Currently the main solid state laser facilities used for plasma physics research in the United Kingdom are the VULCAN laser at the Rutherford Appleton Laboratory and the HELEN facility at the Atomic Weapons Establishment. In the future it is proposed to replace HELEN with a new 100 TW facility to come on line early in the next century.

The activation of a full scale single beam prototype of a multipass amplifier cavity based fusion class laser has been completed. A 35 multiplied by 35 cm² beam is amplified during four passes through an 11 slab long amplifier in a cavity, and is switched out using a full aperture Pockels cell and polarizer. Further amplification is achieved in a five slab long booster amplifier, before being frequency tripled by a Type I/Type II frequency converter. We present initial performance results of this laser system, called Beamlet. At 1 omega, energies up to 17.3 kJ have been generated in a 10 ns pulse, and frequency tripled beams up to 8.3 kJ in a 3 ns pulse.

A novel four-pass amplifier with SBS phase conjugation mirror was conceptually designed. It has the advantage of compactness, lower cost, and higher reliability. A special architecture of active double-cell SBS mirror was suggested. By means of active seed injection, deeply saturating amplification, and 'addition pulse' method, the operative stability and adaptability for shaped pulse can be obtained. It seems that SBS mirror has further applied potentiality to an ICF driver.

The main goal of this report is to analyze the feasibility to use phase conjugation (PC) in megajoule class lasers for inertial confinement fusion (ICF). Phase conjugation has the potential for relieving the fabrication requirements to any optical elements and for compensation of residual thermo-optical distortions. The key problems for phase conjugation are the dynamic range over which phase conjugation operates efficiently, the reflected energy limit, the effect of the laser light bandwidth, and the possibility to mount the PC subsystem into a real megajoule laser. Analytical results of the possibility of use of a classical stimulated Brillouin scattering (SBS) mirror and based on a nonlinear crystal phase conjugator in ICF- laser are presented.

The proposed National Ignition Facility is a 192 beam Nd:glass laser system capable of driving targets to fusion ignition by the year 2005. A key factor in the flexibility and performance of the laser is a front-end system which provides a precisely formatted beam to each beamline. Each of the injected beams has individually controlled energy, temporal pulseshape, and spatial shape to accommodate beamline-to-beamline variations in gain and saturation. This flexibility also gives target designers the options for precisely controlling the drive to different areas of the target. The design of the front-end laser is described, and initial results are discussed.

We describe the performances we obtained in a variety of high power diode pumped Nd³⁺:LNA heads. Two pumping structures are analyzed, which were designed for cylindrical and square cross section rods. The cylindrical rods are tested with 1 joule pumping energy during 200 microsecond(s) . They provide high regenerative gains, in the range 10⁷ to 10⁸ at 20 mJ output energy, with less than 20 passes. The gain transverse distribution looks like a nonuniform cross distribution, at the opposite of that obtained with the square head. The central volume in the rod, whose diameter typically equals 2 mm, only participates to amplification. The purpose of the square structure is both to solve this problem and to optimize the temperature behavior at 10 Hz. Preliminary results are given. In a last step, we also present some spatio-temporal computations in order to understand the growth of the amplified beam as a function of the number of passes in the rod.

Flashlamp pumping of the large aperture multi-segment NIF amplifiers will result in large amounts of energy being deposited as heat in the amplifier components. The magnitude of the heating and the nonuniform distribution result in a delay time between shots due to wavefront distortion and steering error. A NIF requirement is that the thermal wavefront recovery must occur in less than six hours. The principal cause of long-term wavefront distortion is the thermal gradient produced in the slab as heat diffuses from the edge cladding into the pumped volume. Thermal equilibrium is established through conduction, convection, and exchange of thermal radiation. Radiative exchange between glass components, such as flashlamps, blast shields, and laser slabs is especially effective because of the large surface areas of these components and the high emissivity of the glass. Free convection within the amplifier enclosure is also important but is on the order of a 10 to 20% effect compared to radiation for the major surfaces. To evaluate the NIF design, the amplifier was modeled to calculate the thermal response of a single laser element. The amplifier is cooled by flowing room- temperature air or nitrogen through the flashlamp cassettes. Active cooling of the flashlamps and blast shields serves two purposes; the energy deposited in these components can be removed before it is transferred to the amplifier optical components, and the cooled blast shield provides a large area heat sink for removal of the residual heat from the laser slabs. Approximately 50 to 60% of the flashlamp energy is deposited in the flashlamps and blast shields. Thus, cooling the flashlamp cassette is a very effective method for removing a substantial fraction of the energy without disturbing the optical elements of the system. Preliminary thermal analysis indicates that active cooling with flow rates of 10 CFM per flashlamp is sufficient to meet the six hour thermal equilibrium requirement. An experiment was run with a scaled down version of the NIF laser slab to measure the thermal recovery time. This experiment was also modeled and the results from the model are compared with the thermal recovery experiment data.

In a plasma-electrode Pockels cell (PEPC), plasma discharges serve as transparent electrodes on each side of an electro-optic crystal such as KDP. These plasmas facilitate rapid and uniform charging and discharging of the crystal. We describe PEPC technology deployed on Beamlet and envisioned for the National Ignition Facility. Performance on Beamlet is discussed in detail. We also discuss models which have shed light on PEPC operation. These models describe both the high-voltage sheath that forms near the crystal surface and the characteristics of the bulk plasma column.

In order to protect most the final optics hardware of megajoule-class lasers from the radiations produced by ignition shots, it is desirable to steer the laser beams after frequency conversion. Using a transmission grating etched in fused silica is an attractive solution for 3 omega beam steering. The conceptual design of a focusing system based on this device is reviewed. The issues of pulse temporal shear and beam transport are addressed. The compatibility of 3 omega gratings with other beam steering devices is assessed in view of possible evolution of the focusing system during the lifetime of the laser facility.

The baseline design of the National Ignition Facility (NIF) calls for sampling gratings to provide third-harmonic energy diagnostics in the highly constrained area of the target chamber. These 40 multiplied by 40 cm transmission gratings are to diffract at (order plus 1) nominally 0.3% of the incident 351 nm light at a small angle onto a focusing mirror and into a calorimeter. The design calls for a plane grating of 500 lines/mm, and approximately 30 nm deep, etched into a fused silica focusing lens and subsequently overcoated with a sol-gel antireflective coating. Gratings of similar aperture and feature size have been produced for other applications by ion etching processes, but, in an effort to reduce substantially the cost of such optics, we are studying the feasibility of making these gratings by wet chemical etching techniques. Experimentation with high-quality fused silica substrates on 5 and 15 cm scale has led to a wet etching process which can meet the design goals and which offers no significant scaleup barriers to full sized optics. The grating is produced by holographic exposure and a series of processing steps using only a photoresist mask and a final hydrofluoric acid etch. Gratings on 15 cm diameter test substrates exhibit absolute diffraction efficiencies from 0.2 - 0.4% with a standard deviation of about 15% of the mean over the full aperture. The efficiency variation is due to variation in linewidth caused by spatial nonuniformities in exposure energy. Uniformity improvements can be realized by using a smaller, more uniform portion of the exposure beam and exposing for longer times. The laser damage threshold for these gratings has been measured at LLNL and found to be identical to that of the fused silica substrate. Scaleup to full-sized substrates will use techniques such as meniscus coating for photoresist, large-aperture holography and other processes already established at LLNL for optics of this size. A prototype sampling grating to be installed on the Beamlet laser will be produced in early 1996.

