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.     

Contribution of different mechanisms of energy transfer in the development of the thermonuclear combustion wave upon fast ignition of ICF-targets

Temporal characteristics of the thermonuclear combustion wave, critical parameters of the igniter, and the total energy yield were computed using numerical modeling of the fast ignition of the spherically symmetric inertial confinement fusion (ICF) target of the reactor type taking into account different mechanisms of energy transfer from the central igniter to the main mass of fusionable fuel of the target. The program TERA was used for mathematical modeling. Along with complete calculations (including all known mechanisms of energy transfer), model computations with consecutive disengagement of energy transfer by thermonuclear charged particles (local energy deposition approximation) and by neutrons were also carried out. Our computations showed that the main effect consists in variation of the temporal characteristics of the combustion wave. Unlike the diagnostic-type targets, in the case of the reactor targets, energy transfer by neutrons exerts the main influence, and the second in importance is nonlocality of the energy deposition by charged thermonuclear particles.     

The influence of the state of a compressed thermonuclear substance on the combustion of inertial fusion targets under direct ignition by a short radiation pulse

One-dimensional numerical calculations were performed to study the dependence of conditions for initiating thermonuclear combustion and of the target gain of direct-ignition inertial fusion targets ignited by a short radiation pulse on the initial temperature of a preliminarily compressed fuel and the initial heat energy distribution between plasma electrons and ions in the ignition region (igniter). The igniter parameters at which an effective thermonuclear target explosion with a G ～ 10³ target gain occurred were shown to substantially depend on the initial temperature of the major fuel fraction and the initial heat energy distribution between igniter electrons and ions. The heat energy of the igniter passed a minimum as the size of the igniter decreased. The dependences of these minimum energies on the temperature of the major fuel fraction at various initial energy distributions between igniter electrons and ions were determined. An increase in the temperature of the major fuel fraction was shown to decrease the target gain.     

Effect of the initial phase state of DT-matter on the compression of inertial fusion targets

We investigate the efficiency of inertial fusion target compression, where at the initial time moment the thermonuclear fuel is in a two-phase state and has the form of two adjacent layers — the external DT-liquid layer and the internal DT-ice layer. We study this problem for the fast ignition targets, where the ultimate final density of the thermonuclear matter is of a special importance. We take the simplest type of a fast ignition target, which corresponds to the technical justification of the HiPER Project aimed at demonstrating fast ignition at the compressing laser pulse energy ~100 kJ. Such a target presents a spherical DT-ice shell coated with a thin polymer film. We obtain the dependence of the final target density on the mass fraction of the DT-matter liquid phase and formulate the requirements on the admissible concentration of liquid phase if the decrease in the DT-fuel final density does not exceed 10%. We find the criterion for choosing the laser-pulse duration which provides the minimum decrease in the final density of the target containing DT-matter in the initial two-phase state.     

In depth fusion flame spreading with a deuterium-tritium plane fuel density profile for plasma block ignition

<正>Solid-state fuel ignition was given by Chu and Bobin according to the hydrodynamic theory at x = 0 qualitatively. A high threshold energy flux density,i.e.,E^* = 4.3×10¹² J/m²,has been reached.Recently,fast ignition by employing clean petawatt-picosecond laser pulses was performed.The anomalous phenomena were observed to be based on suppression of prepulses.The accelerated plasma block was used to ignite deuterium-tritium fuel at solid-state density. The detailed analysis of the thermonuclear wave propagation was investigated.Also the fusion conditions at x≠0 layers were clarified by exactly solving hydrodynamic equations for plasma block ignition.In this paper,the applied physical mechanisms are determined for nonlinear force laser driven plasma blocks,thermonuclear reaction,heat transfer, electron-ion equilibration,stopping power of alpha particles,bremsstrahlung,expansion,density dependence,and fluid dynamics.New ignition conditions may be obtained by using temperature equations,including the density profile that is obtained by the continuity equation and expansion velocity.The density is only a function of x and independent of time.The ignition energy flux density,E_t^*,for the x≠0 layers is 1.95×10¹² J/m².Thus threshold ignition energy in comparison with that at x = 0 layers would be reduced to less than 50 percent.     

