A BEAM SHAPER FOR THE OPTICAL BEAMLINE OF RF PHOTOINJECTORS

A high performance of SASE FELs requires a low emittance of the electron bunches that emerge from the photoinjector. This in turn can be favoured by irradiating the cathode by a laser beam with a spatial and temporal flat-top profile [2]. A simple way to illuminate the photocathode with a uniform beam profile is to strongly expand the laser beam with a telescope and select the very central region of the magnified beam by means of a small “beam shaping aperture”. This method which is presently implemented at PITZ, inherently gives rise to a large loss of the incident UV-laser power. At PITZ the transmission amounts to approximately 20%, and 80% of the UV laser radiation are lost at th e beam shaping aperture. Thus, a more efficient technique for optical beam-shaping is desirable.