Laser "drill" sets new world record for laser electron acceleration

Laser-plasma experiments, conducted at the Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab), are pushing for more compact and affordable particle acceleration types to power high-energy exotic machines such as laser lasers. X-ray free electrons and particle colliders – which could enable researchers to see molecules, atoms and even subatomic particles more clearly on the scale.

The new electron-propulsion record at 7.8 billion electron-volts (7.8 GeV) at the Berkeley Lab Laser Acceleration Center (BELLA) exceeds the results of 4.25 GeV announced by BELLA in 2014. The latest research is detailed in the February 25 edition of the journal Letters of physical examination. The record result was reached in the summer of 2018.

The experiment used incredibly intense and short laser "attack" pulses, each having a peak power of about 850 trillion watts and a duration of about 35 quadrillion seconds (35 femtoseconds). Peak power equates to lighting approximately 8.5 trillion 100 watt bulbs simultaneously, although the bulbs are only lit for dozens of femtoseconds.

Each intense pilot laser pulse produced a powerful "kick" that lifted a wave into a plasma – a gas heated enough to create charged particles, including electrons. The electrons have traveled the peak of the plasma wave, like a surfer on an ocean wave, to reach unprecedented energies in a sapphire tube 20 centimeters long.

"Creating large plasma waves was not enough," said Anthony Gonsalves, lead author of the latest study. "We also had to create these waves over the entire length of the 20-centimeter tube to accelerate the electrons to such a high energy."

To do this, it required a plasma channel, which contained a laser pulse in the same way that a fiber optic cable channels the light. But unlike a conventional optical fiber, a plasma channel can withstand the ultra-intense laser pulses needed to accelerate the electrons. To form such a plasma channel, you have to make the plasma less dense in the center.

In the 2014 experiment, an electric shock was used to create the plasma channel, but to reach higher energies, the researchers needed the plasma density profile to be deeper – less dense in the center of the plasma. channel. In previous attempts, the laser had lost its accuracy and damaged the sapphire tube. Gonsalves noted that even the weakest areas of the focus of the laser beam – his so-called "wings" – were strong enough to destroy the sapphire structure with the previous technique.

Eric Esarey, Director of the BELLA Center, said the solution to this problem was inspired by a 1990s idea of ​​using a laser pulse to heat the plasma and form a channel. This technique has been used in many experiments, including a 2004 effort by the Berkeley Laboratory, which produced high quality beams of up to 100 million electron volts (100 MeV).

The 2004 team and the team involved in the last effort were both led by former ATAP and BELLA director Wim Leemans, who works at the DESY laboratory in Germany. The researchers realized that the combination of the two methods – and the installation of a heating beam in the center of the capillary – further deepens and narrows the plasma channel. This paved the way for the realization of higher energy beams.

In the latest experiment, Gonsalves said: "The electric shock has provided us with exquisite control to optimize the plasma conditions for the laser pulse of the heating element. 39, impulse of the heating element and the impulse of the pilot was crucial. "

The combined technique radically improved the confinement of the laser beam, preserving the intensity and focus of the pilot laser, and limiting the size of its point, or diameter, to a few tens of millions of meters during its movement in the tube with plasma. This allowed to use a lower density plasma and a longer channel. The previous record of 4.25 GeV used a 9-centimeter channel.

The team needed new digital models (codes) to develop the technique. A collaboration involving Berkeley Lab, the Keldysh Institute of Applied Mathematics in Russia and the ELI-Beamlines project in the Czech Republic has adapted and integrated several codes. They combined MARPLE and NPINCH, developed at the Keldysh Institute, to simulate channel formation; and INF & RNO, developed at the BELLA center, to model laser-plasma interactions.

"These codes have allowed us to quickly see what makes the biggest difference – allowing you to guide and accelerate," said Carlo Benedetti, lead developer of INF & RNO. Once the codes were in agreement with the experimental data, it became easier to interpret the experiments, he noted.

"It's now that the simulations can lead us and tell us what to do next," said Gonsalves.

Benedetti noted that the heavy calculations in the codes made use of the resources of the National Energy Research Scientific Computing Center (NERSC) at Berkeley Lab. Future work to accelerate energy acceleration may require much more sophisticated calculations based on a regime known as exascale computing.

"Today, the beams produced could allow the production and capture of positrons," which are electron – charged equivalents, Esarey said.

He indicated that BELLA's goal was to reach 10 GeV energies in electron acceleration, and that future experiments will target that threshold and beyond.

"In the future, several high-energy electron-accelerating steps could be coupled to achieve an electron-positron collider to explore fundamental physics with new precision," he said. declared.

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More information:
A. J. Gonsalves et al., Laser guidance and electron beam acceleration of Petawatt up to 8 GeV in a laser heated capillary discharge waveguide, Letters of physical examination (2019). dx.doi.org/10.1103/PhysRevLett.122.084801