When one of the Hye-Sook Park experiments goes well, everyone nearby knows it. “We can hear Hye-Sook screaming,” he heard his colleagues say.
No wonder he can’t contain his excitement. He is closely watching the physics of exploding stars or supernovae, a phenomenon so immense that their power is difficult to express.
Instead of studying these explosions remotely through telescopes, Park, a physicist at Lawrence Livermore National Laboratory in California, creates something similar to these paroxysmal explosions using the world’s highest-energy lasers.
About 10 years ago, Park and his colleagues embarked on a quest to understand a fascinating and misunderstood feature of supernovae: the shock waves that form as a result of explosions can increase particles, such as protons and electrons, to extreme energies.
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“Supernova collisions are considered some of the most powerful particle accelerators in the universe,” says plasma physicist Frederico Fiuza of SLAC National Accelerator Laboratory in Menlo Park, California, one of Park’s collaborators.
Some of those particles eventually fall against Earth, after a fast-paced marathon through cosmic distances. Scientists have long been baffled about how these waves give energy particles their huge increase in speed. Now, Park and his colleagues have finally created a supernova-style shockwave in the lab and watched as they sent particles wounding, revealing possible new hints about how this happens in the cosmos.
Bringing supernova physics to Earth could help solve other mysteries in the universe, such as the origins of cosmic magnetic fields. And there’s a more existential reason physicists are fascinated by supernovae. These explosions provide some of the basic elements necessary for our existence. “The iron in our blood comes from supernovae,” says plasma physicist Carolyn Kuranz of the University of Michigan at Ann Arbor, who also studies supernovae in the lab. "We are literally created from stars."
As a graduate student in the 1980s, Park worked on an experiment 600 meters underground in a salt mine in operation under Lake Erie in Ohio. Called IMB by Irvine-Michigan-Brookhaven, the experiment was not designed to study supernovae. But investigators had a stroke of luck. A star exploded in a Milky Way satellite galaxy and IMB captured catapulted particles from that eruption. Those messengers of the cosmic explosion, light subatomic particles called neutrinos, revealed a wealth of new information about supernovae.
But supernovae in our cosmic vicinity are rare. So decades later, Park is not expecting a second event.
on the left: John Van der Velde; Lanie L. Rivera / Lawrence Livermore National Laboratory
Instead, his team and others are using extremely powerful lasers to recreate the physics observed after supernova explosions. Lasers vaporize a small lens, which can be made of various materials, such as plastic. The blow produces a rapidly moving plasma explosion, a mixture of charged particles, that mimics the behavior of the erupting plasma from supernovae.
Stellar explosions are triggered when a massive star runs out of fuel and its core falls and bounces. The outer layers of the star explode outward in an explosion that can unleash more energy than the sun will release throughout its 10 billion-year life. The output stream has an unfathomable 100 quintillions of kinetic energy yottajoules (SN: 2/8/17, p. 24).
Supernovae can also occur when a dead star called a white dwarf is revived again, for example after ripping off the gas from a companion star, causing an explosion of nuclear reactions that spiral out of control (SN: 30/04/16, p .20).
NASA, CXC, MIT L. Lopez et al. (X-rays), Dovecote (infrared), VLA / NRAO / NSF (radio)
In both cases, things actually cook up when the explosion sends a plasma explosion coming out of the star and its surroundings, the interstellar medium, essentially another ocean of plasma particles. Over time, a turbulent, expanding structure called a supernova remnant is formed, which generates a beautiful spectacle of light, tens of light-years in diameter, that can persist in the sky for thousands of years after the initial explosion. It’s that crowned remnant that Park and his colleagues are exploring.
Studying supernova physics in the lab is not the same as real business, for obvious reasons. “We can’t really create a supernova in the lab, otherwise we would all explode,” Park says.
Instead of self-annihilating, Park and others focus on versions of supernovae that shrink, both in size and time. And instead of reproducing the entirety of one supernova at a time, physicists try in each experiment to isolate interesting components of the physics that are taking place. Of the immense complexity of a supernova, “we’re studying a little bit of that, really,” Park says.
For explosions in space, scientists are at the mercy of nature. But in the lab, “you can change parameters and see how the shocks react,” says astrophysicist Anatoly Spitkovsky of Princeton University, who collaborates with Park.
Laboratory explosions occur in an instant and are small, only a few inches in diameter. For example, in Kuranz's experiments, the equivalent of 15 minutes in the life of a real supernova can take only 10 trillionths of a second. And a section of a stellar explosion larger than the diameter of the Earth can be reduced to 100 micrometers. “The processes that occur in both are very similar,” Kuranz says. "Turn my head."
