O.I. Louksha, B. Piosczyk*, G.G. Sominski, M. Thumm*, D.B. Samsonov

Saint-Petersburg State Polytechnical University, St. Petersburg, Russia
*Forschungszentrum Karlsruhe, Association EURATOM-FZK,

Institut für Hochleistungsimpuls- und Mikrowellentechnik (IHM), Karlsruhe, Germany

Enhancement of gyrotron efficiency can be achieved by improvement of the quality of the helical electron beam (HEB) used in gyrotrons as the active medium for energy transfer. One of the important factors preventing the achievement of high-quality HEB's is inhomogeneous electron emission from the thermionic cathode of magnetron-injection guns. Typically, these guns operate in the regime of temperature limited emission. Therefore, the inhomogeneities of both work function and emitter temperature result in a non-uniform distribution of current density in the HEB cross-section. The beam spatial non-uniformities cause a degradation of the gyrotron efficiency due to the increase of velocity spread and of excitation of parasitic modes in the resonator. Another factor of HEB quality deterioration are parasitic low-frequency oscillations (LFOs) which are excited in the space charge, accumulated in the trap between the cathode and the magnetic mirror. These oscillations result in HEB velocity and energy spreads, in electron bombardment of the cathode surface and in appearance of secondary electrons.

This paper reports the results of experiments performed with a pulse 74.2 GHz, 100 kW gyrotron at the St. Petersburg State Polytechnical University. Two LaB6 cathodes and two impregnated dispenser cathodes, differing greatly in emission homogeneity, have been used for studying of the emission inhomogeneity effect on the LFO amplitude and the threshold pitch-factor corresponding to LFO appearance, on the HEB spatial structure in the collector region as well as on the gyrotron output power and efficiency. The amount of transverse velocity spread related with inhomogeneity of the cathode emission was estimated using the data on electron energy distributions measured in the gyrotron. The obtained results allowed to determine the requirements for emission homogeneity of gyrotron cathodes.


This study was supported by the Forschungszentrum Karlsruhe and by the INTAS Grant 03-51-3861.