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Mode density is very relevant for harmonic gyrotron cavity. Theoretical investigations suggest that quasi-optical confocal waveguide performs low mode density and good mode-selective character. By selecting the appropriate mode and optimizing the cavity parameters, the quasi-optical confocal cavity is suitable for high-harmonic terahertz gyrotron w...
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... Utilizing a confocal waveguide as the interaction structure for a gyrotron was first proposed at MIT, and experimentally demonstrated later by a 140 GHz fundamental gyrotron oscillator [22] and a 140 GHz gyrotron traveling wave amplifier (gyro-TWA) [23]. As for the harmonic gyrotron, a 0.4 THz gyrotron with a confocal cavity was developed at TRC-UESTC and, experimentally, achieved an output power of 6.44 kW operating at the second cyclotron harmonic [24]. ...
... Under the geometrical optical approximation for a high frequency wave, the membrane function for the transverse electric (TE) mode in the open waveguide can be obtained by solving the Helmholtz equation in the elliptic coordinate system [24]. The numerically calculated results of the electron field distribution for the TE06 mode are shown in Figure 1. ...
... As shown in Figure 2, a TE06 mode frequency-tunable quasi-optical cavity for a high-power sub-THz gyrotron is designed and studied in this paper. This cavity is similar to the 0.4 THz second harmonic confocal cavity reported previously [24]. The mirror radius in the straight section is set to 4.20 mm, corresponding to a cut-off frequency of 223.06 GHz for a TE06 mode under a rigorously confocal situation. ...
Motivated by some emerging high-frequency applications, a high-power frequency-tunable sub-THz quasi-optical gyrotron cavity based on a confocal waveguide is designed in this paper. The frequency tuning characteristics of different approaches, including magnetic field tuning, mirror separation adjustment, and hybrid tuning, have been investigated by particle-in-cell (PIC) simulation. Results predict that it is possible to realize a smooth continuous frequency tuning band with an extraordinarily broad bandwidth of 41.55 GHz, corresponding to a relative bandwidth of 18.7% to the center frequency of 0.22 THz. The frequency tunability is provided by varying the separation distance between two mirrors and correspondingly adjusting the external magnetic field. During the frequency tuning, the output power remains higher than 20 kW, which corresponds to an interaction efficiency of 10%. Providing great advantages in terms of broad bandwidth, smooth tuning, and high power, this research may be conducive to the development of high-power frequency-tunable THz gyrotron oscillators.
... 9 In recent years, special attention has been paid to the gyrotrons with confocal resonators. [9][10][11] In addition to single confocal resonators formed by one pair of mirrors, 9-11 also double confocal resonators formed by two pairs of confocal mirrors attracted some interest. 12,13 Cross-sections of the interaction space of such gyrotrons are shown in Fig. 2 reproduced from Ref. 13. ...
... Note that in a number of studies (see Refs. 7,18 and references therein), the theory of quasi-optical gyrotrons was developed by using the general theory of conventional axis-symmetric gyrotrons, but under certain restrictions on the geometry of electron beams. The studies [9][10][11][12]19 were focused on consideration of annular electron beams only. The formulation presented below is applicable to devices with an arbitrary transverse geometry of the beams and resonators. ...
In recent years, in addition to the development of traditional gyrotrons with an axially symmetric interaction space, there has been strong interest in developing gyrotrons, whose interaction space is not axially symmetric (e.g., gyrotrons with confocal resonators). The theory of such gyrotrons is presented in this paper. First, equations describing such gyrotrons in the cold-cavity approximation are formulated. Then, the linear theory is developed, which is followed by a simple, single-mode, nonlinear theory. After that, the theory describing the mode interaction in such gyrotrons is presented. Then, an alternative concept of gyrotrons with single and multicavity confocal resonators and sheet electron beams is proposed. This paper also includes the analysis of diffractive losses in confocal resonators and the estimates for curling of the ends of sheet beams of gyrating electrons caused by the space charge forces.
... The intermediate frequency (IF) signal output from the mixer was recorded by an Agilent DSO80604B oscilloscope with a standard bandwidth of 6 GHz and the highest sampling rate up to 40 GSa/s. The detailed methods of pulse power evaluating and radiation frequency measuring have been introduced in [28]. ...
To explore the approach for generating high-power frequency-tunable subterahertz (sub-THz) to THz radiation, a 220-GHz high-order axial modes (HOAMs) gyrotron is developed, and the proof-of-principle experiment is presented in this paper. Experimental results demonstrate that driven by an electron beam of high voltage and high current, HOAMs are successfully excited in a long cylindrical cavity. By tuning the operating magnetic field, a mode transition between the first four axial modes was experimentally observed with a stepwise frequency tuning range of 0.79 GHz around 219 GHz, while a 0.58-GHz frequency tuning range via beam voltage tuning by operating in the first three axial modes. During the process of frequency tuning, the output power was kept not less than 0.45 kW, while the maximum output power was 3.80 kW with an output efficiency of 6%. These experimental results of mode transition and mode competition between axial modes in high-power millimeter-wave gyrotron could be beneficial for the development of frequency-tunable gyrotron toward some up-and-coming novel THz applications.
