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As a promising new way to generate a controllable strong magnetic field, laser-driven magnetic coils have attracted interest in many research fields. In 2013, a kilotesla level magnetic field was achieved at the Gekko XII laser facility with a capacitor–coil target. A similar approach has been adopted in a number of laboratories, with a variety of targets of different shapes. The peak strength of the magnetic field varies from a few tesla to kilotesla, with different spatio-temporal ranges. The differences are determined by the target geometry and the parameters of the incident laser. Here we present a review of the results of recent experimental studies of laser-driven magnetic field generation, as well as a discussion of the diagnostic techniques required for such rapidly changing magnetic fields. As an extension of the magnetic field generation, some applications are discussed.
PDF全文 (下载：147)   HTML全文 (浏览：15) High Power Laser Sci. Eng., 2018年第6卷第3期 pp.e38
C. C. Kuranz
M. J. MacDonald
B. A. Remington
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A novel laboratory experimental design is described that will investigate the processing of dust grains in astrophysical shocks. Dust is a ubiquitous ingredient in the interstellar medium (ISM) of galaxies; however, its evolutionary cycle is still poorly understood. Especially shrouded in mystery is the efficiency of grain destruction by astrophysical shocks generated by expanding supernova remnants. While the evolution of these remnants is fairly well understood, the grain destruction efficiency in these shocks is largely unknown. The experiments described herein will fill this knowledge gap by studying the dust destruction efficiencies for shock velocities in the range $｛\sim｝10｛-｝30~\text｛km｝/\text｛s｝$ ($\unicode[STIX]｛x03BC｝\text｛m｝/\text｛ns｝$), at which most of the grain destruction and processing in the ISM takes place. The experiments focus on the study of grain–grain collisions by accelerating small ($｛\sim｝1~\unicode[STIX]｛x03BC｝\text｛m｝$) dust particles into a large ($｛\sim｝5｛-｝10~\unicode[STIX]｛x03BC｝\text｛m｝$ diameter) population; this simulates the astrophysical system well in that the more numerous, small grains impact and collide with the large population. Facilities that combine the versatility of high-power optical lasers with the diagnostic capabilities of X-ray free-electron lasers, e.g., the Matter in Extreme Conditions instrument at the SLAC National Accelerator Laboratory, provide an ideal laboratory environment to create and diagnose dust destruction by astrophysically relevant shocks at the micron scale.
PDF全文 (下载：96)   HTML全文 (浏览：3) High Power Laser Sci. Eng., 2018年第6卷第3期 pp.e39
摘要 + ：(浏览：289次) | 请注意：下载全文可能需要登录或支付相关费用
Ytterbium-doped fibers have become the optimum gain media of high-power fiber lasers thanks to a simple energy structure, which strongly reduces the excited state absorption, and a low quantum defect and a high optic–optic conversion efficiency, which means the low thermal load. In this paper, we take a review of the current state of the art in terms of $\text｛Yb｝^｛3+｝$ doped fibers for high-power fiber lasers, including the development of the fabrication techniques. The research work to overcome the challenges for $\text｛Yb｝^｛3+｝$ doped fibers, which affect the stability of output power and beam quality, will be demonstrated. Direction of further research is presented and the goal is to look for a fiber design, to boost single fiber output power, stabilize the laser power and support robust single-mode operation.
PDF全文 (下载：146)   HTML全文 (浏览：4) High Power Laser Sci. Eng., 2018年第6卷第3期 pp.e40
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The nonlinear absorption (NLA) properties of potassium dideuterium phosphate crystals at 515 nm under different excitation laser intensities are investigated with the Z-scan technique. Two critical intensities are highlighted: the critical intensity for exciting the NLA and the critical intensity of the multiphoton absorption mechanism transition. Experimental results indicate the existence of defect states located in the band gap, which can be manipulated by varying laser intensity. A model based on the change of multiphoton absorption mechanism induced by the transformation of defect species is proposed to interpret the experiments. Modeling results are in good agreement with the experiment data.
PDF全文 (下载：68)   HTML全文 (浏览：1) High Power Laser Sci. Eng., 2018年第6卷第3期 pp.e41
摘要 + ：(浏览：320次) | 请注意：下载全文可能需要登录或支付相关费用
An all-fiberized high-average-power narrow linewidth ns pulsed laser with linear polarization is demonstrated. The laser system utilizes a typical master oscillator power amplifier (MOPA) configuration. The stimulated Brillouin scattering (SBS) is effectively suppressed due to the short fiber length and large mode area in the main amplifier, combined with the narrow pulse duration smaller than the phonon lifetime of SBS effect. A maximal output power of 466 W is obtained with a narrow linewidth of $｛\sim｝$203.6 MHz, and the corresponding slope efficiency is $｛\sim｝$80.3%. The pulse duration is condensed to be $｛\sim｝$4 ns after the amplification, corresponding to the peak power of 8.8 kW and the pulse energy of $46.6~\unicode[STIX]｛x03BC｝\text｛J｝$. Near-diffraction-limited beam quality with an $M^｛2｝$ factor of 1.32 is obtained at the output power of 442 W and the mode instability (MI) is observed at the maximal output power. To the best of our knowledge, this is the highest average output power of the all-fiberized narrow linewidth ns pulsed fiber laser with linear polarization and high beam quality, which is a promising source for the nonlinear frequency conversion, laser lidar, and so on.