Powerful and mysterious stellar explosions are difficult to understand from afar, so researchers have figured out how to recreate the extreme physics of supernovae in the lab and study how the explosions sow the cosmos with energy elements and particles.
To reproduce the physics of a supernova, laboratory explosions must create an extreme environment. To do this, you need a really large laser, which can only be found in some parts of the world, such as NIF, Lawrence Livermore’s National Ignition Facility, and the OMEGA Laser Facility at the University of Rochester in New York.
In both places, a laser is divided into many beams. The largest laser in the world, in NIF, has 192 beams. Each of these beams is amplified to increase its energy exponentially. Afterwards, some or all of these beams are trained on a small carefully designed target. The NIF laser can deliver more than 500 trillion watts of power for a brief instant, momentarily surpassing total power consumption in the United States by a factor of a thousand.
A single experiment in NIF or OMEGA, called a shot, is a laser blast. And every shot is a great production. Opportunities to use such advanced facilities are scarce and researchers want every detail to be worked out to be sure the experiment will be a success.
When a shot is about to take place, there is a space launch environment. Operators control the installation from a control room full of screens. As the moment of laser burst approaches, a voice begins to countdown: "Ten, nine, eight …"
“When they count back for the shot, your heart beats,” says plasma physicist Jena Meinecke of Oxford University, who has worked on experiments at the NIF and other laser facilities.
At the time of the shooting, “he wants the Earth to shake,” Kuranz says. But instead, you can only hear for a moment: the sound of the discharge from capacitors that store huge amounts of energy for each shot.
Then comes a madman to review the results and determine if the experiment was successful. “It’s a lot of adrenaline,” Kuranz says.
Lawrence Livermore National Laboratory
Lasers are not the only way to investigate the physics of supernovae in the laboratory. Some researchers use intense bursts of electricity, called pulsed energy. Others use small amounts of explosives to fire. The various techniques can be used to understand different stages in the life of supernovae.
A real shock
The park is filled with cosmic levels of enthusiasm, ready to explode in response to a new amount of data or a new success in their experiments. Re-creating part of a supernova's physics in the lab 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 collision-free Astrophysics collaboration with lasers or ACSEL, the mission Park began a decade ago. Their focus is the shock waves.
The result of a violent inflow of energy, the shock waves are marked by a sharp rise in temperature, density and pressure. On Earth, shock waves cause the sound boom of a supersonic jet, the thunder blow in a storm, and the damaging pressure wave that can break windows after a massive explosion. These shock waves form when air molecules collide with each other, accumulating molecules in a wave of high density, high pressure, and high temperature.
In cosmic environments, shock waves do not occur in air, but in plasma, a mixture of protons, electrons, and ions, electrically charged atoms. There, the particles can be diffuse enough not to collide directly as in air. In such a plasma, the accumulation of particles occurs 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, an Oxford University physicist who is part of ACSEL.
But exactly how those fields form and grow and how such a shock wave results, was hard to decipher. Researchers have no way of seeing the process in real supernovae; the details are too small to observe with telescopes.
These shock waves, known as collision-free shock waves, fascinate physicists. “The particles from these shocks can reach amazing energies,” Spitkovsky says. In supernova remnants, particles can gain up to 1,000 trillion volts of electrons, far exceeding the several trillion volts of electrons reached in the largest man-made particle accelerator, the Large Hadron Collider near Geneva. But the way particles can navigate supernova shocks to reach their amazing energies has been mysterious.
Origins of the magnetic field
To understand how supernova shock waves increase particles, you need 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 cannot form.
The electric and magnetic fields are closely intertwined. When electrically charged particles move, they form small electric currents, which generate small magnetic fields. And the magnetic fields themselves send charged particles by pulling out the cork stopper, bending their trajectories. Moving magnetic fields also create electric fields.
The result is a complex process of feedback of particles and fields that cause a shock wave. "That's why it's so fascinating. It's a self-modulating, self-controlled, self-reproducing structure," says Spitkovsky. "It's like he's almost alive."
All this complexity can only develop after a magnetic field is formed. But the random movements of individual particles generate only small, transient magnetic fields. To create a meaningful field, some process within a supernova remnant must reinforce and amplify the magnetic fields. It has long been expected that a theoretical process called Weibel instability, first thought of in 1959, would do just that.
In a supernova, the plasma flowing outward in the explosion meets the plasma in the interstellar medium. According to the theory behind Weibel's instability, the two sets of plasma break into filaments as they flow together, like two hands with fingers intertwined. These filaments act as wires that carry current. And where there is current, there is a magnetic field. The magnetic fields of the filaments strengthen the currents, further improving the magnetic fields. Scientists suspected that the electromagnetic fields could then be strong enough to redirect and retard the particles, causing them to accumulate in a shock wave.
