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Using a laser pulse to heat up and “drill” through a plasma, scientists at Lawrence Berkeley National Laboratory have nearly doubled the previous record for laser-driven particle acceleration. The new record of propelling electrons to 7.8 electron volts (7.8 GeV) surpasses the 4.25-GeV result achieved in 2014.
The team at Berkeley Lab shot intense, short IR laser pulses, each with a peak power of about 850 terawatts — the equivalent to lighting up about 8.5 trillion 100-watt light bulbs simultaneously — lasting just 35 femtoseconds, into a 0.8 millimeter-wide, 20-centimeter long sapphire tube filled with hydrogen. Each intense laser pulse delivered a “kick” that stirred up a wave inside a plasma. Electrons rode the crest of the plasma wave to reach record-breaking energies within the sapphire tube.
A snapshot of a plasma channel’s electron density profile (blue) formed inside a sapphire tube (gray) with the combination of an electrical discharge and an 8-nanosecond laser pulse (red/yellow). Courtesy of Gennadiy Bagdasarov/Keldysh Institute of Applied Mathematics and Anthony Gonsalves and Jean-Luc Vay/Lawrence Berkeley National Laboratory.
“Just creating large plasma waves wasn’t enough,” said Anthony Gonsalves. “We also needed to create those waves over the full length of the 20-centimeter tube to accelerate the electrons to such high energy.” This required a plasma channel that could withstand the ultra-intense laser pulses needed to accelerate electrons. To form such a channel, the team needed to make the plasma less dense in the middle.
In the team’s 2014 experiment, an electrical discharge was used to create a plasma channel, but to go to higher energies, the researchers needed the plasma’s density profile to be deeper. To resolve this issue, the team used a laser pulse to heat the plasma. Putting a heater beam down the center of the channel further deepened and narrowed it. This provided a path to achieving higher-energy beams.
“The electrical discharge gave us exquisite control to optimize the plasma conditions for the heater laser pulse,” Gonsalves said. “The timing of the electrical discharge, heater pulse, and driver pulse was critical.”
This combination of techniques radically improved the confinement of the laser beam, preserving the intensity and the focus of the driving laser, and confining its spot size to just tens of millionths of a meter as it moved through the plasma tube. This enabled the use of a lower-density plasma and a longer channel. The previous 4.25-GeV record had used a 9-centimeter channel. With a 20-centimeter channel, the team achieved an energy level of 7.8 GeV.
A collaborative team including scientists from Berkeley Lab, the Keldysh Institute of Applied Mathematics, and the ELI-Beamlines Project adapted and integrated several new numerical models to develop the technique. “These codes helped us to see quickly what makes the biggest difference — what the things that allow you to achieve guiding and acceleration are,” Carlo Benedetti said. Once the codes were shown to agree with the experimental data, it became easier to interpret the experiments, he said.
“Now it’s at the point where the simulations can lead and tell us what to do next,” Gonsalves said.
A 20-centimeter plasma tube as used for the world record. Courtesy of Marilyn Chung/Lawrence Berkeley National Laboratory.
Scientist Eric Esarey said that the team’s goal is to reach 10-GeV energies in electron acceleration. Future experiments will target this threshold and beyond. “In the future, multiple high-energy stages of electron acceleration could be coupled together to realize an electron-positron collider to explore fundamental physics with new precision,” he said.
Laser plasma acceleration uses an intense, high-energy laser pulse that plows through a plasma, creating plasma waves that can accelerate particles hundreds of times stronger than conventional accelerators. The more powerful the laser pulse is, the stronger the acceleration in the plasma. The laser-plasma experiments conducted at the Berkeley Lab are pushing toward more compact and affordable types of particle acceleration to power exotic, high-energy machines that could enable researchers to see more clearly at the scale of molecules, atoms, and even subatomic particles.
The team was led by Wim Leemans, then head of the Berkeley Lab Laser Accelerator (BELLA) Center and now accelerator director at Deutsches Elektronen-Synchrotron (DESY).
The research was published in Physical Review Letters (https://doi.org/10.1103/PhysRevLett.122.084801).READ MORE