A novel electrostatic lens module in carbon nanotube field-emission electron guns for energy-variable X-ray sources

We have designed and fabricated a novel electrostatic lens module composed of a micro- and macro-focusing lens in carbon nanotube field-emission electron guns for energy-variable X-ray sources maintaining their focal spot size. The effective focal spot was observed to be small and isotropic by the micro-focusing lens to suppress the divergence of an individual electron beam and the elliptical macro-focusing lens to guide a circular focal spot through the tilted anode target. We could retain a constant focal spot under various anode voltages by changing the bias voltages applied to the electrostatic lens.