Numerical modeling analysis of 0.22 THz sheet beam TWT circuit

Extensive numerical analysis has resulted in the conclusion that the THz (H-band) sheet beam traveling wave tube (TWT) amplifier circuit, comprised of a staggered double grating array waveguide, has the very broad bandwidth (~ 30 %) of the fundamental passband (TE-mode) with a 7 : 1 aspect ratio sheet beam without excitation of n = 1 space harmonic backward wave modes. Particle-in-cell (PIC) simulations utilizing MAGIC3D and CST PS predict that the designed circuit produces 150 ~ 300 W output power, corresponding to ~ 3-5.5 % intrinsic electronic efficiency (35 ~ 38 dB gain from 50 mW input driving power), over ~ 25 % bandwidth, which are in good agreement with Christine-1D code simulation results. Simulations, using a perfectly matched layer (PML) boundary (~ -30 dB return loss), show the circuit stably operates without noticeable oscillation. With a more realistic matching condition with ~ -9 dB return loss, it becomes unstable. However, simulations show that the incorporation of a sever with tapered conductivity fully suppresses the instability in tube operation.