Abstract: An electron beam vacuum chamber is provided with an elevator mechanism within the chamber including a platform closing off an opening within a horizontal vacuum chamber wall and separating the vacuum chamber from an overlying antechamber partially defined by a vertically displaceable lid overlying the opening and moving towards and away from the opening. Resilient peripheral seals are fixed to the lid facing the chamber wall and the elevator platform. A coil spring partially compressible upon contact with the first peripheral seal carried by the lid with the face of the vacuum chamber wall forms a first mechanical override and an annular spring metal diaphragm fixedly mounted about its outer peripheral edge to the vacuum chamber adjacent the opening has its inner peripheral edge freely flexible and is contacted by the resilient peripheral seal of the platform to form a second mechanical override.