The story goes that after Ernest Lawrence came up for the design for the first cyclotron, he raced from the Berkeley library shouting, “I’m going to be famous!” His prediction was spot on: the cyclotron was the first particle accelerator, the first machine that could study matter on its smallest scales. Since it was became the model for all subsequent accelerators, its invention established Lawrence’s place as one of the most important and influential physicists of the 20th century.
Eighty years later, accelerators range from the relatively low-energy machines used to treat cancer in single hospital rooms to the Large Hadron Collider, which crosses an international border and gets us to energy levels last seen fractions of second after the Big Bang. Up until now bigger has meant better in terms of accelerators, but as we look forward to the proposed International Linear Collider and beyond, many physicists are investigating how to fit the biggest of Big Science onto a tabletop.
Of course, very fast moving particles are not an anomaly in our world, even without particle accelerators. More than 50 million neutrinos traveling at almost the speed of light pass through your body every second without causing a second glance, while cosmic rays racing through the atmosphere were among the first particles we learned to “see” in cloud chambers and, later, bubble chambers. What is special about a particle accelerator is that you know exactly what you’ve put into it, and you know exactly how fast those particles are going to go once you start the machine.
A particle accelerator is also the first step in building a particle collider, in which extremely fast moving subatomic particles are slammed into each other. In a beautiful practical application of Einstein’s famous e=mc2, the energy released by that collision is transformed into a shower of other subatomic particles, which are then picked up by sophisticated detectors. The content of that particle shower is what physicists use to study the basic building blocks of the universe and the forces that hold them together.
A subatomic particle in a cyclotron travels through an alternating current, which pushes and pulls the charged particle to ever-greater speeds. Lawrence read about this novel way to accelerate particles in a paper by the Norwegian physicist Rolf Winderöe and was immediately intrigued, although a bit discouraged by the fact that in order to generate enough alternating current to accelerate particles to the required speeds, he would need several meters of charged cylinders lined up in a row—too long, he thought, for a laboratory. His breakthrough was realizing that if he applied a perpendicular magnetic field to the particles in the accelerator, they would travel in a spiral path, thereby saving space and cutting down on the amount of equipment needed to build his visionary machine. Indeed, the first cyclotron, which was completed in 1931, was so compact it could be held in the palm of your hand.
As technology and funding improved, Lawrence built increasingly large cyclotrons, eventually reaching 184 inches in diameter. His initial worry about a two- or three-meter linear accelerator being too long to fit in any laboratory would, in just a few decades, come to seem laughable: SLAC’s linear accelerator, completed in 1966, is two miles long, and the proposed ILC is ten times that. Rather than worrying about how the accelerator would fit in a room, labs started being built to house increasingly large accelerators (case in point: Fermilab, home to the Tevatron and its four-mile ring).
But after the disastrously expensive development and subsequent cancellation of the Superconducting Super Collider in 1993, building a new lab every time you wanted a more powerful accelerator became an impossibility. The SSC’s replacement/successor, the LHC, fits neatly into the tunnel that previously housed CERN’s Large Electron-Positron Collider (LEP). As physicists look ahead to building the International Linear Collider, the main stumbling block is not that they don’t know how to build the next big machine. It’s that if they use current technologies, the ILC would be 20 miles long and could cost as much as $25 billion to construct—figures that might once again be physically and financially too big for any laboratory, or even any one country.
ILC development suffered some setbacks at the National Labs during the particularly lean years leading up to the stimulus package, and the international physics community can’t afford to get too excited about a machine they aren’t sure they will get to build. But as the LHC starts to produce results, the ILC could be indispensable for understanding the new science coming out of CERN. Aware that this could well be the next major step in high energy physics, Fermilab is pushing ahead with ILC development. Kurt Riesselmann from the Public Affairs Office took us on a tour of a building that was used for fixed-target experiments in the early days of lab and has recently been converted to house accelerator R&D.
The Fermilab team is working on building the type of superconducting cavities that would be used to build the ILC. Kurt called these cyromodules “bagel structures” and showed us a few of them in various stages of construction. “The challenge for the ILC, why it’s so expensive, is that you need something like 10,000 of the bagel structures, each of them about a meter long,” he explained. Furthermore, “At the moment it’s like putting together an airplane to make one of these. Then you need 1000 or 2000 of them.” If the construction process for each bagel structure could be simplified and sped up to under a month, the ILC’s anticipated price tag would fall dramatically.
But the cryomodules wouldn’t just be used for the ILC—they are also a major component of Project X, the high-intensity accelerator Fermilab is building to aid in its mission to scale up its neutrino physics program. After Project X’s initial acceleration phase, the next half-mile will use exactly these cryomodules. As exciting as Project X is by itself, it can also be seen as a huge step forward in ILC development. Kurt laid out a convincing argument for the lab’s foresight: “Once you’ve built Project X, not only have you built a machine that is really ready for neutrino physics and other experiments we haven’t talked about, but you can also say that we have built 1/40 of the ILC. And we can tell it costs this much, we’ve learned this, we can do it faster. At that time the answer might still be the ILC is too expensive, but at that time we also will know what the LHC has produced, so we will know if it’s actually worthwhile doing the ILC.”
While Fermilab is directing its ILC development efforts towards perfecting current technology, other labs are taking an even longer view and are working on developing completely new accelerator technology in hopes of making future machines smaller and cheaper. At Argonne National Lab, postdoctoral fellow Sergei Antipov showed us around the building where he experiments with building wakefield accelerators.
