Electron gun with electron-beam-heated point cathode: numerical analysis of electron beam for cathode tip heating

Electron beam heating of a point cathode tip, which is utilized in a high brightness thermionic emission gun for the improvement of the cathode life at high temperatures, has been studied with numerical methods. The heating beam is provided by an annular electron gun (5 kV, ≤ 1 mA) placed inside the Wehnelt cylinder. The trajectory and broadening of some typical pencils of rays (beamlets) were made clear by tracing rays, where the electric field was computed with a surface charge method and the influence of a magnetic field in the annular gun was taken into account. The beam heated position and area of the cathode were estimated from the traced rays, and the beam current distribution was estimated by integrating the partial current distributions of many beamlets. The results show that the beam hits a wide area of the cathode tip part under practical operating conditions, and that the magnetic field has no large influence on the heating.