Laser-driven Inertial Confinement Fusion Research Articles

Mitigation of mode-one asymmetry in laser-direct-drive inertial confinement fusion implosions

Nonuniformities present in the laser illumination and target in laser-driven inertial confinement fusion experiments lead to an asymmetric compression of the target, resulting in an inefficient conversion of shell kinetic energy to thermal energy of the hot-spot plasma. In this paper, the effects of asymmetric compression of cryogenic deuterium tritium laser-direct-drive implosions are examined using a suite of nuclear and x-ray diagnostics on the OMEGA laser. The neutron-averaged hot-spot velocity (u→hs) and apparent ion temperature (Ti) asymmetry are determined from neutron time-of-flight measurements of the primary deuterium tritium fusion neutron energy spectrum, while the areal density (ρR) of the compressed fuel surrounding the hot spot is inferred from measurements of the scattered neutron energy spectrum. The low-mode perturbations of the hot-spot shape are characterized from x-ray self-emission images recorded along three quasi-orthogonal lines of sight. Implosions with significant mode-1 laser-drive asymmetries show large hot-spot velocities (>100 km/s) in a direction consistent with the hot-spot elongation observed in x-ray images, measured Ti asymmetry, and ρR asymmetry. Laser-drive corrections have been applied through shifting the initial target location in order to mitigate the observed asymmetry. With the asymmetry corrected, a more-symmetric hot spot is observed with reduced u→hs, Ti asymmetry, ρR asymmetry, and a 30% increase in the fusion yield.

Suppressing the enhancement of stimulated Raman scattering in inhomogeneous plasmas by tuning the modulation frequency of a broadband laser

The stimulated Raman scattering (SRS) instability can inhibit the performance of laser-driven inertial confinement fusion (ICF) implosions by scattering light into unwanted directions or by generating hot electrons that preheat the target fuel. In principle, ICF target designs can avoid parameter regimes conducive to large, linear SRS gains. In practice, kinetic inflation—the nonlinear enhancement of SRS due to electron trapping in the excited plasma wave—makes this difficult. Here, we show that laser bandwidth in the form of frequency modulation can either decrease or increase the inflationary SRS (iSRS) threshold in inhomogeneous plasmas depending on the maximum chirp of the laser pulse. The threshold, mapped out by a series of particle-in-cell simulations, exhibits a minimum when the frequency change within the pulse cancels the spatial detuning due to density inhomogeneities along the trajectory of the scattered light. By tuning the pump laser parameters away from this minimum, the iSRS threshold can be larger than at zero bandwidth, providing a path to mitigating kinetic inflation in ignition relevant plasmas.

Laser plasma instabilities and their suppression strategies

The issue of laser plasma instabilities (LPIs) including stimulated Raman scattering, stimulated Brillouin scattering and so on is one of the most fascinating subjects in laser plasma physics. In particular, LPIs may cause significant laser energy loss and produce hot electrons to preheat fusion targets, which affect target compression and fusion energy gain in laser-driven inertial confinement fusion. Recent experiments carried out on the National Ignition Facility, the largest laser facility in the world for laser fusion, indicate that the understanding and the control of LPIs are essential to the realization of laser fusion. In this paper, we present a review on recent studies of LPIs. Firstly, we retrospect the classical theoretical model of LPIs, which offers a good estimation of growth rate in the linear development stage. Then, we discuss some progresses on the understanding of LPIs in more complex and real scenarios, such as LPI development in the nonlinear regions, cascaded LPIs, multi-beam LPIs, and nonlinear couplings between LPIs. Following the exploration of LPI physics, we emphasize on the strategies for the control of LPIs, including beam smoothing techniques, temporal profile shaping, broadband laser, laser polarization rotation, external magnetic field and so on.

Crossed beam energy transfer between optically smoothed laser beams in inhomogeneous plasmas.

