Research on Sheet Electron Beam Quadrupole Permanent Magnet Focusing System for Terahertz Vacuum Devices

Practical development of terahertz technology requires higher power radiation sources. The sheet electron beam vacuum device is an effective solution of increasing the output power of terahertz radiation sources, but faces the difficulty of stable transmission of the beam. In this paper, a compact quadrupole permanent magnet (QPM) focusing system for terahertz sheet beam devices is designed, and a practical focusing system is constructed into a prototype for beam transmission verification. In the experiment, 16 pieces of high-performance NdFeB permanent magnets were adopted with a total weight of about 10 kg. The magnetic field test of the system was carried out and the results show that the system can provide a uniform high-intensity magnetic field of over 0.95 T within an axial length of 20 mm. With the tested QPM magnetic field configuration, PIC simulation of the sheet beam transmission was implemented, indicating that a sheet electron beam with a 20 kV voltage and 15 mA current can travel through a beam tunnel of a cross-section 0.1 mm × 0.05 mm, with a transmission ratio of 98.5%.