A device for replenishing ionizable material in a field ionization apparatus is disclosed. The device comprises a heatable reservoir containing the ionizable material, a field ionization electrode structure, and a channel being in fluid communication with said heatable reservoir. The heatable reservoir and the channel are designed and constructed such that when the heatable reservoir is heated to an evaporation temperature of the ionizable material, a flux of vaporized ionizable material is directed along the channel to a tip of the field ionization electrode structure.