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Gyrotron, as a most likely practical high-power terahertz (THz) radiation source, has been widely applied to THz dynamic nuclear polarization-enhanced nuclear magnetic resonance (DNP-NMR) spectroscopy. In this article, an iris cavity with high-quality factor is theoretically designed for a frequency-tunable THz gyrotron, which could be tuned at three frequency regimes: 140, 250, and 263 GHz, respectively. Based on the self-developed nonlinear code, the beam wave interactions in the designed cavity are studied in detail when the corresponding high-order axial modes (HOAMs) are TE−6.1.q, TE0.4.q', and TE8.2.q. By tuning the applied magnetic field under the condition that the beam voltage U0 is 20 kV and beam current I0 is 0.10 A, the output power and frequency tuning ranges could achieve 0.75 kW and 1.15 GHz, 0.63 kW and 3.10 GHz, and 0.59 kW and 2.54 GHz. Meanwhile the calculation results are compared with the 3-D particle software CST, which have confirmed that the frequency and power tuning ranges of TE−6.1.q, TE0.4.q', and TE8.2.q'' could reach 0.46 kW and 1.07 GHz, 0.37 kW and 2.24 GHz, and 0.34 kW and 1.96 GHz. These results could be beneficial for the development of frequency-tunable gyrotron toward some up-and-coming novel THz applications. [ABSTRACT FROM AUTHOR]

A dual-band gyrotron traveling wave amplifier (Gyro-TWT) can reduce the size, cost, and weight of a transmitter in dual-band radar and communication systems. In this paper, a dual-mode input coupler for K/Ka dual-band gyrotron traveling wave amplifier (Gyro-TWT) is designed. This structure is composed of two different types of input couplers, one is the coaxial input coupler for the Ka-band TE 2 , 1 Gyro-TWT and the other is a Y-type input coupler for the K-band TE 1 , 1   Gyro-TWT. For reducing the backward wave of the   TE 2 , 1 mode reflecting into the Y-type input coupler to influence the operating bandwidth, a Bragg reflector with a strong mode selective characteristic is inserted between these two couplers, which can make the reflection coefficient of the TE 2 , 1 mode better than −1 dB and the phase matched in the whole bandwidth, and the transmission coefficient of the TE 1 , 1 mode can reach better than −1 dB. Based on the simulation results, the −1 dB bandwidth of the Ka-band TE 1 , 0 ⬜ - TE 2 , 1 ◯ mode input coupler reaches 3.32 GHz and the −1 dB bandwidth of K-band TE 1 , 0 ⬜ - TE 1 , 1 ◯ mode input coupler reaches 3.15 GHz. The designed dual-mode input coupler has the advantages of broad bandwidth and low loss and can be well used in dual-band Gyro-TWTs. [ABSTRACT FROM AUTHOR]