PDF全文 (下载：104)   HTML全文 (浏览：1) High Power Laser Sci. Eng., 2018年第6卷第3期 pp.e42
L. Van Box Som
J. M. Bonnet-Bidaud
J. E. Cross
R. P. Drake
C. C. Kuranz
M. J.-E. Manuel
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The influence of a strong external magnetic field on the collimation of a high Mach number plasma flow and its collision with a solid obstacle is investigated experimentally and numerically. The laser irradiation ($I\sim 2\times 10^｛14｝~\text｛W｝\cdot \text｛cm｝^｛-2｝$) of a multilayer target generates a shock wave that produces a rear side plasma expanding flow. Immersed in a homogeneous 10 T external magnetic field, this plasma flow propagates in vacuum and impacts an obstacle located a few mm from the main target. A reverse shock is then formed with typical velocities of the order of 15–20 $\pm$ 5 km/s. The experimental results are compared with 2D radiative magnetohydrodynamic simulations using the FLASH code. This platform allows investigating the dynamics of reverse shock, mimicking the processes occurring in a cataclysmic variable of polar type.
PDF全文 (下载：119)   HTML全文 (浏览：4) High Power Laser Sci. Eng., 2018年第6卷第3期 pp.e43
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The physics of compressible turbulence in high energy density (HED) plasmas is an unchartered experimental area. Simulations of compressible and radiative flows relevant for astrophysics rely mainly on subscale parameters. Therefore, we plan to perform turbulent hydrodynamics experiments in HED plasmas (TurboHEDP) in order to improve our understanding of such important phenomena for interest in both communities: laser plasma physics and astrophysics. We will focus on the physics of supernovae remnants which are complex structures subject to fluid instabilities such as the Rayleigh–Taylor and Kelvin–Helmholtz instabilities. The advent of megajoule laser facilities, like the National Ignition Facility and the Laser Megajoule, creates novel opportunities in laboratory astrophysics, as it provides unique platforms to study turbulent mixing flows in HED plasmas. Indeed, the physics requires accelerating targets over larger distances and longer time periods than previously achieved. In a preparatory phase, scaling from experiments at lower laser energies is used to guarantee the performance of future MJ experiments. This subscale experiments allow us to develop experimental skills and numerical tools in this new field of research, and are stepping stones to achieve our objectives on larger laser facilities. We review first in this paper recent advances in high energy density experiments devoted to laboratory astrophysics. Then we describe the necessary steps forward to commission an experimental platform devoted to turbulent hydrodynamics on a megajoule laser facility. Recent novel experimental results acquired on LULI2000, as well as supporting radiative hydrodynamics simulations, are presented. Together with the development of LiF detectors as transformative X-ray diagnostics, these preliminary results are promising on the way to achieve micrometric spatial resolution in turbulent HED physics experiments in the near future.
PDF全文 (下载：62)   HTML全文 (浏览：1) High Power Laser Sci. Eng., 2018年第6卷第3期 pp.e44
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Astrophysical collisionless shocks are amazing phenomena in space and astrophysical plasmas, where supersonic flows generate electromagnetic fields through instabilities and particles can be accelerated to high energy cosmic rays. Until now, understanding these micro-processes is still a challenge despite rich astrophysical observation data have been obtained. Laboratory astrophysics, a new route to study the astrophysics, allows us to investigate them at similar extreme physical conditions in laboratory. Here we will review the recent progress of the collisionless shock experiments performed at SG-II laser facility in China. The evolution of the electrostatic shocks and Weibel-type/filamentation instabilities are observed. Inspired by the configurations of the counter-streaming plasma flows, we also carry out a novel plasma collider to generate energetic neutrons relevant to the astrophysical nuclear reactions.
PDF全文 (下载：43)   HTML全文 (浏览：0) High Power Laser Sci. Eng., 2018年第6卷第3期 pp.e45
摘要 + ：(浏览：300次) | 请注意：下载全文可能需要登录或支付相关费用
High power superfluorescent fiber sources (SFSs), which could find wide applications in many fields such as middle infrared laser generation, Raman fiber laser pumping and spectral beam combination, have experienced a flourishing time in recent years for its unique properties, such as short coherence length and high temporal stability. The challenge for performance scalability of powerful SFS mainly lies on the physical issues including parasitic laser oscillation and modal instability (MI). In this contribution, by employing in-band pumping avenue and high-order transverse-mode management, we explore a high power SFS with record power, near-diffraction-limited beam quality and spectral manipulation flexibility. An ultimate output power of 3.14 kW can be obtained with high temporal stability and a beam quality of $M^｛2｝=1.59$ for the amplified light. Furthermore, the dynamics of spectral evolutions, including red-shifting of central wavelength and unsymmetrical broadening in spectral wings, of the main amplifier with different seed linewidths are investigated contrastively. Benefiting from the unique high pump brightness and high MI threshold of in-band pumping scheme, the demonstrated system also manifests promising performance scaling potential.
