Second-harmonic Gyrotron Research Articles

Experimental Study on Continuous Frequency Tuning of a Second-Harmonic 800-GHz Gyrotron

Experimental Study on Continuous Frequency Tuning of a Second-Harmonic 800-GHz Gyrotron

A Novel Complex Cavity for Second-Harmonic Subterahertz Gyrotrons: a Tradeoff Between Engineering Tolerance and Mode Selection

A Novel Complex Cavity for Second-Harmonic Subterahertz Gyrotrons: a Tradeoff Between Engineering Tolerance and Mode Selection

Phase-Locking of Second-Harmonic Gyrotrons for Providing MW-Level Output Power

We propose the method of providing megawatt (MW)-level output power in continuous wave (CW) or long-pulse second-harmonic (SH) gyrotrons using phase-locking by a weak external monochromatic signal. Typically, the power of harmonic gyrotrons is hindered due to excitation of spurious fundamental-harmonic modes at the front of the accelerating voltage and the beam current. Injection of a seeding signal at the operating SH mode (priming) can suppress the spurious generation during the startup process. Within the frame of a self-consistent multimode model, we demonstrate the applicability of this approach to provide ~1-MW radiation in a 230-GHz SH gyrotron with the high-order TE_34,14 operating mode which is standardly unattainable for harmonic operation.

A 250-Watts, 0.5-THz Continuous-Wave Second-Harmonic Gyrotron

To cover the so-called terahertz gap in powerful sources of coherent electromagnetic waves, a second-harmonic gyrotron operating in a 10 T liquid helium-free superconducting magnet has been designed, manufactured, and tested. With an operating voltage of 15 kV and a beam current of 0.6 A, the gyrotron generated coherent continuous-wave radiation at a frequency of 0.526 Terahertz with an output power of 250 Watts and an efficiency of 2.7%.

Multifunctional Coaxial Insert With Distributed Impedance Corrugations for Cavities of Broadband Tunable Second-Harmonic Gyrotrons

A coaxial insert with distributed impedance corrugations for cavities of subterahertz second-harmonic gyrotrons is investigated. The insert is designed to serve multiple functions: suppression of all first- and second-harmonic competing modes, attainment of high peak power, and widening of continuous frequency tunability. In the design study, advantages are taken of distributed shallow and deep corrugations of the coaxial insert. The multifunctional corrugated insert for a coaxial cavity is used to improve performance of a second-harmonic 527-GHz gyrotron. It is shown that the designed insert enables stable single-mode operation of this gyrotron with peak output power of 66 W and frequency bandwidth of 3 GHz. The sensitivity of the gyrotron performance to machining errors in the multifunctional corrugated insert is discussed.

Starting currents of modes in cylindrical cavities with mode-converting corrugations for second-harmonic gyrotrons

A self-consistent system of equations (known as single-mode gyrotron equations) is extended to describe the beam-wave interaction in a cylindrical gyrotron cavity with mode-converting longitudinal corrugations, which produce coupling of azimuthal basis modes. The system of equations is applied to investigate the effect of corrugations on starting currents of the cavity modes. For these modes, eigenvalues, ohmic losses, field structure, and beam-wave coupling coefficients are investigated with respect to the corrugation parameters. It is shown that properly sized mode-converting corrugations are capable of improving the selectivity properties of cylindrical cavities for second-harmonic gyrotrons.

