Review of Japanese fusion program and role of inertial fusion

The high compression of 600 times liquid density and the recent fast heating of a compressed core to 1-keV temperature have provided proof-of-principle of the fast ignition concept, and these results have significantly contributed to approve first phase of the Fast Ignition Realization EXperiment (FIREX) project. The goal of FIREX-I is to demonstrate fast heating of a fusion fuel up to the ignition temperature of 5–10 keV. Although the fuel size of FIREX-I is too small to ignite, sufficient heating will provide the scientific viability of ignition-and-burn by increasing the laser energy thereby the fuel size. Based on the result of FIREX-I, the decision of the start of FIREX-II to achieve ignition-and-burn can be made. The FIREX program is under the collaboration of the Institute of Laser Engineering and the National Institute for Fusion Science.     

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.     

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.     

Optimum hot electron production with low-density foams for laser fusion by fast ignition

We propose a foam cone-in-shell target design aiming at optimum hot electron production for the fast ignition. A thin low-density foam is proposed to cover the inner tip of a gold cone inserted in a fuel shell. An intense laser is then focused on the foam to generate hot electrons for the fast ignition. Element experiments demonstrate increased laser energy coupling efficiency into hot electrons without increasing the electron temperature and beam divergence with foam coated targets in comparison with solid targets. This may enhance the laser energy deposition in the compressed fuel plasma.     

Update on design simulations for NIF ignition targets, and the rollup of all specifications into an error budget

Targets intended to produce ignition on NIF are being simulated and the simulations are used to set specifications for target fabrication and other program elements. Recent design work has focused on designs that assume only 1.0 MJ of laser energy instead of the previous 1.6 MJ. To perform with less laser energy, the hohlraum has been redesigned to be more efficient than previously, and the capsules are slightly smaller. Three hohlraum designs are being examined: gas fill, SiO₂ foam fill, and SiO₂ lined. All have a cocktail wall, and shields mounted between the capsule and the laser entrance holes. Two capsule designs are being considered. One has a graded doped Be(Cu) ablator, and the other graded doped CH(Ge). Both can perform acceptably with recently demonstrated ice layer quality, and with recently demonstrated outer surface roughness. Complete tables of specifications are being prepared for both targets, to be completed this fiscal year. All the specifications are being rolled together into an error budget indicating adequate margin for ignition with the new designs. The dominant source of error is hohlraum asymmetry at intermediate modes 4–8, indicating the importance of experimental techniques to measure and control this asymmetry.     

Tunable Ti: Sapphire regenerative amplifier for femtosecond Chirped-Pulse Amplification

We describe a tunable Ti:Sapphire regenerative amplifier which is used to amplify 120 fs pulses from a self-mode-locked Ti:Sapphire laser to energies in the range of 7–12 mJ from 760 nm to 855 nm. We have used three sets of cavity mirrors in the regenerative amplifier to vary the output wavelength of the laser.On leave from Institute of Laser Engineering, Osaka University, 2-6, Yamada-oka, Osaka 565, Japan (Fax: +81-6/877-4799)     

快点火锥壳靶压缩对称性及燃料分布测量实验研究北大核心CSCD

快点火激光惯性约束聚变将压缩与点火过程分开,与中心点火方式相比,大大放宽了对压缩对称性和能量的要求,是国际惯性约束聚变研究的热点方向。在神光Ⅱ装置上开展的快点火锥壳靶预压缩实验中,背光分幅图像显示导引锥及输运丝对靶丸预压缩过程无明显影响,实验结果与一维数值模拟结果吻合也证明了该结论;实验中通过调节导引锥尺寸和相对靶丸的位置,可保证最大压缩时刻导引锥保存完好,这对超热电子的输运以及能量沉积是至关重要的。     

Fast ignition of asymmetrically compressed targets for inertial confinement fusion

It is shown that fast ignition can ensure the combustion of asymmetrically compressed targets for inertial confinement fusion with an efficiency close to the combustion of one-dimensionally compressed targets. This statement is valid not only for targets specially designed for fast ignition. Fast heating by an external energy source can ensure the ignition of a target designed for spark ignition, but where this ignition does not occur because inhomogeneities are formed in the temperature and density distributions owing to the development of hydrodynamic instabilities. The condition for ignition is the fast heating of the plasma in the combustion initiation region whose size is comparable with the sizes of compression inhomogeneities. Thus, fast ignition not only significantly reduces the ignition energy, but also is possibly a necessary stage in the inertial confinement fusion scheme when the spherically symmetric compression of a target requires very high engineering and financial expenses. The studies are based on the numerical simulation of the compression and combustion of inertial confinement fusion targets with one- and two-dimensional hydrodynamic codes.     

