Update on ignition studies at CEA

This article sums up the theoretical and experimental studies about ignition. Three experiments are salient this year on the Omega laser in collaboration with DOE laboratories (1) 3 cones of beams allow to mimic the LMJ configuration and to get symmetry measurements. (2) We measured perturbations due to hydro-instability in CHGe planar samples with face-on and side-on radiographs. (3) We improved our nuclear diagnostics, particularly the neutron image system tested on direct drive implosions. As far as LMJ target design is concerned, we defined a preliminary domain corresponding to the possible operation at 2ω. At 3ω we studied the low mode instability effects on the DT deformation (due to the laser or to the target) and on the yield. The stability is clearly improved with graded doped CH for our nominal capsule L1215.     

点火黑腔二维模拟设计

基于二维激光靶耦合流体力学程序,研究设计惯性约束聚变间接驱动点火黑腔的方法.提出先调控X射线驱动温度再调控辐照均匀性的设计顺序.给出总激光功率(特别是主脉冲激光功率)的设计方法.模拟表明,填充气体密度的上限受到槽脉冲P²辐照不均匀性的限制.另外,增加腔长可以抑制靶丸烧蚀层物质的膨胀.最后给出点火黑腔二维设计结果.     

神光-Ⅲ主机黑腔腔形研究与设计

利用三维视角因子软件IRAD3D研究了神光-Ⅲ主机靶丸表面辐射驱动时变对称性和驱动强度随黑腔腔形和排布方式的变化。模拟结果表明:采用椭球形黑腔能显著减小黑腔壁面积从而减小腔壁能量消耗并提高靶丸消耗份额;不同腔形和排布方式下均可通过调节腔长使勒让德不对称性模P2时间积分量较小;采用双环辐照和椭球腔形都利于缩短腔长;四环排布利于调节P4分量,但环向不对称性M4较差;椭球腔双环排布能兼顾极向不对称性P2,P4和环向不对称性M4,且靶丸表面X光驱动强度相对柱腔双环提高25%~27%。     

A new ignition hohlraum design for indirect-drive inertial confinement fusion

In this paper,a six-cylinder-port hohlraum is proposed to provide high symmetry flux on capsule.It is designed to ignite a capsule with 1.2-mm radius in indirect-drive inertial confinement fusion(ICF).Flux symmetry and laser energy are calculated by using three-dimensional view factor method and laser energy balance in hohlraum.Plasma conditions are analyzed based on the two-dimensional radiation-hydrodynamic simulations.There is no Y_(lm)(l≤4) asymmetry in the six-cylinder-port hohlraum when the influences of laser entrance holes(LEHs) and laser spots cancel each other out with suitable target parameters.A radiation drive with 300 eV and good flux symmetry can be achieved by using a laser energy of 2.3 MJ and peak power of 500 TW.According to the simulations,the electron temperature and the electron density on the wall of laser cone are high and low,respectively,which are similar to those of outer cones in the hohlraums on National Ignition Facility(NTF).And the laser intensity is also as low as those of NIF outer cones.So the backscattering due to laser plasma interaction(LPI) is considered to be negligible.The six-cyliner-port hohlraum could be superior to the traditional cylindrical hohlraum and the octahedral hohlraum in both higher symmetry and lower backscattering without supplementary technology at an acceptable laser energy level.It is undoubted that the hohlraum will add to the diversity of ICF approaches.     

Influence analysis of symmetry on capsule in six-cylinder-port hohlraum

We have investigated the flux symmetry on the capsule in a six-cylinder-port hohlraum for improving the design of the hohlraum. The influence factors of drive symmetry on the capsule in the hohlraum are studied, including laser power,laser beams arrangement, hohlraum geometric parameters, plasma condition, capsule convergence, etc. The x-ray radiation flux distribution on the capsule is obtained based on the three-dimensional view factor model. In the six-cylinder-port hohlraum, the main drive asymmetry is the C40 mode asymmetry. When the C40 mode asymmetry approaches zero, the drive symmetry on the capsule is optimal. Our results demonstrate that in order to have a high flux symmetry on the capsule in the laser main-pulse stage, more negative initial C40 modes are needed, which can be realized by adjusting the hohlraum geometry parameters. The hohlraum with column length L_H = 4.81 mm has an optimal symmetry in the laser main-pulse stage.     

