Abstract: An ion source for generating ions of a gas or vapor, especially for thinning solid state samples, includes a housing, an arrangement for introducing the gas or vapor into the housing and an anode positioned within the housing. The anode has a rotationally symmetrical cavity which is open at both sides along the axis of the source. First and second electrooptical mirrors are disposed along the source axis and define therebetween a space in which the anode is positioned The mirrors produce an electrostatic field to cause electrons to oscillate between them. At least one of the mirrors is apertured for exit therethrough of a fraction of ions generated in the space. An electron generating arrangement is disposed at one side of the cavity externally of the space between the mirrors and further, an arrangement causes the electrons to move into the cavity.