Abstract: To prevent damage to electron beam apparatus from inadvertent exposure of parts of the apparatus to the electron beam, there is provided a beam catcher which has a catcher plate that seals off the evacuated work chamber of the apparatus from a space containing gas under substantially higher pressure than normally prevailing in the work chamber. When the electron beam inadvertently impinges on the beam catcher, it is able to puncture the catcher plate, thereby permitting the gas to pass through the puncture and enter the work chamber, causing defocusing of the electron beam which prevents a progression of the puncture and also causing a pressure increase in the work chamber, resulting in further defocusing of the electron and in automatic shutoff of the accelerating potential.