3D hohlraum model assists indirect drive implosions at NIF



This graph on the left shows the angular distribution of the measured neutron efficiency against direction in spherical polar coordinates as a colored plot. The red dot represents a region of the compressed fuel that is thinner and has a higher detected neutron efficiency. The graph on the right shows parts of the simple pattern which approximates 192 beam spots and 2-3 diagnostic windows on the hohlraum wall that produce a net mode 1 in the capsule which is correlated with the direction and amplitude of the hot spot speed vector.

Scientists from Lawrence Livermore National Laboratory (LLNL) and the Laser energy laboratory (LLE) described a simple 3D model in hohlraums and capsules for inertial confinement fusion (ICF) implosions. The model will help provide the required implosion symmetry on deuterium-tritium (DT) layered implosions for ignition.

The results of the work remove much of the mystery associated with the observed variability in the direction and magnitude of hot spot velocity of layered DT implosions in ignition experiments.

Brian MacGowan, LLNL scientist and lead author of the article featured in High energy density physics, said the findings reveal an important source of 3D asymmetry in ICF implosions and establish the framework through which all of the known root causes were discovered.

“The paper quantifies the sensitivity of the measured velocity of the compressed hot spot in DT implosions in cryogenic layers, to the mode 1 asymmetry of the X-ray flux at the capsule level, in the indirectly driven hohlraums, which are laser-heated cavities that produce an X-ray of radiation that implodes a capsule filled with deuterium, ”he said.

A mode 1 asymmetry of 1% implies that the extremely symmetrical ablation of the capsule surface by x-rays actually pushes 1% harder in a particular direction rather than being spherically symmetrical. This difference is sufficient for the hot spot to have a residual speed of up to 100 km / s when the imploding shell stagnates.

The paper also establishes a method to understand the variation of the mode 1 asymmetry of the flux at the capsule level due to variations in measured laser performance, known target construction and expected variability in alignment. beam and target.

“We can relate the asymmetry in the delivery of the laser and the construction of the target to the asymmetry of the X-ray flow at the capsule level which implodes to generate the conditions necessary for nuclear fusion,” he said. declared. “Prior to this analysis, the variability of asymmetry in the compressed hotspot was considered idiopathic, eg without explanation. Now, this is something that can be understood and potentially controlled.

Understanding the 3D hohlraum models of asymmetries in implosions

MacGowan says it’s important to understand the sources of 3D asymmetry in hohlraums and capsules for ICF implosions in order to provide the required implosion symmetry on layered DT implosions for ignition.

Poor symmetry during the convergence of the envelope leads to reduced pressure and reduced confinement in the hot spot and therefore reduced neutron efficiency. The main diagnosis of mode 1 symmetry in a DT shell implosion is the velocity of the neutrons emitted by the hotspot. This velocity has been found to be variable in amplitude and direction and is an indicator of the hot spot’s loose movement in a particular direction due to asymmetric implosion.

“Understanding the sources also makes it possible to define appropriate specifications to correct them or to implement design mitigations before the experiment, such as lower loss windows,” he said.

Understanding the asymmetry in a particular target construction prior to the experiment, including shell thickness excluding rounding at the +/- 0.5 micron level, also allows for the implementation of delivery adjustments. laser at a few percent that use mode 1 produced by laser cancel the asymmetry of the thickness of the capsule. Since the laser adjustment is a systematic change to 192 beams, the net asymmetry imposed in mode 1 of the laser can be very precise and directed accordingly, even when the usual variability in power supplied by the National Ignition Facility ( NIF) is added.

This concept was demonstrated in a recent experiment at the NIF which produced an almost record neutron yield. The laser was adjusted to produce an expected asymmetry in mode 1 of +/- 1.2% of the total incident X-ray flux in the right direction to compensate for a large asymmetry in the thickness of the capsule.

Analysis of experiences since 2016

The work was carried out through the analysis of the asymmetry sources in mode 1 from the performance of the laser and the construction of the target, as well as measurements of the amplitude and direction of the speed of the hot spot. from 50 DT layered implosion experiments carried out since 2016. The speed of the hot spot was measured by comparing the emitted energy spectra. neutrons from four different angles of view, then deriving the average speed of the hot spot. Learn more about neutron time-of-flight (nToF) measurements here.

The work was a detective work process to identify sources and sensitivities based on the data available. There were very few shots where a major change was made to any of the Mode 1 sources that would isolate the effect of a setting, as the sources were generally close to the specification for that particular source.

The model benefits future work

MacGowan explains that the development and validation of the Mode 1 asymmetry model benefits LLNL by allowing the explanation of the observed hot spot speed variability and compressed fuel density asymmetry in terms of mode sources. 1 in the laser and the target which can be derived from diagnostic measurements.

“Ultimately, this work helps us understand a principle of target performance degradation and a source of performance variability from shot to shot,” he said. “The work establishes a bargaining chip for precision specifications in laser performance and alignment, target construction and alignment, and facility stability. “

The model can be used in sensitivity studies and Monte Carlo calculations and allows easy propagation of uncertainties for different sources. For example, the impact of any correlated drift in the alignment of NIF beams due to thermal effects in NIF laser arrays can be compared to the impact of target fabrication errors or instability in the generation system. ‘NIF pulses. They are all compared through their impact on the asymmetry of mode 1 at the capsule level and ultimately related to the speed of the hot spot they generate.

By quantifying the laser and induced windows in mode-1 and subtracting them from the observed hot spot velocity, it is possible to isolate the effect of a new source such as capsule thickness in mode-1. This work made it possible to quantify the effect of the mode-1 capsule thickness described in an article referenced here.

In addition to MacGowan, the article’s co-authors include Nino Landen, Dan Casey, Chris Young, Debbie Callahan, Ed Hartouni, Rober Hatarik, Matthias Hohenberger, Tammy Ma, Derek Mariscal, Alastair Moore, Ryan Nora, Hans Rinderknecht, Dave Schlossberg and Bruno Van Wonterghem.

/ Public distribution. This material is from the original organization and may be ad hoc in nature, edited for clarity, style and length. View full here.

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