The technology of plasma-based acceleration promises to deliver a new generation of powerful and compact particle accelerators. Prior to applying this new technology, however, various obstacles must be overcome. In particular precise control of the acceleration process itself must be achieved. Using an innovative technique researchers at DESY have now succeeded in measuring the accelerating plasma wake with previously unattained precision. Their method allows the shape of the effective accelerating field to be determined with a resolution on the order of femtoseconds (billionths of a millionth of a second) so that the acceleration process can be studied in great detail, thereby paving the way for the controlled and optimised operation of future plasma accelerators, as the team led by DESY’s Jens Osterhoff explains in the journal Nature Communications.

A plasma is a gas but with its molecules stripped of their electrons. A high-energy laser or particle beam can force these freely-moving plasma electrons to oscillate, which results in strong electric fields. These can then be used to accelerate charged particles. To achieve this DESY’s FLASHForward facility fires bunches of electrons into a plasma at close to the speed of light. “A wake of plasma electrons forms behind the electron bunch and another electron bunch can surf along on this wake, whereby it is accelerated in the process: like a wakeboarder riding the wake of a boat,” explains Osterhoff. “This is why the technique is also known as plasma wakefield acceleration.”

The acceleration produced by the plasma wake can be up to a thousand times greater than that of the strongest conventional systems currently in operation. “To achieve optimal acceleration the electron bunches and the wake need to be precisely tuned to each other,” explains Sarah Schröder, the principal author of the paper, who works at DESY and the University of Hamburg. “To do that you have to be able to measure the shape of the wake precisely and this is very challenging due to its small dimensions, being just a few thousandths of a millimetre long.”

So the team developed a method whereby the accelerated electrons themselves are used to reveal the shape of the plasma wake’s accelerating field. To achieve this, the electron bunch is first rotated by a magnetic chicane. Thin slices can then be removed from the bunch tail by transversely inserting a piece of metal. Finally the electron bunch is rotated back again.

The resulting energy spectrum of the outgoing electron bunch is, therefore, altered due to the missing electrons, allowing the strength of the accelerating field at the location where part of the bunch was removed to be deduced. If the bunch is sliced thinly enough, the profile of the effective accelerating field in the plasma wake can be determined with a temporal resolution of femtoseconds. In the experiment, the team was able to achieve a resolution of 15 femtoseconds – corresponding to a spatial resolution of around 5 thousandths of a millimetre in the wake. The researchers believe that even higher resolutions are possible.

“For the first time we have precisely measured the effective electric field responsible for the acceleration,” Schröder points out. “Using this technique the interaction between the individual experimental components and the process of acceleration can now be studied in detail.”

Other experimental facilities for plasma acceleration also stand to benefit from the new technology. “Our method is an important step on the path to a detailed understanding of the plasma wake and to optimising it,” explains Osterhoff.

“This experiment demonstrates the exquisite precision that can be achieved in measuring the accelerating fields and paves the way for the new era of control and stability that plasma accelerators are entering,” adds Wim Leemans, Director of DESY’s accelerator division.

 

Reference:
High-resolution sampling of beam-driven plasma wakefields; S. Schröder, C.A. Lindstrøm, S. Bohlen, G. Boyle, R. D’Arcy, S. Diederichs, M.J. Garland, P. Gonzalez, A. Knetsch, V. Libov, P. Niknejadi, K. Põder, L. Schaper, B. Schmidt, B. Sheeran, G. Tauscher, S. Wesch, J. Zemella, M. Zeng, and J. Osterhoff; Nature Communications, 2020; DOI: 10.1038/s41467-020-19811-9