R. B. Ehrlich

Experimental observation and suppression of transverse stimulated Brillouin scattering in large optical components

Using 2.4-nsec pulses at 350 nm, we have observed transverse stimulated Brillouin scattering (SBS) in fused-silica optical components. Transverse SBS sets in when the product of laser fluence and growth time for the scattered optical wave exceeds ∼2.3 J nsec/cm2. An increase in laser bandwidth to 8.3 GHz suppresses SBS losses up to approximately twice the SBS threshold. We review the theory of transverse, broadband SBS and its scaling with experimental parameters.

Harmonic conversion of large-aperture 1.05-μm laser beams forinertial-confinement fusion research

To provide high-energy, high-power beams at short wavelengths for inertial-confinement fusion experiments, we routinely convert the 1.05-microm output of the Nova, Nd:phosphate-glass, laser system to its second- or third-harmonic wavelength. We describe the design and performance of the 3 x 3 arrays of potassium dihydrogen phosphate crystal plates used for type-II-type-II phase-matched harmonic conversion of the Nova 0.74-m diameter beams. We also describe an alternate type-I-type-II phasematching configuration that improves third-harmonic conversion efficiency. These arrays provide conversion of a Nova beam of up to 75% to the second harmonic and of up to 70% to the third harmonic.

Hohlraum symmetry measurements with surrogate solid targets (invited)

Time-resolved radiographic imaging of low density, solid spherical surrogate targets has been used to provide a time-dependent measurement of drive pressure symmetry in cylindrical hohlraums on both the Nova and Omega lasers. The experiments replace the usual capsule at the center of a gold hohlraum with a sphere of SiO2 foam (ρ=0.3 g/cm3). The laser generates an x-ray drive inside the hohlraum which does not produce perfectly symmetric drive pressure on a spherical target, giving rise to a distorted shock traveling radially inward. The rarefaction behind the shock generated in this sphere produces a rapid rise in x-ray transmission which is easily detectable experimentally by radiography. The position of this feature may be determined to within a few microns in our experimental setup using a gated x-ray pinhole camera. Time-dependent control of drive symmetry in a hohlraum requires the ability to adjust the laser power as a function of both time and position along the hohlraum axis. We have implemented t...

Power, energy, and temporal performance of the Nova laser facility with recent improvements to the amplifier system

High-powered glass-laser systems with multiple beams, frequency-conversion capabilities, and pulseshaping flexibility have made numerous contributions to the understanding of inertial confinement fusion and related laser-plasma interactions. The Nova laser at Lawrence Livermore National Laboratory is the largest such laser facility. We have made improvements to the Nova amplifier system that permit increased power and energy output. We summarize the nonlinear effects that now limit Novas performance and discuss power and energy produced at 1.05-, 0.53-, and 0.35-microm wavelengths, including the results with pulses temporally shaped to improve inertial confinement fusion target performance.

Modeling of large-aperture third-harmonic frequency conversion of high-power Nd:glass laser systems

To provide high-energy, high-power beams at short wavelengths for inertial-confinement-fusion experiments the authors rountinely converted the 1.053-micrometers output of the Nova, Nd:phosphate-glass, laser system to its third-harmonic wavelength. We describe performance and conversion efficiency modeling of the 3 X 3 arrays potassium-dihydrogen-phosphate crystal plates used for type II/type II phase-matched harmonic conversion of Nova 0.74-m diameter beams, and an alternate type I/type II phase-matching configuration that improves the third-harmonic conversion efficiency. These arrays provide energy conversion of up to 65% and intensity conversion to 70%.

Acoustic Damage To Large-Aperture Optics

We have observed damage to 80-cm-diam fused-silica disks and lenses subjected to high-fluence pulses (up to 2.3 J/cm2) from an upgraded Nova laser beamline (wavelength 351 nm; pulse duration 2.35 ns; beam diameter 70 cm; energy up to 8 kJ). Damage occurred in the center of each element, where a 6-cm-wide obscuration prevented direct illumination. We believe that light strongly scattered by transverse stimulated Brillouin scattering (SBS) interacts with the surface and with the bulk of the substrate, producing two kinds of acoustic waves that propagate to its center, where they become strong enough to do damage. In the surface interaction, scattered light is absorbed by an 0-ring near the perimeter of the optic, creating a Rayleigh wave that propagates along the surface to the center of the optic. The resulting damage takes the form of crater-shaped fractures about 8 mm in diameter and 4 mm deep. In the bulk interaction, transverse SBS strongly compresses the optic in large regions transverse to the direction of beam polarization at the perimeter of the beam. The compression may result from electrostriction: the SBS intensity is several times that of the incident beam. Compressive waves resulting from the relaxation of these regions propagate to the perimeter of the optic, where they are reflected as bulk tensile waves. The focusing of these tensile waves in the center of the optic results in cracks along the direction of polarization. Up to 25 percent of the incident beam energy is lost to SBS at these high fluences. Frequency chirping of the laser beam by 45 GHz strongly suppresses the SBS, and reduces the amplitude of the stress waves by about an order of magnitude; no energy loss, cratering, or cracking occurs under these conditions. We propose design rules for avoiding acoustic damage in large optics and compare observed thresholds for transverse SBS with predictions in the literature.

