Glass Laser Facility Research Articles

M-shaped beam profile output from an off-axis relay-imaging eight-pass Nd:glass laser preamplifier

In ultra-high-power/energy solid-state Nd:glass laser facilities, the gain distribution of the main amplifier system (MAS) is nonuniform owing to strong amplified spontaneous emissions in the MAS. A shaped beam profile from a preamplifier (PreAMP) should be set opposite to the gain profile in the MAS to obtain a high-power beam with an approximately flat beam profile. This flat beam profile can result in the high efficiency of the frequency-conversion crystals. In this study, the laser beam was spatially formatted as an M-shaped beam with precompensation (PreCOM) for the nonuniform gain profile of the MSA. Modulation of the cross-sectional gain distribution of the PreAMP was demonstrated by selecting the absorption coefficient of the Nd:glass rod and pump configuration. Using the proposed off-axis relay-imaging eight-pass amplifier, an M-shaped beam with an output pulse energy of 1 J and a repetition rate of 1 Hz was experimentally achieved. With an Nd3+ concentration of 1.2 wt% and Nd: glass rod diameter of 18 mm, an M-shaped beam profile with a 4.3:1 ratio of intensity at the margin to that at the center was successfully obtained. Such controllable M-shaped beams can be optionally introduced to the MAS to compensate for non-uniform spatial gain profiles.

Large aperture and non-critical phase-matched fourth harmonic generation of Nd:Glass lasers

A fourth harmonic generation (FHG) scheme in focusing beams is proposed and demonstrated for large aperture Nd:glass laser facilities. By placing the focusing lens before the FHG crystal, the problem of ultraviolet damage can be overcome, largely without affecting FHG conversion efficiency owing to the large angular acceptance of the non-critical phase matching technique. A numerical simulation of the FHG process indicates that angular acceptance can be appropriately increased by lowering the working temperature and jointing the two adjacent compensating angles, so that FHG in focusing beams with relatively small F numbers becomes feasible. With a 170 mm × 170 mm × 7 mm and 65% deuterated potassium dihydrogen phosphate crystal mounted in a high-precision, temperature-controlled system, high-efficiency FHG has been demonstrated in the focusing beam with a full beam convergence angle of 36 mrad. When driven with a 223 J, second harmonic radiation (2ω), 1 ns flat-top pulse with a beam area of 130 cm2, corresponding to 1.7 GW/cm2 2ω input intensity, 182 J of fourth harmonic radiation (4ω) were generated.

Transmittance in a thin aluminum layer at nanosecond pulsed laser ablation

AbstractSince the beginning of the 1960s and up to the present time, laser ablation from metal surfaces under the action of short light pulses has been the topic of many researches. One of the first objects in the early ablation experiments presented thin metal films evaporated by radiation, which were used in lasers with nanosecond pulses as optical gates for Q-switching in resonators or for decoupling in amplifiers. Bleaching of the gates based on metal layers with a time constant τt ≈ 10−8 s observed in a number of experiments, was usually considered as a result of a simple evaporation of matter. We analyze the data of the experiments with a mylar tape [≈0.05 µm aluminum (Al) layer on ≈20 µm lavsan substrate] used as a gate for optical isolation in one of the first Nd: glass laser facilities with a power of ≈1 GW. That gate was irradiated by pulses of Q-switched oscillators: pulse duration 10−7–10−8 s, intensity 107–108 W/cm2. A jump in the transmittance of the expanding Al layer was registered (from ≈0.1 to 50% at τt ≈ 10−9 s). The present study shows that one should consider the metal–insulator phase transition in a superheated liquid metal layer as the mechanism of the fast (up to 10–10–10–11 s) increase in the transmittance of the Al film gate at nanosecond-pulsed laser ablation.

Wavefront correction by target-phase-locking technology in a 500 TW laser facility

We demonstrate a novel approach termed target-phase-locking that could improve the entire beam wavefront quality of a 500 TW Nd3+:phosphate glass laser facility. The thermal and static wavefront from front-end to target is corrected by using one deformable mirror that receives feedback from both the focal-spot sensor and wavefront sensor, and only the main laser of the laser system is employed in the correction process, with auxiliary calibration light no longer necessary. As a result, a static focal spot with full width at half maximum of 8.87 × 5.74 µm is achieved, the thermal wavefront induced by flash-lamp-pumped Nd3+:phosphate glass is compensated with PV from 3.54–0.43 µm, and a dynamic focal spot with intensity exceeding 1020 W cm−2 is precisely predicted at the target with such an approach.

