A magnetically insulated coaxial vacuum diode providing a reduced energy spread in the leading edge of a high-current electron beam

Abstract

A magnetically insulated vacuum diode was modified to reduce the fraction of low-energy electrons in the leading edge of a high-current beam. The voltage pulse of duration ≈1 ns, rise time ≈250 ps, and amplitude ≈ –230 kV that arrives at the diode cathode from a transmission line is split in coupled coaxial lines (CLs) into two pulses with an amplitude ratio U2/U1 >1. The end of the common electrode of the CLs is close in profile to an equipotential surface and forms two gaps in the diode. In the first injection gap, at U0 < U1, explosive electron emission starts at the cathode. The pulse U1 arrives here with a delay at least equal to the duration of the leading edge, since there is a dielectric insert in the inner CL. The beam enters, through the window in the intermediate electrode, the acceleration gap, where the leading pulse U2, delivered from the outer CL, has already peaked. In the proposed device, unlike the one-gap prototype diode, electrons start from the cathode at a much smaller spread of the leading edge voltage. As a result, after acceleration of these electrons in the second gap, their energy spread relative to the maximum energy decreases multiply. This is confirmed by calculations and measurements of the beam current for a beam passed through aluminum filters.