Effects of the Magnetic Field Taper on a Continuously Frequency-Tunable Gyrotron

Generally, the magnetic field profile in the gyrotron beam-wave interaction section is usually considered to be constant in the theoretical investigation, but actually it is approximately linearly tilted slightly due to the limitation of the superconducting magnet coils. The effects of the weak operating magnetic field taper on the operating frequency, the beam-wave interaction efficiency, and the frequency-tuning range for a continuously frequency-tunable gyrotron are investigated with a self-consistent nonlinear beam-wave interaction theory. It is found that the tapered operating magnetic field has a significant effect on the operating frequency, the frequency tuning bandwidth, and the beam-wave interaction efficiency. In addition, the effect of the weak operating magnetic field taper on the starting current is studied based on the linear theory. It is found that the values of not only the starting current but also the operating magnetic field are obviously different from those when the tapered operating magnetic field is taken into account. With the increase in the slope of the tapered operating magnetic field, the operating frequency and the frequency-tuning range are decreased, but the beam-wave interaction efficiency is increased. When the slope of the tapered operating magnetic field is negative, the variations in the operating frequency, the frequency-tuning range, and the beam-wave interaction efficiency are more obvious. The effects of the tapered magnetic field on different order axial modes are different, and the effect on a higher order axial mode is more notable.