BELLA, the Incredible Shrinking Particle Accelerator, Is Ready for Its Close-Up
Atoms will reveal their secrets—you just need enough speed to coerce them. Scientists have known this since at least the 1920s, when they first started firing particles at nuclei via large tubes powered by high-voltage capacitors. The reactions they observed were nothing short of revolutionary. They swung open the doors to the subatomic world and, for the first time in history, humans could peek inside.
But there was a problem. New discoveries required faster and more powerful particle acceleration than what was thought to be possible at the time. Even if scientists could drum up the voltage needed to boost particles to the appropriate speeds, the devices would be far too unwieldy to build and observe—aqueduct-sized cannons that would stretch longer than any university campus.
One evening in 1929, while reading a theoretical article in a journal about high-energy particles and electrodes, a young associate professor at UC Berkeley named Ernest O. Lawrence had an epiphany. Running back to his office at the physics department to hone his idea, Lawrence bumped into a colleague's wife and told her, "I'm going to be famous.”
By 1931, Lawrence had a prototype for his device. It was roughly the size of a barstool cushion and was made up of about $25 worth of metal, wax, wires, and glass. In theory, the machine would race ions in a loop, much like cyclists around a velodrome, and electromagnetic forces would boost their energies after each pass. He figured the technology could achieve unprecedented speeds in a relatively small area. The prototype may have looked like a stitched-together whoopie cushion, but it proved his theory: the thing he dubbed a "proton merry-go-round" worked. Officially, he called it the cyclotron.
From there, Lawrence continued to develop and build bigger and more powerful cyclotrons, bus-sized machines inside brand new, state-of-the-art facilities that dotted the Berkeley Hills. These devices would go on to foster the atomic age and inspire the mechanics behind today's accelerators. Cyclotron technology helped create the first artificial radioisotopes to be used in medical research and cancer treatment. Bigger cyclotrons, like Lawrence's 184-inch-diameter machine built in 1942, paved the way to nuclear reactions and the creation of radioactive elements needed for the atomic bomb. The results were so impressive, size was no longer a hindrance: Going big was worthwhile, and, as time progressed, scientists and engineers continued to build them larger and larger.
Today's particle accelerators and particle colliders are inherently funny things. Like Laurel and Hardy, they operate on a comedically mismatched scale. These structures are often large enough to encircle multiple towns, yet they exist to fire particles that are far too small to be seen through even the most powerful microscopes.
CERN's Large Hadron Collider, the largest and most famous collider in the world, has a circumference of 17 miles. It's so big that it crosses international borders; its tunnel lies beneath both France and Switzerland. The Large Hadron Collider needs to be huge in order to fire protons at insanely high speeds with tremendous precision. These collisions help scientists reveal hitherto unknown phenomena and forces like the Higgs boson, the so-called "God particle" that reinforces once-theoretical ideas about why things have mass.
It is, for lack of a better term, a big deal, and these exciting discoveries are the kinds that, according to The New York Times, “could also elevate proposals now on drawing boards in China and elsewhere to build even larger, more powerful colliders.”
But not everyone is focused on going bigger. Some are headed in the opposite direction, like the team at Lawrence Berkeley Labs working to shrink the technology smaller than ever before. Notably, they're doing this on the same hills where Lawrence made his breakthrough, and to reach the lab where electrical engineer Dr. Wim Leemans is directing this ambitious (and ambitiously small) project, I make my way up a winding, quiet route named Cyclotron Road.
BELLA, A DIMINUTIVE DEBUTANTE
“There’s going to be a point where the machines are so big we simply can’t afford them anymore,” Leemans tells me in his office perched high in the Berkeley Hills. Leemans is the director of accelerator technology and applied physics at Lawrence Berkeley National Lab, and it's his job to shrink accelerators back down again.
Leemans isn't a particle physicist himself; technically, he's an electrical engineer, one who's won the Department of Energy's E.O. Lawrence Award and the Prize for Achievement in Accelerator Physics and Technology from the U.S. Particle Accelerator School. “I’m, if you wish, the tool provider for the particle physicists," Leemans says. "I think about building new tools for particle physicists who make discoveries.”
To that end, Leemans and his team have created BELLA (short for Berkeley Lab Laser Accelerator), a device so small it has been dubbed a “table-top accelerator.” Like Lawrence's cyclotron, BELLA has the potential to eventually hit the reset button on the way accelerators and colliders are made.
Besides being a tool for high-energy physics, particle accelerators have practical applications in medical, industrial, or any other field that can use high-energy electron beams (think super-powerful X-rays or gamma rays). BELLA's technology points the way towards things like improved radiotherapy and imaging, or portable scanners to search for concealed nuclear material.
