4GLS: a new type of fourth generation light source facility

Consideration is now being given in the UK to the provision of an advanced facility at lower energy to complement the DIAMOND x-ray light source. The proposed solution, 4GLS, is a superconducting energy recovery linac (ERL) with an output energy around 600 MeV, delivering both CW beam currents up to 100 mA and alternatively high charge bunches for FEL applications. Production and manipulation of short electron bunches (fs) is a vital part of the source specification. In addition to beam lines from undulator sources in the ERL recovery path there will be three FELs: two will be oscillator types in the infrared and VUV respectively, and the third will be a high gain system for XUV output. The project is outlined, together with its status and the R&D challenges posed. A funded prototype based on a 50 MeV ERL is also described.