Crossed beam energy transfer, CBET, in high-intensity laser–plasma interaction is investigated for the case of optically smoothed laser beams. In the two approaches to laser-driven inertial confinement fusion experiments, the direct-drive and the indirect-drive, CBET is of great importance because it governs the coupling of laser energy to the plasma. We use the two-dimensional wave-coupling code Harmony to simulate the transfer between two laser beams with speckle structure that overlap in a plasma with an inhomogeneous flow profile. We compare the CBET dynamics for laser beams with spatial incoherence and with spatio-temporal incoherence; in particular we apply the smoothing techniques using random phase plates (RPPs) and smoothing by spectral dispersion (SSD), respectively. It is found that for laser beams (wavelength λ0) with intensities (IL) above IL ∼ 2 × 1015 W cm−2(λ0/0.35 µm)−2(Te/keV), both the so-called plasma-induced smoothing as well as self-focusing in intense laser speckles induce temporal incoherence; the latter affects the CBET and the angular distribution of the light transmitted behind the zone of beam overlap. For RPP-smoothed incident beams, the resulting band width of the transmitted light can already be of the same order as the effective band width of the SSD available at major laser facilities. We examine the conditions when spatio-temporal smoothing techniques become efficient for CBET.This article is part of a discussion meeting issue ‘Prospects for high gain inertial fusion energy (part 1)’.

An efficient radiation analysis approach through compressive model for laser driven inertial confinement fusion

Radiation symmetry evaluation is critical to the laser driven Inertial Confinement Fusion (ICF), which is usually obtained through a view-factor equation model. The model is nonlinear, and its equation number can be very large when the size of discrete mesh element is very small to achieve a prescribed accuracy, which may lead to an intensive equation solving process. In this paper, an efficient radiation symmetry analysis approach is presented through equation model compression, in which, (1) the radiation symmetry analysis is carried out for two kinds of geometric shapes: cylindrical capsule and spherical capsule. For the structure of cylindrical capsule, the Legendre–Fourier basis and Zernike basis are applied to represent the radiation flux of the capsule. And for the structure of spherical capsule, the spherical harmonics is employed to sparsely represent the radiation flux on the capsule and spherical cavity. Then the nonlinear energy equilibrium equations are transformed into the equations with sparse coefficients, which means there are many redundant equations, (2) Only a few equations are uniformly selected to recover such sparse coefficients with a Latin hypercube sampling scheme, (3) A Modified Conjugate Gradient-Iteration Hard Thresholding (MCG-IHT) algorithm is then given to rapidly solve such sparse coefficients equations with as few as possible iterations. Finally, the presented approach is validated with two experiment targets for Shenguang III-YX and Shenguang II laser facilities in China. The equation model is reduced much more, only a few of equations are required to achieve the radiation flux with comparable accuracy within much less of computation time. And the solution is much more efficient as the size of discrete mesh element decreases, in which, almost 1.1% computation time is required to obtain the comparable result.

Recent Advances in Laser-Induced Surface Damage of KH2PO4 Crystal

As a hard and brittle material, KDP crystal is easily damaged by the irradiation of laser in a laser-driven inertial confinement fusion device due to various factors, which will also affect the quality of subsequent incident laser. Thus, the mechanism of laser-induced damage is essentially helpful for increasing the laser-induced damage threshold and the value of optical crystal elements. The intrinsic damage mechanism of crystal materials under laser irradiation of different pulse duration is reviewed in detail. The process from the initiation to finalization of laser-induced damage has been divided into three stages (i.e., energy deposition, damage initiation, and damage forming) to ensure the understanding of laser-induced damage mechanism. It is clear that defects have a great impact on damage under short-pulse laser irradiation. The burst damage accounts for the majority of whole damage morphology, while the melting pit are more likely to appear under high-fluence laser. The three stages of damage are complementary and the multi-physics coupling technology needs to be fully applied to ensure the intuitive prediction of damage thresholds for various initial forms of KDP crystals. The improved laser-induced damage threshold prediction can provide support for improving the resistance of materials to various types of laser-induced damage.

