Radiation-driven hydrodynamics of high- hohlraums on the national ignition facility

The first hohlraum experiments on the National Ignition Facility (NIF) using the initial four laser beams tested radiation temperature limits imposed by plasma filling. For a variety of hohlraum sizes and pulse lengths, the measured x-ray flux shows signatures of filling that coincide with hard x-ray emission from plasma streaming out of the hohlraum. These observations agree with hydrodynamic simulations and with an analytical model that includes hydrodynamic and coronal radiative losses. The modeling predicts radiation temperature limits with full NIF (1.8 MJ), greater, and of longer duration than required for ignition hohlraums.     

Wire initiation critical for radiation symmetry in z-pinch-driven dynamic hohlraums

Axial symmetry in x-ray radiation of wire-array z pinches is important for the creation of dynamic hohlraums used to compress inertial-confinement-fusion capsules. We present the first evidence that this symmetry is directly correlated with the magnitude of the negative radial electric field along the wire surface. This field (in turn) is inferred to control the initial energy deposition into the wire cores, as well as any current shorting to the return conductor.     

Measurement of the absolute hohlraum-wall albedo under ignition foot drive conditions

We present measurements of the absolute albedos of hohlraums made from gold or from high-Z mixtures. The measurements are performed over the range of radiation temperatures (70-100 eV) expected during the foot of an indirect-drive temporally shaped ignition laser pulse, where accurate knowledge of the wall albedo (i.e., soft x-ray wall reemission) is most critical for determining capsule radiation symmetry. We find that the gold albedo agrees well with calculations using the supertransition array opacity model, potentially providing additional margin for inertial confinement fusion ignition.     

Demonstration of ignition radiation temperatures in indirect-drive inertial confinement fusion hohlraums

We demonstrate the hohlraum radiation temperature and symmetry required for ignition-scale inertial confinement fusion capsule implosions. Cryogenic gas-filled hohlraums with 2.2 mm-diameter capsules are heated with unprecedented laser energies of 1.2 MJ delivered by 192 ultraviolet laser beams on the National Ignition Facility. Laser backscatter measurements show that these hohlraums absorb 87% to 91% of the incident laser power resulting in peak radiation temperatures of T(RAD)=300 eV and a symmetric implosion to a 100 μm diameter hot core.     

4MA以下电流驱动垂直动态黑腔的自适应性

在4 MA以下电流驱动Z箍缩黑腔辐射特性和耦合效率研究牵引下,研制出了柱型动态黑腔。利用CHO低密度泡沫的自持能力,结合辅助夹具解决了泡沫柱的装配及定位问题。基于负载丝受小应变作用,丝阵和泡沫柱通过μm量级的自由滑动和转动,具备自动适应靶室环境变化能力。实验结果表明:研制出的动态黑腔在Angara-5-1装置环境中具有自动适应能力,各项参数满足物理实验的需求,获得的最高动力学黑腔X光辐射温度为62.7 eV,实验结果重复性优于91.5%。     

强激光间接驱动材料动态破碎过程的实验技术研究

强激光驱动加载已成为冲击波作用下材料动态破碎过程研究的一种有效手段.采用间接驱动方式,设计合适的腔型进行物理实验研究,可实现更大且更均匀的冲击加载一维区.采用数值模拟和物理实验方法,研究强激光间接驱动材料动态破碎过程的实验技术.首先,利用IRAD程序设计适用于开展动态破碎过程研究的半柱腔,其直径为2 mm、腔长为2 mm;进而通过物理实验获得此腔型下多个激光能量点、脉宽2 ns和3 ns条件下辐射峰值温度和波形;最后,利用流体模拟方法给出多种辐射波形下的冲击加载波形.利用高能X射线成像和光子多普勒干涉仪诊断给出间接驱动加载下层裂过程的物理图像和速度历史.经分析发现,间接驱动的加载一维区达到2 mm,平面性优于5%,能有效地开展相关物理实验研究.研究结果为新型柱腔设计、冲击加载技术及动态破碎过程研究提供了重要的研究基础.     

