The embodiments of the present disclosure provide a charged particle assessment system for projecting a beam of charged particles towards a sample. The system comprises a sample holder configured to hold a sample; a charged particle optical system configured to project a beam of charged particles from a charged particle source downbeam towards the sample and comprising a cleaning target; and a cleaning device. The cleaning device is configured to supply cleaning medium in a cleaning flow towards the cleaning target incident on the cleaning target so that the cleaning flow approaches the cleaning target from downbeam of the cleaning target, and to stimulate the cleaning medium at or near the cleaning target such that the cleaning medium cleans at least a portion of the surface of the cleaning target.