This paper reports on the development of several diffractive optical elements (DOE) to fulfill applications on high power Nd glass laser systems. The measured performance for those components realized is discussed. These are focusing beam samplers, beam shapers, and harmonic separation filters (HSF). Designs of more demanding components operating in the resonance domain are also presented. These are linear polarizing elements and beam deflectors.

We present fully continuous phase screens for producing super-Gaussian focal-plane irradiance profiles. Such phase screens are constructed with the assumption of either circular symmetric near-field and far-field profiles or a separable phase screen in Cartesian co-ordinates. In each case, the phase screen is only a few waves deep. Under illumination by coherent light, such phase screens produce high order super-Gaussian profiles in the focal plane with high energy content. Effects of beam aberrations on the focal plane profiles and their energy content are also discussed.

The requirements for laser uniformity are discussed in terms of the l-mode spectrum. It is shown that the choice of smoothing methods can significantly alter this spectrum and that this choice should be made in the context of the target physics. Although two dimensional smoothing by spectral dispersion yields a high quality near field beam profile, it results in poor smoothing for low spatial frequency. The partially coherent light method (fiber smoothing) leads to superior smoothing at low spatial frequencies, but has very poor near field beam quality. As a result, it may be desirable to use partially coherent light during the drive pulse foot (at low intensity and when minimizing the laser imprint is critical) and smoothing by spectral dispersion during the main pulse.

The National Ignition Facility (NIF) is a proposed 1.8 MJ laser facility for carrying out experiments in inertial confinement fusion, currently designed for indirect drive experiments. The direct drive approach is being pursued at the 30 kJ Omega facility at the University of Rochester. In this paper we discuss the modifications to the NIF laser that would be required for both indirect and direct drive experiments. A primary concern is the additional cost of adding direct drive capability to the facility.

The Phebus laser system has been mainly devoted to plasma physics experiments such as implosion and hydrodynamical instability studies since it was completed in 1985. But during the last two years, the three Phebus beamlines (2 main beams and a backlighter beam) are also utilized to perform some laser physics studies in view of the megajoule laser project. The goal of the laser physics experiments conducted at the Phebus facility in 1994-1995 is to validate some design issues of the megajoule laser project concerning namely power balance and frequency conversion.

To improve the symmetry of x-ray drive on indirectly driven ICF capsules, we have increased the accuracy of operating procedures and diagnostics on the Nova laser. Precision Nova operations include routine precision power balance to within 10% rms in the 'foot' and 5% rms in the peak of shaped pulses, beam synchronization to within 10 ps rms, and pointing of the beams onto targets to within 35 micrometer rms. We have also added a 'fail-safe chirp' system to avoid stimulated Brillouin scattering (SBS) in optical components during high energy shots.

Preliminary investigations of a potential broad band oscillator for the HELEN laser facility and its proposed upgrade are described. The reasons for the need of broad bandwidth and the choice of commercial technology to achieve it are discussed. The characterization of the device and the diagnostics used for the investigations are described. Small signal amplification of the bandwidth by a glass amplifier was also performed along with investigations of the effect of various bandwidths on the far field beam quality when using random phase plates.

A novel four-color beam smoothing scheme with a capability similar to that planned for the proposed National Ignition Facility has been deployed on the Nova laser, and has been successfully used for laser fusion experiments. Wavefront aberrations in high power laser systems produce nonuniformities in the energy distribution of the focal spot that can significantly degrade the coupling of energy into a fusion target, driving various plasma instabilities. The introduction of temporal and spatial incoherence over the face of the beam using techniques such as smoothing by spectral dispersion (SSD) can reduce these variations in the focal irradiance when averaged over a finite time interval. One of the limitations of beam smoothing techniques used to date with solid state laser systems has been the inability to efficiently frequency convert broadband pulses to the third harmonic (351 nm). To obtain high conversion efficiency, we developed a multiple frequency source that is spatially separated into four quadrants, each containing a different central frequency. Each quadrant is independently converted to the third harmonic in a four-segment Type I/Type II KDP crystal array with independent phase-matching for efficient frequency conversion. Up to 2.3 kJ of third harmonic light is generated in a 1 ns pulse, corresponding to up to 65% intrinsic conversion efficiency. SSD is implemented by adding limited frequency modulated bandwidth to each frequency component. This improves smoothing without significant impact on the frequency conversion process. The measured far field irradiance shows 25% rms intensity variation with four colors alone, and is calculated to reach this level within 3 ps. Smoothing by spectral dispersion is implemented during the spatial separation of the FM modulated beams to provide additional smoothing, reaching a 16% rms intensity variation level. Following activation the four-color system was successfully used to probe NIF-like plasmas, producing less than 1% SBS backscatter at greater than 2 multiplied by 10¹⁵ W/cm². This paper discusses the detailed implementation and performance of the segmented four-color system on the Nova laser system.

A computer model of third-harmonic conversion of Nd:glass laser radiation in KDP, including paraxial diffraction, walkoff, arbitrary temporal dependence, and B-integral effects, has been developed. The code is four-dimensional in that it includes the spatial field variations along and transverse to the propagation direction as well as temporal variations. A split-step algorithm based on the fast Fourier transform and a Runge-Kutta integrator is employed for forward stepping in space and time. The code has been benchmarked against results of simplified codes in the plane-wave or monochromatic limits, and predictions for conversion efficiencies are in good agreement with experimental results at Livermore. Spatial phase ripples and temporal bandwidth of the input wavefront are much more important in the tripling crystal(s) than in the doubler(s). Two-doubler designs allow for high tripling efficiencies over a broad range of intensities, while large bandwidths with high conversion efficiencies can be realized with two triplers.

A first phase modulator was made of a couple of 3 multiplied by 3 multiplied by 22 mm³ KTP crystals set into an open rectangular coaxial line in such a way that the electric field in the active medium was parallel to the crystallographic Z axis. The line was driven by a single 4 kV, 140 ps FWHM pulse through an inductive connector in the middle, giving rise to damped oscillations at 4 GHz. Phase modulation of laser beams passing through the 44 mm of KTP was due to an efficient Pockels effect owing to similar electric and optical wave velocities in the device. This latter was presented at IAEA Conference, Paris, France November 1994. The main characteristics are reviewed here, however, two noticeable modifications are introduced. The first modification is in the choice of the connector inductance to get the best compromise between voltage transmission and damping rate. The second modification consists in using a second high voltage pulse delayed and synchronized in resonance with the first one to add the respective effects. Nevertheless the combination of both pulses in the same feedthrough involves a reduction of pulse amplitude down to 2.5 kV. Fabry-Perot spectra are shown for 1.5 ns, 1053 nm single frequency laser beams entering the modulator at different working times, less than 20 ns. The measured bandwidths are between 25 and 60 pm but 2 time greater values are expected from a new design driven by two independent pulses of 4 kV.

VULCAN is a multi-beam, multi-terawatt laser facility based on Nd:glass operating at 1053 nm. The system is highly versatile, supplying four experimental areas with laser radiation at a range of pulse durations from 700 fs to 20 ns, at fundamental frequency, frequency doubled, or, as a limited option, frequency tripled wavelengths. Beams are available in a number of geometries dictated by the university based programs, which at present include: cluster; line focus including x-ray laser oscillator/amplifier geometry; backlighting; probing; and chirped pulse amplification (CPA) configurations. The system has eight beams which can deliver synchronized long and short pulses including two beams which can deliver subpicosecond CPA pulses. The CPA capabilities on VULCAN are an integral part of the laser system, not only delivering sub-picosecond pulses, but allowing uncompressed pulses and multi-pulses to be delivered to the target areas synchronized with the nanosecond pulses. This paper describes the system configuration, details the means of pulse synchronization and presents some of the pulse manipulation techniques used on VULCAN to provide the laser requirements for the experimental program.

