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Ion back bombardment is believed to be a main cause of degradation of quantum efficiency of photocathodes in fixed-voltage photoguns. Ions produced in collisions of the electron beam with residual gas in the cathode-anode gap are accelerated towards the cathode and bombard the cathode surface, reducing its quantum efficiency. In addition to ions produced in the accelerating gap, there is another source of ions that has been overlooked so far in existing models of ion bombardment: ions produced in a beam transport line beyond the anode and trapped in the electron beam. These ions can drift back to the cathode and increase the rate of ion bombardment. This article shows that the flux of trapped ions can significantly exceed the flux of ions produced directly in the accelerating cathode-anode gap. The paper also describes a simple and effective solution to the problem of trapped ions, a positive potential barrier

Ion trapping and cathode bombardment by trapped ions in dc photoguns