PDF全文 (下载：79)   HTML全文 (浏览：0) High Power Laser Sci. Eng., 2018年第6卷第3期 pp.e46
摘要 + ：(浏览：314次) | 请注意：下载全文可能需要登录或支付相关费用
The Orion facility at the Atomic Weapons Establishment in the United Kingdom has the capability to operate one of its two 500 J, 500 fs short-pulse petawatt beams at the second harmonic, the principal reason being to increase the temporal contrast of the pulse on target. This is achieved post-compression, using 3 mm thick type-1 potassium dihydrogen phosphate crystals. Since the beam diameter of the compressed pulse is $｛\sim｝600$ mm, it is impractical to achieve this over the full aperture due to the unavailability of the large aperture crystals. Frequency doubling was originally achieved on Orion using a circular sub-aperture of 300 mm diameter. The reduction in aperture limited the output energy to 100 J. The second-harmonic capability has been upgraded by taking two square 300 mm $\times$ 300 mm sub-apertures from the beam and combining them at focus using a single paraboloidal mirror, thus creating a 200 J, 500 fs, i.e., 400 TW facility at the second harmonic.
PDF全文 (下载：65)   HTML全文 (浏览：3) High Power Laser Sci. Eng., 2018年第6卷第3期 pp.e47
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Laser-driven magnetic reconnection (LDMR) occurring with self-generated B fields has been experimentally and theoretically studied extensively, where strong B fields of more than megagauss are spontaneously generated in high-power laser–plasma interactions, which are located on the target surface and produced by non-parallel temperature and density gradients of expanding plasmas. For properties of the short-lived and strong B fields in laser plasmas, LDMR opened up a new territory in a parameter regime that has never been exploited before. Here we review the recent results of LDMR taking place in both high and low plasma beta environments. We aim to understand the basic physics processes of magnetic reconnection, such as particle accelerations, scale of the diffusion region, and guide field effects. Some applications of experimental results are also given especially for space and solar plasmas.
PDF全文 (下载：45)   HTML全文 (浏览：1) High Power Laser Sci. Eng., 2018年第6卷第3期 pp.e48
A. F. A. Bott
T. G. White
D. Q. Lamb
D. H. Froula
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Magnetic field measurements in turbulent plasmas are often difficult to perform. Here we show that for $｛\geqslant｝$kG magnetic fields, a time-resolved Faraday rotation measurement can be made at the OMEGA laser facility. This diagnostic has been implemented using the Thomson scattering probe beam and the resultant path-integrated magnetic field has been compared with that of proton radiography. Accurate measurement of magnetic fields is essential for satisfying the scientific goals of many current laser–plasma experiments.
PDF全文 (下载：85)   HTML全文 (浏览：1) High Power Laser Sci. Eng., 2018年第6卷第3期 pp.e49
X. T. He
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Direct numerical simulation of intense laser–solid interactions is still of great challenges, because of the many coupled atomic and plasma processes, such as ionization dynamics, collision among charged particles and collective electromagnetic fields, to name just a few. Here, we develop a new particle-in-cell (PIC) simulation code, which enables us to calculate laser–solid interactions in a more realistic way. This code is able to cover almost ‘all’ the coupled physical processes. As an application of the new code, the generation and transport of energetic electrons in front of and within the solid target when irradiated by intense laser beams are studied. For the considered case, in which laser intensity is $10^｛20｝~\text｛W｝\cdot \text｛cm｝^｛-2｝$ and pre-plasma scale length in front of the solid is $10~\unicode[STIX]｛x03BC｝\text｛m｝$, several quantitative conclusions are drawn: (i) the collisional damping (although it is very weak) can significantly affect the energetic electrons generation in front of the target, (ii) the Bremsstrahlung radiation will be enhanced by 2–3 times when the solid is dramatically heated and ionized, (iii) the ‘cut-off’ electron energy is lowered by an amount of 25% when both collision damping and Bremsstrahlung radiations are included, and (iv) the resistive electromagnetic fields due to Ohmic heating play nonignorable roles and must be taken into account in such interactions.
