Cavity Gyrotron Research Articles

Smooth Wideband Frequency Tuning in Low-Voltage Gyrotron With Cathode-End Power Output

The possibility of a smooth electronic frequency tuning for medium-power gyrotrons operated at the fundamental cyclotron resonance has been demonstrated in the numerical simulations. The use of a low operating voltage allows a drastic increase in the gyrotron operation efficiency at the higher axial modes and, hence, in the bandwidth of continuous frequency tuning. A gyrotron cavity with power output in the direction of the cathode end can provide a smoother power versus frequency dependence in the gyro-backward wave oscillator regime than a cavity with the conventional output in the direction of the collector. Simulations for a 2-kV gyrotron with a frequency of about 264-GHz demonstrate 3-GHz full-width at half-maximum frequency tuning with a power level of 15 to 30 W, which are attractive parameters for spectroscopic applications.

Electron Gun and Output Coupling System for a 220-/251.5-GHz, 2-MW Triangular Corrugated Coaxial Cavity Gyrotron

In this paper, design studies of a common electron gun and output coupling system for the dual regime operation of a 2-MW coaxial cavity gyrotron are presented. The operating frequencies (220/251.5 GHz) are selected, such that the proposed gyrotron can be used for plasma heating applications in the future experimental fusion reactors. A coaxial magnetron injection gun (MIG) is designed for the generation of hollow electron beam with the desired parameters. Initial geometry of the electron gun is determined through our in-house code Gyrotron Design Suite version 4.1 2016 (GDSv4.1 2016). The MIG is further optimized and simulated through the beam trajectory code ESRAY. A nonlinear taper (NLT) is designed with maximum transmission efficiency at both the operating frequencies. A quasi-optical launcher (QOL) is designed and numerically optimized through the commercial software, launcher optimization tool. The designed launcher efficiently converts both the operating modes into Gaussian-like mode. A single-disk RF window is designed for the extraction of high-power output beam from the gyrotron at their respective operating frequencies. The design of the NLT and the RF window are carried out through GDSv4.1 2016.

RF Behavior of a 220/251.5-GHz, 2-MW, Triangular Corrugated Coaxial Cavity Gyrotron

In this paper, RF behavior studies of a dual regime coaxial cavity gyrotron designed for electron cyclotron resonance heating and current drive of magnetically confined plasmas in the future fusion reactors are presented. Considering all the design constraints, the mode pair is chosen as TE48,30 and TE55,34 for operation at 220 and 251.5 GHz, respectively. The interaction circuit is initially designed through the cold cavity design. A triangular corrugated insert offers good mode selection and also reduces the localized heating problem. Single mode computations are carried out to optimize the beam parameters for the maximum efficiency of the chosen mode pair. Time-dependent multimode simulations are carried out to conform the power in the desired mode pair and the possibility of power in the competing modes. Start-up analyses are performed before and after space-charge neutralization with nonuniform magnetic field using nominal electron beam parameters obtained from magnetron injection gun calculations. These studies ensure that the continuous wave operation of a coaxial cavity gyrotron is possible with the output power of ≈2 MW with the chosen mode pair.

Magnetically shielded electron–optical system of a continuous gyrotron with an operating frequency of 24 GHz

It is shown that it is possible to use warm solenoids with copper winding screened with ferromagnetic shields in order to reduce the solenoid supply power by a factor of 1.8–2 while retaining the strength of the magnetic field in the working space for continuous gyrotrons operating in a frequency range of 24–30 GHz. The configuration of the shields as well as the shape and location of the correcting solenoids providing an extended (up to 10 wavelengths long) section of a uniform field in the region of the gyrotron cavity have been determined. It has been shown that introduction of an additional cathode coil into the magnetic system reduces the degree of nonadiabaticity of the magnetic field approximately by an order of magnitude and thereby makes it possible to use the magnetron injection gun for formation of a helical electron beam. Optimization of the configuration of the gun electrodes based on the trajectory analysis makes it possible to obtain an electron beam whose parameters are as good as parameters of the electron beams formed in classical adiabatic electron–optical systems of gyrotrons.

A 0.4-THz Second Harmonic Gyrotron with Quasi-Optical Confocal Cavity

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 without mode competition. In order to verify the theoretical analysis, a 0.4-THz second harmonic gyrotron has been designed and experimented. Driven by a 40-kV, 4.75-A electron beam and 7.51-T magnetic field, the gyrotron prototype could generate 6.44 kW of output power at 395.35 GHz, which corresponds to an electron efficiency of 3.4%. There is no mode competition between the second harmonic and fundamental observed in the experiments.

Effect of Cavity Ohmic Losses on Efficiency of Low-Power Terahertz Gyrotron

The problem of power degradation due to the dominant ohmic losses in gyrotron cavity is considered. As an example, the operating performances of the FU CW III gyrotron of Fukui University are investigated numerically. It is found that, for this gyrotron, there is a significant divergence between the classical and the self-consistent computations of the loss-induced power degradation. The reason is that the former computations ignore the effect of the cavity ohmic losses on beam-wave interaction efficiency.

