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.     

Effect of spatial nonuniformity of heating on compression and burning of a thermonuclear target under direct multibeam irradiation by a megajoule laser pulse

Direct-drive fusion targets are considered at present as an alternative to targets of indirect compression at a laser energy level of about 2 MJ. In this approach, the symmetry of compression and ignition of thermonuclear fuel play the major role. We report on the results of theoretical investigation of compression and burning of spherical direct-drive targets in the conditions of spatial nonuniformity of heating associated with a shift of the target from the beam center of focusing and possible laser radiation energy disbalance in the beams. The investigation involves numerous calculations based on a complex of 1D and 2D codes RAPID, SEND (for determining the target illumination and the dynamics of absorption), DIANA, and NUT (1D and multidimensional hydrodynamics of compression and burning of targets). The target under investigation had the form of a two-layer shell (ablator made of inertial material CH and DT ice) filled with DT gas. We have determined the range of admissible variation of compression and combustion parameters of the target depending on the variation of the spatial nonuniformity of its heating by a multibeam laser system. It has been shown that low-mode (long-wavelength) perturbations deteriorate the characteristics of the central region due to less effective conversion of the kinetic energy of the target shell into the internal energy of the center. Local initiation of burning is also observed in off-center regions of the target in the case of substantial asymmetry of irradiation. In this case, burning is not spread over the entire volume of the DT fuel as a rule, which considerably reduces the thermonuclear yield as compared to that in the case of spherical symmetry and central ignition.     

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.     

Propagation of ion shock in solid DT target with nonlinear force-driven plasma blocks

The plasma block (piston) with pressure P ₁ is generated as a result of the nonlinear (ponderomotive) force in laser–plasma interaction. The plasma block can be used for the ignition of a fusion flame front in a solid density deuterium–tritium (DT) target by compressing the fuel that creates an ion shock propagating with velocity u _{ion? shock} in the inside of a solid DT target. The ignition is achieved by creating an ion shock during the final stages of the implosion. We estimated the effect of an ion shock in solid DT target at an early stage with no compression and at the last stage with compression, where density increases by a factor of solid-state density. According to the theoretical model, a large target with a very thin layer of fuel (high-aspect ratio target) would be ideal to obtain the very strong shocks. Results indicate that the maximum compression even by an infinitely strong single shock can never produce more than four times the initial density of DT fuel. The results reported that the threshold ignition energy in a solid DT target is reduced by a factor of 4.     

Shock ignition of thermonuclear fuel with high areal density

A novel method by C. Zhou and R. Betti [Bull. Am. Phys. Soc. 50, 140 (2005)] to assemble and ignite thermonuclear fuel is presented. Massive cryogenic shells are first imploded by direct laser light with a low implosion velocity and on a low adiabat leading to fuel assemblies with large areal densities. The assembled fuel is ignited from a central hot spot heated by the collision of a spherically convergent ignitor shock and the return shock. The resulting fuel assembly features a hot-spot pressure greater than the surrounding dense fuel pressure. Such a nonisobaric assembly requires a lower energy threshold for ignition than the conventional isobaric one. The ignitor shock can be launched by a spike in the laser power or by particle beams. The thermonuclear gain can be significantly larger than in conventional isobaric ignition for equal driver energy.     

Numerical Simulation of the Response of an Aluminum Target Impacted by an Intense Laser Driven Flyer

Numerical calculations of the response of an aluminum target impacted by an aluminum flyer driven by an intense laser are presented. The state of the accelerated flyer and the characteristics of shock wave propagation in the target are described in detail. If the parameters of laser and flyer-target structure are selected reasonably, an approximately symmetric impact can be realized between the flyer and the target, also the shock wave in the target has a wide stable range. Therefore the absolute measurement for the equation of state (EOS) can be almost achieved in laser EOS experiments with the bothside-step-target of suitable thickness.     

Numerical Simulation of the Response of an Aluminum Target Impacted by an Intense Laser Driven Flyer

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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.     

Fast ignition by detonation in a hydrodynamic flow

A general concept of fast ignition by a hydrodynamic pulse is developed. The main statements of the concept are formulated having in mind the need to ignite the pre-compressed thermonuclear fuel of the inertial confinement fusion (ICF) target. Initially, combustion must be initiated inside the hydrodynamic flow during its action on the target. The conditions for propagating a self-sustaining thermonuclear-detonation wave from an igniter on the thermonuclear fuel of the ICF-target must be provided. For this, the deuterium–tritium (DT) igniter placed in the forward part of the hydrodynamic flow should not only be heated up to thermonuclear temperature, but also compressed to a density close to the density of the ICF-target fuel. It is shown that the detonation of the multilayer conical target (containing DT-ice and a heavy pusher) enables fast ignition of the ICF target fuel of 200–500 g/cm³ density at an implosion velocity of 300–500 km/s.     

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.     

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.     

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.     

激光驱动高压下材料状态方程实验研究进展

利用激光驱动冲击波进行材料状态方程实验研究已逐渐成为实验室获得材料TPa以上高压状态方程数据的重要途径.文章简述了激光状态方程实验研究方面的进展,重点介绍了激光驱动冲击波的基本特性,激光状态方程实验测量方法,激光驱动冲击波平面性、稳定性及干净性研究,以及激光状态方程实验研究等方面的成果.     

