Deuterium-tritium Ice Research Articles

Analysis of Dynamic Behavior of the Target and the Deuterium-Tritium Ice in Magnetic-Field Assisted Implosions

The use of strong magnetic fields to augment the output energy of inertial confinement fusion experiments at the National Ignition Facility is of high interest. It offers the potential of reducing electron thermal conduction and increasing hot-spot alpha heating with little to no change in hohlraum behavior. In these magnetically assisted ignition experiments, the ultimate goal is to add a B-field in the form of a pulse ranging from 25 to 60 T to a high-performing hohlraum implosion several microseconds before impingement of the laser beams. This requires eliminating metallic components in the target and replacing them with electrically nonconducting materials. However, the strong eddy currents generated by the rapidly increasing high B-field, which were calculated to be as high as 2000 K, can heat the hohlraum. In this paper, we examine the transient effects of this rapid temperature change on the behavior of the target as well as the fuel layer composed typically of deuterium and tritium. Using simulations and calculations for limiting case scenarios, we find that the effect of the heating is not restrictive toward the performance of the target or the quality of the deuterium-tritium ice.

Shock-Augmented Ignition Approach to Laser Inertial Fusion

Shock ignition enables high gain at low implosion velocity, reducing ablative Rayleigh-Taylor instability growth, which can degrade conventional direct drive. With this method, driving a strong shock requires high laser power and intensity, resulting in inefficiencies in the drive and the generation of hot electrons that can preheat the fuel. A new "shock-augmented ignition" pulse shape is described that, by preconditioning the ablation plasma before launching a strong shock, enables the shock ignition of thermonuclear fuel, but importantly, with substantially reduced laser power and intensity requirements. The reduced intensity requirement with respect to shock ignition limits laser-plasma instabilities, such as stimulated Raman and Brillouin scatter, reducing the risk of hot-electron preheat and restoring the laser coupling advantages of conventional direct drive. Simulations indicate that, due to the reduced power requirements, high gain (∼100) ignition of large-scale direct drive implosions (outer radius ∼1750 μm, deuterium-tritium ice thickness ∼165 μm) may be possible within the power and energy limits of existing facilities such as the National Ignition Facility. Moreover, this concept extends to indirect drive implosions, which exhibit substantial yield increases at reduced implosion velocity. Shock-augmented ignition expands the viable design space of laser inertial fusion.

Enhanced laser-energy coupling with small-spot distributed phase plates (SG5-650) in OMEGA DT cryogenic target implosions

Cryogenic deuterium–tritium ice target implosions on OMEGA with new small-spot (SG5-650) distributed phase plates (DPPs) achieved an (11±4)% increase in energy coupling compared to implosions with larger-spot SG5-850 DPPs by decreasing the ratio of the laser spot diameter to the target diameter from 0.93 to 0.75. The SG5-650 DPPs provide a focus spot size of 674 μm, which is defined as the diameter that encircles 95% of the measured beam energy compared to 834 μm for the SG5-850, which are the standard DPPs in cryogenic target implosions on OMEGA. The hydrodynamic efficiency, defined as the ratio of the kinetic energy in the imploding shell to the laser energy, increased from 4.5% to 5.0% based on radiation-hydrodynamic calculations benchmarked to shell trajectory and bang-time measurements. The higher coupling came with a trade-off of an increased hot-electron production as well as increased hydrodynamic instabilities seeded by a larger mode-10 amplitude from the beam port geometry, both of which may have reduced the fusion neutron production and areal density.

Comparison of Shadowgraphy and X-Ray Phase Contrast Methods for Characterizing a DT Ice Layer in an Inertial Confinement Fusion Target

Shadowgraphy and X-ray phase contrast (XPC) imaging are two techniques that are used for characterizing the deuterium-tritium ice layer in inertial confinement fusion targets. Each technique has limitations that affect how accurately they can characterize small crystalline defects and measure the ice thickness nonuniformities that may be only a few micrometers in height. The concern is that shadowgraphy may be overly sensitive to the shape and depth of defects in the ice surface and insufficiently sensitive to the shape of longer wavelength roughness, while XPC may be too insensitive to defects in the ice surface. Multiple ice layers with different thicknesses (40 to 63 μm), thickness uniformities (peak-to-valley variations that range from < 2 to 12 μm), and crystal defects were analyzed using shadowgraphy and XPC techniques. The results from each method agree when the ice layer is uniformly thick and the crystal lacks defects. That agreement worsens as the number of defects in the surface of the ice layer increases, and the roughness (that is determined from a shadowgram image of the target’s limb) becomes greater than can be justified by the number of defects that are seen in the target’s front and rear surfaces. The XPC technique is considerably less sensitive to surface defects, in part because of the poorer dynamic range and image resolution compared to shadowgraphy. Localized regions of the ice layer that are thicker or thinner than the average thickness of the layer are reported by shadowgraphy to be smaller in height and footprint (by up to 30%) than by XPC. As a result, the two techniques report different ice layer thicknesses that can vary by up to 10%. Shadowgraphy, which results from two caustics that trace different paths through the target, and in theory, image the same ice/vapor surface (but reflect from either the vapor or ice side of the interface), did not consistently characterize the size or shape of ice features to be the same magnitude. The XPC technique provides the best assessment of low-mode (l < 7) roughness in the ice layer. Shadowgraphy results using the strongest caustic is best for detecting the presence of grooves in the ice, although not for quantifying the size of them. If multiple grooves are present, it is best to discard and reform the ice layer.

