If one of the experiments at Hye-Sook Park goes well, everyone nearby will know. “We can hear Hye-Sook screaming,” she heard colleagues say.
It’s no surprise that she can’t hold back her excitement. She gets a close look at the physics of exploding stars or supernovae, a phenomenon so immense that its power is difficult to put into words.
Instead of examining these explosions remotely through telescopes, Park, a physicist at Lawrence Livermore National Laboratory in California, uses the world’s most energetic lasers to create something similar to these paroxysmal explosions.
About 10 years ago, Park and colleagues began their search for a fascinating and poorly understood feature of supernovae: shock waves that form after the explosions can excite 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 the SLAC National Accelerator Laboratory in Menlo Park, California, who works for Park.
Some of these particles eventually hit the earth after a rapid marathon over cosmic distances. Scientists have long pondered how such waves give energetic particles their massive bursts of speed. Now Park and colleagues have finally created a supernova-style shockwave in the lab and watched it blow up particles, revealing possible new clues as to how this is happening in the cosmos.
When you bring supernova physics to Earth, you can solve other puzzles of 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 building blocks necessary for our existence. “The iron in our blood comes from supernovae,” says plasma physicist Carolyn Kuranz from the University of Michigan at Ann Arbor, who also studies supernovae in the laboratory. “We are literally made of stars.”
As a PhD student in the 1980s, Park worked on an experiment 600 meters underground in a working salt mine below Lake Erie in Ohio. The experiment, called IMB for Irvine-Michigan-Brookhaven, was not designed to study supernovae. But the researchers were lucky. A star exploded in a satellite galaxy in the Milky Way, and IMB captured particles catapulted from that eruption. These cosmic explosion messengers, light subatomic particles called neutrinos, revealed a wealth of new information about supernovae.
But supernovae in our cosmic environment are rare. Decades later, Park is not waiting for a second happy event.
Instead, her team and others are using extremely powerful lasers to restore physics after the supernova explosions. The lasers vaporize a small target that can be made from various materials such as plastic. The blow creates an explosion of fast-moving plasma, a mixture of charged particles that mimics the behavior of plasma erupting from supernovae.
The stellar explosions are triggered when a massive star runs out of fuel and its core collapses and rebounds. The star’s outer layers explode outward in an explosion that can release more energy than is released by the sun during its entire 10 billion year lifespan. The runoff has an unfathomable kinetic energy of 100 trillion yottajoules (SN: 02/08/17, p. 24).
Supernovas can also occur when a dead star known as a white dwarf is reignited, such as after sipping gas from a companion star, causing a burst of nuclear reactions that get out of hand (SN: 04/30/16, p. 20th).
In either case, it really gets boiling when the explosion sends an explosion of plasma from the star into its surroundings, the interstellar medium – essentially another ocean of plasma particles. Over time, a turbulent, expanding structure known as the supernova remnant forms, creating a beautiful light show several tens of years in diameter that can remain in the sky for many thousands of years after the first explosion. It is this seething remnant that Park and his colleagues are exploring.
Studying supernova physics in the lab isn’t quite the same as doing actual business for obvious reasons. “We can’t create a supernova in the laboratory, otherwise we would all have exploded,” says Park.
Instead of self-annihilation, Park and others are focusing on versions of supernovae that are shrinking both in size and time. And instead of reproducing the entirety of a supernova 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 really only study a tiny piece of it,” says Park.
For explosions in space, scientists are at the mercy of nature. In the lab, however, you can “change parameters and see how shocks react,” says Princeton University astrophysicist Anatoly Spitkovsky, who works with Park.
The laboratory explosions are instantaneous and tiny, only inches wide. For example, in Kuranz’s experiments, the equivalent of 15 minutes in the life of a real supernova can be as little as 10 billionths of a second. And a section of a stellar explosion larger than the diameter of the earth can be reduced to 100 microns. “The processes that take place in both are very similar,” says Kuranz. “It blows me away.”
In order to recreate the physics of a supernova, laboratory explosions must create an extreme environment. For that you need a really big laser that can only be found in a few places in the world, e.g. B. at NIF, the National Ignition Facility in Lawrence Livermore and the OMEGA Laser Facility at the University of Rochester in New York.
A laser is split into many beams at both points. The largest laser in the world at the NIF has 192 beams. Each of these rays is amplified to increase its energy exponentially. Then some or all of these rays are trained on a small, carefully designed target. NIF’s laser can deliver more than 500 trillion watts of power for a brief moment, exceeding total power consumption in the US by a factor of a thousand.
