OMEGA EP装置利用激光系统模型PSOPS提升装置运行性能与灵活性

梁彦 朱坪

在激光聚变、高能量密度物理、实验室天体物理、光学材料激光处理等研究领域中,能够输出复杂脉冲形状对激光装置非常重要。想要能够精确输出复杂脉冲形状,并使装置稳定运行、实验效率最大化,对激光系统时空输出特性的精确实时预测十分关键。近期,来自罗切斯特大学OMEGA EP装置的研究团队在High Power Laser Science and Engineering 2020年第1期的论文中,报导了一种基于OMEGA EP装置的半解析MATLAB模型PSOPS,能够实时预测激光输出性能,为提升激光系统的参数精确性和运行灵活性提供依据(M. J. Guardalben, M. Barczys, B. E. Kruschwitz, et al. Laser-system model for enhanced operational performance and flexibility on OMEGA EP[J]. High Power Laser Science and Engineering, 2020, 8(1): 010000e8)。

图1 OMEGA EP激光装置系统示意图。

PSOPS模型将能量传输方程的解析解与小信号增益的测量、饱和通量的校准相结合,能够在几分钟之内基于前一发次输出特性实现对后一发次脉冲形状、能量、近场光束分布等特性的预测,耗时远远小于90 min的发次间隔时间。

该模型程序结构如图2所示,具有以下四个特点:1)实时、准确地预测每一路激光的性能;2)操作界面直观,运行人员容易上手;3)可针对实验数据进行反馈优化调节;4)具备前向和后向两种预测能力。

图2 PSOPS模型程序结构。(a)前向光传输预测:使用注入光参数与放大器构型作为输入预测紫外光束输出能力。(b)逆向光传输预测:使用紫外光参数和放大器构型作为输入预判注入光特性。

基于OMEGA EP第三路激光,开展了PSOPS模型对激光系统空间和时间特性的前向预测和逆向预测的实验验证研究。实验结果表明,计算与测量结果表现出了良好的一致性。图3对比了PSOPS正向仿真的放大光近场光斑、时间波形、能量与对应的激光实验测量结果。基频:测量值3112 J,仿真值3102 J;三倍频:测量值453 J,仿真值452 J。图4对比了PSOPS逆向仿真的注入光近场光束轮廓、波形、能量与对应的激光实验测量结果,测量值79.5 mJ,仿真值76.9 mJ。

图3 PSOPS正向仿真与实验结果对比。

图4 PSOPS逆向仿真与实验结果对比。

此外,该模型还有一个独特之处,PSOPS具有改善紫外脉冲能量和波形精确度、提升实验灵活性、增加有效脉冲宽度、提高系统准直性能等功能,使装置运行的精确性与灵活性大幅度提高:1)调整前端参数和脉冲波形,补偿光路中的动态损耗、增益损耗、脉冲时空变化等;2)基于用户对实验数据的实时分析,调整到靶能量和脉冲波形;3)通过精确拼接多束激光脉冲实现有效脉宽的增加,增加范围高达4倍;4)提升系统准直能力,减少近场调制量,并有助于理解增益改变的原因。

最后,该研究团队展望了PSOPS模型的后期升级工作。一方面,会在模型中增加激光增益、有效截面积、饱和通量的光谱维度;另一方面,使用闭环的任意波形发生器将有助于进一步减少脉冲波形设计与产生的所需时间。

 

On the Cover of HPL: Laser-system model for enhanced operational performance and flexibility on OMEGA EP

M. J. Guardalben

The ability of high-energy laser systems to provide complex laser pulse shapes has growing importance in many research disciplines such as laser fusion, high-energy-density physics, laboratory astrophysics, and laser conditioning of optical materials. In such laser facilities, accurate real-time predictions of laser performance are critical for maximizing experimental and operational effectiveness and flexibility. This is particularly important when real-time guidance is required by the laser facility to satisfy the demands of rapidly evolving experimental campaign needs. For example, x-ray diffraction of ramp-compressed crystalline solids can probe high-pressure solid–solid phase transformations that are inaccessible with shock compression. In this case, the laser pulse shape must be tailored to provide a specific pressure-loading profile that prevents the melting of the material due to an increase in entropy and temperature. Additionally, different ramped pulse shapes may be requested during an experimental campaign when exploring the location of phase boundaries within a complex phase diagram. To provide such laser pulse-shape flexibility over a wide range of energies requires a stable, well-characterized laser system, and an agile laser prediction model that can be optimized in real time to compensate for any drifts that may occur in laser system performance.

