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Feature - Calming the wakefield

Feature - Calming the wakefield

A snapshot of a simulation of the wakefield generated by a particle bunch moving through a series of ILC cavities, from three different perspectives. The colors represent the magnitude of the fields, with warmer colors representing the strongest fields.

For the International Linear Collider to run at maximum performance, each of its 27,000 cavities must be designed as precisely as possible.

It is very time consuming and costly, however, to produce physical prototypes, so researchers at SLAC National Accelerator Laboratory decided to use a supercomputer to create and test virtual prototypes of the cavities.

The ILC, which is in its design phase, will use superconducting cavities to accelerate electrons and their antimatter partners, positrons, to nearly the speed of light before colliding them. By studying these collisions, researchers will be able to probe more deeply into the subatomic world.

As particle bunches travel through the accelerator cavities, they produce electromagnetic wakes behind them, much like the waves left behind as a speedboat races across the water.

These wakefields interfere with bunches that follow, causing some particles to stray off course, degrading the electron beam's quality. The wakefields also produce heat that can damage cavities and cause the accelerator to malfunction.

The more powerful the beam, the stronger the wakefield. And with expected collision energies of around 500 billion electronvolts, the ILC definitely has the potential to produce strong wakefields.

"From our simulations, we can measure the wakefield and modify the cavity's design until it is within a certain limit," said SLAC researcher Cho Ng. "By doing this, you can bypass the process of developing a prototype several times and reduce cost."

The first step in producing a simulation is to create a computerized mesh representation of the cavities. Then the researchers must simulate a particle beam going through the cavities, including a simulation of how the individual particles behave. From this, they can calculate the strength of the wakefields created and tweak the cavity design to minimize the effect.

The SLAC group's simulations revealed that ILC designers can considerably reduce wakefields by using damping mechanisms - devices placed inside and outside the cavities that absorb lingering electromagnetic wakes.

This isn't the kind of simulation they can simply run on their desktop, however. Each simulation requires the processing of about 10 terabytes of data, said SLAC researcher Lie-Quan Lee.

To generate the simulations, the SLAC group has been allotted 12.5 million processor hours on Jaguar, a U.S. Department of Energy supercomputer at Oak Ridge National Laboratory that can perform up to 1.64 quadrillion operations per second. Using Jaguar, each simulation takes about a week to complete and is much less expensive than producing physical prototypes.

"We use our simulations to make sure the ILC will operate according to design," Ng said. "We don't want to come back later with a cavity that doesn't work and have to redesign it because that is very costly."

-Amelia Williamson, iSGTW

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