The laser incoupler comprises a set of two mirrors, mounted off axis in a CF40 cross, at an angle of 45° with the beamline axis. A femtosecond UV laser pulse entering through a quartz window in one of the CF40 flanges can be directed via one mirror towards the photo-cathode, allowing photoemission at near-normal incidence. The part of the femtosecond beam reflected from the photocathode is guided out of the beamline via the other mirror, through the same quartz window, thus preventing unwanted photoemission from other surfaces. The near-normal incidence of the photoemission femtosecond UV laser pulse allows operation in the bunch blow-out regime which is beneficial for beam quality. The UV laser can be outlined on the center of the photo-cathode by observation of the (very weak) circular machining grooves, which are visible in the reflected laser light. The beam will still wander off the mechanical axis due to the earth magnetic field (and possibly incompletely screened fields from magnetic materials in pumps etc.). To correct for this, a steering coil is foreseen.
|Size||Standard CF40 cross|
|Mirror size||12.7 mm square|