An electron exit window foil for use with a high performance electron beam generator operating in a corrosive environment is provided. The electron exit window foil comprises a sandwich structure having a film (202) of Ti, a first layer (204) of a material having a higher thermal conductivity than the Ti, and a flexible second layer (206) of a material being able to protect said film (202) from said corrosive environment, wherein the second layer (206) faces the corrosive environment.