Kenneth R. Manes

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.

Kinoform phase plates for focal plane irradiance profile control

A versatile, rapidly convergent, iterative algorithm is presented for the construction of kinoform phase plates for tailoring the far-field intensity distribution of laser beams. The method consists of repeated Fourier transforming between the near-field and the far-field planes with constraints imposed in each plane. For application to inertial confinement fusion, the converged far-field pattern contains more than 95% of the incident energy inside a desired region and is relatively insensitive to beam aberrations.

CHARACTERIZATION OF LASER-PRODUCED PLASMA X-RAY SOURCES FOR USE IN X-RAY RADIOGRAPHY.

We report the absolute conversion efficiency ξx from the incident laser light energy to x‐ray photons for laser‐produced plasmas. Potential x‐ray backlighting (radiography) line sources having photon energies from 1.4 to 8.6 keV are studied as a function of laser wavelength, pulsewidth, and intensity. The laser intensity and pulsewidth range from 1014 to 1016 W/cm2, 100 ps to 2 ns and include incident wavelengths of 1.06, 0.53, and 0.35 μm. We found that K‐shell x‐ray line emission ξx : (1) decreases with increasing x‐ray energy, (2) decreases with increasing laser intensity, (3) decreases rapidly with pulselength, and (4) moderately increases with decreasing laser wavelength. On the contrary, for Au M band emission, at a fixed laser intensity and pulsewidth, ξx significantly increases (∼25×) upon decreasing the laser wavelength from 1.06 to 0.35 μm.

Hot images from obscurations

Certain damage observed on the optics in NOVA is consistent with a phenomenon akin to holographic imaging. (NOVA is the Lawrence Livermore National Laboratorys 10-beam Nd:glass laser used for inertial confinement fusion research.) The minimization of similar damage in next-generation laser systems is sought by first identifying the sources for these holographic images, specifying glass parameters appropriately, and staging the amplifier chain to circumvent the problem. The insight gained has lead to an explanation for a 20-year-old puzzle.

National Ignition Facility wavefront requirements and optical architecture

With the first four of its eventual 192 beams now executing shots, the National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory is already the worlds largest and most energetic laser. The optical system performance requirements that are in place for NIF are derived from the goals of the missions it is designed to serve. These missions include inertial confinement fusion (ICF) research and the study of matter at extreme energy densities and pressures. These mission requirements have led to a design strategy for achieving high quality focusable energy and power from the laser and to specifications on optics that are important for an ICF laser. The design of NIF utilizes a multipass architecture with a single large amplifier type that provides high gain, high extraction efficiency and high packing density. We have taken a systems engineering approach to the practical implementation of this design that specifies the wavefront parameters of individual optics in order to achieve the desired cumulative performance of the laser beamline. This presentation provides a detailed look at the causes and effects of performance degradation in large laser systems and how NIF has been designed to overcome these effects. We will also present results of spot size performance measurements that have validated many of the early design decisions that have been incorporated in the NIF laser architecture.

Compact, Efficient Laser Systems Required for Laser Inertial Fusion Energy

Abstract This paper presents our conceptual design for laser drivers used in Laser Inertial Fusion Energy (LIFE) power plants. Although we have used only modest extensions of existing laser technology to ensure near-term feasibility, predicted performance meets or exceeds plant requirements: 2.2 MJ pulse energy produced by 384 beamlines at 16 Hz, with 18% wall-plug efficiency. High reliability and maintainability are achieved by mounting components in compact line-replaceable units that can be removed and replaced rapidly while other beamlines continue to operate, at up to ˜13% above normal energy, to compensate for neighboring beamlines that have failed. Statistical modeling predicts that laser-system availability can be greater than 99% provided that components meet reasonable mean-time-between-failure specifications.

Studies of Raman scattering from overdense targets irradiated by several kilojoules of 0.53 μm laser light

In this paper a study of stimulated Raman scattering (SRS) from relatively planar plasmas irradiated with short‐wavelength (0.53 μm) laser light is reported. The Novette Laser Facility [Laser Part. Beams 3, 173 (1985)] produced several kilojoules of light in 1 nsec, which allowed it to irradiate a large spot with enough intensity to produce significant Raman scattering. These experiments measured the fluence, angular distribution, spectrum, and timing of the Raman light, as a function of the average laser intensity. Reductions in the Raman fluence at low laser intensity are attributed to collisional damping. The measured SRS fluence was larger than that predicted from convective amplification of bremsstrahlung noise, as calculated using the average properties of the laser beam and the plasma. Possible contributions to the observed scattering from enhanced noise, Raman scattering within filaments, and the absolute Raman instability at density extrema are discussed.

Light–plasma interaction studies with high-power glass laser*

High-intensity (1013–1017 W/cm2) 1.06 μm laser light absorption experiments with spherical and planar targets suggest that inverse bremsstrahlung absorption is not the dominant absorption mechanism. Evidence is presented that resonance absorption together with ponderomotive force effects such as filamentation and density profile steepening strongly influence the laser light absorption.

Comparison of Nd:phosphate glass, Yb:YAG and Yb:S-FAP laser beamlines for laser inertial fusion energy (LIFE) [Invited]

We present the results of performance modeling of diode-pumped solid state laser beamlines designed for use in Laser Inertial Fusion Energy (LIFE) power plants. Our modeling quantifies the efficiency increases that can be obtained by increasing peak diode power and reducing pump-pulse duration, to reduce decay losses. At the same efficiency, beamlines that use laser slabs of Yb:YAG or Yb:S-FAP require lower diode power than beamlines that use laser slabs of Nd:phosphate glass, since Yb:YAG and Yb:S-FAP have longer storage lifetimes. Beamlines using Yb:YAG attain their highest efficiency at a temperature of about 200K. Beamlines using Nd:phosphate glass or Yb:S-FAP attain high efficiency at or near room temperature.

Evidence of localized heating in CO2‐laser‐produced plasmas

Polyethylene foils and parylene disks have been irradiated by CO2 (λ∼10.6 μm) and Nd : YAG‐glass (λ∼1.06 μm) laser pulses focused to flux levels in the 1013‐ and 1014‐W/cm2 range. X‐ray pinhole photographs of the CO2‐laser‐produced plasmas exhibit intense localized emission regions whose characteristic dimensions are smaller than the nearly diffraction‐limited focal spot of the laser. Corresponding photographs of the glass‐laser‐produced plasmas show no evidence of localized emission. These experimental results are consistent with theoretical predictions for laser‐beam trapping and/or filamentation in laser‐produced plasmas.