CLARA is a FEL test facility currently under construction at Daresbury Laboratory. CLARA stands for Compact Linear Accelerator for Research and Applications. The maximum electron beam energy is 250 MeV and the FEL wavelength range is 100-400 nm.

The aim of CLARA is to demonstrate novel FEL capabilities and accelerator technologies which have prospects for enabling new science at X-ray FEL facilities, such as the proposed UK XFEL. These include:

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• The generation of ultra-short photon pulses. For example, we aim to generate pulse durations as short as 1 femtosecond by applying the idea of Mode-Locking to the FEL. Once demonstrated, this technique could then be applied on an X-ray FEL, such as SwissFEL, where because the wavelength is so short the pulse duration could be less than a single attosecond. This is a factor of 100 shorter than the pulses available from FELs today and would allow users to study processes that occur on a faster timescale than has ever been possible before. For more information see:

• https://doi.org/10.1103/PhysRevLett.100.203901

• https://doi.org/10.1103/PhysRevLett.110.104801

• The improvement of pulse quality. For example we plan to demonstrate a technique called High-Brightness SASE (HB-SASE) which is a new method for reducing the bandwidth of the FEL output. This increases the spectral brightness (so that all the photons are essentially at exactly the same wavelength) making the FEL a more powerful scientific tool. The way bandwidth reduction is currently achieved at X-ray FELs is using a technique called self-seeding. This uses optical elements to reduce the bandwidth of the FEL emission partway along the FEL before it is amplified up to full power. These optical elements are carefully chosen and adjusted depending on the wavelength of the FEL. In HB-SASE there are no optical elements - the bandwidth is reduced just by using magnets to slow down the electrons as they go through the FEL. Because this technique uses no optical elements it can be applied at any wavelength and there are no restrictions on how many FEL pulses per second that can be produced which may otherwise be a constraint for the current self-seeding due to heating up of the optics. For more details see:

• https://doi.org/10.1103/PhysRevLett.110.134802

• Testing of novel undulator technologies. For example, we are prototyping a short period, narrow gap superconducting undulator specifically optimised for FELs. This type of undulator has the potential to generate far higher magnetic fields than alternative technologies and so enable X-ray FELs to cover a much broader range of wavelengths. For more details see here:

• http://fel2017.jacow.de/papers/wec02.pdf

Several review articles are available which offer excellent overviews of Free-Electron Lasers. For example:

• https://doi.org/10.1038/nphoton.2010.239

• https://doi.org/10.1103/PhysRevSTAB.10.034801

• https://doi.org/10.1016/j.nima.2010.02.274