Recent research progress of optical Thomson scattering in laser-driven inertial confinement fusion

Currently, laboratory created energy density of laser-driven inertial confinement fusion (ICF) is extremely close to that for ignition, while the divergence between experiment and simulation is increasing. One of the key issues is the lack of advanced knowledge of laser-hohlraum coupling process, which has shown the complexity of hohlraum environment. Optical Thomson scattering (OTS) becomes the standard technique for diagnosing the ICF hohlraum plasma parameters, due to its capability of providing unperturbed, local and precise measurement. The development of OTS in China is closely related with the Shenguang series laser facilities, on which most of the ICF experiments are carried out. In recent years, 4ω(263 nm) Thomson scattering technique has been set up on Shenguang-III prototype and 100 kJ-level laser facility, the corresponding results help the understanding of ICF physics. In the near future, several novel methods will be developed, for high-precision diagnostics of ICF ignition hohlraum plasmas and the research of new physical phenomena.

Nonlinear transmission of laser light through coronal plasma due to self-induced incoherence.

The success of direct laser-driven inertial confinement fusion (ICF) relies critically on the efficient coupling of laser light to plasma. At ignition scale, the absolute stimulated Raman scattering (SRS) instability can severely inhibit this coupling by redirecting and strongly depleting laser light. This article describes a new dynamic saturation regime of the absolute SRS instability near one-quarter of the critical density. The saturation occurs when spatiotemporal ion-acoustic fluctuations in the plasma density detune the instability resonance. The dynamic saturation mitigates the strong depletion of laser light and enhances its transmission through the instability region, explaining the coupling of laser light to ICF targets at higher plasma densities.

Untrafast smoothing scheme based on dynamic interference structure between beamlets of laser quad

Aiming at the high requirements for illumination uniformity on the target in laser-driven inertial confinement fusion (ICF) facilities, an ultrafast smoothing method based on dynamic interference structure between beamlets of a laser quad is proposed. The basic principle of this scheme is to use a conjugate phase plate array to add the conjugate phase modulation to the multiple beamlets of a laser quad with a certain wavelength difference. Consequently, every two beamlets are coherently superposed in the far field to generate a dynamic interference pattern, resulting in the fast redistribution of the speckles introduced by continuous phase plate inside the focal spot and further improving the illumination uniformity on the target on a picosecond timescale. The coherent beamlets with a certain wavelength difference can be generated by using a broadband seed laser. Taking the laser quad of the typical ICF facilities for example, the physical model of the ultrafast smoothing method based on dynamic interference structure of beamlets is built up. The influences of the phase-plate type, the peak-to-valley value of the phase modulation and the wavelength difference between the beamlets are analyzed quantitatively, and the smoothing characteristics of the focal spot are discussed in detail and compared with those from the traditional temporal smoothing scheme such as smoothing by spectral dispersion. The results indicate that the directions of the moving speckles in the focal spot are determined by the phase-plate type. However, the required time to achieve stable illumination uniformity, i.e, the decay time, is determined by the wavelength difference between the beamlets. Moreover, the illumination uniformity on the target becomes better with the increase of peak-to-valley value of the phase modulation at first and then remains almost the same. Thus, the ultrafast smoothing method based on dynamic interference structures with well-designed phase arrays and wavelength combinations of the beamlets can realize the multi-directional and multi-dimensional speckle sweeping inside the focal spot, and further improving the irradiation uniformity on the target within several picoseconds or sub-picoseconds. Combining with the traditional beam smoothing scheme, better illumination uniformity can be achieved on an ultrashort timescale. This novel scheme can be used as an effective supplement to the existing temporal beam smoothing techniques.

A coupled model of electromagnetic and heat on nanosecond-laser ablation of impurity-containing aluminum alloy.

