Emittance-Limited Beam Radius of Pencil Beams in Terahertz Vacuum Electron Devices

Abstract

As vacuum electron devices (VEDs) push toward the terahertz band, the interaction-circuit beam tunnel must shrink with frequency, and the transverse dimension of the electron beam becomes increasingly affected by emittance. To estimate the minimum achievable beam radius (MABR) of pencil beams in terahertz VEDs, we perform numerical simulations of a 20-kV solenoid-confined pencil beam under idealized injection and focusing conditions, scanning magnetic field 0.3–0.8 T, normalized rms emittance 1–200 nm, and beam current 5–30 mA. Across the full scan, the MABR is a universal function of a single dimensionless space-charge-to-emittance ratio: it decreases from about 3.1 times the matched rms envelope radius in the emittance-dominated limit to about 2.2 times in the space-charge-dominated limit. In the emittance-dominated regime relevant to terahertz operation, the matched radius—and hence the MABR—is governed primarily by emittance and magnetic field rather than by beam current or voltage.