The first NIF Discovery Science experiment designed to create and study fully formed collisionless shocks, such as those responsible for the properties of many astrophysical phenomena including supernova remnants, gamma-ray bursts, jets from active galactic nuclei, and cosmic ray acceleration, was performed on July 29.
Studying astrophysics with laboratory experiments can help answer questions about micro-physics in astrophysical objects that are far beyond the reach of direct measurements. Most shock waves in astrophysics are collisionless from high plasma flow velocities—they form due to plasma instabilities and self-generated magnetic fields. Laser-driven plasma experiments can study the micro-physics of plasma interaction and instability formation (known as filamentary Weibel instability) under controlled conditions. NIF is the only facility that can create the proper plasma conditions to generate fully formed collisionless shocks and strong magnetic fields.
The NIF experiment, conducted by an international team of physicists comprising the Astrophysical Collisionless Shock Experiments with Lasers (ACSEL) campaign, builds on simulations and a number of previous experiments at the University of Rochester’s OMEGA Laser Facility. It was designed to investigate high-Mach-number non-relativistic collisionless shock formations. The experiment also collected data for the study of self-generated magnetic fields from the Weibel instability in counter-streaming plasma flows, and magnetic field generation and amplification in turbulent flows.
Supported by LLNL’s Hye-Sook Park and Steven Ross, the ACSEL team used the NIF lasers to irradiate the inner surface of two deuterated plastic foils doped with iron and nickel to create high-velocity counter-streaming plasmas. All 60 requested NIF beams delivered 307 kilojoules (kJ) of 3ω (ultraviolet) light to the targets in a 64.5-terawatt (TW) peak power pulse. The two resulting plasmas interacted at high velocity in a collisionless shock.
Neutron-yield diagnostics and x-ray spectral and imaging diagnostics were tested to evaluate the interaction region of the two counterpropagating plasma discs. Stimulated Raman scattering was measured from four laser probe beams.
“The experiment yielded excellent results,” Park said. “The team observed a high number of neutrons and observed that neutrons came at a relatively late time, which may indicate that they were produced in a shock. We also observed strong x-ray brightening from hot plasmas in the center of the experiment that had never been seen previously. The backscatter measurements delivered good results as well.”
Park added that the suite of diagnostics in the experiment performed extremely well, producing a copious amount of data that is now being processed. “With the neutron yield, the delayed neutron production and the x-ray brightening, we are studying whether these signals could be consistent from the shock,” she said. “However, the team needs to confirm these results with physics ‘controlled reference’ shots with a single disc and non-deuterated discs. These shots are planned for this fall. Self-generated magnetic field measurements from the collisionless shock will be done when proton backlighter capability is available on NIF next year.”
The ACSEL collaboration is led by LLNL, Princeton University, Osaka University, and Oxford University, with many other universities participating.
Lawrence Livermore scientists have, for the first time, experimentally recreated the conditions that exist deep inside giant planets, such as Jupiter, Saturn, and many of the planets recently discovered outside our solar system.
Researchers can now recreate and accurately measure material properties that control how these planets evolve over time, information essential for understanding how these massive objects form. This study focused on carbon, the fourth most abundant element in the cosmos (after hydrogen, helium, and oxygen) with an important role in many types of planets within and outside our solar system. The research appears in the July 17 edition of the journal Nature.
Using the largest laser in the world, the National Ignition Facility, teams from Lawrence Livermore National Laboratory (LLNL), the University of California, Berkeley, and Princeton University squeezed samples to 50 million times Earth’s atmospheric pressure, comparable to the pressures at the center of Jupiter and Saturn. Of the 192 lasers at NIF, the team used 176 with exquisitely shaped energy versus time to produce a continually increasing, albeit shockless pressure wave until the last instant of the experiment, where the pressure wave steepens to produce a massive shock, causing the sample to vaporize. The whole experiment lasts 20 billionths of a second.
Though diamond is the least compressible material known, the researchers were able to compress it to an unprecedented density, greater than lead at ambient conditions.
“The experimental techniques developed here provide a new capability to experimentally reproduce pressure-temperature conditions deep in planetary interiors,” said Ray Smith, LLNL physicist and lead author of the paper.
Such pressures have been reached before only with shock waves that also create high temperatures—hundreds of thousands of degrees or more—that are not realistic for planetary interiors. The technical challenge was keeping temperatures low enough to be relevant to planets. The problem is similar to moving a plow slowly enough to push sand forward without building it up in height. This was accomplished by carefully tuning the rate at which the laser intensity changes with time.
“This new ability to explore matter at atomic scale pressures, where extrapolations of earlier shock and static data become unreliable, provides new constraints for dense matter theories and planet evolution models,” said Rip Collins, another Lawrence Livermore physicist on the team.
The data described in this work are among the first tests for predictions made in the early days of quantum mechanics, more than 80 years ago, routinely used to describe matter at the center of planets and stars. While agreement between these new data and theory are good, there are important differences discovered, suggesting potential hidden treasures in the properties of diamond compressed to such extremes. Future experiments on NIF are focused on further unlocking these mysteries.
