“…It is also clear that a cavity hugely increases the complexity and alignment around the conversion and interaction points, where it is also necessary to extract the electron and laser beams and have space for a particle physics detector. There are many concerns for cavity based solutions about issues such as damage, longevity and reflectivity reduction for optics exposed to a radiation hard environment, locking of the cavity length precisely to the electron bunch repetition rate and stabilisation and maintaining the spatial overlap of the laser pulse with the electron bunch 13,14 . Given these limitations and the additional complexity of an optical cavity around the conversion and interaction points, it seems that a preferable solution would be to have a single pass interaction geometry and not recycle the laser pulse, if at all possible.…”