A new approach for a multistage depressed collector for gyrotrons

A feasibility study for a two-stage depressed gyrotron collector has been performed. A new approach for an adiabatic magnetic decompression of the hollow electron beam has been used. It permits control of the radius of the constant magnetic flux surface, which determines the radial extension of the electron beam. Independent of the value of the magnetic field around the beam. For this purpose, either solenoidal coils or a ferromagnetic insert can be placed inside the hollow electron beam. Thus, the radial dimensions of a multistage depressed collector of a high-power high-frequency gyrotron can be kept within limits given by technological constraints. The energy sorting of the electron beam is improved by using electrodes inside the hollow electron beam for controlling the potential distribution. The additional control electrodes make it possible to eliminate almost all of the effect of secondary electrons on the operation of the collector. In order to demonstrate the proposed approach, a compact two-stage depressed collector has been designed for a 1.5-MW coaxial cavity gyrotron operating at 165 GHz in the transverse electric (TE)_31,17 mode, which is under development at FZK, Karlsruhe, Germany. Including the effect due to secondary electrons, a collector efficiency of 73% has been calculated with an average and peak heat dissipation density of about 240 W/cm² and 500 W/cm², respectively. This results in an increase of the output gyrotron efficiency from 36.5% to 62.6% when internal radio frequency (RF)-losses inside the gyrotron tube of 15% are taken into account