Inertial Confinement Fusion Experiments Research Articles

Absolute stimulated Raman side scatter in direct-drive laser-produced plasmas

Production of suprathermal electrons by stimulated Raman scattering (SRS) is a principal concern for contemporary direct-drive inertial confinement fusion experiments at the National Ignition Facility and similar systems since such electrons penetrate and preheat the target core, preventing efficient implosion. The higher temperatures and longer scale lengths in these experiments favor SRS over two-plasmon decay, which predominated in earlier experiments. In particular, current experiments are expected to exceed the threshold for absolute Raman side scatter, which would then dominate the interaction since it grows temporally until saturated by nonlinear mechanisms such as hot-electron production. Until recently, analyses of SRS side scatter have treated the case of a single laser beam incident on a plasma, but the direct-drive approach to laser fusion employs a multitude of beams to drive the implosion. Here, we present an analysis that can be applied to an arbitrary number of beams with varied angles of incidence and polarizations. In the case of a single beam, it allows a physically motivated derivation and verification of an analytic threshold formula. In the general case of multiple beams and arbitrary orientation and polarizations, the threshold is found by numerical integration of a set of first-order linear partial differential equations.

Static and dynamic properties of multi-ionic plasma mixtures.

Complex plasma mixtures with three or more components are often encountered in astrophysics or in inertial confinement fusion (ICF) experiments. For mixtures containing species with large differences in atomic number Z, the modeling needs to consider at the same time the kinetic theory for low-Z elements combined with the theory of strongly coupled plasma for high-Z elements, as well as all the intermediate situations that can appear in multicomponent systems. For such cases, we study the pair distribution functions, self-diffusions, mutual diffusion, and viscosity for ternary mixtures at extreme conditions. These quantities can be produced from first principles using orbital free molecular dynamics at the computational expense of very intensive simulations to reach good statistics. Utilizing the first-principles results as reference data, we assess the merit of a global analytic model for transport coefficients, "pseudo-ions in jellium" (PIJ), based on an isoelectronic assumption (iso-n_{e}). With a multicomponent hypernetted-chain integral equation, we verify the quality of the iso-n_{e}prescription for describing the static structure of the mixtures. This semianalytical modeling compares well with the simulation results and allows one to consider plasma mixtures not accessible to simulations. Applications are given for the mix of materials in ICF experiments. A reduction of a multicomponent mixture to an effective binary mixture is also established in the hydrodynamic limit and compared with PIJ estimations for ICF relevant mixtures.

A suite of neutron time-of-flight detectors to measure hot-spot motion in direct-drive inertial confinement fusion experiments on OMEGA

A suite of six neutron time of flight detectors capable of measuring the primary DT neutron energy spectrum has recently been completed on the OMEGA Laser System. The detectors are positioned along multiple quasi-orthogonal lines of sight (LOS’s) in either a stand-alone, collinear, or antipodal configuration. The collinear detector configuration enables one to measure the neutron velocity along the detector LOS independent of the neutron-production history, while the antipodal detector configuration enables one to directly measure of the hot-spot velocity along the detector axis and Gamow velocity shift due to the plasma ion temperature. Using these six measurements of the primary DT neutron energy spectrum, the neutron-averaged hot-spot velocity and Gamow velocity shift of DT fusion neutrons have been measured in cryogenic experiments on OMEGA for the first time. The detector suite is operable between DT neutron yields of 1 × 1013 to 2 × 1014 with an uncertainty of ∼14 km/s in the neutron-averaged hot-spot velocity measurement.

Study of the kinetic effects in indirect-drive inertial confinement fusion hohlraums

Kinetic physics has the potential to impact the performance of indirect-drive inertial confinement fusion (ICF) experiments. In near vacuum hohlraums (or vacuum hohlraums), the high-Z plasma expands from the hohlraum wall and collides with the blow-off from the capsule (or the low-density fill-gas). Such collision produces conditions in which kinetic effects may dominate since the ion-ion mean free paths are larger than the size of the interaction region. In the present work, we present the experimental evidence supported by simulations of kinetic effects launched in the interpenetration layer between the laser-driven hohlraum plasma bubbles and the corona plasma of the compressed pellet on the Shenguang-III prototype laser facility. Our study showed kinetic shocks arisen in the hohlraum wall/ablator (or the low-density fill-gas) interpenetration region, which result in efficient acceleration of high energy ions. When these high energy ions were deposited inside the capsule, it resulted in significant low-mode asymmetry of the implosion capsule since there are no high energy ions from the laser entrance holes. These studies provide novel insight into the interactions and dynamics of a vacuum hohlraum and near vacuum hohlraum.

