Stability of a 95-GHz slotted third-harmonic gyro-TWT amplifier

Modal Analysis of Azimuthally Periodic Vane-Loaded Cylindrical Waveguide Interaction Structure for Gyro-TWT

“…The strength of interaction in a vane-loaded gyro-TWT, which depends on the energy stored per unit interaction length U l , say, may be estimated by the interaction impedance of the structure. Since the interaction impedance is inversely proportional to U l , and the vanes reduce RF fields in the outer region between vanes of the structure, the value of U l is significantly reduced, and, consequently, that of the interaction impedance increased by the presence of vanes [9]. Also an expression for the azimuthal interaction impedance of the vaneloaded cylindrical waveguide is obtained, which measures the available azimuthal RF electric field for a given power propagating down the waveguide for electrons to interact with field [25].…”

“…Such selective excitation of the desired mode is an extremely difficult problem due to the presence of many competing modes [3][4][5][6][7][8][9][10][11][12][13]18]. Therefore, study of the startup scenario in the design of such a gyrotron amplifier operating at high-order modes is extremely important.…”

“…At beam energies considered practical for microwave devices, cyclotron interaction weakens rapidly as one increases the harmonic number for reducing the magnetic field requirement [4,6,[8][9][10][11][12][13][14][15][16][17][18][19]. Moreover, at high harmonics severe mode competition occurs [12,19,22].…”

“…Similarly, for large gains, a resistive wall waveguide was used in order to stabilize the device against spurious oscillations [7][8][9][10] or use a number of stages with severs [16]. Furthermore, design considerations were made like reducing the required background magnetic field in the gyro-TWT experiment by operating the device at high cyclotron beam-harmonic resonance in a whispering gallery waveguide mode in the large-orbit configuration [8][9][10].…”

“…ω c (= |e| B 0 /m e0 ) is the non-relativistic cyclotron angular frequency, corresponding to the rest mass m e0 of an electron that carries a charge of e, with B 0 as the background magnetic flux density. The waveguide-mode dispersion relation is [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19]:…”