TW and temporal pulse shapes spanning 2 orders of magnitude at the third harmonic (351 nm or 3omega) of the laser wavelength. The focal-spot fluence distribution of these pulses is carefully controlled, through a combination of special optics in the 1omega (1053 nm) portion of the laser (continuous phase plates), smoothing by spectral dispersion, and the overlapping of multiple beams with orthogonal polarization (polarization smoothing). We report performance qualification tests of the first eight beams of the NIF laser. Measurements are reported at both 1omega and 3omega, both with and without focal-spot conditioning. When scaled to full 192 beam operation, these results demonstrate, to the best of our knowledge for the first time, that the NIF will meet its laser performance design criteria, and that the NIF can simultaneously meet the temporal pulse shaping, focal-spot conditioning, and peak power requirements for two candidate indirect drive ignition designs.

SERRATED-APERTURE APODIZERS FOR HIGH-ENERGY LASER SYSTEMS

Spatial beam apodization is a critical part of the design of high-energy solid-state laser systems. Standard methods of making apodizers include photographic and metal-vapor-deposition techniques. Apodizers fabricated with these methods are subject to damage and deterioration from high-intensity laser pulses. An alternative approach is to use a serrated-edge aperture in conjunction with the spatial filter. This system can produce beams with smooth edge profiles. We present the theory of operation of the serrated aperture along with some useful design rules and describe the successful application of a serrated-aperture apodizer in the Beamlet laser system.

Multicrystal designs for efficient third-harmonic generation.

Efficient frequency tripling of high-fluence, narrow-band laser pulses is routinely accomplished with a doubling crystal and a sum-frequency mixer. The addition of a second mixer can dramatically improve conversion efficiencies for the large bandwidths of interest for inertial confinement fusion. Designs that involve two doublers similarly offer a higher dynamic range of conversion efficiency versus intensity than the usual two-crystal design.

NIF final optics system: frequency conversion and beam conditioning

Installation and commissioning of the first of forty-eight Final Optics Assemblies on the National Ignition Facility was completed this past year. This activity culminated in the delivery of first light to a target. The final optics design is described and selected results from first-article commissioning and performance tests are presented.

Modeling of frequency doubling and tripling with measured crystal spatial refractive-index nonuniformities.

Efficient frequency doubling and tripling are critical to the successful operation of inertial confinement fusion laser systems such as the National Ignition Facility currently being constructed at the Lawrence Livermore National Laboratory and the Omega laser at the Laboratory for Laser Energetics. High-frequency conversion efficiency is strongly dependent on attainment of the phase-matching condition. In an ideal converter crystal, one can obtain the phase-matching condition throughout by angle tuning or temperature tuning of the crystal as a whole. In real crystals, imperfections in the crystal structure prohibit the attainment of phase matching at all locations in the crystal. We have modeled frequency doubling and tripling with a quantitative measure of this departure from phase matching in real crystals. This measure is obtained from interferometry of KDP and KD*P crystals at two orthogonal light polarizations.

Paraxial Wave Theory of Second and Third Harmonic Generation in Uniaxial Crystals: I. Narrowband Pump Fields

Abstract We present a paraxial wave formulation of three-wave mixing in a negative uniaxial crystal, including effects of diffraction and transverse walkoff. The theory, though general, is applied specifically to second harmonic generation in KDP with Nd:glass laser radiation, followed by the mixing of the second harmonic with the fundamental in a second KDP crystal to produce third harmonic radiation near 0·35 μm. For applications of interest, which can involve third harmonic conversion efficiencies approaching 90%, walkoff is potentially much more deleterious to conversion than diffraction. However, walkoff is negligible when the angular spectrum of the pump field does not have substantial contributions from spatial frequencies ≳ (aL)−1, where L is the crystal length and a depends upon the derivative of each extraordinary refractive index with respect to the angle between the optic axis and the direction of wave propagation. A similar result, scaling as L− 1/2 rather than L −1, holds for diffraction. We...

NIF optical specifications: the importance of the RMS gradient

The performance of the National Ignition Facility (NIF), especially in terms of laser focusability, will be determined by several key factors. One of these key factors is the optical specification of the thousands of large aperture optics that will comprise the 192 beamlines. We have previously reported on the importance of the specification of the power spectral density (PSD) on NIF performance. Recently, we have been studying the importance of long spatial wavelength phase errors on focusability. We have concluded that the preferred metric for determining the impact of these long spatial wavelength phase errors is the rms phase gradient. In this paper, we outline the overall approach to NIF optical specifications, detail the impact of the rms phase gradient on NIF focusability, discuss its trade-off with the PSD in determining the spot size, and review measurements of optics similar to those to be manufactured for NIF.

Closure phenomena in pinholes irradiated by Nd laser pulses

An experimental investigation has been made on plasma closure in pinholes irradiated by Nd glass laser pulses; 300micro500-microm diam pinholes of various materials and thicknesses have been irradiated by 20-100-J 300-psec FWHM pulses on the Janus laser system. Calorimetry measurements have yielded data on pinhole energy transmission and intensity loading on the periphery of the pinhole. Ultrafast streak photography measurements indicate effective closure velocities of 2-5 x 10(7) cm/sec. Scattered light measurements have shown the transmission loss through a typical spatial filter configuration to be primarily refractive in nature.

Frequency-conversion modeling with spatially and temporally varying beams

A computer model of third-harmonic conversion of Nd:glass laser radiation in KDP, including paraxial diffraction, walkoff, arbitrary temporal dependence, and B-integral effects, has been developed. The code is four-dimensional in that it includes the spatial field variations along and transverse to the propagation direction as well as temporal variations. A split-step algorithm based on the fast Fourier transform and a Runge-Kutta integrator is employed for forward stepping in space and time. The code has been benchmarked against results of simplified codes in the plane-wave or monochromatic limits, and predictions for conversion efficiencies are in good agreement with experimental results at Livermore. Spatial phase ripples and temporal bandwidth of the input wavefront are much more important in the tripling crystal(s) than in the doubler(s). Two-doubler designs allow for high tripling efficiencies over a broad range of intensities, while large bandwidths with high conversion efficiencies can be realized with two triplers.

Frequency converter development for the National Ignition Facility

The design of the NIF incorporates a type I/type II third harmonic generator to convert the 1.053-micrometers fundamental wavelength of the laser amplifier to a wavelength of 0.351 micrometers for target irradiation. To understand and control the tolerances in the converter design, we have developed a comprehensive error budget that accounts for effects that are known to influence conversion efficiency, including variations in amplitude and phase of the incident laser pulse, temporal bandwidth of the incident laser pulse, crystal surface figure and bulk non-uniformities, angular alignment errors, Fresnel losses, polarization errors and crystal temperature variations. The error budget provides specifications for the detailed design of the NIF final optics assembly and the fabrication of optical components. Validation is accomplished through both modeling and measurement, including full-scale Beamlet tests of a 37-cm aperture frequency converter in a NIF prototype final optics cell. The prototype cell incorporates full-perimeter clamping to support the crystals, and resides in a vacuum environment as per the NIF design.