A free-electron laser (FEL) that is driven by a plasma-based electron accelerator has been unveiled by Sam Barber at Lawrence Berkeley National Laboratory and colleagues. The device is a promising step towards compact, affordable free-electron lasers that are capable of producing intense, ultra-short X-ray laser pulses. It was developed in collaboration with researchers at Berkeley Lab, University of California Berkeley, University of Hamburg and Tau Systems.
A FEL creates X-rays by the rapid back-and-forth acceleration of fast-moving electron pulses using a series of magnets called an undulator. These X-rays are emitted at a narrow wavelength and then interact with the pulse as it travels down the undulator. The result is a bright X-ray pulse with laser-like coherence.
What is more, wavelength of the emitted X-rays can be adjusted simply by changing the energy of the electron pulses, making FELs highly tuneable.
Big and expensive
FELs are especially useful for generating intense, ultra-short X-ray pulses, which cannot be produced using conventional laser systems. So far, several X-ray FELs have been built for this purpose – but each of them relies on kilometre-scale electron accelerators costing huge amounts of money to build and maintain.
To create cheaper and more accessible FELs, researchers are exploring the use of laser-plasma accelerators (LPAs) – which can accelerate electron pulses to high energies over distances of just a few centimetres.
Yet as Barber explains, “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. But sustained research across the international landscape continues to drive improvements in all aspects of LPA performance.”
Recently, important progress was made by a group at the Chinese Academy of Sciences (CAS), who used an LPA to create FEL pulses by a factor of 50. Their pulses have a wavelength of 27 nm – which is close to the X-ray regime – but only about 10% of pulses succeeded.
Very stable laser
Now, the team has built on this by making several improvements to the FEL setup, with the aim to enhance its compatibility with LPAs. “On our end, we have taken great pains to ensure a very stable laser with several active feedback systems,” Barber explains. “Our strategy has essentially been to follow the playbook established by the original FEL research: start at longer wavelengths where it is easier to optimize and learn about the process and then scale the system to the shorter wavelengths.”
With these refinements, the team amplified their FEL’s output by a factor of 1000, achieving this in over 90% of their shots. This vastly outperformed the CAS result – albeit at a longer wavelength. “We designed the experiment to operate the FEL at around 420 nm, which is not a particularly exciting wavelength for scientific use cases – it’s just blue light,” Barber says. “But, with very minor upgrades, we plan to scale it for sub-100 nm wavelength where scientific applications become interesting.”
The researchers are optimistic that further breakthroughs are within reach, which could improve the prospects for LPA-driven FEL experiments. One especially important target is reaching the “saturation level” at X-ray wavelengths: the point beyond which FEL amplification no longer increases significantly.
“Another really crucial component is developing laser technology to scale the current laser systems to much higher repetition rates,” Barber says. “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.”
The research is described in Physical Review Letters.
The post New laser-plasma accelerator could soon deliver X-ray pulses appeared first on Physics World.
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