Laser Preheat Research Articles

Development of a high performance MagLIF target platform using high aspect ratio coated liners and low-mix laser preheat

Development of a high performance MagLIF target platform using high aspect ratio coated liners and low-mix laser preheat

Demonstration of improved laser preheat with a cryogenically cooled magnetized liner inertial fusion platform.

We report on progress implementing and testing cryogenically cooled platforms for Magnetized Liner Inertial Fusion (MagLIF) experiments. Two cryogenically cooled experimental platforms were developed: an integrated platform fielded on the Z pulsed power generator that combines magnetization, laser preheat, and pulsed-power-driven fuel compression and a laser-only platform in a separate chamber that enables measurements of the laser preheat energy using shadowgraphy measurements. The laser-only experiments suggest that ∼89% ± 10% of the incident energy is coupled to the fuel in cooled targets across the energy range tested, significantly higher than previous warm experiments that achieved at most 67% coupling and in line with simulation predictions. The laser preheat configuration was applied to a cryogenically cooled integrated experiment that used a novel cryostat configuration that cooled the MagLIF liner from both ends. The integrated experiment, z3576, coupled 2.32 ± 0.25 kJ preheat energy to the fuel, the highest to-date, demonstrated excellent temperature control and nominal current delivery, and produced one of the highest pressure stagnations as determined by a Bayesian analysis of the data.

Exploring the parameter space of MagLIF implosions using similarity scaling. III. Rise-time scaling

Magnetized liner inertial fusion (MagLIF) is a z-pinch magneto-inertial-fusion concept studied at the Z Pulsed Power Facility of Sandia National Laboratories. Two important metrics characterizing current delivery to a z-pinch load are the peak current and the current-rise time, which is roughly the time interval to reach the peak current. It is known that, when driving a z-pinch load with a longer current-rise time, the performance of the z-pinch decreases. However, a theory to understand and quantify this effect is still lacking. In this paper, we utilize a framework based on similarity scaling to analytically investigate the variations in the performance of MagLIF loads when varying the current-rise time, or equivalently, the implosion timescale. To maintain similarity between the implosions, we provide scaling prescriptions of experimental input parameters defining a MagLIF load and derive the expected scaling laws for stagnation conditions and for various performance metrics. We compare predictions of the theory to 2D numerical simulations using the radiation, magneto-hydrodynamic code hydra. For several metrics, we find acceptable agreement between the theory and simulations. Our results show that the voltage φload near the MagLIF load follows a weak scaling law φload∝tφ−0.12 with respect to the characteristic timescale tφ of the voltage source, instead of the ideal φload∝tφ−1 scaling. This occurs because the imploding height of the MagLIF load must increase to preserve end losses. As a consequence of the longer imploding liners, the required total laser preheat energy and delivered electric energy increase. Overall, this study helps understand the trade-offs of the MagLIF design space when considering future pulsed-power generators with shorter and longer current-rise times.

Effect of laser preheat in magnetized liner inertial fusion at OMEGA

Integrated magnetized liner inertial fusion (MagLIF) experiments on the OMEGA laser system are modeled to investigate the effects of laser preheat on implosion performance. In both simulations and experiments, preheating the fuel with a laser enhances the neutron yield, with the maximum yield occurring at an optimal preheat laser energy. Increasing the preheat energy past the optimal value reduces the neutron yield. In simulations, the rate of the drop in neutron yield depends on the inclusion of the Nernst effect. The Nernst effect advects the magnetic field away from the center of the fuel region during the preheat stage of MagLIF on OMEGA and weakens magnetic-flux compression. If the Nernst effect is not included, the simulated drop in yield past the optimal preheat laser energy is more gradual as opposed to the sharper drop seen in experiments. Simulations are able to model trend in measured ion temperatures seen in experiments. A mix model is used to show that including mix from the wall in the simulated fuel region further degrades the yield and lowers the optimal preheat laser energy. Simulations with mix predict that increasing the initial axial magnetic field may still enhance yield performance in integrated implosions.

Investigation of surface integrity on laser pre-heat assisted diamond turning of binderless tungsten carbide

With the widespread application of binderless tungsten carbide (WC) moulds, optical finish with ideal surface integrity is in great demand. Although Pre-heat laser assisted turning (Pre-LAT) is a potential method to achieve optical surface on hard and brittle materials, there is less investigation on the surface integrity, which needs to be studied for better understanding of Pre-LAT mechanism. In this article, the critical depth of no observed surface crack on binderless WC was experimentally studied with the laser pre-heat assistance. The subsurface damage is analyzed in detail, and the material removal mechanism during Pre-LAT is subsequently revealed.

