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THE most extreme conditions that can be created in the laboratory are in the high-energy-density (HED) regime, where materials often behave in unexpected ways. HED research involves examining materials under pressures and densities found until recently only in the cosmos—in stars and the cores of giant planets—or on Earth only in detonating nuclear weapons. The advent of Lawrence Livermore’s 192‑beam National Ignition Facility (NIF), the world’s largest and most energetic laser, now makes it possible to test materials in HED regimes that are inaccessible to scientists by any other means.
Inside NIF’s 10-meter-diameter target chamber, laser beams converge on meticulously engineered targets to obtain extremely accurate data on the HED conditions created. The pulses in a NIF beam typically last only a few nanoseconds (billionths of a second) and are spaced at intervals of less than 20 picoseconds (trillionths of a second). Together, the beams can deliver a combined energy of up to 1.8 megajoules and a peak power of 500 terawatts (trillions of watts), creating pressures greater than 100 terapascals (1 billion times Earth’s atmospheric pressure), temperatures of up to 100 million kelvins, and densities up to 500 grams per cubic centimeter. Not only can scientists define total laser energy and power by selecting the number of laser beams to use and the energy of each individual beam, but NIF’s pulse-shaping ability allows them to vary the power of an individual pulse over time. Furthermore, some beams can be separately targeted at thin foils or other secondary targets to generate diagnostic streams of x rays, protons, or other particles that assess what is occurring in the primary target.
NIF has assembled a specialized team to carry out these HED materials experiments, which fall broadly into three categories: equation of state (EOS), or the relationship between pressure, temperature, and density; strength, that is, the speed and extent of the deformation of a solid; and phase changes, such as from a solid to a liquid. Livermore scientist Tom Arsenlis, who heads the team, explains, “No previous data exist for these very difficult regimes we are accessing.” Physicist Damian Swift notes that although NIF creates HED conditions of extremely short duration, the same physics apply to the long-lasting conditions found in stars and planets. Indeed, NIF provides the ability to reproduce the extreme conditions in myriad celestial phenomena, such as magnetic fields that stretch hundreds of light years, the interiors of stars, and the cores of planets inside and outside our solar system.
Such fundamental HED experiments on NIF are conducted as part of the Discovery Science Program, which provides academic users access to NIF’s HED regimes and enhances collaborations between Livermore scientists and academia. The program constitutes about 8 percent of NIF’s more than 400 annual shots. A review committee of outside experts from different branches of physics advises NIF managers on the relative scientific merit of proposals submitted by investigators.
Discovery Science participants include researchers from other national laboratories and from leading universities worldwide. They include graduate students and postdoctoral researchers. Many prepare for years to conduct experiments at NIF. Discovery Science teams typically are allocated a few days of access each year, with two to three laser shots conducted in a 24-hour day. “The entire facility is theirs, so they can, within safety limits, fire any kind of laser energy and pulse shape and power they would like,” says Bruce Remington, head of the Discovery Science Program. The program encourages Livermore HED researchers to engage the broader academic community in planning and executing Discovery Science experiments, publish their research in the open literature, and elicit comments from their peers. Collaborations also serve to attract the next generation of HED scientists to Livermore.
Discovery Science experiments are conducted on such materials as hydrogen, carbon, iron, copper, magnesium, and various silica compounds. (See S&TR December 2015, Smashing Science.) Physicist Jon Eggert says, “It is important that we do experiments on a wide variety of materials to make sure our results are consistent.” An experiment is conducted on a “platform,” which comprises the laser configuration (energy, power, and duration), an extraordinarily precise target measuring a few centimeters in size, and ultrahigh-resolution diagnostic instruments, all tailored to an experiment’s needs.
Preliminary experiments are often conducted at lower energy research centers such as the Omega Laser Facility at the University of Rochester and the Z Pulsed Power Facility at Sandia National Laboratories in Albuquerque, New Mexico. “Omega allows us to conduct many experiments in one day,” explains Arsenlis. “We can test an idea there before graduating to NIF. Despite fielding more than 400 shots per year, demand for NIF shots always exceeds supply.” The Z facility uses high magnetic fields associated with high electrical currents to produce powerful x rays and extreme temperatures and pressures that last 100 times longer than NIF’s. Arsenlis explains, “NIF experiments are over in tens of nanoseconds. Z offers a valuable window into a longer timescale, although at somewhat lower energy densities.”
Discovery Science experiments probe such mysteries as the source of ultrahigh-energy cosmic rays; the formation and evolution of collisonless astrophysical shocks; the creation mechanisms for the magnetic fields that thread the universe; nuclear reactions in HED plasmas; how the elements of the universe are forged in the interiors of stars; how stars form in molecular hydrogen clouds, such as the famous “Pillars of Creation” in the Eagle Nebula; and the likely composition of the cores of exoplanets, those planets located outside our solar system.
