Measurements of the I-V Characteristic of Short-Pulse (10-15 ns) Electron Beams

Abstract

It has been experimentally demonstrated [1] that the currentvoltage characteristic of a short-pulse electron beam (when the electron beam pulse duration is less than or comparable with the electron time-of-flight between the cathode and anode in the planar geometry of the electron diode) is considerable higher then the conventional Child-Langmuir limit. It can be projected, also, that in the coaxial geometry of electron beam formation, where the Fedosov-Belomytsev current [2] is an analog of the Child-Langmuir current, the I-V characteristic of a short-pulse electron beam should similarly higher. This means that, when the electron-beam pulse duration is less than or comparable with the characteristic time determining the formation of the steady-state space charge distribution along the electron-beam drift path downstream from the cathode, the total electron beam current should be higher than the appropriate Fedosov-Belomytsev limit. We measured the current-voltage characteristic of nanosecond-duration (10-15 ns) thin tubular relativistic (300-600 keV) electron beams accelerated in vacuum along the axis of a smooth uniform metal tube immersed in a strong axial magnetic field. Results of these measurements as well as results of computer simulations performed using PIC code show that the I-V characteristic at the front of the nanosecond-duration electron-beam pulse is different from the analogous dependence measured at the flat part of the pulse. In the steady-state (flat) part of the pulse, the measured electron-beam current is close to Fedosov-Belomytsev current [2], which is governed by the conservation law of electron momentum flow for any constant voltage. In the non steady-state part (front) of the pulse, the electron-beam current is considerably higher than the appropriate steady-state Fedosov-Belomytsev current and is close to the space-charge-limiting current determining the maximum electron beam current that a metallic tube is capable of supporting.