Mixing in ICF implosions on the National Ignition Facility caused by the fill-tube

The micrometer-scale tube that fills capsules with thermonuclear fuel in inertial confinement fusion experiments at the National Ignition Facility is also one of the implosion's main degradation sources. It seeds a perturbation that injects the ablator material into the center, radiating away some of the hot-spot energy. This paper discusses how the perturbation arises in experiments using high-density carbon ablators and how the ablator mix interacts once it enters the hot-spot. Both modeling and experiments show an in-flight areal-density perturbation and localized x-ray emission at stagnation from the fill-tube. Simulations suggest that the fill-tube is degrading an otherwise 1D implosion by ∼2×, but when other degradation sources are present, the yield reduction is closer to 20%. Characteristics of the fill-tube assembly, such as the through-hole size and the glue mass, alter the dynamics and magnitude of the degradation. These aspects point the way toward improvements in the design, some of which (smaller diameter fill-tube) have already shown improvements.

Pulsed power indirect drive approach to inertial confinement fusion

We propose an approach to Inertial Confinement Fusion (ICF) that could potentially lead to high levels of thermonuclear burn in the laboratory. The proposal is based upon the concept of pulsed power X ray driven hohlraums together with indirectly-driven ICF capsule designs that are scaled up in size from those typically used in laser indirect-drive ICF experiments at the National Ignition Facility (NIF). We propose that near-term NIF experiments can be used to explore some important aspects of the X ray driven hohlraum ICF concept.

Polarization of x-ray radiation after electron-impact excitation of ions in plasmas

A new relativistic model for calculating (magnetic sublevel) electron-impact excitation process in plasmas is proposed in this paper, which is essential to predict the polarization property of x-ray radiation. Our model focuses on weakly coupled plasmas (assuming that the plasma is of low density and very hot), where we use a Debye-Hückel potential to represent the interactions among the charged particles, in order to calculate plasma shielding effects. As a practical example, we present results of accurate calculations of transition energy, cross section, and linear polarization of the photon emission following EIE of 1s2→1s2p3,1P1 lines of He-like Ar16+ ion over a broad range of energies and Debye lengths, i.e., 16 times ionized atom of Ar. Such highly-ionized species can exist in many environments, such as in laser-produced plasmas, x-ray laser, astrophysics, and inertial confinement fusion experiments. The energies and cross sections are found to be decreased with decreasing Debye length, these general characteristics also lead to a reduction in the linear polarization of the photon emission. The present approach compares well, with existing methods and experimental data. Apart from other applications, results obtained in this work should be useful in plasma diagnostic.

A neutron time-of-flight system for inertial confinement fusion.

A neutron time-of-flight (nTOF) system has been implemented at the largest laser facility in China. The nTOF system is used to measure neutron spectra in inertial confinement fusion experiments. The nTOF system consists of 11 fast plastic scintillation detectors. The detectors employed three designs to measure neutron yield, ion temperature, and neutron bang time. The nTOF system is capable of measuring the primary neutron yield from 107 to 1013, secondary DT neutron yield from 106 to 108, and ion temperature and neutron bang time yields from 108 to 1013. The accuracies of the nTOF system are about 10% for neutron yield and ion temperature measurements and better than 60 ps for neutron bang time measurements. The nTOF system has become one of the most important diagnostics for implosions, and it is used for more than 200 shots per year.

Observation of persistent species temperature separation in inertial confinement fusion mixtures

The injection and mixing of contaminant mass into the fuel in inertial confinement fusion (ICF) implosions is a primary factor preventing ignition. ICF experiments have recently achieved an alpha-heating regime, in which fusion self-heating is the dominant source of yield, by reducing the susceptibility of implosions to instabilities that inject this mass. We report the results of unique separated reactants implosion experiments studying pre-mixed contaminant as well as detailed high-resolution three-dimensional simulations that are in good agreement with experiments. At conditions relevant to mixing regions in high-yield implosions, we observe persistent chunks of contaminant that do not achieve thermal equilibrium with the fuel throughout the burn phase. The assumption of thermal equilibrium is made in nearly all computational ICF modeling and methods used to infer levels of contaminant from experiments. We estimate that these methods may underestimate the amount of contaminant by a factor of two or more.

