During the more than 60 years from the invention of the laser to demonstration of ignition on NIF, scientists, engineers and technicians at LLNL developed incrementally larger and more energetic/powerful laser systems designed for inertial confinement fusion. Each step forward brought new understanding of issues and new concepts for their solution. A continual effort to solve new and often surprising incidents of optical damage supported this evolution. Insights into problems of scale learned from previous lasers, including Argus (1977), Shiva (1978), Nova (1984) and Beamlet (1995) were combined successfully in NIF to provide this platform for the recent ignition demonstration.
This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract No. DE-AC52-07NA27344; LLNL-ABS-849703
The National Ignition Facility (NIF) at Lawrence Livermore National Laboratory contains a 192-beam 4.2 MJ neodymium glass laser (around 1053 nm or 1w) that is frequency converted to 351nm light or 3w. It was built to access the extreme high energy density conditions needed to support the nation’s nuclear stockpile in the absence of further underground nuclear tests, including studying Inertial Confinement Fusion (ICF) and ignition in the laboratory.
Over the last year, important results have been obtained demonstrated a fusion yield of 1.35MJ with 1.9MJ of laser energy (and 440 TW power) injected in the target, bringing the NIF to the threshold of ignition [2-3]. As the yield curve near ignition is steep, the laser performance team has focused on providing improved power accuracy and precision (better shot-to-shot reproducibility) with a high-fidelity pulse shaping system (HiFiPS), and also on extending the NIF operating power and energy space by 15% to 2.2MJ and 500TW.
The National Ignition Facility (NIF) at Lawrence Livermore National Laboratory contains a
192-beam 4.2 MJ neodymium glass laser (around 1053 nm or 1w) that is frequency converted to
351nm light or 3w. It has been designed to support the study of Inertial Confinement Fusion (ICF)
and High Energy Density Physics (HEDP). The NIF Precision Diagnostic System (PDS) was reactivated and new
diagnostic packages were designed and fielded that offer a more comprehensive suite
of high-resolution measurements. The current NIF laser performance will be presented as well as the preliminary results obtained with the various laser experimental campaigns using the new diagnostic tool suites.
The National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory (LLNL)
routinely fires high energy shots (> 6 kJ per beamline) through the final optics, located on the
target chamber. After a high fluence shot, exceeding 4J/cm2 at 351 nm wavelength, the final
optics are inspected for laser-induced damage. The FODI (Final Optics Damage Inspection)
system has been developed for this purpose, with requirements to detect laser-induced damage
initiation and to track and size it's growth to the point at which the optic is removed and the site
mitigated. The FODI system is the "corner stone" of the NIF optic recycle strategy. We will
describe the FODI system and discuss the challenges to make optics inspection a routine part of
NIF operations.
The National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory (LLNL) will routinely
fire high energy shots (approaching 10 kJ per beamline) through the final optics, located on the target
chamber. After a high fluence shot, exceeding 4J/cm2 at 351 nm wavelength, the final optics will be
inspected for laser-induced damage. The FODI (Final Optics Damage Inspection) system has been
developed for this purpose, with requirements to detect laser-induced damage initiation and to track and size
it's the growth to the point at which the optic is removed and the site mitigated. The FODI system is the
"corner stone" of the NIF optic recycle strategy. We will describe the FODI system and discuss the
challenges to make optics inspection a routine part of NIF operations.
We examine the effect of lattice temperature on the probability of surface damage initiation for 355nm, 7ns laser pulses for surface temperatures below the melting point to temperatures well above the melting point of fused silica. At sufficiently high surface temperatures, damage thresholds are dramatically reduced. Our results indicate a temperature activated absorption and support the idea of a lattice temperature threshold of surface damage. From these measurements, we estimate the temperature dependent absorption coefficient for intrinsic silica.
Growth of laser initiated damage plays a major role in determining optics lifetime in high power laser systems. Previous
measurements have established that the lateral diameter grows exponentially. Knowledge of the growth of the site in the
propagation direction is also important, especially so when considering techniques designed to mitigate damage growth,
where it is required to reach all the subsurface damage. In this work, we present data on both the diameter and the depth
of a growing exit surface damage sites in fused silica. Measured growth rates with both 351 nm illumination and with
combined 351 nm and 1054 nm illumination are discussed.
