Watching wakefields
to keep particles on track
Posted April 14, 2009
When you’re building a $6.75 billion, 31-kilometer device to crash subatomic particles, you want to be sure it works the best it can. “Do-overs” can be expensive – or impossible.
A team from the SLAC National Accelerator Laboratory (SLAC) in the San Francisco area is applying major computer power to help head off problems. The researchers are using one of the world’s fastest computers to investigate the design of that aforementioned huge project, the International Linear Collider (ILC).
High-energy physicists from around the world envision the ILC as a companion to the Large Hadron Collider (LHC), a proton-proton particle collider in Switzerland that launched in 2008. They characterize the ILC as a “precision machine” that will help them fill in blanks the LHC leaves behind and help answer fundamental questions about the natures of matter, space and time.
The ILC will consist of two linear accelerators that face each other. Each is a 14-kilometer string of connected cavities called cryomodules, which are chilled to near absolute zero to make them superconducting. Radio frequency (RF) units stationed along the cavities will generate radio waves that change the electric fields in the cavities. “Bunches” of particles – negatively charged electrons in one accelerator and positrons, their positively charged counterparts, in another – will ride the waves to ever-higher energy levels. Tens of billions of particles, at energy levels of 500 billion electron volts (GeV), will crash into each other around 14,000 times a second, creating new particles and radiation.
But like boats on a river, the particle bunches kick up an electromagnetic (EM) wake as they zoom toward their high-energy collision, making the way rough for those bunches that follow. A trailing bunch can go off course, degrading the beam quality and causing heat that could damage the accelerator cavity, says Lie-Quan Lee, a SLAC computational scientist. Higher currents – that is, more particles in the beam – lead to improved collision rates but also create stronger wakefield effects.
Lee, who also is computational mathematics group leader in SLAC’s Advanced Computations Department, is principal investigator on a project designed to understand wakefield effects by creating detailed computer simulations of ILC cryomodules.
The research team uses millions of computer processor hours granted by the Innovative and Novel Computational Impact on Theory and Experiment (INCITE) program. The program, overseen by the Office of Advanced Scientific Computing Research under the Department of Energy’s Office of Science, is designed to attack computationally intensive, large-scale research projects with breakthrough potential.
The simulations also rely heavily on codes developed under another DOE Office of Science program: Scientific Discovery through Advanced Computing (SciDAC). See sidebar.
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