Modulated electron beam produced by a thermionic cathode electron gun for particle accelerator applications

Electron injectors used in radiation sources such as Free-electron lasers (FELs) and medical linear accelerators (LINACs) can generate high peak current and low emittance electron beams. There are different types of electron injectors depending on their cathode. These are thermionic cathodes, photocathodes, and field emission cathodes. Each one of them has its own advantages and disadvantages. In this thesis, considering the advantages of a long lifetime, large current density and being cost effective, a thermionic cathode gridded electron gun for a particle accelerator was designed and modelled. Both theoretical work and numerical simulations were carried out to explore the relationship between important parameters like the bunch charge and the bunch length of the modulated beam. Two 2D simulation packages, the DC electron trajectory solver TRAK and Particle-In-Cell code MAGIC were utilised initially to optimise the Pierce-type electron gun and to simulate the RF field applied to the grid. Similar to existing guns the electron energy, pulse duration and charge of the electron beam bunch were predicted but the significant deviation from existing guns was the pulse length as the function of the bunch charge. The beam dynamics simulations showed that a minimum pulse length of 106 ps could be achieved with a bunch charge of 33 pC when the driving RF frequency was 1.5 GHz. Simulations at a higher RF frequency did not significantly reduce the micro-pulse length and the normalised emittance was measured to be 5.6 mmmrad obtained from the particle-in-cell simulations. The results obtained with 2D simulation packages were compared with 3D simulations using CST Particle Studio with similar values for the pulse length at a value of 102 ps for 35 pC bunch charge observed. Different designs of grids, the spider web grid and the pepper pot grid were simulated and their performance was examined and compared. The comparison resulted in similar values for the peak current and the bunch charge. Furthermore, other techniques for reduction of the bunch length like higher harmonics of the fundamental frequency, specifically the idealised case of a square wave, using CST Particle Studio are presented. Overall, the notable advance in science lies in obtaining the bunch length as the function of the bunch charge which enabled the calculation of the emittance as a function of time by postprocessing the output of the numerical simulations. This is vital information to be passed to the designers of the S-band (3GHz) LINAC as it enables the performance of the LINAC in terms of capture of electrons to be predicted.Electron injectors used in radiation sources such as Free-electron lasers (FELs) and medical linear accelerators (LINACs) can generate high peak current and low emittance electron beams. There are different types of electron injectors depending on their cathode. These are thermionic cathodes, photocathodes, and field emission cathodes. Each one of them has its own advantages and disadvantages. In this thesis, considering the advantages of a long lifetime, large current density and being cost effective, a thermionic cathode gridded electron gun for a particle accelerator was designed and modelled. Both theoretical work and numerical simulations were carried out to explore the relationship between important parameters like the bunch charge and the bunch length of the modulated beam. Two 2D simulation packages, the DC electron trajectory solver TRAK and Particle-In-Cell code MAGIC were utilised initially to optimise the Pierce-type electron gun and to simulate the RF field applied to the grid. Similar to existing guns the electron energy, pulse duration and charge of the electron beam bunch were predicted but the significant deviation from existing guns was the pulse length as the function of the bunch charge. The beam dynamics simulations showed that a minimum pulse length of 106 ps could be achieved with a bunch charge of 33 pC when the driving RF frequency was 1.5 GHz. Simulations at a higher RF frequency did not significantly reduce the micro-pulse length and the normalised emittance was measured to be 5.6 mmmrad obtained from the particle-in-cell simulations. The results obtained with 2D simulation packages were compared with 3D simulations using CST Particle Studio with similar values for the pulse length at a value of 102 ps for 35 pC bunch charge observed. Different designs of grids, the spider web grid and the pepper pot grid were simulated and their performance was examined and compared. The comparison resulted in similar values for the peak current and the bunch charge. Furthermore, other techniques for reduction of the bunch length like higher harmonics of the fundamental frequency, specifically the idealised case of a square wave, using CST Particle Studio are presented. Overall, the notable advance in science lies in obtaining the bunch length as the function of the bunch charge which enabled the calculation of the emittance as a function of time by postprocessing the output of the numerical simulations. This is vital information to be passed to the designers of the S-band (3GHz) LINAC as it enables the performance of the LINAC in terms of capture of electrons to be predicted