Terahertz gyrotrons in harmonic operation offer the magnetic-field reduction, but they can also still easily generate various competing modes that operate at low harmonics. In this paper, an injection-locking technique for phase control and spectral purity is employed to enhance the operating modes and suppress the competing modes in gyrotrons. The simulation results, using a multi-mode time-independent code, show that gyrotrons driven by backward or forward waves cannot avoid mode competition wherever the input power increases. To avoid the fundamental harmonic competing mode, a second-harmonic gyrotron traveling-wave amplifier (gyro-TWA) with a severed section is used instead of the driven gyrotrons. The gyro-TWA operates at a slightly low external magnetic field and has a uniform interaction structure. The simulation results reveal that the fundamental harmonic TE3,5 competing mode does not occur at Ib< 4.2 A; meanwhile, the second-harmonic TE8,9-mode gyro-TWA can yield a stable output power. The ampli...

A study of a terahertz gyrotron traveling-wave amplifier

The harmonic multiplying operation in a gyrotron traveling-wave amplifier (gyro-TWA) permits magnetic field reduction and frequency multiplication. This study presents a comparative analysis of fourth-harmonic multiplying gyro-TWAs with three schemes of operation. An improved mode-selective circuit using circular waveguides with various radii provides the rejection points within the range of operating frequencies to suppress the competing modes of gyro-TWAs. The simulated results reveal that gyro-TWAs are the most susceptible to the fundamental-harmonic TE11 competing mode, regardless of the operating scheme, and that the mode-selective circuit can provide an attenuation of more than 20 dB to suppress the competing modes. The amplification of the waves in a gyro-TWA depends on the lengths of the sections, and the simulated results show that the gain increases for all schemes, as the length of the lossy section or the length of the copper section increases. All schemes exhibit nearly the same saturated out...

Comparative analysis of fourth-harmonic multiplying gyrotron traveling-wave amplifiers operating at different frequency multiplications

In the gyrotron traveling wave amplifier (gyro-TWA), high loss dielectric materials loaded in a cylindrical waveguide are adopted to suppress the unwanted parasitic oscillations. It is of great importance to accurately understand the relative permittivity r and tan δ for studying the microwave and millimeter wave dispersion, and loss properties of a specific mode. The high lossy dielectric loaded circuits of the gyro-TWAs made of the BeO-SiC ceramic are theoretical calculated, simulated, and measured. The field distribution, dispersion, and loss properties of three different dielectric loaded circular HE12, HE d 22, and TE d 02 modes (corresponding to the TE11, TE21, and TE01 modes in the smooth hollow cylindrical waveguide respectively) in different frequency bands are respectively investigated. The theoretical analysis, simulation, and measurement results have a good agreement. This work has clear guiding significance for the stable work of gyro-TWAs.

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Theory, Simulation and Millimeterwave Measurement of the Operating and Parasitic Modes in a High Loss Dielectric Loaded Gyrotron Traveling Wave Amplifier (Invited)

A comparative analysis of fourth-harmonic multiplying gyrotron traveling-wave amplifiers (gyro-TWAs) with three schemes of operation is presented. The simulated results show that the gyro-TWAs are most susceptible to the fundamental harmonic TE11 competing mode, regardless of the operating scheme, and the mode-selective circuit can provide an attenuation of more than 20 dB to suppress the competing modes. The amplification of the wave in a gyro-TWA depends on the lengths of sections, and the gain increases for all schemes as the length of the lossy section or the length of the copper section increases. All schemes exhibit almost same saturated output powers and bandwidths while the saturated gain of the scheme at a high frequency multiplication ratio is less than that of the scheme at a low frequency multiplication ratio. Extensive numerical calculations of the power and gain scaling are conducted for all schemes.

A study of fourth-harmonic multiplying gyrotron traveling-wave amplifiers operating at different frequency multiplications

Harmonic multiplying operation in a gyrotron traveling-wave amplifier (gyro-TWA) permits for magnetic field reduction and frequency multiplication. Lowering a beam voltage is an important step toward miniaturization of a harmonic multiplying gyro-TWA. However, the additional degree of freedom that is provided by the multitude cyclotron harmonics in a low-voltage harmonic multiplying gyro-TWA still easily generates various competing modes. An improved mode-selective circuit, using circular waveguides with various radii, can provide the rejection points within the frequency range to suppress competing modes. Simulated results reveal that the mode-selective circuit can provide an attenuation of more than 14 dB to suppress the competing modes. Furthermore, the performance of the gyro-TWA is analyzed for studying the sensitivity of the saturated output power and full width at half maximum bandwidth of the gyro-TWA to the beam voltage and the magnetic field. A stable low-voltage harmonic multiplying gyro-TWA with the mode-selective circuit is predicted to yield a peak output power of 24 kW at 200.4 GHz, corresponding to a saturated gain of 56 dB at an interaction efficiency of 20%. The full width at half maximum bandwidth is 3.0 GHz.

B. H. Kao

Papers

Low-voltage harmonic multiplying gyrotron traveling-wave amplifier in G band