CompactLight completes the Conceptual Design Report for an advanced and compact hard X-ray facility

The EU-funded project held the promise of designing more compact and cost-effective linac-based photon sources.


The planned facility allows the production of X-rays up to 16 keV within about 400 m of length, including the experimental hall: half the length of equivalent facilities.

Synchrotron Radiation (SR) is a fundamental and indispensable research tool in a wide spectrum of scientific and technological fields. The latest generation of SR sources is based on Free Electron Lasers (FELs) driven by linacs. These facilities, with sub-picosecond pulse-lengths and wavelengths down to the hard X-ray range, feature unprecedented performance in terms of peak brilliance, exceeding by many orders of magnitude that of third generation synchrotrons.

Despite the great scientific and technological benefits that X-ray FELs can provide, only very few such facilities are currently in operation worldwide, due to the high costs and complexity preventing their wide diffusion.

The CompactLight collaboration during the second annual review meeting in Helsinki.

For four years, the CompactLight collaboration (XLS) aimed to facilitate the widespread development of X-ray FEL facilities across Europe and beyond. This design study funded by the European Union under the Horizon 2020 programme gathered partnership of 23 international laboratories and academic institutions, 3 private companies and 5 third parties, bringing together the world’s leading experts in this field. Its strategy? Make X-ray FEL facilities more affordable, more compact, more power efficient, and more performing, by using an optimum combination of emerging and innovative accelerator technologies.

The FEL specifications targeted by the XLS design were determined by inquiring the potential user community directly. In the early stages of the project, a dedicated workshop was held at CERN to survey the characteristics of the X-rays needed by the users, for their current as well as for their future experiments.

To fulfil the user requests, while preserving the facility compact and affordable, the CompactLight collaboration has based its design on the latest concepts for bright electron photoinjectors, high-gradient X-band structures at 12 GHz, and innovative super-conductive short-period undulators.

After four years of work, at the end of 2021, the main deliverable of the CompactLight collaboration was completed: the CompactLight Conceptual Design Report (CDR), a 360 pages long document describing in detail the proposed facility.

“Compared to existing facilities, for the same operating wavelengths, the technical solutions adopted ensure that the CompactLight facility can operate with a lower electron beam energy and will have a significantly more compact footprint” explains Gerardo D’Auria, CompactLight Project Coordinator. “All these enhancements make the proposed facility more attractive and more affordable to build and operate”.

CompactLight has been designed as a hard X-ray facility, covering the wavelength range from 0.8 Å up to 5nm (16 keV to 0.25 keV) with two separate FEL beamlines:

  1. a soft X-ray (SXR) FEL able to deliver photons from 5.0 nm to 0.6 nm (0.25 keV to 2 keV) operating up to 1 kHz repetition rate (high rep rate)
  2. a hard X-ray FEL source (HXR) ranging from 6.0 Å to 0.8 Å (2 keV to 16 keV) with maximum 100 Hz repetition rate (low rep rate).

Key elements proposed in the design are the dual-bunch photoinjector and the two-beam deflectors adopted for the linac. “Both elements give a huge flexibility for the facility operation, with different combinations of SXR and HXR operating modes, at high and low repetition rates, as requested by the users” highlights Gerardo.

The design presented in the CDR includes a facility baseline layout and two main upgrades, with the most advanced option allows the simultaneous operation of both FEL beamlines, in SXR/HXR pump-probe configuration, at 100 Hz repetition rate.

The XLS CDR also includes preliminary evaluations of a soft X-ray FEL, and an extremely compact and relatively inexpensive photon source based on Inverse Compton Scattering (ICS), both using the CompactLight technology. Compared with the full CompactLight facility, this soft X-ray FEL can be considered an affordable solution in terms of cost and complexity in case of limited investment capabilities. In addition, the ICS source, with its wide range of applications and reduced cost, can be easily installed and operated in university campuses, small laboratories, and hospitals.

The XLS Collaboration is determined to continue its activities well beyond the end of its H2020 contract, improving the partnership and maintaining its leadership in compact acceleration and light production, strong of the experience gained in the four years of the project. Periodic workshops will be organized to promote exchanges among the members of the XLS Collaboration and the Scientific Community to foster further developments in the field of very compact photon sources. Afterall, the XLS technology is already being adopted even beyond Europe: SXFEL and DCLS in China, for example, have recently completed the manufacturing of components for a C-band photoinjector as well as an X-band linearizer inspired by XLS.