Lasergate: A windowless gas target for enhanced laser preheat in magnetized liner inertial fusion

At the Z Facility at Sandia National Laboratories, the magnetized liner inertial fusion (MagLIF) program aims to study the inertial confinement fusion in deuterium-filled gas cells by implementing a three-step process on the fuel: premagnetization, laser preheat, and Z-pinch compression. In the laser preheat stage, the Z-Beamlet laser focuses through a thin polyimide window to enter the gas cell and heat the fusion fuel. However, it is known that the presence of the few μm thick window reduces the amount of laser energy that enters the gas and causes window material to mix into the fuel. These effects are detrimental to achieving fusion; therefore, a windowless target is desired. The Lasergate concept is designed to accomplish this by “cutting” the window and allowing the interior gas pressure to push the window material out of the beam path just before the heating laser arrives. In this work, we present the proof-of-principle experiments to evaluate a laser-cutting approach to Lasergate and explore the subsequent window and gas dynamics. Further, an experimental comparison of gas preheat with and without Lasergate gives clear indications of an energy deposition advantage using the Lasergate concept, as well as other observed and hypothesized benefits. While Lasergate was conceived with MagLIF in mind, the method is applicable to any laser or diagnostic application requiring direct line of sight to the interior of gas cell targets.

Deep-learning-enabled Bayesian inference of fuel magnetization in magnetized liner inertial fusion

Fuel magnetization in magneto-inertial fusion (MIF) experiments improves charged burn product confinement, reducing requirements on fuel areal density and pressure to achieve self-heating. By elongating the path length of 1.01 MeV tritons produced in a pure deuterium fusion plasma, magnetization enhances the probability for deuterium–tritium reactions producing 11.8−17.1 MeV neutrons. Nuclear diagnostics thus enable a sensitive probe of magnetization. Characterization of magnetization, including uncertainty quantification, is crucial for understanding the physics governing target performance in MIF platforms, such as magnetized liner inertial fusion (MagLIF) experiments conducted at Sandia National Laboratories, Z-facility. We demonstrate a deep-learned surrogate of a physics-based model of nuclear measurements. A single model evaluation is reduced from O(10–100) CPU hours on a high-performance computing cluster down to O(10) ms on a laptop. This enables a Bayesian inference of magnetization, rigorously accounting for uncertainties from surrogate modeling and noisy nuclear measurements. The approach is validated by testing on synthetic data and comparing with a previous study. We analyze a series of MagLIF experiments systematically varying preheat, resulting in the first ever systematic experimental study of magnetic confinement properties of the fuel plasma as a function of fundamental inputs on any neutron-producing MIF platform. We demonstrate that magnetization decreases from BR∼0.5 to BR∼0.2 MG cm as laser preheat energy deposited increases from Epreheat∼460 J to Epreheat∼1.4 kJ. This trend is consistent with 2D LASNEX simulations showing Nernst advection of the magnetic field out of the hot fuel and diffusion into the target liner.

Scaling laser preheat for MagLIF with the Z-Beamlet laser

Optimizing the performance of the Magnetized Liner Inertial Fusion (MagLIF) platform on the Z pulsed power facility requires coupling greater than 2 kJ of preheat energy to an underdense fuel in the presence of an applied axial magnetic field ranging from 10 to 30 T. Achieving the suggested optimal preheat energies has not been experimentally achieved so far. In this work, we explore the preheat design space for cryogenically cooled MagLIF targets, which represent a viable candidate for increasing preheat energies. Using 2D and 3D HYDRA MHD simulations, we first discuss the various physical effects that occur during laser preheat, such as laser energy deposition, self-focusing, and filamentation. After identifying the changes that different phase plates, gas-fill densities, and magnetic fields bring to the aforementioned physical effects, we, then, consider higher laser energies that are achievable with modest upgrades to the Z Beamlet laser. Finally, with a 6.0-kJ upgraded laser, 3D calculations suggest that it is possible to deliver 4.25 kJ into the MagLIF fuel, resulting in an expected deuterium neutron yield of YDD≃1.5×1014, or roughly 50 kJ of DT equivalent yield, at 20-MA current drive. This represents a 10-fold increase in the currently achieved yields for MagLIF.