Energetic efficiency of laser thermonuclear targets with ablator shells made of beryllium materials: A comparative analysis

The energetic characteristics of the compression and burning of targets with beryllium and beryllium deuteride shells are compared. The characteristics considered include the hydrodynamic efficiency, the efficiency of energy transmission to the thermonuclear fuel, and the gain factor found from numerical simulation using the ‘Diana’ one-dimensional mathematical code. The calculations are carried out for direct-drive cryogenic laser targets with the ablator shells made of beryllium or beryllium deuteride with parameters corresponding to the third harmonic of energy of the neodymium-laser radiation with a pulse energy of 1–3 MJ. It is proved that the gain of beryllium hydride targets can be brought to the level of beryllium targets due to variations in the geometrical parameters of BeD₂ targets. It is shown that the fission of BeD₂ or BeDT ablators in reactor-scale targets could significantly contribute to the final thermonuclear yield. __________ Translated from Preprint No. 20 of the P. N. Lebedev Physical Institute, Moscow (2001).     

一种激光驱动高压状态方程绝对测量方法的探索

数值模拟了激光直接驱动铝飞片空腔靶的物理过程,模拟结果表明,如果靶结构参数与激光条件匹配,飞片与靶可以近似实现对称碰撞,而且冲击波在靶中存在稳定传播区,采用结构参数合理的飞片双面台阶靶,可能实现状态方程的绝对测量.同时进行了实验探索,实验结果与数值模拟结果基本吻合. 关键词： 绝对测量 飞片双面台阶靶 数值模拟 状态方程     

用于冲击诊断的成像速度干涉仪

为了诊断测量激光驱动冲击波,研制了具有空间分辨力的成像型速度干涉仪.该干涉仪主要包括输入部分、像传递部分及干涉部分,探测光采用波长为532 nm的单纵模激光.在靶位放置一分线板,经过测量,其空间分辨力小于10μm.基于天光KrF准分子激光系统的参数,设计并自制含有烧蚀层的单台阶金属靶,利用成像型速度干涉仪测量到了金属铝...     

高压氘氚靶球的制备与拉曼光谱研究

氢同位素的定量分析与监测在能源与环境领域都有着重要的意义。激光拉曼光谱由于其可以无损分析氢同位素分子,已经成为一种重要的方法,在国际热核聚变实验反应堆(ITER)和美国萨凡纳河工厂得到了广泛应用。利用高压充气装置得到了惯性约束聚变(ICF)高压靶丸,并对靶丸内气体进行原位拉曼光谱测量,通过对高压下氘氚混合气体的拉曼光谱进行分析得到了靶丸内气体的成分比例,验证了靶丸充气工艺参数。实验表明,在CCD的积分时间延长到1 min时,氘(DD),氘氚(DT)和氚(TT)的测量精度可以达到1%,同时对不同时刻靶丸内气体组分的拉曼光谱进行测量,实验结果表明在氘氚渗透和氚衰变两者共同作用下,靶丸内总气体压力随时间不断下降,但是气体组成基本不发生变化。     

Lasers for ICF with a Controllable Function of Mutual Coherence of Radiation

The current status of laser thermonuclear fusion research in the leading world scientific centers is characterized by the development of superhigh-power multi-channel laser facilities of megajoule pulse-energy level. The development of such laser installations operating in the pulse-repetition mode with a large number of laser beams, which are necessary for high-symmetry irradiation of a spherical thermonuclear target, is an extremely difficult physical and engineering problem. The concept of a special laser with a controllable function of mutual coherence of radiation is proposed. The studies performed demonstrate that a laser based on such a principle has a number of advantages as compared to the conventional schemes of lasers. In particular, the optical scheme of the laser is significantly simplified, and the cost of the output-energy unit is reduced by several times. The influence of radiation coherence on the homogeneity of the thermonuclear target irradiation is analyzed. The feasibility of suppressing the small-scale self-focusing without application of spatial filtration is shown. A module of the laser facility has been triggered to check the validity of the principles proposed for constructing a laser driver for power stations, and the first experimental results are reported. The possibility of controlling the coherence of laser beams used in ICF experiments without violation of the laser--target system matching is demonstrated, as well as controlling the distribution of the laser radiation intensity in the lens focus.     

Compression and combustion of non-cryogenic targets with a solid thermonuclear fuel for inertial fusion

Variants of a target with a solid thermonuclear fuel in the form of deuterium-tritium hydrides of light metals for an inertial fusion have been proposed. The laser-pulse-induced compression of non-cryogenic targets, as well as ignition and combustion of such targets, has been examined. The numerical calculations show that, despite a decrease in the caloric content of the fuel and an increase in the energy losses on intrinsic radiation in the target containing deuterium-tritium hydrides of light metals as compared to the target containing deuterium-tritium ice, the non-cryogenic target can ensure the fusion gain sufficient for its use in the energy cycle of a thermonuclear power plant based on the inertial plasma confinement method.     

Numerical Investigation Into the Highly Nonlinear Heat Transfer Equation with Bremsstrahlung Emission in the Inertial Confinement Fusion Plasmas

A highly nonlinear parabolic partial differential equation that models the electron heat transfer process in laser inertial fusion has been solved numerically. The strong temperature dependence of the electron thermal conductivity and heat loss term (Bremsstrahlung emission) makes this a highly nonlinear process. In this case, an efficient numerical method is developed for the energy transport mechanism from the region of energy deposition into the ablation surface by a combination of the Crank‐Nicolson scheme and the Newton‐Raphson method. The quantitative behavior of the electron temperature and the comparison between analytic and numerical solutions are also investigated. For more clarification, the accuracy and conservation of energy in the computations are tested. The numerical results can be used to evaluate the nonlinear electron heat conduction, considering the released energy of the laser pulse at the Deuterium‐Tritium (DT) targets and preheating by heat conduction ahead of a compression shock in the inertial confinement fusion (ICF) approach. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)