World Record in Laser-Driven Electron Acceleration

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, orange, and yellow). This plasma channel was used to guide femtoseconds-long “driver” laser pulses from the BELLA petawatt laser system, which generated plasma waves and accelerated electrons to 8 billion electron volts in just 20 centimeters. (Source: G. Bagdasarov, Keldysh Inst. of Appl. Mathematics / A. Gonsalves & J.-L. Vay, Berkeley Lab)

Combining a first laser pulse to heat up and drill through a plasma, and another to accelerate electrons to incredibly high energies in just tens of centi­meters, scientists have nearly doubled the previous record for laser-driven particle acceleration. The laser-plasma experiments, conducted at the Department of Energy’s Lawrence Berkeley National Labora­tory (Berkeley Lab), are pushing toward more compact and affor­dable types of particle acceleration to power exotic, high-energy machines – like X-ray free-electron lasers and particle colliders – that could enable researchers to see more clearly at the scale of molecules, atoms, and even subatomic particles.

The new record of propel­ling electrons to 7.8 GeV at the Berkeley Lab Laser Acce­lerator (BELLA) Center surpasses a 4.25 GeV result at BELLA announced in 2014. The record result was achieved during the summer of 2018. The experiment used incre­dibly intense and short driver laser pulses, each with a peak power of about 850 trillion watts and confined to a pulse length of about 35 femto­seconds.

Each intense driver laser pulse delivered a heavy kick that stirred up a wave inside a plasma. Electrons rode the crest of the plasma wave, like a surfer riding an ocean wave, to reach record-breaking energies within a 20-centi­meter-long sapphire tube. “Just creating large plasma waves wasn’t enough,” noted 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.”

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

In the 2014 experiment, an elec­trical discharge was used to create the plasma channel, but to go to higher energies the researchers needed the plasma’s density profile to be deeper – so it is less dense in the middle of the channel. In previous attempts the laser lost its tight focus and damaged the sapphire tube. Gonsalves noted that even the weaker areas of the laser beam’s focus were strong enough to destroy the sapphire structure with the previous technique.

Eric Esarey, BELLA Center Director, said the solution to this problem was inspired by an idea from the 1990s to use a laser pulse to heat the plasma and form a channel. This technique has been used in many experiments, including a 2004 Berkeley Lab effort that produced high-quality beams reaching 100 MeV. Both the 2004 team and the team involved in the latest effort were led by former ATAP and BELLA Center Director Wim Leemans, who is now at the DESY labora­tory in Germany. The researchers realized that combining the two methods – and putting a heater beam down the center of the capillary – further deepens and narrows the plasma channel. This provided a path forward to achieving higher-energy beams.

In the latest experiment, Gonsalves said, “The electrical discharge gave us exqui­site control to optimize the plasma conditions for the heater laser pulse. The timing of the elec­trical discharge, heater pulse, and driver pulse was critical.” The combined technique radically improved the confinement of the laser beam, preser­ving the intensity and the focus of the driving laser, and confining its spot size, or diameter, 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.

The team needed new numerical models to develop the technique. A colla­boration including Berkeley Lab, the Keldysh Institute of Applied Mathe­matics in Russia, and the ELI-Beamlines Project in the Czech Republic adapted and inte­grated several codes. They combined MARPLE and NPINCH, developed at the Keldysh Insti­tute, to simulate the channel formation; and INF&RNO, developed at the BELLA Center, to model the laser-plasma interactions. “These codes helped us to see quickly what makes the biggest difference – what are the things that allow you to achieve guiding and acceleration,” said Carlo Benedetti, the lead developer of INF&RNO. Once the codes were shown to agree with the expe­rimental data, it became easier to interpret the experi­ments, he noted. “Now it’s at the point where the simulations can lead and tell us what to do next,” Gonsalves said. (Source: LBL)

Reference: A. J. Gonsalves et al.: Petawatt Laser Guiding and Electron Beam Acceleration to 8 GeV in a Laser-Heated Capillary Discharge Waveguide, Phys. Rev. Lett. 122, 084801 (2019); DOI: 10.1103/PhysRevLett.122.084801

Link: Berkeley Lab Laser Accelerator BELLA Center, Berkeley, USA

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