In 2015 at Nature Physics, the ACSEL team reported a look at Weibel’s instability in an OMEGA experiment. The researchers detected magnetic fields, but did not directly detect current filaments. Finally, this year, in the May 29 physical review letters, the team reported that a new experiment would produce the first direct measurements of currents forming as a result of Weibel's instability, confirming scientists' ideas about how fields could form. strong magnetic. in supernova remnants.
For that new experiment, also at OMEGA, ACSEL researchers launched seven lasers each on two opposing targets. This resulted in two plasma currents flowing at each other up to 1,500 kilometers per second, a speed fast enough to orbit the Earth twice in less than a minute. When the two fluxes were found, they separated into current filaments, as expected, producing magnetic fields of 30 tesla, approximately 20 times the strength of the magnetic fields in many MRI machines.
“What we found was basically this image from the textbook that has been out for 60 years, and now we’ve finally been able to see it experimentally,” Fiuza says.
Surfing a shock wave
Once the researchers saw magnetic fields, the next step was to create a shock wave and observe it by accelerating the particles. But, says Park, "no matter how hard we tried with OMEGA, we couldn't create the shock."
They needed the national ignition installation and its largest laser.
There, the researchers hit two disk-shaped targets with 84 laser beams each, or nearly half a million joules of energy, roughly the same as the kinetic energy of a car traveling a road at 60 miles per hour.
Two plasma streams emerged from each other. The researchers reported in the September issue of the journal Nature Physics that plasma density and temperature rose where the two collided. “There’s no question about that,” Park says. The group had seen a shock wave, especially the no-collision type found in supernovae. In fact, there were two shock waves, each moving away from the other.
Learning the results unleashed a moment of joyful celebration, Park says: five years for all.
“This is some of the first experimental evidence of the formation of these collision-free shocks,” says plasma physicist Francisco Suzuki-Vidal of Imperial College London, who did not participate in the study. "This is something that was very difficult to reproduce in the lab."
The team also found that electrons had been accelerated by shock waves, reaching energies more than 100 times higher than those of ambient plasma particles. For the first time, scientists have seen particles surfing shock waves like those found in supernova remnants.
But the group still didn’t understand how that was going.
In a supernova remnant and in the experiment, a small number of particles are accelerated as they cross the shock wave, going back and forth repeatedly to accumulate energy. But to cross the shock wave, electrons need some energy to get started. Fiuza says he’s like a big-wave surfer trying to get through a big swell. There is no way to catch such a big wave simply by paddling. But with the energy provided by a Jet Ski towing surfers in place, they can harness the energy of the wave and ride the tide.
F. Fiuza / SLAC National Accelerator Laboratory
"What we're trying to understand is, what is our jet ski? What happens in this environment that allows these little electrons to become energetic enough to be able to ride this wave and accelerate in the process?" Fiuza says.
The researchers performed computer simulations that suggested that the shock wave has a transition region in which magnetic fields are turbulent and disordered. This suggests that the turbulent field is Jet Ski: some of the particles disperse in it, giving them enough energy to cross the shock wave.
Call for attention
Huge laser facilities like NIF and OMEGA are usually built to study nuclear fusion, the same source of energy that powers the sun. Using lasers to compress and heat a target can cause the nuclei to fuse with each other, 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 hazardous nuclear waste (SN: 20/04/13, p. 26). But so far, scientists have yet to draw more energy from fusion than they put in, a necessity for practical power generation.
Thus, these laser installations dedicate many of their experiments to pursuing fusion power. But sometimes researchers like Park have the opportunity to study issues not based on resolving the global energy crisis, but on curiosity, wondering what happens when a star explodes, for example. Still, in a roundabout way, understanding supernovae could help make fusion power a reality as well, as celestial plasma exhibits some of the same behaviors as plasma from fusion reactors.
At NIF, Park also worked on fusion experiments. He studied a wide variety of subjects from his undergraduate studies, from working on the U.S. "Star Wars" missile defense program, to designing a camera for a satellite sent to the moon, to searching for high-energy cosmic light sources. flares called gamma ray bursts. Although he is passionate about every subject, "of all those projects," he says, "this collision-free shock project is my love."
Early in his career, back in that experiment at the salt mine, Park had a first taste of the thrill of discovery. Even before IMB captured neutrinos from a supernova, another unexpected neutrino appeared in the detector. The particle had traversed the entire Earth to reach the experiment from the bottom. Park found the neutrino as he analyzed the data at 4 a.m. and woke up all his collaborators to talk to them. It was the first time anyone working on the experiment had seen a particle emerge from below. “I still clearly remember the time when I was seeing something no one saw,” Park recalls.
Now, he says, he still has the same feeling. Screams of joy erupt when he sees something new that describes the physics of unimaginably vast explosions.
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