The concept of a three-pass amplification system was developed. There was no large aperture Pockels cell and polarizer in the system. A special beam transformer was adopted to ensure the beam match and diminish self-oscillating in the cavity by insertion of a small size optical switch. Some preliminary simulation results are given to optimize the design.

A novel active double-cell SBS mirror with an injected Stokes seed in SBS oscillator is suggested. These analyzed results show that such an SBS mirror can achieve a high phase fidelity and an ability of shaped pulse output.

The possibility to control an intensity distribution in the far field of a powerful laser system by high-frequency moving the position of a focal spot is considered. Quadruple electro-optic deflector (beam wiggler) on the base of LiNbO₃ crystal installed in resonance cavity has been developed and constructed. The main parameters of the device are as follows: amplitude of the angular deflection plus or minus 4 dif.limit, clear aperture 1 cm, the deflector is designed for operation at 6.5 GHz powerful microwave source. Results of beam wiggler dynamic testing are presented and discussed.

The work is devoted to the investigation of a wide-aperture amplifier which is intended to be installed in the laser system Nova upgrade. The amplifier should meet rather severe requirements. We have carried out the experimental investigation of gain nonuniformity over all the aperture of the amplifier and estimated depolarization and phase distortions to determine the sizes of the amplifier aperture operating zone.

The design and performance of xenon flashlamps fabricated by Russian industry and successfully used in powerful Nd-glass laser systems for ICF experiments are described. Results of flashlamp testing in specific conditions of large-aperture multilamp optical amplifiers with slab active elements are presented. The external triggering of flashlamp array is shown to be more preferable than the internal triggering of each flashlamp. It was also recognized that flashlamps with cap-like hermetization units were more reliable and less expensive. The failure probability for these lamps is estimated to be about 10^-3 for 1000 test shots at an energy loading factor of 0.2.

The tolerance of the circuit topology proposed for the National Ignition Facility (NIF) power conditioning system to specific fault conditions is investigated. A new pulsed power circuit is proposed for the NIF which is simpler and less expensive than previous ICF systems. The inherent fault modes of the new circuit are different from the conventional approach, and must be understood to ensure adequate NIF system reliability. A test-bed which simulates the NIF capacitor module design was constructed to study the circuit design. Measurements from test- bed experiments with induced faults are compared with results from a detailed circuit model. The model is validated by the measurements and used to predict the behavior of the actual NIF module during faults. The model can be used to optimize fault tolerance of the NIF module through an appropriate distribution of circuit inductance and resistance. The experimental and modeling results are presented, and fault performance is compared with the ratings of pulsed power components. Areas are identified which require additional investigation.

ShenGuang II (SG-II) is a 6 kJ ICF driver with 8 beams. In this paper we report the design method and the layout of the target beam path. The beam-targeting system for the ICF experiment is designed using 'center position sensor,' a high precision spatial coordinate sensor, to position target, and align and focus the laser beam. The scheme and results of the experiment for the prototype are included.

Reflections from lens surfaces create parasitic beams that can damage optics in high-powered laser systems. These parasitic beams are low in energy initially, because of the low reflectivity of antireflection (AR) coated lens surfaces and because they are clipped by spatial filter pinholes, but subsequent amplification can raise them to damage fluence levels. Also, some of the pencil beams in multipass laser systems become pre-pulses at the output by by-pass of one or more of the passes, arriving at the output ahead of the main pulse in time. They are insidious because pencil beams that are not initially a problem can become so due to a slow degradation of the AR coatings. Both the Nova and Beamlet laser systems at LLNL have had optics damaged by pencil beams. The best solution for pencil beams is to tip the lenses far enough to eliminate them altogether. This is the approach taken for the National Ignition Facility (NIF).

The microwave transmission properties of a high-purity (greater than or equal to 40 k(Omega) (DOT)cm) single-crystal-silicon, photoconductive (PC) switch were measured while the switch was optically activated. The switch was 2.3 mm wide (the width of the microstrip electrode), 2 mm long, and 0.5 mm thick with a 0.5-mm photoconductive gap and was mounted in a 50-(Omega) microstrip transmission line. The switch was irradiated uniformly with a 150-ns FWHM pulse from a Nd:YAG laser (wavelength equals 1.064 micrometer). The insertion loss of the optically activated PC switch was nearly constant minus 0.7 dB across the measurement system bandwidth (9 GHz). Under these illumination conditions, the switch exhibited negligible bandwidth limitations. The microstrip structure by itself had an insertion loss that increased from minus 0.4 dB at 1 GHz to minus 1.4 dB at 9 GHz.

We developed an efficient electro-optic phase modulator for deep microwave modulation to control temporal coherence of laser light. Deep modulation was achieved by quasi-velocity matching between modulation microwave and light with periodic domain inversion of a bulk electro-optic crystal. The interaction length is only limited by a diffraction loss in the bulk modulator. Sideband broadening up to 3 THz by 16.25 GHz modulation was achieved in the experiment with an Ar laser. Effective coherence time and length of the modulated light are 0.3 ps and 0.1 mm, respectively.

Two dimensional smoothing by spectral dispersion is analyzed by using diffraction theory calculations. It is shown that by using standard frequency modulated light one can obtain bandwidth limited smoothing over integration times relevant to inertial confinement fusion (about 1 nsec) with modest induced beam divergence. At longer integration times one can obtain bandwidth limited smoothing by increasing the divergence and/or by using more advanced phase modulation methods.

In this paper we present experimental measurements and theoretical modeling of third harmonic (3(omega) ) conversion efficiency with optical bandwidth. Third harmonic conversion efficiency drops precipitously as the input bandwidth significantly exceeds the phase matching limitations of the conversion crystals. For Type I/Type II frequency tripling, conversion efficiency begins to decrease for bandwidths greater than approximately 60 GHz. However, conversion efficiency corresponding to monochromatic phase-matched beams can be recovered provided that the instantaneous propagation vectors are phase matched at all times. This is achieved by imposing angular spectral dispersion (ASD) on the input beam via a diffraction grating, with a dispersion such that the phase mismatch for each frequency is zero. Experiments were performed on the Optical Sciences Laser (OSL), a 1 - 100 J class laser at LLNL. These experiments used a 200 GHz bandwidth source produced by a multipassed electro-optic phase modulator. The spectrum produced was composed of discrete frequency components spaced at 3 GHz intervals. Angular dispersion was incorporated by the addition of a 1200 gr/mm diffraction grating oriented at the Littrow angle, and capable of rotation about the beam direction. Experiments were performed with a pulse length of 1-ns and a 1(omega) input intensity of approximately 4 GW/cm² for near optimal dispersion for phase matching, 5.2 (mu) rad/GHz, with 0.1, 60, and 155 GHz bandwidth, as well as for partial dispersion compensation, 1.66 (mu) rad/GHz, with 155 GHz and 0.1 GHz bandwidth. The direction of dispersion was varied incrementally 360 degrees about the beam diameter. The addition of the grating to the beamline reduced the narrowband conversion efficiency by approximately 10%. Sufficient dispersion to allow nearly full phase-matching of all frequency components along the sensitive axis of the tripler allowed recovery of the narrow band conversion efficiency with bandwidth. However, even partial dispersion compensation was shown to significantly increase broadband 3(omega) conversion efficiency.