One-dimensional modeling and simulation of end loss effect in magnetized liner inertial fusion

Benefiting from laser preheat and magnetization, magnetized liner lnertial fusion (MagLIF) has a promising potential because theoretically it can dramatically lower the difficulties in realizing the controlled fusion. In this paper, the end loss effect caused by laser preheat in MagLIF process is chosen as an objective to explore its influences, and a one-dimensional and heuristic model of this effect is proposed based on the jet model of ideal fluid, in which the high-dimensional influences, such as geometric parameters and sausage instability, are taken into consideration. To complete the verification progress, the calculation results of one-dimensional MIST code and two-dimensional programs TriAngels and HDYRA are compared, and the application scopes of this heuristic model are discussed and summarized. Based on this model, the key parameters and influences of the end loss effect on the MagLIF implosion process and pre-heating effect are obtained. The calculation results show that the MagLIF load maintains a similar hydrodynamic evolution process in most of the implosion processes with different laser entrance radii, and experiences the same percentage of mass (～16%) lost during stagnation stage. With the same driving current, the fuel temperature will rise higher in the model with more mass losing, so the fusion yields do not change too much. The mass loss ratio seems to play a dominant role. It is recommended to design the laser entrance hole as small as possible in the experiment to increase the yield. The predictions obtained after considering the end loss effect lower the preheating temperature and fusion yield, but no change happens to the regularity trend. As the liner height increases, the preheating temperature, peak current, fuel internal energy, and fusion yield each still show a monotonically downward trend. Therefore, under the premise of fixed driving capability and laser output capability, it is suggested that the liner height in MagLIF load design should be as short as possible. The established heuristic model and conclusions are helpful in better understanding the physical mechanism in the process of MagLIF preheat and end loss.

Characterizing laser preheat for laser-driven magnetized liner inertial fusion using soft x-ray emission

Laser heating of a gas in a cylindrical liner held in by a thin foil window is a critical process in the MagLIF (magnetized liner inertial fusion) concept [S. A. Slutz and R. A. Vesey, Phys. Rev. Lett. 108, 025003 (2012)]. Window burnthrough and gas heating for OMEGA scale MagLIF cylinders as a function of time have been determined using spectrally integrated soft x-ray diagnostics. Window laser absorption is classified in terms of the emitted x-rays from the window plasma as a function of laser energy and shows that the laser energy absorbed is weakly dependent on incident intensity. Radiation–hydrodynamic simulations overestimate the amount of laser energy absorbed by the window as evidenced by the increase in x-ray radiation across several photon energy bands compared to experiments. Gas temperatures inferred from soft x-ray emission from the front 1 mm of the liner are shown to evolve in time in a similar manner to simulation predictions. Soft x-ray emission from the gas within the region of the liner that is normally imploded is shown to meet the 100 eV requirements set by the initial point design for laser-driven MagLIF.

The effect of laser entrance hole foil thickness on MagLIF-relevant laser preheat

The magnetized liner inertial fusion (MagLIF) scheme relies on coupling laser energy into an underdense fuel raising the fuel adiabat at the start of the implosion. To deposit energy into the fuel, the laser must first penetrate a laser entrance hole (LEH) foil which can be a significant energy sink and introduce mix. In this paper, we report on experiments investigating laser energy coupling into MagLIF-relevant gas cell targets with LEH foil thicknesses varying from 0.5 μm to 3 μm. Two-dimensional (2D) axisymmetric simulations match the experimental results well for 0.5 μm and 1 μm thick LEH foils but exhibit whole-beam self-focusing and excessive penetration of the laser into the gas for 2 μm and 3 μm thick LEH foils. Better agreement for the 2 μm-thick foil is found when using a different thermal conductivity model in 2D simulations, while only 3D Cartesian simulations come close to matching the 3 μm-thick foil experiments. The study suggests that simulations may over-predict the tendency for the laser to self-focus during MagLIF preheat when thicker LEH foils are used. This effect is pronounced with 2D simulations where the azimuthally symmetric density channel effectively self-focuses the rays that are forced to traverse the center of the plasma. The extra degree of freedom in 3D simulations significantly reduces this effect. The experiments and simulations also suggest that, in this study, the amount of energy coupled into the gas is highly correlated with the laser propagation length regardless of the LEH foil thickness.

Performance Scaling in Magnetized Liner Inertial Fusion Experiments.

We present experimental results from the first systematic study of performance scaling with drive parameters for a magnetoinertial fusion concept. In magnetized liner inertial fusion experiments, the burn-averaged ion temperature doubles to 3.1keV and the primary deuterium-deuterium neutron yield increases by more than an order of magnitude to 1.1×10^{13} (2kJ deuterium-tritium equivalent) through a simultaneous increase in the applied magnetic field (from 10.4 to 15.9T), laser preheat energy (from 0.46 to 1.2kJ), and current coupling (from 16 to 20MA). Individual parametric scans of the initial magnetic field and laser preheat energy show the expected trends, demonstrating the importance of magnetic insulation and the impact of the Nernst effect for this concept. A drive-current scan shows that present experiments operate close to the point where implosion stability is a limiting factor in performance, demonstrating the need to raise fuel pressure as drive current is increased. Simulations that capture these experimental trends indicate that another order of magnitude increase in yield on the Z facility is possible with additional increases of input parameters.

