A recent breakthrough in computer modeling may help hasten the day when accelerators thousands of times more powerful than current accelerators can be built in a fraction of the space—and for significantly less money.
Laser wakefield acceleration works by shooting powerful laser pulses through a cloud of ionized gas (plasma). The pulse creates a wave (or wake) on which introduced electrons “surf,” much as human surfers ride ocean waves. Using this method, researchers have demonstrated acceleration gradients 1,000 times greater than conventional methods. Experiments using laser wakefields have so far spanned no more than a few centimeters, but if scientists successfully extend that to meters and can string several laser wakefield stages together, an accelerator more powerful than the LHC would theoretically require a wake path only 10-100 meters long.
Completing the calculations for modeling longer wakes produced by lasers, around a meter, would require a prohibitive amount of computing power, not to mention time.
To address this problem, a team led by Mori and Silva turned to a quirk of special relativity. Here’s how it works: When objects are moving at or near the speed of light, each experiences time, space, sometimes even event-order differently. Yet even in this mixed up state, the same laws of physics apply. So, rather than carrying out their computer simulations in a standard frame of reference—the plasma being more or less stationary and the laser beam moving through it at the speed of light—the researchers used a moving frame. That is, they simulated the plasma moving towards the beam and just under the speed of light.
Due to a relativistic effect called Lorentz contraction, the plasma in this configuration shrinks ten times shorter and the laser’s wavelength grows ten times longer. In this “Lorentz-boosted” frame of reference, the code need calculate only about 1/100th the number of time-slices normally necessary to create a simulation. Depending on the simulation, Mori’s team was able calculate simulations at rates 100 to 300 times faster than in a standard frame-of-reference. That’s something like recording a 100-minute feature film in a minute or less.The Lorentz-boosted frame makes it possible to simulate longer stretches of plasma than ever. “With this code we can simulate lasers that aren’t yet possible to build,” said Silva. That’s crucial to scientists who are racing to build lasers powerful enough to keep electrons surfing longer and thus reaching higher speeds. The team’s code can help inform critical design decisions: “For example, you could tightly focus a laser or make the spot size wider; there are tradeoffs to doing each of these for a given laser that could be developed,” said Silva.” Speeding up simulations lets researchers test a wider range of possibilities in less time.
Traditional radio frequency accelerators coax particles to higher energies through long particle racetracks, or wave paths. The Stanford Linear Accelerator, for example, can boost particles to energies of up to 50 billion electron volts, 5 GeV, along its 3.2 km length. Laser wakefield experiments, meanwhile, have achieved 1 GeV in about 1/100,000th that stretch, 3.3 cm.
Not Just Atom Smashers
Thousands of electron accelerators are in routine use everyday. These do everything from providing radiation treatments for cancer patients to sterilizing food and treating industrial materials and nuclear waste. Accelerators also help to provide the x-rays that scientists use to probe nano materials, proteins, and even archaeological artifacts. In the near term, laser wakefield technology holds out the promise of smaller, more powerful and more affordable electron accelerators for the generation of x-rays. It will probably take more time for laser wakefield acceleration to make its way into large-scale particle physics experiments akin to the LHC, the ultimate aim of much laser wakefield acceleration research.