This PDF file contains the front matter associated with SPIE Proceedings Volume 8602, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and Conference Committee listing.

The National Ignition Facility (NIF) is the world’s most energetic laser, having demonstrated in excess of 1.9MJ @351nm with Inertial Confinement Fusion pulse-shapes in July, 2012. First commissioned with 192 operational beamlines in March, 2009, NIF has since transitioned to routine operation for stockpile stewardship, inertial confinement fusion research, and basic high energy density science.

Because germanium and silicon may be used as dopants in the ablator of ignition target, the knowledge of their opacities is crucial. We have calculated the opacity by using two approaches. The first one utilizes a detailed line calculation in which the atomic database is provided by the MCDF code. A lineshape code was then adapted to the calculation of opacity profiles. Because the calculation time is prohibitive when the number of lines is huge, a second approach, combining detailed line calculations and statistical calculations is used. This approach necessitates much smaller calculation than the first one and is then well suited for extensive calculations. The monochromatic opacity and the Rosseland and Planck mean opacities are calculated for various relevant densities and temperatures.

The Orion laser facility at AWE in the UK began operations at the start of 2012 to study high energy density physics. It consists of ten nanosecond beam lines and two sub-picosecond beam lines. The nanosecond beam lines each deliver 500 J per beam in 1ns at 351nm with a user-definable pulse shape between 0.1ns and 5ns. The short pulse beams each deliver 500J on target in 500fs with an intensity of greater than 10²¹ Wcm^-2 per beam. All beam lines have been demonstrated, delivering a pulse to target as described. A summary of the design of the facility will be presented, along with its operating performance over the first year of experimental campaigns. The facility has the capability to frequency-double one of the short pulse beams, at sub aperture, to deliver a high contrast short pulse to target with up to 100J. This occurs post-compression and uses a 3mm thick, 300mm aperture KDP crystal. The design and operational performance of this work will be presented. During 2012, the laser performance requirements have been demonstrated and key diagnostics commissioned; progress of this will be presented. Target diagnostics have also been commissioned during this period. Also, there is a development program under way to improve the contrast of the short pulse (at the fundamental) and the operational efficiency of the long pulse. It is intended that, from March 2013, 15% of facility operating time will be made available to external academic users in addition to collaborative experiments with AWE scientists.

We present the design parameters of a diode-pumped 100J-class multi-slab Yb:YAG laser at 10 Hz scalable to the kJ regime. Results of detailed energetics and thermo-optical modeling confirm the viability of cryogenic helium-gas cooling approach to drastically reduce thermally-induced distortions in the laser slabs. In addition, a comparison of spectral measurements from laser-diode stacks and Yb:YAG crystals validates the feasibility of highly efficient diode-pumped solid-state lasers at cryogenic temperatures.

In order to avoid propagation nonlinearities (Kerr effect, Raman and Brillouin scattering) and optical damage, nanosecond high power lasers such as the Laser MegaJoule (LMJ) amplify quasi-monochromatic pulses. But they generate a static speckle pattern in the focal spot. This speckle pattern needs to be smoothed in order to lower high intensity peaks which are detrimental during the propagation and the interaction with the plasma in the target. Different techniques are implemented to smooth the intensity nevertheless all high power lasers carry at least smoothing by spectral dispersion. It consists in broadening the spectrum through a phase modulator and focusing the different wavelengths at slightly different positions using a diffractive element such as a grating. In the temporal domain, it has been theoretically shown that the pulse power is thus filtered between near field and far field [1, 2]. The filtering allows techniques such as “picket fence” to increase conversion efficiency [1] and reduces detrimental effects of unwanted intensity distortions called FM-AM conversion [2, 3]. Here, to the best of our knowledge we show the first experimental measurement of the frequency transfer function of this filtering. Measurements are in perfect agreement with the numerical calculations.

Programmable spatial shapers using liquid-crystal-based spatial-light-modulators in the National Ignition Facility lasers enable spatial shaping of the beam profile so that power delivered to the target can be maximized while maintaining system longevity. Programmable spatial shapers achieve three objectives: Introduce obscurations shadowing isolated flaws on downstream optical elements that could otherwise be affected by high fluence laser illumination; Spatial shaping to reduce beam peak-to-mean fluence variations to allow the laser to operate at higher powers so that maximum power can be delivered to the target; And finally gradually exposing the optical regions that have never seen laser light because they have always had shadowing from a blocker that is no longer needed. In this paper, we describe the control and image processing algorithms that determine beam shaping and verification of the beam profile. Calibration and transmittance mapping essential elements of controlling the PSS are described along with spatially nonlinear response of the device such as scale and rotation.