Numerical Research on a 170-GHz Time-Dependent Multimode Coaxial Gyrotron With Outer Corrugation

In this paper, a coaxial gyrotron with outer corrugation is considered. Its multimode electron equation of motion is derived from the Lorentz equation, and its time-dependent amplitude and phase equation for the $\mathrm {TE}_{mn}$ mode is obtained from the wave equation. Using the time-dependent multimode beam–wave interaction equations, the time evolution processes of the time-dependent multimode coaxial cavity gyrotron with outer corrugation are investigated by numerical calculation. Results show that a 170-GHz coaxial gyrotron with outer corrugation can start up and operate stably at the $\mathrm {TE}_{34,19}$ mode and its competing modes are suppressed under different initial field amplitude conditions. By the calculation of the time-dependent multimode and single-mode coaxial gyrotron with outer corrugation, it is found that the output power and the output efficiency of the $\mathrm {TE}_{34,19}$ mode are 1.856 MW and 32.59%, respectively. These studies show the mode competition processes of the coaxial gyrotron with outer corrugation at the beginning start-up stage.

Multi-physics analysis of a 1 MW gyrotron cavity cooled by mini-channels

The interaction cavity of the European 170GHz, 1MW Continuous Wave (CW) gyrotron for ITER, which could also be water-cooled using mini-channels as recently proposed, experiences during operation a very large heat load (>15MW/m2) localized on a very short (<1cm) axial length. Such heat loads are typical for high power gyrotrons.As the thermal deformation of the cavity influences the electromagnetic field structure and consequently the gyrotron operation, the analysis of the cavity performance requires the simultaneous solution of the coupled thermal-hydraulic, thermo-mechanic and electro-magnetic fields. In this paper, the thermal behaviour of the cavity under nominal heat load is computed first by CFD. Then a 3D thermo-mechanical model of the cavity is developed, based on the temperature maps computed by CFD, to evaluate the resulting deformation of the inner cavity surface. Finally the deformation is used to compute the updated heat load coming from the electromagnetic field generated by the electron beam in the deformed cavity, which becomes the input for a new iteration of the thermal-hydraulic, thermal-mechanical and electromagnetic analyses. It is shown that this iterative procedure converges to a self-consistent heat-load/temperature-field/deformation-field picture in nominal operating conditions, without exceeding a temperature of ∼230°C on the inner surface of the cavity.

Full Wave Analysis of Plasma Loaded Coaxial Gyrotron Cavity With Triangular Corrugations on the Insert

The presence of plasma inside the gyrotron interaction structure alters the mode field. The presence of a magnetic field changes the plasma to an anisotropic media. Field analysis of a plasma loaded coaxial gyrotron cavity with triangular corrugations on the insert is undertaken using full wave approach. Modes inside a plasma loaded interaction structure have all six nonzero field components and hence are hybrid modes. Plasma modes (space charge modes), cyclotron modes, and waveguide EH and HE modes are the three families of modes that can exist in plasma loaded waveguide. Inside the gyrotron interaction structure, the cyclotron mode and the desired mode (HE) couple and this coupling leads to change in the eigenvalue of the modes. In this paper, a full wave approach has been used to analyze the dispersion relation and calculate eigenvalue of the desired HE mode.

Cylindrical Cavity with Distributed Longitudinal Corrugations for Second-Harmonic Gyrotrons

Metallic cavity with distributed longitudinal corrugations is proposed and studied for the use in a subterahertz second-harmonic gyrotron. The corrugated conducting walls are treated as a homogeneous surface with effective (averaged) anisotropic impedance. The theoretical study incorporates both single-mode and coupled-mode approaches. It is shown that the distributed longitudinal corrugations provide several-fold increase in the Q-value of the operating mode with respect to that of the fundamental competing mode at a reasonable level of ohmic losses and mode conversion in the gyrotron cavity.

A 170 GHz time-dependent multi-mode coaxial gyrotron with inner corrugation

In this paper, a time-dependent multi-mode coaxial gyrotron with inner corrugation is introduced. The time-dependent amplitude and phase equation of the mode TEmn and the multi-mode electron motion equation are derived from the wave equation and the Lorentz equation, respectively. Using these equations, the time evolution of the time-dependent multi-mode coaxial cavity gyrotron with inner corrugation is studied by numerical calculations. Researches show that a 170 GHz coaxial gyrotron with inner corrugation can start up and stably operate in the mode TE32,17 and its competition modes are depressed under the given operation parameters. It is found that the output power and the output efficiency of the coaxial gyrotron with inner corrugation are 2.205 MW and 40%, respectively. These studies display the mode competition process of the time-dependent multi-mode coaxial gyrotron with inner corrugation. It is helpful to further research on the beam-wave interaction of the gyrotron.

