Design of electron gun and novel PPM focusing system for W-band sheet beam TWT

Summary form only given. For W-band and THz frequencies, sheet beam travelling wave tubes (SBTWT) are emerging as high power oscillator and amplifier. Sheet beam transport is the critical issue in such devices^1-4. An 20 kV, 210 mA electron gun for a SBTWT is designed. BFE and anode dimensions are optimized using CST code to converge the beam coming from planar cathode of rectangular size 1.2 mm×1 mm. The beam waist is formed at 11.2 mm from the cathode with the beam size of 1.2 mm× 0.22 mm. Novel PPM focusing system is designed for stable transport of the beam. The designed focusing system is the modified form of the staggered PCM with extended pole-piece as discussed in literature. In this case no side magnets are used and pole-pieces are extended after beam edge. Absence of side magnets will reduce the cost of magnetic material, weight of the tube and make easier to insert couplers. The horizontal position of extended pole-piece is after 5 mm from the side wall of the drift tube tunnel in order to make mechanically possible to extend the pole-pieces. NdFeB magnates with Br value 1.4 T is considered. The shielding pole piece is extended towards cathode region to get Brillouin flow. Beam transmission is achieved more than 95% up to 40 mm and 90% up to 70 mm in a tunnel suitable for W-band sheet beam TWT.