How to make great even better – European XFEL receives new electron source “GUN5”

The European XFEL, the world’s largest X-ray laser, is taking another leap forward. On 17 September, a brand-new electron source, known as “GUN5.2”, was delivered to Hamburg after years of development and rigorous testing at DESY’s Photo Injector Test Facility (PITZ) in Zeuthen. During the current extended maintenance period, the source is being installed in the accelerator’s injector – a critical upgrade that will directly enhance the laser’s experimental capabilities.

For a free-electron laser to work, one factor is key: the density of electrons in each accelerated bunch. The denser the bunch, the more efficiently it can interact with the self-generated X-ray light in the undulator, creating the ultrashort, brilliant flashes of light that make the European XFEL unique.

Remarkably, the crucial parameters are set within the very first 30 centimetres of acceleration – a tiny section that ultimately determines the success of experiments taking place over three kilometres away at the facility in Schenefeld.

Developing reliable and powerful electron sources – called “guns” – has therefore been essential to building and operating free-electron lasers. At European XFEL and its sister facility FLASH, both based on superconducting accelerator technology, these sources have been a cornerstone since the 1990s.

Inside an XFEL gun, an intense laser beam frees electrons from a specially coated metal surface, the cathode, via the photoelectric effect. These electrons are then rapidly accelerated by strong radiofrequency fields in a copper cavity. The process has to happen in fractions of a second: if electrons spread out too much, the bunch density is lost. The rapid acceleration process benefits from a relativistic effect that limits the repulsion between the electrons. This allows the researchers to keep the electron bunches very compact and therefore the charge density very high.

The development of the sources began in the 1990s, together with research into superconducting accelerator technology, when DESY decided to build a free-electron X-ray laser. The new generation, GUN5, builds on these decades of expertise at PITZ in Zeuthen and DESY in Hamburg. While the fourth generation (GUN4) has been in use since the start of European XFEL operations in 2017, plans for improvements were already under discussion during commissioning. “Out of these discussions came the fifth generation, with a refined shape, integrated field probes, enhanced cooling, improved mechanics for swapping cathodes, and a double input window. These advancements allow the gun to be more stable and reliable in the future,” says Frank Stephan, leader of PITZ.

The most visible change for the experiments is the 30% extension of the high-frequency pulse and thus the maximum length of the electron train. This increases the maximum number of light flashes from 27 000 to 36 000 per second. This means that many experiments can obtain more data per measurement time, or more different experiments can be carried out.

Before the new source can prove its worth in Hamburg, a lot of installation work still needs to be done in the seventh basement of the European XFEL injector complex. “We hope to be able to gather initial operating experience from November onwards,” explains DESY researcher Frank Brinker, who is coordinating the installation and operation of the gun in Hamburg.

“With the new electron gun that our teams have built for the European XFEL, we will further improve the operational reliability and performance of the accelerator, which is already functioning extremely well. I am very proud of what our teams have achieved,” says Wim Leemans, Director of the DESY Accelerator Division.

“The upgrade underlines the importance of the European XFEL as a global research facility. By providing brighter, faster, and more stable X-ray flashes, it enables scientists from around the world to investigate matter at the atomic level – from the dynamics of chemical reactions and the behaviour of quantum materials to the structures of viruses and biomolecules”, says Thomas Feurer, Chairman of the Management Board of European XFEL.