PDF全文 (下载：73)   HTML全文 (浏览：1) High Power Laser Sci. Eng., 2018年第6卷第3期 pp.e50
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Stimulated Raman scattering (SRS) effect is considered to be one of the main obstacles for power scaling in general-type fiber lasers. Different from previous techniques that aim at suppressing SRS, nonlinear fiber amplifier (NFA), which manipulates and employs the SRS for power scaling in rare-earth-doped fiber, is under intensive research in recent years. In this paper, the authors will present an all-round study on this new kind of high-power fiber amplifier. A theoretical model is proposed based on the rate equation and amplified spontaneous emission (ASE), with random noise taken into account. By numerical solving of the theoretical model, the power scaling potential, heat analysis and advantages in suppressing the undesired backscattering light are quantificationally analyzed for the first time. Then two different types of high-power NFAs are demonstrated individually. Firstly, a laser diode pumped NFA has reached kilowatt output power, and the results agree well with theoretical predictions. Secondly, a tandem-pumped NFA is proposed for the first time and validated experimentally, in which 1.5 kW output power has been achieved. The authors also briefly discuss several new issues relating to the complex nonlinear dynamics that occur in high-power NFAs, which might be interesting topics for future endeavors.
PDF全文 (下载：71)   HTML全文 (浏览：1) High Power Laser Sci. Eng., 2018年第6卷第3期 pp.e51
A direct prejudgement strategy that takes the diffraction ring as the analysis target is put forward to predict hot images induced by defects of tens of microns in the main amplifier section of high power laser systems. Analysis of hot-image formation process shows that the hot image can be precisely calculated with the extracted intensity oscillation of the diffraction ring on the front surface of the nonlinear plate. The gradient direction matching (GDM) method is adopted to detect diffraction rings. Recognition of simulated diffraction rings shows that it is feasible to directly prejudge hot images induced by those closely spaced defects and the defects that are far apart from each other. Image compression and cluster analysis are utilized to optimize the performance of the GDM method in recognizing actually collected diffraction images. Results show that hot images induced by defects of tens of microns can be directly prejudged without redundant information.
PDF全文 (下载：55)   HTML全文 (浏览：2) High Power Laser Sci. Eng., 2018年第6卷第3期 pp.e52
摘要 + ：(浏览：17次) | 此论文可免费阅读 (可能需要登录)
This special issue of the journal High Power Laser Science and Engineering will widely present the recent work at NLHPLP. Two review articles on the facilities are contained in this issue and other article introduce some critical subsystem and technology developments such as the injection subsystem,final optics subsystem, alignment technique,beam controlling technique, amplifier cleaning technique and so on.
PDF全文 (下载：4) High Power Laser Sci. Eng. ，年第卷第期 pp.
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In this paper, we reported both the experimental demonstration and theoretical analysis of a Raman fiber laser based on a master-oscillator-power-amplifier configuration. The Raman fiber laser adopted the dual-wavelength bidirectional pumping configuration, utilizing 976 nm laser diodes and 1018 nm fiber lasers as the pump sources. A 60-meter-long 25/400 μm ytterbium -doped fiber was used to convert the power from 1070 to 1124 nm, realizing a maximum power output of 3.7 kW with a 3 dB spectral width of 6.8 nm. Moreover, we developed a multi-frequency model taking into consideration the Raman gain spectrum and amplified spontaneous emission. The calculated spectral broadening of both the forward and backward laser was in good agreement with the experimental results. Finally, a 1.5 kW, 1183 nm second-order Raman fiber laser was further experimentally demonstrated by the addition of a 70-meter-long germanium-doped passive fiber.
PDF全文 (下载：0) High Power Laser Sci. Eng. ，年第卷第期 pp.
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In order to improve the damage threshold and enlarge the aperture of a laser beam shaper, photolithographic patterning technology is adopted to design a new type of liquid crystal binary mask. The inherent conductive metal layer of commercial liquid crystal electro-optical spatial light modulators is replaced by azobenzene-based photoalignment layers patterned using noncontact photolithography. Using the azobenzene-based photoalignment layer, a liquid crystal binary mask for beam shaping is fabricated. In addition, the shaping ability, damage threshold, write/erase flexibility, and stability of the liquid crystal binary mask are tested. Using a 10-Hz near-IR (1064-nm) laser, the multiple-shot nanosecond damage threshold of the liquid crystal mask is measured to be higher than 15 J/cm<sup>2</sup>. The damage threshold of the azobenzene-based photoalignment layer is higher than 50 J/cm<sup>2</sup> under the same testing conditions.
PDF全文 (下载：3) High Power Laser Sci. Eng. ，年第卷第期 pp.
A high-power, Joule-class, nanosecond temporally shaped multi-pass ring laser amplifier system with two Nd: glass (neodymium-doped phosphate glass) laser heads is demonstrated. The laser amplifier system consists of three parts: all fiber structure seeder, diode pumped Nd: glass regenerative amplifier, and multi-pass ring amplifier, where the thermally induced depolarization of two laser heads is studied experimentally and theoretically. Following the injection of a square pulse with the pulse energy of 0.9 mJ and pulse width of 6 ns, a 0.969 J high-energy laser pulse at 1 Hz was generated, which had the ability to change the waveform arbitrarily based on the all fiber structure front-end. The experimental results show that the proposed laser system is promising to be adopted in the pre-amplifier of high power laser facilities.
