A 0.22 THz half-period-staggered double grating circuit with a sheet electron beam: A computer simulation study

Computer simulations of a half-period-staggered double grating circuit conceived by the Y.M. Shin and L. Barnett with a sheet electron beam are performed in the terahertz frequency band. Terahertz vacuum electron devices have advantages such as high data rate communications capability and resolving power. However, there are challenging problems at the same time, e.g. high ohmic loss and poor RF energy transmission, fabrication errors, and difficult electron beam formation. When the gratings are shifted by a half-period, the EM energy is concentrated primarily in the beam channel with essentially only point contact on the vane tips, which reduce ohmic losses and enhance RF energy transmission. In a half-period-staggered double grating circuit of Professor Luhmann's laboratory, the small scale, high current density electron beam is achieved by using a sheet beam instead of the conventional circular beam. Moreover, a micro-fabrication method under investigation at UC Davis reduces fabrication errors and produces a small scale vacuum electron circuit.The dispersion relation characteristics are examined from HFSS computer simulations and analytic calculations. HFSS simulations show that the return losses are less than -5 dB in the frequency range of 0.19-0.26 THz. Therefore, in this circuit, oscillation might occurs because the return losses are more than -7dB. MAGIC-3D computer simulations with the electron beam predict the gain of more than 30 dB and 36 dB for the beam current density of 357 A/cm2 and 750 A/cm2 respectively over the frequency band of 0.20-0.27 THz.The effect on performance due to fabrication-errors such as slanted vanes and misaligned interaction circuit is investigated by the use of HFSS and MAGIC-3D computer codes. According to simulation results, the effect of slanted vanes is insignificant unless the vanes are slanted by more than 20 degree in counter clockwise direction. However, even though there are fabrication-errors, the vanes are slanted by less than 10 degree. Therefore, the effect of slanted vane is negligible.Computer simulation results show that the misaligned interaction circuit has no significant deleterious effect on the circuit performance. However, the shorter gaps between an electron beam and vanes due to misalignment might cause the deformation of the circuit and bad circuit performance. In addition, axial misplacement might have effect on the circuit performance. Therefore, simulations of effects of axial misalignment and shorter gap between an electron beam and vanes might be helpful for investigation of a half-period-staggered double vane structure as the next step.