快点火锥壳靶内爆自发光图像数据处理

为研究快点火预压缩过程中导引锥对压缩靶丸芯部辐射对称性的影响,在神光Ⅲ原型上开展了快点火预压缩的自发光测量实验。采集了相同长度黑腔中不同类型靶丸的内爆自发光图像和不同长度黑腔中同类靶丸的内爆自发光图像,利用最小二乘椭圆拟合方法对芯部的自发光半峰值强度等高线进行椭圆拟合,通过椭圆特征来分析靶丸在最强发光时刻的压缩对称性。分析结果表明:导引锥的存在会使发光区域沿着赤道方向拉长,降低了压缩对称性;有锥靶丸在1600μm黑腔内的形变小于在1500μm和1700μm黑腔内的形变,压缩对称性可以通过调整黑腔长度来控制。     

腔内辐射温度的实验研究

介绍了利用两台软X射线能谱仪和多针孔软X射线条纹相机配合诊断腔内的辐射温度。实验解决了因等离子体喷射引起的诊断口缩孔带来的测不准问题,测到了诊断孔的二维缩孔图像和缩孔时间过程,从而给出了腔内辐射温度随时间的变化关系。通过实验数据分析,建立了初步的诊断孔缩孔模型。 关键词：     

X光分幅相机在黑腔等离子体填充特性研究中的应用

 在神光Ⅱ装置上2.4 ns长脉冲三倍频激光（激光能量8×300 J）与腔靶耦合实验研究中,X光分幅相机通过激光注入孔观测获得了3种腔尺寸腔内Au等离子体径向运动时空分辨图像。用MATLAB对图像进行了定量处理,结合时间分辨辐射温度测量结果分析表明：在腔内不充气、无低Z衬垫情况下,标准腔（800 μm×1 350 μm）在激光脉冲作用到约1.5 ns时出现明显的Au等离子体堵腔效应;当腔尺寸放大到1.25倍（1 000 μm×1 800 μm）和1.5倍（1 200 μm×2 100 μm）时,腔内等离子体基本不堵腔。给出了3种腔尺寸不同时期腔内Au等离子体径向聚心速度,分析表明：大腔的聚心速度比小腔的慢,后期比初期慢。     

强光1号装置上钨丝阵列的Z箍缩实验研究

本文报道了2003年在强光一号装置上进行的钨丝阵列的Z箍缩实验研究的主要结果。为了研究钨丝阵列的内爆过程和x光辐射特性，使用了x光功率谱仪和一维时空分辨x光成像系统等诊断设备。实验中由32根5微米钨丝组成的直径8mm长20mm的优化负载获得了最高能量为36.6kJ的x光产额，一维时空分辨x光成像系统的数据显示x光辐射区域以6.4×10⁶cm/s的速度减小，同时x光峰值时刻出现在等离子体被压缩到近轴区域之前。     

X-ray spatial harmonic imaging of phase objects

Refractive index gradients in materials or at material interfaces lead to x-ray diffraction. Interference of this radiation with adjacent x-ray waves causes phase contrast that can be used for imaging purposes if an x-ray source with sufficient spatial coherence is used. The imaging modality presented here uses hard x radiation diffracted at interfaces, but requires only little spatial coherence. We report experiments showing, first, that image contrast is not diminished by motional blurring, and second, that contrast can be increased by orders of magnitude relative to in-line x-ray phase-contrast imaging. These properties substantially broaden the applicability of phase-sensitive imaging to moving samples and very weak density gradients.     

Hohlraum-driven high-convergence implosion experiments with multiple beam cones on the omega laser facility

High-convergence implosion experiments have been performed on the Omega laser facility [T.R. Boehly, Opt. Commun. 133, 495 (1997)]] using cylindrical gold hohlraums with 40 drive beams arranged into multiple cones. These experiments make use of improved hohlraum radiation symmetry conditions [T.J. Murphy, Phys. Rev. Lett. 81, 108 (1998)]] to demonstrate near predicted primary (2.45 MeV) neutron production from single-shell implosions with measured deuterium fuel convergence ratios exceeding 20 at an ignition-relevant hohlraum case-to-capsule ratio approximately 3.     

Z箍缩动态黑腔形成过程和关键影响因素数值模拟研究

在Z箍缩动态黑腔研究中, 认识黑腔形成物理过程及主要特征, 明确优化黑腔辐射的关键参数, 是实验物理设计的重要基础. 本文针对W丝阵填充CH泡沫转换体的负载构型, 利用一维辐射磁流体程序, 在8 MA驱动电流条件下开展了动态黑腔形成过程和关键影响因素的数值模拟研究. 结果表明, 丝阵等离子体与泡沫转换体相互作用产生局部高压力区是驱动冲击波传播和形成动态黑腔的关键物理过程. 由于辐射超声速传播及其与冲击波波阵面的空间分离, 产生了辐射温度较高而物质未受明显压缩的动态黑腔区域. 丝阵等离子体碰撞泡沫转换体前的状态分布决定了动态黑腔辐射场的主要特征, 可以通过改变负载参数调整并优化黑腔辐射波形. 综合考虑黑腔峰值辐射温度和有效维持时间两个参数, 选择匹配质量的丝阵和泡沫, 使丝阵质量略小于泡沫, 可以获得相对优化的动态黑腔辐射波形. 同时, 合适的丝阵/泡沫初始半径比也是优化动态黑腔辐射的重要影响因素.     

