Watching wakefields
to keep particles on track
(Page 3 of 4)
Using T3P, one of the SciDAC codes, the simulation found several wakefield radio frequency peaks. These wave modes can remain trapped in the cavity long after the particle bunch that caused them has moved through, possibly contributing to heating and beam instability, Ng says.
The researchers used another SciDAC/SLAC code, Omega3P, to focus on the waveforms. “T3P is a way to study the bigger spectrum – the whole spectrum” of electromagnetic waves, Lee says. Omega3P analyzes those modes one by one.
The Omega3P simulation indicated one wave peak, at 2.95 gigahertz (GHz), causes few problems, but the researchers are still studying the effect of another wave that peaked at 3.85 GHz.
Accelerator designers must worry about more than just how one particle bunch affects the bunches to follow. There also are short-range wakefields that don’t linger in the cavity. “Their effect is on the bunch itself,” Ng says. “This field will kick out the particles at the end of the bunch.” This “intrabunch effect” also can degrade beam quality.
To capture this short-range wakefield the researchers developed a windowing technique that mathematically constructs a small mesh just near the beam but ignores the area outside it. That reduces the size of the problem and uses computer resources more efficiently, Lee says.
The researchers also evaluated the effect of the Lorentz force – an attraction or repulsion caused by the interaction of an electric charge such as an electron or positron as it moves through a magnetic field. “It’s like a pressure on the cavity wall” that can deform its shape, possibly “detuning” the radio frequencies that drive the particles, Ng says. “We wanted to see how this Lorentz force would affect cavity acceleration.”
The researchers used TEM3P, another SciDAC-developed code, for the calculations. TEM3P is a multiphysics code that portrays and integrates electromagnetic, thermal and mechanical effects. The simulation showed the Lorentz force displaced the cavity wall by less than 1 micron, too small to significantly change the EM frequency.
Simulations are critical to understanding wakefields’ effects on cavity heating and beam degradation, says Tor Raubenheimer, a member of the accelerator advisory panel of the ILC Global Design Effort, an international collaboration to hammer out a blueprint. Both have been difficult to resolve in experiments.
Experiments linking three cryomodules are being constructed, says Raubenheimer, who also heads SLAC’s accelerator research division. However, using computer models to study and tweak the design is “an enormous cost- and time-saver.”
Raubenheimer says the simulations already have generated data that could influence the ILC design. He expects even more.