点火黑腔等离子体定标关系及350 eV靶黑腔设计

点火黑腔内环激光通道内靠近黑腔壁的区域是内环激光受激拉曼散射(SRS)背向散射产生与发展的主要区域。根据内环通道在该区域满足通道内外压力平衡和能量平衡,提出了间接驱动惯性约束聚变点火黑腔等离子体定标关系。该定标关系在一定程度上统筹考虑靶丸性能、激光器指标和激光等离子体相互作用(LPI)。在此基础上,根据靶丸抑制流体不稳定的需求,提出了一个350eV点火黑腔设计,该设计可以较好地抑制内环LPI的发展,并对激光器设计提出了更高的要求。     

间接驱动的内爆不对称性随腔长和时间变化的研究

在激光间接驱动惯性约束聚变中, 为达到高密度压缩以实现点火, 对靶丸内爆对称性和黑腔辐射场均匀性有严格的要求. 为了研究内爆不对称性随腔长和时间的变化, 实验中采用了三种不同尺寸的黑腔, 利用X光分幅相机观测了靶丸燃料区自发光, 获得了压缩变形过程和椭圆度变化规律, 初步判断了在三种腔型中腔长1700 μm 的黑腔较接近神光Ⅲ原型装置内爆对称性要求. 根据视角因子程序计算得到辐射流不对称性随时间变化情况, 通过一个简化的解析模型推导出内爆形变不对称性随时间变化过程, 与实验结果大致符合. 由此进一步分析了黑腔辐射场不均匀性的演化导致内爆不对称性随腔长和时间变化的物理机制. 关键词： 内爆不对称性 黑腔辐射场不均匀性 腔长 视角因子程序     

ICF点火靶定标关系与稳定性优化设计研究

激光等离子体相互作用(LPI)和瑞利-泰勒流体不稳定性(RTI)是影响间接驱动惯性约束聚变成功的两个主要不确定性因素。点火黑腔内环激光通道在靠近黑腔壁的区域是内环激光SRS背反产生与发展的主要区域。内环通道在该区域满足通道内外压力平衡和能量平衡条件。据此提出了间接驱动惯性约束聚变点火黑腔等离子体定标关系。结合描述靶丸内爆飞行阶段物理以及内爆性能的两个定标关系,提出了描述稳定性相对性能的指标。该指标可以指导点火靶设计,为LPI和RTI提供需要的裕量空间,是点火阈值因子(ITF)的补充。     

Demonstration of radiation symmetry control for inertial confinement fusion in double Z-pinch hohlraums

Simulations of a double Z-pinch hohlraum, relevant to the high-yield inertial-confinement-fusion concept, predict that through geometry design the time-integrated P2 Legendre mode drive asymmetry can be systematically controlled from positive to negative coefficient values. Studying capsule elongation, recent experiments on Z confirm such control by varying the secondary hohlraum length. Since the experimental trend and optimum length are correctly modeled, confidence is gained in the simulation tools; the same tools predict capsule drive uniformity sufficient for high-yield fusion ignition.     

Progress in direct-drive inertial confinement fusion research at the laboratory for laser energetics

Direct-drive inertial confinement fusion (ICF) is expected to demonstrate high gain on the National Ignition Facility (NIF) in the next decade and is a leading candidate for inertial fusion energy production. The demonstration of high areal densities in hydrodynamically scaled cryogenic DT or D₂ implosions with neutron yields that are a significant fraction of the “clean” 1-D predictions will validate the ignition-equivalent direct-drive target performance on the OMEGA laser at the Laboratory for Laser Energetics (LLE). This paper highlights the recent experimental and theoretical progress leading toward achieving this validation in the next few years. The NIF will initially be configured for X-ray drive and with no beams placed at the target equator to provide a symmetric irradiation of a direct-drive capsule. LLE is developing the “polar-direct-drive” (PDD) approach that repoints beams toward the target equator. Initial 2-D simulations have shown ignition. A unique “Saturn-like” plastic ring around the equator refracts the laser light incident near the equator toward the target, improving the drive uniformity. LLE is currently constructing the multibeam, 2.6-kJ/beam, petawatt laser system OMEGA EP. Integrated fast-ignition experiments, combining the OMEGA EP and OMEGA Laser Systems, will begin in FY08.     

Prolate-spheroid ("rugby-shaped") hohlraum for inertial confinement fusion

A novel rugby-ball shaped hohlraum is designed in the context of the indirect-drive scheme of inertial-confinement fusion (ICF). Experiments were performed on the OMEGA laser and are the first use of rugby hohlraums for ICF studies. Analysis of experimental data shows that the hohlraum energetics is well understood. We show that the rugby-ball shape exhibits advantages over cylinder, in terms of temperature and of symmetry control of the capsule implosion. Simulations indicate that rugby hohlraum driven targets may be candidates for ignition in a context of early Laser MegaJoule experiments with reduced laser energy.     

Impeding hohlraum plasma stagnation in inertial-confinement fusion

This Letter reports the first time-gated proton radiography of the spatial structure and temporal evolution of how the fill gas compresses the wall blowoff, inhibits plasma jet formation, and impedes plasma stagnation in the hohlraum interior. The potential roles of spontaneously generated electric and magnetic fields in the hohlraum dynamics and capsule implosion are discussed. It is shown that interpenetration of the two materials could result from the classical Rayleigh-Taylor instability occurring as the lighter, decelerating ionized fill gas pushes against the heavier, expanding gold wall blowoff. This experiment showed new observations of the effects of the fill gas on x-ray driven implosions, and an improved understanding of these results could impact the ongoing ignition experiments at the National Ignition Facility.     

