Scientists put additive manufactured foams to the test

Scientists Lawrence Livermore National Laboratory (LLNL) recently published the results of a three-week experimental project at the Jupiter Lab Laser Facility to test the performance of manufactured gases with laser heating.

The project will help support two key Laboratory focus areas, including helping to advance additive manufacturing and enabling improvements in hohlraums performance – a the caves are laser-heated which emits an X-ray radiation driver that attaches a deuterium-filled capsule.

The work also supports the state’s advancement in the science of high energy density. In particular, by enabling more efficient hohlraums, it should help to achieve the goal of the inertial fusion (ICF) program for achieving mitigation in the laboratory.

Oggie Jones, lead author of the work featured in it Plasma Physics, according to the team ‘s experience, this was the first time that laser fabricated structural fabrication was tested.

The main findings of the research showed that laser-heated manufactured beams behaved in many ways similar to chemical gases (airgel) of equal density. The amount of backscattered laser light for a particular laser intensity and thermal wave propagation speed although the plasma was similar.

“This was true even though the manufactured foams have filamentary structures 100 times thicker than chemical gases of the same density,” Jones said. “The manufactured rays themselves were also found to be behaving independently of the size of the scale.”

The team tested geometric-like manufactured foams, one with 0.5-micron-thick filaments and one with 10-micron-thick filaments. The backscatter signatures and X-ray image are almost unrecognizable. The team found that published analytical models were generally capable of interpreting the measured thermal multiplication speed and measured temperature in the experiments.

Jones explained that the use of foam materials in hohlraums opens up new design opportunities in indirect driving in inertial fusion. In particular, foams can be placed inside the hohlraum to stretch the walls.

“If the density of the foam is carefully selected, it is possible to change how the hohlraum wall material expands over time and thus improves the symmetry of the radiation drive on the ICF capsule,” he said.

In addition, very low density gases with several elements can be used to plot the position of plasma within the hohlraum and to attenuate plasma laser interactions (laser backscatter). Added manufactured foams allow optimal control of the plasma conditions. Density and dopant gradients can be built into the foam. Because these stems are inside the hohlraum, the way in which they are heated by the laser is crucial to understanding their overall effect on hohlraum performance.

The experiments used a single 527-nanometer laser beam (green). The laser pulse was 200 goules, about two nanoseconds in length and resulted in a maximum laser intensity of 3×1014 W / cm² on the foam targets. During a week of beam time, the team fired around 20 different foam targets.

Elijah Kemp was the lead experimenter on this project and coauthors included Steve Langer, Benjamin Winjum, Dick Berger, James Oakdale, Mikhail Belyaev, Juergen Biener, Monika Biener, Derek Mariscal, Jose Milovich, Michael Stadermann, Phil Sterne and Scott Wilks.

A second paper on this research, focusing on numerical simulations of these experiments, was approved for publication by Plasma Physics and Controlled Fusion. Authors include Jose Milovich, Ogden Jones, Dick Berger, Elijah Kemp, James Oakdale, Juergen Biener, Mike Belyaev, Derek Mariscal, Steve Langer, Phil Sterne, Scott Sepke and Michael Stadermann.

The novel foam targets were produced at LLNL by a group led by Stadermann, Juergen Biener and Oakdale.

The work was funded by the LLNL Weapons and Complex Integration Laboratory Directed Research (LDRD) program titled “Foams in Hohlraums.”

This research has led to an ongoing LDRD project titled “Fills Foam for LPI Suppression.” In this project, researchers will study low-density foam filling arrangements that lead to a reduced backscatter in ICF hohlraums.

“If successful, this research could allow hohlraums to operate at filling densities that did not work with simple helium gas fillings,” Jones said. “This would open up a range of previously closed design space due to excessive laser backscatter.”