At present, coaxial cavity gyrotrons are among the most promising sources of microwave radiation, which can be used in modern and forthcoming fusion machines. Investigations on the characteristics and factors, which affect the gyrotron power and efficiency, are now of great importance.
Fig. 1 – Cavities formed by longitudinally inhomogeneous waveguides
The main mechanisms of power limitation in coaxial gyrotrons have been investigated on the basis of the general theory for the longitudinally inhomogeneous waveguides with impedance walls.
For the first time, a system of nonlinear gyrotron equations has been obtained with the self-consistent and rigorous consideration for the linear mode conversion and the ohmic losses in a coaxial gyrotron cavity.
It has been shown that the linear conversion of the operating mode (TE34,19) of coaxial gyrotron to the TE34,18 and TE34,20radial satellites and the ohmic wall losses lead to a distinct drop in the quality factor of the operating mode, while the operating frequency and the field distribution in the cavity change only slightly.
Linear mode conversion and ohmic losses in the gyrotron cavity notably affect the mode competition scenario during the gyrotron start-up, as well as the efficiency and the output power in the stationary regime of generation. This effect depends strongly on the parameters of the electron beam and mainly on the pitch factor and the accelerating voltage.
For small pitch factors α ≤ 1 and moderate values of the accelerating voltage V ≤ 75 kV, the mode conversion and the ohmic wall losses lead to a 20% reduction in the gyrotron power and efficiency. For some intensities of the external magnetic field this causes the mode hoping, which qualitatively coincides with the experimental results.
For large voltages V ≥ 80 kV and large pitch factors α ≥ 1.2, the mode conversion and the ohmic losses enlarge the gyrotron output power and efficiency, and also make broader the operating region. Note that the increase in output power for the operating mode under the action of competing modes was observed before (and is known as the mode cooperation).
The results obtained can be rather useful in the conceptual design of a new generation of coaxial gyrotrons for the future fusion project - DEMO.