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Abstract A new generation of high power laser facilities will provide laser pulses with extremely high powers of 10 Peta-Watt (PW) and even 100 PW, capable of reaching 1023 W/cm2 in the laser focus. These ultrahigh intensities are nevertheless lower than the Schwinger intensity IS = 2.3x1029 W/cm2 at which the theory of Quantum Electrodynamics (QED) predicts that a large part of the energy of the laser-photons will be transformed to hard gamma-ray photons and even to mass, via electron-positron pair production. To enable the investigation of this physics at the intensities achievable with the next generation of high power laser facilities, an approach involving the interaction of two colliding PW laser pulses
is being adopted. Theoretical simulations predict strong QED effects with colliding laser pulses of ≥10 PW focused to intensities ≥1022 W/cm2.
Key words: Quantum Electrodynamics; Colliding PW Laser Pulses; Peta-Watt Laser Facilities; Electron-Positron Pairs; Nonlinear Breit-Wheeler process;
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Muons produced by the Bethe-Heitler process from laser wakefield accelerated electrons interacting with high Z materials
have velocities close to the laser wakefield. It is possible to accelerate those muons with laser wakefield directly.
Therefore for the first time we propose an all-optical “Generator and Booster” scheme to accelerate the produced muons
by another laser wakefield to supply a prompt, compact, low cost and controllable muon source in laser laboratories. The
trapping and acceleration of muons are analyzed by one-dimensional analytic model and verified by two-dimensional
particle-in-cell (PIC) simulation. It is shown that muons can be trapped in a broad energy range and accelerated to
higher energy than that of electrons for longer dephasing length. We further extrapolate the dependence of the maximum
acceleration energy of muons with the laser wakefield relativistic factor γ and the relevant initial energy E0. It is shown
that a maximum energy up to 15.2 GeV is promising with γ = 46 and E0 = 1.45 GeV on the existing short pulse laser
PDF全文 (下载：5) High Power Laser Sci. Eng. ，年第卷第期 pp.
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The paper presents a review of dynamic stabilization mechanisms for plasma instabilities. One of the dynamic stabilization mechanisms for plasma instability was proposed in the paper [Phys. Plasmas 19, 024503(2012)], based on a perturbation phase control. In general, instabilities emerge from the perturbations. Normally the perturbation phase is unknown, and so the instability growth rate is discussed. However, if the perturbation phase is known, the instability growth can be controlled by a superimposition of perturbations imposed actively. Based on this mechanism we present the application results of the dynamic stabilization mechanism to the Rayleigh-Taylor instability (RTI) and to the filamentation instability as typical examples in this paper. On the other hand, in the paper [Comments on Plasma Phys. Controlled Fusion 3, 1(1977)] another mechanism was proposed to stabilize RTI, and was realized by the pulse train or the laser intensity modulation in laser inertial fusion [Phys. Rev. Lett. 71, 3131(1993)]. In this latter mechanism, an oscillating strong force is applied to modify the basic equation, and consequently the new stabilization window is created. Originally the latter was proposed by P. L. Kapitza. We review the two stabilization mechanisms, and present the application results of the former dynamic stabilization mechanism.
PDF全文 (下载：1) High Power Laser Sci. Eng. ，年第卷第期 pp.
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The spatial-intensity profile of light reflected during the interaction of an intense laser pulse with a microstructured target is investigated experimentally and the potential to apply this as a diagnostic of the interaction physics is explored numerically. Diffraction and speckle patterns are measured in the specularly reflected light in the cases of targets with regular groove and needle-like structures, respectively, highlighting the potential to use this as a diagnostic of the evolving plasma surface. It is shown, via ray-tracing and numerical modelling, that for a laser focal spot diameter smaller than the periodicity of the target structure, the reflected light patterns can potentially be used to diagnose the degree of plasma expansion, and by extension the local plasma temperature, at the focus of the intense laser light. The reflected patterns could also be used to diagnose the size of the laser focal spot during a high intensity interaction when using a regular structure with known spacing.
Zuegel Jonathan D.
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Optical parametric chirped-pulse amplification (OPCPA) implemented using multikilojoule Nd:glass pump lasers is a promising approach to produce ultra-intense pulses (>10<sup>23</sup> W/cm<sup>2</sup>). Technologies are being developed to upgrade the OMEGA EP Laser System with the goal to pump an optical parametric amplifier line (EP OPAL) with two of the OMEGA EP beamlines. The resulting ultra-intense pulses (1.5 kJ, 20 fs, 10<sup>24</sup> W/cm<sup>2</sup>) would be used jointly with picosecond and nanosecond pulses produced by the other two beamlines. A mid-scale OPAL pumped by the Multi-Terawatt (MTW) laser is being constructed to produce 7.5-J, 15-fs pulses and demonstrate scalable technologies suitable for the upgrade. MTW OPAL will share a target area with the MTW laser (50 J, 1 to 100 ps), enabling several joint-shot configurations. We report on the status of the MTW OPAL system, and the technology development required for this class of all-OPCPA laser system for ultra-intense pulses.
