Characteristics of modulated field emission in helix TWT

Summary form only given, as follows. The steady evolution of the FEAs (Field Emitter Arrays), enabled the parameters such as current density, total current, and packing density, to have their values reached meeting the moderate power TWT (Traveling Wave Tube) requirements. Due to their compactness, light-weight, and low power consumption, FEAs became a promising alternative to the thermionic emitters. Also, the advance in FEAs microfabrication technique revived the concept of "density modulation" from the cathode, which was suggested and built by Haeff at 1940s using grid controlled thermionic cathode. The usage of FEAs enables the high frequency density-modulation on the order of 10 GHz due to the reduction of the transit time between cathode and grid, therefore, eliminates the need for the portion of the helix used to modulate the electron beam. The research on the emission-gated, FEA-based TWT (twystrode) was performed using Cassandra and CHRISTINE with next generation FEA parameters. In that simulation, he modeled beam pattern as gated emission (class C operation), which is used extensively for conventional power vacuum triode to increase the efficiency. However, significant distortion of the wave form is expected because of the nonlinear current-voltage characteristics of the Fowler-Nordheim equation. The harmonic contents in the premodulated electron beam for given FEAs parameters is investigated varying the modulation depth. MAGIC is used employing a rigorous emission model. The harmonic contents of current cause the premature saturation of induced electromagnetic wave in the helical slow wave structure. Simulation result reveals that tight bunch from the class C operation does not guarantee high efficiency, as does in conventional power triodes. Rather class A operation can reach the desirable output power within comparable circuit length.