Crafting the World's Smallest Beam
The Accelerator Test Facility 2 in Japan reached a milestone last month when an international team of researchers—including several SLACers—successfully narrowed the beam down to a height of 310 nanometers, about 1 percent the diameter of a human hair.
ATF2 began operation in 2008 as a major test bed for future linear colliders. It sends a beam of electrons along a 139-meter racetrack before funneling them into the final focus test region, which simulates the collision point in a linear collider. The ultimate goal is to narrow the beam to the record-breaking tiny size needed in a future collider: about 37 nanometers—about 0.1 percent the diameter of a human hair. For comparison, colliding beams were on the order of 650 nanometers in the smallest direction in the SLAC Linear Collider.
As a pulse of the ATF2 beam heads toward the final focus test region, it looks a bit like a short needle, similarly tiny in the horizontal and vertical, and a little longer in the longitudinal direction. The goal of last month's tests was to narrow the beam's height even further so that it resembles a miniscule ribbon more than a needle.
The tests were a good starting point, said SLAC Physicist Glen White, who traveled to the KEK physics lab in Tsukuba, Japan to join in the week-long R&D session devoted solely to tuning the beam.
"This was our first real attempt at putting everything together. Really for the first time we started using some of our beam tuning technology" in situ, he said. "It was very exciting."
That new tuning technology includes a comprehensive beam monitoring system that can directly measure the beam size down to 20 nanometers. With this tracking ability, advanced software can change the strength and orientation of a complicated assortment of magnets and optical devices to narrow the beam further.
"Because it's a non-linear system"—in other words, because the system is so complex that it can't be described by a linear, or simple, combination of independent variables—"this is a very long procedure," White said.
Although the team still has a long way to go before it reaches the 37 nanometer goal, White said he is pleased by the results of these tests.
ATF2, which is located at the Japanese high-energy physics laboratory KEK, will shut down in July for the remainder of the summer. When the machine starts up again this autumn, the team will try to get a little closer to their 37 nanometer spot size goal.
That minuscule beam is needed for next-generation colliders including the International Linear Collider and the Compact Linear Collider. Packing more electrons and positrons into a thinner beam makes it more likely that individual particles will collide, increasing the number of collision events recorded. With more recorded events, researchers are more likely to be able to see new and interesting physical processes take place.
SLAC first became involved in the ATF design in the 1990s, and has since provided and supported many of the machine's hardware subsystems. The lab also contributes to the software that runs the machine. In addition, SLAC has supported operational aspects of the project, with about 5 to 10 SLAC staff members currently involved in ATF2.
The continuation of these tests is quite important for future accelerators, White said, not only to learn how tuning tools can create a very small beam but also to train new accelerator physicists.
"ATF2 is a very good training ground," he said. "When there's a long lag between the construction of new machines, it's important to keep the knowledge going."
ATF2 is a collaboration of many different collaborating institutions including the Budker Institute of Nuclear Physics, Brookhaven National Laboratory, Cambridge University, CERN, Cornell University, DESY, DL, Fermilab, Institute of High Energy Physics, Laboratoire de L'accelerateur Lineaire, Lawrence Berkeley National Laboratory, Lawrence Livermore National Laboratory, National Science Center Kharkov Institute of Physics and Technology, Physics Advanced Laboratory, Paul Scherrer Institute, Queen Mary-University of London, Royal Holloway-University of London, SLAC National Accelerator Laboratory, Tomsk, UC Berkeley, London's Global University, University of Notre Dame, Oxford University, Osaka University, Kyoto University, JAEA, Kobe University, Shinshu University, JASRI, Seikei University, The Graduate University for Advanced Studies, University of Tokyo, Institute for Solid State Physics of the University of Tokyo, Tokyo Metropolitan University, Science University of Tokyo, Toho University, Tohoku University, Tohoku Gakuin University, Nagoya University, Hiroshima University, Yokohama National University, RIEKN and Waseda University.