直接驱动球内爆点火的数值模拟

 实现中心点火的基本条件是在内爆中心形成面密度0.3 g/cm²，温度10 keV的点火热斑。减速阶段流体不稳定性的增长，会破坏对称压缩，减小热斑体积，直接破坏点火热斑的形成，对点火构成威胁。在原有LARED-S程序的基础上，加入热核反应和α粒子加热过程程序模块，对直接驱动ICF球内爆过程进行数值模拟研究，1维模拟结果与NIF直接驱动点火靶的设计基本相符，显示α粒子加热对边缘点火起重要作用；2维模拟表明减速阶段流体不稳定性对点火有重要影响。     

激光惯性约束聚变靶靶丸制备与表征

激光惯性约束聚变的核心思想是利用球形内爆技术对聚变燃料进行增压,使热核燃料达到高温、高密度的等离子体状态,进而实现聚变点火。基于对称压缩、流体界面不稳定性和实验诊断的考虑,ICF实验对作为热核燃料容器的空心微球的品质在球形度、壁厚均匀性、表面粗糙度以及掺杂水平等方面提出了严格的要求。为满足这些要求,陆续发展了乳液微封装技术、降解芯轴技术、低压等离子体聚合/掺杂技术、干凝胶玻璃微球制备技术等用于多层塑料微球和空心玻璃微球的研制。另一方面,针对ICF靶丸量小、质轻以及表面要求高的特点,发展了相应的非破坏性靶丸参数表征技术,如X光照相技术、4π形貌表征技术、微球掺杂水平测量技术以及微球内燃料负载水平快速测试技术。基于这些制备与表征技术,初步实现了多层塑料微球、玻璃微球、聚-!-甲基苯乙烯芯轴微球、梯度掺杂CH微球的研制,满足了"神光Ⅱ"、"神光Ⅲ原型"及"神光Ⅲ主机"上开展的一系列内爆物理实验的要求,同时为未来点火物理实验用靶丸的研制提供了技术支撑。     

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.     

激光等离子体中高能电子各向异性压强的粒子模拟

直接驱动惯性约束聚变(ICF)的实现需要对靶丸进行严格的对称压缩,以达到自持热核反应(点火)所需的条件.快点火方案的应用降低了对靶丸压缩对称性以及驱动能量的要求,但压缩及核反应过程中良好的靶丸对称性无疑有助于核反应增益的提高.本文研究了快点火方案中高能电子注入高密等离子体后导致的各向异性电子的压强张量.这一现象存在于ICF快点火方案中的高能电子束"点火"及核反应阶段.鉴于高能电子加热离子过程以及靶丸核反应自持燃烧过程的时间较长,高密靶核会由于超高的各向异性压强的作用破坏高密靶丸的对称性,降低核燃料密度,进而降低了核燃料燃烧效率以及核反应增益.     

Hydrodynamique de l'implosion d'une cible FCI

We consider the hydrodynamic behaviour of an imploding ICF target. After recalling the requirements for thermonuclear ignition, we analyse in detail the two phases of the implosion. First, the acceleration, with its two important ignition parameters, the implosion velocity and the entropy generated in the DT. Second, the slowing down which, through electronic conduction, allows the creation of a central hot spot of sufficient mass. The method of successive shocks allows the détermination of the laser pulse shape which, by insuring an isentropic acceleration, optimises the energy delivered to the DT. We obtain the implosion velocity as a function of the initial capsule parameters and the maximum incident radiation. We clarify the hydrodynamical reason which makes the ignition threshold sensitive to the entropy generated during the acceleration. These elements constitute the basis of a global indirect drive ICF model, allowing target design optimisation.     

基于现役装置的冲击点火可行性概念研究

以冲击点火物理特性的研究为基础, 分析冲击点火对高功率激光驱动器的物理需求, 然后从总体层面概括给出基于现役装置(神光III等间接驱动中心点火高功率激光装置) 研究冲击点火面临的关键技术问题. 研究表明, 基于现役装置的冲击点火主要面临两个层面的问题, 首先是非均匀光路排布下实现均匀辐照的工程层面问题, 其次是在现役装置上高效实现冲击点火激光脉冲的激光技术层面问题. 通过研究 分别对两个层面的问题提出相应的解决思路, 为后续研究奠定基础.     

Thermonuclear gain of a two-cascade laser-fusion target

One-dimensional numerical calculations are used to explore the possibility of thermonuclear fuel “ignition” (achieving an energy gainG ～ 1) in two-cascade laser-fusion targets with a relatively small aspect ratio for the inner shell. It is demonstrated that the parameters of the laser-produced thermonuclear plasma for a laser pulse energy of 200 kJ, various wavelengths of the laser radiation, and a simple pulse shape closely correspond to the “ignition” state for a target with an inner shell having an aspect ratio of ～ 3–10. This is indicative of the high energy efficiency of two-cascade targets that appear to be characterized by high reliability with respect to evolution of hydrodynamic instabilities.     

