Making Small Free-Electron Lasers More Reliable
For two decades, researchers have sought to make free-electron lasers (FELs) cheaper and smaller by replacing the conventional radio-frequency accelerators with more efficient and compact laser-plasma accelerators (LPAs). Sam Barber at Lawrence Berkeley National Laboratory (LBNL) in California and colleagues have made progress toward that goal by demonstrating an LPA-driven FEL with record performance and reliability [1]. This advance could help make FELs, which are invaluable tools for materials research, more widely available.
The active medium in an FEL is a bunched beam of relativistic electrons that is buffeted by a sequence of static magnetic fields in a series of “undulators.” Ideally, the resultant jiggling triggers an electron–radiation interaction that feeds on itself, producing a laser’s characteristic coherent, self-amplifying emission. Whereas an FEL’s line of undulators can be as long as 100 m, the radio-frequency accelerators currently used to boost electrons to the necessary energies can be over a kilometer in length. LPAs can accelerate electrons to the same energies over lengths of a few centimeters. But electron beams generated by LPAs are generally less consistent from bunch to bunch and have wider energy spreads, impairing their ability to achieve self-amplification.
Barber and colleagues have been steadily improving the performance of LPA-driven FELs for years. Their latest experiment incorporated several active feedback systems to help stabilize the LPA’s internal laser. As a result, they reached the self-amplifying state 90% of the time while firing their device once per second for an hour. They also achieved a gain of 1000, which represents the ratio of the coherent output energy to the initial synchrotron emission energy. Other groups have previously obtained values of 10% for the success rate and 50 for the gain. Barber points out, however, that the LBNL test operated at about 420 nm. Minor upgrades will reduce the wavelength to less than 100 nm. “That’s where scientific applications become interesting,” he says.
–Marric Stephens
Marric Stephens is a Corresponding Editor for Physics Magazine based in Bristol, UK
References
- S. K. Barber et al., “Greater than 1000-fold gain in a free-electron laser driven by a laser-plasma accelerator with high reliability,” Phys. Rev. Lett. 135, 055001 (2025).