Microstructure and morphology evolution of high-Z/low-Z gradient coatings on PAMS microspheres

Gradient coatings exhibit excellent implosion performance in double-shell targets for Inertial Confinement Fusion (ICF). Here, we present the controllable preparation of high-Z/low-Z (Z is the atomic number) gradient alloy coatings which can be used for ICF double-shell targets. Tungsten/carbon (W/C), tungsten/silicon (W/Si) and tungsten/aluminum (W/Al) gradient coatings were successfully deposited on silicon substrates and poly-alpha-methyl-styrene (PAMS) spheres by direct-current magnetron sputtering. The morphology, component and surface roughness of gradient coatings were characterized. Although there are significant differences in morphology and surface roughness compared with planar gradient coatings, the spherical gradient coatings still have a dense microstructure and smooth surface. The root-mean-square (RMS) roughness of our gradient coatings experience an improvement compared with the previous films which used in ICF experiments. The gradient coatings deposited by co-sputtering of high-Z and low-Z elements in this work provide competitive candidate materials for ICF double-shell targetsee.

Predicting spherical symmetry degeneration of non-infrared deuterium ice layer in a cryogenic capsule

A cylindrical cryogenic target containing deuterium fuel acts as an important surrogate to help understand implosion physics before the deuterium-tritium capability is brought online. Uniformity of the deuterium ice thickness is a key parameter for the inertial confinement fusion (ICF) experiments. Achieving and retaining a uniform deuterium ice layer in capsule without infrared radiation is difficult in engineering. The method used to calculate the ice thickness deviation of deuterium–tritium fuel is invalid when the bulk heat generation is equal to zero. Appearance solutions of the deuterium ice in steady state conclude that a uniform ice layer cannot be retained for long without infrared radiation. A transient algorithm by integrating heat transfer theory, the equation derived from Stefan problem and mass conservation with moving mesh technics in a finite element model can be applied to predict deuterium ice spherical symmetry degeneration. It is certified with good reliability by comparing the simulated results with theoretical and experimental data. As for the deuterium targets, the characteristics of linear approximation and integrability avert heavy work of moving mesh in analyses of stable and unstable scenarios. The work has great support for the cryogenic processing and engineering design of ICF targets.

Transfer Learning to Model Inertial Confinement Fusion Experiments

Inertial confinement fusion (ICF) experiments are designed using computer simulations that are approximations of reality and therefore must be calibrated to accurately predict experimental observations. In this article, we propose a novel technique for calibrating from simulations to experiments, or from low fidelity simulations to high fidelity simulations, via “transfer learning” (TL). TL is a commonly used technique in the machine learning community, in which models trained on one task are partially retrained to solve a separate, but related task, for which there is a limited quantity of data. We introduce the idea of hierarchical TL, in which neural networks trained on low fidelity models are calibrated to high fidelity models, then to experimental data. This technique essentially bootstraps the calibration process, enabling the creation of models which predict high fidelity simulations or experiments with minimal computational cost. We apply this technique to a database of ICF simulations and experiments carried out at the Omega laser facility. TL with deep neural networks enables the creation of models that are more predictive of Omega experiments than simulations alone. The calibrated models accurately predict future Omega experiments, and are used to search for new, optimal implosion designs.

Experimental investigation of the collective stimulated Brillouin and Raman scattering of multiple laser beams in inertial confinement fusion experiments

