Session PO7: Hohlraum and X-Ray Cavity Physics II

Hybrid strategy for increasing fusion performance and stagnation pressure in x-ray driven inertially confined fusion implosions on the NIF

Post NIC (2012), more stable and lower convergence implosions were developed and used as part of a `basecamp' strategy to identify obstacles to further performance. From 2013-2015 by probing away from a conservative working implosion {\it in-steps} towards conditions of higher velocity and compression, `Fuel Gain' and alpha-heating were obtained. In the process, performance cliffs unrelated to `mix' were identified the most impactful of which were symmetry control of the implosion and hydro seeded by engineering features. From 2015-2017 we focused on mitigating poor symmetry control and engineering improvements on fill-tubes and capsule mounting techniques. The results were more efficient implosions that can obtain the same performance levels as the earlier implosions, but with less laser energy. Presently, the best of these implosions is poised to step into a burning plasma state. Here, we describe the next step in our strategy that involves using the data we've acquired across parameter space to make a step to the largest symmetric implosions that can be fielded on NIF with the energy available. We describe the key principles that form the foundation of this approach.   

New and improved CH implosions at the National Ignition Facility

Improvements to the hohlraum for CH implosions [1] have resulted in near-record hot spot pressures, \textasciitilde 225 Gbar.~ Implosion symmetry and laser energy coupling are improved by using a hohlraum that, compared to the previous high gas-fill hohlraum [2], is longer, larger, at lower gas fill density, and is fielded at zero wavelength separation to minimize cross-beam energy transfer [3].~ With a capsule at 90{\%} of its original size in this hohlraum, implosion symmetry changes from oblate to prolate, at 33{\%} cone fraction. Simulations highlight improved inner beam propagation as the cause of this symmetry change. These implosions have produced the highest yield for CH ablators at modest power and energy, i.e., 360 TW and 1.4 MJ. Upcoming experiments focus on continued improvement in shape as well as an increase in implosion velocity. Further, results and future plans on an increase in capsule size to improve margin will also be presented. [1] D. E. Hinkel \textit{et al}., \textit{Phys. Rev. Lett.} \textbf{118}, 089902 (2017). [2] O. A. Hurricane \textit{et al.,} \textit{Nature} \textbf{506}, 342, (2014). [3] P. Michel \textit{et al.,} \textit{Phys. Rev. Lett. }\textbf{102}, 025004 (2009).   

Design and Follow-on from \textasciitilde 50 kJ Fusion Yield using High-Density Carbon Capsules at the National Ignition Facility

We have demonstrated nearly 3x alpha-heating at the National Ignition Facility by using tungsten-doped High-Density Carbon (HDC) capsules in low gasfill, unlined, uranium (DU) hohlraums. Shot N170601 achieved a primary neutron yield of 1.5 x 10$^{\mathrm{16}}$ neutrons with a Deuterium-Tritium ion temperature of 4.7 keV. Predecessor experiments demonstrated high-performing, efficient performance, as noted through high neutron yield production per laser energy input. Building on these `subscale' results, follow-on experiments utilize an 8{\%} larger target than the predecessor campaign, to increase the capsule surface area and absorbed energy. The capsule fill tube has been reduced in size from 10 to 5 micron diameter, and the laser design implements a new, ``drooping'' technique for the end of the pulse, to reduce the time between laser shut-off and capsule peak emission while still maintaining capsule mass remaining. Design of the current platform as well as avenues to potentially improve performance based on these experiments will be discussed.   

Experimental measurements of the conditions of implosion reaching \textasciitilde50 KJ of fusion yield using High Density Carbon on the National Ignition Facility

Building on our experimental and modelling effort over the last three years, we have found a capsule/hohlraum combination enabling us to drive a symmetrical implosion to convergence 27 with minimal Laser Plasma Interaction (LPI). The experimental platform consists of a low gas fill unlined DU hohlraum driving a W-doped High-Density Carbon (HDC) capsule. With the symmetry in control, the campaign is now moving forward on increasing the neutron yield by increasing the energy absorbed by the capsule. A series of experiments have been carried out first to test how the symmetry of the implosion was preserved at larger capsule and hohlraum scale and then to test the performance of the high convergence cryogenic DT-layered implosion. Using this platform, a record primary neutron yield of 1.47e16, with a DSR of 3.29{\%} an ion temperature of 4.7 keV at 1.55 MJ was achieved (shot N170601). Details of the implosion conditions of the high performer shot will be presented. This work was performed under the auspices of the U.S. Department of Energy by the Lawrence Livermore National Laboratory under contract DE-AC52-07NA27344   

