Beam Dynamic Analysis of RF Modulated Electron Beam Produced by Gridded Thermionic Guns

A thermionic cathode gridded electron gun used in injectors for different types of circular and linear particle accelerators and for energy recovery configurations was studied. Both theory and numerical simulation were used to explore the relationship between the bunch charge and bunch length. The electron gun is based on a Pierce-type geometry. It was initially designed using Vaughan synthesis followed by optimization using a 2D electron trajectory solver TRAK. After optimization, the grid in front of the cathode was inserted and the RF field was introduced through a coaxial waveguide structure. The complete gun was simulated using the PIC code MAGIC. High duty cycle operations at frequencies 1.5 GHz and 3.0 GHz, were investigated using different combinations of both the bias and the RF voltage applied between the cathode and the grid. The beam dynamics results from the PIC showed that a minimum bunch length of 106 ps could be achieved with a bunch charge of 33 pC when the driving RF frequency was 1.5 GHz. Operating at the higher RF frequency of 3GHz did not significantly reduce the bunch length. The normalized emittance of about 5.6 mm-mrad was demonstrated in PIC simulations.