One series of experiments is devoted to understanding ultrahigh-energy cosmic rays. Most cosmic rays are protons or helium nuclei—two protons bound to two neutrons—originating mainly from outside the solar system. Scientists do not know the origin of these rays or how they gain their incredible energy, which is largely dissipated when they enter Earth’s atmosphere. In the experiments, conducted by an international collaboration called Astrophysical Collisionless Shock Experiments with Lasers (ACSEL), researchers are investigating the possible role of collisionless shocks and related magnetic fields in the acceleration of cosmic rays.
In collisionless shocks, clouds of rapidly moving plasma particles pass through each other largely without colliding but create shocks by means of collective plasma instabilities, such as the Weibel instability. Collisionless shocks appear in violent solar flares, outbursts from galaxies, supernova remnants, protostellar jets, and gamma-ray bursts. NIF is the only facility capable of creating plasma streams with sufficiently high density (greater than 1020 particles per cubic centimeter), velocity (greater than 1,000 kilometers per second), and temperature (greater than 1,000 electronvolts) to approximate these energetic astrophysical conditions. “We are seeing that magnetic fields can be generated from these high-velocity interacting plasmas. The magnetic fields definitely play an important role in the acceleration of cosmic-ray particles,” says Hye-Sook Park, leader of the ACSEL experimental team. The NIF targets used in the experiments consist of two opposing plastic foil discs spaced 10 millimeters apart. When struck by NIF laser beams, the foils emit high-velocity plasma flows that generate shock, largely without ion–ion collisions. The associated magnetic fields produced by the interpenetrating plasmas are probed with protons created by other NIF beams imploding a tiny capsule known as an “exploding pusher,” which is filled with a mixture of helium-3 and deuterium (a hydrogen isotope with one neutron).
A related series of experiments, called Turbulent Dynamo, or TDyno, is aimed at understanding how magnetic fields created in the early universe grew to their current size and strength. Led by a team from the University of Oxford in the United Kingdom, the TDyno researchers are studying how small “seed” magnetic fields are amplified by a turbulent dynamo—a kind of plasma characterized by turbulent eddies. Collisionless shocks are thought to be one of the mechanisms that create seed magnetic fields, which a turbulent dynamo then amplifies. The NIF experiments are re-creating the dynamos on a much smaller scale by striking two parallel plastic foils in a fashion that produces highly turbulent plasmas. As with the collisionless shock experiments, an exploding pusher creates a proton source to diagnose the magnetic fields as they are amplified by the turbulent plasma.
Other Discovery Science experiments are aimed at understanding the upwards of 4,000 exoplanets discovered to date orbiting more than 2,600 stars. Scientists aim to determine exoplanets’ internal structure, including whether they could have plate tectonics and a magnetodynamo, both considered essential to supporting some form of life. Indeed, identifying habitable planets has been deemed one of the top three scientific goals for both astrophysics and planetary science. Livermore physicist Robert Rudd states that the search aims to find a “Goldilocks” exoplanet that is not too hot or cold, has some water, and possesses a magnetosphere—a magnetic field generated by a magnetodynamo, shielding potential life forms from damaging cosmic radiation. Scientists may have found one such exoplanet in August 2016—a roughly Earth-sized planet orbiting Proxima Centauri, the star nearest to our solar system. The planet, Proxima b, is located at a distance from its parent star that allows temperatures mild enough for liquid water to exist on its surface.
Exoplanet-related experiments on NIF are designed to replicate the conditions believed to exist in exoplanetary cores, where estimated pressures range from 350 gigapascals to 7 terapascals—the pressures found inside Earth and Jupiter, respectively. “Deep inside a planet, the properties of the constituent materials are strongly modified by extreme density, pressure, and temperature,” explains Livermore physicist Marius Millot. Even those conditions can be re-created using NIF.
The experiments also seek to accurately determine an exoplanet’s composition, which is largely a matter of comparing its mass and radius—deduced from dips in the parent star’s light as the planet crosses in front—to the composition suggested by the planet’s distance from its star. The mass–radius relationship relies heavily on the EOS, which in turn is determined experimentally from a material’s compressibility under HED conditions. Squeezed to extreme pressures by a single shock wave or a sequence of staged shocks, the material changes to a new state at higher density, temperature, and pressure. The resulting EOS data refines computational models. “We need data from NIF experiments to validate our calculations,” says Swift.
An extremely useful technique for obtaining EOS and material strength data is ramp compression, in which a material is pressurized gradually, so that it heats to lower temperatures and thereby retains its structural integrity. In contrast, a standard, nearly instantaneous shock raises temperatures significantly, limiting the study of fluid properties to those beyond 200 gigapascals. A realistic model of a planet’s interior cannot be based on an EOS ascertained at high temperatures because compression in these planets is caused by shock-free gravitational forces acting over millions of years, leading to high densities at relatively low temperatures.