A single experiment at NIF or OMEGA called a shot is an explosion of the laser. And every shot is a big production. Opportunities to take advantage of such advanced facilities are rare, and researchers want all the details ironed out to be sure the experiment will be a success.
When a shot is imminent, there is a space launch atmosphere. The operators monitor the system from a control room filled with screens. As the time of the laser beam approaches, a voice begins to count down: “Ten, nine, eight …”
“When you count down for your shot, your heart beats,” says plasma physicist Jena Meinecke from Oxford University, who has worked on experiments at the NIF and other laser systems.
At the moment of the recording, “you want the earth to shake,” says Kuranz. Instead, you might just hear a crackle – the sound of capacitors discharging, storing large amounts of energy with each intake.
Then a crazy shot comes in to check the results and see if the experiment was successful. “It’s a lot of adrenaline,” says Kuranz.
Lasers aren’t the only way to study supernova physics in the laboratory. Some researchers use intense electrical surges called pulsed power. Others use small amounts of explosives to cause explosions. The various techniques can be used to understand different stages in the life of supernovas.
A real shock
Park is bursting with cosmic excitement and ready to break out in her experiments in response to a new data nugget or success. It really is as remarkable as it sounds to recreate some of the physics of a supernova in the laboratory, she says. “Otherwise I wouldn’t work on it.” Along with Spitkovsky and Fiuza, Park is one of more than a dozen scientists involved in the Astrophysical Collisionless Shock Experiments with Lasers (ACSEL) collaboration, the quest that Park began a decade ago. Your focus is on shock waves.
As a result of violent energy input, shock waves are characterized by sudden increases in temperature, density, and pressure. On Earth, shock waves cause the sonic boom of a supersonic jet, the thunder of a storm, and the noxious pressure wave that can shatter windows after a massive explosion. These shock waves form when air molecules hit each other and molecules pile up into a wave with high density, high pressure, and high temperature.
In cosmic environments, shock waves do not occur in the air but in the plasma, a mixture of protons, electrons and ions, electrically charged atoms. There particles can be so diffuse that they do not collide directly as in the air. In such a plasma, the accumulation of particles occurs indirectly, which is due to electromagnetic forces that push and pull the particles. “If a particle changes its trajectory, it’s because it feels a magnetic field or an electric field,” says Gianluca Gregori, a physicist at Oxford University who is part of ACSEL.
However, it was difficult to decipher how these fields form and grow and how such a shock wave results. Researchers have no way of seeing the process in real supernovae; The details are too small to be observed with telescopes.
These shock waves, known as collision-free shock waves, fascinate physicists. “Particles in these shocks can reach amazing energies,” says Spitkovsky. In supernova remnants, particles can gain up to 1,000 trillion electron volts, far exceeding the several trillion electron volts achieved in the largest man-made particle accelerator, the Large Hadron Collider near Geneva. But how particles could surf supernova shock waves to reach their amazing energies has remained a mystery.
Magnetic field origins
To understand how supernova shock waves stimulate particles, you need to understand how shock waves form in supernova remnants. To get there, one has to understand how strong magnetic fields are created. Without them, no shock wave can form.
Electric and magnetic fields are closely related. When electrically charged particles move, they form tiny electrical currents that create small magnetic fields. And magnetic fields themselves let charged particles corkscrew and curve their trajectories. Moving magnetic fields also create electric fields.
The result is a complex feedback process in which particles and fields are jostled and ultimately a shock wave is generated. “That’s why it’s so fascinating. It’s a self-modulating, self-regulating, self-reproducing structure,” says Spitkovsky. “It’s like it’s almost alive.”
All of this complexity can only develop after a magnetic field has formed. However, the random movements of individual particles only generate small, transient magnetic fields. In order to create a significant field, a process within a supernova remnant must amplify and amplify the magnetic fields. A theoretical process called Weibel instability, first developed in 1959, has long been expected to do just that.
In a supernova, the plasma flowing outwards during the explosion meets the plasma of the interstellar medium. According to the theory behind the Weibel instability, the two plasma sets break into filaments when they flow past each other, like two hands with folded fingers. These filaments act like current-carrying wires. And where there is electricity, there is a magnetic field. The magnetic fields of the filaments amplify the currents and further strengthen the magnetic fields. Scientists suspected that the electromagnetic fields could then become strong enough to redirect and slow down particles, causing them to pile up into a shock wave.
