Computation of space charge in a point cathode thermionic emission gun

Space charge in a tungsten point cathode gun has been studied with numerical method. The method is an iterative one consisting of the field determination using the integral form of Poissonʹs equation, the direct ray tracing in 3D, and the estimation of the space charge from the traced rays. The space charge is treated as a number of the coaxial charged rings with different radii and positions. Using the method, the space charge effects on the potential distribution around the cathode tip were examined. The method makes clear the variation of the emission area at different cathode temperatures.