SLAC at the LHC: Accelerator Systems
SLAC researchers are currently involved in several accelerator projects for the Large Hadron Collider through the LHC Accelerator Research Program, or LARP—a consortium of SLAC, Brookhaven, Fermi and Lawrence Berkeley national laboratories. The most developed SLAC efforts include the design of rotatable collimators to absorb any stray protons, and simulation of beam-beam interactions.
"We're using tools developed at SLAC and applying them at CERN," said Tom Markiewicz, Accelerator Systems leader for U.S. LARP and head of the LHC Accelerator Research department in the SLAC Accelerator Research division.
The LHC beams contain an enormous amount of stored energy: about 5000 times as much as the PEP-II beams and 250 times as much as the Fermilab Tevatron proton beam. Collimators, an integral part of most accelerators, are designed to "catch" particles when they go astray and thereby protect the remainder of the accelerator and detectors. The copper rotatable collimators being designed and tested at SLAC will complement carbon collimators currently in use at the LHC, offering improved performance through higher efficiency and lower impedance.
Each of the LARP-designed collimators consists of a vacuum box with two cooled copper cylinders inside. The price of copper’s higher efficiency and lower impedance is the damage that will surely occur if the collimator is made to stop a number of full-intensity proton bunches. CERN anticipates that equipment failure may cause the collimator system to endure about five such hits in the lifetime of the machine. After such a hit, researchers could then simply rotate the copper cylinder so that an undamaged portion faces the beam—a task much simpler than replacing the entire collimator. The collimators designed by LARP should be able to handle about 20 such events.
The first full-scale device is complete and undergoing testing at SLAC. If the tests are successful, SLAC may be asked to build upwards of 30 collimators as part of future LHC upgrades. CERN accelerator physicists are currently designing two other, complementary approaches to the collimator problem. The final design—or possibly designs—to be installed in the LHC will be chosen in early 2010.
LARP is also heavily involved in simulations to predict the behavior of the LHC proton beams near the collision point. As two bunches of positively charged particles approach and eventually cross, they naturally begin to repel one another. This "beam-beam interaction" degrades the density of the beam and lowers the number of particle interactions—the "luminosity."
SLAC's Andreas Kabel and colleagues are running computer simulations that test two different techniques to combat this effect. The first uses an electron lens to push the beams into a tighter alignment as they approach the collision point; the second relies on a thin, electrically charged wire to counter the repulsion. These simulations will help determine whether either idea is feasible.
In addition to these projects, SLAC researchers are also collaborating on commissioning LHC’s synchrotron light, beam loss and luminosity monitors. Knowledge gained at PEP-II in particular is being applied to LHC’s low level radio frequency system and to efforts to measure and mitigate electron cloud instabilities. The researchers are also applying codes and models developed for the International Linear Collider’s crab cavities to those proposed for a future LHC luminosity upgrade. LARP currently has a proposal before the Department of Energy that specifies several options where the U.S. can contribute to the future LHC upgrade effort. A first decision on technology, budgets and timescale is expected this fall.
"SLAC and the other LARP collaborators can offer very unique expertise," said Uli Wienands of the Accelerator Systems Division. "It's always of interest to be able to apply your knowledge in a different context."