Effects of thermal conductivity of liquid layer in NIF wetted foam experiments

Numerical simulation of inertial confinement fusion (ICF) capsule implosion experiments requires many plasma parameters corresponding to different materials and their mixtures for a wide range of densities and temperatures. Thermal conduction plays a crucial role in coupling energy to the capsule, is one of the primary mechanisms of energy loss during implosion, has a significant effect on hot-spot formation, and impacts the growth of hydrodynamic instabilities. The determination of accurate thermal conductivity of ICF relevant materials is thus important for understanding capsule performance. Analytic models such as Spitzer or Lee-More models have been extensively used in simulations due to the limited availability of experimental data. First principles calculations have shown that these analytic models tend to underestimate electron thermal conductivity in the warm dense plasma regime for ICF related materials. In this paper, we numerically investigate the effects of different models for the electron heat conductivity coefficients, including both analytic and Quantum Molecular Dynamics (QMD)-based models, for mixed materials in ICF. We also investigate the impact of how conductivities are calculated in mixed cells from constituent material conductivities. We apply this to the modeling of recent wetted foam capsule implosions on the National Ignition Facility, in which a foam layer on the inside of the capsule is wetted with deuterium-tritium (DT) liquid. We have found that electron heat conductivity affects the initial hot-spot formation and its evolution. Strikingly, we observe that capsule performance is more sensitive to the method used to mix material conductivities in mixed cells than how individual material conductivity coefficients are calculated. We have also found that using the first principles QMD-based conductivity model along with an appropriate model for mixed-cell conductivities yields better agreement with experimental results compared to the established modeling strategies. We also investigate the impact of mixed material conductivity modeling on the process of ablator material mixing with DT ice in a plastic ice-layer capsule. In our simulations, the heat conductivity model affects the calculated mix widths at the fuel-ablator interface, particularly near the tent scar. Additional mixing between the DT fuel and the ablator in turn increases the implosion adiabat, which results in a lower hot-spot pressure.

Numerical analysis of dynamic heating modulation during rapid cooling of fuel layer in an indirect-drive cryogenic target

The thermal shimming technology is initially used to adjust the temperature field around the capsule in order to obtain an isothermal distribution and meet the requirement of a high-quality uniform deuterium-tritium (DT) ice layer. In this paper, we focus on the asymmetric heating scheme on upper and lower heaters, which are used both in the quasi-steady layering process and the rapid-cooling process. A dynamic modulation scheme with asymmetric heating is first proposed to attenuate the cooling-induced thermal perturbation around the capsule. Thermal and hydrodynamic simulations of the cryogenic target are numerically conducted. An axisymmetric model based on the NIF-scale indirect-drive target is built and used in simulations. A variety of dynamic modulation schemes are studied and compared with each other. In the layering process, the asymmetric heating scheme is validated to be effective in reducing the low-mode roughness of the DT ice layer. In the rapid cooling, the thermal uniformity around the capsule can be greatly improved by using the dynamic heating modulation scheme when the DT layer is frozen to 1.5 K below its triple point temperature. At a constant cooling rate of 0.4 K/s, the temperature difference on the capsule outer surface reaches ∼24.5 mK without dynamic heating. By employing a nonlinear modulation with symmetric heating, this value can be decreased by 25%. Based on these results, further improvement can be achieved by optimizing the modulation with asymmetric heating and the temperature difference can be significantly reduced to ∼6.0 mK, which is a factor of four lower than the original value.

