Session CO05: ICF: Holraums and Magnetic Fields

Live

The quartraum platform is a high-Z cylinder with a back window (or wall) which is typically one-quarter the length of a full hohlraum deployed at the National Ignition Facility (NIF). When coupled to NIF geometry, laser beams enter through a laser entrance hole (LEH) located on the cylinder endcap opposite the back wall, blow down the window covering that LEH, and propagate into the target, which is filled with helium gas. NIF's outer beams strike the inside of the barrel of the cylinder, and the inner beams strike the back wall. Thin-wall imaging techniques are utilized to deduce laser intensity at the walls of the quartraum through a calibration of laser-to-x-ray intensity.[1] This platform is under consideration as a vehicle to research laser entrance hole physics, provided such a platform can briefly emulate plasma conditions in an ignition target at the laser entrance hole. Results and design analyses will be presented, with proposed improvements and enhancements. [1] L. A. Pickworth \textit{et al.}, High Energy Density Physics \textbf{23} (2017) 159-166.   

Live

A major upgrade of the gated laser-entrance-hole imager on the National Ignition Facility (NIF) now allows for 8 frames of 1-2 ns integration time gated at variable intervals.~ The frames show much improved uniformity and the diagnostic is now radiation hardened for high yield shots. These new capabilities enable imaging the hohlraum LEH over the entire time history of a NIF inertial confinement fusion (ICF) experiment,~from the low power ``foot'' of the laser drive through capsule stagnation after the laser turns off. Example data and their interpretation from a range of NIF experiments will be presented, contributing towards a better understanding of NIF hohlraum physics.~ This work was performed under the auspices of the U.S. Department of Energy by LLNS, LLC, under Contract No. DE-AC52- 07NA27344.   

Live

A more complete understanding of laser-driven hohlraum plasmas is critical for the continued development and improvement of indirect-drive ICF experiments. For such plasmas, hydrodynamic calculations are very successful in describing the evolution of the plasma at early times. However, at late times kinetic effects become dominant and the hydrodynamic description is insufficient. In these hohlraums, self-generated electric and magnetic fields also play an important role in determining plasma dynamics and evolution; however, it has largely been uncertain whether electric fields or magnetic fields dominate these systems. To explore this question, we conducted several experiments at OMEGA, using tri-particle monoenergetic proton and deuteron radiography to probe laser-driven vacuum-filled gold and plastic hohlraums. In our analysis, we utilized reconstructive methods ¬to infer information about the structure of electromagnetic fields in the hohlraum, as well as to quantify the relative magnitudes of proton deflections due to electric and magnetic fields, respectively.   

Live

In ICF hohlraums, high laser intensities drive multiple types of instabilities capable of accelerating electrons to supra-thermal velocities. These hot electrons I nteract with the hohlraum plasma and walls to produce hard x-ray bremsstrahlung emission that can be seen in various diagnostics like NIF's FFLEX. This hot electron flux can be confined by spontaneously generated magnetic fields within the hohlraum plasma and result in x-ray spectra with features that give information about the magnetic and electric fields within the hohlraum. Recent work by Dewald et al. [PRL 116, 075003 (2016)] has shown that for low gas fill hohlraums during the picket pulse, the predominant LPI mechanism results in a highly directional hot electron flux emanating from the laser entrance hole, which is simpler to model than the main pulse. In this work, we develop a model for plasma bremsstrahlung emission in this regime and compare it to measured FFLEX data to provide physical insight into the features seen.   

Live

Hohlraums with lower fill pressure (\textless 0.6 mg/cc) have been used frequently because of their improved energy coupling. However, because of less tamping, plasma ablated off the hohlraum wall and the capsule fills the interior of the hohlraum at the later stage of the pulse. The inner cone beams are absorbed by inverse Bremsstrahlung, which has a steep temperature and density dependence. This change of power delivery can interfere with the uniform compression of the capsule. The plasma conditions of this hohlraum and ablator plasma strongly depends on heat transport. To validate the heat transport models used in out simulations, we measure the electron temperature of the wall plasma using mid-Z dopants in a part of the hohlraum wall. X-ray line spectrum produced is compared with simulations. We also developed a semi-empirical model of inner cone beam obscuration. This model facilitates understanding how the hohlraum fill pressure affects the time transport of the inner cone beams.   

