The present invention relates to a charged particle beam generator (50) suitable for use in a charged particle lithography system (10), the charged particle beam generator comprising: a charged particle source (52) for generating a charged particle beam (54) along an optical axis (A); a collimator electrode stack (70) for collimating the charged particle beam, where the electrode stack spans a collimator height (Hc) along an optical axis; a generator vacuum chamber (51) for accommodating the charged particle source (52) and the collimator electrode stack (70); and at least one vacuum pump system (122, 123) disposed at a distance (delta Rp) from an outer periphery (85) of the collimator electrode stack inside the generator vacuum chamber (51), wherein the at least one vacuum pump system spans an effective pumping surface (122a, 123a) oriented substantially parallel to the optical axis (A), and wherein the effective pumping surface has a surface height (Hp) that spans at least a portionof the collimator height (Hc).