Simulation analysis of nano-CNC fabricated 220 GHz ultra wide band TWTA

We report detailed simulation analysis for the 220 GHz sheet beam TWTA being fabricated using nano-CNC technology[1]. The single cell dispersion analysis incorporating an inevitable 100 μm edge radius demonstrated the shift in the synchronism condition with the slow space charge wave (20 kV, 0.25 A beam) for the 467 μm period and 320 μm vane depth design. This case was further analyzed for beam tunnel widths of 770 μm and 800 μm. To regain synchronism, it was established that the 800 μm beam tunnel width is a better choice as the phase velocity matching condition occurs for a relatively low beam energy (i.e ~ 19 kV). This also correlates well with the S-matrix simulation analysis of the TWT double-vane half-period staggered structure including sever and couplers. The 100 μm corner radius shifts the passband by ~ 5 GHz to higher frequency which is compensated for by the relatively wide 800 μm beam tunnel. Particle-in-cell simulations supported the single cell dispersion analysis and synchronism shift due to corner radius. At 220 GHz, with 100 μm radius, beam-wave interaction was seen to be completely deteriorated. In the same model, reducing the radius to ~ 1 μm resulted in P_out ~ 80 W and P_ref ~ 3mW for a peak input RF power of ~ 100 mW. For the 100 μm radius case, the synchronism was regained by increasing the beam energy to ~ 20.7 kV and PIC simulation predicted an output power of ~ 32 W. Further investigations are underway for an optimum robust design for a beam of 18 kV that takes into account realistic fabrication errors in the simulation modeling analysis.