In early June the NIF Team conducted two successful x-ray shots to performance-commission two of the new NIF x-ray spectrometers (NXS’s) funded by the Defense Threat Reduction Agency and built by the Laboratory for Laser Energetics at the University of Rochester.
The NXS measures time-resolved x-ray spectra in various spectral regions from two to 18 keV (thousand electron volts). The instruments’ Bragg crystal spectrometers were qualified by taking x-ray spectral data from a chromium-nickel-zinc alloy target and streaking the data on the diagnostic instrument manipulator (DIM) insertable streak cameras (DISCs), each in a different spectral configuration. Good x-ray spectra were recorded from both instruments on both shots.
The NIF Team conducted 28 target shots in 17 shot days in June for an average of 1.65 target shots per shot day. Here are some highlights:
High-Foot Convergent Ablator Experiment: On June 1-2, the team conducted a successful high-foot (high initial laser pulse) convergent ablator experiment. The goal was to measure the implosion velocity for a high energy/high power drive similar to the high-neutron-yield high-foot deuterium-tritium experiment last November.
At right is the streaked radiograph of the in-flight imploding capsule ablator over a 2.8-nanosecond window, from a radius of about 500 microns to 500 picoseconds after bang time, showing the excellent data acquired from the experiment.
TARDIS Experiment: On June 6, the team completed a successful target diffraction in-situ (TARDIS) experiment studying the structure of lead driven to a pressure of 2.5 megabars (Mbar) in the body-centered cubic (BCC) crystalline phase. Sixteen drive beams and eight backlighter beams delivered 30.6 kilojoules (kJ) of 3ω (ultraviolet) energy to the TARDIS package. All diagnostics triggered and good data were collected.
This high-energy-density shot supported the development of a direct-drive experimental platform to measure high-pressure, high-strain-rate x-ray diffraction using a ramped drive. All objectives of the experiment were met, including evaluating planar drive qualification, study of lead in the BCC phase at 2.5 Mbar, improved TARDIS diffraction data shielding, and evaluation of an improved efficiency germanium x-ray diffraction source.
Pleiades Radiation Transport Experiments: A “mini-campaign” of four Los Alamos National Laboratory (LANL) Pleiades radiation transport experiments was conducted by a team from LANL and the UK’s Atomic Weapons Establishment during the week of June 9. A foam tube was driven by a 3.5-millimeter-diameter “halfraum” (half hohlraum) heated by 80 beams from the lower hemisphere in the NIF Target Chamber to measure burnthrough in radiatively heated chlorinated plastic and silicate foams. The series currently under way introduces a late-time timing beam to cross-calibrate the channels of the upper-hemisphere Dante time-resolved x-ray spectrometer. One shot also measured the time-resolved spectrum from the foam. Good data were obtained on all four shots.
Adiabat-Shaping Experiment: On June 11-12, the team conducted the first adiabat-shaping experiment for the inertial confinement fusion (ICF) program. This is a new campaign that uses adiabat (internal capsule energy) shaping to demonstrate the benefits of both the high-foot (high initial laser pulse) implosion scheme with its reduced ablator-fuel mix and the low-foot pulse with its higher compression.
A slightly modified low-foot laser pulse produced a decaying shock in the ablator to create a high adiabat at the ablator surface for increased stability, while maintaining the fuel on a lower adiabat for improved compression and a better-performing implosion. The shot measured shock timing, temporal behavior, and drive symmetry in a plastic capsule in a gas-filled hohlraum. Good Velocity Interferometer System for Any Reflector (VISAR) data of the shock propagation were obtained.
Polar Direct-Drive Experiments: The team conducted two polar direct-drive (PDD) experiments in collaboration with the Laboratory for Laser Energetics at the University of Rochester on June 12-13. They tested the use of iron and copper backlighters to obtain radiographs of the imploding capsule at different x-ray energies to characterize symmetry. Good images of the imploding shell were recorded in both experiments.
With the completion of the two PDD shots, the NIF team had conducted nine successful target experiments within four shot days and one hour; five shots were high energy density program shots and four were ICF program experiments. NIF Operations Manager Bruno Van Wonterghem said that only once in the history of NIF—in March 2011 as part of the radiation transport campaign—had nine target shots been fired in a shot week, but those shots required five days and were simpler shots.
“This is a fantastic achievement for the operations, engineering, optics and experimental teams required to support these shots,” he said. “All experimental teams were very satisfied with the results and the operations.”
Material Strength Experiments: On June 16-17, the team completed two experiments in a material strength tantalum Rayleigh-Taylor (TaRT) mini-campaign for the high energy density (HED) program. The campaign’s goal is to characterize the strength of high-Z (high atomic number) materials at high pressures by measuring the growth of Rayleigh-Taylor hydrodynamic instabilities. The experiments measure growth in tantalum foils by x-ray radiography. The first shot did not produce useful data due to a target fabrication error. The second shot showed the expected modulation in the radiographs from the growth of pre-imposed modulations on the target capsule. The second shot also tested the efficiency for sample recovery using radioactive tracers in the sample.
TARDIS Uranium Diffraction Shots: The team conducted two TARDIS uranium diffraction experiments as part of the HED material strength campaign on June 17-18. The first shot acquired diffraction of germanium x rays by an un-driven uranium sample. Four NIF beams delivered 10.5 kJ of 3ω light to a single-sided germanium backlighter at a peak power of 4.2 terawatts (TW). Excellent diffraction data were obtained. The second shot acquired diffraction data from a uranium target driven to 2.5 megabars of pressure from a zirconium backlighter; 16 heater beams and eight backlighter beams delivered 36 kJ of 3ω light to the target.
MDA Source Development Experiment: On June 20, the team completed a Missile Defense Agency (MDA) source development shot as part of a national security campaign to design and characterize a long-pulse, high-intensity x-ray source on NIF. In this shot, 188 beams delivered 1.33 megajoules of 3ω light to a thin-walled 4×14-millimeter krypton/xenon-filled gas pipe. The pulse duration was 14 nanoseconds and the peak power was 115 TW. The target’s dimpled shield was coated with parylene to optimize the x-ray spectrum for effects testing. Good data were acquired on the gated x-ray detectors.
Subscale Symcap Experiment: The team conducted the first symmetry capsule (symcap) experiment in the subscale platform campaign, which is aimed at developing a warm, low-energy alternate platform for conducting rapid low-impact tuning shots on NIF, on June 25. All 192 beams delivered 879 kilojoules of 3ω light to the drive hohlraum in a 233-TW pulse. Energy was within 0.2 percent of the request. Good data were acquired.
Zirconium Backlighter Mini-Campaign: On June 27, the team completed a zirconium backlighter mini-campaign by firing a record four system shots in 20 hours. The experiments collected backscatter measurements from a zirconium foil driven at high irradiance, and 16 keV x-ray production from a zirconium foil at various irradiances. The shots used various quads of beams, continuous phase plates, and pulse length and shape. Using different quads from different clusters on each shot enabled the team to bypass amplifier cooling between shots. Excellent data were obtained on all the shots.