A post mass selection decel lens (9) is located between the exit aperture (55) of the mass selection chamber (47) and the entry (74) to the electron confinement tube (69) of the PFS. The lens comprises a first electrode (65) at the substrate potential, a second electrode (60) at the flight tube potential, and a field electrode (61) between them at a relatively high (negative) potential sufficient to provide focusing of the ion beam at the first electrode. The first electrode is larger than the beam to avoid deflecting ions at the periphery of the aperture out of the beam. The first electrode has an aperture which is smaller than that of the field electrode. The field electrode is at least -5 kV relative to the flight tube, that is substantially more than required for electron suppression. Additional apertures are provided between the process chamber and the mass selection chamber to improve evacuation.