PDF全文 (下载：2) High Power Laser Sci. Eng. ，年第卷第期 pp.
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We developed a systematic experimental method to demonstrate that damage threshold ?uence(DTF) for fused silica changes with the number of femtosecond laser (800 nm, 65±5 fs, 10 Hz and 600 Hz) pulses. Based on the experimental data, we were able to develop a model which indicates that the change in DTF varies with number of shots logarithmically (lnp) up to a critical value. Above this value, DTF approaches an asymptotic value. Both DTF for a single shot and the asymptotic value as well as the critical value where this happens, are extrinsic parameters dependent on the con?guration (repetition rate, pressure and geometry near or at the surface). These measurements indicate that the power of this dependence (p) is an intrinsic parameter independent of the con?guration.
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A multichannel calorimeter system is designed and constructed which is capable of delivering single-shot and broad-band spectral measurement of Terahertz radiation generated in intense laser-plasma interactions. The generation mechanism of backward THz radiation is studied using the multichannel calorimeter system in an intense picosecond laser-solid interaction experiment. The dependence of the BTR energy and spectrum on laser energy, target thickness and pre-plasma scale length is obtained. These results indicate that coherent transition radiation is responsible for the low frequency component (< 1 THz) of BTR. It is also observed that a largescale pre-plasma primarily enhance the high frequency component (> 3 THz) of BTR.
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We report on the energetic and beam quality performance of the second to the last main amplifier section HEPA I of the PEnELOPE laser project. A polarization coupled double-12-pass scheme to verify the full amplification capacity of the last two amplifiers HEPA I and II was used. The small signal gain for a narrow band cw laser was 900 and 527 for a broadband nanosecond pulse, demonstrating 12.6 J of output pulse energy. Those pulses, being spectrally wide enough to support equivalent 150 fs long ultrashort pulses, are shown with an excellent spatial beam quality. A first active correction of the wavefront using a deformable mirror resulted in a Strehl ratio of 76% in the single-12-pass configuration for HEPA I.
Paul David Samuel
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Development of high energy laser sources with nano second pulses at several hertz of repetition rate has been very attractive in recent years due to their great potential for practical applications. With the recent advancement in fabricating large size laser quality transparent ceramics, diode pumped solid state laser generating pulse energy of 100 J at 10 Hz has been recently realized at HiLASE center using Yb:YAG ceramic with Cr:YAG cladding. This review discusses Yb based high energy lasers, specific laser geometries for efficient thermal management and the role of transparent ceramics in such diode pumped high-energy class solid-state lasers around the world.
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Pulse contrast is a crucial parameter of high peak-power lasers since the pre-pulse noise may disturb laser-plasma interactions. Contrast measurement is thus a prerequisite to tackle the contrast challenge in high peak-power lasers. This paper presents the progress review of single-shot cross-correlator (SSCC) for real-time contrast characterization. We begin with the key technologies that enable an SSCC to simultaneously possess high dynamic range (10^10), large temporal window (50-70 ps) and high fidelity. We also summarize the instrumentation of SSCC prototypes and their applications on five sets of petawatt laser facilities in China. Finally, we discuss how to extend contrast measurements from time domain to spatiotemporal domain. Real-time and high-dynamic-range contrast measurements, provided by SSCC, can not only characterize the various complex noise in high peak-power lasers but also guide the system optimization.
PDF全文 (下载：7) High Power Laser Sci. Eng. ，年第卷第期 pp.
De Vido Mariastefania
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In this paper we review the design and development of a 100 J, 10 Hz nanosecond pulsed laser, codenamed DiPOLE100X, being built at the Central Laser Facility (CLF). This 1 kW average power diode pumped solid state laser (DPSSL) is based on a master oscillator power amplifier (MOPA) design, which includes two cryogenic gas cooled amplifier stages based on DiPOLE multi-slab ceramic Yb;YAG amplifier technology developed at the CLF. The laser will produce pulses between 2 and 15 ns in duration with precise, arbitrarily selectable shapes, at pulse repetition rates up to 10 Hz, allowing real time shape optimisation for compression experiments. Once complete, the laser will be delivered to the European X-ray Free Electron Laser (XFEL) facility in Germany as a UK funded contribution-in-kind, where it will be used to study extreme states of matter at the High Energy Density (HED) instrument.
Yu Tae Jun
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We have optimized the input pulse width and injection time to achieve the highest possible output pulse energy in a double-pass laser amplifier using two Nd:YAG rods. For this purpose, we have extended the Frantz-Nodvik equation by simultaneously including both spontaneous emission and pump energy variation. The effective pump energy of the flash-lamp was 8.84 <i></i>J for each gain media. The energy of 1 <i></i>J could be amplified to output energy of 12.17 <i></i>J with the maximum achieved extraction efficiency of 63.18% when an input pulse having a pulse width of 168 <i></i>μs is sent 10 <i></i>μs after the absorbed pump energy becomes the maximum value.
