Oscillation starting condition of an oversized backward wave oscillator is investigated experimentally. Periodically corrugated slow wave structure (SWS) is used, which is designed for K-band operation in a weakly relativistic region. The upper frequency of fundamental axisymmetric mode (TM01) is about 24 GHz. The beam voltage is weakly relativistic, i.e., 30-80 kV. The beam current is in the range of 50-200 A. It is shown experimentally that there exist critical values of SWS length and beam voltage for the meaningful radiation. The oscillation starting voltage is about 40 kV for 20-period SWS and decreases to about 30 kV for 50-period SWS.
The output powers up to about 100 kW are obtained. The typical frequency observed is about 23 GHz. The output modes are examined by measuring the radiation patterns. Non-axisymmetric hybrid mode as well as axisymmetric TM mode are observed. The hybrid mode has both TE and TM components, which are in the same order of magnitude and have the azimuthal mode number m=1. The modes are controlled by the length of SWS and the strength of magnetic field. The radiation modes are estimated as TM01 mode for the axisymmetric case and as HE11 mode for the non-axisymmetric case, by comparing the numerically obtained dispersion curves.
The oscillation starting condition is investigated numerically by considering the effect of finite length SWS [1] and compared with the experiments. The starting beam energy depends on the SWS length and on the beam radius and is qualitatively in agreement with the experimental value.

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