After the successful completion of its programme in December 2016, the probe-beam line of the CLIC Test Facility (CTF3) was converted into the CERN Linear Electron Accelerator for Research (CLEAR). The necessary hardware modifications were carried at the beginning of 2017 and the first beam was delivered in August 2017.

The 20-metre-long linear accelerator (or linac) of CLEAR produces bunched electron beams from a photocathode coated with cesium telluride (Cs₂Te) and accelerates them up to an energy of about 220 MeV. The linac is followed by a 20-metre-long experimental beamline that can host several experiments. The facility is conveniently located on the surface at CERN’s Meyrin site next to the ring of the Proton Synchrotron (PS). Easy access to the accelerator hall and the possibility to quickly adapt the beamline to an experiment’s needs make CLEAR a unique facility for fast and efficient beam-based research.

The CLEAR programme covers two key priorities identified by the 2020 Update of the European Strategy for Particle Physics: prototyping and validation of accelerator components, and studies of high-gradient acceleration methods. The latter include studies of acceleration using X-band RF components (typically around 12 GHz) and novel concepts such as the use of plasma or terahertz-wavelength radiation for charged-particle acceleration.

Two of the main experiments installed on the CLEAR experimental line are the plasma lens and the CLIC test stand. The aim of the plasma-lens experiment is to develop a compact and strong focusing component that will be a key ingredient for possible future accelerators based on plasma acceleration. On the CLIC test stand, high gradient (>100 MV/m) accelerating structures and compact, high-precision beam position monitors for the CLIC design are being tested, pushing the limits of the traditional accelerating structures and beam instrumentation.

Researchers are also pursuing two main branches of irradiation tests: : radiation hardness of electronic components (initiated through a collaboration with the European Space Agency) for space and high-energy physics, and dosimetry for medical applications, in particular cancer therapy.

CLEAR is supported by a broad community of internal and external users and experts. It covers several activities, including irradiation tests, wakefield and impedance studies, plasma-lens experiments, beam-diagnostics development and terahertz-emission studies.