The direct and indirect drive schemes for inertial confinement fusion (ICF) make use of a large number of laser beams arranged in a symmetric angular distribution. The preferential decay geometry of the three waves resonant couplings, mainly responsible for backscattered light in single beam experiments, may then be deeply modified in the region of crossing beams where collective laser plasma instabilities could develop. Such instabilities can occur for laser beams having a common symmetry axis along which they drive a common daughter wave. The collective coupling results in an increase of the growth gain with the increase of the number of interacting beams and produce energy losses in new backward directions. We have taken advantage of the multiple beams of the Omega laser facility and of its large battery of diagnostics to study the physics related to this multiple beams interaction in the regimes of high temperature plasmas relevant of the direct and indirect drive schemes to ICF. Experiments performed in a planar open geometry have evidenced the large amplification of stimulated Raman scattering (SRS) electromagnetic waves almost transverse to the density gradient as theoretically predicted 40 years ago. This was achieved in long scale-length high-temperature plasmas in which two beams couple to the same scattered electromagnetic wave further demonstrating this multiple-beams collective SRS interaction. The collective nature of the coupling and the amplification at large angles from the density gradient increase the global SRS losses and produce light scattered in novel directions out of the planes of incidence of the beams. Indirect drive experiments were performed in rugby ball shaped Hohlraum irradiated by 40 beams. Large instantaneous (peak reflectivity >30%) Brillouin sidescattering was evidenced to originate from the collective Brillouin amplification of a shared ion acoustic wave driven along the Hohlraum axis by a cone of 10 beams. In this paper, the scattering geometry is detailed for the two types of collective instabilities showing that they produce light scattered in novel very precise directions located far from the original aperture of the beams where the diagnostics are usually set-up. This scattered light could be measured on Omega thanks to the flexibility of the facility. Key features of the light scattered by collective instabilities are identified that would allow to recognize their signatures in more complex, less diagnosed experiments.

Magnesium oxide at extreme temperatures and pressures studied with first-principles simulations.

We combine two first-principles computer simulation techniques, path integral Monte Carlo and density functional theory molecular dynamics, to determine the equation of state of magnesium oxide in the regime of warm dense matter, with densities ranging from 0.35 to 71 g cm-3 and temperatures ranging from 10 000 K to 5 × 108 K. These conditions are relevant for the interiors of giant planets and stars as well as for shock wave compression measurements and inertial confinement fusion experiments. We study the electronic structure of MgO and the ionization mechanisms as a function of density and temperature. We show that the L-shell orbitals of magnesium and oxygen hybridize at high density. This results in a gradual ionization of the L-shell with increasing density and temperature. In this regard, MgO behaves differently from pure oxygen, which is reflected in the shape of the MgO principal shock Hugoniot curve. The curve of oxygen shows two compression maxima, while that of MgO shows only one. We predict a maximum compression ratio of 4.66 to occur for a temperature of 6.73 × 107 K. Finally, we study how multiple shocks and ramp waves can be used to cover a large range of densities and temperatures.

Impact of the Langdon effect on crossed-beam energy transfer

The prediction that laser plasma heating distorts the electron distribution function away from Maxwellian and towards a super-Gaussian distribution dates back four decades1. In conditions relevant to inertial confinement fusion, however, no direct evidence of this so-called ‘Langdon effect’ has previously been observed. Here we present measurements of the spatially and temporally resolved Thomson scattering spectrum that indicate the presence of super-Gaussian electron distribution functions consistent with existing theory2. In such plasmas, ion acoustic wave frequencies increase monotonically with the super-Gaussian exponent3. Our results show that the measured power transfer between crossed laser beams mediated by ion acoustic waves requires a model that accounts for the non-Maxwellian electron distribution function, whereas the standard Maxwellian calculations overpredict power transfer over a wide region of parameter space. Including this effect is expected to improve the predictive capability of crossed-beam energy transfer modelling at the National Ignition Facility in California and may restore a larger operable design space for inertial confinement fusion experiments. This is also expected to motivate further inquiry in other areas affected by non-Maxwellian electron distribution functions, such as laser absorption, heat transport and X-ray spectroscopy. In inertial confinement fusion experiments, the effect of the overlapping laser beams on the plasma is predicted to lead to a distortion of the electron distribution function, which has now been observed in experiments.

Transfer Learning as a Tool for Reducing Simulation Bias: Application to Inertial Confinement Fusion

We adopt a technique, known in the machine learning community as transfer learning, to reduce the bias of computer simulation using very sparse experimental data. Unlike the Bayesian calibration, which is commonly used to estimate the simulation bias, the transfer learning approach discussed in this article involves calculating an artificial neural network surrogate model of the simulations. Assuming that the simulation code correctly predicts the trends in the experimental data but it is subject to unknown biases, we then partially retrain, or transfer learn, the initial surrogate model to match the experimental data. This process eliminates the bias while still taking advantage of the physics relations learned from the simulation. Transfer learning can be easily adapted to a wide range of problems in science and engineering. In this article, we carry out numerical tests to investigate the applicability of this technique to predict the observable outcomes of inertial confinement fusion (ICF) experiments under new conditions. Using our synthetic validation data set, we demonstrate that an accurate predictive model can be built by retraining an initial surrogate model with experimental data volumes so small that they are relevant to the ICF problem. This opens up new opportunities for knowledge transfer and building predictive models in physics. After implementing transfer learning in a standard neural network, we successfully extended the method to a more complex, generative adversarial network architecture, which will be needed for predicting not only scalars but also diagnostic images in our future work.

