When one of Hye-Sook Park’s experiments goes well, everyone nearby knows. “We can hear Hye-Sook screaming,” she’s heard colleagues say.
It’s no surprise that she can’t contain her excitement. She’s getting a closeup look at the physics of exploding stars, or supernovas, a phenomenon so immense that its power is difficult to put into words.
Rather than studying these explosions from a distance through telescopes, Park, a physicist at Lawrence Livermore National Laboratory in California, creates something akin to these paroxysmal blasts using the world’s highest-energy lasers.
About 10 years ago, Park and colleagues embarked on a quest to understand a fascinating and poorly understood feature of supernovas: Shock waves that form in the wake of the explosions can boost particles, such as protons and electrons, to extreme energies.
“Supernova shocks are considered to be some of the most powerful particle accelerators in the universe,” says plasma physicist Frederico Fiuza of SLAC National Accelerator Laboratory in Menlo Park, Calif., one of Park’s collaborators.
Some of those particles eventually slam into Earth, after a fast-paced marathon across cosmic distances. Scientists have long puzzled over how such waves give energetic particles their massive speed boosts. Now, Park and colleagues have finally created a supernova-style shock wave in the lab and watched it send particles hurtling, revealing possible new hints about how that happens in the cosmos.
Bringing supernova physics down to Earth could help resolve other mysteries of the universe, such as the origins of cosmic magnetic fields. And there’s a more existential reason physicists are fascinated by supernovas. These blasts provide some of the basic building blocks necessary for our existence. “The iron in our blood comes from supernovae,” says plasma physicist Carolyn Kuranz of the University of Michigan in Ann Arbor, who also studies supernovas in the laboratory. “We’re literally created from stars.”
As a graduate student in the 1980s, Park worked on an experiment 600 meters underground in a working salt mine beneath Lake Erie in Ohio. Called IMB for Irvine-Michigan-Brookhaven, the experiment wasn’t designed to study supernovas. But the researchers had a stroke of luck. A star exploded in a satellite galaxy of the Milky Way, and IMB captured particles catapulted from that eruption. Those messengers from the cosmic explosion, lightweight subatomic particles called neutrinos, revealed a wealth of new information about supernovas.
But supernovas in our cosmic vicinity are rare. So decades later, Park isn’t waiting around for a second lucky event.
Instead, her team and others are using extremely powerful lasers to re-create the physics seen in the aftermath of supernova blasts. The lasers vaporize a small target, which can be made of various materials, such as plastic. The blow produces an explosion of fast-moving plasma, a mixture of charged particles, that mimics the behavior of plasma erupting from supernovas.
The stellar explosions are triggered when a massive star exhausts its fuel and its core collapses and rebounds. Outer layers of the star blast outward in an explosion that can unleash more energy than will be released by the sun over its entire 10-billion-year lifetime. The outflow has an unfathomable 100 quintillion yottajoules of kinetic energy (SN: 2/8/17, p. 24).
Supernovas can also occur when a dead star called a white dwarf is reignited, for example after slurping up gas from a companion star, causing a burst of nuclear reactions that spiral out of control (SN: 4/30/16, p. 20).
In both cases, things really get cooking when the explosion sends a blast of plasma careening out of the star and into its environs, the interstellar medium — essentially, another ocean of plasma particles. Over time, a turbulent, expanding structure called a supernova remnant forms, begetting a beautiful light show, tens of light-years across, that can persist in the sky for many thousands of years after the initial explosion. It’s that roiling remnant that Park and colleagues are exploring.
Studying supernova physics in the lab isn’t quite the same thing as the real deal, for obvious reasons. “We cannot really create a supernova in the laboratory, otherwise we would be all exploded,” Park says.
In lieu of self-annihilation, Park and others focus on versions of supernovas that are scaled down, both in size and in time. And rather than reproducing the entirety of a supernova all at once, physicists try in each experiment to isolate interesting components of the physics taking place. Out of the immense complexity of a supernova, “we are studying just a tiny bit of that, really,” Park says.
For explosions in space, scientists are at the mercy of nature. But in the laboratory, “you can change parameters and see how shocks react,” says astrophysicist Anatoly Spitkovsky of Princeton University, who collaborates with Park.
The laboratory explosions happen in an instant and are tiny, just centimeters across. For example, in Kuranz’s experiments, the equivalent of 15 minutes in the life of a real supernova can take just 10 billionths of a second. And a section of a stellar explosion larger than the diameter of Earth can be shrunk down to 100 micrometers. “The processes that occur in both of those are very similar,” Kuranz says. “It blows my mind.”
To replicate the physics of a supernova, laboratory explosions must create an extreme environment. For that, you need a really big laser, which can be found in only a few places in the world, such as NIF, the National Ignition Facility at Lawrence Livermore, and the OMEGA Laser Facility at the University of Rochester in New York.
