Design and simulation of cloverleaf TWT slow wave structure

Summary form only given. AFRL is designing an S-band MW-class TWT amplifier. Cold test simulations using ICEPIC have mapped out dispersion characteristics of the fundamental TM
₀₁
and the next higher TM
₁₁
modes inside the cloverleaf slow wave structure. The cold (no beam) fractional bandwidth, Δf/f, of the slow wave structure is >17%. ICEPIC cold test simulations indicate interaction impedance of 91Ω and electric field amplitudes inside the slow wave structure well below the Kilpatrick breakdown limit at MW-class power levels. ICEPIC hot (beam included) test simulations indicate a problem with parasitic π-mode oscillations which overwhelm the amplified RF signal. A combined pitch- and impedance-taper at the end of the slow wave structure is being used to suppress π-mode oscillations. Couplers for insertion and extraction of RF into and out of the slow wave structure were designed with CST Microwave Studio. The couplers use an adjustable tuning ring and inductive doors to minimize return loss across a broad bandwidth. Our couplers have |S
₁₁
|<;-17 dB over a fractional bandwidth of ~11%.