Testing Alternatives to Tents in NIF Implosions

LLNL researchers have reported on their progress in the months-long effort to mitigate the adverse effects on NIF implosion performance of the gossamer-thin membranes known as "tents" that support the target capsule in the hohlraum.

Experiments have shown that the tents cause undesirable distortions of the capsule as it implodes during NIF inertial confinement fusion (ICF) experiments (see "How NIF Works"). The results of tests of several alternative techniques to support the capsules were described in a Physics of Plasmas paper published online on July 23.

In the paper, LLNL physicist Vladimir Smalyuk and his colleagues from the Laboratory, General Atomics, and Schafer Corp. reviewed the results of experiments using hydrodynamic growth radiography (HGR) and self-backlighting techniques to measure the growth of hydrodynamic instabilities caused by five alternative capsule support methods.

"While some of these methods completely eliminated tents," the researchers said, "other concepts still used tents, but concentrated on mitigating their impact. The tent-less methods included ‘fishing pole’ reinforced fill tubes, cantilevered fill tubes, and thin-wire ‘tetra-cage’ supports. Other concepts used ‘polar tents’ and a foam shell to mitigate the effects of the tents. These concepts were downselected from more than 30 concepts based on physics and target fabrication considerations, investigated in x-ray radiography experiments, and compared with perturbations from standard tent support."

The paper also briefly discussed the use of magnetic levitation to support the capsules, a concept currently under development and testing (see "Superconductive Coatings May Float NIF Targets").

The "Fishing-Pole" Fill Tube

In the "fishing-pole" configuration for capsule support, a 10-micron-diameter fill tube was inserted inside a 30-micron fill tube for extra support, with the connection point located 300  microns away from the plastic capsule surface (a human hair is about 60 microns in diameter). The thicker fill tube was expected to improve the robustness of the capsule mounting to help with target handling and layering of cryogenic deuterium-tritium (DT) fuel.

The measured "fishing-pole" perturbation was no worse than that with a nominal fill tube at the same capsule convergence, the researchers reported. The fill-tube perturbation decreased by about 3 times as the convergence ratio increased from 1.9 to 3.0. "During this time, the perturbations from x-ray shadows grew around the fill tube," they said. "It was predicted that the fill-tube perturbation reversed its sign in the acceleration phase, sending a jet of material to the center of the capsule near peak compression.

"The exact evolution of this perturbation will be a subject of future studies," they said. "It is important to understand and mitigate it since it was shown to have a significant effect on the neutron yield in recent layered DT implosions with HDC (high-density carbon, or diamond) ablators."

Cantilevered Fill Tubes

In the cantilevered fill-tube configuration, the fill tubes were supported by 12-micron-thick silicon carbide (carborundum) rods. The tubes were attached to the capsule with a 40° separation, with one 20° above the equator and the second 20° below the equator. The support rods were offset from the capsule surface by 100  microns and 200  microns in one shot and by 200  microns and 300 microns in a second shot.

According to the researchers, the perturbations were sensitive to the distance of the wire to the surface of the capsule. The size and shape of the perturbations from the 200-micron cantilevered fill tubes were similar in the two shots, showing that the perturbations were reproducible, they said. At 300 microns off-set, the residual perturbation due to the support appeared to be much reduced relative to the tent support.
 

The "Tetra-Cage" Support

The concept of the "tetra-cage" mounting replaces tents with four 2.5-micron-thick wires (carbon nanotube yarns) that reduce contact area. A pair of parallel wires support the capsule from the top and the other pair of parallel wires, perpendicular to the top pair, support the capsule from the bottom. The overall affected area of four tetra-cage wires is reduced by 5 times compared to two standard tents.

Carbon nanotubes for "tetra-cage" targets are produced in a densely packed forest. Individual strands are separated and pulled together into a wedge shape, then twisted into a yarn. Four yarns make up the "tetra-cage" formation, with the capsule suspended between two pairs. Tests of the "tetra-cage" support were encouraging, the researchers said, and preparations for layered DT implosions are under way. In addition, development and testing of roughly 1-micron-thick wires are in progress which may reduce the "tetra-cage" perturbations further in the future.

Polar Tent" Experiments

The "polar-tent" concept is based on reducing the contact area between the capsule and the tent. The contact radius was reduced from about 700 to 800  microns in the standard configuration to about 200  microns for the polar tent.

The polar tent consisted of 43-nanometer-thick polyimide foil coated by 8 nanometers of carbon at a density of about 1.8 to 2.0 grams per cubic centimeter to increase its stiffness. The carbon overcoat was on the side of the polar tent away from the capsule. This mitigation method was tested in radiography experiments using a self-backlighting platform and later in layered DT implosions.

The researchers said the polar-tent mitigation technique could not be tested in HGR experiments due to limited field of view of the HGR platform and therefore could not be directly compared to other mitigation techniques. "However," they said, "x-ray radiography experiments near peak implosion velocity and first tests in layered DT implosions provided a useful first assessment of polar tents."
 

"Foam-Shell" Support

In the "foam-shell" concept, the tents were offset from the plastic capsule surface by a 200-micron-thick layer of silicon dioxide foam with a density of 30  milligrams per cubic centimeter in an effort to mitigate their effects. The laser pulse shape was slightly modified for this experiment relative to the nominal pulse by increasing the energy in the first picket (initial shock) and advancing the rest of the drive to compensate for the propagation of the shocks in the added foam layer.

The "foam-shell" HGR experiment showed that tent perturbations were mitigated when the tents were offset from the capsule surface by the foam layer, the researchers said. "However," they added, "measured 3D broadband perturbations remain a concern for this type of tent mitigation technique. The origin of these 3D perturbations was not quite understood and required further investigations."

In summary, the researchers concluded that measured perturbations in the "fishing pole," cantilevered fill tube, and "tetra-cage" concepts were smaller than nominal tent perturbations and were recommended for further testing for feasibility in layered DT implosions. The polar tents were tested in layered DT implosions with a relatively stable "high-foot" (high initial laser pulse) drive showing an improvement in neutron yield in one experiment compared to companion implosions with nominal tents.

Smalyuk was joined on the paper by LLNL colleagues Harry Robey, Peter Amendt, Chantel Aracne-Ruddle, John Bigelow, Tom Bunn, Dan Casey, Dan Clark, Sean Felker, John Field, Steve Johnson, Steve Haan, Bruce Hammel, Alex Hamza, Chuck Heinbockel, Warren Hsing, Jeremy Kroll, Sergei Kucheyev, Nino Landen, Xavier Lepro-Chavez, Andrew MacPhee, David Martinez, Jose Milovich, Abbas Nikroo, Louisa Pickworth, Michael Stadermann, David Steich, and Chris Weber and by Cresenciano Alday, Kuo-Chen Chen, Jay Crippen, Silverio Diaz, Mike Farrell, Javier Jaquez, Martin Havre, Karina Kangas, and Neal Rice from General Atomics and Jimmy Cortez from Schafer Corp.