Observation of hohlraum-wall motion with spectrally selective x-ray imaging at the National Ignition Facility

The high fuel capsule compression required for indirect drive inertial confinement fusion requires careful control of the X-ray drive symmetry throughout the laser pulse. When the outer cone beams strike the hohlraum wall, the plasma ablated off the hohlraum wall expands into the hohlraum and can alter both the outer and inner cone beam propagations and hence the X-ray drive symmetry especially at the final stage of the drive pulse. To quantitatively understand the wall motion, we developed a new experimental technique which visualizes the expansion and stagnation of the hohlraum wall plasma. Details of the experiment and the technique of spectrally selective x-ray imaging are discussed.

On the system stability and calibration of the image plate/scanner system for plasma diagnosis at the National Ignition Facility

At the National Ignition Facility (NIF), storage phosphor image plates (IP) are used extensively for recording x-rays, charged particles, and neutrons. For x-ray imaging and spectroscopy, absolute and relative calibrations are important for extracting plasma information from the diagnostics. We use Fuji MS, SR, and TR image plates that have been cut to fit custom diagnostic envelopes. The image plates are scanned on a General Electric FLA 7000 IP flying spot scanner. Calibrations for sensitivity, spatial scale, and temperature dependent fade are applied. During a set of recent calibrations, we noticed large shifts in the absolute calibration of the image plate system. The possible source of these shifts is discussed. We discuss scanner stability and a method for calibration. We discuss the fade and temperature effects of the image plates and how this correction is applied within the NIF environment. We also compare our NIF GE FLA 7000 IP scanner with a new General Electric Amersham Typhoon IP scanner.

Implementation and performance of beam smoothing on 10 beams of the Nova Laser

Recent simulations and experiments on Nova indicate that some level of smoothing may be required to suppress filamentation in plasmas on the National Ignition Facility, resulting in the addition of 1D smoothing capability to the current baseline design. Control of stimulated Brillouin scattering and filamentation is considered essential to the success of laser fusion because they affect the amount and location of laser energy delivered to the x-ray conversion region (holhraum wall) for indirect drive and to the absorptive region for direct drive. Smoothing by spectral dispersion (SSD), reduces these instabilities by reducing nonuniformities in the focal irradiance when averaged over a finite time interval. We have installed SSD on Nova to produce beam smoothing on all 10 beam lines. A single dispersion grating is located in a position common to all 10 beam lines early in the preamplifier chain. This location limits the 1(omega) bandwidth to 2.2 angstroms with sufficient dispersion to displace the speckle field of each frequency component at the target plane by one half speckle diameter. Several beam lines were modified to allow orientation of the dispersion on each arm relative to the holhraum wall. After conversion to the third harmonic the beam passes through a kinoform phase plate (KPP) designed to produce an elliptical spot at best focus. The KPPs produce a focal spot having an elliptical flat-top envelope with a superimposed speckle pattern. Over 93% of the energy is contained in the central 400 micrometers . Calculations indicate a 16% rms intensity variance will be reached after 330 ps for a single beam.

A four-color beam smoothing irradiation system for laser-plasma interaction experiments at LLNL

A novel four-color beam smoothing scheme with a capability similar to that planned for the proposed National Ignition Facility has been deployed on the Nova laser, and has been successfully used for laser fusion experiments. Wavefront aberrations in high power laser systems produce nonuniformities in the energy distribution of the focal spot that can significantly degrade the coupling of energy into a fusion target, driving various plasma instabilities. The introduction of temporal and spatial incoherence over the face of the beam using techniques such as smoothing by spectral dispersion (SSD) can reduce these variations in the focal irradiance when averaged over a finite time interval. One of the limitations of beam smoothing techniques used to date with solid state laser systems has been the inability to efficiently frequency convert broadband pulses to the third harmonic (351 nm). To obtain high conversion efficiency, we developed a multiple frequency source that is spatially separated into four quadrants, each containing a different central frequency. Each quadrant is independently converted to the third harmonic in a four-segment Type I/Type II KDP crystal array with independent phase-matching for efficient frequency conversion. Up to 2.3 kJ of third harmonic light is generated in a 1 ns pulse, corresponding to up to 65% intrinsic conversion efficiency. SSD is implemented by adding limited frequency modulated bandwidth to each frequency component. This improves smoothing without significant impact on the frequency conversion process. The measured far field irradiance shows 25% rms intensity variation with four colors alone, and is calculated to reach this level within 3 ps. Smoothing by spectral dispersion is implemented during the spatial separation of the FM modulated beams to provide additional smoothing, reaching a 16% rms intensity variation level. Following activation the four-color system was successfully used to probe NIF-like plasmas, producing less than 1% SBS backscatter at greater than 2 multiplied by 1015 W/cm2. This paper discusses the detailed implementation and performance of the segmented four-color system on the Nova laser system.