Like Oak Ridge, Argonne National Laboratory serves as a living witness to the continuity of American 20th century physics: after its first incarnation as part of the Manhattan Project’s Metallurgical Laboratory (the group that first successfully isolated Plutonium), it was the first research site to be designated a National Laboratory after the war. In the sixty-five years between some of the world’s first nuclear reactor research and today’s most cutting-edge accelerator development, there was hardly a science-and-technology subject in which Argonne didn’t have a hand.
This history is written all over the lab, even as it is already carving itself a place in the 21st century:
Like the PHENIX and STAR experiments at Brookhaven, Illinois’s Argonne and Fermilab share a faux-bitter rivalry. Argonne’s history gives it a unique place in American physics, and it is considerably larger than its neighbor in the Chicago suburbs; but particularly after the Tevatron was completed at Fermilab in the 1980s, the younger, smaller lab has received the lion’s share of media attention. That a proposed expansion of Argonne had to be rejected in the 1960s in favor of the entirely new particle physics lab just a few miles down the road naturally fueled the fire.
But as with most combative siblings, the rivalry is little more than the face of a close working relationship. Argonne’s proximity to the nation’s (and until just recently, the world’s) largest particle collider has made it a perfect site for the development of new accelerator techniques and the construction of accelerator parts. (More on this later: accelerator development is so multilaboratory in nature that Lizzie and I will be doing a separate post on the subject.) As early work on the International Linear Collider (ILC) progresses, Argonne and Fermilab work with the same components and problems, each responsible for their own pieces in the intricate process.
When Lizzie and I visited in July, detector and accelerator designer Tom Fields, who has worked off-and-on at Argonne since the 1960s (and who helped start the MINOS/Soudan neutrino experiment), had just compiled a complete list of the accelerator projects housed at Argonne since the lab’s inception. Over the last forty years, these fourteen projects not only advanced fundamental physics and technology, but laid the groundwork for the user-based approach to research that the National Labs nearly all employ to some extent. With the help of others at the lab, this list (along with explanations) has now been made available on the Argonne Accelerator Institute’s website.
As Fields’s list also makes clear, Argonne isn’t defined by a single large-scale accelerator, as Fermilab was by the Tevatron. Indeed, as a multidisciplinary lab, it isn’t even solely focused on high energy physics. The Advanced Photon Source is the brightest source of x-rays in the western hemisphere, and like most contemporary synchrotrons it is used for virtually every field of science. And after weathering all the shifts in Atomic Energy Commission and Department of Energy mission over the last 60 years, the lab has played host to the development and improvement of just about every source of energy on the market. Four generations of nuclear reactor, as well as the reactors used by submarines and aircraft carriers; the windmills we saw around the country on our trip; even more obscure concepts like solar ponds. All were developed or substantively improved at Argonne.
On a more personal note, one of the more distinctive side-effects of visiting eleven physics-related sites in a short span of time is that you start seeing physics machines everywhere: from construction equipment on the highways to the oil rigs of Texas and the wind farms of California, suddenly every strange piece of machinery looked like it belonged in an underground lab. Argonne helped to make this actually the case.
Physicists benefit from this arrangement as much as anyone, as there are tangible rewards for locating a small particle physics division in this larger, multidisciplinary environment. As Harry Weerts, the director of the Argonne team working on the International Linear Collider, remarked to us: nine out of ten solutions to a specialized problem will come from the discipline that needs it; but that one time where expertise from another discipline turns out to be relevant can make all the difference in a major project’s success.
One of Argonne’s current undertakings is the perfection of advanced materials science techniques for use in the construction of more efficient radio frequency cavities for accelerators. To make it happen, scientists from one part of the lab simply had to walk to another building and talk to their colleagues.
Likewise, construction of the country’s first non-reactor neutron source – the predecessor of the larger but similarly designed Spallation Neutron Source we visited at Oak Ridge – was instigated by a solid-state researcher from the University of Michigan who pitched his idea to an enthusiastic Argonne scientist. There may be good reason to concentrate particle physics research in a dedicated facility like Fermilab; but with successes like these, there will always be a place for multidisciplinary labs like Argonne as well.
Along with teams at Brookhaven, SLAC and Berkeley, Argonne physicists contribute to the ATLAS experiment at the Large Hadron Collider (Fermilab works by itself on a CMS collaboration). But there hasn’t been a major high energy research accelerator at Argonne since the Zero Gradient Synchrotron (ZGS), which operated from 1963 to 1979 at 12 GeV. With a fifteen-year run as one of the first large-scale accelerators in the world, the ZGS holds a hallowed space in the history of particle physics.
