Possible gyrotron operation in the no start current zone caused by the axial dependence of the phase of the resonator field

It is known that gyrotrons (as well as other electron beam driven microwave and millimeter-wave oscillators) can operate in the regime of either soft or hard self-excitation. In the regime of soft self-excitation, the beam current exceeds its starting value; thus, the oscillations can start to grow from the noise produced by electrons. In the regime of hard self-excitation, the beam current is less than its starting value. Therefore, for exciting the oscillations, a certain start-up scenario is required, which may include the variation of the mod-anode and/or beam voltage or the guiding magnetic field. It was found recently [O. Dumbrajs and G. S. Nusinovich, Phys. Plasmas 25, 013121 (2018)] that some gyrotrons can also operate in the region of magnetic fields where there is no start current at all. In the present paper, it is shown that this sort of operation can be attributed to the presence of the axial dependence of the phase of the resonator field. Published by AIP Publishing.