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High-power microwave radiation of 9.5–13.4 GHz was generated in a plasma-loaded backward-wave oscillator employing a relativistic electron beam of 400–500 keV and 1–3 kA. This experiment was to re-examine and confirm the previous works that had been done in other various institutions. The relativistic electron beam, which was produced from a helium gas-loaded foil-less diode, was injected into the helium gas-loaded rippled-wall waveguide of the backward-wave oscillator, generating a plasma in the waveguide and then the high-power microwave radiation. A sharp increase in the output microwave power has been observed in a narrow range of the helium gas pressure, and two dips in the microwave emission have also been found in a certain range of the axial magnetic field. Additionally, at certain values of the helium gas pressure and guide magnetic field, the total microwave emission of the plasma-loaded backward-wave oscillator was found to be seven times as large as that of the vacuum case. The highest interaction efficiency was estimated to be about 29% for the present experiment. © 2000 American Institute of Physics.