High power terahertz (THz) sources and amplifiers hold the potential to greatly improve remote sensing and high bandwidth communication. To enable these applications, a Traveling Wave Tube (TWT) operating at 0.22 THz and a multi-cathode Field Emission (FE) electron gun are developed and characterized using a Particle-in-Cell simulation. Three candidate high current density cathode materials, Halfnium Carbide (HfC), carbon fibers, and Carbon Nanotubes (CNTs) were tested, characterized and their emission properties compared and used to verify simulations. A current of 3.0 mAmps for a single 100 micron diameter single walled nanotube rope was experimentally achieved and used as the basis of the FE gun design. Simulations of the FE gun and THz TWT were coupled and the effects of multiple and single tip FE gun beam characteristics on the TWT gain, bandwidth, and efficiencies are examined for several beam optic configurations.