in accordance with the present invention parts made from magnetic material such as soft iron are shown with dense crosshatching in fig1 and 2 referring to fig1 and 2 nonmagnetic weld flange 108 is sealed to nonmagnetic ring 204 by weld joint 202 to produce a hermetic seal to support a vacuum in the gun and input section regions a magnetic gun adjustment disk 206 is supported by nonmagnetic ring 204 and can be used to adjust the focus of the electron beam emitted by the cathode 102 magnetic gun polepiece 212 is situated in proximity to the focus electrode 104 cathode 102 and anode 214 translating the polepiece 212 axially toward or away from cathode 102 changes the focusing behavior of the electron beam to enable axial movement of the polepiece and anode with respect to the cathode a flexible bellows 208 is used to attach the magnetic gun adjustment disk 206 and a structural body adjustment disk 210 that is preferably made from a nonmagnetic material it is preferred that the bellows 208 is made from a nonmagnetic material because it is known in the art that bending of magnetic parts can cause work hardening of the material which.