One thing I was quick to learn during my visit is that, in the world of particle physics, matters of size and scale routinely dip out of the realm of everyday comprehension. In other words: appreciate clear, analogical terms like “table-top accelerator,” for they are few and far between.
That’s not to say Leemans has an overly technical parlance (at least not when talking to a layman like me). He thoughtfully explains the technology he has been working on for over 20 years like someone discussing a weekend woodworking project.
BELLA, the latest tool in Leemans’s shed, works by shooting a laser through plasma. “The plasma is the medium that converts the laser peak power into a wave,” he says, “and electrons can surf on that wave.” Whereas conventional accelerators use kilometers-long tubes lined with massive magnets and radio frequency structures to boost particle energies, a laser plasma accelerator achieves similar results in a tube that’s just a few centimeters in length.
WHY GO SMALL?
Like "tabletop," the terms "accelerator" and "collider" are mercifully self-explanatory. One makes particles go fast, the other makes them crash into one another (while also going very, very fast). So, while all colliders are accelerators, not all accelerators are colliders.
BELLA is an accelerator, not a collider. "Colliders need high average power," Leemans explains. While it's still the most powerful compact accelerator in the world (a record it achieved in 2014), BELLA cannot yet create the kind of sustained power made by the likes of the Large Hadron Collider. "That’s one of the challenges we’re starting to embark on—how do we do that?"
Being small opens up a lot of avenues for BELLA, ones that aren't necessarily dedicated to particle physics. "There are other applications where our technology could become competitive at a much earlier state," Leemans explains, "We’re working on another application that would use the electrons directly to do medical treatments. We had an idea a number of years ago: Could you make our devices small enough that you could enter them into the body?"
Think about it: a particle accelerator about the size of a grain of rice that could be maneuvered directly next to a tumor. "It would be arthroscopically bringing an accelerator into the body," Leemans says, "powered by an optical fiber." This in-body accelerator could bombard the cancerous cells directly without subjecting the rest of the patient's organs and unrelated tissue to its high-powered beams.
It sounds like we're in Magic School Bus territory here, but Leemans and his team already own the patent for this technology. "We’re working with a couple companies that are very excited about this application," he says.
Beyond the world of medicine, BELLA has promising applications in other fields, like nuclear nonproliferation (handheld devices to "look at what’s inside containers, what’s inside of radioactive waste vessels, maybe into even nuclear reactors"). The key to making this groundbreaking technology work? "It all starts with the laser."
The laser used by BELLA is so powerful, Leemans had to appear at city council meetings to assure the residents of Berkeley that their city wouldn’t go dark every time he turned it on. “There were certainly other people who thought we would suck all of the energy out of the Gulf Stream,” he says with a chuckle, recalling some of the more outlandish concerns. Ridiculous, sure, though the amount of power produced by BELLA’s laser is referred to in measurements and terms usually reserved for things like the Sun.
BELLA uses the world’s highest repetition rate petawatt laser, a petawatt being a unit of energy equal to 10^15 watts. “We can reach 1.3 petawatts, which is 1300 terawatts," Leemans says. "The sun emits 100,000 terawatts. The total consumption of electrical power in the U.S. is on the order of maybe as high as 10 terawatts, if you combine all the power." According to the journal Physics of Plasmas, BELLA's laser "generates 400 times more power than all the world's power plants combined."
The key to how BELLA can be so powerful without causing Berkeley or the world to go dark lies within its insanely short pulses. Each burst lasts for about 30 femtoseconds. A femtosecond is 10^-15 of a second, or a quadrillionth of a second. In other words, one femtosecond is to one second as one second is to 31.71 million years.
Right now, the laser can only produce about 10 of these bursts per second. If you were a creature whose sense of consciousness and time were at the femtosecond level, which is to say you perceived these units as actual seconds, then you could live next to the laser for 31.71 million years and only observe its sustained firing for a cumulative time of 5 minutes.
While these technological feats are quantifiable, they're also largely incomprehensible. That's the word that keeps popping up in my head. Femtoseconds are essentially incomprehensible. Petawatts are incomprehensible. How does something create all that power? Or, better yet, where does that power come from? Surely you can't just plug the laser into the wall?
“It comes out of the wall,” Leemans says, smiling, about the source of the laser's electricity. For all this talk of petawatts and femtoseconds, “the average power used is about that of a lightbulb.”
This is done by compression. Energy made by multiple laser pulses is stored and then combined into one powerful burst. “You essentially start with a really short little pulse,” Leemans says, “and then you start stretching that laser light out in time, and you put energy into the laser pulse, and then at the very end, you make sure that everything gets compressed in time.”
