An X-ray tube (1) comprising a cathode (3), an anode (5) and a further electrode (7) is proposed. Therein, the further electrode is arranged and adapted such that, due to impact of 'free electrons (27) coming from the anode (5), the further electrode (7) negatively charges to an electrical potential lying between a cathode's potential and an anode's potential. The further electrode (7) may be passive, i.e. substantially electrically isolated and not connected to an active external voltage supply. The further electrode (7) may act as an ion pump removing ions from within a primary electron beam (21) and furthermore also removing atoms of residual gas within the housing (11) of the X-ray tube (1). In order to further increase the ion pumping capability of the further electrode (7), a magnetic field generator (61) can be arranged adjacent to the further electrode (7).