Live

A model with a fixed set of physics assumptions and resolution was employed to create radiation sources to drive very high resolution calculations of several high yield NIF implosions. We will discuss the justification for the various model choices. The current model agrees fairly well with smaller scale hohlraums that typically employ lower laser intensities, but drive errors increase with hohlraum scale. We will discuss the extent to which the model agrees with radiation drive magnitude, spectrum, and angular dependence (inferred from the low mode shape of various types of capsule implosion data). The model must be adjusted at some level to adequately match the available drive and low mode shape tuning data for a given DT implosion. The fuel velocity and fuel adiabat are sensitive to how well the data are matched. The sources of random low-mode asymmetry in NIF capsule implosions are the subject of a separate investigation. Here, we focus only on the lowest even Legendre mode asymmetry in the radiation drive, P2. We compare the model's P2 predictions to data and empirical scaling curves. Recently we have updated the computational grid to include the hardware rings that clamp the laser entrance hole windows to the hohlraum. This external hardware interacts with the incoming laser. We show that it can change the P2 drive symmetry by altering the calculated crossed beam energy transfer and improves the agreement with shape data.   

Live

Reaching high radiation temperatures (~300 eV) is important in indirect-drive, ICF hohlraums because the capsule absorbed energy scales as radiation temperature to the fourth power. Simulating drive in a hohlraum using a radiation-hydrodynamics code is complicated because of the variety of physics processes that go into determining the drive – x-ray conversion, wall opacity (LTE and NLTE), heat transport, laser-plasma-instabilities, and hydrodynamic motion of the wall and laser entrance hole. While progress continues in improving our hohlraum simulations, we have developed a simpler model that can be used to estimate the radiation temperature in the hohlraum, based on the laser pulse shape and hohlraum geometry. This model, while simple, can capture the drive in many of ICF experiments on NIF. In this talk, we will describe the model and its applicability, and compare with NIF data.   

Live

The first magnetized, hohlraum-driven implosions on NIF -- or any facility -- are planned for the end of 2020. The goal is to demonstrate field compression in the implosion, following the MHD frozen-in law, which will be diagnosed by improved capsule nuclear performance (higher yield and temperature). These will be deuterium gas-filled capsules based on the ``BigFoot'' campaign, and use about 900 kJ of laser energy (about half of NIF's maximum). To facilitate soak-through of the externally-imposed, predominantly axial field, especially against inward radial JxB motion, a hohlraum with much higher electrical resistivity than the typical gold or uranium is needed. A novel gold-tantalum alloy has been developed for this purpose, with the first shots using a 2:8 Au:Ta mixture (atomic). We present radiation-MHD modeling of this design with the Lasnex code. Based on this, we expect DD neutron yields (2-4) x 10$^{\mathrm{13}}$ and burn-weighted ion temperature of 3.5 -- 4 keV, with an imposed 30 Tesla field. This should be 30-50{\%} higher in yield, and 0.5 keV in T$_{\mathrm{ion}}$, than an unmagnetized analog. The first two shots will vary the laser cone fraction (inner-beam vs. total power), to tune the implosion shape, and the third shot will be an unmagnetized repeat of one of these shots.   

Live

An external seed magnetic field applied to an inertial confinement fusion (ICF) indirect drive target is expected to increase the ion temperature by \(\ge\) 0.5 keV and the neutron yield by 30 – 50\% due to reduced electron thermal conduction. Room temperature implosions using a 30 T seed field and a D2 filled capsule will start in the fall of this year. Multiple changes to the hohlraum target and the NIF facility are required to successfully apply a sufficient field to the fuel capsule at shot time. We describe progress on developing a high electrically resistive hohlraum, the experiment design, effects of magnetization on shock propagation and dynamic effects on the cryogenic DT fuel layer.   

Live

The contributions of the electrothermal and resistivity tensors to magnetic-field evolution in inviscid magnetohydrodyamics are considered. Resistivity leads to anisotropic advection diffusion, with advection due to resistivity gradients that moves the magnetic field to regions of lower resistivity, and an increase in the Hall term. The electrothermal tensor leads to magnetic-field generation due to gradients in effective$ Z$ perpendicular to electron temperature gradients, and advection of the magnetic field with electron heat flow. Braginskii's fits do not provide adequate information for $Z$ \textgreater 4, overestimate the perpendicular resistivity, and underestimate advection with perpendicular electron heat flow (Nernst) at $Z \quad =$ 1. Epperlein--Haines' fits give perpendicular resistivity that is discontinuous in $Z$, causing errors in the gradient, and overestimate advection with cross-field electron heat flow. Ji and Held\footnote{ J.-Y. Ji and E. D. Held, Phys. Plasmas \textbf{20}, 042114 (2013).} give the most accurate fits, valid for any $Z$ from 0 to 100. None of the fits give adequate results for advection with cross-field electron heat flow, so a new fit is provided based on a direct numerical solution of the Fokker--Planck equation. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856.   

