Berkeley Lab

Beam Stabilization Key to Unlocking Potential of Next-Generation Accelerators

By creating secondary “ghost” beams to stabilize high-power lasers, ATAP researchers have laid the foundations for new applications of plasma-based accelerators.

Samuel Barber (left) inspects the 100-terawatt laser system at the BELLA Center with graduate student Fumika Isono (middle) and Jeroen van Tilborg (right). (Credit: Berkeley Lab/Marilyn Sargent)

By Carl A. Williams, April 19, 2023

Researchers from ATAP have developed a technique for stabilizing the lasers that drive plasma-based particle accelerators. The work could pave the way for the use of this technology—which offers much smaller machines—as an enabling technology for next-generation colliders and light sources, opening the door to new areas of research and applications in energy, materials science, and medicine.

Laser-plasma accelerators, or LPAs, use powerful laser bursts passing through a plasma to create a moving wave capable of accelerating electrons up to a thousand times faster than conventional accelerators. They could provide a new source of high-energy electrons for conducting fundamental scientific research, developing new materials, and enable new medical treatments.

However, many design considerations and challenges unique to the beam dynamics of LPAs “must be resolved before LPAs are capable of delivering these intense, high-quality electron beams,” says Samuel Barber, a Research Scientist at ATAP’s Berkeley Lab Laser Accelerator (BELLA) Center, who is a key member of the team that developed the technique for stabilizing lasers used to drive LPAs.

“For example, fluctuations and drifts in the laser caused by mechanical vibrations, changes in airflow, and thermal variations to optical surfaces, as well as other factors can lead to instabilities in these electron beams, which significantly limits their usefulness.”

Resolving these instabilities, he says, is critical for developing LPAs capable of producing electron beams “with the properties needed for accelerator-based applications like compact light sources, which require high-quality, high-energy beams.”

“Ghost” beams and active feedback

To improve beam stability, the researchers first trained a beam from a 100-terawatt laser at the BELLA Center onto a specially designed wedged mirror to make a spatially-separated, low-power copy of the laser’s final focus, referred to as the “ghost” beam. This ghost beam was then directed to a pair of position-sensitive detectors that monitor the beam’s position in the near and far field. The signal on these detectors, generated by a kHz-repetition-rate pilot beam that co-propagates with the high-intensity laser beam, is then used in a feedback system that employs fast-acting corrective optics to stabilize the LPA laser.

A schematic (left) showing how the detection of the ghost beam’s focus yields a measurement of the high-power laser focus without interrupting the laser-plasma accelerator. The vertical position of the electron beam (right) at the LPA source jitters and drifts in time (orange circles) but greatly improves in stability when the active stabilization system of the on-target background laser is turned on (blue circles). (Credit: Berkeley Lab/Samuel Barber)

Images of the resulting electron beam taken by a magnetic spectrometer showed a “four-fold reduction in the fluctuations of the electron beams’ transverse properties,” says Barber.

“This significant improvement in the transverse stability and dispersion of the beam is an important step in improving the stability of the beams produced by LPAs, making them potential candidates for use in future applications such as plasma-based colliders or next-generation light source applications like X-ray free-electron lasers, or XFELs.”

The work presented here appears in a recently published paper. Curtis Berger, the lead author of the paper and a graduate student researcher at the University of California, Berkeley, was mentored and supervised by Barber.

Curtis Berger (front) with his mentor Samuel Barber inside the BELLA Center control room. (Credit: Berkeley Lab/Jeroen van Tilborg)

“Our focus is to continue to improve the stability and reliability of LPA sources and to develop novel schemes for producing the highest quality beams in LPAs by improving injection mechanisms for delivering electrons to the plasma wave,” says Barber.

If successful, he says the work could provide “a blueprint for designing new LPA-based electron beam injectors that would have far-reaching impacts on the future of XFELs and other applications of high-brightness electron beams.”

Commenting on the research, Jeroen van Tilborg, Staff Scientist and Deputy Director for Experiments at the BELLA Center  said, “The ongoing research could provide the next level of beam stability and precision control needed for LPAs to be a viable enabling technology for new light source applications. Seeing Sam lead his team to take this concept to fruition is a promising sign that more innovations will follow.” 


To learn more …

Berger, C., Barber, S., Isono, F., Jensen, J., Natal. J., Gonsalves, A., and van Tilborg, J. “Active nonperturbative stabilization of the laser-plasma-accelerated electron beam source,” Rev. Accel. Beams 26 (2023),


Written by Carl A. Williams or other authors as credited.

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