Direct observation of highly nonlinear plasma waves

Direct observation of highly nonlinear plasma waves

The highly nonlinear plasma wave (green) driven by a strong laser pulse reaches the breaking point of the wave, where a fraction of the plasma electrons (red) are captured by the wake field and accelerated. Credit: Igor Andriyash, Yang Wan and Victor Malka.

In recent decades, physicists and engineers have sought to create ever more compact laser-plasma accelerators, a technology to study the interactions of matter and particles produced by interactions between ultrafast laser beams and plasma. These systems are a promising alternative to existing large-scale machines based on radio frequency signals, as they can be much more efficient at accelerating charged particles.

While laser-plasma accelerators are not yet in widespread use, several studies have highlighted their value and potential. To optimize the quality of the accelerated laser beam produced by these devices, however, researchers will need to be able to monitor several ultrafast physical processes in real time.

Researchers at the Weizmann Institute of Science (WIS) in Israel recently devised a method to directly observe laser-driven and nonlinear relativistic plasma waves in real time. Using this method, introduced in a paper published in The physics of naturewere able to characterize the nonlinear plasma at incredibly high temporal and spatial resolutions.

“Imaging a micrometer laser-driven plasma wave traveling at the speed of light is very difficult, involving the use of ultrashort pulses of light or bundles of charged particles,” Yang Wan, one of the researchers who conducted the study. said “While light can reveal structures in the plasma density, particle beams probe the internal fields of plasma waves and could thus give us much more information about the state of these waves, i.e. their ability to inject and accelerate electrons in the plasma” .

Wan and his colleagues’ recent work builds on an earlier proof-of-principle study he conducted with his former research team at Tsinghua University in China. This previous study essentially confirmed the imaging feasibility of weaker linear sine waves (ie, natural representations of how things and systems in nature change state over time).

“To directly observe the highly nonlinear plasma wave, which is most popularly used to accelerate electrons, we built two high-power laser-plasma accelerators using our dual 100 TW laser system at WIS,” explained Wan. “This system produces a high-energy electron probe, and the other produces a highly nonlinear plasma wake field to be probed. In this exploratory study, we tested this new imaging technique to its limits, searching for fine field structures inside nonlinear plasma waves.”

The original goal of the experiment by Wan and his colleagues at WIS was to observe plasma waves in detail. After doing so, however, the team realized that the nonlinear plasma waves deflected the probe particles in more interesting and surprising ways, acting through both electric and magnetic fields.

“When we deciphered this information with theoretical and numerical models, we identified features that directly correlate with the electron-dense peak at the back of the formed ‘plasma bubble,'” Wan said. “To our knowledge, this is the first measurement of such fine structures inside the nonlinear plasma wave.”

Wan and his colleagues later increased the power of the driver laser used in their experiment. This allowed them to identify the so-called “wave break”, the condition after which a plasma wave can no longer grow, so instead it captures plasma electrons in its acceleration field. Wavebreaking is a fundamental physical phenomenon, especially in plasma.

“The first important achievement of our work is the imaging of the extremely strong fields of relativistic plasmas, as it exploits a unique feature of such laser-plasma accelerators – the beam duration of a few femtoseconds and the beam source size of micrometers, which provides ultra-spatial resoln. large timescale for capturing microscopic phenomena moving at the speed of light,” Wan said. “Through the plasma wave image, I also directly observed the subtle process of ‘wave breaking’, which in itself was a wonderful experience.”

Remarkably, the measurement collected by this team of researchers would be impossible to achieve using any of the existing conventional accelerators based on radio frequency technology. In the future, their work could thus inspire other teams to develop similar experimental methods to further observe the many nuances of plasma.

“Wave breaking is also crucial for plasma-based accelerators due to the production of relativistic electrons from self-injection,” Wan said. “This injection mechanism is quite important in single-stage multi-GeV accelerators, where it is difficult to maintain controlled injection over a long period of operation.”

This recent work by Wan and his colleagues could have many important implications for the development and use of laser-plasma accelerators. In particular, it introduces a valuable tool to identify the electron self-injection process in real time, which would allow researchers to fine-tune accelerators and improve the quality of their beams.

“We now have a unique and powerful tool to explore extreme fields to investigate many other fundamental questions in a wider range of plasma parameters that are relevant to physics, including the particle beam-driven wake field, the beam-plasma interaction and fusion-related plasma dynamics.” Prof. Victor Malka, the principal investigator of the study and the principal investigator of the group, told “The future is very exciting and we look forward to going deeper into exploring the rich phenomena of plasma physics.”

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More information:
Yang Wan et al, Direct observation of relativistic broken plasma waves, The physics of nature (2022). DOI: 10.1038/s41567-022-01717-6

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