In 2015 in Natural physicsThe ACSEL team reported in an experiment at OMEGA about a look at the Weibel instability. The researchers discovered magnetic fields, but were unable to detect the current filaments directly. Finally, this year, on May 29th Physical Examination LettersThe team reported that a new experiment provided the first direct measurements of the currents formed as a result of the Weibel instability and confirmed the scientists’ ideas about how strong magnetic fields might form in supernova remnants.
For this new experiment, also at OMEGA, ACSEL researchers shone seven lasers each on two mutually facing targets. This resulted in two streams of plasma converging at speeds of up to 1,500 kilometers per second – a speed fast enough to orbit the earth twice in less than a minute. When the two currents met, they separated into current filaments, as expected, creating magnetic fields of 30 Tesla, which is roughly 20 times the strength of the magnetic fields in many MRI machines.
“What we found was basically this textbook picture that has been around for 60 years, and now we could finally see it experimentally,” says Fiuza.
Surfing a shock wave
After seeing magnetic fields, the next step was to create a shock wave and watch it accelerate particles. But Park says, “No matter how much we tried with OMEGA, we couldn’t create the shock.”
They needed the National Ignition Facility and their bigger laser.
There, the researchers hit two disk-shaped targets each with 84 laser beams, or nearly half a million joules of energy, roughly the same as the kinetic energy of a car racing down a highway at 60 miles per hour.
Two streams of plasma converged. The density and temperature of the plasma rose where the two collided, the researchers reported in September Natural physics. “No doubt about it,” says Park. The group had seen a shock wave, particularly the collision-free type found in supernovas. In fact, there were two shock waves moving away from each other.
Learning the results sparked a moment of joyous celebration, Park says: high fives for everyone.
“This is one of the first experimental evidence for the origin of these collision-free shocks,” says the plasma physicist Francisco Suzuki-Vidal of Imperial College London, who was not involved in the study. “This is something that was really difficult to reproduce in the laboratory.”
The team also discovered that electrons were accelerated by the shock waves and reached energies more than 100 times that of particles in the surrounding plasma. For the first time, scientists had observed particles surfing shock waves such as those found in supernova remnants.
But the group still didn’t understand how this happened.
In a supernova remnant and experiment, a small number of particles accelerate as they cross the shock wave and move back and forth repeatedly to build up energy. In order to overcome the shock wave, the electrons first need some energy. It’s like a big wave surfer trying to catch massive swell, says Fiuza. There is no way to catch such a big wave by simply paddling. With the energy provided by a jet ski surfer, they can use the energy of the wave and handle the swell.
“We try to understand: what is our jet ski? What happens in this environment in which these tiny electrons become so energetic that they ride this wave and can be accelerated in the process? “Fiuza says.
The researchers ran computer simulations that indicated that the shock wave has a transition area where magnetic fields become turbulent and messy. That suggests that the turbulent field is the jet ski: some of the particles scatter in it, giving them enough energy to cross the shock wave.
Wake up call
Huge laser systems like the 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 nuclei to fuse together, releasing energy in the process. The hope is that such research could lead to fusion power plants that could provide energy without emitting greenhouse gases or dangerous nuclear waste (SN: 04/20/13, p. 26th). So far, however, scientists have not gained more energy from the fusion than they invested – a necessity for practical power generation.
Therefore, these laser systems dedicate many of their experiments to the hunt for fusion power. But sometimes researchers like Park have the opportunity to explore questions that are based not on solving the world’s energy crisis but on curiosity. For example, you might wonder what happens when a star explodes. In a roundabout way, understanding supernovae could also help make the force of fusion a reality, as this heavenly plasma exhibits some of the same behaviors as the plasma in fusion reactors.
At the NIF, Park also worked on fusion experiments. Since her school days, she has studied a variety of topics, from working on the American anti-missile defense program “Star Wars” to developing a camera for a satellite sent to the moon and searching for sources of high-energy cosmic light torches, so-called gamma-ray bursts. Although she is passionate about every topic, “of all of these projects,” she says, “this particular collision-free shock project just happens to be my love.”
Early in her career, when she was experimenting again in the salt mine, Park got her first taste of the thrill of discovery. Even before IMB caught neutrinos from a supernova, another unexpected neutrino appeared in the detector. The particle had traveled all over the earth to reach the experiment from below. Park found the neutrino while analyzing data at 4 a.m. and woke up all of her coworkers to tell them about it. It was the first time anyone working on the experiment saw a particle emerge from below. “I remember the time I saw something that nobody saw,” said Park.
Now, she says, she still feels the same way. Cries of joy break out when she sees something new that describes the physics of unimaginably large explosions.