More compact and cost-effective accelerators? These two important features are fundamental to make accelerators more affordable drivers for research, industry, and medicine. Stemming from the CLIC R&D for high-gradient, normal-conductive RF acceleration in the X band of frequencies, the CompactLight (XLS) collaboration brings together 26 of the world's leading research centres in the field. It proposes a flexible design for two facilities as the primary result of the study: a soft X-ray FEL and a hard X-ray FEL. Both designs are based on the latest technologies to make XFEL facilities more compact, energy-efficient, and affordable. Their goal is to make expensive facilities such as XFELs affordable at the regional level.

After four years of work, the EU-funded CompactLight project is coming to an end and brings forward one instrument that could change the game in the field of high-brilliance X-ray production: a new generation of compact X-ray free-electron lasers (XFELs). The project's main deliverable, conceptual design for a compact and cost-effective free electron laser, will be ready by the end of 2021.

The CompactLight's key enabling technologies are: (1) its 6 GHz C-band photoinjector, capable of operating at a 1 kHz repetition rate with sub-micron emittance; (2) a very high gradient X-band linac at 12 GHz accelerating the electron bunches up to 5.5 GeV energy; (3) a Ka-band (36 GHz) lineariser; and (4) an ultra-short superconducting undulator. These ingredients make the CompactLight XFEL at the forefront of accelerator and undulator technology, capable of producing X-rays up to 16 keV within about 400 m of length, including the experimental hall (see Figure). The CompactLight XFEL facility will be less than half the length of equivalent facilities based on S- or C-band RF frequencies. In addition, unique features such as two-colour operation and simultaneous production of soft and hard X-rays are some of the relevant potentialities offered by the XLS design. Detailed cost estimates, civil engineering studies, and realistic performance studies of the proposed facility will be part of the CDR.

The CompactLight collaboration did not limit itself to the full-fledged XFEL. The collaboration studied even more compact light sources based on Inverse Compton Scattering (ICS). Designed as a toolbox, the XLS injector gun and a few X-band linac modules can be combined to drive an ICS as an X-ray source. These are extremely compact facilities that use electron beam energies of a few hundred MeV to produce very high energy photons (in the MeV energy range), with application possibilities in medicine, material science and nuclear photonics. Preliminary beam dynamics simulations show that commercially available 1 kW lasers and a slightly modified beam structure (50 bunches per pulse, instead of 2, at the cost of a moderately higher normalised emittance), fluxes of the order of 2 x 1012 ph/s with a brightness of 3 x 1013 ph / (s mm2mrad2 0.1% BW) can be achieved. These are unprecedented figures for a normal-conductive, linac-based ICS.

On 8 and 9 November 2021, the CompactLight organised an online workshop dedicated to ultra-compact implementations of XLS technology. With 114 registered participants from all over the world, the two-day workshop offered a comprehensive overview of applications and ongoing projects related to this dynamic research area. The workshop, structured in four sessions, began by reviewing medical and industrial needs from X-ray sources. For instance, room-sized X-ray imaging facilities based on K-edge subtraction and phase-contrast techniques could revolutionise the field of cancer detection and diagnosis even in hospitals. The workshop continued with a review of the latest advancements in laser technology. Several current design studies for ICS were presented on the second day and reports on existing facilities, such as the CLS in Munich. The last session of the workshop was dedicated to complementary projects where very high energy electrons could be directly used for oncological treatment via the so-called FLASH therapy.

The XLS collaboration is active and strong and is clearly determined to continue its activities well beyond the official end of its H2020 contract in 2022.

The CompactLight project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 777431. It brought together a consortium of 21 leading European institutions, including Elettra, CERN, PSI, KIT and INFN, in addition to seven universities and three industry partners (Kyma, VDL ETG, and Bilfinger Noell).