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In this study, we investigate a new simple scheme using a planar undulator (PU) together with a properly dispersed electron beam (e beam) with a large energy spread (~1%) to enhance the free-electron laser (FEL) gain. For a dispersed e beam in a PU, the resonant condition is satisfied for the center electrons, while the frequency detuning increases for the off-center electrons, inhibiting the growth of the radiation. The PU can act as a filter for selecting the electrons near the beam center to achieve the radiation. Although only the center electrons contribute, the radiation can be enhanced significantly owing to the high peak current of the beam. Theoretical analysis and simulation results indicate that this method can be used for the improvement of the radiation performance, which has great significance for short-wavelength FEL applications.
The study of structure, thermodynamic state, equation of state (EOS) and transport properties of Warm Dense Matter (WDM) has become one of the key aspects of laboratory astrophysics. This field has demonstrated its importance especially concerning the internal structure of planets, but also other astrophysical bodies such as brown dwarfs, crusts of old stars or white dwarf stars. Recent years have shown a rapid increase in interest and activity in this field owing to many technological advances including the commissioning of high energy optical laser systems, z-pinches, free electron x-ray lasers, but also short-pulse laser facilities capable of generation of novel particle and x-ray sources. Many new diagnostic methods have been developed recently to study WDM in its full complexity. Even non-equilibrium dynamics has been accessed for the first time thanks to sub-picosecond laser pulses achieved at new facilities. Recent years saw a number of major discoveries with direct implications to astrophysics such as the formation of diamond at pressures relevant to interiors of frozen giant planets like Neptune, metallic hydrogen under conditions such as those found inside Jupiter’s dynamo or formation of lonsdaleite crystals under extreme pressures during asteroid impacts on celestial bodies.
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We present the design and experiment of a broadband optical parametric chirped pulse amplifier (OPCPA) which provide high conversion efficiency and good beam quality at 808 nm wavelength. Using a three-dimensional spatial and temporal numerical model, several design considerations necessary to achieve high conversion efficiency, good beam quality, and good output stability are discussed. To improve the conversion efficiency and broaden the amplified signal bandwidth simultaneously, the nonlinear crystal length and OPCPA parameters are analyzed and optimized with the concept of dissipating amplified idler between optical parametric amplification (OPA) of two crystals configuration. In the experiment, an amplifier consisting of two OPCPA stages of “L” type configuration was demonstrated by using the optimized parameters. An amplified signal energy of 160 mJ was achieved with a total pump-to-signal efficiency of 35% (43% efficiency for the OPCPA stage 2). The output bandwidth of signal pulse reached 80 nm and the signal pulse was compressed to 24 fs. The energy stability reached 1.67% RMS at 3% pump energy variation. The optimized OPCPA amplifier operates at a repetition rate of 1 Hz and is used as a front-end injection for the main amplifier of SG-II 5PW laser facility.
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An all-fiberized and narrow-bandwidth master oscillator power amplification (MOPA) system with 4 kW-level and slope efficiency of 78% is demonstrated. Tandem pumping strategy is tentatively introduced for thermally-induced mode instability (TMI) suppression. The stimulated Brillouin scattering effect is balanced by simply one-stage phase modulation technique. With different modulation signals, SBS limited output powers of 336 W, 1.2 kW and 3.94 kW are respectively achieved with spectral bandwidths account for 90% power of ~ 0.025, 0.17 and ~ 0.89 nm. Compared with our previously 976 nm pumping system, TMI threshold is overall boosted to be > 5 times in which tandem pumping increases the TMI threshold of > 3 times. The beam quality (M2 factor) of the output laser is well within 1.5 below the TMI threshold while it is ultimately saturated to be 1.86 with the influence of TMI at maximal output power. Except for SBS and TMI, stimulated Raman scattering (SRS) effect will be another challenge for further power scaling. In such a high power MOPA system, multi-detrimental effects (SBS, SRS, and TMI) will coexist and may be mutual-coupled, which could provide a well platform for further comprehensively investigating and optimizing the high power, narrow bandwidth fiber amplifiers
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Compared with end pumping fiber combiner, one of the advantages for side pumping combiner is the unlimited pumping points, which means multi-point or cascaded side-pumping can be realized. However, the loss mechanism of the cascaded structure is seldom discussed. In this paper, we present the numerical and experimental investigation about the loss mechanism of a two-stage-cascaded side pumping combiner based on tapered-fused technique. The influence of loss mechanism on the coupling efficiency and thermal load of the fiber coating are analyzed according to simulations and experiments with different tapering ratios for the first stage. Based on the analysis, a cascaded component with total pump coupling efficiency of 96.4% handling a pump power of 1088 W is achieved by employing 1018 nm fiber laser as the pump source. Future work to further improve the performance of a cascaded side pumping combiner is discussed and prospected.
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In 2018 the journal High Power Laser Science and Engineering produced a Special Issue on Fibres for High Power Lasers. Nowadays, fibre based laser sources have found extensive applications both in industry and in scientific research. The scope of the special issue was to span the latest developments on the fast developing fibre based high power lasers and amplifers.
