New Plasma Accelerator Paves Way for Compact X-ray Lasers

A team led by Sam Barber at the Lawrence Berkeley National Laboratory has unveiled a groundbreaking free-electron laser (FEL) driven by a plasma-based electron accelerator. This innovation represents a significant advancement towards developing compact and affordable FELs capable of generating intense, ultra-short X-ray laser pulses. The project involved collaboration with researchers from the University of California Berkeley, University of Hamburg, and Tau Systems.

FELs produce X-rays by rapidly accelerating electron pulses back and forth using a series of magnets, known as undulators. This process generates X-rays at a narrow wavelength that subsequently interact with the pulse as it travels through the undulator, resulting in a coherent, laser-like X-ray pulse. A distinct advantage of FELs is their ability to adjust the wavelength of emitted X-rays simply by altering the energy of the electron pulses, making them highly tunable.

Traditionally, the construction of X-ray FELs has been hampered by the need for large, expensive electron accelerators, often spanning several kilometers. These systems require significant financial investment both to build and maintain. In response, researchers are investigating laser-plasma accelerators (LPAs) as a more cost-effective alternative. LPAs can accelerate electron pulses to high energies over distances of just a few centimeters.

Barber noted the historical challenges associated with LPAs, stating, “LPAs have had a reputation for being notoriously hard to use for FELs because of things like parameter jitter and the large energy spread of the electron beam compared to conventional accelerators.” Despite these hurdles, ongoing international research continues to enhance LPA performance.

Recent advancements from a team at the Chinese Academy of Sciences have demonstrated the potential of LPAs, successfully generating FEL pulses that are 50 times more effective than previous attempts. Although their pulses achieved a wavelength of 27 nm, which is in the X-ray range, only about 10% of the attempts were successful.

Enhancements to FEL Performance

The research team has made notable improvements to their FEL setup to increase compatibility with LPAs. Barber explained, “We have taken great pains to ensure a very stable laser with several active feedback systems.” Their approach follows a methodical process, beginning with longer wavelengths to optimize the system before scaling it down to shorter wavelengths.

With these enhancements, the team has amplified their FEL’s output by a factor of 1000, achieving this success in over 90% of their attempts. This performance surpassed the results of the Chinese Academy of Sciences, although the wavelength operated at was around 420 nm, considered less optimal for scientific applications. Barber indicated that minor upgrades could soon facilitate operation at sub-100 nm wavelengths, where the scientific potential becomes much more exciting.

The researchers express optimism about future breakthroughs that could further improve LPA-driven FEL experiments. A key target is reaching the “saturation level” for X-ray wavelengths, which marks the threshold beyond which amplification of the FEL no longer significantly increases.

Barber highlighted the importance of advancing laser technology to elevate the current systems to much higher repetition rates. He stated, “Right now, the typical laser used for LPAs can operate at around 10 Hz, but that will need to scale up dramatically to compare to the performance of existing light sources that are pushing megahertz.”

This research is detailed in the journal Physical Review Letters, marking a significant stride in the development of compact X-ray laser technology. The implications of this work could pave the way for more accessible and versatile applications in various scientific fields.