Flattening the wave

New result at FLASHForward marks major step forward for plasma acceleration

A 50-mm-long plasma accelerator module in operation. The blue light is produced during the recombination of the argon plasma after a high-voltage discharge. The magnification shows a simulated plasma wakefield corresponding to that of the experiment. It is generated by a leading electron bunch moving to the right; a trailing electron bunch is accelerated quickly and efficiently in it (photomontage: C. Lindstrøm/DESY).

View along the FLASHForward beamline (right in the picture). It is located next to the second undulator line of the free-electron laser FLASH (photo: D. Nölle/DESY).

The technology of plasma-based acceleration promises to deliver a new generation of powerful and compact particle accelerators. An international team of researchers has now made a major leap forward: at the FLASHForward beam line at DESY’s FLASH facility, the team created precisely tailored particle bunches to drive a plasma accelerator. For the first time, the researchers were able to preserve a sharp energy spectrum within the accelerated particle bunch while simultaneously accelerating particles with record high energy efficiency: both prerequisites for a sustainable application in compact next-generation colliders and brilliant photon sources.

Plasma-based accelerators promise to drastically reduce the size of large-scale particle accelerator facilities such as linear colliders and free-electron lasers. A high-energy laser or particle beam shot through a plasma can cause a strong electromagnetic wakefield which can be used to accelerate charged particles. At FLASHForward, the wakefield in the plasma is formed by an electron bunch fired into the plasma close to the speed of light. The electrons of this driving beam force the freely-moving plasma electrons to oscillate, which results in strong electric fields. These fields accelerate electrons in the beam travelling right behind. The acceleration produced by the plasma wake can be up to a thousand times higher than that of conventional systems, promising a new generation of more powerful, compact and versatile accelerators. The accelerated electrons or X-rays produced by them can be used for scientific research as well as in industrial or medical applications.

However, today´s experiments at accelerators and colliders are demanding high energy efficiency and have strict requirements for beam quality, such as the spread of energies within the beam. This is particularly challenging in plasma accelerators since their high-frequency wakefields typically vary significantly over the length of even short particle bunches, resulting in a spread of the electric fields that accelerate the electrons, again leading to a spread of energies within the bunch. The theoretical solution to this problem was already identified in the 1980s, shortly after the discovery of plasma wakefields: precise tailoring of the time structure of the accelerating bunches. If done correctly, an accelerating bunch can drive its own wakefield to destructively interfere with the wakefield from the driving beam, in such a way that all particles are accelerated uniformly. Researchers at FLASHForward have now achieved this goal for the first time.

For their experiments, the researchers made use of high-quality and ultra-stable electron bunches from the superconducting accelerator of the free-electron laser facility FLASH, which supplies the FLASHForward beam line with electrons. Using a magnetic chicane, the team divided a FLASH bunch in two – one that creates the plasma wakefield and one that is accelerated. Utilising a new technique of measuring the wave in the plasma very precisely, which was recently developed by the FLASHForward team, the team was able to shape the electron bunches that travel through the plasma so precisely that the gradient of its wakefield was flattened over the area where the accelerating beam was accelerated. “Imagine the electron bunches as snow ploughs pulling through the plasma, ripping apart the electrons and ions in it and thus forming the accelerating electric field. We are now able to adjust very accurately how these snow ploughs look like, so that the plasma is ideally prepared to uniformly and efficiently accelerate electrons,” says Carl Lindstrøm from DESY’s FLASHForward team, first author of the publication now published in Physical Review Letters.

In their experiments the team accelerated the 1-gigaelectronvolt (GeV) electrons of FLASH by 45 megaelectronvolts (MeV), while preserving the energy spread within the bunch in the per-mille regime. “I am very proud of this result, which is a major step forward in plasma accelerator technology and one of the main goals FLASHForward was originally designed for,” says Jens Osterhoff, head of the plasma accelerator group at DESY and co-author of the paper.

The precisely tailored accelerating bunch not only led to a “flattened” wakefield — the method also results in a high energy efficiency. “In these initial experiments, the efficiency of energy transfer from wakefield to particles was more than 40 %, a factor of almost 2 better than before,and we aim to improve this further,” underlines Osterhoff.

“This result enabled by the high quality performance of the electron beams produced by the FLASH facility at DESY demonstrates that plasma accelerators can be designed to preserve energy spread and operate with high efficiency, which is yet another important step in the realization of plasma accelerators for practical applications,” says Wim Leemans, DESY´s Director of the Accelerator Division.



Energy-Spread Preservation and High Efficiency in a Plasma-Wakefield Accelerator; C. A. Lindstrøm, J. M. Garland, S. Schröder, L. Boulton, G. Boyle, J. Chappell, R. D’Arcy, P. Gonzalez, A. Knetsch, V. Libov, G. Loisch, A. Martinez de la Ossa, P. Niknejadi, K. Põder, L. Schaper, B. Schmidt, B. Sheeran, S. Wesch, J. Wood, and J. Osterhoff, Phys. Rev. Lett. 126, 014801