Inertial Confinement Fusion Laser Systems Research Articles

Optimization of irradiation configuration using spherical t-designs for laser-direct-drive inertial confinement fusion

A new class of beam configurations is proposed for symmetric-direct-drive inertial confinement fusion laser systems. These configurations are based on spherical t-designs that are studied in spherical design theory in mathematics (Delsarte et al 1977 Geom. Dedicata 6 363). Employing t-design configurations offers elimination of spherical-harmonic intensity modulations for modes . Additionally, these configurations provide fast decay of intensity nonuniformities with increasing number of beams and symmetric intensity patterns on the surface of the target. Methods developed in spherical design theory offer a convenient, systematic way of obtaining beam configurations for an arbitrary number of beams.

Improving phase retrieval accuracy of optical parallel plate by adjusting exposure time of CCD

A new phase retrieval method is investigated to reconstruct wavefronts of optical parallel plates with the use of a group of diffraction patterns. More specifically, the improved phase retrieval algorithm is realized by converting the measuring plane wave into a spherical wave and by using single or double discrete Fourier transforms to calculate both far- and near-field diffraction propagations, and the accuracy is improved by adjusting the exposure time of the charge-coupled device according to the distance between the focal plane and diffraction plane. Both the theoretical and experimental results are consistent with the results obtained using conventional wavelength-modulated phase-shifting interferometry. The new method opens new doors for realizing wavefront measurement of optical parallel plates with high accuracy, especially for large-aperture parallel plates in inertial confinement fusion laser systems.

Surface error modeling, evaluation and optimization of large optics in inertial confinement fusion laser system

Surface errors of mounted large optics have significant impacts on laser performance in high power laser system. However, in SG-III inertial confinement fusion (ICF) laser system, it is really tough to realize the minimized surface error and satisfy the design criterions. In this paper, the compositions and corresponding causes of complicated surface topographies are analyzed systematically. Surface decoupling, reconstruction and fitting methods are proposed successively and constitute an integrated surface modeling methodology to visualize and evaluate both offline and online mirror surfaces. Further, we present a closed-loop “mounting – inspection – optimization” assembly process based on the combination of interferometric measuring, finite element simulation and digital reconstruction, which will lead the mounting induced surface error of the large optics to meet the wavefront criterions. Through a case study of the large aperture laser transport mirror, the proposed approach is proved to have good performance on both improving optical mounting performance and obtaining accurate surface features.

FM-to-AM modulations induced by a weak residual reflection stack of sine-modulated pulses in inertial confinement fusion laser systems

We report on frequency to amplitude modulation (FM-to-AM) conversion induced by a weak residual reflection stack of sine-modulated pulses in a complex laser system. Theoretical and experimental investigations reveal that when weak residual reflected pulses stack on the main pulse, the spectral intensity changes in the stacked region, which then converts to obvious AM. This kind of FM-to-AM effect often occurs in the tail of the pulse and cannot be eliminated by common compensation methods, which even enhance the modulation depth. Furthermore, the actual intensity modulation frequency and depth induced by the residual reflection stack are much higher and deeper than observed on the oscilloscope, which is harmful for safe operation of the laser facility and the driving power balance during inertial confinement fusion. To eliminate this kind of FM-to-AM effect, any possible on-axis and near-axis residual reflection in laser systems must be avoided.

ON INTRODUCTION OF SCATTERING COEFFICIENT FOR SOLVING DIFFRACTION PROBLEMS

The paper deals with diffraction theory with the most of diffraction problems solved nowadays. The authors proposed and carried out experiments that couldn’t be explained in terms of diffraction theory. The results of experiments with diffraction image contrast varies from zero to the values exceeding those observed in classical diffraction experiments have been presented. It is discussed the possibility of creating an improved theory of diffraction in which real physical processes taking place when light beams are limited by diaphragms or mirrors. One of the potential ways of enhancing the method of solving diffraction problems is introduction of dispersion factor. The developed method can be used in solving the wide range of diffraction problems, as well as in calculating soft apertures for inertial confinement fusion laser systems – one of the main directions of alternative energy sources search. The novel view on diffraction mechanisms may lead to understanding the other light phenomena and development of specific optical tools, e.g. unique soft apertures for inertial confinement fusion purposes.

Fundamental-frequency-absorbed oxyfluoride glass in a high-power laser.

A high-power third-harmonic laser faces challenges in the filtering remnant unconverted fundamental frequency, which is from the frequency converting crystal. In this work, a novel fundamental-frequency-absorbed oxyfluoride glass has been prepared, which provides a possible option to solve the problem. By being doped with Fe2+ ion, the glass shows strong absorption property at 1053 nm, and the glass's transmittances at 351 and 1053 nm are stable with changing the laser power or increasing the irradiation times under high-power laser irradiation. Meanwhile, the laser-induced damage threshold of the glass is 12.5 J/cm2 at 351 nm, which is two times higher than that of fused silica whose threshold is 6.2 J/cm2 in the same testing condition. The glass also exhibits a higher laser-induced damage threshold as well as 36.6 J/cm2 at its absorption wavelength of 1053 nm. The results indicate that this glass is promising as a color-separation optic, thus allowing a novel design for the final optics assembly in an inertial confinement fusion laser system.