We examine the effect of pulse duration on both density and morphology of laser-induced damage in
KDP and SiO2. In both materials the density of damage sites scales with pulse duration to the ~ 0.4
power for 351-nm pulses between 1 and 10 ns. In SiO2 three types of damage sites are observed. The
sizes of the largest of these sites as well as the size of KDP damage sites scale approximately linearly
with pulse duration. Similarities of damage in very different materials points to properties of laser-induced
damage which are material independent and give insight to the underlying physics of laser-induced
damage.
Growth of laser initiated damage is a potential lifetime limiter of laser optics. While laser initiated damage occurs most often on the exit surface of optical components, some damage sites can occur on the input surface. We have investigated the growth of laser initiated damage in fused silica when the damage occurs on the input surface of the optic. We have measured both the threshold for growth as well as the lateral growth rate at 351 nm. The lateral growth of damage on the input surface is best described as having a linear dependence on shot number. The rate of growth has a linear dependence on fluence, with an extrapolated threshold of approximately 6 J/cm2. This behavior will be contrasted to growth of damage when located on the exit surface. The behavior will be compared to growth of input surface damage when the irradiation wavelength is 1053 nm or 527 nm.
We summarize recent investigations of the density and morphology of bulk damage in KDP crystals as a function of pulse duration, temporal profile, wavelength, and energy fluence. As previously reported by Runkel et al.1, we also find that the size of bulk damage sites varies roughly linearly with pulse duration for pulses between 1 ns and 9 ns. However this trend no longer applies at pulse durations below 1 ns. Experiments measuring the damage density and size distribution as a function of wavelength confirm many previous works which indicated a strong dependence of damage density with wavelength. However, we also find that the size of damage sites is relatively insensitive to wavelength. Further we see damage due to Flat-In-Time (FIT) pulses has different pulse length and fluence dependence than Gaussian pulses. We demonstrate that a simple thermal diffusion model can account for observed differences in damage densities due to square and Gaussian temporally shaped pulses of equal fluence. Moreover, we show that the key laser parameter governing size of the bulk damage sites is the length of time the pulse remains above a specific intensity. The different dependences of damage density and damage site size on laser parameters suggest different absorption mechanisms early and late in the damaging pulse.
In laser systems using frequency conversion, multiple wavelengths will be present on optical components. We have investigated the growth of laser initiated damage in fused silica in the presence of multiple wavelengths. In particular, we measured growth at 351 nm in the presence of 1053 nm near the threshold of growth for 351 nm alone. The data shows that the sum fluence determines the onset of growth as well as the growth rate. The measured growth coefficient is consistent with all the energy being delivered at 351 nm. Additionally, we measured growth at 527 nm in the presence of 1053 nm near the threshold of growth at 527 nm alone. In this case, the sum fluence also determines the growth coefficient but the rate is consistent with all the energy being delivered at 1053 nm. We present the measurements and discuss possible reasons for the behavior.
An experimental technique has been developed to measure the damage density ρ(Φ) variation with fluence from scatter maps of bulk damage sites in plates of KD2PO4 (DKDP) crystals combined with calibrated images of the damaging beam's spatial profile. Unconditioned bulk damage in tripler-cut DKDP crystals has been studied using 351 nm (3ω) light at pulse lengths of 0.055, 0.091, 0.30, 0.86, 2.6, and 10 ns. It is found that there is less scatter due to damage at fixed fluence for longer pulse lengths. The results also show that for all the pulse lengths the scatter due to damage is a strong function of the damaging fluence. It is determined that the pulse length scaling for bulk damage scatter in unconditioned DKDP material varies as τ0.24±0.05 over two orders of magnitude of pulse lengths. The effectiveness of 3ω laser conditioning at pulse lengths of 0.055, 0.096, 0.30, 0.86, 3.5, and 23 ns is analyzed in term of damage density ρ(Φ) at 3ω, 2.6 ns. The 860 ps conditioning to a peak irradiance of 7 GW/cm2 had the best performance under 3ω, 2.6 ns testing. It is shown that the optimal conditioning pulse length appears to lies in the range from 0.3 to 1 ns with a low sensitivity of 0.5 J/cm2/ns to the exact pulse length.
With the first four of its eventual 192 beams now executing shots and generating more than 100 kJ of laser energy at its primary wavelength of 1.06 µm, the National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory is already the world's 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 to achieve the desired cumulative performance of the laser beamline. This paper 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 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.
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 NIF's 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.
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 world's 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.
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