The trajectory and width of the pencils of rays in a point cathode thermionic emission gun have been analyzed with numerical methods. The numerical model of the gun consists of the point cathode with the tip radius of 0.4 micrometers , the wehnelt with the aperture diameter of 1 mm, and the anode. The electric field was computed with a surface charge method, which is one of the integral methods of the charge distribution on the electrode surface. A direct ray-tracing was done in 3D from the axial and off-axial positions on the spherical tip part of the cathode for different emitting conditions. The shape of each pencil of rays was estimated from these rays. Some 3D views of the pencils are given, together with the electrode geometry. The pencils have large widths when they pass through the wehnelt aperture, and then focus onto different positions with different cross- sections. The methods and results are presented.
Ryo Iiyoshi, Ryo Iiyoshi,
"Numerical analysis of pencils of rays in a point cathode thermionic emission gun", Proc. SPIE 2858, Charged-Particle Optics II, (25 October 1996); doi: 10.1117/12.255509; https://doi.org/10.1117/12.255509