NIF experiments in late August tested the performance of two alternative target designs, one with an alternate capsule ablator and another with modified hohlraum geometry. Beryllium capsule in the hohlraum of a keyhole
shock-timing target. Keyhole experiments measure the strength (velocity) and
timing of the shock waves from the laser pulse as they transit the capsule. The
viewing cone for the Velocity Interferometer System for Any Reflector (VISAR)
diagnostic, shown mounted above the hohlraum, is inserted through the side of
the hohlraum wall and into the capsule. (Credit: Jason Laurea) Cryogenic Systems Operator Eric Mertens installs the
first NIF beryllium capsule target in the target positioner (TARPOS) for the
Aug. 29-30 LANL ICF campaign shot. (Inset) The beryllium capsule subassembly
mounted on the VISAR keyhole cone. This was the first shot in the development
of a promising new ablator, spearheaded by the LANL ICF team, that has been in
the works at LLNL and LANL for more than a decade. In the experiment, all 192 NIF beams delivered 561 kilojoules of ultraviolet light to the drive hohlraum in a 12.2-nanosecond laser pulse with 288 terawatts of peak power. Good diagnostic data were acquired; very low stimulated Brillouin and Raman scattering was measured. Excellent pole and equator VISAR data was obtained during both the foot and the main laser pulse, and good shock symmetry was observed. Target capsules in previous NIF experiments have been made of germanium, silicon-doped plastic, or undoped diamond-like high density carbon (HDC). ICF researchers believe beryllium could present a more attractive ablator material option. The low opacity and relatively high density of Be may lead to higher rocket efficiencies, Revision 6” beryllium capsule. The capsule has an outer radius of
1,051 microns and consists of a 180-micron-thick layer of graded copper
(Cu)-doped Be ablator, a 90-micron-thick layer (about 0.2 milligrams) of
deuterium-tritium (DT) ice, and a spherical volume of DT gas. The ablator layer
comprises five sublayers (from outside in): an undoped 150.5-micron layer, a
1.2-percent doped 4.5-micron layer, a 3-percent doped 16-micron layer, a
1.2-percent doped 4.5-micron layer, and an undoped 4.5-micron layer.giving a higher fuel implosion
velocity for a given x-ray drive; and to higher ablation velocities, providing
more ablative stabilization and reducing the effect of hydrodynamic
instabilities on implosion performance. Be ablator advantages provide a larger
target design optimization space and potentially could improve NIF’s
experimental margin for ignition. An optimized Be ignition target design known as “NIF Revision 6” takes advantage of knowledge gained from recent NIF experiments, including more realistic levels of laser-plasma energy backscatter, degraded hohlraum-capsule coupling, and the presence of cross-beam energy transfer. “The first NIF Be target experiments will study the associated energy backscatter, hohlraum-capsule coupling, Be grain and imperfection effects on the capsule performance, and thereby benchmark the design work and confirm experimentally the Be ablator advantages,” said Andrei Simakov, a lead LANL researcher. Be is coated over two weeks of 24-hour-per-day machine operation in LLNL’s Bldg. 298 on batches of 15 plastic mandrels, made by the team at General Atomics, which also characterizes the shells and machines the keyhole. The capsules are finally assembled on cones and cryo-tested in LLNL’s Bldg. 381 cleanroom. “This experiment represents a very significant new facility capability that was put in place,” said NIF Operations Manager Bruno Van Wonterghem. “The NIF Target Operations and Radiation Control teams worked safely and diligently through the first target and target diagnostic recovery activities in full radiation and beryllium contamination protocol, including respirators and personal air samplers,” he said. “The good news is that initial indications are that all swipes show very low levels of Be, well below the release level of 0.2 micrograms per 100 square centimeters. Be air samples were well below the action level. “This
bodes well for both Be and high-Z shots (experiments using high-atomic-number
materials),” Van Wonterghem said. “These first data need to be confirmed in
subsequent shots and future Be shots. Once validated over a statistically
relevant range, controls will be adjusted to these findings.” Radiation Control Technician Brian Daniels surveys
respirator masks in the Hazardous Materials Management Area radiation lab. The
use of dispersible beryllium targets requires respiratory controls to process
Target Chamber systems such as diagnostic instrument manipulators and target
positioners. (Left) The rugby-shaped hohlraum keyhole target. (Right)
A rugby-shaped hohlraum in the thermomechanical package during final assembly in
Bldg. 381. Experiments using hohlraums shaped like rugby balls began on NIF in March of this year in collaboration with CEA, the French Alternative Energies and Atomic Energy Commission. Rugby-shaped hohlraums are being tested to determine if they can improve drive symmetry control on the target capsule. Compared to standard cylindrical hohlraums, rugby-shaped hohlraums increase the volume around the fuel capsule, making it easier for the laser beams to propagate without increasing the overall area of the hohlraum and the energy required to drive it. In experiments performed on the OMEGA Laser Facility at the University of Rochester comparing rugby and cylindrical gas-filled hohlraums driven by high-contrast laser pulses, the rugby-shaped hohlraums showed nearly 20 percent more x-ray drive energy than standard cylindrical hohlraums, and the high-performance design of the capsules (both cylinder and rugby) provided nearly 20 times more deuterium neutrons than in any previous OMEGA hohlraum campaigns. |
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