A perturbation theory has been developed to calculate the transfer of electric field amplitude and phase ripples from the first harmonic to either the second harmonic or the third harmonic. The theory is restricted to steady-state conversion processes. In the case of small phase gradients, the real and imaginary parts of the output harmonic ripple are related to the real and imaginary parts of the input perturbation by a 2 multiplied by 2 matrix. To confirm the validity of the perturbation theory, we have performed an initial set of experiments on the Optical Sciences Laser to investigate the transfer of a weak ripple from the first harmonic to the second harmonic.

Conditions which seed the self focussing of high-power broadband laser beams are determined by examining growth rates for plane-wave perturbations on a strong pump field as a function of frequency and angle. Measurements verifying predictions of growth based on the linearized stability analysis of Bespalov and Talanov extended to broadband fields are reported.

In connection with an elaboration of selective pumping techniques for solid-state laser-drivers a stored energy formation in solid state media under absorption of a narrow-band pumping radiation has been considered. The calculations demonstrate the possibility of the inversion profile smoothing in the slab-like Nd- and Yb-doped active elements pumped from excited levels of activator's or sensitizer's ions. A possibility of the Nd:glass and iodine lasers usage to carry out modelling experiments on selective pumping at several kJ energy level is discussed.

This report presents results of a study of reabsorbed spontaneous emission (RSE) influence on the spatial inversion distribution in an active medium of large phosphate slab. Experimental results are also presented for a model case of active medium pumping by monopulse second harmonic of Nd-glass laser having specific profile of spatial distribution. RSE is observed for this case. A model is proposed for RSE simulation in solid bodies of arbitrary shape. It is shown that RSE influence is taken into account completely by first approximation, considering the rest fluorescence part of active medium to be an additional source of pump in order to define inversion in the arbitrary point of active medium. Fluorescence kinetics coincides with numerical results of a model experiment. Calculations are carried out of inversion redistribution due to RSE in a large aperture slab of phosphate glass demonstrating that spatial distribution distortions of inversion are small by the moment when inversion maximum is achieved, however, 10 - 12% of stored energy may relate to RSE effect at the absence of amplified spontaneous emission (ASE). Fluorescence kinetics measurements are carried out over slab cross-section demonstrating good agreement with model results.

Transverse stimulated Brillouin scattering (TSBS) is an important parasitic effect in high- power laser facilities for ICF applications. In this report computer models of this phenomenon in uniaxial crystals-converters and some computer runs results are presented.

Nonlinear self-focusing in laser glass imposes limits on the energy fluence that can be safely transmitted without risking damage. For this reason, it is desirable to strictly limit the peak to average spatial variations of fluence by smoothing schemes such as smoothing by spectral dispersion (SSD). While spatial variations are problematic, the same is not necessarily true of temporal variations since normal group velocity dispersion tends to smooth out temporal peaks caused by spatial self-focusing. Earlier work indicated that increased bandwidth can delay the onset of self focusing. Indeed, a point can be reached at which self phase modulation nonlinearly increases the bandwidth, changing the speckle statistics along with suppressing self focusing. Unfortunately, this study found that a large initial bandwidth (compared with the gain bandwidth) was necessary to achieve this suppression under practical conditions. The full calculation for modulated beams was carried out for one transverse dimension. Two transverse dimensional calculations only treated symmetric beams. The present work reexamines the question of self focusing threshold increases due to high bandwidth by investigating another source of such increase in three dimensional beam breakup -- the bending instability. For simplicity, we consider the behavior of a single space-time speckle. Normal dispersion can lead to splitting of the pulse and delay of self focusing for short enough pulses as noted above. In addition to the self focusing instability, the laser beam is also subject to the so-called bending (sausage like) instability which can spatially disperse the field maxima over time. Because the bending instability breaks an initial axial symmetry, a full three dimensional numerical simulation is required to study it accurately. Such calculations are possible, but costly. We have used a modified 2D nonlinear Schrodinger equation with a high power nonlinearity since this mimics the 3D behavior of the competition between self focusing and bending. This allows a semi-quantitative estimate to be made of the possible significance of the bending instability for suppression of self focusing.

Successful operation of large-scale high-power lasers, such as those in use and planned at LLNL and elsewhere, require optical elements that can withstand extremely high fluences without suffering damage. Of particular concern are gratings used for pulse compression. Laser induced damage to bulk dielectric material originates with coupling of the electric field of the radiation to bound electrons, proceeding through a succession of mechanisms that couple the electron kinetic energy to lattice energy and ultimately to macroscopic structural changes (e.g. fracture, melting, ablation, etc.). The constructive interference that is responsible for the diffractive behavior of a grating or the reflective properties of a multilayer dielectric stack can enhance the electric field above values that would occur in unstructured homogeneous material. The presence of nonuniform electric fields, resulting from diffractive coherence, has the potential to affect damage thresholds. We describe aspects of LLNL work directed towards understanding the influence of dielectric structures upon damage, with particular emphasis on electric fields within multilayer dielectric stacks.

Chirped pulse amplification is increasingly used to produce intense ultrashort laser pulses. When high efficiency gratings are the dispersive element, as in the LLNL Petawatt laser, their susceptibility to laser induced damage constitutes a limitation on the peak intensities that can be reached. To obtain robust gratings, it is necessary to understand the causes of short-pulse damage, and to recognize the range of design options for high efficiency gratings. Metal gratings owe their high efficiency to their high conductivity. To avoid the inevitable light absorption that accompanies conductivity, we have developed designs for high efficiency reflection gratings that use only transparent dielectric materials. These combine the reflectivity of a multilayer dielectric stack with a diffraction grating. We report here our present understanding of short-pulse laser induced damage, as it applies to dielectric gratings.

Two types of solid-state lasers have served as key elements in the development of laser fusion: tunable lasers, such as Ti:sapphire, and lasers with discrete emissions based on neodymium. These lasers have been utilized for research, diagnostics, and as oscillators (i.e., Nd:YLF) in the first stage. Crystal-line phosphates were studied in depth many years ago for laser applications, but these crystals generally fell into disfavor when they could not be easily commercialized. A class of self-activated materials, referred to as stoichiometric phosphates, were particularly interesting, since they could operate efficiently at high active ion concentrations without fluorescence quenching. Neodymium pentaphosphate (NdP₅O₁₄) initiated this interest, but the potential for rare-earth orthophosphate (REOP) crystals was not seriously considered at that time. Extrinsic effects observed during some fundamental studies of REOP crystal properties, such as by electron paramagnetic resonance (EPR), may heighten the interest in using these latter materials for far-ranging laser applications, including laser fusion.

The problem of creating highly effective technology to fabricate crystal elements for high- energy laser systems is observed. The principal technology is developed which permits us to grow crystal blanks with given shapes, dimensions, and orientations. The growth rates exceed the conventional ones by more than 10 times. The (101) orientation KDP crystal with dimensions 380 by 450 by 50 mm has been obtained. The possibility of use 'skewed' elements is considered.