Constraining preheat energy deposition in MagLIF experiments with multi-frame shadowgraphy

A multi-frame shadowgraphy diagnostic has been developed and applied to laser preheat experiments relevant to the Magnetized Liner Inertial Fusion (MagLIF) concept. The diagnostic views the plasma created by laser preheat in MagLIF-relevant gas cells immediately after the laser deposits energy as well as the resulting blast wave evolution later in time. The expansion of the blast wave is modeled with 1D radiation-hydrodynamic simulations that relate the boundary of the blast wave at a given time to the energy deposited into the fuel. This technique is applied to four different preheat protocols that have been used in integrated MagLIF experiments to infer the amount of energy deposited by the laser into the fuel. The results of the integrated MagLIF experiments are compared with those of two-dimensional LASNEX simulations. The best performing shots returned neutron yields ∼40–55% of the simulated predictions for three different preheat protocols.

Diagnosing and mitigating laser preheat induced mix in MagLIF

A series of Magnetized Liner Inertial Fusion (MagLIF) experiments have been conducted in order to investigate the mix introduced from various target surfaces during the laser preheat stage. The material mixing was measured spectroscopically for a variety of preheat protocols by employing mid-atomic number surface coatings applied to different regions of the MagLIF target. The data show that the material from the top cushion region of the target can be mixed into the fuel during preheat. For some preheat protocols, our experiments show that the laser-entrance-hole (LEH) foil used to contain the fuel can be transported into the fuel a significant fraction of the stagnation length and degrade the target performance. Preheat protocols using pulse shapes of a few-ns duration result in the observable LEH foil mix both with and without phase-plate beam smoothing. In order to reduce this material mixing, a new capability was developed to allow for a low energy (∼20 J) laser pre-pulse to be delivered early in time (−20 ns) before the main laser pulse (∼1.5 kJ). In experiments, this preheat protocol showed no indications of the LEH foil mix. The experimental results are broadly in agreement with pre-shot two-dimensional HYDRA simulations that helped motivate the development of the early pre-pulse capability.

Self-Similar Solutions With Electrothermal Processes for Plasmas of Arbitrary Beta

Self-similar magnetohydrodynamic (MHD) solutions are developed for a cold planar wall next to a hot plasma with an embedded magnetic field parallel to the wall, including the relevant electrothermal terms. Velikovich et al. (Phys. Plasmas, vol. 22, no. 4, 042702, 2015) studied the Nernst electrothermal effect in the induction equation for such a problem under the assumption that the ratio of thermal to magnetic pressure ( $\beta$ ) was large. Other electrothermal processes, such as the Ettingshausen and plasma Thomson effects, vary inversely with $\beta$ , as does Joule heating, and may impact the plasma evolution at low $\beta$ . To study all these processes, we have extended the self-similar formulation to allow for an arbitrary $\beta$ . The self-similar ansatz requires constant total pressure, i.e., thermal plus magnetic. Self-similar solutions are presented with the density and temperature boundary conditions characteristic of the plasma immediately following laser preheat in the Magnetized Liner Inertial Fusion experiment, and with the far-field magnetic field equal to that at the wall. Four cases are examined: two with a large $\beta$ and two with a moderate $\beta$ , and for each of these with and without electrothermal terms. Solutions for the density, temperature, and velocity depend on the choice of $\beta$ but are largely insensitive to the electrothermal effects. However, for the same value of $\beta$ , whether large or moderate, the profile of the magnetic field changes significantly when the electrothermal effects are included. In the case of moderate $\beta$ and electrothermal effects, the magnetic field profile has two extrema. This interesting case is further simulated with a 1-D MHD code that allows for total pressure changes through the momentum equation. Comparison with the self-similar solution shows that the simulation results approach the self-similar solution between the wall and an outward propagating rarefaction wave. The solutions are proposed as verification tests for advanced, multiphysics MHD codes.

