The Research on the High-Current-Density Shielded Sheet Electron Beam Matching Focusing Magnetic Field

Increasing beam current density is a very effective approach to boost the output power in millimeter wave and terahertz vacuum devices. However, the current density of sheet electron beam (SEB) is severely limited, due to instabilities in the SEB transport, especially in the case of high current density. To overcome this limitation, a new approach for stably transporting a high-current-density SEB, named shielded SEB matching focusing magnetic field (shielded SEB-MFM), is presented in this article. This new focusing magnetic field consists of a small transverse magnetic field and a conventional shielded uniform magnetic field (shielded UM). Compared with the conventional shielded UM, about double the maximum beam current density can be transported by the shielded SEB-MFM, in the same strength as the magnetic field. In addition, an implementable scheme of an electron optical system based on the shielded SEB-MFM is designed and calculated to verify the feasibility of the shielded SEB-MFM. The results indicate that the SEB with a 636-A/cm2 current density can be well focused by a 0.8-T shielded SEB-MFM in a 40-mm tunnel. To achieve the same expansion degree of thickness, the conventional shielded UM must be around 1.5 T.