Optimization of millimeter wave microfabricated folded waveguide traveling-wave tubes

With broad bandwidth and relatively high power, microfabricated folded waveguide is an advanced slow-wave structure for millimeter and sub-millimeter wave traveling-wave tubes (TWTs). In this paper, two different ways were used to improve the output power and circuit gain of folded waveguide slow-wave structure. Pierce small signal analysis showed that the sidewall surface roughness will greatly undermine the circuit gain. And the surface roughness reduction measure was effectively proven by experiments. Atomic force microscope (AFM) analysis indicated that the rms surface roughness of our microfabricated structure is below 8 nm. In loss-free particle-in-cell (PIC) simulation, the linearity and efficiency were greatly improved by phase velocity taper of electromagnetic wave on the rear half of the circuit. The saturated output power and efficiency were respectively improved from 49 W to 73.4 W, and 5.7% to 8.7%, for a 35.8 mm-long (64 periods) loss-free and concentrative-attenuated circuit.