In the emerging field of laser-driven inertial confinement fusion, Joule heating generated via electromagnetic heating of the metal frame is a critical issue. However, there are few reported models explaining thermal damage to the aluminum alloy. The aim of this study was to build a coupled model for electromagnetic radiation and heat conversion of an ultrashort laser pulse on an aluminum alloy based on Ohm's law. Additionally, the application SiO2 films on aluminum alloy to improve the laser-induced damage threshold (LIDT) were simulated, and the effects of metal impurities in the aluminum alloy were analyzed. A model examining the relation between electromagnetic radiation and heat for a nanosecond laser irradiating an aluminum alloy was developed using a coupled model equation. The results obtained using the finite difference time domain (FDTD) algorithm can provide a theoretical basis for future improvement of the aluminum alloy LIDT.

LPSE: A 3-D wave-based model of cross-beam energy transfer in laser-irradiated plasmas

The new component of the “laser–plasma simulation environment” (LPSE) described here is a practical numerical model that solves the coupled vector equations for the propagation of nearly monochromatic, coherent, electromagnetic waves in inhomogeneous unmagnetized plasmas. It operates efficiently in the numerically challenging semiclassical regime where characteristic plasma scale lengths are many times greater than the wavelength of the light and solutions are highly oscillatory. Solutions can be obtained in one, two, or three spatial dimensions and time. The model includes the effects of nonlinear coupling of electromagnetic waves to the low-frequency plasma perturbations (i.e., ion-acoustic response) that are responsible for stimulated Brillouin scattering. Induced plasma perturbations are assumed to be imposed on a prescribed large-scale inhomogeneous background that includes spatially varying plasma density and flow. Our code is directly relevant to the problem of cross-beam energy transfer in laser-driven inertial confinement fusion. It may also be applicable in other areas where eikonal solutions of multicomponent wave equations (or coupled wave equations) are insufficient, such as optical scattering from ultrasound, electron dynamics in quantum devices or in nanoscale light–matter interactions.

Method of statistically characterizing target plane light field properties in inertial confinement fusion device

In the laser-driven inertial confinement fusion facilities, the irradiation uniformity of the laser beams on the target is a key factor affecting the effective compression of the target. At present, a variety of beam-smoothing techniques have been developed to control the spatiotemporal characteristics of the focal spots. However, many optical components involved in optical transmission links and complex transmission transformations often lead to complex optical transmission. Moreover, when using the diffraction optical method to analyze the shape and characteristics of the focal spots, a lot of data are needed to be processed and calculated, resulting in large calculation and low computational efficiency. It is urgent to find a new and fast method to describe the statistical properties of the focal spots. In addition, in the beam-smoothing technique, since the phase distribution of the continuous phase plate is obtained by multiple iterations of random numbers, although the details of focal spots obtained by different continuous phase plates are not the same, they all have similar statistical properties. Therefore, the modulation of the laser beam by the continuous phase plate can be regarded as the transmission process of the laser beam through a random surface. Although the intensities of the speckle within the focal spot at different locations have the strong randomness, and the random distributions of the target speckles obtained by different beam-smoothing methods are different, the overall distribution satisfies a certain statistical law. In this paper, the light-field properties of the focal spot are described by the statistical characterization method. The circular complex Gaussian random variables are used to directly describe the statistical properties of the target surface light field, and the far-field focal spots obtained by the diffractive optical method and those by the statistical characterization method are compared with each other and analyzed based on the typical focal spot evaluation parameters. The results show that the instantaneous properties of the focal spots obtained by the diffractive optical method and those obtained by the statistical characterization method are basically identical, but their time-integrated far-field focal spots are different. The correlation coefficient can be further used to describe the time-varying properties of the far-field focal spots. Compared with the diffractive optical method, in the numerical calculation process, the statistical characterization method of light field properties can directly obtain the analytical expression of the statistical distribution of the light field according to the statistical properties of the continuous phase plate surface shape. Secondly, this method can avoid the numerical calculation process from near field to far field. Last but not least, there is no need to perform data processing on each point of the light field, which makes things simple and effective and does not require large-scale data storage and processing.

