Conditions in our vast universe can be very different. The violent falls of celestial bodies leave scars on the surface of planets. Nuclear reactions in the hearts of stars generate enormous amounts of energy. Giant explosions will catapult matter far into space. But how exactly do processes like these proceed? What do they tell us about the universe? Can their power be used for the benefit of humanity?
To find out, scientists at the SLAC National Accelerator Laboratory have conducted sophisticated experiments and computer simulations that recreate the harsh space conditions at the laboratory's micro-scale.
“The field of laboratory astrophysics is growing at a rapid pace and is fueled by a number of technological breakthroughs,” says Siegfried Glenzer, head of the high energy density science division at SLAC. “We now have powerful lasers for creating extreme states of matter, advanced X-ray sources to analyze these states at the atomic level, and high-performance supercomputers for complex simulations that guide and help explain our experiments. With vast opportunities in these areas, SLAC is becoming a particularly fertile ground for this kind of research.”
Three recent studies highlighting this approach involve meteor strikes, giant planet cores and cosmic particle accelerators millions of times more powerful than the Large Hadron Collider, the largest particle accelerator on Earth.
Cosmic "trinkets" indicate meteors
It is known that high pressure can transform the soft form of carbon - graphite, which is used as a lead - into an extremely heavy form of carbon, diamond. Could this happen if a meteor hits graphite on the ground? Scientists believe they can, and that these falls, in fact, could be powerful enough to produce what is called lonsdaleite, a special form of diamond that is even stronger than a regular diamond.
"The existence of lonsdaleite has been disputed, but now we have found compelling evidence for this," says Glenzer, principal investigator of the paper, published in March in Nature Communications.
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The scientists heated the surface of the graphite with a powerful optical laser pulse, which sent a shock wave into the sample and quickly compressed it. By shining bright, ultrafast LCLS X-rays through the source, the scientists were able to see how the shock altered the atomic structure of the graphite.
“We saw lonsdaleite form in some graphite samples in a few billionths of a second and at a pressure of 200 gigapascals (2 million times the atmospheric pressure at sea level),” says lead author Dominik Krautz of the German Helmholtz Center, based in California. University at Berkeley at the time of research. "These results strongly support the idea that violent impacts can synthesize this form of diamond, and this, in turn, can help us identify meteor impact sites."
Giant planets turn hydrogen into metal
The second study, published recently in Nature Communications, focuses on another important transformation that could take place inside giant gas planets like Jupiter, the interior of which is mostly liquid hydrogen: at high temperature and pressure, this material passes from "normal", electrically insulating state into metallic, conductive.
“Understanding this process provides new details about planetary formation and the evolution of the solar system,” says Glenzer, who was also one of the principal investigators of the work. "Although such a transition was already predicted in the 1930s, we never opened a direct window to atomic processes."
That is, it was not opened until Glenzer and his fellow scientists conducted an experiment at Livermore National Laboratory (LLNL), where they used a high-power Janus laser to quickly squeeze and heat a sample of liquid deuterium, a heavy form of hydrogen, and create an X-ray burst., which revealed consistent structural changes in the sample.
Scientists have seen that above a pressure of 250,000 atmospheres and a temperature of 7,000 degrees Fahrenheit, deuterium does change from a neutral insulating liquid to an ionized metallic one.
"Computer simulations show that the transition coincides with the separation of two atoms, usually bonded together in deuterium molecules," says lead author Paul Davis, a graduate student at the University of California, Berkeley at the time of writing. "Apparently, the pressure and temperature of the laser-induced shock wave rips the molecules apart, their electrons become unbound and can conduct electricity."
In addition to planetary science, this research could also aid research aimed at using deuterium as nuclear fuel for thermonuclear reactions.
How to build a space accelerator
The third example of an extreme universe, a universe "on the brink", is incredibly powerful space particle accelerators - near supermassive black holes, for example - spewing streams of ionized gas, plasma, hundreds of thousands of light years into space. The energy contained in these streams and their electromagnetic fields can be converted into incredibly energetic particles, which produce very short but intense bursts of gamma rays that can be detected on Earth.
Scientists would like to know how these energy accelerators work, as it will help understand the universe. In addition, fresh ideas for building more powerful accelerators could be drawn from this. After all, particle acceleration is at the heart of many fundamental physics experiments and medical devices.
Scientists believe that one of the main driving forces behind space accelerators could be "magnetic reconnection" - a process in which magnetic field lines in a plasma break up and reconnect in a different way, releasing magnetic energy.
“Magnetic reconnection has previously been observed in the laboratory, for example, in experiments with the collision of two plasmas that were created using high-power lasers,” says Frederico Fiutsa, a scientist at the High Energy Density Science Division and principal investigator of the theoretical paper published in March in Physical Review Letters. … “Nevertheless, none of these laser experiments have observed nonthermal acceleration of particles - acceleration not associated with plasma heating. Our work shows that with a certain design, our experiments should see it."
His team ran a series of computer simulations that predicted how plasma particles should behave in such experiments. The most serious calculations, based on 100 billion particles, required over a million CPU hours and over a terabyte of memory on the Mira supercomputer at Argonne National Laboratory.
“We have identified key parameters for the required detectors, including the energy range in which they will operate, the required energy resolution and the location in the experiment,” said lead author Samuel Totorika, a graduate student at Stanford University. "Our results represent a recipe for designing future experiments that will want to know how particles get energy from magnetic reconnection."