The National Ignition Facility and the National Ignition Campaign

The National Ignition Facility (NIF), the world's largest and most powerful laser system for inertial confinement fusion (ICF) and experiments studying high-energy-density (HED) science, is now operational at Lawrence Livermore National Laboratory (LLNL). NIF construction was certified by the Department of Energy as complete on March 27, 2009. NIF, a 192-beam Nd:glass laser facility, will ultimately produce 1.8 MJ 500 TW of 351-nm third-harmonic ultraviolet light. On March 10, 2009, a total of 192-beam energy of 1.1 MJ was demonstrated; this is approximately 30 times more energy than ever produced in an ICF laser system. The principal goal of NIF is to achieve ignition of a deuterium-tritium (DT) fuel capsule and provide access to HED physics regimes needed for experiments related to national security, fusion energy, and broader frontier scientific exploration. NIF experiments in support of indirect-drive ignition began in August 2009. These first experiments represent the next phase of the National Ignition Campaign (NIC). The NIC is a national effort to achieve fusion ignition and is coordinated through a detailed execution plan that includes science, technology, and equipment. The equipment required for ignition experiments includes diagnostics, a cryogenic target manipulator, and user optics. Participants in this effort include LLNL, General Atomics, Los Alamos National Laboratory, Sandia National Laboratory, and the University of Rochester Laboratory for Energetics. The primary goal for NIC is to have all of the equipment operational, integrated into the facility, and ready to begin a credible ignition campaign in 2010. With NIF now operational, the long-sought goal of achieving self-sustained nuclear fusion and energy gain in the laboratory is much closer to realization. Successful demonstration of ignition and net energy gain on NIF will be a major step toward demonstrating the feasibility of inertial fusion energy (IFE) and will likely focus the world's attention on the possibility of an ICF energy option. NIF experiments to demonstrate ignition and gain will use central-hot-spot (CHS) ignition, where a spherical fuel capsule is simultaneously compressed and ignited. The scientific basis for CHS has been intensively developed. Achieving ignition with CHS will open the door for other advanced concepts, such as the use of high-yield pulses of visible wavelength rather than ultraviolet and fast-ignition concepts. Moreover, NIF will have important scientific applications in such diverse fields as astrophysics, nuclear physics, and materials science. The NIC will develop the full set of capabilities required to operate NIF as a major national and international user facility. A solicitation for NIF frontier science experiments is planned for summer 2009. This paper summarizes the design, performance, and status of NIF and plans for the NIF ignition experimental program. A brief summary of the overall NIF experimental program is also presented.

Way of detecting entire beam path aberrations of laser systems based on a phase-retrieval method

A method is presented to detect entire beam path aberrations of many laser systems, especially the SG-III inertial confinement fusion (ICF) laser system, which is under construction in China. Instead of a Hartman-Shack (HS) sensor, a CCD will be introduced into the target chamber of the ICF laser system. The HS sensor and the CCD could record the near-field wavefront distributions and the intensity signals of their corresponding focal spots, respectively. An amendatory phase-retrieval method is introduced to reconstruct the aberrations of the entire beam path from near-field wavefront distributions and their corresponding focal spots intensity. Principle experimental results indicate that the aberrations of the entire beam path of a laser system can be precisely detected based on this method.

Modeling of frequency doubling and tripling with measured crystal spatial refractive-index nonuniformities

Efficient frequency doubling and tripling are critical to the successful operation of inertial confinement fusion laser systems such as the National Ignition Facility currently being constructed at the Lawrence Livermore National Laboratory and the Omega laser at the Laboratory for Laser Energetics. High-frequency conversion efficiency is strongly dependent on attainment of the phase-matching condition. In an ideal converter crystal, one can obtain the phase-matching condition throughout by angle tuning or temperature tuning of the crystal as a whole. In real crystals, imperfections in the crystal structure prohibit the attainment of phase matching at all locations in the crystal. We have modeled frequency doubling and tripling with a quantitative measure of this departure from phase matching in real crystals. This measure is obtained from interferometry of KDP and KD*P crystals at two orthogonal light polarizations.

The spatial location of laser-driven, forward-propagating waves in a National-Ignition-Facility-relevant plasma

Ion acoustic and electron plasma waves, associated with backward-propagating stimulated Brillouin scattering and stimulated Raman scattering, have been diagnosed in a long-scale-length, nearly homogenous plasma with transverse flow. Thomson scattered light from a probe beam is employed to show that these waves are well localized in space and for a time much shorter than the laser pulse duration. These plasma conditions are relevant to hohlraum design for the National Ignition Facility inertial confinement fusion laser system. [R. Sawicki et al., Fusion Technol. 34, 1097 (1998)].