Properties of vapor-deposited and solution-processed targets for laser-driven inertial confinement fusion experiments

Targets for low-adiabat direct-drive-implosion experiments on OMEGA must meet rigorous specifications and tight tolerances on the diameter, wall thickness, wall-thickness uniformity, and presence of surface features. Of these, restrictions on the size and number of defects (bumps and depressions) on the surface are the most challenging. The properties of targets that are made using vapor-deposition and solution-based microencapsulation techniques are reviewed.Targets were characterized using confocal microscopy, bright- and dark-field microscopy, atomic force microscopy, electron microscopy, and interferometry. Each technique has merits and limitations, and a combination of these techniques is necessary to adequately characterize a target.The main limitation with the glow-discharge polymerization (GDP) method for making targets is that it produces hundreds of domes with a lateral dimension of 0.7–2 μm. Polishing these targets reduces the size of some but not all domes, but it adds scratches and grooves to the surface. Solution-made polystyrene shells lack the dome features of GDP targets but have hundreds of submicrometer-size voids throughout the wall of the target; a few of these voids can be as large as ∼12 μm at the surface.

Ray-based modeling of cross-beam energy transfer at caustics

Cross-beam energy transfer (CBET) is a laser-plasma instability that significantly impacts laser energy deposition in laser-driven inertial confinement fusion (ICF) experiments. Radiation-hydrodynamics simulations, which are used to design and tune ICF implosions, use ray-based CBET models, but existing models require artificial multipliers to conserve energy and to obtain quantitative agreement with experiments. The discretization of the ray trajectories in traditional ray-based CBET models does not account for the rapid variation in CBET gain as rays pass through caustics. We introduce a model that allows one to treat caustics much more accurately and greatly improves energy conservation. The ray-based CBET calculations show excellent agreement with laser absorption from two-dimensional wave-based calculations (0.3% difference) and a three-dimensional 60-beam OMEGA implosion (2.4% difference) without artificial multipliers.

Fabrication of UV-curable silicone coating with high transmittance and laser-induced damage threshold for high-power laser system

Third harmonic generating (THG) element is an important component for high-power laser system and its incident side is in need of high transmittance and laser-induced damage threshold (LIDT) at specified wavelengths (1064 nm and 532 nm) to meet the demand of laser-driven inertial confinement fusion. An UV-curable organic–inorganic hybrid silicone coating was fabricated for this THG element and its relevant properties have been investigated. Si–O–Si backbone structure in combination with characteristics from a mixture of tripropyleneglycoldiacrylate (TPGDA) as a reactive diluent and Darocur 1173 (HMPP) as a photoinitiator provided qualified adhesion, hardness, transmittance, and laser damage resistance for the UV-cured coating. Based on this design, THG element with both high transmittance and LIDT at specified wavelengths was successfully achieved after the solidification of coatings by means of UV irradiation, which provides an alternate option to cure coatings upon the temperature-sensitive substrates (like potassium dideuterium phosphate (DKDP), ammonium dihydrogen phosphate (ADP), and so on) without heating, and meanwhile greatly reduces the time consumption of curing process.

A GPU based iteration approach to efficiently evaluate radiation symmetry for laser driven inertial confinement fusion

• Efficient computing approach presented for laser driven inertial confinement fusion • Guaranteed symmetry, strictly diagonally dominant and positive definite properties. • GPU based Preconditioned Conjugate gradient iteration approach. • The radiation models are efficiently solved and validated with two examples. Radiation computation is very important for high energy density experiments design in the laser-driven Inertial Confinement Fusion. The view-factor based models are often used to calculate the radiation on the capsule inside a hohlraum. However, it usually takes much time to solve them when the number of equations is very large. In this paper, an efficient iteration approach GPU is presented. The core idea is: (1) guaranteed symmetry, strictly diagonally dominant, and positive definite properties underlying the models are described, (2) a preconditioned conjugate gradient iteration approach is presented to compute the radiation based on such guaranteed properties, and (3) such approach is then parallelized and implemented for GPU so that the large scale models, especially for the non-linear model, can be efficiently solved in reasonable time. Finally, two experimental targets for Shenguang laser facilities built in China are demonstrated and compared to validate the efficiency of the presented approach. The results show that, the models’ computation (1) can be speeded up with successive over-relax iteration method by eight times as compared with Cholesky factorization based direct approach, (2) can be accelerated more with the preconditioned conjugate gradient iteration approach by almost eight times, and (3) can be further accelerated about 2 to 4 times as it parallelized and run on the GPU, which enables the large scale models, can be efficiently solved in reasonable time on the usual desktop computers.