Live

Simulations with a 50 T seed B-field is observed to make the requirements for ignition less stringent. (Ref 1) For high-yield implosions, a seed field does not increase the yield but increases the the implosion robustness. This allows implosion designs to consider trade-offs between reducing the laser energy while preserving the implosion robustness and yield, or using the same laser energy but increasing the implosion robustness and yield by using a larger capsule/hohlraum system. We also report novel MHD features here. (1) The ellipticity of the prolate imploding shock in magnetized implosions is higher for ice-layered capsules than for warm symcaps because of the difference in MHD shock propagation behavior. Analytical theory agrees well with simulations for the shape of the prolate shock. (2) The ellipticity of the shock creates an oblate shape in the hotspot ion and electron temperature contours. The elongation of the oblate shape is more pronounced for ice-layered capsule than for warm symcap. 1 Perkins, Ho, Logan, Zimmerman et al., Phys. Plasmas 24, 062708 (2017).   

Live

The Rayleigh-Taylor (RT) and Darrieus-Landau (DL) instabilities are discussed in an inertial confinement fusion context within the framework of small critical-to-shell density ratio and weak acceleration regime. The quasi-isobaric analysis in Sanz et al. [Phys. Plasmas 13, 102702 (2006)] is completed with the inclusion of non-isobaric and self-generated magnetic field effects. The use of a Sharp Boundary Model leads to a single analytical expression of the dispersion relation encompassing both instabilities. The two new effects come into play by modifying the perturbed mass and momentum fluxes at the ablation front. The momentum flux (perturbed pressure at the spike) is the predominant stabilizing mechanism in the RT limit and the driving mechanism in the DL limit. The non-isobaric effects modify notably the scaling laws in the DL limit (k\textless \textless 1). The magnetic fields are generated due to misalignment between pressure and density gradients (Biermann Battery effect). They affect the hydrodynamics by bending the heat flux lines. Within the framework of this paper, they enhance ablation, resulting in a stabilizing effect that peaks for perturbation wavelengths comparable to the conduction layer width.   

Live

The pre-magnetization of inertial confinement fusion capsules is a promising avenue for reaching ignition as magnetic fields reduce electron thermal conduction losses during hotspot formation. However, to reach very high yields the burn-up of remaining cold fuel must be as efficient as possible. This work investigates the potential suppression of burn in a magnetized plasma utilizing the radiation-MHD code ‘Chimera’, developed at Imperial College London. This code includes extended MHD effects, such as the Nernst term, and a Monte-Carlo model for magnetized alpha particle transport and heating. Simulations are carried out in planar geometry to investigate burn front dynamics. 1D simulations show a reduction in burn propagation rate with increasing magnetization. These studies also show the possibility of forming a ‘transport barrier’ where electron magnetization grows faster at the burn front than in the hot fuel, suppressing heat flow through it. This barrier may grow unstably due to the action of Nernst and magnetic field advection. 2D simulations are used to study the evolution of this transport barrier and how it is affected by hydrodynamical instabilities. Integrated capsule simulations are also carried out to investigate the relevance of these effects to ICF experiments.   

Live

Magnetization of ICF implosions is a pathway to increasing fusion yields by reducing thermal hot-spot losses [1,2]. However, simulations presented here indicate that high-gain spherical direct-drive implosions require greater constraints on the laser heating uniformity when magnetized. At the capsule pole, where the magnetic field is normal to the ablator surface, the field remains in the conduction zone and suppresses non-radial thermal conduction; in unmagnetized implosions this non-radial heat-flow is crucial in mitigating laser heating imbalances. Single-mode simulations show the magnetic field particularly amplifying short wavelength perturbations, whose behavior is dominated by thermal conduction. The most unstable wavelength can also become shorter. 3D multi-mode simulations of the capsule pole reinforce these findings, with increased perturbation growth anticipated across a wide range of scales. Potential experiments to verify these results will be proposed, including a magnetic field applied normal to an ablating foil. [1] -- Chang et al, Physical Review Letters 107 035006 (2011) [2] -- Walsh et al, Physics of Plasmas 26 022701 (2019)   

Live

The development of advanced targets capable of achieving ignition, with improved energy gain at lower driver energies, is one of four key technical challenges to be solved in order to realize economical inertial fusion energy [J. H. Nuckolls, J. Phys.: Conf. Ser. 244, 012007 (2010)]. We determine the minimum energy necessary for a small hemispherical mass of high-density DT fuel to explosively ignite a significantly larger hemispherical mass of separately assembled cold fuel with much lower mass density. Propagating fusion burn sensitivity to the rapid alpha-particle and conduction heating, as well as to a flux-compressed magnetic field connecting the two regions, has been investigated. As the strength of the magnetic field increases, thermal conduction losses are suppressed and alpha-particle propagation is better confined, so the burn rate improves and lower energy states become more effective. The imploded fuel reservoir available in the lower-density, larger-mass region of the steep density gradient determines whether the yield is several MJ or up to a GJ. The improved energy gain over the conventional spark-ignited approach is presented, as well as a discussion of integrated design simulations to realize the desired stagnation state.