Nonadiabatic effects in the electron-optical system of a relativistic millimeter-wave gyrotron

Abstract

Gyrotrons based on relativistic electron flows generated by thermionic magnetron injection guns are promising pulsed sources of millimeter and submillimeter radiation with multi-megawatt output power. Obviously, to increase the output power of such devices, it is necessary to provide a high beam current of more than 100 A. At the selected emission density, this forces the use of an emitter with a sufficiently large area. As is well known, wide emitters produce beams with a large velocity spread, which reduces the efficiency of the electron-wave interaction. Therefore, it is necessary to increase the diameter and, accordingly, increase the distance from the cathode to the gyrotron cavity, which leads to an increase in the magnetization reversal coefficient. Thus, the magnetic field in the region of beam formation becomes weak, and the Larmor radius and the turn pitch of the electron trajectory become large compared to the geometry of the surrounding electrodes, which can cause nonadiabatic effects. In this work, we consider one of these effects, which consists in the emergence of a range of anode voltage values in which the monotonic growth of the pitch factor stops. The results of trajectory analysis using current tube and discrete source methods based on the ANGEL software package are presented, as well as an assessment of the influence of a magnetic lens on the properties of an electron beam based on a simple analytical model.