Coupling 'Tabletop' Laser-Plasma Accelerators

Estimated reading time: 7 minutes

The staging system was ready for its first test. In the gas-jet LPA, the first laser pulse created an electron beam that passed through the tape, while the plasma mirror reflected the second laser pulse. Electron beam and laser pulse both entered the stage 2 capillary.

No beam came out.

“We were stunned,” says Jeroen van Tilborg, a long-time member of the BELLA Center and its predecessor, the LOASIS Program, where he earned his PhD from Eindhoven Technical University. “Suddenly there were four or five of us sitting around scribbling on the backs of envelopes.”

ATAP scientists had used discharge capillaries to inject and accelerate electrons for over 10 years, but this was the first time anyone had shot an external electron beam into one. They’d never dealt with the full effects of the powerful discharge current: it ionizes the gas and forms an optical waveguide through the plasma, but also creates a strong magnetic field that can blow apart a pre-existing electron beam.

Or, more optimistically, can shape and focus it. Van Tilborg called dibs on studying the problem and soon realized the pulsed magnetic field would make an excellent plasma lens. Such a fast-acting lens could find many uses, for example by conditioning beams of existing free-electron lasers. Its immediate application was to tightly focus the staging experiment’s injector beam.

The final configuration—gas-jet injector, plasma lens, plasma mirror, discharge capillary second stage, and diagnostics—showed energy gains, for significant portions of the electron beam, of around a hundred million electron-volts.

The success of the experiment resulted from on-the-job discoveries plus continuous feedback between experimental observations and computer modeling. Running on a Cray supercomputer at DOE’s National Energy Research Scientific Computing Center (NERSC) at Berkeley Lab, the highly efficient INF&RNO code for modeling laser and plasma interactions could turn a day’s experimental data into a simulation almost overnight, like “dailies” on a movie set. Among many other questions, intricacies of laser timing could be explored; focusing the energetic but ragged beam from the gas jet could be simulated even as the serendipitous discovery of how to actually do it was becoming a reality.

“Through matching to the experimental observations, simulation can see everything,” says Carlo Benedetti of the BELLA Center’s simulation team, who led development of INF&RNO. “We can see how the laser beam is behaving and understand which electrons are the ones being accelerated.”

The first successful coupling of two independent laser-plasma accelerators has proven the principle. Next comes the real thing.

“We’re ready for staging BELLA,” says Leemans, using two charge-capillary LPAs. “We’ll split the BELLA laser beam,” capable of a quadrillion watts (a petawatt) per 40-femtosecond pulse every second. “The first stage should bring up the beam to about 5 GeV. We will do the bunch transport with our capillary lens and play around with the timing of the second pulse. We should come out of the second stage with 10 GeV. And, while in the staging experiment we’re only trapping about three or four percent of the electrons available, in BELLA we’ll be able to trap 100 percent of the charge.”

Even better, says Steinke, “BELLA is much simpler. The effects of the tape on beam quality should be much less, and the beam is much ‘stiffer,’ easier to handle.”

Van Tilborg concurs: “At 5 GeV per stage there may be no problem. The higher energy saves you.”

“Many groups around the world are working on different aspects of LPA development, and I am confident that we will see the first applications of LPAs in the coming decade,” comments James Symons, Associate Laboratory Director for Physical Sciences at Berkeley Lab. “As with all new technologies, the nature of those applications may surprise us.” Challenges remain, but the era of accelerators that are not only compact but can achieve extraordinary energies is upon us.

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