Solid state pulse generators with controlled multi-kilovolt outputs are now production items. The range of applications within the field of lasers has increased so that they can control laser pulse width and shape, cavity dumping and seeding, stage isolation and coherence reduction for smoothing irradiation. Such pulse generators can now be built with embedded computer systems for remote control, interrogation and diagnosis of pulser parameters. Diagnostic equipment to monitor laser beam profiles with respectable time resolution also employs these pulsed generators.

Timely and repeatable alignment of the 192 beam National Ignition Facility (NIF) laser will require an automatic system. Demanding accuracy requirements must be met with high reliability at low cost while minimizing the turn-around time between shots. We describe an approach for internally self-consistent alignment of the mirrors in the laser chains using a network of local light sources that serve as near field and far field alignment references. It incorporates a minimum number of alignment lasers, handles many beams in parallel, and utilizes simple control algorithms.

A new nonlinear optical crystal, CsLiB₆O₁₀ (CLBO), is described, which is a congruently melting crystal and can realize fourth harmonic and fifth harmonic generations of the 1.064 micrometer Nd:YAG laser radiation with type-I phase matching. A large and high quality single crystal with dimensions of 14 by 11 by 11 cm³ was obtained by the top- seeded Kyropoulos method. CLBO showed higher fourth harmonic generation efficiency from the second harmonic of Nd:YAG laser output compared to beta-BaB₂O₄.

In preparation for beginning the design of the National Ignition Facility (NIF) in the United States and the Laser Mega-Joule (LMJ) in France, we are in the process of deriving new specifications for the large optics required for these facilities. They are currently being evaluated through modeling and experimentation. These specifications will be ready for general release by the end of the year. Traditionally, specifications for transmitted wavefront and surface roughness of large ICF optics have been based on parameters which were easily measured during the early 1980s, such as peak-to-valley wavefront error (PV) and root-mean- square (rms) surface roughness, as well as wavefront gradients in terms of waves per cm. While this was convenient from a fabrication perspective, since the specifications could be easily interpreted by fabricators in terms which were understood and conventionally measurable, it did not accurately reflect the requirements of the laser system. In some cases, optics which were not adequate for a given application which was particularly sensitive to periodic errors, were fabricated acceptably in terms of the optics specifications. For the NIF and LMJ laser systems, we have availed ourselves of advances in metrology and interferometry and an enhanced understanding of laser system performance to derive specifications which are based on power spectral densities (PSDs). Such requirements can more accurately reflect the requirements of the laser system for minimizing the amplitude of mid- and high-spatial frequency surface and transmitted wavefront errors, while not over constraining the fabrication in terms of low spatial frequencies, such as residual coma or astigmatism, which are typically of a very large amplitude compared to periodic errors. In order to study the effect of changes in individual component tolerances, it is most useful to have a model capable of simulating real behavior. The basis of this model is discussed in this paper, outlining the general approach to the 'theoretical' study of ICF optics specifications, and an indication of the type of specification to be expected is shown, based upon existing ICF laser optics. The problem of specifying optics for high energy lasers is more difficult than for 'classical' optical systems for many reasons, which are discussed as well.

This paper discusses the techniques, developed over the past year, for high spatial resolution measurement and analysis of the transmitted and/or reflected wavefront of large aperture ICF optical components. Parts up to 400 mm by 750 mm have been measured and include: laser slabs, windows, KDP crystals and lenses. The measurements were performed using state-of- the-art commercial phase shifting interferometers at a wavelength of 633 micrometer. Both 1 and 2-D Fourier analysis have been used to characterize the wavefront; specifically the power spectral density (PSD) function was calculated. The PSDs of several precision optical components are shown. The PSD(nu) is proportional to the (amplitude)² of components of the Fourier frequency spectrum. The PSD describes the scattered intensity and direction as a function of scattering angle in the wavefront. The capability of commercial software is limited to 1-D Fourier analysis only. We are developing our own 2-D analysis capability in support of work to revise specifications for NIF optics. Two-dimensional analysis uses the entire wavefront phase map to construct 2-D PSD functions. We have been able to increase the signal-to-noise relative to 1-D and can observe very subtle wavefront structure. The physics of the NIF laser design dictate partitioning the wavefront into three regimes of spatial wavelength (or spatial frequency). We discuss the data in terms of the following three scale length regimes: (1) short scale, or 'micro roughness,' having scale lengths less than 120 micrometer; (2) mid-spatial scale, with scale lengths from 0.12 to 33 mm; and (3) long scale, or conventional 'optical figure/curvature,' having scale lengths greater than 33 mm. Regular repetitive wavefront structure has been observed in all three regimes, ranging from 10 micrometer to 100 mm in scale length. The magnitude of these structures are typically from lambda/100 to lambda/20. Structure has been detected in optical materials and on the surfaces of finished parts. We believe the sources of these structures are small fabrication errors. The Modeling Group at LLNL is using this data in beam propagation codes to assist in optimizing laser system design and to develop optics specifications for the NIF.

The author derived an equation between the glass volume of a slab and the volume of a stirring chamber in the continuous melting furnace and proved the equation can be applied to get 10^-6 homogeneity for glasses.

Due to the relationship between Judd-Ofelt intensity parameter and covalency, new laser glasses have been developed which have low expansion coefficients (85 to 91 multiplied by 10^-7/cm degrees Celsius, 0 to 70 degrees Celsius) and high emission cross sections. They have good chemical properties, high Young's modulus, and high thermal conductivities.

It is well established by manufacturers and users that optical coatings are generally prepared by the well known physical vapor deposition (PVD) technology. In the authors' opinion sol-gel technology is an effective and competitive alternative. The aim of this paper is to emphasize the sol-gel thin film work carried out at Centre d'Etudes de Limeil-Valenton (CEL-V) and concerning the technology for high power lasers. We briefly discuss the chemistry of the sol- gel process, the production of optical coatings, and the related deposition techniques. Finally, the paper describes the preparation and performance of sol-gel optical coatings we have developed to fulfill the requirements of a future 2 MJ/500 TW (351 nm) pulsed Nd:glass laser so-called LMJ (Laser MegaJoules). This powerful laser is to be used for our national inertial confinement fusion (ICF) program, to demonstrate at the laboratory scale, ignition of deuterium-tritium fusion fuel. Moreover, the aim of this article is, hopefully, to provide a convincing argument that coatings and particularly optical coatings, are some of the useful products available from sol-gel technology , and that exciting developments in other areas are almost certain to emerge within the coming decade.

Aqueous-inorganic (AIZ) and organic (OZ) zirconia sols have been synthesized by hydrothermal and alcothermal techniques. The size and structure of the ZrO₂ nanoparticles of which the sols are composed are dependent on the reaction precursors and the synthesis conditions used. AIZ produced crystallites which are a mixture of monoclinic and metastable tetragonal ZrO₂ while the OZ route yielded mainly amorphous particles. ZrO₂ coatings deposited from both sols show comparable laser-induced damage thresholds to SiO₂ sol-gel anti-reflective (AR) coatings. Thus, they could be candidates for the high index component in alternating SiO₂/ZrO₂ stacks for highly reflective mirrors. Here we describe sol-gel routes to ZrO₂ coatings and the benefits in terms of resistance to laser damage of the addition of polyethylene glycol (PEG). The relevant microchemistry is discussed.