At both places, one laser is split into many beams. The biggest laser in the world, at NIF, has 192 beams. Each of those beams is amplified to increase its energy exponentially. Then, some or all of those beams are trained on a small, carefully designed target. NIF’s laser can deliver more than 500 trillion watts of power for a brief instant, momentarily outstripping the total power usage in the United States by a factor of a thousand.
A single experiment at NIF or OMEGA, called a shot, is one blast from the laser. And each shot is a big production. Opportunities to use such advanced facilities are scarce, and researchers want to have all the details ironed out to be confident the experiment will be a success.
When a shot is about to happen, there’s a space-launch vibe. Operators monitor the facility from a control room filled with screens. When the time of the laser blast nears, a voice begins counting down: “Ten, nine, eight …”
“When they count down for your shot, your heart is pounding,” says plasma physicist Jena Meinecke of the University of Oxford, who has worked on experiments at NIF and other laser facilities.
At the moment of the shot, “you kind of want the Earth to shake,” Kuranz says. But instead, you might just hear a snap — the sound of the discharge from capacitors that store up huge amounts of energy for each shot.
Then comes a mad dash to review the results and determine if the experiment has been successful. “It’s a lot of adrenaline,” Kuranz says.
Lasers aren’t the only way to investigate supernova physics in the lab. Some researchers use intense bursts of electricity, called pulsed power. Others use small amounts of explosives to set off blasts. The various techniques can be used to understand different stages in supernovas’ lives.
A real shocker
Park brims with cosmic levels of enthusiasm, ready to erupt in response to a new nugget of data or a new success in her experiments. Re-creating some of the physics of a supernova in the lab really is as remarkable as it sounds, she says. “Otherwise I wouldn’t be working on it.” Along with Spitkovsky and Fiuza, Park is among more than a dozen scientists involved in the Astrophysical Collisionless Shock Experiments with Lasers collaboration, or ACSEL, the quest Park embarked upon a decade ago. Their focus is shock waves.
The result of a violent input of energy, shock waves are marked by an abrupt increase in temperature, density and pressure. On Earth, shock waves cause the sonic boom of a supersonic jet, the clap of thunder in a storm and the damaging pressure wave that can shatter windows in the aftermath of a massive explosion. These shock waves form as air molecules slam into each other, piling up molecules into a high-density, high-pressure and high-temperature wave.
In cosmic environments, shock waves occur not in air, but in plasma, a mixture of protons, electrons and ions, electrically charged atoms. There, particles may be diffuse enough that they don’t directly collide as they do in air. In such a plasma, the pileup of particles happens indirectly, the result of electromagnetic forces pushing and pulling the particles. “If a particle changes trajectory, it’s because it feels a magnetic field or an electric field,” says Gianluca Gregori, a physicist at the University of Oxford who is part of ACSEL.
But exactly how those fields form and grow, and how such a shock wave results, has been hard to decipher. Researchers have no way to see the process in real supernovas; the details are too small to observe with telescopes.
These shock waves, which are known as collisionless shock waves, fascinate physicists. “Particles in these shocks can reach amazing energies,” Spitkovsky says. In supernova remnants, particles can gain up to 1,000 trillion electron volts, vastly outstripping the several trillion electron volts reached in the biggest human-made particle accelerator, the Large Hadron Collider near Geneva. But how particles might surf supernova shock waves to attain their astounding energies has remained mysterious.
Magnetic field origins
To understand how supernova shock waves boost particles, you have to understand how shock waves form in supernova remnants. To get there, you have to understand how strong magnetic fields arise. Without them, the shock wave can’t form.
Electric and magnetic fields are closely intertwined. When electrically charged particles move, they form tiny electric currents, which generate small magnetic fields. And magnetic fields themselves send charged particles corkscrewing, curving their trajectories. Moving magnetic fields also create electric fields.
The result is a complex feedback process of jostling particles and fields, eventually producing a shock wave. “This is why it’s so fascinating. It’s a self-modulating, self-controlling, self-reproducing structure,” Spitkovsky says. “It’s like it’s almost alive.”
All this complexity can develop only after a magnetic field forms. But the haphazard motions of individual particles generate only small, transient magnetic fields. To create a significant field, some process within a supernova remnant must reinforce and amplify the magnetic fields. A theoretical process called the Weibel instability, first thought up in 1959, has long been expected to do just that.
In a supernova, the plasma streaming outward in the explosion meets the plasma of the interstellar medium. According to the theory behind the Weibel instability, the two sets of plasma break into filaments as they stream by one another, like two hands with fingers interlaced. Those filaments act like current-carrying wires. And where there’s current, there’s a magnetic field. The filaments’ magnetic fields strengthen the currents, further enhancing the magnetic fields. Scientists suspected that the electromagnetic fields could then become strong enough to reroute and slow down particles, causing them to pile up into a shock wave.
