The invention relates to charged particle beam generator comprising a charged particle source for generating a charged particle beam, a collimator system comprising a collimator structure with a plurality of collimator electrodes for collimating the charged particle beam, a beam source vacuum chamber comprising the charged particle source, and a generator vacuum chamber comprising the collimator structure and the beam source vacuum chamber within a vacuum, wherein the collimator system is positioned outside the beam source vacuum chamber. Each of the beam source vacuum chamber and the generator vacuum chamber may be provided with a vacuum pump.