Assessment of the impact that the capsule fill tube has on implosions conducted with high density carbon ablators

In recent inertial confinement implosion experiments conducted at the National Ignition Facility, bright and spatially localized x-ray emission within the hot spot at stagnation has been observed. This emission is associated with higher Z ablator material that is injected into the hot spot by the hydrodynamic perturbation induced by the 5-10 um diameter capsule fill tube. The reactivity of the DT fuel and subsequent yield of the implosion are strongly dependent on the density, temperature, and confinement time achieved throughout the stagnation of the implosion.~ Radiative losses from higher Z ablator material that mixes into the hot spot as well as non-uniformities in the compression and confinement induced by the fill tube perturbation can degrade the yield of the implosion.~ This work will examine the impact to conditions at stagnation that results from the fill tube perturbation. This assessment will be based from a pair of experiments conducted with a high density carbon ablator where the only deliberate change was reduction in fill tube diameter from 10 to 5 um. An estimate~of the radiative losses and impact on performance from ablator mix injected into the hot spot by the fill tube perturbation will be presented.~   

Symmetry control strategies in low gas-fill hohlraum

The primary neutron yield record, to-date, for an ICF implosion on the NIF (1.47*1016) has been achieved using a doped HDC capsule (D$=$1.82 mm) in an unlined DU hohlraum (D$=$6.20 mm, L $=$ 11.3 mm) filled with a low He gas-fill (0.3 mg/cc). This platform uses a new ``drooping'' pulse designed to keep high remaining mass and short coasting time. Prior to the high convergence (27x) cryogenic DT implosion, our ability to tune hot spot symmetry using this new pulse was tested at lower convergence (15x) using DD gas-filled capsules. Hot spot symmetry was tuned using beam pointing, gas-fill density, and power balance between outer and inner beams. The main metrics to assess the efficiency of each change are the implosion shape (time resolved X-ray emission of the hot spot) and DD neutron yield. In addition, we will describe the irradiation pattern obtained in each case using X-ray (soft and hard) diagnostics and the laser coupling to the hohlraum.   

Testing low-mode symmetry control with low-adiabat, extended pulse-lengths in BigFoot implosions on the National Ignition Facility

The Bigfoot approach to indirect-drive inertial confinement fusion (ICF) has been developed as a compromise trading high-convergence and areal densities for high implosion velocities, large adiabats and hydrodynamic stability. Shape control and predictability are maintained by using relatively short laser pulses and merging the shocks within the DT-ice layer. These design choices ultimately limit the theoretically achievable performance, and one strategy to increase the 1-D performance is to reduce the shell adiabat by extending the pulse shape. However, this can result in loss of low-mode symmetry control, as the hohlraum ``bubble,'' the high-Z material launched by the outer-cone beams during the early part of the laser pulse, has more time to expand and will eventually intercept inner-cone beams preventing them from reaching the hohlraum waist, thus losing equatorial capsule drive. We report on experimental results exploring shape control and predictability with extended pulse shapes in BigFoot implosions. Prepared by LLNL under Contract DE-AC52-07NA27344.   

Simultaneous visualization of hohlraum-wall motion and inner-cone beam transport using mid-Z tracers and thin-wall patches at the National Ignition Facility

The shorter drive of the High-density carbon (HDC) ablator design allows us to use Intermediate gas-Fill Hohlraums (IFH, 0.3 \textasciitilde 0.6 mg/cc). Due to its reduced initial electron density, IFHs have lower backscatter, lower hot-electrons, and do not require CBET for radiation symmetry control. However, reduced tamping by the hohlraum gas allows more expansion of the hohlraum wall and the ablator. Therefore, the beam transport can be affected by the plasma filling of the hohlraum and the drive symmetry can be altered dynamically. We developed a method to visualize the energy deposition of the inner-cone beams by using thin-wall patches on the hohlraum. The inner-cone beams absorbed on the gold wall create \textasciitilde 11 keV x-rays which are imaged though the thin-wall patches on the equator of the hohlraum. Clipping and absorption of the inner cone beams in the hohlraum is clearly observed with temporal resolution. Comparison of experimental data and rad-hydro simulation will be presented.   

