May 29, 2014
Reduced Precision Unlikely To Negatively Impact Laser-Plasma Particle Accelerators
redOrbit Staff & Wire Reports - Your Universe Online
Creating your own tabletop particle accelerator just got a little bit easier, according to scientists from the Lawrence Berkeley National Laboratory (Berkeley Lab), who report that the laser lights used in these miniature units do not have to be as precise as previously believed.When most people think of particle accelerators, things like the Large Hadron Collider that was used in the discovery of the Higgs boson come to mind. Those devices require high-powered radio-frequency waves in order to excite electrons, but a new type of accelerator known as a laser-plasma accelerator uses laser light pulses to cause plasma motion, the Berkeley Lab’s Kate Greene explained in a statement Wednesday.
By doing so, these new particle accelerators allow electrons riding on top of water-like waves to reach high speeds. Unfortunately, it can be difficult to create a laser pulse powerful enough to be a match for traditional accelerators, as lasers have to be able to fire high-energy pulses thousands of time per second, said Greene. Modern lasers are only capable of producing one pulse per second at the energy levels required for particle acceleration.
“If you want to make a device that’s of use for particle physics, of use for medical applications, of use for light source applications, you need repetition rate,” explained Wim Leemans, a physicist at the Berkeley Lab and one of the authors of a new Physics of Plasmas study that demonstrates how some of the requirements for the lasers required for these small-area particle accelerators can be relaxed significantly.
Leemans and co-authors Carlo Benedetti, Carl Schroeder and Eric Esarey hope that their discovery could usher in a new era of accelerators requiring only a few meters to successfully speed-up particles, rather than the several kilometers required by traditional accelerators such as the Large Hadron Collider.
During a US Department of Energy workshop held in January 2013, Leemans and his colleagues attempted to tackle the issue of how to improve upon the current tabletop particle acceleration technology. Conventional wisdom suggests combining several smaller lasers to create one extremely powerful pulse, but actually putting this theory into practice appeared difficult for many reasons.
Among those reasons was the notion that the light from the smaller lasers would have to precisely match in terms of color, phase and other properties in order to accelerate electron motion in the plasma. However, the Berkeley Lab researchers behind the new study have found that this isn’t true. By testing several different scenarios using computer models, they found that the colors and phases of the laser light did not matter to the plasma.
“The plasma is a medium that responds to a laser, but it doesn’t respond immediately,” Benedetti told Greene, explaining that the light operates on a faster time scale and a smaller length scale. Ultimately, the various patterns of interference and electromagnetic fields average out in the slower-responding medium of plasma. In layman’s terms, once the laser light actually gets inside the plasma, the majority of the potential problems are no longer factors.
“As an experimentalist for all these years we’re trying to make these perfect laser pulses, and maybe we didn’t need to worry so much,” added Leemans. “I think this will have a big impact on the laser community and laser builders because all of a sudden, they’ll think of approaches where before hand all of us said, ‘No, no, no. You can’t do that.’ This new result says, well maybe you don’t have to be all that careful.”
While he and his colleagues have previously demonstrated a laser with a petawatt pulse (one quadrillion watts) lasting 40 femtoseconds and capable of generating a 10 GeV beam in experiments, they said they are at least five years away from creating a high-repetition rate, 10-GeV laser-plasma accelerator that can fire at least one thousand pulses or more per second.
However, Leemans added that his team is in the process of developing a new project that will use combined, less precise laser light sources in order to produce faster and more powerful laser pulses. “Once we synthesize a pulse at higher repetition rates, we will be on our way towards a kilohertz GeV laser plasma accelerator,” he said.