Researchers at the University of Rochester's Laboratory for Laser Energetics (LLE) describe an agile laser-system prediction model developed for the OMEGA EP Laser System that has enabled rapid and flexible laser pulse shaping, published in High Power Laser Science and Engineering, Vol. 8, Issue 1, 2020 (M. J. Guardalben, M. Barczys, B. E. Kruschwitz, M. Spilatro, L. J. Waxer, E. M. Hill. Laser-system model for enhanced operational performance and flexibility on OMEGA EP[J]. High Power Laser Science and Engineering, 2020, 8(1): 010000e8).

Essential features of the model are (1) accurate, real-time predictions of expected performance of all four OMEGA EP beamlines within a small fraction of the OMEGA EP shot cycle; (2) an intuitive, easy-to-use interface for laser operators; (3) rapid optimization capability of the code between laser shots to fine-tune predictions based on shot performance; and (4) forward and backward prediction capabilities. These features allow laser-system operators to quickly and accurately optimize laser pulse shape, energy, and laser diagnostic filtrations prior to each OMEGA EP shot. The unique features of the model have also provided greater performance accuracy and flexibility by enabling rapid optimization in the following key areas:

Determination of laser front-end throttle and pulse-shape adjustments required to compensate for such issues as changes in passive loss through a beamline, loss of gain from amplifier flash-lamp degradation, spatial variations in saturated gain resulting from changes in injected beam profile, and spatiotemporal variations in front-end laser performance. This has improved OMEGA EP's ability to accurately produce users' requested laser energies and pulse shapes.

Adjustments to on-target energy and pulse shape within predetermined allowances based on a user's real-time analysis of experimental data.

Increased effective pulse-duration range through precise concatenation of pulses across multiple beams.

Improved system alignment. As a post-shot analysis and diagnostic tool, the laser performance model has been used to guide alignment of beam-shaping apodizers in the front end of OMEGA EP and to understand the effects of beamline-centering errors in order to optimize the fill factor of the amplified beam, reduce near-field modulation, and help elucidate causes of beamline gain changes.

The model's ability to fine-tune complex laser pulse shapes in real time has benefitted several user experiments on OMEGA EP, such as experiments to investigate the transition of alkali metals from conducting, metal-like behavior to nonconducting, insulating behavior at high pressure; experiments to investigate the dominant mechanism in laser–plasma instabilities at intermediate regimes of electron temperature and density scale length; and experiments that probe the crystalline structure and solid-to-liquid ratio of diamond along different temperature/pressure pathways. Future research that will benefit from OMEGA EP's pulse-shaping capability include experiments to tune the energy density of matter into a high-energy-density quantum regime to understand extremes of quantum matter behavior, properties, and phenomena. These compression experiments will tune the distance between atoms, thereby unlocking a new quantum behavior at unprecedentedly high temperatures, transferring quantum phenomena to the macroscale, and opening the potential for hot superconductors, superconducting-superfluid plasma, transparent aluminum, insulating plasma, and potentially more.

"The improved OMEGA EP pulse shaping capabilities now allow us to more accurately probe new states of matter at more extreme conditions than before. This opens fantastic new opportunities for research in Inertial Confinement Fusion (ICF), fundamental condensed matter, planetary sciences and astronomy with capabilities that are very complementary to the National Ignition Facility", stated Dr. Marius Millot, Research Scientist at the Lawrence Livermore National Laboratory, and one of the many users of the OMEGA EP Laser System.

A recent upgrade to the model accounts for the spectral dependence of beamline gain for shots that require spectrally tunable ultraviolet on-target irradiation to mitigate cross-beam energy transfer that reduces the on-target irradiation uniformity required for spherically symmetric implosion of laser-fusion targets. This will be refined to account for the spectral dependence of effective emission cross section and saturation fluence. Due to the success of the model on the OMEGA EP Laser System, a modified version is being adapted for use on LLE's 60-beam OMEGA Laser System.

(a) The OMEGA EP Laser System showing the booster amplifiers in the foreground and target chamber in the distance. (b) Pre-shot prediction and (c) measurement of 27-ns composite ultra-violet pulse formed by incoherent addition of individual beamline pulses. The cover photo was taken from the target chamber area and shows the main-cavity amplifiers firing during a four-beam target shot. (photo credits: Eugene Kowaluk)