Characterizing NIF hohlraum energy and particle transport using mid-Z spectroscopic tracer materials

Line emission from mid-Z dopants placed at several spatial locations is used to determine the electron temperature ($T_e$) and plasma flow in NIF hohlraums. Laser drive ablates the dopant and launches it on a trajectory recorded with a framing camera. Analysis of temporally streaked spectroscopy provides an estimate of the time-resolved $T_e$. The estimated temperature gradients show evidence for significantly restricted thermal conduction. Non-local thermal conductivity can account for part of this; additional effects due to magnetic fields, return-current instabilities, ion acoustic turbulence and other physics are considered. We describe our findings and discuss interpretations. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.   

Experimental study of the plasma conditions in gas-filled hohlraum

Hohlraum plasma and its kinetic behavior are vital to study the laser heated hohlraum, affecting the temporal, spatial and spectral features of the x-ray source. Accurate measurements of the plasma conditions in gas-filled hohlraum have been achieved using the recently set-up 4$\omega $ Thomson scattering diagnostic at Shenguang-III prototype laser facility. The plasma evolution and kinetic behavior for different locations inside the hohlraum are explored through compareing the theoretical Thomson-scattering spectra based on radiation hydrodynamic code to the experiment result.   

Spherical hohlraum energetics studies on the SG series laser facility

The integrated experiments at the National Ignition Facility indicates that the radiation asymmetry control in the cylindrical hohlraums is an extremely challenging problem in achieving ignition by using indirect drive. Recently, Lan et al. proposed the octahedral spherical hohlraum which has the natural superiority in providing high radiation symmetry. As new and promising hohlraums, the performance of spherical hohlraum attracts much research interests. Hohlraum energetics is one of the fundamental problems in indirect drive inertial confinement study. We report on the spherical hohlraum experiments performed at the SG series laser facility. At the SGIII-prototype laser facility, we performed the first spherical energetics experiment. The radiation temperature is measured by using an array of flat-response x-ray detectors through a laser entrance hole at different angles. The radiation temperature and M-ban fraction inside the hohlraum are determined by the shock wave technique. At the SGIII laser facility, we performed the first octahedral spherical hohlraum energetics experiment. The 32 of 48 laser beams enter the hohlraum through six laser entrance holes. The radiation flux is measured by 5 FXRDs at different angles. And the radiation temperature inside the hohlraum is determined by the shock wave technique. The repetition of the experimental results is excellent.   

BigFoot: a program to reduce risk for indirect drive laser fusion

The conventional approach to inertial confinement fusion (ICF) with indirect drive is to design for high convergence (40), DT areal density, and target gain. By construction, this strategy is challenged by low-mode control of the implosion (Legendre P2 and P4), instability, and difficulties interpreting data. Here we consider an alternative - an approach to ICF that emphasizes control. To begin, we optimize for hohlraum predictability, and coupling to the capsule. Rather than focus on density, we work on making a high-energy hotspot we can diagnose and ``tune'' at low convergence (20). Though gain is reduced, this makes it possible to study (and improve) stagnation physics in a regime relevant to ignition (1E16-1E17). Further improvements can then be made with small, incremental increases in areal density, target scale, etc. Details regarding the ``BigFoot'' platform and pulse are reported, including recent findings. Work that could enable additional improvements in capsule stability and hohlraum control will also be discussed. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.   

Low fuel convergence path to ignition on the NIF

A novel concept for achieving ignition on the NIF is proposed that obviates current issues plaguing single-shell high-convergence capsules. A large directly-driven Be shell is designed to robustly implode two nested internal shells by efficiently converting 1.7MJ of laser energy from a 6 ns, low intensity laser pulse, into a 1 ns dynamic pressure pulse to ignite and burn a central liquid DT core after a fuel convergence of only 9. The short, low intensity laser pulse mitigates LPI allowing more uniform laser drive of the target and eliminates hot e-, preheat and laser zooming issues. Preliminary rad-hydro simulations predict ignition initiation with 90{\%} maximum inner shell velocity, before deceleration Rayleigh-Taylor growth can cause significant pusher shell mix into the compressed DT fuel. The gold inner pusher shell reduces pre-ignition radiation losses from the fuel allowing ignition to occur at 2.5keV. Further 2D simulations show that the short pulse design results in a spatially uniform kinetic drive that is tolerant to variations in laser cone power. A multi-pronged effort, in collaboration with LLE, is progressing to optimize this design for NIF's PDD laser configuration.