Investigation and optimisation of a plasma cathode electron beam gun for material processing applications

Abstract

This thesis describes design, development and testing work on a plasma cathode electron beam gun as well as plasma diagnosis experiments and Electron Beam (EB) current measurements carried out with the aim of maximising the power of the EB extracted and optimising the electron beam gun system for material processing applications. The elements which influence EB gun design are described and put into practice in a thermionic EB gun case study. The relevant principles of plasma EB gun systems, such low-temperature, low-pressure, RF excitation, are described along with the test rigs developed to investigate different plasma cathode configurations. The first experimental setup was for optical spectroscopy measurements of the light emitted from the plasma and the second included current measurements from EBs generated at –30 and –60 kV as well as the spectroscopic measurements. Comparison of EB current measurements with different plasma cathode configurations and correlation with spectroscopic measurements are presented. The maximum current extracted from the Radiofrequency (RF) gun was 38 mA at –60 kV using a hollow cathode geometry and permanent magnets for electron confinement. The RF gun was compared to a Direct Current (DC) gun which generated higher currents. This was reflected in the spectra which indicated a higher ionisation level than in the RF plasma. Simulation work carried out using Opera-2d to model beam trajectories indicated that the beam shape is largely influenced by the plasma boundary. Particle In Cell (PIC) simulations of a parallel plate RF plasma cathode demonstrated that higher excitation frequencies produced higher ionisation, however the RF sheaths were larger and thus the current extracted may be limited in practice due to fewer electrons being available near the aperture. The sheath thickness decreased in the simulations as the discharge gap was increased. RF plasma also produced larger currents from larger plasma chambers.