Coupling 2 ‘Tabletop’ Laser-Plasma Accelerators, a Decisive First Step Toward Tomorrow’s Ultrapowerful Compact Machines
Laser-plasma accelerators (LPAs) got the nickname “tabletop” because, as shown by the unique BELLA accelerator at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab), they can boost electron beams to multibillion electron-volt energies (GeVs) in a few centimeters—a distance thousands of times shorter than conventional accelerators.
Past those few centimeters, however, the laser pulse weakens and energy gain stalls. LPAs will have to get off the tabletop if they are to rival proposed conventional colliders, such as 30-kilometer-long electron-positron linear colliders or circular proton colliders 100 kilometers in circumference, with electron-volt energies in the trillions (TeVs), not billions. Only by coupling a hundred LPAs in series, each powered by a BELLA-class laser in series, and accelerating a well-shaped beam from one stage to the next, will such high energies be achieved.
“Long before planning began for BELLA, we’d set our sights on staging as the way to achieve energies needed for compact particle colliders, free-electron lasers, and other tools of future science,” says Wim Leemans, Director of Berkeley Lab’s Accelerator Technology and Applied Physics Division (ATAP)and Director of the BELLA Center. But because of the daunting technical challenges, including maintaining electron beams with dimensions measured in millionths of a meter and laser pulses measured in quadrillionths of a second (femtoseconds), Leemans says, “Lots of people told us we’d never be able to do it.”
In an experiment packed with scientific firsts, Leemans and his BELLA Center colleagues have now demonstrated that a laser pulse can accelerate an electron beam and couple it to a second laser plasma accelerator, where another laser pulse accelerates the beam to higher energy—a fundamental breakthrough in advanced accelerator science. The results are reported in the Feb. 1 issue of Nature.
Stable beams, disposable mirrors
Sven Steinke, lead author of the Nature paper, says that to achieve staging wasn’t about huge energy gains; the challenge was handing off a useful beam. “A billion electron-volts wouldn’t matter,” he says. “What mattered was stability,” an experiment that would work reliably for days at a time and many thousands of laser shots. “You don’t want to spend three-quarters of your day tuning your beam injector, with no time left to do an experiment.”
The solution was to use two different kinds of LPA. The more advanced but more finicky type is a discharge capillary, a block of sapphire with a thin horizontal tube through it. Hydrogen gas fills the tube; a potent electrical discharge ionizes it, separating electrons from their nuclei and forming a plasma. Almost instantly this discharge arc heats the plasma and forms a laser waveguide, a cylindrical channel of thinner plasma in the center; the incoming laser pulse drives through it like a speedboat on water, picking up free electrons in its wake and hurling them forward like a surfer on a following wave.
Another kind of LPA is a jet of supersonic gas a few hundred micrometers in diameter. The laser pulse drills through the gas, simultaneously ionizing it to form a plasma and leaving a wake to accelerate the free electrons.
The gas jet, conceptually simple but still capable of beam energies of over a hundred million electron-volts, was the team’s choice for stage 1, the beam injector. The more powerful discharge capillary, similar to the kind used in BELLA, would be Stage 2.
A critical challenge was how to introduce the second laser pulse, using a mirror, within the few-millimeter space between the two stages. The electron beam would have to pass through a hole in the mirror. The reflected laser pulse would come close behind. Unfortunately, to focus enough power to accelerate the electron beam, the laser focus would have to be so close to the mirror it would blow it to pieces.
“We decided from the beginning of the project that instead of worrying about blowing up the mirror, we’d blow it up with every shot,” says Leemans. They first developed a prototype mirror of water film, he says, “but settled for much more robust VHS tape.”
Video cassette players may be out of fashion, but VHS tape is thin, stretch-resistant, and capable of running for hours at a time. The electron beam pierces the tape virtually untouched. On the opposite side, in the merest fraction of a second before the laser pulse can penetrate the tape, it ionizes the surface to form a dense, perfectly flat plasma: a highly efficient mirror.
Steinke, whose dissertation involved plasma mirrors and who was a postdoc at the Max Born Institute in Berlin before joining the BELLA Center, characterized the mirror system for the staging experiment. Previous plasma mirrors were based on expensive solid optics made for completely different purposes. Steinke and Leemans agree: “This was the first use of a continuous, high-repetition-rate, disposable plasma mirror.”
A serendipitous plasma lens
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.
The Latest on: Laser plasma accelerators
via Google News
The Latest on: Laser plasma accelerators
- New plasma wave accelerator propels electrons to record speedson October 20, 2019 at 5:00 pm
The wall of plasma surrounding the channel then helps to focus the subsequent laser pulses. By concentrating the energy of the laser pulses, the accelerator can achieve greater electron beam energies ...
- High energy proton micro-bunches from a laser plasma acceleratoron September 25, 2019 at 3:33 am
Recent advances on laser-driven ion accelerators have sparked an increased ... The use of mixed-gas targets will enable high repetition rate operation of these accelerators, free of plasma debris and ...
- Electron energy increase in a laser wakefield accelerator using up-ramp plasma density profileson August 2, 2019 at 10:32 am
The phase velocity of the wakefield of a laser wakefield accelerator can, theoretically, be manipulated by shaping the longitudinal plasma density profile, thus controlling the parameters of the ...
- The age of giant particle accelerators like the LHC may be overon July 24, 2019 at 10:14 am
This is more than wishful thinking. Plasma accelerators have been advancing steadily over the past few decades, and while they have yet to pose a serious threat to the dominance of conventional ...
- Science: 'Photon accelerator' could boost laser lighton July 3, 2019 at 5:58 am
Now John Dawson and his colleagues from the University of California, Los Angeles, and the Los Alamos National Laboratory in New Mexico have proposed what they call a ‘photon accelerator ... a pulse ...
- Miniature beam-driven Plasma ACceleratorson July 1, 2019 at 5:50 pm
In the M-PAC project, I propose to power plasma accelerators with laser-accelerated electron beams based on 100-TW-class laser systems, so as to miniaturize the so-called “beam-driven plasma ...
- Shrunken accelerator blasts like the big boyson June 20, 2019 at 5:00 pm
U. TEXAS-AUSTIN (US) — A new tabletop particle accelerator is a step toward having multi-gigaelectronvolt laser plasma accelerators in research labs around the world. “We have accelerated about half a ...
- UT Austin laser-based tabletop particle accelerator reaches major milestoneon June 19, 2019 at 5:00 pm
The conventional accelerators he speaks of cost hundreds of millions of dollars to build. The results, which were published in Nature Communications, mark a major milestone in the advance toward the ...
- The potential of plasma wakefield accelerationon March 14, 2019 at 9:29 am
All PWFA experiments require lasers to create a plasma—that’s how they ionize gas. But laser wakefield accelerators also use a laser as a drive beam. The radiation pressure from the laser pushes ...
- Physicists set world record in tabletop plasma acceleration with laser drillon February 27, 2019 at 5:07 am
By contrast, plasma wakefield acceleration involves firing very intense ... Fortunately, LBNL has one of the world's most powerful and precise lasers in BELLA (Berkeley Lab Laser Accelerator). But ...
via Bing News