A research team at the DESY’s FLASHForward plasma accelerator experiment has for the first time been able to preserve an important parameter of the particle beam during the plasma acceleration process. They kept the emittance of the high-quality bunches from the FLASH linear accelerator at the same level during the acceleration process. The team has published their experiments recently in the scientific journal Nature Communications.

Plasma wakefield accelerators boost the energy of electron bunches in gigavolt-per-metre electric fields, which are up to three orders of magnitude stronger than the fields found in commonly-used radiofrequency accelerators. The first particle bunch injected into a plasma, called a “driver bunch”, generates a density wave (a wakefield) in the plasma, much like a boat creating waves in water. A subsequent “accelerating bunch” can then gain energy from the strong electric fields of this wave. An ideal accelerator would add energy to each electron in the accelerating bunch, while maintaining all of its other properties, such as its charge and energy spread. While researchers have previously shown that a plasma accelerator could preserve these quantities, their results until now were accompanied by a degradation of the transverse properties of the bunch, something that generally does not happen high-quality radiofrequency accelerators such as FLASH.

The transverse quality of a particle bunch is assessed via a quantity called its emittance. The difference between high- and low-emittance bunch is analogous to the difference between a light bulb and a laser beam.  Light from a light bulb rapidly expands in many directions and cannot be focussed to a small spot, while laser light stays collimated over long distances and can easily be focussed to the micrometre scale. Similar to a laser beam, low emittance particle bunches can be focussed to very small sizes, which is beneficial for particle colliders, or they can remain close to the same size over long distances, which is essential for utilizing them to produce bright radiation in free-electron lasers.

It is therefore critical that a plasma accelerator is able preserve the small emittance of the bunch it accelerates. While this was thought to be possible from a theoretical standpoint, until now this had not been demonstrated in an experiment. A research team lead by Carl Lindstrøm at the FLASHForward plasma accelerator experiment at DESY was able to demonstrate emittance preservation during the plasma acceleration process using the high-quality bunches from the FLASH linear accelerator. This first required precise control over the electron bunch and plasma properties to first preserve the charge and energy spread of the bunch. The researchers then varied the size of the accelerating bunch in the plasma, as well as its alignment with respect to the driver bunch, until the bunch experienced no detrimental effects on its emittance. This methodology and the measured tolerances will inform future experiments and facilities as progress is made towards realising high-energy-gain, quality preserving plasma accelerators. With the same electron bunch properties as in the experiment, a 500 mm long plasma could boost the electron energy from 1.05 to 1.84 GeV while still preserving the emittance, according to simulations.

“With the crucial parameter of emittance being preserved, plasma accelerators could for instance be used as “energy boosters” in free-electron laser facilities increasing the particle energy in a very compact stage behind the main accelerator and thus increasing the attainable X-ray energy”, says Jonathan Wood, head of the FLASHForward research team.

“This result is a critical step forward towards building a plasma accelerator that has results similar to a conventional one,” says DESY Accelerator Division Director Wim Leemans. Our work continues to advance this exciting area so that we can open up accelerator technologies to a much wider arena of applications.”

 

Reference

"Emittance preservation in a plasma-wakefield accelerator", C. A. Lindstrøm, J. Beinortaitė, J. Björklund Svensson, L. Boulton, J. Chappell, S. Diederichs, B. Foster, J. M. Garland, P. González Caminal, G. Loisch, F. Peña, S. Schröder, M. Thévenet, S. Wesch, M. Wing, J. C. Wood, R. D’Arcy & J. Osterhoff, Nature Communications 15, 6097 (2024). https://doi.org/10.1038/s41467-024-50320-1