Sensitivity of Perveance to Cathode Placement in a Low Perveance Electron Gun

The SLAC ELECTRON TRAJECTORY PROGRAM1 (EGUN) has been used to simulate a low perveance electron gun which will produce an 800 mA space charge limited beam at 130 kV. The simulations indicate that axial displacements of the cathode by + 0.5 mm from its nominal position can produce a factor of two variation in the perveance of the electron gun. This sensitivity is due to the boundary conditions of the electrostatic potential near the cathode. Movement of the cathode relative to a fixed focus electrode produces an enhanced variation of the electric field near the cathode surface over that which occurs when both cathode and electrode are moved together. The simulations are in agreement with experimental data.