... 7 and 8. This dependence calculated with the use of spheroidal wave functions is presented in Fig. 11 in Ref. 7 (see also Ref. 9 where the losses are given as the functions of the mirror width 2a). ...
Low-voltage gyrotrons operating at near-terahertz frequencies are of great interest for numerous applications. Development of such gyrotrons in conventional configurations faces significant difficulties, especially in the case of operation at cyclotron harmonics. Some of these problems can be mitigated by using confocal resonators formed either by one pair of mirrors (a single configuration) or two pairs of mirrors (a double configuration). This paper describes the study of the effect of transverse nonuniformity of the resonator field on the gyrotron efficiency in both configurations. The study includes the analysis of operation in soft and hard self-excitation regimes.
... According to the geometrical optics method in [12], the membrane function ψ mn (x, y) for TE mn mode in quasi-optical waveguide can be expressed as [13]: ...
... In generalized gyrotron theory [15], the beam-wave interaction is characterized by the interaction coupling coefficient [13] ...
A design of high-order mode input coupler for 220-GHz confocal gyrotron travelling wave tube is proposed, simulated, and demonstrated by experimental tests. This input coupler is designed to excite confocal TE06 mode from rectangle waveguide TE10 mode over a broadband frequency range. Simulation results predict that the optimized conversion loss is about 2.72 dB with a mode purity excess of 99%. Considering of the gyrotron interaction theory, an effective bandwidth of 5 GHz is obtained, in which the beam-wave coupling efficiency is higher than half of maximum. The field pattern under low power demonstrates that TE06 mode is successfully excited in confocal waveguide at 220 GHz. Cold test results from the vector network analyzer perform good agreements with simulation results. Both simulation and experimental results illustrate that the reflection at input port S11 is sensitive to the perpendicular separation of two mirrors. It provides an engineering possibility for estimating the assembly precision.
A new high-
$\textit{Q}$
dielectric-loaded cavity is considered an alternative to conventional cavities for terahertz gyrotrons. The cavity consists of a two-layered distributed Bragg reflector (DBR) resonator jointed to standard input and output cavity sections. The DBR is formed by a cylindrical dielectric tube and a hollow layer. The main function of the DBR is to shield the metal wall of the resonator from the field of the selected operating mode. It is shown that this mode can be efficiently excited by the same electron beam in both the conventional uniform resonator and the DBR resonator. It is found that the mode conversion at the junctions between DBR resonator and standard cavity sections is low, resulting in more than 99% output purity of the operating mode. It is established that the characteristics of both the conventional and dielectric-loaded cavities exhibit similar robustness against dimensional errors. It is shown that, unlike the competing modes, the operating mode benefits from exceptionally low ohmic losses, which favors improved mode selection in the dielectric-loaded cavity. Using the cavity of an existing second-harmonic 0.5-THz gyrotron as an example, it is shown that sapphire DBR makes it possible to increase the ohmic quality factor of the operating mode and gyrotron efficiency by a factor of 8 and 6, respectively.
In this article, the design of a fourth-harmonic gyrotron with 1-kW-level output power at 1 terahertz (THz) is presented. A unique advantage of this design is that, with a well-optimized magnetic-cusp large-orbit electron gun, the designed gyrotron can operate from first-harmonic to fourth-harmonic in multiple discrete bands by varying the confining magnetic field strength. By carefully balancing the competing modes, the gyrotron can be tuned to operate at six candidate modes, including TE
<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1,2</sub>
for fundamental harmonic, TE
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and TE
<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2,4</sub>
for second-harmonic, TE
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and TE
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for third-harmonic, and TE
<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4,8</sub>
for fourth-harmonic interactions. As the main operating mode, the TE
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can generate a peak output power of 1.68 kW, with an output efficiency of about 2.1%, and a magnetically controlled frequency tuning range of about 1.5 GHz around 1 THz. The impact of the longitudinal nonuniformity of the cavity at high-harmonic interaction was also studied. It shows that a radial tolerance of several micrometers will significantly elevate the start-oscillation current and deteriorate output performance. This design is to produce a THz source with ultra-wideband tuning capability ranging from the submillimeter-wave to 1-THz bands.
The low-voltage compact gyrotron is suitable for industrial applications. However, the beam-wave interaction efficiency is low in conventional low-voltage gyrotron. To improve the whole tube efficiency, a compact depressed collector is introduced in developing a 75 GHz low-voltage compact gyrotron. The compact depressed collector is directly connected to the output waveguide. It is grounded and isolated with the cavity by a ceramic ring which is easy to be connected with the application system. The design of the original tube electron beam voltage and electron beam current are 10 kV and 1.2 A. In the particle-in-cell (PIC) simulation, the operating mode is TE0,1 and the generated power is 1.2 kW operated at the frequency of 75.5 GHz, which corresponds to an electron efficiency of 10%. When the depressed collector is performed and the electron reflux is under 5%, the efficiency of the whole tube can reach 30%, and when the reflux rate is controlled at about 15%, the efficiency of the whole tube can reach 50%. The dissipation power would be sufficiently reduced.