While most of today’s accelerators use electro-magnetic fields to accelerate and control a single beam of particles, wakefield accelerators use the “wake” left behind by one beam of electrons to accelerate a second. As you may imagine, this process is all but impossible to describe without a few helpful surfing metaphors, which Sergei was happy to provide: “You take the first cloud of electrons and pass it through some structure. It leaves this wake behind it, just like a boat. You take a smaller electron bunch and pass it behind it, and sort of like a surfer on those waves, the smaller bunch will get accelerated by the wake.”
The upside is that wakefield accelerators are much more efficient than other accelerators, since the first beam, which can be relatively low energy and quite large, does most of the hard work of getting the second beam up to very high energies. This makes it relatively easy to increase the acceleration gradient of the machine, which means the accelerator itself can be shorter—a major goal for scientists working towards the ILC and beyond. The appeal of smaller accelerators reaches far beyond the high energy physics community, since they would be easier to install everywhere from hospitals (to treat cancer) to food processing plants (to irradiate produce).
The downside of wakefield accelerators is that we don’t yet know how to build structures that can withstand the huge energies the second beam can reach. Sergei is experimenting with many different kinds of ceramic structures and even working with some material scientists to determine how to ensure that the accelerator itself won’t break apart when the second beam travels through it.
Another important part of constructing a working wakefield accelerator is making sure you can get a big enough bunch of electrons into the first beam. As Sergei would say, a huge boat leaves a big wake, helping the surfer reach higher energies. He creates this “boat,” or the first beam of electrons, by hitting a cathode with a laser and capturing the electrons that are released. He needs to get as many electrons into the machine as he can in order to create the highest energy wake possible, so he is also doing painstaking experiments to create the most efficient cathode. This part of his work, part of which involves evaporating cesium telluride so that the molecules with drift over and stick to the surface of the cathode, seemed a little unreal even to Sergei, who said, “It’s still alchemy, we just have better equipment.”
Another way to create a wake for electrons to ride on is by using laser pulses. At Lawrence Berkeley National Lab, we visited the Laser Optics and Accelerator Systems Integrated Studies group, or L’OASIS, which is working on developing the technology needed for laser wakefield acceleration. Fittingly, Berkeley Lab was founded by Ernest Lawrence as his Radiation Laboratory, or Rad Lab for short, in order to house his increasingly large cyclotrons (it was designated a National Laboratory and renamed after Lawrence’s death in 1959).
Before going inside the rooms that house the L’OASIS experiments, we had to put on the most intense clean suits of our trip.
Once inside, L’OASIS scientist Nicholas Matlis gave us a tour of the lab’s unique two-story laser system. In order to create a high enough quality beam for laser wakefield acceleration to work, it needs to be amplified and compressed many times along the way. At L’OASIS, the beam travels through two amplifying systems: Godzilla and T-Rex. (There is also a third and much smaller laser system named Chihuahua.) They look like a combination of a Rube Goldberg machine and the best maze a lab rat could ever hope to run.
Once the laser beam has been amplified to its fullest, it is shot through a plume of gas to ionize it, creating a plasma and leaving a wake in the form of a hugely powerful electron field. A beam of electrons can then follow the laser into the plasma and be accelerated by that electric field to energies that conventional metal structure could never support. (For a more detailed description of this process, check out this article by Paul Preuss, our wonderful guide at Berkeley Lab.)
L’OASIS made the cover of Nature in 2004 by proving that they could make useful electron beams through laser wakefield accelerator. But there are still many problems to be solved. As Nicholas told us, “At the moment there’s still a lot of physicists who sort of condescend and look down on this technology as not being real. It was only in 2004 that first demonstrated that you can make electrons of interesting quality with these lasers. But now they’re still at an energy that’s much lower than conventional accelerators and still not as high quality. We have yet to prove to the rest of the accelerator community to take us completely seriously and these experiments were designed to do that.”
One of the biggest problems the L’OASIS group is confronting is staging. Just like a car has to shift gears in order to accelerate from 0 to 60 mph, particles must go through several different “stages” of an accelerator before they reach their top speed. A new laser beam must be used for each stage of a laser wakefield accelerator, which creates the challenge of getting the timing just right so that the new laser hits the gas and makes a plasma tunnel just an instant before the existing high energy electron beam is ready to travel to through it. The previous laser beam must also be deflected very quickly in a way that won’t interfere with the electron beam riding on its wake. L’OASIS scientists are currently experimenting with the laser-reflecting properties of small water jets and – you guessed it – aluminum foil.
If the L’OASIS group is able to show staging, it could open the door for the development of a full-fledged accelerator that is a lot smaller than today’s conventional accelerators – think the size of a football stadium rather than the size of the city of Dallas.
It’s hard to believe that high energy physicists are already looking beyond the still hypothetical ILC, especially since we haven’t even seen any new results from the LHC. But as Fermilab’s Kurt Riesselmann said, “For the last 70 to 80 years we’ve developed accelerator technology, pushed it forward, and now it doesn’t make sense to all of a sudden say, OK, let’s drop it. We probably don’t know some of the applications that we still will discover for accelerators, so pushing the limits and making it more powerful and more precise some day will pay off.”
It has been suggested that the LHC might be the most powerful accelerator we ever build, especially if it doesn’t turn up the exciting results that physicists are hoping for. But each time a more powerful accelerator is built, not only does it offer a window onto previously unseen aspects of our universe, but it pushes the limits of human collaboration and international cooperation to new heights. Harry Weerts, director of the Argonne team working on the ILC, summed up the grand appeal of looking one step ahead to the next great machine when he said, “It sets a goal for everybody, that we as a human race can do something like this.”
For more on the science behind many different types of accelerators, check out Nick’s post about Argonne National Lab.