An X-ray tube assembly is provided including an emitter configured to emit an electron beam, an emitter focusing electrode, an extraction electrode, and a downstream focusing electrode. The emitter focusing electrode is disposed proximate to the emitter and outward of the emitter in an axial direction. The extraction electrode is disposed downstream of the emitter and the emitter focusing electrode. The extraction electrode has a negative bias voltage setting at which the extraction electrode has a negative bias voltage with respect to the emitter. The downstream focusing electrode is disposed downstream of the extraction electrode, and has a positive bias voltage with respect to the emitter. When the extraction electrode is at the negative bias voltage setting, the electron beam is emitted from an emission area that is smaller than a maximum emission area from which electrons may be emitted.