Lasers without mirrors,
designed by supercomputer
Posted October 14, 2009
Sometimes it takes a big machine to understand the tiniest details.
That’s the case with free electron lasers (FELs). The powerful X-rays they generate can probe matter directly at the level of atomic interactions and chemical-bond formation, letting scientists observe such phenomena as chemical reactions in trace elements, electric charges in photosynthesis and the structure of microscopic machines. FELs have the potential to address a host of research challenges in physics, chemistry and material and biological sciences.
To generate a free electron laser, electrons are accelerated and made to emit X-ray light. (Click on image to enlarge and for more information.)
FELs share many optical properties with conventional lasers. The main difference: In conventional solid-state and gas lasers, electrons are bound and bounce off mirrors. FELs enlist an electron beam that moves freely through an oscillating magnet, accelerating the electrons to near light speed, where they release photons. By using free electrons and eliminating mirrors, FELs produce coherent light at wavelengths shorter than conventional lasers – at the X-ray scale.
To succeed, FEL designs must be optimized to produce and preserve high-brightness electron beams, characterized by high current and “low emittance,” or when particles are bunched and move together. At the Department of Energy’s Lawrence Berkeley National Laboratory (LBNL) a team of researchers is using high-performance computers to shed light on these designs.
A billion points of flight
Using code he wrote for use on Franklin, the Cray XT computer at DOE’s National Energy Research Scientific Computing Center (NERSC) at LBNL, staff scientist Ji Qiang, John Corlett, head of LBNL’s Center for Beam Physics, and colleagues will model 1 billion electrons as they pick up speed in a linear accelerator, or linac. Another computer code will track these electrons through an X-ray FEL.
Qiang and his co-investigators will simulate evolution of the electron beam from its origin to its fate as a photon source. He and four other LBNL scientists will scrutinize the model for phenomena that can hamper beam quality.
“To build a next-generation X-ray FEL source, we need a better understanding of beam dynamics,” Qiang says. And that requires supercomputing to model “multiple billion electrons through the different accelerator stages and track the details.”
Franklin, Qiang says, will give the researchers computer power that can run their model at 10 times the time, space and energy resolution of previous machines.