The National Ignition Campaign: status and progress

The National Ignition Facility (NIF) at Lawrence Livermore National Laboratory (LLNL) has been operational since March 2009 and a variety of experiments have been completed and many more are planned in support of NIF's mission areas: national security, fundamental science, and fusion energy. NIF capabilities and infrastructure are in place to support all of its missions with nearly 60 x-ray, optical and nuclear diagnostic systems and the ability to shoot cryogenic targets and DT layered capsules. The NIF has also been qualified for the use of tritium and other special materials as well as to perform high-yield experiments and classified experiments. Implosions with record indirect-drive neutron yield of 7.5 × 1014 neutrons have been achieved. NIF, a Nd : Glass laser facility, is routinely operating at 1.6 MJ of ultraviolet (3ω) light on target with very high reliability. It recently reached its design goal of 1.8 MJ and 500 TW of 3ω light on target, and has performed target experiments with 1.9 MJ at peak powers of 410 TW. The National Ignition Campaign (NIC), an international effort with the goal of demonstrating thermonuclear burn in the laboratory, is making steady progress towards achieving ignition. Other experiments have been completed in support of high-energy science, materials equation of state, and materials strength. In all cases, records of extreme temperatures and pressures, highest neutron yield and highest energy densities have been achieved. This paper describes the unprecedented experimental capabilities of the NIF and the results achieved so far on the path towards ignition.

The NIF: An international high energy density science and inertial fusion user facility

The National Ignition Facility (NIF), a 1.8-MJ/500-TW Nd:Glass laser facility designed to study inertial confinement fusion (ICF) and high-energy-density science (HEDS), is operational at Lawrence Livermore National Laboratory (LLNL). A primary goal of NIF is to create the conditions necessary to demonstrate laboratory-scale thermonuclear ignition and burn. NIF experiments in support of indirect-drive ignition began late in FY2009 as part of the National Ignition Campaign (NIC), an international effort to achieve fusion ignition in the laboratory. To date, all of the capabilities to conduct implosion experiments are in place with the goal of demonstrating ignition and developing a predictable fusion experimental platform in 2012. The results from experiments completed are encouraging for the near-term achievement of ignition. Capsule implosion experiments at energies up to 1.6 MJ have demonstrated laser energetics, radiation temperatures, and symmetry control that scale to ignition conditions. Of particular importance is the demonstration of peak hohlraum temperatures near 300 eV with overall backscatter less than 15%. Important national security and basic science experiments have also been conducted on NIF. Successful demonstration of ignition and net energy gain on NIF will be a major step towards demonstrating the feasibility of laser-driven Inertial Fusion Energy (IFE). This paper will describe the results achieved so far on the path toward ignition, the beginning of fundamental science experiments and the plans to transition NIF to an international user facility providing access to HEDS and fusion energy researchers around the world.

High-energy Nd:glass laser facility for collisionless laboratory astrophysics

A kilojoule-class laser (Raptor) has recently been activated at the Phoenix-laser-facility at the University of California Los Angeles (UCLA) for an experimental program on laboratory astrophysics in conjunction with the Large Plasma Device (LAPD). The unique combination of a high-energy laser system and the 18 meter long, highly-magnetized but current-free plasmawill support a new class of plasma physics experiments, including the first laboratory simulations of quasi-parallel collisionless shocks, experiments on magnetic reconnection, or advanced laser-based diagnostics of basic plasmas. Here we present the parameter space accessible with this new instrument, results from a laser-driven magnetic piston experiment at reduced power, and a detailed description of the laser system and its performance.

Picosecond optical parametric chirped pulse amplifier as a preamplifier to generate high-energy seed pulses for contrast enhancement

We present the design, implementation, and testing of a novel picosecond optical parametric preamplifier system to generate high-energy seed pulses for the Vulcan laser facility. The preamplifier amplifies 100 fs pulses stretched to 3 ps pulses from 10 pJ to 70 μJ in a single stage of amplification before the pulses are further amplified in the Vulcan high-power Nd:glass laser facility to the petawatt power level. This increased seed energy has led to an improvement of the nanosecond amplified spontaneous emission contrast intensity to 10(-10) of the main pulse, without degrading the output of the 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.