The National Ignition Facility (NIF) is producing experimental results for the study of Inertial Confinement Fusion (ICF). The Gamma Reaction History (GRH) diagnostic at NIF can detect gamma rays to measure fusion burn parameters such as fusion burn width, bang time, neutron yield, and areal density of the compressed ablator for cryogenic deuterium-tritium (DT) implosions. Gamma-ray signals detected with this diagnostic are inherently distorted by hardware impulse response functions (IRFs) and gains, and are comprised of several components including gamma rays from laser-plasma interactions (LPI). One method for removing hardware distortions to approximate the gamma-ray reaction history is deconvolution. However, deconvolution of the distorted signal to obtain the gamma-ray reaction history and its associated parameters presents an ill-posed inverse problem and does not separate out the source components of the gamma-ray signal. A multi-dimensional parameter space model for the distorted gamma-ray signal has been developed in the literature. To complement a deconvolution, we develop a multi-objective optimization algorithm to determine the model parameters so that the error between the model and the collected gamma-ray data is minimized in the least-squares sense. The implementation of the optimization algorithm must be suffciently robust to be used in automated production software. To achieve this level of robustness, impulse response signals must be carefully processed and constraints on the parameter space based on theory and experimentation must be implemented to ensure proper convergence of the algorithm. In this paper, we focus on the optimization algorithm's theory and implementation.

A one-dimensional (1-D) smoothing by spectral dispersion (SSD) system for smoothing focal-spot nonuniformities using multiple modulation frequencies has been commissioned on one long-pulse beamline of OMEGA EP, the first use of such a system in a high-energy laser. Frequency modulation (FM) to amplitude modulation (AM) conversion in the infrared (IR) output, frequency conversion, and final optics affected the accumulation of B-integral in that beamline. Modeling of this FM-to-AM conversion using the code Miró [Morice, O., “Miró: Complete modeling and software for pulse amplification and propagation in high-power laser systems,” Opt. Eng. 42(6), 1530−1541 (2003).] was used as input to set the beamline performance limits for picket (short) pulses with multi-FM SSD applied. This article first describes that modeling. The 1-D SSD analytical model of Chuang [Chuang, Y.-H., “Amplification of broad-bandwidth phase-modulated laser counterpropagating light waves in homogeneous plasma,” Ph.D. thesis, University of Rochester (September 1991).] is first extended to the case of multiple modulators and then used to benchmark Miró simulations. Comparison is also made to an alternative analytic model developed by Hocquet et al. [Hocquet, S., Penninckx, D., Bordenave, E., Gouédard, C. and Jaouën, Y., “FM-to-AM conversion in high-power lasers,” Appl. Opt. 47(18), 3338−3349 (2008).] With the confidence engendered by this benchmarking, Miró results for multi-FM SSD applied on OMEGA EP are then presented. The relevant output section(s) of the OMEGA EP Laser System are described. The additional B-integral in OMEGA EP IR components upstream of the frequency converters due to AM is modeled. The importance of locating the image of the SSD dispersion grating at the frequency converters is demonstrated. Finally, since frequency conversion is not performed in OMEGA EP’s target chamber, the additional AM due to propagation to the target chamber’s vacuum window is modeled.

A one-dimensional smoothing by spectral dispersion (SSD) demonstration system for smoothing focal-spot nonuniformities using multiple modulation frequencies (multi-FM SSD) was commissioned on one long-pulse beamline of OMEGA EP—the first use of such a system in a high-energy laser. System models of frequency modulation-to-amplitude modulation (FM-to-AM) conversion in the OMEGA EP beamline and final optics were used to develop an AM budget. The AM budget in turn provided a UV power limit of 0.85 TW, based on accumulation of B-integral in the final optics. The front end of the demonstration system utilized a National Ignition Facility preamplifier module (PAM) with a custom SSD grating inserted into the PAM’s multipass amplifier section. The dispersion of the SSD grating was selected to cleanly propagate the dispersed SSD bandwidth through various pinholes in the system while maintaining sufficient focal-spot smoothing performance. A commissioning plan was executed that systematically introduced the new features of the demonstration system into OMEGA EP. Ultimately, the OMEGA EP beamline was ramped to the UV power limit with various pulse shapes. The front-end system was designed to provide flexibility in pulse shaping. Various combinations of pickets and nanosecond-scale drive pulses were demonstrated, with multi-FM SSD selectively applied to portions of the pulse. Analysis of the dispersion measured by the far-field diagnostics at the outputs of the infrared beamline and the frequency-conversion crystals indicated that the SSD modulation spectrum was maintained through both the beamline and the frequency-conversion process. At the completion of the plan, a series of equivalent-target-plane measurements with distributed phase plates installed were conducted that confirmed the expected timeintegrated smoothing of the focal spot.