In 2015 in Nature Physics, the ACSEL team reported a glimpse of the Weibel instability in an experiment at OMEGA. The researchers spotted magnetic fields, but didn’t directly detect the filaments of current. Finally, this year, in the May 29 Physical Review Letters, the team reported that a new experiment had produced the first direct measurements of the currents that form as a result of the Weibel instability, confirming scientists’ ideas about how strong magnetic fields could form in supernova remnants.
For that new experiment, also at OMEGA, ACSEL researchers blasted seven lasers each at two targets facing each other. That resulted in two streams of plasma flowing toward each other at up to 1,500 kilometers per second — a speed fast enough to circle the Earth twice in less than a minute. When the two streams met, they separated into filaments of current, just as expected, producing magnetic fields of 30 tesla, about 20 times the strength of the magnetic fields in many MRI machines.
“What we found was basically this textbook picture that has been out there for 60 years, and now we finally were able to see it experimentally,” Fiuza says.
Surfing a shock wave
Once the researchers had seen magnetic fields, the next step was to create a shock wave and to observe it accelerating particles. But, Park says, “no matter how much we tried on OMEGA, we couldn’t create the shock.”
They needed the National Ignition Facility and its bigger laser.
There, the researchers hit two disk-shaped targets with 84 laser beams each, or nearly half a million joules of energy, about the same as the kinetic energy of a car careening down a highway at 60 miles per hour.
Two streams of plasma surged toward each other. The density and temperature of the plasma rose where the two collided, the researchers reported in the September Nature Physics. “No doubt about it,” Park says. The group had seen a shock wave, specifically the collisionless type found in supernovas. In fact there were two shock waves, each moving away from the other.
Learning the results sparked a moment of joyous celebration, Park says: high fives to everyone.
“This is some of the first experimental evidence of the formation of these collisionless shocks,” says plasma physicist Francisco Suzuki-Vidal of Imperial College London, who was not involved in the study. “This is something that has been really hard to reproduce in the laboratory.”
The team also discovered that electrons had been accelerated by the shock waves, reaching energies more than 100 times as high as those of particles in the ambient plasma. For the first time, scientists had watched particles surfing shock waves like the ones found in supernova remnants.
But the group still didn’t understand how that was happening.
In a supernova remnant and in the experiment, a small number of particles are accelerated when they cross over the shock wave, going back and forth repeatedly to build up energy. But to cross the shock wave, the electrons need some energy to start with. It’s like a big-wave surfer attempting to catch a massive swell, Fiuza says. There’s no way to catch such a big wave by simply paddling. But with the energy provided by a Jet Ski towing surfers into place, they can take advantage of the wave’s energy and ride the swell.
“What we are trying to understand is: What is our Jet Ski? What happens in this environment that allows these tiny electrons to become energetic enough that they can then ride this wave and be accelerated in the process?” Fiuza says.
The researchers performed computer simulations that suggested the shock wave has a transition region in which magnetic fields become turbulent and messy. That hints that the turbulent field is the Jet Ski: Some of the particles scatter in it, giving them enough energy to cross the shock wave.
Enormous laser facilities such as NIF and OMEGA are typically built to study nuclear fusion — the same source of energy that powers the sun. Using lasers to compress and heat a target can cause nuclei to fuse with one another, releasing energy in the process. The hope is that such research could lead to fusion power plants, which could provide energy without emitting greenhouse gases or dangerous nuclear waste (SN: 4/20/13, p. 26). But so far, scientists have yet to get more energy out of the fusion than they put in — a necessity for practical power generation.
So these laser facilities dedicate many of their experiments to chasing fusion power. But sometimes, researchers like Park get the chance to study questions based not on solving the world’s energy crisis, but on curiosity — wondering what happens when a star explodes, for example. Still, in a roundabout way, understanding supernovas could help make fusion power a reality as well, as that celestial plasma exhibits some of the same behaviors as the plasma in fusion reactors.
At NIF, Park has also worked on fusion experiments. She has studied a wide variety of topics since her grad school days, from working on the U.S. “Star Wars” missile defense program, to designing a camera for a satellite sent to the moon, to looking for the sources of high-energy cosmic light flares called gamma-ray bursts. Although she is passionate about each topic, “out of all those projects,” she says, “this particular collisionless shock project happens to be my love.”
Early in her career, back on that experiment in the salt mine, Park got a first taste of the thrill of discovery. Even before IMB captured neutrinos from a supernova, a different unexpected neutrino popped up in the detector. The particle had passed through the entire Earth to reach the experiment from the bottom. Park found the neutrino while analyzing data at 4 a.m., and woke up all her collaborators to tell them about it. It was the first time anyone working on the experiment had seen a particle coming up from below. “I still clearly remember the time when I was seeing something nobody’s seen,” Park recalls.
Now, she says, she still gets the same feeling. Screams of joy erupt when she sees something new that describes the physics of unimaginably vast explosions.
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