The NIF Team completed 19 target shots and 4 laser shots in July. Target shots were distributed as follows: High Energy Density (HED) Stewardship Science 8; Ignition 7; Discovery Science 1; National Security 2; Facility 1. Here are some additional highlights:
On June 30 and July 1, the team conducted two complex hydrodynamics crystal ball shock breakout (SBO) measurements. The shots measured the drive symmetry of the shock wave produced in a copper foam ablator in a vacuum hohlraum using the Velocity Interferometer System for Any Reflector (VISAR) diagnostic.
Two two-dimensional (2D) convergent ablator (ConA) implosion experiments to test the effect of gas fill (4He at 0.6 mg/cc) in a near-vacuum hohlraum on drive symmetry and backscatter were completed on July 1 and 2. The hohlraums contained high-density carbon (diamond) capsules and the experiments used a two-shock pulse. Good data were acquired including radiography of the imploding capsule using an iron backlighter and gated x-ray detector.
A materials equation-of-state diamond capsule experiment on July 3 measured the stress-density response of single-crystal diamond to a peak pressure of 10Mbar. The experiment used a strength-drive hohlraum combined with an 18-nanosecond ramped laser pulse to ramp-compress the diamond sample. Diamond is used as the ablator and pusher in high-Z (high atomic number) diffraction experiments. Good VISAR data were obtained measuring the diamond response.
On July 8, the team conducted the thinnest-shell deuterium-tritium (DT) experiment to date. The high-foot (high initial laser pulse) experiment used a plastic capsule with a shell 30 microns (15 percent) thinner than a standard ignition capsule to test mass remaining during implosion, shell breakup, and implosion shape. This shot was designed to probe the limits of increasing implosion velocity by reducing capsule mass, which could make the ablator susceptible to hydrodynamic instabilities. The DT fuel layer was considered to be among the best targets made for NIF. Initial data confirmed expected yield and performance.
From July 11-15, the team conducted a three-shot symcap mix mini-campaign for the HED program. The purpose was to determine the effect deuterated plastic (CD) layers at various depths in the fuel capsule shell would have on the amount of mixing of capsule material with the imploded fusion fuel. The DT neutron yield was used as a signature of mix. These experiments were similar to experiments done last year but with a higher-convergence implosion. The first “control” shot repeated a prior experiment in a gold hohlraum with a tritium-filled capsule with silicon doping but without a CD layer. The second shot used the same hohlraum and tritium fill with a CD dopant layer recessed one micron from the capsule surface. The third shot evaluated mix with the depth of the dopant layer recessed two microns inside the capsule. Good yield data were acquired in all shots and the goals of the experiment were met.
On July 12, the team conducted the first experiment in a four-shot case-to-capsule-ratio campaign to performance qualify the 2D ConA subscale platform. The experiments measure the in-flight symmetry of a plastic capsule in a room-temperature gas-filled hohlraum. The size of both the capsule and hohlraum are scaled to about 80 percent of an ignition target. These experiments require less energy, have less optics impact, and are faster to field than their full-scale cryogenic counterparts, providing a more efficient tuning platform as recommended earlier this year by the 120-day study.
A HED material strength shot was conducted on July 13-14 as part of a campaign to develop a five-megabar quasi-isentropic low-temperature drive for high-Z material strength experiments in Fiscal Year 2015. The experiment measured release and recompression characteristics of a full-stack reservoir with a 30 mg/cc carbon resorcinol foam (CRF) component instead of the 10 mg/CC silicon dioxide component used in previous experiments (the full-stack reservoir contains layers of different foam densities to provide a shaped impulse). The shot also tested the spatial uniformity of the leading shock. Good VISAR data were obtained showing a smooth initial ramp pressure loading of the foam on the tantalum sample.
On July 19, the team completed a keyhole experiment to measure shock timing, temporal behavior and drive symmetry in a plastic capsule driven by a three-shock shaped-adiabat pulse. The pulse is being developed to maintain the fuel on a low adiabat (internal capsule energy) for high compression while having the shell ablator on a higher adiabat to decrease ablation front instability similar to high-foot implosions. This experiment modified the laser pulse compared to an experiment done last month. Good data of the shock history were obtained.
Another 2D ConA experiment was completed on July 20. The shot used a diamond capsule doped with tungsten for x-ray preheat shielding. The in-flight shape and time-integrated self-emission image, backscatter, and bang-time performance will be compared with results of a previous experiment using an undoped diamond capsule to assess the impact of the tungsten dopant on target performance. Good in-flight radiographs were recorded showing prolate implosion symmetry.
A layered cryogenic DT implosion experiment using a diamond capsule was completed on July 23. The experiment used a gas-filled hohlraum driven with a three-shock laser pulse. This was the last of a series of five shots developing the three-shock system for a diamond capsule.
On July 27, the team conducted two National Security Applications experiments for x-ray source development. The first shot marked the first use of a cylindrical target lined with three microns of molybdenum. The goal was to quantify the molybdenum K-shell x-ray flux levels and determine the laser-to-x-ray conversion efficiency to better than 20 percent accuracy. The second shot tested an argon-xenon (Ar:Xe) gas-filled cylinder to generate argon K-shell and xenon L-shell and K-shell radiation. X-ray source strength data were measured on both shots.
The team completed a Los Alamos National Laboratory shock/shear experiment on July 28. The experiment used a new long-delay backlighter (LDBL) pulse to take about 10 nanoseconds of data with the DISC streak camera. The experiment used a longer-duration pulse shape and an aluminum-shielded target on the streak camera to continue LDBL development. Good streak camera data were obtained.