Instability growth seeded by DT density perturbations in ICF capsules

Identifying and controlling hydrodynamic instabilities is vital to inertial confinement fusion. We use simulations to examine the growth of several defects seeded in the deuterium-tritium (DT) fuel layer. First, we examine the growth of bulk density fluctuations in a solid DT ice layer. These density perturbations grow with amplitudes similar to surface defects, however the high-mode (m &amp;gt; 40) growth structures differ. We also consider the wetted foam capsule design, where density perturbations can be seeded by foam inhomogeneity. Simulations show that foam-seeded perturbations grow similarly to pure DT density seeds at low modes (m &amp;lt; 40), but at higher modes, the foam seeds grow significantly more. Next, we simulate the growth of two common multimode ice defects, grooves, and bubbles, and find that bubbles are significantly less harmful than grooves of similar width. Finally, we explore shimming the ablator to counteract surface roughness and show that instability growth from low-mode roughness can be effectively mitigated.

Simulation and analysis of time-gated monochromatic radiographs of cryogenic implosions on OMEGA

Spherical polymer shells containing cryogenic DT ice layers have been imploded on the OMEGA Laser System and radiographed using Si backlighter targets (hν = 1.865keV) driven with 20-ps IR pulses from the OMEGA EP Laser System. We report on a series of implosions in which the deuterium–tritium (DT) shell is imaged for a range of convergence ratios and in-flight aspect ratios. The shadows of the converging DT ice and polymer shells are recorded while the self-emission is minimized using a time-resolved (40-ps) monochromatic crystal imaging system. The images acquired have been analyzed for the level of ablator mixing into the DT fuel (even 0.1% of carbon mix can be reliably inferred). Simulations are compared with measured x-ray radiographs to provide insight into the early time and stagnation stages of an implosion, to guide the modeling efforts to improve the target designs, and to guide the development of this and other imagining techniques, such as Compton radiography.

Monochromatic backlighting of direct-drive cryogenic DT implosions on OMEGA

Backlighting is a powerful technique to observe the flow of cold and dense material in high-energy-density–plasma experiments. High-performance, direct-drive cryogenic deuterium–tritium (DT) implosions are a challenging backlighting configuration because of the low opacity of the DT shell, the high shell velocity, the small size of the stagnating shell, and the very bright self-emission of the hot core. A crystal imaging system with a Si Heα backlighter at 1.865 keV driven by ∼20-ps short pulses from OMEGA EP was developed to radiograph the OMEGA cryogenic implosions. The high throughput of the crystal imaging system makes it possible to record high-quality images with good photon statistics and a spatial resolution of ∼15 μm at 10% to 90% modulation. This imager has been used to study the evolution of preimposed mass-density perturbations in the ablator, to quantify the perturbations caused by the stalk that is used to mount the target, and to study the mix caused by laser imprint or small-scale debris on the target surface. Because of the very low opacity of DT relative to carbon, even 0.1% of mix of carbon into the DT ice can be reliably inferred from the images. With the current implosion designs, mix is only observed for an adiabat below α = 4.

Asymmetric-shell ignition capsule design to tune the low-mode asymmetry during the peak drive

The low-mode radiation flux asymmetry in the hohlraum is a main source of performance degradation in the National Ignition Facility (NIF) implosion experiments. To counteract the deleterious effects of the large positive P2 flux asymmetry during the peak drive, this paper develops a new tuning method called asymmetric-shell ignition capsule design which adopts the intentionally asymmetric CH ablator layer or deuterium-tritium (DT) ice layer. A series of two-dimensional implosion simulations have been performed, and the results show that the intentionally asymmetric DT ice layer can significantly improve the fuel ρR symmetry, hot spot shape, hot spot internal energy, and the final neutron yield compared to the spherical capsule. This indicates that the DT asymmetric-shell capsule design is an effective tuning method, while the CH ablator asymmetric-shell capsule could not correct the fuel ρR asymmetry, and it is not as effective as the DT asymmetric-shell capsule design.

Hydrodynamic instability measurements in DT-layered ICF capsules using the layered-HGR platform

The first measurements of hydrodynamic instability growth at the fuel-ablator interface in an ICF implosion are reported. Previous instability measurements on the National Ignition Facility have used plastic capsules to measure ablation front Rayleigh-Taylor growth with the Hydro.-Growth Radiography (HGR) platform. These capsules substituted an additional thickness of plastic ablator material in place of the cryogenic layer of Deuterium- Tritium (DT) fuel. The present experiments are the first to include a DT ice layer, which enables measurements of the instability growth occurring at the fuel-ablator interface. Instability growth at the fuel-ablator interface is seeded differently in two independent NIF experiments. In the first case, a perturbation on the outside of the capsule feeds through and grows on the interface. Comparisons to an implosion without a fuel layer produce a measure of the fuel's modulation. In the second case, a modulation was directly machined on the inner ablator before the fuel layer was added. The measurement of growth in these two scenarios are compared to 2D rad-hydro modeling.

