The quality factor and oscillating mode of a gyrotron cavity are essential parameters to consider when trying to obtain a high power (>500 kW), high efficiency (∼50%) microwave source, which oscillates in a stable manner in the principal mode of the cavity. The study and development of an 8 GHz gyrotron whose resonant cavity is formed by a cylindrical waveguide of slowly varying radius, is undertaken. The study is principally concerned with the phenomena associated with the low quality factor of the TE°011 mode of the cavity. The power at optimal efficiency of a given resonant mode (TE°011 ) is increased when the diffractive quality factor (Qdiff) of the mode is decreased. Experiments performed with the gyrotron delivering power into a matched load show that during the process of decreasing Qdiff, the chosen mode of oscillation is perturbed by the appearance of oscillations in parasitic modes of the cavity; composed of higher order longitudinal modes (TE°O1q'q=2.3), and a propagating mode TE°21' gyro BWO), corresponding to an absolute instability of the system. These modes limit the power and efficiency of the principal mode by either oscillating simultaneously with, or in place of, the principal mode. When the gyro BWO mode oscillates, even at low power (several kilowatts), the efficiency of the principal mode of a cavity with Qdiff=160 is limited to less than 25%. This parasitic oscillation can be eliminated by the addition of a gradient in the magnetic field over the length of the interaction region. The maximum efficiency of the principal mode is then greater thon 40%; however, the power delivered remains lower than that of the cavity with Qdiff=225. The study of the influence of power reflections on the oscillation characteristics of the gyrotron reveals that the lowering of the quality factor of the cavity increases the sensitivity of the gyrotron to power reflections due to mismatched loads. A gyrotron with a quality factor which is too low can only deliver its maximum power to a matched load. In the presence of power reflections, the mode of oscillation changes, or the pulse is interrupted by an arc in the interior of the gyrotron. Even though a maximum power of 310 kW at an efficiency of 35% is obtained by the cavity with Qdiff=225, the sensitivity of this gyrotron to power reflections could limit the possibility of its use as a source for current generation or heating in a plasma, for example. These results show that there is a lower limit to the quality factor of a gyrotron cavity. This limit is situated between 410 and 225 for the TE°011 mode at 8 GHz. Below this limit the power in the mode decreases, the oscillation is perturbed or eliminated by the simultaneous oscillation of parasitic modes, and the sensitivity to power reflections could render the tube unusable for practical applications. A determination of the average velocity ratio α of the electrons in the electron beam (α=
/ ) is made by measuring the oscillation frequency of the gyro BWO mode. The results obtained using a non-linear simulation of the interaction between the gyro BWO and the electron beam show that the value of α at the entrance of the cavity corresponds to the α-value which the electron gun is supposed to produce. It seems, however, to indicate the presence of a larger velocity dispersion than expected. This original method of measuring the α of an electron beam can be applied to any source in which a gyro BWO mode is observed, or directly as a method of characterising the beam generated by a given electron gun.