实验靶对半黑腔辐射源辐射温度的影响

间接驱动惯性约束聚变和高能量密度物理实验中使用黑腔作为强X射线辐射源,黑腔源靶的等效辐射温度受安装在辐射出口的实验用靶的影响而与单独进行辐射源特性研究时有所不同。通过对几种不同辐射输运靶对半黑腔辐射源的影响进行的实验研究发现,添加不同填充材料的输运管能够使测量到的黑腔辐射源的辐射温度升高3~8 eV。实验结果的统计显示,输运管越长,填充材料密度越大,辐射温度越高。     

Demonstration of a Ni-like Kr optical-field-ionization collisional soft x-ray laser at 32.8 nm

We report the first experimental demonstration of a Ni-like optical-field ionization collisional soft x-ray laser. The amplifying medium is generated by focusing a circularly polarized 760 mJ, 30 fs, 10-Hz Ti:sapphire laser beam in a few mm cell filled with krypton. We have measured a gain coefficient of 78 cm(-1) on the 3d(9)4d 1S0-3d(9)4p(1)P1 transition at 32.8 nm, which is here amplified for the first time. This radiation source represents the shortest wavelength optical-field ionization collisional soft x-ray laser ever produced. The influence of the gas pressure and the pumping energy on the lasing output are also presented.     

Demonstration of the density dependence of x-ray flux in a laser-driven hohlraum

Experiments have been conducted using laser-driven cylindrical hohlraums whose walls are machined from Ta2O5 foams of 100 mg/cc and 4 g/cc densities. Measurements of the radiation temperature demonstrate that the lower density walls produce higher radiation temperatures than the high density walls. This is the first experimental demonstration of the prediction that this would occur [M. D. Rosen and J. H. Hammer, Phys. Rev. E 72, 056403 (2005)10.1103/PhysRevE.72.056403]. For high density walls, the radiation front propagates subsonically, and part of the absorbed energy is wasted by the flow kinetic energy. For the lower wall density, the front velocity is supersonic and can devote almost all of the absorbed energy to heating the wall.     

Hohlraum-driven ignitionlike double-shell implosions on the omega laser facility

High-convergence ignitionlike double-shell implosion experiments have been performed on the Omega laser facility [T. R. Boehly, Opt. Commun. 133, 495 (1997)] using cylindrical gold hohlraums with 40 drive beams. Repeatable, dominant primary (2.45 MeV) neutron production from the mix-susceptible compressional phase of a double-shell implosion, using fall-line design optimization and exacting fabrication standards, is experimentally inferred from time-resolved core x-ray imaging. Effective control of fuel-pusher mix during final compression is essential for achieving noncryogenic ignition with double-shell targets on the National Ignition Facility [Paisner, Laser Focus World 30, 75 (1994)].     

靶丸变形实验的多群扩散整体数值模拟

神光Ⅱ装置上靶丸压缩实验中,黑腔长度的变化会影响靶丸赤道和两极受到的辐照强度,从而导致燃料区被压缩成不同的形状。介绍了辐射多群扩散建模下的辐射流体力学方程组及其数值方法。采用最近研制的二维总体LARED集成程序,对神光Ⅱ不同长度黑腔靶丸压缩变形实验进行了整体数值模拟。结果表明辐射多群扩散建模可以反映腔内辐照均匀性变化情况,靶丸压缩形状与神光Ⅱ实验定性一致。     

Parametric x-ray radiation generated by 5.7-MeV electrons in a pyrolytic-graphite crystal

Parametric x-ray radiation is observed in a mosaic pyrolytic-graphite crystal irradiated with a 5.7-MeV electron beam. The first-order (002) reflection of parametric x-ray radiation is detected in Bragg geometry. In the measured spectra, there is a peak at about 8 keV. The detector used is arranged at an angle of Θ_D=2Θ_B=27° with respect to the electron beam, where Θ_B is the Bragg angle. The target thickness is 0.3 mm. The spectra of parametric x-ray radiation are measured at various angles of target orientation with respect to the axis of the incident-electron beam.     

数值模拟辐射驱动2维内爆压缩过程

采用2维三温非平衡辐射流体力学程序LARED-H数值模拟辐射驱动内爆过程。针对2维三温能量方程九点差分格式离散后的线性方程组,采用了高效的Krysolv子空间迭代解法,改进了代数解法器。将1维间接驱动内爆总体程序CFJ与LARED-H程序的计算结果进行比对,验证了LARED-H程序数值模拟1维内爆问题的正确性。并数值模拟了不同腔长辐射温度源驱动下的2维靶球运动,数值结果显示:随着腔长的增加,高压缩内爆燃料区分别被压缩成香肠形、球形和铁饼形,数值模拟结果与神光Ⅱ的实验结果定性上相同。