The megajoule laser program — ignition at hand

The French Commissariat à l'énergie Atomique (CEA) is currently building the Laser MegaJoule (LMJ), a 240-beam laser facility, at the CEA Laboratory CESTA near Bordeaux. LMJ will be a cornerstone of CEA's “Programme Simulation”, the French Stockpile Stewardship Program. LMJ is designed to deliver about 2 MJ of 0.35 μm light to targets for high energy density physics experiments, among which fusion experiments. LMJ technological choices were validated with the Ligne d'Intégration Laser (LIL), a scale 1 prototype of one LMJ bundle, built at CEA/CESTA. Plasma experiments started at the end of 2004 on LIL, which is already open to the scientific community through the Plasma and Lasers Institute. The construction of the LMJ building itself started in March of 2003. LMJ will be gradually commissioned from early 2011, and after an experimental program to progress toward fusion, the first fusion experiments will begin late 2012.     

Comparison of ion acceleration from ultra-short intense laser interactions with thin foil and small dense target

The comparative efficiency and beam characteristics of high-energy ions generated from the interaction of a petawatt laser pulse with thin foil target and a small solid-density plasma bunch target have been studied by particle-in-cell simulation under identical conditions. It is shown that thin foil and small solid dense target of micrometer size can be efficiently accelerated when irradiated by a laser pulse of intensity >10²¹?W/cm². Using direct beam measurements, we find that small solid dense target acceleration produces higher energy particles with smaller divergence and a higher efficiency compared to thin foil target acceleration. The merits of small solid target acceleration can be exploited for potential applications such as its role as ignitor for fast ignition in inertial confinement fusion.     

神光Ⅱ升级装置锥壳靶间接驱动快点火集成实验

在神光Ⅱ升级装置上完成了国际上首次间接驱动快点火集成实验。实验采用双台阶脉冲整形激光注入黑腔产生X射线准等熵压缩锥壳靶,实现了高密度压缩,然后采用皮秒超短脉冲激光注入加热燃料。实验中观测到中子产额由皮秒激光注入前的5×103增加到2.2×105,中子产额增益达到44倍,实验证实了皮秒激光具有明显燃料加热效果。该实验为进一步开展快点火热斑形成效率和相关物理研究奠定了基础。     

强激光与细锥靶相互作用产生强流高能电子束的研究

本文提出采用了强激光与细锥形靶作用, 产生大量定向高能电子, 用于快点火激光聚变方案研究. 通过PIC 模拟, 研究了细锥靶和激光脉冲的各项参数, 对产生高能电子的影响. 模拟发现, 细锥靶开口10° 时能够产生较多的高能电子, 当开口角度逐渐增大时, 高能电子的能量和数目都有一定程度下降. 若为细锥靶加上预等离子体, 产生的高能电子的数目将大大提高, 而最高的电子能量将会下降. 中等能量的电子加速主要由于激光有质动力加速, 而高能量的电子加速主要由于电子感应加速. 随着激光脉宽的增加, 高能电子的数量直线上升. 关键词： 细锥形靶 电子加速 感应共振加速     

间接驱动快点火锥壳靶锥体材料燃料混合问题研究

与中心点火相比,快点火将压缩和点火过程分开,大大放宽了对压缩对称性和驱动能量的要求。通过在神光Ⅱ激光装置上开展了快点火锥壳靶预压缩实验研究,利用X射线背光分幅照相方法观察到了清晰完整的快点火锥壳靶内爆压缩过程,并利用阿贝反演结合剩余烧蚀质量的方法得到了不同时刻燃料密度、面密度分布数据,当前实验条件下获得的最大压缩密度和面密度分别为30g/cm3和50mg/cm2;为解决金柱腔M带对导引锥的预热以及由此导致的燃料-锥体材料混合问题,提出了一种在锥体表面镀低Z材料的方法,实验和辐射流体数值模拟结果验证了该方法的有效性,该方法的成功解决了间接驱动快点火激光聚变的重要关键技术问题。     