The National Ignition Facility and the Golden Age of High Energy Density Science

The National Ignition Facility (NIF) is a 192-beam Nd:glass laser facility being constructed at the Lawrence Livermore National Laboratory to conduct research in inertial confinement fusion (ICF) and high energy density science. When completed, NIF will produce 1.8 MJ, 500 TW of ultraviolet light, making it the world's largest and highest energy laser system. The NIF is poised to become the world's preeminent facility for conducting ICF and fusion energy research and for studying matter at extreme densities and temperatures.

Construction of the 1 kJ Nd: glass laser facility at KAERI

We report on the design and present status of a 1 kJ Nd:Glass laser facility for basic research on quantum engineering at KAERI (Korea Atomic Energy Research Institute). By applying a newly designed spatial filter with a serrated aperture, we improved the diffracted Gaussian spatial profile of an oscillator into a flat-top one. The laser system consists of 4 beam lines, each with the energies of more than 200 J at the nano-second regime. We measured the gain and spatial profiles of each amplification stage. A spectral shaping by a two-stage OPCPA (Optical Parametric Chirped Amplifier) for a pico-second front end was studied to compensate for gain narrowing in multi-stage amplifier chains.

Generation of a single hot spot by use of a deformable mirror and study of its propagation in an underdense plasma

Adaptive optics systems offer the prospect of significantly increasing the capabilities of high-power laser focusability, which is currently limited by thermal distortions. Using novel wave-front measurement techniques that improve the stability of such systems and a downstream large-aperture deformable mirror that does not bear the usual limitations associated with precompensation, we have improved the focusability of a high-power (6×100-J, 1-ns) Nd:glass laser facility by a factor of 6. Measuring the wave front and the on-target focal spot at full power, we obtained after correction focal spots with a best Strehl ratio of 0.6. The pulse peak intensity could thus be increased to ∼2×1016 W/cm2, a level beyond reach of the usual focal spot shaping techniques. We then used the near-diffraction-limited focal spots produced by this system to measure the laser–plasma coupling for a single, controlled filament of light and to underline the importance of the coupling among the numerous speckles within conventional multispeckled beams.

Cost of electricity difference for direct and indirect drive targets for inertial fusion energy using a diode pumped solid state laser driver

A detailed systems analysis code has been used to compare the projected cost of electricity (COE) for inertial fusion energy for direct drive (DD) and indirect drive (ID) target scenarios, based on a diodepumped solid state laser driver with Yb:S-FAP (Yb doped Sr5(PO4)3F) gain media.Previously published target gain curves which resulted in a target gain at the optimal DD operating point that is 30% higher than that for the ID scenario have been used. This gain advantage for DD is offset by a requirement for improved beam smoothing, which was obtained via smoothing by spectral dispersion (SSD) with a 1 THz bandwidth at 349 nm. Such a large SSD bandwidth has a number of effects on laser performance, including greater risk of optics damage from non-linear effects, lowered harmonic conversion efficiency, altered extraction parameters and higher front-end costs. The non-linear effects, which contribute to optical component damage by amplification of intensity non-uniformities, were parameterized through a constraint on the maximum allowable B integral (i.e. the total average phase retardation due to the non-linear indices of all materials traversed by the beam). If we constrain B to be no larger than 1.8 rad, which is the presently accepted safe value based on observations in single shot glass laser facilities, the COEs for DD and ID are predicted to be the same within the uncertainties. If technology permits the B limit to be raised, the optimized COE for DD is predicted to decrease relative to that for ID.

Wave-front evaluation of the Ni-like Ag laser

Coherent x-ray lasers are desired for various applications such as holography of biological samples and diagnostics of laser-produced plasmas. However, the shape of the x-ray laser had not been examined. Information about its wave-front shape is also important for future application of x-ray lasers in research and industry. In this paper, we report the evaluation of the shape of the wave-front of the x-ray laser. Ni-like Ag lasers at 13.9 nm were examined in IV-01-GXII experiment at GEKKO XII glass laser facility at ILE, Osaka Univ. The fringe patterns generated using crossed wires located in the beam were recorded on the x-ray CCD. Our experimental results indicate that x-ray laser emitted from an end of a plasma column has a property of a point light source.