Because iron is a major component of Earth and other planets, understanding the phases and melt curve of the metal at high pressure advances our understanding of Earth and Earthlike exoplanets. Studying iron at Earth-core conditions requires pressures of 130 to 360 gigapascals and temperatures of over 6,000 kelvins. These conditions are near melting, and in fact Earth’s core is separated into a solid iron inner core surrounded by a rapidly convecting liquid iron outer core. The magnetosphere that protects life on Earth from cosmic radiation is generated by a magnetodynamo driven by this convective flow of electrically conducting molten iron. Iron’s melting transition is therefore extremely important for determining the habitability of large terrestrial exoplanets, the so-called “super-Earths” that have up to 10 times the mass of our planet. (Beyond this mass, planets tend to be gas giants such as Jupiter.) Super-Earths are calculated to have core pressures of up to 3.6 terapascals, but researchers presently have no idea whether such exoplanets will also have a liquid outer core and a magnetosphere.
A team of scientists led by Livermore physicist Richard Kraus is conducting experiments to measure iron’s melting curve at pressures from 500 gigapascals to 2 terapascals, an area where no experimental data currently exist. If the iron core of a super-Earth is completely solid, it may not generate a magnetosphere, making it more difficult for life to evolve on the planet’s surface. Kraus explains, “We need to know more about the liquid–solid transition in iron cores to determine whether rocky planets larger than Earth could have a protective magnetosphere.” NIF has also advanced investigations of the phase changes of materials under HED conditions. Kraus and his team are using a novel experimental technique, developed at Omega, in which a sample is initially shocked into the liquid phase with high heat and low pressure and then subsequently ramp-compressed back into the solid phase at high pressure, with only a small increase in temperature. They use x-ray diffraction to confirm whether a 20-micrometer-thick iron foil was melted by the initial shock and then resolidified by the ramp-compression wave. This technique was proven on Omega at pressures lower than 500 gigapascals, but now researchers need access to higher pressures along the melting curve. Upcoming tests on NIF will provide the crucial data.
The integrated platform used in these experiments features a target and a diagnostic named Target Diffraction In Situ. In this setup, as many as 24 NIF laser beams strike an iron sample, followed nanoseconds later by up to 16 additional beams that illuminate a backlighter metal foil mounted on a stalk. Collimated x rays from the backlighter probe the sample, producing a diffraction pattern that serves as a “fingerprint” revealing the structure (phase) of the iron. (See S&TR, January/February 2016, A Growing Family of Targets for the National Ignition Facility.) Kraus notes that the experiments are highly sensitive to the laser pulse’s temporal shape, that is, how its power varies over time. He reflects that a pulse lasting all of 30 nanoseconds provides reliable data on processes that occur in the cosmos over tens of thousands of years.
In previous experiments on the Z facility, Kraus and colleagues determined the impact conditions needed to vaporize iron in meteors and other objects that collided with an ancient Earth. “We need to model iron during impact events because it is a major component of planets, and its behavior is critical to how we understand planet formation,” explains Kraus. The experiments showed that these violent collisions generated significant amounts of iron vapor, and that iron vaporizes at much lower impact speeds than previously thought. As a result, more iron was probably vaporized during Earth’s period of formation and spread over the young planet’s surface instead of immediately sinking down to Earth’s incipient core.
Most exoplanets discovered so far are gas giants similar to Jupiter and Saturn, along with even larger “hot Jupiters,” so called because of their proximity to the stars they orbit. Gas giants are believed to be composed largely of hydrogen and helium. Hydrogen is the simplest element (one proton orbited by one electron) yet exhibits complex behavior under HED conditions, with at least four different solid phases, perhaps two different liquid phases, and a gas phase.
Some scientists postulate that the hydrogen and helium swirling around together in gas giants could separate when hydrogen becomes metallic under terrific pressures. Such a separation could affect the internal core structure and influence how the planets evolved over the past 4.5 billion years. Previous experiments and models produced conflicting results about hydrogen’s sudden transition from an insulating molecular fluid to a metallic atomic fluid at around 100 to 300 gigapascals and at a relatively cool 1,000 to 2,500 kelvins.
