DESY scientists have proposed a new concept for so-called plasma accelerators that could overcome some of the current limitations of this technology. These novel devices are currently in the spotlight due to their potential for significantly shrinking the size and cost of future accelerator facilities. However, this technology faces a number of issues that currently prevent plasma accelerators to be widely used for applications. The newly presented concept, published in the journal Physical Review Letters (PRL), addresses the energy spread, one of the most pressing challenges of this novel technology, and could enable applications such as compact free-electron lasers, measuring just dozens of meters instead of kilometres as today.

These plasma devices use high-intensity laser pulses to generate waves in a plasma channel, that electrons can ride like a surfer rides an ocean wave. The resulting accelerating fields are up to a thousand times higher than with current technology. However, a well-known problem directly linked to these high accelerating gradients is that the electrons forming the accelerated beam experience a different energy gain depending on their individual position on the wave. This leads to the development of a significant correlated energy spread along the beam which is detrimental to its quality and severely limits its applications.

“What we propose in our concept is performing the acceleration process in two plasma stages which are then joined by an array of magnets known as chicane.” says Angel Ferran Pousa, a PhD student from University Hamburg at DESY and lead author of the study. “This chicane inverts the energy correlation along the beam and therefore allows it to be naturally compensated for in the second stage. As a result, the energy spread of the beam is drastically reduced.”

Overview of a conceptual implementation of the proposed scheme. The two plasma acceleration stages (blue) are joined by a magnetic chicane. Two active plasma lenses (red) are used for beam transport and plasma mirrors are used for the laser in- and outcoupling. The trajectory of the laser pulses and the electron beam is represented in red and white, respectively. Credit: DESY/Universität Hamburg, Angel Ferran Pousa

Results from numerical simulations show that the electron beams produced with this method exhibit an energy spread which is improved by more than an order of magnitude when compared to current plasma accelerators. The study describes a first conceptual implementation of the scheme performed for the EU project EuPRAXIA in the framework of the Horizon2020 programme of the European Union. The simulated 1.5-m-long setup predicts for a beam energy of 5.5 giga electron volts (GeV) a residual relative energy spread of 0.1 per cent for the total beam and just 0.03 per cent for a short slice of the beam.

“This new concept of a low energy spread plasma accelerator and the underlying improved understanding of the relevant accelerator physics phenomena is the culmination of several years of work by a team of scientists. It could be a major step forward in building a plasma-based free-electron laser at multi-GeV beam energy but the validation of the concept requires experimental tests which could be partly carried out in the ATHENA project”, comments Ralph Assmann, EuPRAXIA coordinator and DESY leading scientist for accelerator research and development, in whose group this work was performed.

This work was supported by EU funding for the Horizon2020 Design Study EuPRAXIA and by the Helmholtz Association (via project IuVF ZT-0009).