Beam transport modeling of PPM focused THz sheet beam TWT circuit

We report periodic permanent magnet modeling for an efficiently focused sheet beam (~7:1) transport achieved in a relatively long (40 mm) drift tube. To perform a quick pulsed device (TWT) test, it is planned for a high aspect ratio electron gun (12:1, designed by CPI¹ for solenoidal field focused beam) to be used for currently designed PPM focused ~7:1 aspect ratio sheet beam, with drift tube to act as an aperture to the incoming planar stream of electrons. Extensive simulations have been conducted to elucidate beam trajectory analysis and parasitic beam interception with drift tube walls. Two different permanent magnet materials were considered (a) SmCo (Br =1.15T) and(b) NdFeB (Br =1.4T). For Case (a), we obtained beam transmission of 80% with spacing between parallel magnet stacks of 2.6 mm and for case (b) we obtained beam transmission of ~73.35% that includes tuning voltage on Focus Electrode, for an increased stack spacing of 4 mm that is crucial, in order to provide sufficient space for the slow wave structure/couplers fabrication. This corresponds to 190 mA transmitted current. Particle in Cell (PIC) simulations were also conducted to analyze beam-wave interaction at a significantly lower current ~ 90 mA (approx 50% of what anticipated) to analyze a worst case scenario. The currently designed actual PPM B-Field was employed. The PIC simulation result was very promising, showing ~ 10 dB gain and maximum output of 16 W for an input drive of 1 W at 0.22THz. For 190 mA transmitted current, the anticipated maximum output power is 75 W with a gain of 17 dB.