Improved inertial confinement fusion gamma reaction history 12C gamma-ray signal by direct subtraction.

The Gamma Reaction History (GRH) diagnostic located at the National Ignition Facility (NIF) measures time resolved gamma rays released from inertial confinement fusion experiments by converting the emitted gamma rays into Cherenkov light. Imploded capsules have a bright 4.4 MeV gamma ray from fusion neutrons inelastically scattering with carbon atoms in the remaining ablator. The strength of the 4.4 MeV gamma ray line is proportional to the capsule's carbon ablator areal density and can be used to understand the dynamics and energy budget of a carbon-based ablator capsule implosion. Historically, the GRH's four gas cells use the energy thresholding from the Cherenkov process to forward fit an estimation of the experiment's complete gamma ray spectrum by modeling the surrounding environment in order to estimate the 4.4 MeV neutron induced carbon gamma ray signal. However, the high number of variables, local minima, and uncertainties in detector sensitivities and relative timing had prevented the routine use of the forward fit to generate carbon areal density measurements. A new, more straightforward process of direct subtraction of deconvolved signals was developed to simplify the extraction of the carbon areal density. Beryllium capsules are used as a calibration to measure the capsule environment with no carbon signal. The proposed method is then used to appropriately subtract and isolate the carbon signal on shots with carbon ablators. The subtraction algorithm achieves good results across all major capsule campaigns, achieving similar results to the forward fit. This method is now routinely used to measure carbon areal density for NIF shots.

Using Sparse Gaussian Processes for Predicting Robust Inertial Confinement Fusion Implosion Yields

Here we present the application of an advanced Sparse Gaussian Process based machine learning algorithm to the challenge of predicting the yields of inertial confinement fusion (ICF) experiments. The algorithm is used to investigate the parameter space of an extremely robust ICF design for the National Ignition Facility, the `Simplest Design'; deuterium-tritium gas in a plastic ablator with a Gaussian, Planckian drive. In particular we show that i) GPz has the potential to decompose uncertainty on predictions into uncertainty from lack of data and shot-to-shot variation, ii) permits the incorporation of science-goal specific cost-sensitive learning e.g. focussing on the high-yield parts of parameter space and iii) is very fast and effective in high dimensions.

Three-dimensional particle-in-cell modeling of parametric instabilities near the quarter-critical density in plasmas.

The nonlinear regime of laser-plasma interactions including both two-plasmon decay (TPD) and stimulated Raman scattering (SRS) instabilities has been studied in three-dimensional (3D) particle-in-cell simulations with parameters relevant to the inertial confinement fusion (ICF) experiments. SRS and TPD develop in the same region in plasmas, and the generation of fast electrons can be described accurately with only the full model including both SRS and TPD. The growth of instabilities in the linear stage is found to be in good agreement with analytical theories. In the saturation stage the low-frequency density perturbations driven by the daughter waves of the SRS side scattering can saturate the TPD and consequently inhibit the fast-electron generation. The fast-electron flux in 3D modeling is up to an order of magnitude smaller than previously reported in 2D TPD simulations, bringing it close to the results of ICF experiments.

Analysis of spherical and comatic aberrations in a pulse-dilation framing camera

A pulse-dilation framing camera is a two-dimensional ultrafast diagnostic tool that is used to research spatiotemporal evolution of plasmas through pinhole array images in inertial confinement fusion experiments. This imaging technique uses a pulse dilated with magnetic focusing imaging. Aberrations exist in the images due to the influence of long drift area and magnetic focusing on photoelectron motion. These aberrations were simulated and measured in this study in order to further understand aberrations in a pulse-dilation framing camera. The results show that the spherical and comatic aberrations primarily affect the imaging quality during imaging of camera. The spherical aberration produces diffuse spots and reduces the spatial resolution for any point on the object (i.e., regardless of whether or not it lies on the optical axis). Meanwhile comatic aberration especially obvious occurs for off-axis points on the object. These results raise an important question regarding the development of larger object planes and multiple frames of cameras that remains to be resolved.