Speed of sound in solid molecular hydrogen-deuterium: Quantum Molecular Dynamics Approximation

Uniformity of the solid layer is one of the critical points for an efficient ignition of the Deuterium-Tritium (DT) target. During the compression process this layer, perturbations grow as the Rayleigh-Taylor instability. Knowing the mechanical properties of this layer and its thermo-mechanical limits is necessary if we want to control or to minimize these instabilities. In this work we have used a simplified approach, replacing the DT ice system with a mixture of hydrogen-deuterium (HD) because beta decay of tritium complicates the analysis in the former case. Through simulation with ab initio methods we have calculated the elastic constants, the bulk modulus and sound velocity for hydrogen isotopes in solid molecular state. In this work we present the results for hydrogen-deuterium mixtures 50%-50%, at 15 K and with a compression which covers the range of 1 to 15 GPa. This system is interesting for study the early stages of the dynamic compression and provides conditions that are close to the manufacture of DT target in inertial confinement fusion. Discontinuities in the curve that have been observed on pure hydrogen, which are associated with phase transitions and the phase hysteresis.

Advances in shock timing experiments on the National Ignition Facility

Recent advances in shock timing experiments and analysis techniques now enable shock measurements to be performed in cryogenic deuterium-tritium (DT) ice layered capsule implosions on the National Ignition Facility (NIF). Previous measurements of shock timing in inertial confinement fusion (ICF) implosions were performed in surrogate targets, where the solid DT ice shell and central DT gas were replaced with a continuous liquid deuterium (D2) fill. These previous experiments pose two surrogacy issues: a material surrogacy due to the difference of species (D2 vs. DT) and densities of the materials used and a geometric surrogacy due to presence of an additional interface (ice/gas) previously absent in the liquid-filled targets. This report presents experimental data and a new analysis method for validating the assumptions underlying this surrogate technique.

Scaling magnetized liner inertial fusion on Z and future pulsed-power accelerators

The MagLIF (Magnetized Liner Inertial Fusion) concept [S. A. Slutz et al., Phys. Plasmas 17, 056303 (2010)] has demonstrated fusion–relevant plasma conditions [M. R. Gomez et al., Phys. Rev. Lett. 113, 155003 (2014)] on the Z accelerator with a peak drive current of about 18 MA. We present 2D numerical simulations of the scaling of MagLIF on Z as a function of drive current, preheat energy, and applied magnetic field. The results indicate that deuterium-tritium (DT) fusion yields greater than 100 kJ could be possible on Z when all of these parameters are at the optimum values: i.e., peak current = 25 MA, deposited preheat energy = 5 kJ, and Bz = 30 T. Much higher yields have been predicted [S. A. Slutz and R. A. Vesey, Phys. Rev. Lett. 108, 025003 (2012)] for MagLIF driven with larger peak currents. Two high performance pulsed-power accelerators (Z300 and Z800) based on linear-transformer-driver technology have been designed [W. A. Stygar et al., Phys. Rev. ST Accel. Beams 18, 110401 (2015)]. The Z300 design would provide 48 MA to a MagLIF load, while Z800 would provide 65 MA. Parameterized Thevenin-equivalent circuits were used to drive a series of 1D and 2D numerical MagLIF simulations with currents ranging from what Z can deliver now to what could be achieved by these conceptual future pulsed-power accelerators. 2D simulations of simple MagLIF targets containing just gaseous DT have yields of 18 MJ for Z300 and 440 MJ for Z800. The 2D simulated yield for Z800 is increased to 7 GJ by adding a layer of frozen DT ice to the inside of the liner.

Shock timing measurements and analysis in deuterium-tritium-ice layered capsule implosions on NIF

Recent advances in shock timing experiments and analysis techniques now enable shock measurements to be performed in cryogenic deuterium-tritium (DT) ice layered capsule implosions on the National Ignition Facility (NIF). Previous measurements of shock timing in inertial confinement fusion implosions [Boehly et al., Phys. Rev. Lett. 106, 195005 (2011); Robey et al., Phys. Rev. Lett. 108, 215004 (2012)] were performed in surrogate targets, where the solid DT ice shell and central DT gas were replaced with a continuous liquid deuterium (D2) fill. These previous experiments pose two surrogacy issues: a material surrogacy due to the difference of species (D2 vs. DT) and densities of the materials used and a geometric surrogacy due to presence of an additional interface (ice/gas) previously absent in the liquid-filled targets. This report presents experimental data and a new analysis method for validating the assumptions underlying this surrogate technique. Comparison of the data with simulation shows good agreement for the timing of the first three shocks, but reveals a considerable discrepancy in the timing of the 4th shock in DT ice layered implosions. Electron preheat is examined as a potential cause of the observed discrepancy in the 4th shock timing.

