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Plasma accelerators recover in a flash


image: The two FLASHForward plasma cells observed through a vacuum window. The cells are filled with argon gas and then ionized with a high voltage electrical discharge to form a plasma. When the plasma recombines, it emits light in the blue wavelength range. The two plasma cells, 50 and 195 millimeters long, can then be used for plasma acceleration of electron bunches in acceleration gradients of gigavolts per meter.
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Credit: DESY, CA Lindstrøm

An international team of researchers led by DESY scientists demonstrated for the first time in the FLASHForward experiment that it is in principle possible to operate plasma accelerators at the repetition rates desired by particle physicists and scientists. photons. This opens up the possibility of using such high-gradient accelerators as booster stages in existing high-repetition-rate facilities, such as the European FLASH and XFEL large-scale X-ray lasers, to significantly increase the energy of long trains of particles over short distances. The team presents the results of its studies in the journal Nature today.

Plasma acceleration is an innovative technology to be applied to the next generation of particle accelerators due to both its compactness and its versatility, the aim being to use the accelerated electrons for various fields of application in scientific, industrial and medical fields. The acceleration takes place in an extremely fine channel – typically only a few centimeters – which is filled with an ionized gas, the plasma. A high-energy laser or particle beam shot through the plasma can excite a strong electromagnetic field, a kind of “wake”, which can be used to accelerate charged particles. In this way, plasma accelerators can achieve acceleration gradients up to a thousand times higher than the most powerful accelerators in use today. They could thus drastically reduce the size of kilometric installations such as particle colliders or free electron lasers.

Modern accelerators for cutting-edge science must also meet high demands in terms of efficiency, beam quality and the number of accelerated bunches per second. In order to generate a particularly high number of light flashes or particle collisions in the shortest possible time, thousands or even millions of dense particle bunches must be propelled through accelerators in a single second. Plasma accelerators should therefore achieve a similar repetition rate to be competitive with state-of-the-art particle accelerator technology. Current test facilities for plasma acceleration typically operate at much slower repetition rates in the range of one to ten accelerations per second. The team led by DESY researcher Jens Osterhoff has now proven 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 plasma acceleration processes,” says Osterhoff.

At FLASHForward, the accelerating wave – the so-called wake field in the plasma – is generated by a bundle of electrons from the FLASH accelerator that travels through the plasma at almost the speed of light. The electrons in this “drive beam” cause the free electrons in the plasma to oscillate in its wake and thus generate very powerful electric fields. These fields accelerate the electrons of a bunch of particles flying directly behind the conducting group. “Unlike conventional accelerators, where long-lived electromagnetic waves stored in a resonant cavity can accelerate multiple packets of particles in rapid succession, the electromagnetic fields generated in the plasma decay very rapidly after each acceleration process,” explains Richard D’Arcy, first author of the study. “To start a new, similar acceleration process, the electrons and ions in the plasma must then have ‘recovered’ to approximately their original state so that the acceleration of the next pair of particle packets is not changed by that of the previous one.” In their experiments, the scientists took advantage of the highly flexible FLASH superconducting accelerator to generate bunches of particles with extremely short temporal spacings. The first bunch generated passed through the plasma, driving a high-intensity wakefield and thus disrupting the plasma in its wake. At varying intervals thereafter, pairs of particle packets were sent through the plasma cell; the first driving a second wake field and the second being accelerated by the resulting fields. The properties of these subsequent packets were precisely measured by the experimenters and compared to those of packets that had undergone this process in undisturbed plasma. Result: after about 70 billionths of a second (70 nanoseconds), it was no longer possible to distinguish whether the second acceleration had taken place in a previously perturbed or unperturbed plasma. “We were able to observe precisely the decay of the disturbance, which was completed within the first 70 nanoseconds, and explain it exactly in simulations,” says D’Arcy. “In subsequent measurements, we want to check how different framework conditions in the setup influence the recovery time of the plasma wave.” For example, heating of the plasma medium due to high frequency operation can influence how quickly the plasma takes to recover.

The current findings, involving scientists from DESY, University College London and the universities of Oxford and Hamburg, open the door to equipping today’s particle accelerators, which operate at repetition rate in the kilohertz-megahertz regime, with plasma from the accelerator modules acting as booster stages to significantly increase the energy of the particles over the shortest distance. “The findings have a profound impact on the potential for implementing plasma technology towards the future high repetition rate facilities for which DESY is world famous,” concludes Wim Leemans, Director of the Accelerator Division at DESY.


Wake plasma accelerator recovery time; R. D’Arcy, J. Chappell, J. Beinortaite, S. Diederichs, G. Boyle, B. Foster, MJ Garland, P. Gonzalez Caminal, CA Lindstrøm, G. Loisch, S. Schreiber, S. Schröder, RJ Shalloo, M. Thévenet, S. Westch, M. Wing and J. Osterhoff; „Nature», 2022; DOI: 10.1038/s41586-021-04348-8

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