Recent results and future prospects of laser fusion research at ILE, Osaka

Reviewed are the present status of the fast ignition researches. Since 1997, the fast ignition experiment and theory researches have been extensively continued at the Institute of Laser Engineering of Osaka University. In particular, the cone-shell target experiments and simulation research have been progressing. In order to demonstrate heating of imploded high density plasma to the ignition temperature, in the April of 2003, the construction of heating laser of 10 kJ/10 ps/1.06 μm (Laser for Fusion Experiment; LFEX), for FIREX-I (Fast Ignition Realization Experiment) has started. The fabrication of DT foam cryogenic cone target is also under development as a collaboration program between Osaka University and NIFS (National Institute for Fusion Science). The LFEX will be completed in 2008. After the completion of LFEX, the foam cryogenic cone shell target experiment will start in 2008. As a new approach toward a compact ignition, an impact fusion has been proposed, where the ablative acceleration to the order of 10⁸ cm/s is the key issue. The ablation acceleration related to the impact fusion has been explored by experiments.     

黑腔中等离子体相互作用的流体力学现象观测

激光间接驱动惯性约束聚变实验中,黑腔内情况复杂,在激光烧蚀和辐射烧蚀等的驱动下,光斑区、冕区、纯辐射烧蚀区、射流区的多种等离子体以不同规律运动.发展了X光双能段窄能带的时间分辨成像方法,用以观测黑腔内多种等离子体的运动情况.在真空黑腔中观测到清晰的射流,分析了射流产生机制及其速度;在黑腔中充气,能有效消除射流和抑制冕区等离子体运动,但两种物质界面处可能会出现流体力学不稳定性等现象,分析了界面处的压力平衡关系和密度陡变情况.     

视角因子方法在间接驱动ICF中的应用

采用视角因子方法,对间接驱动惯性约束聚变(ICF)过程中从腔内壁辐射到靶丸表面的辐射进行了计算,并分析了激光原型装置上的一个实验的对称性环境。间接驱动ICF系统的不对称性一般来自于激光焦斑和腔体的几何结构,利用视角因子方法,计算X光能量在腔体内的强度分布,将靶丸表面的驱动对称性表示成腔体结构和激光焦斑几何参数的函数。结果表明,采用该方法的程序模拟能够评估腔靶设计的主要参数。     

神光Ⅲ主机装置首次间接驱动内爆集成实验

基于新建成的神光Ⅲ主机装置开展了首次激光间接驱动内爆集成实验。国内首次采用多环脉冲整形激光注入黑腔产生X光辐射驱动内爆,通过优化激光打靶参数控制驱动不对称性,演示了以惯性压缩为主、收缩比约15倍的DT靶丸内爆实验能力, 实现了准一维的高静产额(YOC)和高中子产额的物理指标;其中,真空黑腔DT靶丸最高中子产额为1.91012,YOC达到60%;充气黑腔DT靶丸最高中子产额为2.41012,YOC大约70%。该实验为未来开展多台阶整形辐射驱动、更高倍数收缩比的高压缩内爆综合实验、验证点火靶物理设计和关键调控措施有效性奠定了基础。     

The national ignition facility: path to ignition in the laboratory

The National Ignition Facility (NIF) is a 192-beam laser facility presently under construction at LLNL. When completed, NIF will be a 1.8-MJ, 500-TW ultraviolet laser system. Its missions are to obtain fusion ignition and to perform high energy density experiments in support of the US nuclear weapons stockpile. Four of the NIF beams have been commissioned to demonstrate laser performance and to commission the target area including target and beam alignment and laser timing. During this time, NIF demonstrated on a single-beam basis that it will meet its performance goals and demonstrated its precision and flexibility for pulse shaping, pointing, timing and beam conditioning. It also performed four important experiments for Inertial Confinement Fusion and High Energy Density Science. Presently, the project is installing production hardware to complete the project in 2009 with the goal to begin ignition experiments in 2010. An integrated plan has been developed including the NIF operations, user equipment such as diagnostics and cryogenic target capability, and experiments and calculations to meet this goal. This talk will provide NIF status, the plan to complete NIF, and the path to ignition.     

激光间接驱动聚变柱形腔靶内壁光斑参数计算

 用几何光学计算了柱形腔壁上的激光光斑参数, 包括光斑的位置、形状、大小以及光斑内的光强分布, 同时还计算了在激光注入孔的光斑参数, 这对分析靶丸的初始辐射均匀性、确定腔壁上的辐照强度大小和判断能否将激光有效地注入腔内都是非常重要的。     

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.     

Progress toward ignition with noncryogenic double-shell capsules

Inertial confinement fusion implosions using capsules with two concentric shells separated by a low density region (double shells) are reported which closely follow one dimensional (1D) radiatively driven hydrodynamics simulations. Capsule designs which mitigate Au M-band radiation asymmetries appear to correspond more closely to 1D simulations than targets lacking mitigation of hohlraum drive M-band nonuniformities. One capsule design achieves over 50% of the unperturbed 1D calculated yield at a convergence ratio of 25.5, comparable to that of a double-shell design for an ignition capsule at the National Ignition Facility.