Measurement of High-Pressure Shock Waves in Cryogenic Deuterium-Tritium Ice Layered Capsule Implosions on NIF

The first measurements of multiple, high-pressure shock waves in cryogenic deuterium-tritium (DT) ice layered capsule implosions on the National Ignition Facility have been performed. The strength and relative timing of these shocks must be adjusted to very high precision in order to keep the DT fuel entropy low and compressibility high. All previous measurements of shock timing in inertial confinement fusion implosions [T. R. Boehly etal., Phys. Rev. Lett. 106, 195005 (2011), H. F. Robey etal., Phys. Rev. Lett. 108, 215004 (2012)] have been performed in surrogate targets, where the solid DT ice shell and central DT gas regions were replaced with a continuous liquid deuterium (D2) fill. This report presents the first experimental validation of the assumptions underlying this surrogate technique.

Experimental investigation of bright spots in broadband, gated x-ray images of ignition-scale implosions on the National Ignition Facility

Bright spots in the hot spot intensity profile of gated x-ray images of ignition-scale implosions at the National Ignition Facility [G. H. Miller et al., Opt. Eng. 443, (2004)] are observed. X-ray images of cryogenically layered deuterium-tritium (DT) and tritium-hydrogen-deuterium (THD) ice capsules, and gas filled plastic shell capsules (Symcap) were recorded along the hohlraum symmetry axis. Heterogeneous mixing of ablator material and fuel into the hot spot (i.e., hot-spot mix) by hydrodynamic instabilities causes the bright spots. Hot-spot mix increases the radiative cooling of the hot spot. Fourier analysis of the x-ray images is used to quantify the evolution of bright spots in both x- and k-space. Bright spot images were azimuthally binned to characterize bright spot location relative to known isolated defects on the capsule surface. A strong correlation is observed between bright spot location and the fill tube for both Symcap and cryogenically layered DT and THD ice targets, indicating the fill tube is a significant seed for the ablation front instability causing hot-spot mix. The fill tube is the predominant seed for Symcaps, while other capsule non-uniformities are dominant seeds for the cryogenically layered DT and THD ice targets. A comparison of the bright spot power observed for Si- and Ge-doped ablator targets shows heterogeneous mix in Symcap targets is mostly material from the doped ablator layer.

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.

Fast ignition upon the implosion of a thin shell onto a precompressed deuterium-tritium ball

Fast ignition of a precompressed inertial confinement fusion (ICF) target by a hydrodynamic material flux is investigated. A model system of hydrodynamic objects consisting of a central deuterium-tritium (DT) ball and a concentric two-layer shell separated by a vacuum gap is analyzed. The outer layer of the shell is an ablator, while the inner layer consists of DT ice. The igniting hydrodynamic flux forms as a result of laser-driven acceleration and compression of the shell toward the system center. A series of one-dimensional numerical simulations of the shell implosion, the collision of the shell with the DT ball, and the generation and propagation of thermonuclear burn waves in both parts of the system are performed. Analytic models are developed that describe the implosion of a thin shell onto a central homogeneous ball of arbitrary radius and density and the initiation and propagation of a thermonuclear burn wave induced by such an implosion. Application of the solution of a model problem to analyzing the implosion of a segment of a spherical shell in a conical channel indicates the possibility of fast ignition of a spherical ICF target from a conical target driven by a laser pulse with an energy of 500–700 kJ.

Change in Inertial Confinement Fusion Implosions upon Using anAb InitioMultiphase DT Equation of State

Improving the description of the equation of state (EOS) of deuterium-tritium (DT) has recently been shown to change significantly the gain of an inertial confinement fusion target [S. X. Hu et al., Phys. Rev. Lett. 104, 235003 (2010)]. Here we use an advanced multiphase EOS, based on abinitio calculations, to perform a full optimization of the laser pulse shape with hydrodynamic simulations starting from 19K in DT ice. The thermonuclear gain is shown to be a robust estimate over possible uncertainties of the EOS. Two different target designs are discussed, for shock ignition and self-ignition. In the first case, the areal density and thermonuclear energy can be recovered by slightly increasing the laser energy. In the second case, a lower in-flight adiabat is needed, leading to a significant delay (3ns) in the shock timing of the implosion.