CHARGED PARTICLE OPTICAL DEVICE, ASSESSMENT APPARATUS, METHOD OF ASSESSING A SAMPLE

The present disclosure relates to charged particle devices for projecting charged particles towards a sample and methods of assessing a sample using charged particles. A charged particle optical element directs beams of charged particles towards a sample. The charged particle optical element comprising a plate in which is defined beam apertures and a plurality of vent apertures. A beam tube defines an inner tube volume comprising paths up-beam of the plate of charged particles of the beams, and an outer tube region that is outside of the beam tube. The beam apertures are for passage towards the sample of charged particles of the beams, from the inner tube volume to a down-beam volume on an opposite side of the plate to the inner tube volume. Vent apertures fluidically connect the down-beam volume to the outer tube region.