Effect of the azimuthal inhomogeneity of electron emission on gyrotron operation

Abstract

Gyrotrons are typically driven by electron beams produced by magnetron-type thermionic electron guns operating in the regime of temperature limited emission. Very often, the current density in such annular electron beams is azimuthally nonuniform. To describe the effect of this nonuniformity on gyrotron operation, the code MAGY [M. Botton et al., IEEE Trans. Plasma Sci. 26, 882 (1998)], which is widely used for modeling of slow and fast microwave sources, was properly modified. The results of numerical simulations demonstrate the effect of azimuthal inhomogeneity of the emission on the excitation of low- and high-frequency satellites of the operating mode and on the efficiency degradation. The calculations are done for parameters typical for megawatt-class, long-pulse, millimeter-wave gyrotrons, which are currently under development for electron cyclotron plasma heating and current drive experiments in controlled fusion reactors.

Studies of the 1.5-MW 110-GHz Gyrotron Experiment

“…The microwave resonator, as mentioned previously, is an open ended cylindrical cavity, with a linear downtaper from the beam tunnel and a rounded uptaper to the output waveguide. This cavity has been designed based on cold-cavity codes, which solve for the electromagnetic fields in the absence of the electron beam, such that the TE 22,6 mode resonates at 110 GHz with a quality factor, Q, of 935. Figure 2 shows the wall profile and cold-cavity field profile for the TE 22,6 mode.…”

“…This cavity has been designed based on cold-cavity codes, which solve for the electromagnetic fields in the absence of the electron beam, such that the TE 22,6 mode resonates at 110 GHz with a quality factor, Q, of 935. Figure 2 shows the wall profile and cold-cavity field profile for the TE 22,6 mode. Although it is not shown in the schematic, an alpha probe [21] is located immediately before the cavity.…”

“…Second, theory and simulation have shown that this mode provides high coupling with the beam, such that it is a highly efficient mode for energy transfer. Third, the distribution of modes in the frequency spectrum suggests that the TE 22,6 mode is fairly isolated from other modes, implying a stable operating region without much mode competition. Finally, we have previous experience with other tubes which use the TE 22,6 mode, including the robust 1 MW, 110 GHz gyrotron, upon which this experiment is based.…”

“…This cavity profile is crucial in minimizing mode competition and limiting ohmic losses, and is designed to produce a single high-purity output mode. The design parameters for the gyrotron, listed in Table 1, are chosen such that a 96 kV, 40 A electron beam generates 1.5 MW of power in the TE 22,6 cylindrical waveguide mode, corresponding to an efficiency of 39 %. The TE 22,6 mode was chosen for several reasons.…”

“…The design parameters for the gyrotron, listed in Table 1, are chosen such that a 96 kV, 40 A electron beam generates 1.5 MW of power in the TE 22,6 cylindrical waveguide mode, corresponding to an efficiency of 39 %. The TE 22,6 mode was chosen for several reasons. First, the points of highest intensity (the first Bessel function maxima) of the mode pattern are away from the walls of the structure, towards the center of the cavity.…”