A beam-shaping system, based on a liquid-crystal-on-silicon spatial light modulator, has been deployed on two of the long-pulse UV beamlines of the OMEGA EP laser. Simultaneous control of both amplitude and phase with a single spatial light modulator is possible by encoding intensity information on a high-frequency carrier phase, which is subsequently removed by a low-pass spatial filter. The beam-shaping system has been integrated into operations of the existing front-end laser source and has demonstrated improved beam uniformity at multiple points in the laser. The system operates in closed loop to optimize the input infrared beam’s spatial-amplitude profile prior to amplification and frequency conversion. Measured amplified beam profiles from near-field cameras along the laser beam’s path are used to specify the desired input infrared beam shape. The system is used to correct local hot spots in the input beam profile and to refine the amplifier gain precompensation profile that is applied to the input beam with separate static apodizers. At present, the beam-shaping system is used only to correct amplitude variations in the beam profile, but future use may also utilize the system’s capability to apply wavefront corrections to the beam.

The Laser Mégajoule (LMJ) facility has about 40 large optics per beam. For 22 bundles with 8 beams per bundle, it will contain about 7.000 optical components. First experiments are scheduled at the end of 2014. LMJ components are now being delivered. Therefore, a set of acceptance criteria is needed when the optical components are exceeding the specifications. This set of rules is critical even for a small non-conformance ratio. This paper emphasizes the methodology applied to check or re-evaluate the wavefront requirements of LMJ large optics. First we remind how LMJ large component optical specifications are expressed and we describe their corresponding impacts on the laser chain. Depending on the location of the component in the laser chain, we explain the criteria on the laser performance considered in our impact analyses. Then, we give a review of the studied propagation issues. The performance analyses are mainly based on numerical simulations with Miró propagation simulation software. Analytical representations for the wavefront allow to study the propagation downstream local surface or bulk defects and also the propagation of a residual periodic aberration along the laser chain. Generation of random phase maps is also used a lot to study the propagation of component wavefront/surface errors, either with uniform distribution and controlled rms value on specific spatial bands, or following a specific wavefront/surface Power Spectral Distribution (PSD).

A challenging aspect of preparing cryogenic targets for National Ignition Facility (NIF) ignition experiments is growing a single crystal layer (~ 70 m thick) of solid frozen deuterium-tritium (DT) fuel on the inner surface of a spherical hollow plastic capsule 2 mm in diameter. For the most critical fusion experiments, the layer must be smooth, having uniform thickness, and largely free of isolated defects (e.g. grooves). A single target layer typically takes up to 18 hours to form. X-ray images on 3 orthogonal axes are used to monitor the growth of the crystal and evaluate the quality of the layer. While these methods provide a good indicator of target layer condition, new metrics are currently being developed to take advantage of other properties in the x-ray image, which may give earlier indications of target quality. These properties include symmetry of texture, seed formation, and eigenimage analysis. We describe the approach and associated image processing to evaluate and classify these metrics, whose goal is to improve overall layer production and better quantify the quality of the layer during its growth.

Solid-state lasers have been demonstrated as attractive drivers for laser-plasma interaction and have presently been developed for various applications like inertial confinement fusion (ICF) [1], particle acceleration and intense X-ray generation [3]. Viable real world applications like power production at industrial scale will require high laser system efficiency, repetition rate and lifetime which are only possible with semiconductor diode pumping. The paper describes the work conducted with two 20 kW diode laser sources pumping an ytterbium:YAG laser amplifier. The set-up acts as a small scale prototype for the DiPOLE project [2]. This project aims to develop scalable gas cooled cryogenic multi-slab diode pumped solid state lasers capable of producing KJ pulse energy. A scale-down prototype is currently under development at the Central Laser Facility (CLF) designed to generate 10 J at 10 Hz. To secure an efficient pumping process the sources have to fulfill aside power requirement in the spectral and time domain, the claim for high homogenization and low divergence of the spatial and angular beam distribution as well as a minimization of losses within the optical path. The existing diode laser sources designed and built by INGENERIC deliver 20 kW pulsed power, concentrated on a plateau of FWHM dimension of 20 x 20 mm² with a homogeneity of more than 90 %. The center wavelength of 939.5 nm is controlled in a range of ± 0.1 nm. The time and area integrated spectrum of at least 76 % of the total energy is contained within a 6 nm wide wavelength band around the center wavelength. Repetition rates can be adjusted between 0.1 Hz up to 10 Hz with rise and fall times less than 50 μs and pulse durations from 0.2 ms to 1.2 ms. The paper describes the impact of different designs on the performance of pump sources and puts special emphasis on the influence of the optical components on efficiency and performance. In addition the influence of the measuring principle is discussed.