The inertial confinement fusion (ICF) program at LLNL is beginning the design of a 1.8 megajoule, 0.35-micrometer, laser system called the National Ignition Facility (NIF). In order to reduce cost and increase performance, high damage threshold optics are essential. It has been found that the damage threshold of some coatings can be increased by as much as 2 or more times as a result of pre-illumination at incrementally increasing fluences. This process, termed laser conditioning, has been associated with the ejection of damage-initiating defects present in some optical coatings. With current damage thresholds, mirrors and polarizers for the NIF will have to be laser conditioned in order to meet the laser requirements for fluence propagation. LLNL has constructed a system dedicated to laser conditioning of meter-sized HfO₂/SiO₂ multilayer polarizers and mirrors. The optic is moved in a raster pattern through a stationary 10-Hz rep-rated, 1.064 micrometer beam with 10-ns pulses. A scatter measurement diagnostic allows on-the-fly evaluation of laser-induced damage during a scan. This system has been used to laser condition optics as large as 73 cm by 37 cm. Such optics are now being used on the Beamlet laser system at LLNL. With this large area conditioning system, it has been observed that the scattered light from an optical coating increases during the conditioning process. This increase in scattered light has been related to the removal of nodular defects.

Silica sol-gel anti-reflective (AR) coatings have been investigated with particular reference to their laser induced damage threshold (LIDT) when subjected to irradiation from a Nd pulse laser at 1064 nm. Coatings (whose thickness was optimized for minimum reflection at 1064 nm) were deposited by spinning silica sols (average particle size 15 nm) formed by the base (ammonium hydroxide) catalyzed hydrolysis/condensation of TEOS in ethanol. Addition of polyethyleneglycols (PEGs) increased the size of the colloidal silica particles and also induced some particle aggregation in the sol, unlike a similar chain length diol. Increases in the LIDT of the coatings possibly depend upon the impurity levels, the wettability of the substrate, and the presence of PEG. LIDT improvements may be obtained by control of substrate and coating surface wettability, hydrophilicity, and surface chemistry.

Based on studies we have performed with several radiation sources such as pulsed nuclear reactors, we have been able to construct a physical picture and measure quantitative parameters necessary to model the radiation-induced losses expected for fused silica and fused quartz National Ignition Facility (NIF) target area. It is important to note that these surrogate radiation sources do not have identical temporal and spectral characteristics to NIF, therefore caution is in order since the results obtained to date must be extrapolated somewhat to predict NIF performance.

We have explored the major technical and conceptual issues relating to the suitability of a diode-pumped solid state laser as a driver for an inertial fusion energy power plant. While solid state lasers have long served as the workhorse of inertial confinement fusion physics studies, the deployment of a driver possessing adequate efficiency, reliability, and repetition rate for inertial fusion energy requires the implementation of several technical innovations discussed in this article.

We present a conceptual design of a diode-pumped solid-state-laser (DPSSL) driver for an inertial fusion energy (IFE) power plant based on the minimized cost of electricity (COE) as determined in a comprehensive systems study. This study contained extensive detail for all significant DPSSL physics and costs, plus published scaling relationships for the costs of the target chamber and the balance of plant (BOP). Our DPSSL design offers low development cost because it is modular, can be fully tested functionally at reduced scale, and is based on mature solid-state-laser technology. Most of the parameter values that we used are being verified by experiments now in progress. Future experiments will address the few issues that remain. As a consequence, the economic and technical risk of our DPSSL driver concept is becoming rather low. Baseline performance at 1 GW$e) using a new gain medium [Yb³⁺-doped Sr₅(PO₄)₃F, or Yb:S-FAP] includes a product of laser efficiency and target gain of (eta) G equals 7, and a COE of 8.6 cents/kW(DOT)h, although values of (eta) G greater than or equal to 11 and COEs less than or equal to 6.6 cents/kW(DOT)h are possible at double the assumed target gain of 76 at 3.7 MJ. We present a summary of our results, discuss why other more-common types of laser media do not perform as well as Yb:S-FAP, and present a simple model that shows where DPSSL development should proceed to reduce projected COEs.

As the technology associated with the development of solid-state drivers for inertial fusion energy (IFE) has evolved, increased emphasis has been placed on the development of an efficient approach for managing the waste heat generated in the laser media. This paper addresses the technical issues associated with the gas cooling of large aperture slabs, where the laser beam propagates through the cooling fluid. It is shown that the major consequence of proper thermal management is the introduction of simple wedge, or beam steering, into the system. Achieving proper thermal management requires careful consideration of the geometry, cooling fluid characteristics, cooling flow characteristics, as well as the thermal/mechanical/optical characteristics of the laser media. Particularly important are the effects of cooling rate variation and turbulent scattering on the system optical performance. Helium is shown to have an overwhelming advantage with respect to turbulent scattering losses. To mitigate cooling rate variations, we introduce the concept of flow conditioning. Finally, optical path length variations across the aperture are calculated. A comparison of two laser materials (S-FAP and YAG) shows the benefit of a nearly a-thermal material on optical variations in the system.

Although solid-state lasers have been the primary means by which the physics of inertial confinement fusion (ICF) have been investigated, it was previously thought that solid-state laser technology could not offer adequate efficiencies for an inertial fusion energy (IFE) power plant. Orth and co-workers have recently designed a conceptual IFE power plant, however, with a high efficiency diode-pumped solid-state laser (DPSSL) driver that utilized several recent innovations in laser technology. It was concluded that DPSSLs could offer adequate performance for IFE with reasonable assumptions. This system was based on a novel diode pumped Yb-doped Sr₅(PO₄)₃F(Yb:S-FAP) amplifier. Because this is a relatively new gain medium, a project was established to experimentally validate the diode-pumping and extraction dynamics of this system at the smallest reasonable scale. This paper reports on the initial experimental results of this study. We found the pumping dynamics and extraction cross-sections of Yb:S-FAP crystals to be similar to those previously inferred by purely spectroscopic techniques. The saturation fluence for pumping was measured to be 2.2 J/cm² using three different methods based on either the spatial, temporal, or energy transmission properties of a Yb:S-FAP rod. The small signal gain implies an emission cross section of 6.0 multiplied by 10^-20 cm² that falls within error bars of the previously reported value of 7.3 multiplied by 10^-20 cm², obtained from spectroscopic techniques. Up to 1.7 J/cm³ of stored energy density was achieved in a 6 multiplied by 6 multiplied by 44 mm³ Yb:S-FAP amplifier rod. In a free running configuration diode-pumped slope efficiencies up to 43% were observed with output energies up to approximately 0.5 J per 1 ms pulse from a 3 multiplied by 3 multiplied by 30 mm³ rod. When the rod was mounted in a copper block for cooling, 13 W of average power was produced with power supply limited operation at 70 Hz with 500 microsecond pulses.

The use of different shifted narrow band amplifying media in the chirped pulse amplification technique allows us to produce sub-100 TW pulses with duration in the 250 - 300 fs range. By focusing these pulses on target we have obtained peak intensities above 2.10¹⁹ W/cm².