The process is far more complicated than that, of course, given that it relies on devices with names like “titanium sapphire amplifier crystals” and whatnot, but this is still merely the first part of BELLA's equation. The laser is not what makes BELLA an accelerator. That honor goes to something much smaller.
THE JOYS OF PLASMA
While the machinery that makes up BELLA's laser is big enough to fill a room the size of a small high school's cafeteria, the accelerator itself is only about 9 centimeters long. It looks kind of like a bubble level.
The tiny device consists of a tube that is filled with plasma, the process's essential medium. As Leemans describes it, plasma is "essentially a soup of electrons and ions." It's a fundamental state of matter (the others being solid, gas, and liquid), and it exists all over the universe. Capturing plasma here on Earth, however, is like catching lightning in a bottle.
Actually, scratch that: It is catching lightning in a bottle. Literally.
"If you look at a lightning bolt outside, it rips the electrons off of the atoms or the molecules because of the high voltage," Leemans says. This briefly creates plasma. This process is replicated inside the accelerator for a sustained period of time by filling it with gas and then applying a high-voltage pulse. "You actually create a little lightning bolt inside the device."
One can't just catch lightning in a soda bottle, however. The accelerator's walls are made of sapphire, a material with an extremely high melting point.
(Leemans likes sapphire because, as a toolmaker, he can appreciate when something is just right for the job. "The iPhone was going to be a sapphire screen," he tells me, "but there was a problem: the sapphire didn’t survive the drop test." Take note: Just because something can get struck by lightning doesn't mean it can withstand clumsy attempts at sending drunk texts.)
Inside the plasma, a channel about the width of a human hair is created. As the laser's electron beam flows through this tunnel, it "surfs" on the waves formed by the plasma and its speed and energy are greatly increased. BELLA is able to push an electron to 1 billion electron volts in the span of a little more than an inch. For comparison, it takes Stanford's Linear Accelerator Center—the longest linear accelerator in the world—two miles to achieve 50 billion electron volts.
HOW THE SAUSAGE GETS MADE
To get to the laser bay (this is what it's actually called, as if it were on the Death Star), you walk through large hallways festooned with giant pictures of UC Berkeley's famous scientists of yore. There's Ernest O. Lawrence in black and white, standing next to one of his cyclotrons. "This is a building where several of the elements were discovered for the periodic table," Leemans says.
The laser bay is remarkably quiet and sterile. As I put on a hair net before entering, I mention that the preparations one has to take here are not unlike those enforced by the USDA at meat processing plants. "We make a different kind of sausage," Leemans says, securing his own hair net atop his head.
Inside, it looks much like a server room at a big office building. Boxy black machines hum like computers as they work to power the laser. It's currently being fired at a low level for tests, and Leemans proves this by inserting a sheet of film into the machine's bowels. THWACK! He removes the film, showing me the scorched evidence of the beam's existence, and the laser bay returns to its normal quiet hum.
The bay is quiet for a reason. Because the scientists are firing a laser through the insanely narrow capillary of the accelerator, the slightest vibration can disrupt the device's finely tuned components. "We ask people to walk around gingerly," Leemans says.
This is a funny request considering the facility is constructed on one of the most seismically active fault zones in the world. "The system doesn’t like earthquakes," Leemans says, adding that dealing with the occasional tectonic shift is just part of the job—all the lab's machinery is fastened down with large-gauge hardware. "When I visit European labs—and I grew up in Europe—now my first reaction is, ‘Wait a minute, these guys haven’t bolted everything down!’" Leemans, who originally hails from Belgium, says. Because it is so sensitive to vibration, the laser stops working in the event of an earthquake. Leemans sees a bright side to this, though: "You could argue that’s a safety feature."
The laser's machinery snakes around the laboratory and winds up in another room where it points at the accelerator, which sits atop a bolted-down table, as promised. The accelerator isn't on, though I have to take Leemans's word for it—it's not like I'd be able to see electrons surfing on searing waves of plasma with my own eyes anyway.
Upon exiting the laboratory I notice a huge picture hanging in the hallway, near Lawrence and his cyclotron, that I had somehow missed before. It shows Leemans's plasma accelerator emanating a warm purple glow. The photo is enhanced, though Leemans says BELLA does in fact make that color naturally. What's truly unnatural is the size. The image is blown up to fill a large section of the wall, and the hair-thin plasma channel now looks as thick as rebar. I take a picture of the picture which, while redundant, still serves a purpose: Who knows if I'll ever see BELLA that big again?