Looking for signs of this transition was a goal of Discovery Science experiments on NIF conducted by Marius Millot, Peter Celliers, and other Livermore researchers, together with colleagues from the University of California (UC) at Berkeley, the Carnegie Institution, France’s Alternative Energies and Atomic Energy Commission, and the United Kingdom’s Edinburgh University. A series of small shocks of increasing strength was fired at cryogenic liquid deuterium positioned between a copper plate and a lithium fluoride window. This technique kept the temperature lower than would be possible with one large shock, allowing access to unprecedentedly high densities. The team used a diagnostic called the Velocity Interferometer System For Any Reflector (VISAR) to record the velocity of reflected laser light by measuring the light’s Doppler shift. Through VISAR’s transparent lithium fluoride window, they also observed dramatic changes in the optical properties of deuterium. “During these 30-nanosecond-long dynamic compressions, we clearly see hydrogen transitioning from transparent to opaque and then becoming as reflective as a metal foil,” says Millot. Further analysis and comparison with state-of-the-art simulations at Livermore will likely enhance understanding of the behavior of hydrogen and other matter at extreme density.
A research team led by Livermore physicist Ray Smith and including researchers from UC Berkeley and Princeton University has used ramp compression on diamond, the hardest known material and a form of carbon, the fourth most abundant element in the universe. Diamond may reside in the cores of Neptune and Neptune-sized giant planets, which are common in our galaxy. The experiments achieved pressures as high as 5 terapascals—15 times the pressure at Earth’s core and comparable to pressures at the center of Jupiter and Saturn. This pressure was achieved by ramp-compressing diamond to almost four times its normal density and more than six times the pressure previously achieved in similar experiments by the same team at the Omega laser. Eggert notes that at pressures exceeding 1 terapascal, diamond is theorized to transition to a different solid phase called BC8, which may be harder than ordinary diamond. NIF experimenters are attempting to create that phase in the laboratory for the first time.
“We are now measuring material properties on NIF at pressures up to a billion Earth atmospheres,” notes Swift. The improvement in HED understanding has not come without surprises, in part because materials can behave entirely differently under HED environments, whereas materials science theories have been based on experiments conducted at much lower pressures and temperatures. “We sometimes fail to predict complex material behavior,” says Swift. “For example, we’re discovering crystalline structures we didn’t know existed but which have a bearing on material strength.” In addition, some materials can be stronger than simulation codes predict because a shock can damage the crystal lattice, making the material stiffer.
Remington, Rudd, and Justin Wark of the University of Oxford reflected on the growing importance of HED experiments in a paper they published in 2015. “Regimes of science hitherto thought out of reach in terrestrial settings are now being accessed routinely,” they wrote. “There are a multitude of new regimes of science that are now accessible in laboratory settings. Matter can now be studied experimentally with precision at very high pressures and over very short timescales.” Physicist Alan Wan observes, “Thanks to these NIF experiments, we have obtained much greater understanding of HED physics.” He adds, “In HED science, the United States is clearly out front in the world. Lawrence Livermore and the nation need to remain the center of excellence for HED science. We’re doing things no one else can do. It’s a way to leverage the nation’s scientific prowess.”
NIF can transform astronomy into an experimental science to better interpret observations and refine the latest models of the evolution and composition of stars, planets, and cosmic rays, as well as the dynamics of asteroid and meteor impacts, the response of space hardware to dust and debris impacts, and the effect of prolonged radiation exposure on nuclear reactor components. These are indeed exciting times for the astrophysicists, planetary physicists, and materials scientists who are pioneering new directions in HED science and regularly announcing exciting new discoveries. NIF’s rising shot rate—increased from 194 in fiscal year 2014 to more than 400 in fiscal year 2016—is enabling more Discovery Science experiments, shorter down time between them, and greater opportunities to explore new ideas.
Remington credits Discovery Science campaigns with bringing new ideas, science, and diagnostic techniques to NIF through the many outside collaborations. “We inevitably have a better facility and better science coming out of NIF when we have new people coming in with new ideas, creative instincts, and challenges.”
Arsenlis adds that the experiments help recruit, retain, and train a new generation of HED researchers. By their very nature, the experiments attract talented physicists and challenge theories in ways that can be experimentally tested. HED experiments at NIF, he says, are often extremely complex and require researchers to exercise the full range of modern computational and experimental capabilities. “A lot of physics is happening in every experiment, and we have to make sure the conditions are right,” he notes. “At the same time, a huge engineering effort is underway to build the required targets and diagnostics. All this creates a lot of opportunities for talented young researchers.”
Key Words: Astrophysical Collisionless Shock Experiments with Lasers (ACSEL), astrophysics, collisionless shock, cosmic ray, Discovery Science Program, exoplanet, high-energy-density (HED) science, hydrogen, Jupiter, National Ignition Facility (NIF), Omega Laser Facility, Neptune, planetary science, super-Earth, Target Diffraction In Situ, Velocity Interferometer System for Any Reflector (VISAR), Z Pulsed Power Facility.
For further information contact Tom Arsenlis (925) 424-2584 (email@example.com).
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