Pole-Piece With Stepped Hole for Stable Sheet Electron Beam Transport Under Uniform Magnetic Field

An electron gun is designed for a W-band sheet beam TWT using a planar rectangular cathode working in the space charge limited region. A permanent magnet solenoid focusing system is designed to focus the beam. A pole-piece with a novel multisized (stepped) hole is investigated for stable transport of the beam. The size of the hole in the pole-piece is larger near to the magnets and it is gradually decreased stepwise toward the cathode. The pole-piece with the multisized hole reduces both the transverse magnetic field and magnitude of flux leakage toward the cathode. Simulation results show that the sheet beam transports without significant beam ripple and edge curling under the magnetic field of the designed focusing structure with the pole-piece of multisized hole. The thermal velocity of the electrons is also considered in the simulation. The beam transmission up to 70-mm distance is 98.8% and 97.3%, respectively, for nonthermal and thermal beam under the magnetic field of the designed focusing system.