Investigation of the energy balance components for a plane target irradiated with a picosecond laser pulse

The scattering and absorption of a high-power picosecond laser pulse by a solid target were investigated experimentally making use of the 'Progress-P' Nd:glass laser facility (λ = 1053 nm, τ = 1.4 ps) at radiation intensities I = 1016 — 1019 W cm-2 on the target surface. It was found that, for I ≤ 1017 W cm-2, more than 30% of the intensity of the scattered light was contained in the specularly reflected component. The absorption coefficient of the laser radiation with intensities ranging from 1018 to 1019 W cm-2 was higher for targets made of materials with higher atomic numbers.

'Progress-P' laser facility with chirped-pulse amplification in neodymium glass

A Nd:YLF/Nd:glass laser facility with chirped-pulse amplification capable of generating pulses with a duration of 1.5 ps and a power up to 30 TW at the wavelength of 1053 nm was investigated. The facility consists of a initial laser system, producing a chirped pulse with an energy up to ~1 J, a main three-stage rod amplifying channel with a 85-mm aperture of the output stage, a pulse compressor based on 210 mm × 420 mm diffraction gratings, and a focusing axial parabolic mirror (with an aperture ratio of 1:11). Optimisation of the parameters of the laser facility allowed us to generate an output laser beam with an angular divergence close to the diffraction limit and an intensity up to ~1019 W cm-2 . Laser radiation with such parameters can be employed in experiments on the interaction of superstrong laser fields with matter.

Development of XUV lasers at the RAL Central Laser Facility

Recent progress in the development of XUV lasers by research teams using high-power and ultrashort-pulse Nd: glass and KrF laser facilities at the Rutherford Appleton Laboratory is reviewed. Injector-amplifier operation and prepulse enhanced output of the Ge XXIII collisional laser driven by a kilojoule glass laser, enhanced gain in CVI recombination with picosecond CPA drive pulses from a glass laser, and optical field ionization and XUV harmonic generation with a KrF CPA laser are described.

Trident: a versatile high-power Nd:glass laser facility for inertial confinement fusion experiments

<p>The Trident Nd:glass laser system operates as an experimental facility supporting the national Inertial Confinement Fusion program at Los Alamos. The laser has two identical main beam lines with 14-cm-disk final amplifiers. The beams are frequency doubled, expanded to 19.2 cm, and focused on target with a variety of focusing optics. A third beam with 10-cm disk final amplifiers is also frequency doubled and used as a target-shooting or diagnostic beam simultaneously with the other two beams.</p><p>The facility provides a flexible combination of energy, pulse-shaping capabilities, and diagnostic tools for laser-target interaction experiments.</p>

Mechanism of hot electron generation and Stimulated Raman Scattering in laser cavity target plasma

Stimulated Raman Scattering (SRS) and x-ray spectrum emitted from many kinds of targets were experimentally investigated on the Nd:glass laser facility Shenguang-I in Shanghai, China. By theoretical analysis and computer simulation for these experiments, the result showed that the mechanism of hot electron generation in laser cavity target was mainly SRS, according to detailed comparison between the cavity target and the plane target. Other results, such as the energy fraction and temperatures of hot electron, energy and angular distribution and wave spectrum in SRS, were also presented in detail.

ICF-related experiments at CEL-V

Implosion experiments performed at Centre d'Etudes de Limeil-Valenton in the indirect drive scheme using the two-beams Nd:glass laser facility Phebus at the energy level = 6 kJ (blue light) are presented. A final density of compressed DT close to 100 ρ0 has been obtained; it has been deduced from radiochemistry of the activated silicon atoms in the pusher. The best irradiance uniformity on the microballoon was evaluated to = 15% rms. Phebus has also been equipped with an optical fiber oscillator to study the effect of a smoothing technique on coupling processes: It appeared that at 0·53 μm absorption efficiency is increased by =15–20%. With the eight-beams Octal laser, hydrodynamic instabilities development in accelerated planar targets has been investigated both for direct and indirect drives; the mixing zone detected at the light-heavy interface does not present visible bubble-and-spike like structures and is less developed in the indirect configuration. Atomic physics in laser plasmas is also deeply studied; a particular effort has been made on absorption spectroscopy, a powerful diagnostic of ionization dynamics in cold and dense plasmas. Experiments have been realized either in multilayered targets or using rear-side X-ray emission of thin Au foils to heat the samples. To reach fuel ignition conditions, more powerful lasers, in the range of megajoule, will be needed. Their design needs further technological developments to reduce the capital cost in $/W. At Limeil, we work mainly on high-damage threshold optical coatings, using the sol-gel process, high-quality, low-cost mirror fabrication, using the replica technics, and incoherent laser pulse generation for beam smoothing.