Influence of spatial distribution and temporal dynamics of compressed ICF-target parameters on fast ignition efficiency

The fast ignition as, possibly the most efficient method of inertial confinement fusion (ICF) is based on heating a small fusion initiation region (igniter) while the rest of the target is compressed by a compressing driver. We investigate the influence of two factors on the efficiency of fast ignition of ICF-targets. The first factor is the spatial distribution of thermonuclear-fuel parameters formed due to the ICF-target irradiation by the compressing driver. The second one is a mismatch in the time moments of the target maximum compression and the igniter heating by the igniting driver. The main characteristics of fast ignition, namely, the minimum energy of igniter needed to ignite the main ICF-target fuel (ignition energy) and the burn efficiency (ratio between the burnt and initial fuel masses), are investigated in view of numerical simulation. The scale-invariant dependences of the minimum ignition energy E _ig and the burn efficiency are obtained. It is shown that the burn efficiency depends on the spatial heterogeneity of the thermonuclear-fuel parameters much weaker then the ignition energy, but a strong dependence of the burn efficiency on the mismatch in time moments of the maximum compression and ignition is found. In particular, the ignition before the time moment of the maximum compression is more favorable than the ignition at just this moment.     

Heating mechanisms in short-pulse laser-driven cone targets

The fast ignitor is a modern approach to laser fusion that uses a short-pulse laser to initiate thermonuclear burn. In its simplest form the laser launches relativistic electrons that carry its energy to a precompressed fusion target. Cones have been used to give the light access to the dense target core through the low-density ablative cloud surrounding it. Here the ANTHEM implicit hybrid simulation model shows that the peak ion temperatures measured in recent cone target experiments arose chiefly from return current joule heating, mildly supplemented by relativistic electron drag. Magnetic fields augment this heating only slightly, but capture hot electrons near the cone surface and force the hot electron stream into filaments.     

Recent progress of laser driven particle acceleration at Peking University

Recently, radiation pressure acceleration （RPA） has been proposed and extensively studied, which shows that circularly polarized （CP） laser pulses can accelerate mono-energetic ion bunches in a phase-stable-acceleration （PSA） way from ultrathin foils. It is found that self-orgizing proton beam can be stably accelerated to GeV in the interaction of a CP laser with a planar target at 1022 W/cm2. A project called Compact LAser Plasma proton Accelerator （CLAPA） is approved by MOST in China recently. A prototype of laser driven proton accelerator （1 to 15 MeV/1 Hz） based on the PSA mechanism and plasma lens is going to be built at Peking University in the next five years. It will be upgraded to 200 MeV later for applications such as cancer therapy, plasma imaging and fast ignitiou for inertial confine fusion.     

Fast ignition by intense laser-accelerated proton beams

The concept of fast ignition with inertial confinement fusion (ICF) is a way to reduce the energy required for ignition and burn and to maximize the gain produced by a single implosion. Based on recent experimental findings at the PETAWATT laser at Lawrence Livermore National Laboratory, an intense proton beam to achieve fast ignition is proposed. It is produced by direct laser acceleration and focused onto the pellet from the rear side of an irradiated target and can be integrated into a hohlraum for indirect drive ICF.     

超强激光与泡沫微结构靶相互作用提高强流电子束产额模拟研究

利用二维粒子模拟方法,本文研究了超强激光与泡沫微结构镀层靶相互作用产生强流电子束问题.研究发现泡沫区域产生了百兆高斯级准静态磁场,形成具有选能作用的"磁势垒",强流电子束中的低能端电子在"磁势垒"的作用下返回激光作用区域,在鞘场和激光场的共同作用下发生多次加速过程,从而显著提升高能电子产额.还应用单粒子模型,分析了电子在激光场作用下的运动行为,验证了多次加速的物理机理.     

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.     

Collisionless stopping of electron current in an inhomogeneous electron magnetohydrodynamics plasma

A brief review of a recent work on a novel collisionless scheme for stopping electron current pulse in plasma is presented. This scheme relies on the inhomogeneity of the plasma medium. This mechanism can be used for heating an overdense regime of plasma where lasers cannot penetrate. The method can ensure efficient localized heating at a desired location. The suitability of the scheme to the frontline fast ignition laser fusion experiment has been illustrated.