The availability of ultrashort (100 fs) high intensity lasers capable of producing focused intensities in excess of 10¹⁸ W/cm² provides many new opportunities in studying the interaction of radiation with matter. We describe a new laser plasma facility constructed around a Cr:LiSAF laser system currently capable of output energies of approximately 1 J in times of approximately 100 fs. This facility will be directed initially towards basic studies of the interaction of intense ultrashort laser pulses with dense plasmas, and the generation and application of intense hard x-ray point sources. The laser system we have developed for this facility, uses the new solid- state laser material, Cr:LiSAF. This material has many advantages for the generation of intense ultrashort laser pulses. The spectral gain bandwidth is sufficiently broad for the amplification of pulses shorter than 100 fs in duration. Its florescence lifetime is long enough to permit the use of conventional flashlamp pumping, and its emission cross- section is such that it provides small signal gains high enough to allow its use in high powered chirped pulsed amplification laser architectures. The material can now be fabricated into laser rods up to 25 mm in diameter with low scattering losses and its Cr concentrations can be varied to optimize the small signal gain for specific pump cavities. These factors taken together allow the design of a 100 fs high intensity oscillator-laser system that is simpler in architecture than those based on Ti:sapphire or KrF. Moreover the use of Cr:LiSAF laser crystal elements should enable the generation of much higher powers. In principle as high as 1 PW should be achievable with a laboratory with this approach. Modifications currently being implemented onto our Cr:LiSAF laser system include improvements to pulse contrast, energy extraction, beam uniformity and focusability. The system will soon be incorporated with a 52- port precision target chamber that will be equipped with a broad array of x ray and plasma diagnostics. Several research programs are being designed around this facility. Two of these programs, those relating to the use of hard x rays for the analysis of shock phenomena in solids and the general physics of extremely high magnetic fields are presented.

New kinds of optical diffraction dispersive devices which could be possible candidates for the chirp pulse amplification technique used in the ICF fast ignition concept are proposed. These dispersive filters show linear group delay time dispersion versus frequency with either positive and negative slopes avoiding the necessity of slope inversion.

Detailed modeling of beam propagation in Beamlet has been made to predict system performance. New software allows extensive use of optical component characteristics. This inclusion of real optical component characteristics has resulted in close agreement between calculated and measured beam distributions.

The problems of SBS phase conjugation application for large scale fusion facility are discussed. The experimental and computer modeling results show the possibility of SBS phase conjugation application in those lasers. Additional SBS mirror energy scaling investigations should be done for such implementation.

An optical pulse-shaping system will be implemented on the OMEGA laser that is capable of producing complex optical pulse shapes. The pulse-shaping system relies on photoconductive switches that are activated with a pulse compressed by the stimulated Brillouin scattering (SBS) process. The SBS activation pulse provides overall system bandwidth and eliminates laser prepulse noise. The SBS process is modeled in detail to understand the performance and limitations of the OMEGA pulse-shaping system. Experimental results and numerical simulations are presented in the SBS pulse-steepening and pulse-compression regimes. Good qualitative agreement is obtained between theory and experiments.

We present a high yield method of broadband frequency mixing which relies on a matching of induced phase-modulation in order to overcome the normal phase-matching limitations. We analyze it for two cases of linear and sine chirpings.

Transverse SRS with fluence 3.4 J/cm² has been received in KDP crystal. The TSRS amplification increment for KDP, which was prepared as type II, has been calculated according to experimental results g equals 0.5 cm/GW.

Transverse SBS characteristics of the KD*P crystal were determined by the method of SBS generation excitation in the transverse resonator. Fused silica was utilized as the test medium. Experimental oscillograms of Stokes pulses were processed by the method of pulse form approximation using the four-parametric function of time.

The Secretary of the U.S. Department of Energy (DOE) commissioned a conceptual design report (CDR) for the National Ignition Facility (NIF) in January 1993 as part of a key decision zero (KD0), justification of mission need. Motivated by the progress to date by the inertial confinement fusion (ICF) program in meeting the Nova technical contract goals established by the National Academy of Sciences in 1989, the Secretary requested a design using a solid-state laser driver operating at the third harmonic (0.35 micrometer) of neodymium (Nd) glass. The participating ICF laboratories signed a memorandum of agreement in August 1993, and established a project organization, including a technical team from the Lawrence Livermore National Laboratory (LLNL), Los Alamos National Laboratory (LANL), Sandia National Laboratories (SNL), and the Laboratory for Laser Energetics at the University of Rochester. Since then, we completed the NIF conceptual design, based on standard construction at a generic DOE defense program's site, and issued a 7,000-page, 27-volume CDR in May 1994. Over the course of the conceptual design study, several other key documents were generated, including a facilities requirements document, a conceptual design scope and plan, a target physics design document, a laser design cost basis document, a functional requirements document, an experimental plan for indirect drive ignition, and a preliminary hazards analysis (PHA) document. DOE used the PHA to categorize the NIF as a low-hazard, non-nuclear facility. On October 21, 1994 the Secretary of Energy issued a key decision one (KD1) for the NIF, which approved the project and authorized DOE to request Office of Management and Budget-approval for congressional line-item FY 1996 NIF funding for preliminary engineering design and for National Environmental Policy Act activities. In addition, the Secretary declared Livermore as the preferred site for constructing the NIF. In February 1995, the NIF Project was formally submitted to Congress as part of the President's FY 1996 budget. If funded as planned, the Project will cost approximately $1.1 billion and will be completed at the end of FY 2002.

The conceptual design of the National Ignition Facility (NIF) 192 beam laser incorporates a low-power alignment beam injected in the pinhole plane of the final spatial filter with a wavelength intermediate between the 1053 nm laser output and the 351 nm frequency- converted beam that illuminates the target. Choosing the specific wavelength for which the spatial filter plane is reimaged in the same target chamber plane as the frequency-converted main laser pulse, achieves optimum accuracy without the need for additional means to insure precise overlap between the two beams. Insertion of the alignment beam after the last laser amplifier also allows alignment to the target while the amplifiers are still cooling from a previous shot.

We have developed a pulse-shaping system of the partially coherent light with angular spectral dispersion for the application to inertial confinement fusion experiment. The angular spectral dispersion is essential not only to the efficient frequency conversion but also to the reduction of the medium mode irradiation nonuniformity. The pulse-shaping system is based on the temporal stacking of pulses using optical fiber couplers and variable attenuators. This system enables us to generate arbitrarily pulse-shaped partially coherent light with a fast rise time of approximately 50 ps. Using this system we have demonstrated a 1.6-ns flat-top pulse with a 200-ps picket pulse under the operation condition of approximately 1000 J/beam at the fundamental wavelength, and this pulse is being supplied in the current implosion experiment with the isentrope control.

Output laser parameters are enhanced significantly by using laser pumping. An excellent example is usage of laser diodes for solid-state laser pumping. Although there are permanent advances towards development of this technique, its application for laser systems of more than 100 J output requires time, significant effort and expense. I propose another pumping source based on a rather simple and inexpensive technique and admitted scaling up to energy values which are beyond the reach now with the diodes. This is a pulsed chemical oxygen-iodine laser (COIL) with intracavity frequency doubling. The COIL operates on a laser transition of atomic iodine (1.315 micrometer). The upper laser level populates via energy transfer from metastable oxygen molecules (O₂(¹(Delta) )-singlet oxygen) which formed in a rather simple chemical reaction between an alkaline solution of hydrogen peroxide and gaseous chlorine. The COIL is a gas laser of low pressure (not more than several torrs), having high output parameters and efficiency. A peculiar mechanism of inversion formation makes it difficult to realize a pulsed mode operation by conventional techniques. In particular, there is a limitation of energy stored in large volume. This problem has been solved in our laboratory by forming of atomic iodine with external exposure on some iodides. As a result a pulsed COIL system with an external initiation arose. High optical quality of an active medium and rather high intensity permit us to get 100% intracavity frequency doubling. The wavelength (657.5 nm) is suitable for pumping of some efficient laser materials such as Cr:LiSAF, and garnets codoped with Cr³⁺ and TR³⁺ ions. The proposed laser system has the following advantages: (1) scaling by merely increasing the size of the laser, (2) regulated pulse duration from 20 microseconds, (3) well-collimated beam, and (4) repetition rate of about tens Hz. There is a possibility to use the proposed laser system to pump large-size laser elements of laser-drivers for ICF. It is especially interesting to use the proposed pumping source for chirped pulse amplification. Energy of 100 - 200 J can be obtained with currently available pulsed COILs. Thereby a real ability opens for generation of ultrashort pulses of petawatt level output power.