Terahertz gyrotrons with inhomogeneous magnetic fields to suppress mode competition and enhance efficiency

Gyrotrons are promising radiation sources for bridging the terahertz gap. They are based on the instability of electron cyclotron maser, where the harmonic operation is generally necessary to alleviate the need for a strong magnetic field. Unfortunately, the performance of a harmonic gyrotron is extremely sensitive to mode competition and magnetic tuning. In this study, to achieve highly efficient and mode-selective gyrotrons, inhomogeneous magnetic fields are applied to introduce a specified longitudinal distribution of the detuning frequency between the terahertz wave and the gyrating electron beam. This detuning frequency has different influences on the oppositely traveling forward wave (FW) and backward wave (BW) inside the cavity, from which optimized magnetic-field profiles for FW-favored and BW-favored interaction circuits are generalized accordingly. It is proposed that a negatively tapering magnetic field converts the energy-transfer rate of the FW interaction into a positive value, leading to highly efficient FW interaction. By contrast, a positively tapering magnetic field reduces the detuning frequency of BW interaction and extends its effective length. By controlling the detuning frequency, a scenario of suppressed mode competition is proposed in a 330-GHz second-harmonic gyrotron. A universal understanding of the influence of an inhomogeneous magnetic field—i.e., the detuning frequency—on the interaction dynamics would help to develop efficient and broadband tunable terahertz gyrotrons.

Mode-Converting Corrugations for Cavities of Second-Harmonic Gyrotrons with Improved Performance

Mode-converting longitudinal corrugations are used as a means of improving the selectivity properties of cavities for second-harmonic gyrotrons. As an example, 100-kW 0.3-THz second-harmonic gyrotron is considered. For the operating second-harmonic mode and most dangerous first-harmonic competing modes, the eigenvalues, ohmic losses and beam-wave coupling coefficients are investigated with respect to dimensions of a corrugated cavity. The most optimal parameters are found for a gyrotron cavity with mode-converting corrugations, which ensure the widest range of a single mode operation for the 0.3-THz second-harmonic gyrotron. It is shown that, in this range, the gyrotron output power can be increased up to 180 kW. It is found that output mode purity of the 0.3-THz second-harmonic gyrotron falls off due to mode-converting corrugations, which induce undesirable coupling of the operating mode with neighboring Bloch harmonics in the output section of the gyrotron cavity.

Mode Discrimination by Lossy Dielectric Rods in Cavities of Second-Harmonic Gyrotrons

The influence of a coaxial dielectric rod on eigenvalues, ohmic losses, transverse field structure, and beam-wave coupling coefficients is investigated for TE modes of a gyrotron cavity. It is shown that such dielectric insert, when made from a moderate-loss material, results in strong attenuation of all cavity modes, with the exception of those having caustic radii much larger than the insert radius. It is proposed to employ such dielectric loading for selective suppression of competing modes in cavities of second-harmonic gyrotrons. The high performance and flexibility of the proposed method of mode discrimination are demonstrated for the example of the cavity of the high-power 0.39-GHz second-harmonic gyrotron developed at the University of Fukui. In addition, some fascinating capabilities enabled by coaxial inserts made of ultralow-loss dielectrics are discussed.

Demonstration of a Selective Oversized Cavity in a Terahertz Second-Harmonic Gyrotron

A method of increasing the selectivity of highly-oversized open cavities is experimentally demonstrated in the electron cyclotron maser (gyrotron) operating at the second cyclotron harmonic in a high transverse mode (presumably, TE58,13) of a circular cylindrical cavity at a frequency near 1.2 THz in the magnetic field of 24 T. This method is purely electrodynamical and based on a simple deformation of the cavity wall. The proposed approach paves the way for increase of the gyrotron frequencies and for enhancing the selectivity of open cavities in other applications.

Improved Mode Selection in Coaxial Cavities for Subterahertz Second-Harmonic Gyrotrons

A coaxial metal rod with partial dielectric coating is considered as a means for efficient suppression of all volume competing modes in cavities for second-harmonic gyrotrons operated in whispering gallery modes. The rod radius is selected small enough to have only a slight effect on operating mode, which therefore remains insensitive to fabrication tolerances and a misalignment of the coaxial insert. By contrast, for the competing modes such a rod is shown to reduce the effective cavity length, thereby greatly increasing the starting currents. Such a method of mode selection is demonstrated to be more versatile, when compared to that provided by a tapered coaxial conductor. The advantage of a dielectric-coated coaxial insert is illustrated by the example of a cavity for a 100-kW 300-GHz pulsed gyrotron operated in the second-harmonic mode.