Minimizing scatter-losses during pre-heat for magneto-inertial fusion targets

The size, temporal and spatial shape, and energy content of a laser pulse for the pre-heat phase of magneto-inertial fusion affect the ability to penetrate the window of the laser-entrance-hole and to heat the fuel behind it. High laser intensities and dense targets are subject to laser-plasma-instabilities (LPI), which can lead to an effective loss of pre-heat energy or to pronounced heating of areas that should stay unexposed. While this problem has been the subject of many studies over the last decades, the investigated parameters were typically geared towards traditional laser driven Inertial Confinement Fusion (ICF) with densities either at 10% and above or at 1% and below the laser's critical density, electron temperatures of 3–5 keV, and laser powers near (or in excess of) 1 × 1015 W/cm2. In contrast, Magnetized Liner Inertial Fusion (MagLIF) [Slutz et al., Phys. Plasmas 17, 056303 (2010) and Slutz and Vesey, Phys. Rev. Lett. 108, 025003 (2012)] currently operates at 5% of the laser's critical density using much thicker windows (1.5–3.5 μm) than the sub-micron thick windows of traditional ICF hohlraum targets. This article describes the Pecos target area at Sandia National Laboratories using the Z-Beamlet Laser Facility [Rambo et al., Appl. Opt. 44(12), 2421 (2005)] as a platform to study laser induced pre-heat for magneto-inertial fusion targets, and the related progress for Sandia's MagLIF program. Forward and backward scattered light were measured and minimized at larger spatial scales with lower densities, temperatures, and powers compared to LPI studies available in literature.

Development of target fabrication for laser-driven inertial confinement fusion at research center of laser fusion

As the basic conditions for laser inertial confinement fusion (ICF) research, the targets are required to be well specified and elaborately fabricated. Because of the characteristics of the targets, the research and fabrication process is a systematically tough task, which needs fundamental and deep insights into film deposition, mechanical machining, precise measurement and assembly, etc. As a result, knowledge of material science, physics, mechanical as well as electronics is a necessity for target researchers. In this paper, we give introductions to the state of art on target fabrication for ICF research at Research Center of Laser Fusion (RCLF) in China.

Two-plasmon decay instability in inhomogeneous plasmas at oblique laser incidence

The two-plasmon decay (TPD) instability has been studied in the region near the quarter-critical density in the plasmas of the laser-driven inertial confinement fusion for a wide range of laser angles of incidence. The TPD equations for the oblique laser incidence in the inhomogeneous plasmas have been analyzed theoretically. The range of wave vectors for the instability growth has been identified. The theoretical growth rates and thresholds have been compared with the results of the fluid-type simulations, and a good agreement has been found.

Generation of optical pulse packet using a fiber stacker for time fiducial applications

We present the concept of multicolor optical pulse packet generation based on modified fiber stackers featured with reflection geometries. Infrared radiation, visible and ultraviolet time fiducials were obtained by the amplification and frequency-conversion of the pulse packet generated in this fiber stacker. Application for the inertial confinement fusion (ICF) experiment diagnosis with time fiducials was demonstrated. With featured reflection geometries, the shaped packet pulses with uniform sub-pulse polarization states in a temporal window of ns range were generated in the fiber stacker. The design greatly simplifies the packet pulse generation for time fiducial and could be used for laser-driven ICF experimental diagnosis.