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Image of the Week - Relativistic reference frames speed simulations

Relativistic reference frames speed simulations

This simulation was conducted on Franklin at the National Energy Research Scientific Computing Center, NERSC, at Berkeley Lab. Image courtesy of Mori et. al.

Even on a supercomputer, simulating the next generation of laser-plasma accelerators would take months. Luckily, by solving an old problem to create a new way to model these accelerators, an international group of researchers has cut the computation time by a factor of at least 100.

Laser-plasma accelerators shoot powerful laser pulses into a cloud of plasma, creating a wave that electrons can "surf" to accelerate more rapidly than in traditional accelerators. Normally, computer models consider this problem from the experimenter's reference frame, in which the plasma is more or less stationary and the laser is traveling at the speed of light.

This team, jointly led by Warren Mori at the University of California at Los Angeles and Luis Silva at the Instituto Superior Tecnico in Portugal, took a different point of view: a reference frame in which the plasma is moving towards the beam at just under the speed of light. This is commonly known as a Lorentz-boosted frame. According to Einstein's Special Relativity, in the Lorentz-boosted frame the laser will appear to be stretched and the plasma will appear contracted.

In the laboratory the laser is always much smaller than the plasma. But in this new frame of reference the difference in size is reduced. It is this reduced size difference that ultimately results in fewer calculations.

"The idea of using a Lorentz-boosted frame to speed up this type of calculation was initially explored in the 90s by Mori's group," said Samuel Martins, a computational physicist formerly affiliated with the Instituto Superior Tecnico. "But several numerical issues were encountered, and the project was abandoned."

In 2007, Lawrence Berkeley Lab physicist Jean-Luc Vay published a paper describing how he successfully applied this technique to particle-beam interactions. Vay also suggested that Lorentz-boosting could be applied to free electron lasers and laser-plasma accelerators.

Despite Vay's suggestion, until now no-one had ever succeeded at applying the Lorentz-boosting technique to a laser-plasma accelerator simulation. Mori's team is the first, and their result is this week's featured image. To learn more about this research, check out the Berkeley Lab Computing Sciences press release.

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