Investigation of the Alignment Method for High-Frequency Gyrotrons

The successful operation of the gyrotron strongly depends on the proper alignment of the gyrotron with the magnet. The difficulty of the alignment will increase as the frequency becomes higher, especially for the terahertz pulsed gyrotron. In this article, we describe an easily implemented alignment method in detail. A triaxial adjusting flange was designed to align the gyrotron. The alignment of the gyrotron can be identified by the fluorescent effect of the electron beam on the sapphire output window, which will be captured by triggering a charge-coupled device camera. The affine transformation was also used to correct the distorted image. The validity of the proposed alignment method is verified via a prototype of an 800-GHz second-harmonic pulsed gyrotron.

Coaxial Cavity With Stepped Inner Conductor for a Sub-Terahertz Second-Harmonic Gyrotron With Broadband Continuous Frequency Tuning

It is found that a cavity design strategy proposed recently to enhance the continuous frequency tuning band of a sub-terahertz second-harmonic gyrotron aggravates the problem of competition between the operating mode and the first-cyclotron-harmonic (fundamental) modes. A stepped coaxial inner conductor is proposed in this article as a means for a selective suppression of the most dangerous fundamental competitors. It is shown that, with a proper choice of the operating mode, the effect of the inner conductor is only distinct for unwanted competing modes. In this case, the gyrotron performance is slightly sensitive to the longitudinal profile of the coaxial insert. Therefore, this profile can be selected in such a way as to reduce the effective cavity length for the competing modes, together with their diffractive quality factors. As a consequence, starting currents of these modes can be selectively increased. This is illustrated by a realistic design of a 0.4-THz second-harmonic gyrotron with 3-GHz continuous frequency tuning.

Effects of Different Magnetic Field Profiles on Output Power and Efficiency of a Second-Harmonic Gyrotron

As the magnetic field used in nuclear magnetic resonance (NMR) research increases (up to 23 T), the irradiation frequency of the electromagnetic field used for dynamic nuclear polarization (DNP) needs to be in the terahertz band (140–600 GHz). This article analyzed and compared the influence of different external magnetic field profiles on start current and beam–wave interaction of a 394-GHz gyrotron. As the simulation results show, the minimum start current corresponding to the linear and parabolic external magnetic field profile is lower than that corresponding to the uniform magnetic field. The minimum start current in the case of parabolic magnetic field profile is a little lower than that in the case of a linear magnetic field profile. Based on the nonlinear self-consistent calculation, it is observed that the optimum output powers in both linear and parabolic magnetic field profile cases are nearly equal when the beam current is 0.25 A and both of them are larger than the optimum output power when the magnetic field is uniform. In the calculation of orbital efficiency <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\eta _{\bot } $ </tex-math></inline-formula> , the maximum orbital efficiency in the soft-excitation region in the inhomogeneous magnetic field profile cases is larger than that in the case of uniform magnetic field profile. After comprehensive consideration of beam voltage, beam current, and efficiency, the final operation point is located in the place where the beam current is 0.25 A and the beam voltage is 15 kV. The output power is more than 300 W and meets the requirements of DNP-NMR experiment.

Selectivity Properties of Cylindrical Waveguides with Longitudinal Wall Corrugations for Second-Harmonic Gyrotrons

Spatial harmonic method is applied to investigate selectivity properties of longitudinally corrugated waveguides for potential application in second-harmonic gyrotrons. The effect of corrugations on frequencies, ohmic losses, and mode conversion of guiding TE modes is studied in details. Numerical results are presented for operating second-harmonic and competing first-harmonic modes of a 0.4-THz gyrotron with corrugated RF structure. It is shown that longitudinal wall corrugations of proper dimensions can ensure increase in ohmic losses and decrease in beam-wave coupling strength for the first-harmonic modes, while their effect on the operating mode is only slight. This demonstrates improved selectivity properties of corrugated waveguides for second-harmonic gyrotrons.

