Watching wakefields
to keep particles on track
(Page 2 of 4)
The SLAC team’s research could help engineers improve their designs to limit or control wakefield effect problems.
Using algorithms that calculate the physics of particle beams and wakefields, “We can predict results and provide insights,” Lee says. “On the other hand, if we use actual materials and build the structure and test it, it would take a long time and the cost would be very high.”
Accelerator cavities make up about a third of the ILC’s multibillion-dollar cost, Lee adds. “Anything we can do to improve the cavities by some percentage – even 5 percent – will lead to a really huge savings in building costs and also operating costs, because (the linac) can operate over a longer period.”
 Click for larger image |
Fellow project investigator Cho Ng, a physicist and deputy head of the Advanced Computations Department, says although each ILC accelerator is 14 km, “If you can understand the basic unit, then you more or less can have a good understanding of the entire linear accelerator. (Simulations) will give you a very good handle on how good your design is.”
To model the cryomodule, the SLAC team will deploy an INCITE allocation of 8 million processor hours. That’s on top of a 2008 INCITE grant of 4.5 million hours. Both grants were for time on Jaguar, Oak Ridge National Laboratory’s Cray XT computer, rated in 2008 as the world’s most powerful computer for unclassified research with a theoretical peak speed of 1.64 petaflops, or 1.64 quadrillion operations per second. Jaguar’s more than 181,000 cores make it possible to consume millions of processor hours in a relatively short time.
The SLAC researchers need the power partly because their simulations must span a large range of scales in size and time. For example, an accelerator cavity, comprised of nine cells that look like beads on a string, is a meter long. But the simulations also must depict the fine features – only thousandths of a meter in size – of complex couplers at the cavities’ ends.
 Click for larger image |
To capture the interaction of radio waves, wakefields and other physics, the researchers mathematically distribute a mesh of data points throughout the simulated cryomodule. The computer calculates changes in properties like temperature or EM field strength at each data point over time, often taking into account how physics interact. The more data points in the mesh, the more accurate the simulation; the more data points, the greater the demand for computer resources.
With codes developed under SciDAC, the SLAC researchers ran multiple simulations on Jaguar, including the first wakefield analysis of an entire eight-cavity cryomodule. They started with a 3 million-element mesh before running simulations with meshes of 16 million and 80 million elements.