The phenomenon of phase-matching curve transformation under strong energy exchange conditions and its influence on frequency conversion of ultrashort light pulses in nonlinear crystals are discussed. The routes to increase energy conversion efficiency in three-wave interactions are suggested.

The mechanism of eliminating platinum inclusions in laser glass by POCl₃ gas bubbling is discussed from the thermodynamic aspect in the preset paper. It is deduced from calculated reaction free energy that platinum inclusion is ionized after POCl₃ gas bubbling in the existence of water and oxygen. After reaction platinum inclusion is dissolved as platinum (II) metaphosphate in the laser glass matrix.

We present the results of a series of experiments to measure the thermal recovery times of a flashlamp-pumped, Nd:glass multi-segment laser amplifier. In particular, we investigated the thermal recovery times under the following cooling options: (1) passive cooling, (2) active cooling of the flashlamp cassettes, and (3) active cooling of the flashlamp cassettes and gas flow in the pump cavity.

Using adaptive optics we have obtained nearly diffraction-limited 5 kJ, 3 nsec output pulses at 1.053 micrometer from the Beamlet demonstration system for the National Ignition Facility (NIF). The peak Strehl ratio was improved from 0.009 to 0.50, as estimated from measured wavefront errors. We have also measured the relaxation of the thermally induced aberrations in the main beam line over a period of 4.5 hours. Peak-to-valley aberrations range from 6.8 waves at 1.053 micrometer within 30 minutes after a full system shot to 3.9 waves after 4.5 hours. The adaptive optics system must have enough range to correct accumulated thermal aberrations from several shots in addition to the immediate shot-induced error. Accumulated wavefront errors in the beam line will affect both the design of the adaptive optics system for NIF and the performance of that system.

A two-dimensional multi-lens array of 350 mm in diameter with 37 pieces of spherical element lenses is tested for improving the irradiation uniformity of the ICF target. Circular and hexagonal shapes of element lens aperture have been examined. The circular aperture is chosen for eliminating azimuthally asymmetric intensity distribution in the beam pattern. An approximate flat-top intensity distribution has been obtained at the focus of the principal lens. Beam patterns of coherent laser and partially coherent light (PCL) with the random phase plate (RPP) and the multi-lens array (MLA) have been measured at GEKKO XII glass laser system. The irradiation non uniformity of MLA on a spherical target is calculated from the measured beam patterns.

Several key technologies for uniform laser irradiation are reported. This paper includes the uniformity performance as a result of the introduction of the random phase plate, the partially coherent light, and the beam smoothing by spectral dispersion into the New Gekko XII glass laser system. Finally we summarize the overall irradiation uniformity on the spherical target surface by considering the power imbalance effect. The technologies developed for the beam smoothing and the power balance control enable us to achieve the irradiation nonuniformities of around 1% level for a foot pulse and of a few percent for a main drive pulse, respectively.

It is possible to irradiate spherical targets with perfect uniformity from a finite number of finite aperture beams, provided that the intensity profile of each beam is a square-root function of its radius in the target plane. The consequences of this observation are described, some simple examples of geometries that realize perfect uniformity are presented, and heuristic rules for arranging beams with the appropriate symmetry are defined.

The distribution coefficient of deuterium in the crystal depends on that of the growth solution. It was not affected, even if the saturated temperature and the supersaturation of the growth solution were changed. Thirteen percent-deuterated KDP crystal of 9 by 9 by 11 cm size was grown in which the deuterated rate was constant within the measurement error of plus or minus 1%. The excess energy due to an over heating and/or an acoustic energy to the solution and the pH control realized a higher level of supersaturation. KDP crystals were grown up to 50 mm/day.

The Beamlet laser is a full-scale, single-aperture scientific prototype of the frequency-tripled Nd:glass laser for the proposed National Ignition Facility. At aperture sizes of 30 cm by 30 cm and 34 cm by 34 cm using potassium dihydrogen phosphate crystals of 32 cm by 32 cm and 37 cm by 37 cm, respectively, we have obtained up to 8.3 kJ of third harmonic energy at 70% - 80% whole beam conversion efficiency.

The features of liquid purification from molecular and dispersive admixtures are studied. The analysis has revealed the processes (thermal effects, microparticles heating with a subsequent optical breakdown, stimulated Raman scattering) limiting pumping pulse energy. These effects complicate also a realization of a high quality phase conjugation at SBS. The data concerning physical properties of liquid tetrachlorides and freons are presented. The picture of a behavior of liquid under conditions of an optical breakdown is described. Some recommendations regarding a choice of nonlinear media are formulated. The two-cell scheme providing a phase conjugation of powerful short laser pulses is proposed.

Optical components of large-aperture, high irradiance and high fluence lasers can experience significant levels of stimulated scattering along their transverse dimensions. We have observed transverse stimulated Raman scattering in large aperture KDP crystals, and have measured the stimulated gain coefficient.

Optical damage that results in large scale fracture has been observed in the large, high- fluence, fused-silica, spatial filter lenses on the Nova and Beamlet lasers. In nearly all cases damage occurs on the vacuum side of the lenses and because the vacuum side of the lens is under tensile stress this damage can lead to catastrophic crack growth if the flaw (damage) size exceeds the critical flaw size for SiO₂. The damaged 52 cm Nova lenses fracture into two and sometimes three large pieces. Although under full vacuum load at the time they fracture, the Nova lenses do not implode. Rather we have observed that the pieces lock together and air slowly leaks into the vacuum spatial filter housing through the lens cracks. The Beamlet lenses have a larger aspect ratio and peak tensile stress than Nova. The peak tensile stress at the center of the output surface of the Beamlet lens is 1490 psi versus 810 psi for Nova. During a recent Beamlet high energy shot, a damage spot on the lens grew to the critical flaw size and the lens imploded. Post shot data indicate the lens probably fractured into 5 to 7 pieces, however, unlike Nova, these pieces did not lock together. Analysis shows that the likely source of damage is contamination from pinhole blow-off or out-gassing of volatile materials within the spatial filter. Contamination degrades the anti-reflection properties of the sol-gel coating and reduces its damage threshold. By changing the design of the Beamlet lens it may be possible to insure that it fails safe by locking up in much the same manner as the Nova lens.

A high aspect ratio line focus on a target for the x-ray laser experiments is required for obtaining a high gain-length product. A new line focus system is developed to generate a uniform line focus. The system consists of a deformable mirror of a continuous faceplate type which provides an appropriate wavefront distribution for compensating an aberration of a line focus optic. The width and intensity distribution of 18.2 mm long line focus has been improved on 2 times diffraction limit. In another application, a rectangular beam shaping from a circular defocused beam is investigated by the experiment and the diffraction calculation. The controllability of intensity distribution of laser beam by deformable mirror has been demonstrated.