Over the past few decades, physicists and engineers have attempted to create increasingly compact laser-plasma accelerators, a technique for studying matter-particle interactions resulting from interactions between ultrafast lasers and plasmas. These systems are a promising alternative to current large machines based on radio frequency signals, as they can be more efficient at accelerating charged particles.
While laser plasma accelerators are not yet widely used, several studies have highlighted their value and potential. To improve the quality of the accelerated laser beam produced by these devices, researchers will need to be able to monitor many ultra-fast physical processes in real time.
Researchers at the Weizmann Institute of Science (WIS) in Israel recently devised a method for monitoring nonlinear relativistic plasma waves driven by lasers in real time. Using this method, it was presented in a paper published in Nature Physicswere able to characterize nonlinear plasmas with incredibly high temporal and spatial resolution.
“Imaging a plasma wave driven by a micrometric laser operating at the speed of light is very challenging, which means using very short pulses of light or groups of charged particles,” Yang Wan, one of the researchers who conducted the study, told Phys.org. “While light can reveal structures in plasma density, particle beams explore the inner fields of plasma waves and thus can give us more information about the state of these waves, namely their ability to inject and accelerate plasma electrons.”
The latest work by Wan and colleagues builds on a previous proof-of-principle study he conducted with his previous research team at Tsinghua University in China. This previous study essentially confirmed the feasibility of imaging weaker linear sine waves (i.e., natural representations of the number of objects and systems in nature that change over time).
“For direct observation of the high nonlinear plasma wave most commonly used for electron acceleration, we built two high-powered laser plasma accelerators using the 100 TW dual laser system at WIS,” Wan explained. “This system produces one high-energy, highly charged electron probe and the other produces a highly nonlinear plasma field to be investigated. In this exploratory study, we tested this new imaging technology to its limits, searching for precise field structures within nonlinear plasma waves.”
The initial goal of the experiment conducted by Wan and colleagues at WIS was to observe plasma waves in detail. After doing this, the team realized that nonlinear plasma waves deflected the probe particles in more interesting and surprising ways, acting through both electric and magnetic fields.
“When deciphering this information using theoretical and numerical models, we identified features that are directly related to the electron-dense height at the back of the formed plasma bubble,” Wan said. “To our knowledge, this is the first measurement of such minute structures within a nonlinear plasma wave.”
Next, Wan and colleagues increased the power of the laser used in their experiment. This allowed them to identify a so-called “wave break”, the state after which a plasma wave cannot grow, so instead captures the plasma electrons in its accelerating field. Wave penetration is a fundamental physical phenomenon, especially in plasmas.
“The first important achievement of our work is the imaging of the extremely strong fields of relativistic plasmas, as they exploit a unique feature of laser plasma accelerators – the few femtosecond beam duration and micrometer beam source size, which provide ultra-high spatial resolution to capture microscopic phenomena operating in light’s speedWan said, “By imaging the plasma wave, we also directly observed the exact process of ‘wave breaking’, which in itself was a fascinating experience.”
Remarkably, the measurement collected by this team of researchers would be impossible to achieve using any of the conventional accelerators based on radiofrequency technology. In the future, their work could inspire other teams to devise similar experimental methods to further observe the many nuances of plasma.
“Wave breaking is also critical for plasma-based accelerators, due to the production of relativistic electrons from self-injection,” Wan said. “This injection mechanism is somewhat important in multi-stage single-stage GeV accelerators where controlled injection is difficult to maintain over a long operating time.”
This recent work by Wan and colleagues could have several important implications for the development and use of laser plasma accelerators. Notably, it offers a valuable tool for determining the electron self-injection process in real time, allowing researchers to fine-tune accelerators and improve the quality of their beams.
“We now have a unique and powerful tool for exploring extreme fields to investigate many other fundamental questions in a broader range of plasma parameters relevant to physics including the particle beam-driven Wakefield field, beam-plasma interaction and fusion-related plasma dynamics,” said Professor Victor Malka, Study principal investigator and group principal investigator, Phys.org. “The future is very exciting, and we are impatient to delve deeper into the rich phenomena of plasma physics.”
Yang Wan et al., Direct observation of relativistic broken plasma waves, Nature Physics (2022). DOI: 10.1038 / s41567-022-01717-6
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