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Polarization mode dispersion (PMD) in fibers for high-power lasers can induce significant frequency modulation-to-amplitude modulation (FM-to-AM) conversion. However, existing techniques are not sufficiently flexible to achieve efficient compensation for such FM-to-AM conversion. By analyzing the non-uniform transmission spectrum caused by PMD, we found that that the large-scale envelope of the transmission spectrum has more serious impacts on the amount of AM. In order to suppress the PMD-induced FM-to-AM conversion, we propose a novel tunable spectral filter with multiple degrees of freedom based on a half-wave plate, a nematic liquid crystal, and an axis-rotated polarization-maintaining fiber. Peak wavelength, free spectral range (FSR), and modulation depth of the filter are decoupled and can be controlled independently, which is verified through both simulations and experiments. The filter is utilized to compensate for the PMD-induced FM-to-AM conversion in the front end of a high-power laser facility. The results indicate that, for a pulse with phase-modulation frequency of 22.82 GHz, the FM-to-AM conversion could be reduced from 18% to 3.2% in the short term and maintained below 6.5% for 3 h. The proposed filter is also promising for other applications that require flexible spectral control such as high-speed channel selection in optical communication networks.
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A direct prejudgement strategy that takes the diffraction ring as the analysis target is put forward to predict hot images induced by defects of tens of microns in the main amplifier section of high power laser system. Analysis of hot-image formation process shows that the hot image can be precisely calculated with the extracted intensity oscillation of the diffraction ring on the front surface of the nonlinear plate. Gradient direction matching method (GDM) is adopted to detect diffraction rings. Recognition of simulated diffraction rings shows that it is feasible to directly prejudge hot images induced by those closely spaced defects and the defects that are far apart from each other. Image compression and cluster analysis are utilized to optimize the performance of GDM method in recognizing actually collected diffraction images. Results show that hot images induced by defects of tens of microns can be directly prejudged without redundant information.
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Stimulated Raman Scattering (SRS) effect is considered to be one of the main obstacles for power scaling in general-type fiber lasers. Different from previous techniques that aim at suppressing SRS, nonlinear fiber amplifier (NFA), which manipulates and employs the SRS for power scaling in rare-earth doped fiber, is under intensive research in recent years. In this paper, we will present an all-round study on this new kind of high power fiber amplifier. A theoretical model is proposed based on the rate equation and amplified spontaneous emission (ASE), with random noise taken into account. By numerical solving the theoretical model, the power scaling potential, heat analysis and advantages in suppressing the undesired backscattering light are quantificationally analyzed for the first time. Then two different types of high power NFAs are demonstrated individually. Firstly, a laser diode pumped NFA has reached kilowatt output power, and the results agree well with theoretical predictions. Secondly, a tandem pumped NFA is proposed for the first time and validated experimentally, in which 1.5 kW output power has been achieved. We also briefly discuss several new issues relating to the complex nonlinear dynamics that occur in high power NFAs, which might be interesting topics for future endeavors.
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The Orion facility at AWE in the UK has the capability to operate one of its two 500 J, 500 fs short pulse petawatt beams at the second harmonic, the principal reason being to increase the temporal contrast of the pulse on target. This is achieved post-compression, using 3 mm-thick type 1 potassium dihydrogen phosphate (KDP) crystals. Since the beam diameter of the compressed pulse is ~600 mm, it is impractical to achieve this over the full aperture due to the unavailability of the large aperture crystals. Frequency doubling was originally achieved on Orion using a circular sub-aperture of 300 mm diameter. The reduction in aperture limited the output energy to 100 J. The second harmonic capability has been upgraded by taking two square 300 mm x 300 mm sub-apertures from the beam and combining them at focus using a single paraboloidal mirror, thus creating a 200 J, 500 fs, i.e. 400 TW facility at the second harmonic.
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High power superfluorescent fiber sources (SFSs), which could find wide applications in many fields such as middle infrared laser generation, Raman fiber laser pumping, and spectral beam combination, have experienced a flourishing time in recent years for its unique properties, such as short coherence length and high temporal stability. The challenge for performance scalability of powerful SFS mainly lies on the physical issues including parasitic laser oscillation and modal instability (MI). In this contribution, by employing in-band pumping avenue and high-order transverse-mode management, we explore a high power SFS with record power, near-diffraction-limited beam quality and spectral manipulation flexibility. An ultimate output power of 3.14 kW can be obtained with high temporal stability and a beam quality of M2=1.59 for the amplified light. Furthermore, the dynamics of spectral evolutions, including red-shifting of central wavelength and unsymmetrical broadening in spectral wings, of the main amplifier with different seed linewidth are investigated contrastively. Benefiting from the unique high pump brightness and high MI threshold of in-band pumping scheme, the demonstrated system also manifests promising performance scaling potential.
PDF全文 (下载：20) High Power Laser Sci. Eng. ，年第卷第期 pp.