Gain of a compressed DT fuel statistically ignited by a multispark assembly

Most of the current inertial confinement fusion (ICF) schemes are based on the ignition of a high-density DT fuel by a single, high-temperature spherical hot spot (the spark). The spark is self-generated by the implosion process, which is used to bring the fuel to high density. To start ignition the spark has to be dimensioned in such a way that the ion temperature would be greater than 5–7 keV, and that the spark radius would be greater than the α-particle range. A spark with these features is indicated as supercritical. In the scheme based on self-generated spark, ignition can fail to occur when the produced spark strongly deviates from spherical shape, which can make all the surface losses highly relevant. High deformation, or even spark splitting, can occur due to the amplification of initial deviations from spherical shape by hydrodynamic instabilities (or by secular growth) during the implosion process. In principle, ignition can be recovered if the implosion is designed in such a way as to make supercritical at least one of the portions of hot fuel which are produced in this way near stagnation. As a general trend, more compressed final assemblies are required. In this paper we present fuel gain calculations (Gain = Thermonuclear energy/Energy in the compressed fuel) for DT assemblies ignited at the end of an implosion process by a supercritical spark statistically created within a cluster of many subcritical sparks. It is assigned the total number of sparks and the probability of having at least one of them supercritical. As a function of these quantities we calculate, in the framework of an isobaric model, the average thermal energy associated with the spark assembly. The same model is also used to evaluate, by statistical arguments, the areal mass, the burn fraction, and the system’s total fuel gain. It is found that the energy distribution function of the sparks is influenced only by a single global parameter, in which the assigned ignition probability and the number of sparks are also represented. Compared to the single central-spark approach, being the final states with allowed inner turbulence, the multispark scheme is characterized by relaxed initial symmetry requirements. For multispark systems we can realistically consider the achievement of fuel gains comparable or greater than those typical of the single-spark approach, when evaluated for currently accepted spark convergence ratios. With regard to the single spark case, higher cold fuel densities are needed, as expected (typically 2×–3×, for the same gain, depending on the energy distribution function). Zh. éksp. Teor. Fiz. 113, 805–815 (March 1998) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

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.     

Study of Coulomb collisional effects on the propagation of electromagnetic waves in a turbulent plasma

The presence of Weibel instability in laser-irradiated fuel could be detrimental to the process of ablative implosion, which is necessary for achieving thermonuclear fusion reactions. In this paper, the effect of the Coulomb collisional within the turbulent plasma on the Weibel instability growth rate has been investigated for linear and circular polarization. The results indicate that the Weibel instability growth rate at circular polarization near the ignition centre of the fuel fusion (collisional plasma) is about 10⁵ times higher than the collisional Weibel instability growth rate at linear polarization. The Weibel instability growth rate is observed near the critical density of the fuel fusion (collisionless plasma) at linear polarization and enhancement near the foot of the heat in front of the fuel fusion. By increasing the steps of the density gradient plasma in the low-density corona, electromagnetic instability occurs at a higher stress flow. Therefore, the deposition condition of electron beam energy in circular polarization of turbulent plasma can be shifted to the fuel core for suitable ignition.     

Nonequilibrium laser-produced plasma of volume-structured media and inertial-confined-fusion applications

We review the results of experimental and theoretical studies of the properties of a nonequilibrium plasma produced from volume-structured media, containing micro- and nano-size internal elements, under laser-pulse irradiation. We consider two types of materials, i.e., regularly and stochastically structured materials. The first type is either a set of flat layers or cylindrical and spherical shells of micrometer thickness, and the second type is either foams of light elements or light foams containing clusters of heavy elements with dimensions in the range of 10–100 nm. We study the properties of high-temperature laser-produced plasmas of such materials and applications directed to developing the design of inertial confinement fusion (ICF) targets and creating powerful sources of thermonuclear neutron and soft X-ray emission initiated by the laser pulse. The foam materials can be used as absorbers capable of providing homogeneity of laser-energy absorption by the target. A neutron yield up to 1014−1015 DT neutrons per shot can be achieved by heating regularly structured materials using a laser pulse in the regime of the consequent thermal explosions of solid elements containing isotopes of hydrogen. Laser-radiation conversion into soft X-ray emission with the efficiency controlled in a wide range may be realized in laser-produced plasmas of porous media doped with clusters of heavy elements. In particular, such a material can be used as an absorber–converter of laser radiation in inertial confinement fusion targets. Under direct irradiation of an ICF target by a laser pulse, such a converter can provide transformation of 20–30% of the absorbed laser energy into the energy of X-ray radiation transferred to thermonuclear capsules.     

