Detailed Investigations on a Multisection Cavity for a Continuously Frequency-Tunable Gyrotron

The effects of the slightly up-tapered sections in a multisection gyrotron cavity on the resonant frequency and the quality factor are investigated in detail. The main role of these sections is to change the effective cavity length or the total reflection. The slightly up-tapered section near the input section plays a role of increasing the effective cavity length and the diffractive quality factor, and the increase for a higher order axial mode is more obvious. The slightly up-tapered main section plays a role of reducing the diffractive quality factor, and the decrease for a lower order axial mode is more notable. The role of the slightly up-tapered section near the output end is complicated; when the angle is small, the main role of this section is to increase the effective cavity length; however, with the increase of the angle, the main role is to reduce the reflection from the open end. In a conventional three-section gyrotron cavity, the quality factor of the low-order axial mode is dozens of times of that of the high-order axial mode, and in an optimized multisection gyrotron cavity, the quality factor for all axial modes can maintain an order of magnitude in the cold cavity approximation; it means that the operating current for all axial modes can keep an order of magnitude.