EFFECT OF ELECTRON EMISSION INHOMOGENEITY ON ELECTRON BEAM CHARACTERISTICS AND OUTPUT PARAMETERS OF A 4-MM GYROTRON
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.