The invention relates to a collimator electrode stack (70), comprising: - at least three collimator electrodes (71-80) for collimating a charged particle beam along an optical axis (A), wherein each collimator electrode comprises an electrode body with an electrode aperture for allowing passage to the charged particle beam, wherein the electrode bodies are spaced along an axial direction (Z) which is substantially parallel with the optical axis, and wherein the electrode apertures are coaxially aligned along the optical axis; and - a plurality of spacing structures (89) provided between each pair of adjacent collimator electrodes and made of an electrically insulating material, for positioning the collimator electrodes at predetermined distances along the axial direction. Each of the collimator electrodes (71-80) is electrically connected to a separate voltage output (151-160). The invention further relates to a method of operating a charged particle beam generator.