Characterization of Low Emittance Electron Beams Generated by Transverse Laser Beam Shaping

Linac based X-ray free electron laser demand a high beam quality from the electron source, therefore RF photoinjectors are used to generate the electron bunches for state of the art beam brightness. One important figure of merit for these injectors is the transverse emittance of the generated electron beam, which can be minimized by shaping the photocathode laser pulses. Best performance can be achieved with ellipsoidal laser pulses, but 3D shaping is technically challenging. Typically, a quasi-uniform transverse laser profile is truncated from the Gaussian profile generated by the laser with an aperture to reduce the transverse nonlinear space charge forces. This is investigated in detail by optimizing the laser transverse profile at the Photoinjector Test facility at DESY in Zeuthen (PITZ), where photoinjector R is conducted for the E-XFEL and FLASH free electron lasers at DESY in Hamburg. In this contribution we present experimental results at high acceleration gradients (up to 60 MV/m) for both 250 pC and 500 pC. For a bunch charge of 500 pC an emittance reduction of about 30% compared to the commonly used transverse flat-top laser distribution was achieved