Conceived in an Atomic Energy Commission panic over the realization that Russia had built an accelerator of similar power and technology – what if they pointed it at us?! – the ZGS differed from more recent accelerators in that it used “weak focusing” to keep its accelerated particles together in a tight beam. This means that it relied on a single, uniform, strong magnetic field that directed its particles in a circle.
Weak focusing ultimately lost out to alternate-gradient or “strong” focusing, developed at CERN’s Proton Synchrotron (which has been re-purposed as a booster accelerator for the LHC) and Brookhaven’s Alternate Gradient Synchrotron (which now feeds into RHIC), since strong focusing allows for much tighter beam coherence.
But weak-focusing technology had some unintended benefits too: alternating-gradient machines like the AGS couldn’t accelerate polarized particles, so the ZGS proved essential for explorations of polarity, and useful for broader areas of research where a particle’s polarity needed to be preserved. This helped to set a precedent, particularly at Argonne but at other labs as well, of allowing users from a range of fields to use an accelerator for research that had little to do with its original purpose, once that original purpose was fulfilled.
Since we didn’t get a chance to see anything but the outside of the ZGS, I dug up some historical images on Argonne’s website and in the University of Chicago’s Archival Photographic Files online database. (We didn’t actually tour the ZGS, so I’m guessing about some of what follows – it makes some sense to me after seeing a half-dozen accelerators/colliders over the summer, but if anyone reading this knows better, then please correct me. Also, click on any of the images to visit their original site for more information).
Just like the enormous accelerators of today’s physics, the ZGS used a series of acceleration techniques (some old, some new) to bring its particles up to speed. The protons began their journey in a 750-kv Cockcroft-Walton generator, one of the oldest particle accelerator designs.
From there, they were accelerated to 50 MeV by a 110-foot linear accelerator:
After this second acceleration, the particles were dumped into the main synchrotron ring, which relied on weak focusing to keep the beam moving in a circle as a single 20,000-volt radio-frequency cavity kicked the particles into higher energies each time they made a lap.
Finally fully accelerated, the protons would smash into their targets, and the resulting spray of particles would make its way into a detector. In order to observe only what was relevant, though, uninteresting pieces had to be separated out of the mess by electrostatic separators placed just ahead of the detectors.
In the middle days of particle acceleration, detectors were generally bubble chambers – hydrogen-filled containers that responded when a charged particle went through them. Physicists could watch (and photograph) the tracks that the particles made, and by applying a magnetic field they could characterize the different types of particles by the amount that their tracks curved towards or away from the magnet.
(Fun fact about bubble chambers: contrary to popular anecdote, 1960 Nobel laureate Donald Glaser was not inspired to invent them by a glass of beer. But Glaser did put beer into some early prototypes, with little success.)
In all, the ZGS would have looked something like this:
Argonne also happened to be the only opportunity Lizzie and I had to speak with a theoretical physicist. Everywhere else we visited last summer, experimental physicists were exploring the properties of particles predicted by the standard model. But for Ed Berger, the Standard Model is an unsatisfying piece of theory that doesn’t explain the universe so much as just fit our data – and only if we stay within fairly limited assumptions. Even more compelling ideas like supersymmetry and extra dimensions are merely promising “toys” to use in the process of developing a theory that he’s sure will run deeper and stronger than the Standard Model.
With this perspective, the LHC’s new higher-energy data isn’t exciting because it will verify what we know, or even because it will expand it. Rather, it’s exciting because of the possibility that it will contain something that hasn’t been theorized about at all. A wildly new, unexplainable discovery would put new bounds on what theorists can postulate, pushing their explanations of the universe into truly new territory. Right now, as Berger puts it, the theorists need the experimentalists to rescue them, to “get us out of our dark desert.” Until they have more data, the theorists “have too many unconstrained possibilities to help them.”
If the LHC does produce a deep mystery, we might see a theoretical revolution like the one that occurred in the early 1970s with the development of quantum chromodynamics (which explains strong nuclear interactions) and electroweak unification (which unifies electrodynamics with the weak nuclear force). Since almost all of today’s particle physics cam be characterized as a continual refinement of this revolution – finding predicted particles one-by-one, making better measurements – the theoretical possibilities that the LHC represents strike at the very heart of physics: who knows what we’ll find out there, and what it will lead us to understand?