Evolution of Gas Cell Targets for Magnetized Liner Inertial Fusion Experiments at the Sandia National Laboratories PECOS Test Facility

Z-beamlet experiments conducted at the PECOS test facility at Sandia National Laboratories (SNL) investigated the nonlinear processes in laser plasma interaction (or laser-plasma instabilities) that complicate the deposition of laser energy by enhanced absorption, backscatter, filamentation, and beam-spray that can occur in large-scale laser-heated gas cell targets. These targets and experiments were designed to provide better insight into the physics of the laser preheat stage of the Magnetized Liner Inertial Fusion scheme being tested on the SNL Z-machine. The experiments aim to understand the trade-offs between laser spot size, laser pulse shape, laser entrance hole window thickness, and fuel density for laser preheat. Gas cell target design evolution and fabrication adaptations to accommodate the evolving experiment and scientific requirements are described in this paper.

Quantitative texture prediction of epitaxial columnar grains in additive manufacturing using selective laser melting

Metal additive manufacturing (AM) such as selective laser melting (SLM) has the powerful capability to produce very different microstructural features, hence different mechanical properties in metals using the same feedstock material but different values of process parameters. However, the relation between processing-microstructure is mostly investigated by experiments, which is expensive and time-consuming since the parameter space is quite large. The lack of a reliable theoretical model of the processing-microstructure relationship of AM material is preventing AM technology from being widely adopted by the manufacturing community. Hence, the goal of this work is to establish the link between the microstructure (texture) and the process parameters (laser power, scanning speed, preheat and scanning strategy) of a metal SLM process. To achieve the above goal, a quantitative semi-empirical method is proposed to predict the texture of the epitaxial columnar grains grown from polycrystal substrates. Combined with the melt pool prediction by the Rosenthal solution, the processing and microstructure were linked together quantitatively. The proposed method is used to estimate the texture evolution with the number of layers for EOS-DMLS-processed AlSi10Mg (unidirectional scanning direction in one layer and no rotation of scanning direction between layers). The texture reaches a steady state after five layers, and the steady state texture has similar pattern and intensity to that obtained from the experiment using the same process parameter values and scanning strategy.

Evolution of surface structure in laser-preheated perturbed materials.

We report an experimental and computational study investigating the effects of laser preheat on the hydrodynamic behavior of a material layer. In particular, we find that perturbation of the surface of the layer results in a complex interaction, in which the bulk of the layer develops density, pressure, and temperature structure and in which the surface experiences instability-like behavior, including mode coupling. A uniform one-temperature preheat model is used to reproduce the experimentally observed behavior, and we find that this model can be used to capture the evolution of the layer, while also providing evidence of complexities in the preheat behavior. This result has important consequences for inertially confined fusion plasmas, which can be difficult to diagnose in detail, as well as for laser hydrodynamics experiments, which generally depend on assumptions about initial conditions in order to interpret their results.

Design of magnetized liner inertial fusion experiments using the Z facility

The magnetized liner inertial fusion concept has been presented as a path toward obtaining substantial thermonuclear fusion yields using the Z accelerator [S. A. Slutz et al., Phys. Plasmas 17, 056303 (2010)]. We present the first integrated magnetohydrodynamic simulations of the inertial fusion targets, which self-consistently include laser preheating of the fuel, the presence of electrodes, and end loss effects. These numerical simulations provided the design for the first thermonuclear fusion neutron-producing experiments on Z using capabilities that presently exist: peak currents of Imax = 18–20 MA, pre-seeded axial magnetic fields of Bz0=10 T, laser preheat energies of about Elas = 2 kJ delivered in 2 ns, DD fuel, and an aspect ratio 6 solid Be liner imploded to 70 km/s. Specific design details and observables for both near-term and future experiments are discussed, including sensitivity to laser timing and absorbed preheat energy. The initial experiments measured stagnation radii rstag<75 μm, temperatures around 3 keV, and isotropic neutron yields up to YnDD=2×1012, with inferred alpha-particle magnetization parameters around rstag/rLα=1.7 [M. R. Gomez et al., Phys. Rev. Lett. (submitted)].

Deposition Behavior at Different Substrate Temperatures by Using Supersonic Laser Deposition

The supersonic laser deposition (SLD) is a new fabrication process which combines the supersonic powder stream found in cold spray with laser heating of the deposition zone. Because of the instantaneity of particles impact, the deformation behaviors and the adhesion behaviors of particles impacted on the substrate by SLD cannot be well investigated through experiments. Therefore, a finite element model was developed to solve the problems above. Meanwhile, the heat effect of the substrate heated by laser was discussed. The effective plastic strain and the effective stress between the particle and substrate at different laser preheat temperatures were studied. The results show that laser depositing temperatures of 1000 and 1100 °C on substrate would be the optimized for the bonding of particles and substrate. In addition, the simulation results conformed to experimental results.