Study of Magnetic Focusing Structures for 220 GHz Sheet Beam Traveling Wave Tubes

The study of three types of magnetic focusing structures for terahertz band sheet beam traveling wave tubes (SB-TWTs) is presented in this article. Although there are a lot of reported literatures regarding the study of magnetic focusing structures, most of them just focused on a single type of focusing structure and lack of systematic research (for example, only simulation study), with a limited discourse on the comparative performance across different types of focusing structures. In addition, many reported simulations regarding the focusing structures are normally based on the ideal electron beam, resulting in reduced simulation accuracy to some extent. In this article, three types of focusing structures (uniform magnetic (UM) / pole offset periodic cusped magnetic (PO-PCM) / PCM-tunable quadrupolar magnet (PCM-TQM) focusing structures) are studied in detail by theoretical analysis and simulation design. The PO-PCM focusing structure is experimentally studied as a representative. Simulation results based on the electron beam emitted from the designed sheet beam electron gun show that a stable long-distance transmission over 112 mm and 85 mm can be achieved with the UM focusing structure and the PCM focusing structures, respectively. To verify the design, the microfabrication and test of the PO-PCM focusing structure is carried out as a representative. The measured magnetic field matches well with its simulated and theoretically calculated counterparts. The beam-wave interaction co-simulation results based on different focusing structures have a discrepancy among them.