Abstract
A self-focusing field emission (FE) X-ray tube with a large-area cathode design was simulated and fabricated. The designed X-ray tube had a cylindrically symmetric geometry; the diameter of the cathode and the anode was 15 mm, and the cathode-anode distance was 20 mm. Owing to the unique cup-shaped design of the cathode, the electron beam emitted from the large-area cathode was focused onto the anode without using magnetic lenses or extra biased electrodes. Carbon nanocoils, which were grown on the bottom of the circular cup-shaped cathode, were used as electron emitters because of their excellent FE properties. A simulation of the electron trajectories for various cup heights revealed that the optimal focal spot size (0.1 mm) was obtained at a cup height of 5 mm when a voltage of 50 kV was applied. To verify this result, an X-ray tube was fabricated and tested. An X-ray photograph of the tested tooth and electric circuits showed good resolution and X-ray intensity. The large cathode area effectively reduces the current density and thereby improves the lifetime of the cathode.
Highlights
X-ray radiation has long been used in various fields, such as medical diagnosis, industrial applications, material characterization, and scientific research
The red part is the anode, which has a diameter of 15 mm and is biased at an applied voltage of 50 kV; the blue part is the metallic cup for electrostatic focusing which has a diameter of 15 mm with a 12-mm-diameter aperture on the top to focus the emitted electrons
The distance from the electron emitters to the anode was fixed at 20 mm, and the cup height was varied to minimize the size of the focal spot on the anode
Summary
X-ray radiation has long been used in various fields, such as medical diagnosis, industrial applications, material characterization, and scientific research. The metal filament is heated to over 1000 °C to emit electrons, which are accelerated by an applied electric field to bombard the metal target, generating X-rays [1]. These thermionic emission (TE) X-ray tubes have many limitations. In FE, electrons are drawn from high-aspect-ratio or low-work-function cathode materials to the vacuum under a high electric field. The typical FE X-ray tubes may comprise three parts, which are an FE cathode grown with electron emitters, an anode (metal target), and a gate electrode. The FEM is a suitable tool for obtaining a numerical solution for the spatial distribution of the electric field between the cathode and the anode
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