Richard A. Sacks

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.

Design and modeling of ignition targets for the National Ignition Facility

Several targets are described that in simulations give yields of 1–30 MJ when indirectly driven by 0.9–2 MJ of 0.35 μm laser light. The article describes the targets, the modeling that was used to design them, and the modeling done to set specifications for the laser system in the proposed National Ignition Facility. Capsules with beryllium or polystyrene ablators are enclosed in gold hohlraums. All the designs utilize a cryogenic fuel layer; it is very difficult to achieve ignition at this scale with a noncryogenic capsule. It is necessary to use multiple bands of illumination in the hohlraum to achieve sufficiently uniform x‐ray irradiation, and to use a low‐Z gas fill in the hohlraum to reduce filling of the hohlraum with gold plasma. Critical issues are hohlraum design and optimization, Rayleigh–Taylor instability modeling, and laser–plasma interactions.

Experimental comparison of a Shack–Hartmann sensor and a phase-shifting interferometer for large-optics metrology applications

We performed a direct side-by-side comparison of a Shack-Hartmann wave-front sensor and a phase-shifting interferometer for the purpose of characterizing large optics. An expansion telescope of our own design allowed us to measure the surface figure of a 400-mm-square mirror with both instruments simultaneously. The Shack-Hartmann sensor produced data that closely matched the interferometer data over spatial scales appropriate for the lenslet spacing, and much of the <20-nm rms systematic difference between the two measurements was due to diffraction artifacts that were present in the interferometer data but not in the Shack-Hartmann sensor data. The results suggest that Shack-Hartmann sensors could replace phase-shifting interferometers for many applications, with particular advantages for large-optic metrology.

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.

Damage mechanisms avoided or managed for NIF large optics

Abstract After every other failure mode has been considered, in the end, the high-performance limit of all lasers is set by optical damage. The demands of inertial confinement fusion (ICF) pushed lasers designed as ICF drivers into this limit from their very earliest days. The first ICF lasers were small, and their pulses were short. Their goal was to provide as much power to the target as possible. Typically, they faced damage due to high intensity on their optics. As requests for higher laser energy, longer pulse lengths, and better symmetry appeared, new kinds of damage also emerged, some of them anticipated and others unexpected. This paper will discuss the various types of damage to large optics that had to be considered, avoided to the extent possible, or otherwise managed as the National Ignition Facility (NIF) laser was designed, fabricated, and brought into operation. It has been possible for NIF to meet its requirements because of the experience gained in previous ICF systems and because NIF designers have continued to be able to avoid or manage new damage situations as they have appeared.

Wavefront control of high-power laser beams in the National Ignition Facility (NIF)

The use of lasers as the driver for inertial confinement fusion and weapons physics experiments is based on their ability to produce high-energy short pulses in a beam with low divergence. Indeed, the focusability of high quality laser beams far exceeds alternate technologies and is a major factor in the rationale for building high power lasers for such applications. The National Ignition Facility (NIF) is a large, 192-beam, high-power laser facility under construction at the Lawrence Livermore National Laboratory for fusion and weapons physics experiments. Its uncorrected minimum focal spot size is limited by laser system aberrations. The NIF includes a Wavefront Control System to correct these aberrations to yield a focal spot small enough for its applications. Sources of aberrations to be corrected include prompt pump-induced distortions in the laser amplifiers, previous-shot thermal distortions, beam off-axis effects, and gravity, mounting, and coating-induced optic distortions. Aberrations from gas density variations and optic-manufacturing figure errors are also partially corrected. This paper provides an overview of the NIF Wavefront Control System and describes the target spot size performance improvement it affords. It describes provisions made to accommodate the NIFs high fluence (laser beam and flashlamp), large wavefront correction range, wavefront temporal bandwidth, temperature and humidity variations, cleanliness requirements, and exception handling requirements (e.g. wavefront out-of-limits conditions).

The National Ignition Facility (NIF) wavefront control system

A wavefront control system will be employed on NIF to correct beam aberrations that otherwise would limit the minimum target focal spot size. For most applications, NIF requires a focal spot that is a few times the diffraction limit. Sources of aberrations that must be corrected include prompt pump-induced distortions in the laser slabs, thermal distortions in the laser slabs from previous shots, manufacturing figure errors in the optics, beam off-axis effects, gas density variations, and gravity, mounting, and coating-induced optic distortions.

The National Ignition Facility 2007 laser performance status

The National Ignition Facility (NIF) at Lawrence Livermore National Laboratory contains a 192-beam 3.6 MJ neodymium glass laser that is frequency converted to 351nm light. It has been designed to support high energy density science (HEDS), including the demonstration of fusion ignition through Inertial Confinement. To meet this goal, laser design criteria include the ability to generate pulses of up to 1.8-MJ total energy at 351nm, with peak power of 500 TW and precisely-controlled temporal pulse shapes spanning two orders of magnitude. The focal spot fluence distribution of these pulses is conditioned, through a combination of special optics in the 1ω (1053 nm) portion of the laser (continuous phase plates), smoothing by spectral dispersion (SSD), and the overlapping of multiple beams with orthogonal polarization (polarization smoothing). In 2006 and 2007, a series of measurements were performed on the NIF laser, at both 1ω and 3ω (351 nm). When scaled to full 192-beam operation, these results lend confidence to the claim that NIF will meet its laser performance design criteria and that it will be able to simultaneously deliver the temporal pulse shaping, focal spot conditioning, peak power, shot-to-shot reproducibility, and power balance requirements of indirect-drive fusion ignition campaigns. We discuss the plans and status of NIFs commissioning, and the nature and results of these measurement campaigns.

Design modeling of the 100-J diode-pumped solid state laser for Project Mercury

We present the energy, propagation, and thermal modeling for a diode-pumped solid-state laser called Mercury being designed and built at LLNL using Yb:S-FAP [i.e., Yb3+-doped Sr5(PO4)3F crystals] for the gain medium. This laser is intended to produce 100 J pulses at 1 to 10 ns at 10 Hz with an electrical efficiency of approximately 10%. Our modeling indicates that the laser will be able to meet its performance goals.

The commissioning of the advanced radiographic capability laser system: experimental and modeling results at the main laser output

The National Ignition Facility (NIF) at Lawrence Livermore National Laboratory is the first of a kind megajoule-class laser with 192 beams capable of delivering over 1.8 MJ and 500TW of 351nm light [1], [2]. It has been commissioned and operated since 2009 to support a wide range of missions including the study of inertial confinement fusion, high energy density physics, material science, and laboratory astrophysics. In order to advance our understanding, and enable short-pulse multi-frame radiographic experiments of dense cores of cold material, the generation of very hard x-rays above 50 keV is necessary. X-rays with such characteristics can be efficiently generated with high intensity laser pulses above 1017 W/cm² [3]. The Advanced Radiographic Capability (ARC) [4] which is currently being commissioned on the NIF will provide eight, 1 ps to 50 ps, adjustable pulses with up to 1.7 kJ each to create x-ray point sources enabling dynamic, multi-frame x-ray backlighting. This paper will provide an overview of the ARC system and report on the laser performance tests conducted with a stretched-pulse up to the main laser output and their comparison with the results of our laser propagation codes.