Study Demonstrates the Potential of Plasma as Energy Booster for High-Gradient Accelerators

An international team of scientists guided by Deutsches Elektronen-Synchrotron (DESY) researchers has shown, for the first time at the FLASHForward experiment that in principle, it is possible to work plasma accelerators at the repetition rates preferred by particle physicists and photon experts.

This paves the way for utilizing such high-gradient accelerators as booster stages in current high-repetition-rate facilities, such as the European XFEL and the large-scale X-Ray free-electron lasers FLASH, so as to considerably boost the energy of long sequences of particles in short distances. The results of their studies have been presented in the journal Nature.

Plasma acceleration is a groundbreaking technology for application in the next generation of particle accelerators because of its versatility and compactness, with the aim being to exploit the accelerated electrons for numerous fields of application in industry, science and medicine. The acceleration occurs in a very thin channel — usually just a few centimeters in length — which is full of an ionized gas, the plasma.

A high-energy laser or particle beam fired via the plasma can trigger a powerful electromagnetic field — a kind of “wake” — which can be used to speed up charged particles. In this manner, plasma accelerators can realize acceleration gradients up to a thousand times higher than the strongest accelerators in use today. They could thus significantly decrease the size of kilometer-scale facilities such as free-electron lasers or particle colliders.

Advanced accelerators for pioneering science must also match high requirements with regard to beam quality, efficiency and volume of bunches accelerated per second. In order to produce a specifically large number of particle collisions or light flashes in the least amount of time, thousands or even millions of tightly packed particle bunches must be thrust via the accelerators in a single second.

Plasma accelerators would, thus, have to realize a similar repetition rate so as to be competitive with advanced particle-accelerator technology. Existing test facilities for plasma acceleration are commonly operated at much slower repetition rates in the range of one to ten accelerations per second.

The team directed by DESY scientist Jens Osterhoff has currently established that much higher rates are possible.

At FLASHForward we were able to show for the first time that, in principle, repetition rates in the megahertz range are supported by the plasma acceleration processes.

Jens Osterhoff, Study Lead and Researcher, DESY

At FLASHForward, the accelerating wave — the supposed wakefield in the plasma — is produced by an electron bunch from the FLASH accelerator that thrusts via the plasma at nearly the speed of light. The electrons of this “drive beam” cause the freely flowing electrons of the plasma to oscillate in its wake and thus produce highly robust electric fields.

These fields accelerate the electrons of a particle packet flying straight behind the driver bunch.

Unlike in conventional accelerators, where long-living electromagnetic waves stored in a resonating cavity can accelerate several particle bunches in quick succession, the electromagnetic fields generated in plasma decay very quickly after each acceleration process.

Richard D'Arcy, Scientific Coordinator of FLASHForward and Study First Author, DESY

“To start a new similar acceleration process, the plasma electrons and ions must then have ‘recovered’ to approximately their initial state such that the acceleration of the next pair of particle bunches is not modified by that of the previous one,” Richard D'Arcy added.

During their experiments, the researchers made use of the very flexible superconducting FLASH accelerator to produce particle bunches with very short temporal spacings. The first bunch produced thrust through the plasma, steering a high-strength wakefield and thus unsettling the plasma in its wake.

At variable intervals afterward, pairs of particle bunches were thrust through the plasma cell; the first driving a second wakefield and the second being fast-tracked by the resulting fields. The properties of these following bunches were precisely assessed by the experimenters and compared with those of bunches that had undergone this process in an unperturbed plasma.

The result: after approximately 70 billionths of a second (70 nanoseconds), it was not possible to differentiate whether the second acceleration had occurred in a previously perturbed or unperturbed plasma.

We were able to precisely observe the decay of the perturbation, which reached completion within the first 70 nanoseconds, and to explain it exactly in simulations. In subsequent measurements, we want to check how different framework conditions in the setup influence the recovery time of the plasma wave.

Richard D'Arcy, Scientific Coordinator of FLASHForward and Study First Author, DESY

For instance, the heating of the plasma medium because of high-frequency operation may have an impact on how fast the plasma will take to reload.

The present findings, which involved researchers from DESY, University College London, and the Universities of Oxford and Hamburg, pave the way for preparing present-day particle accelerators, which are operated at repetition rates in the kilohertz-to-megahertz zone; with plasma accelerator modules serving as booster stages to considerably increase the particle energy over the least distance.

The findings have a profound impact on the potential for implementation of the plasma technology towards future high repetition rate facilities for which DESY is world renowned.

Wim Leemans, Director of the Accelerator Division, DESY

Journal Reference:

D'Arcy, R., et al. (2022) Recovery time of a plasma-wakefield accelerator. Nature. doi.org/10.1038/s41586-021-04348-8.