MODE SELECTION BY OHMIC LOSSES IN LONGITUDINALLY CORRUGATED CAVITIES OF SUB-THz SECOND-HARMONIC GYROTRONS

Ohmic wall losses are proposed as a means for improving the mode selection in a cylindrical gyrotron cavity with longitudinal wedge-shaped corrugations. Such losses depend on mode frequency and geometrical parameters of the corrugations. For cavity of the 0.4-THz second-harmonic gyrotron we find the corrugation depth, which corresponds to maximum ohmic losses of the competing modes excited at the first (fundamental) cyclotron resonance, as well as to reasonably low losses of the operating mode. For this depth, we determine the number of corrugations and their width, which ensure the maximum enhancement of the ohmic wall losses of the fundamental modes with respect to those of the operating second-harmonic mode, together with minimum conversion of the operating second-harmonic mode to higher Bloch harmonics. Parameters of the corrugations in hand are practicable.

Electromagnetic Modeling of a Complex-Cavity Resonator for the 0.4-THz Second-Harmonic Frequency-Tunable Gyrotron

Complex-cavity resonator with mode conversion is a promising way of improving mode selection properties of a terahertz gyrotron. In this paper, enhancement of the frequency tunability of the second-harmonic 0.4-THz gyrotron by using the complex-cavity resonator is considered. Results of detailed numerical study of the cold-cavity electromagnetic parameters are presented. Using the results of cold-cavity simulations, start-oscillation currents for different modes are calculated.

A study of a terahertz gyrotron traveling-wave amplifier

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&amp;lt; 4.2 A; meanwhile, the second-harmonic TE8,9-mode gyro-TWA can yield a stable output power. The amplification of waves in a gyro-TWA depends on the lengths of the sections. The simulated results, in particular, show that the output power depends on the length of the interaction section, in contrast to those of the drive or sever sections. A stable second-harmonic gyro-TWA is predicted to yield a peak output power of 6.9 kW at 888.7 GHz with an efficiency of 8%, a saturated gain of 45 dB and a bandwidth of 0.7 GHz for a 30-kV, 3-A electron beam with an axial velocity spread of 10%.

Startup and mode competition in a 420 GHz gyrotron

In the experiments of a 420 GHz second-harmonic gyrotron, it is found that the electron beam voltage and current ranges for single mode operation of TE17.4 are slightly narrower than those in the simulation. To explain this phenomenon, the startup scenario has been investigated with special emphasis on mode competition. The calculations indicate that the decreases of the operating ranges are caused by the voltage overshoot in the startup scenario.

The influences of beam quality and Ohmic loss on the beam-wave interaction in a 420 GHz second-harmonic complex-cavity gyrotron

Recently, a 0.42 THz second-harmonic gyrotron with complex cavity has been investigated and measured in the Terahertz Research Center. The experimental results show that the designed gyrotron can operate stably at the candidate mode TE−17.4 with the pulsed power of 19.3 kW, corresponding to an efficiency of 8.6%. However, the measured output power and efficiency are much lower than the theoretical values. In this paper, the influences of beam quality and Ohmic loss on the beam-wave interaction in the designed complex-cavity gyrotron are analyzed explicitly by a nonlinear, time-dependent, multi-mode code. The numerical simulations show that in the low-beam-voltage region, the calculated powers perfectly fit with the experimental results when the electron beam width is 0.195 mm, the electron velocity spread is 6% and the Ohmic quality factor of TE−17.4 is about 75000. Meanwhile it is also found that competing modes are well suppressed although considering the beam quality and Ohmic loss, which reproduces the experimental observation of the high-power single-mode oscillation of the TE−17.4 mode as the design mode.