点火靶高熵内爆的数值模拟

使用一维多群输运程序RDMG与二维少群扩散程序LARED-S对点火靶高脚与低脚内爆进行数值模拟.相对于低熵内爆,高脚高熵内爆通过提高预脉冲的辐射温度使得烧蚀面与物质界面的流体稳定性得到明显的改善,能够抑制流体不稳定的增长与热斑混合的发展.同时,高熵设计导致燃料的压缩变差,阻滞时刻燃料的压缩密度与面密度相应降低,中子产额降低.因此,高脚高熵内爆是通过牺牲燃料的高压缩,来换取靶丸内爆流体稳定性能的改善.     

On the possibility of target plasma ignition under the conditions of inertial nuclear fusion

The thermonuclear gain G for bulk and spark ignitions are calculated using a mathematical simulation of thermonuclear combustion in a DT plasma of laser targets for various parameters of the target plasma and (isobaric and isochoric) ignitors. The critical parameters of ignitors at which an effective nuclear burst occurs with G ～ 100 are calculated. It is shown that a further increase in the temperature and size of the ignitors virtually does not affect the efficiency of DT fuel burnup. Irrespective of the ignition technique, the value of G can be estimated with the help of a simple asymptotic formula. At the same time, the critical parameters of ignitors are determined to a considerable extent by the mode of ignition and by the target parameters. Spark ignition with an isochoric ignitor corresponding to the fast ignition mode is considered in detail. It is shown that the main critical parameter for optimal isochoric ignitors is their thermal energy liberated upon absorption of an auxiliary ultrashort laser pulse. The critical values of this energy are calculated.     

Physical processes in a laser-greenhouse target: Experimental results, theoretical models, and numerical calculations

The paper is devoted to recent results concerning investigation of physical processes occurring in a “laser greenhouse” target. Results of experimental and theoretical studies of laser-pulse interaction with a low-density absorber of the target, namely, with a porous substance having density close to the plasma critical density, are presented. On the basis of a vast cycle of experiments carried out in a number of laboratories, it is shown that the absorption of the laser radiation in porous media, including those with a density exceeding the critical one by at least a factor of 4 to 6, has a bulk nature and is distributed over the target depth. In particular, the laser-radiation absorption region in a porous substance with density 10⁻³–10⁻² g/cm³ is extended into the target 400–100 μm, respectively. The coefficient of absorption of laser radiation with intensity 10¹⁴–10¹⁵ W/cm² in porous substances, including those of the supercritical density, is 70–90%. Experiments have not shown enhanced (compared to a solid-state target) radiation intensity associated with a possible development of parametric instabilities in an extended laser plasma of low-density porous media, as well as noticeable contribution of fast electrons to the energy balance and their effect on the energy transfer. In this paper, theoretical models are developed explaining features of the laser-radiation absorption and energy transfer in porous media. These models are based on the phenomenon of laser-radiation interaction with solid components of a porous substance and plasma production inside pores and cells of the medium. The efficiency of energy conversion in the vicinity of the ignition threshold for the laser-greenhouse target is investigated in the case of an absorber having the above properties. Numerical calculations have shown that a thermonuclear-gain coefficient of 1 to 2 (with respect to the energy absorbed) is attained for a laser-radiation energy of 100 kJ. Translated from Preprint No. 58 of the P. N. Lebedev Physical Institute, Moscow (1999).     

Fabrication of polystyrene hollow microspheres as laser fusion targets by optimized density-matched emulsion technique and characterization

Inertial confinement fusion, frequently referred to as ICF, inertial fusion, or laser fusion, is a means of producing energy by imploding small hollow microspheres containing thermonuclear fusion fuel. Polymer microspheres, which are used as fuel containers, can be produced by solution-based micro-encapsulation technique better known as density-matched emulsion technique. The specifications of these microspheres are very rigorous, and various aspects of the emulsion hydrodynamics associated with their production are important in controlling the final product. This paper describes about the optimization of various parameters associated with density-matched emulsion method in order to improve the surface smoothness, wall thickness uniformity and sphericity of hollow polymer microspheres. These polymer microshells have been successfully fabricated in our lab, with 3–30 μm wall thickness and 50–1600 μm diameters. The sphericity and wall thickness uniformity are better than 99%. Elimination of vacuoles and high yield rate has been achieved by adopting the step-wise heating of W₁/O/W₂ emulsion for solvent removal.