Sever design and full model particle-in-cell simulation analysis for a 220 GHz ultra wideband TWTA

We report a distributed tapered sever design for a 220 GHz sheet beam TWTA circuit, compatible with MEMS precision micro-fabrication process. The loss material (Si/SiC) is introduced gradually into the cavities of the double-vane half period staggered structure of the TWTA starting from a thickness of 10 μm and gradually increased in successive cavities to a maximum value (less than 270 μm which is the cavity depth) and then gradually decreased again. The conductivity and maximum thickness of the sever were optimized for a 12.14 mm long sever and a relatively short length 4.02 mm sever. The return loss of the circuit with the long sever was below -10 dB from 205 GHz-275 GHz except for a peak around 247 GHz, while an isolation of (S₂₁) <; -50 dB was achieved. The return loss for the short sever was <; -10 dB from 205 GHz-275 GHz with an isolation of <; -27.6 dB. Particle-In-Cell (PIC) simulations were conducted on a full 220 GHz TWTA model incorporating circuit, broadband couplers and sever. The PIC analysis without sever model showed an output power of ~ 325 W with a gain of ~ 38 dB for an input drive of 50 mW at 220 GHz. The reverse power at the input port was 2W. After the inclusions of the long sever and conductivity optimization, the initial results showed a clear stability in the output response of the device, with an output power of ~ 25 W and gain ~ 27 dB. The reflected power was reduced significantly to a value of ~ 30 mW. The results are promising and work is in progress on PIC optimization of short sever and its realization by MEMS/DRIE technology.