Full Wave Analysis of Coaxial Gyrotron Cavity With Triangular Corrugations on the Insert

Field analysis of coaxial gyrotron cavity with triangular corrugations on the insert is undertaken using full wave approach. The inclusion of insert offers the advantage of mode selectivity and reduction in voltage depression. The effect of insert can be enhanced by having corrugations on it. Mathematical expressions for eigenvalue and ohmic loss of triangular corrugated coaxial cavity have been derived using full wave approach specifically space harmonics method (SHM). Degeneration of SHM to surface impedance method under specific conditions is also discussed and analyzed. The inclusion of harmonics has a small effect on eigenvalue but the ohmic loss varies significantly. An interaction structure design for 170-GHz, 2-MW coaxial-cavity gyrotron with triangular corrugations on the insert is discussed. Mode competition and ohmic losses for such a structure have also been calculated.

The Nonlinear Designs and Experiments on a 0.42-THz Second Harmonic Gyrotron With Complex Cavity

Terahertz (THz) and sub-THz science and technology are very hot today. However, it is difficult to obtain high-power source of THz wave. Gyrotrons are well known as one of the most promising source for powerful THz radiation. In this paper, a 0.42-THz second harmonic gyrotron with complex cavity operating at TE17.4 is numerically simulated and designed by adopting self-consistent nonlinear theory with the first-order transmission line equations. Meanwhile, the mode competitions in the designed gyrotron have been investigated carefully by using time-dependent multimode nonlinear theory. It is proved that the operating mode can be first excited and other competing modes are well suppressed in the complex cavity. Based on these numerical simulations, the designed complex cavity gyrotron has been manufactured and measured in the Terahertz Science and Technology Research Center at the University of Electronic Science and Technology of China. The measured results show that the designed gyrotron can stably operate at TE17.4 with pulsed output power of 19.3 kW when the beam current ${I}_{{0}}$ is 4.4 A, beam voltage ${U}_{{0}}$ is 51 kV, and magnetic field ${B}_{{0}}$ is 8.03 T. The corresponding frequency and interaction efficiency are 421.645 GHz and 8.6%, respectively.

Simulations of Sectioned Cavity for High-Harmonic Gyrotron

A recently proposed method of decreasing the ohmic losses in long gyrotron cavities is thoroughly studied. Its workability is proved using various simulations. A fourth-cyclotron-harmonic submillimeter-wavelength gyrotron, which utilizes this method, is designed, and its ability to operate with high efficiency is numerically demonstrated.

Numerical Study of a Low-Voltage Gyrotron (“Gyrotrino”) for DNP/NMR Spectroscopy

Feasibility of a gyrotron for the dynamic nuclear polarization (DNP) purpose, integrated with nuclear magnetic resonance (NMR) spectrometer inside a single cryomagnet, is analyzed on the basis of numerical simulations. The necessary condition for DNP is matching of the gyrotrino and DNP frequencies. This imposes a strong restriction on the gyrotron operating voltage, which should be less than 2 kV. The most part of the uniform magnetic field region in the cryomagnet is occupied by a sample with NMR probe, so there is a very limited space for the gyrotron cavity. This dictates a number of peculiarities for the gyrotrino design, in particular, the diffraction power output from the cathode end of the cavity and collecting of a thin electron beam in a strong magnetic field. According to simulations, the gyrotrino operating at the fundamental cyclotron resonance with a voltage of 1.5 kV can provide an output power of 10–20 W at a frequency of 264 GHz, which is suitable for many NMR-DNP experiments.

CFD Analysis of Different Cooling Options for a Gyrotron Cavity

Three different pressurized subcooled water cooling options [hypervapotron, mini-channels (MCs), and meander flow drivers] for a full-size gyrotron cavity are developed and analyzed using the commercial computational fluid dynamics code STAR-CCM+, to guarantee a pressure drop below the maximum acceptable, and the compliance with manufacturing constraints. The full-size designs are then transposed to corresponding mock-ups to identify the most suitable candidate for an experimental test. The conjugate multiphase heat transfer problem is solved in the three mock-up geometries, aimed at comparatively assessing their thermal performance at increasing heat load on the target up to 24 MW/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The most promising option for an effective cooling of the cavity results to be the mini-channels.

Tuning photonic bands in plasma metallic photonic crystals

Introducing plasma in the background provides additional degrees of freedom for tuning dispersion curves of photonic crystals. 2D photonic crystals in triangular lattice arrangements offer more global bandgap regions and thus are of more interest for various applications. The dispersion characteristics of a two-dimensional plasma metallic photonic crystal (PMPC) in square as well as triangular lattice arrangements have been analyzed in this paper using the orthogonal finite difference time domain method. The dispersion characteristics of PMPCs for the range of r/a ratios and plasma frequencies for triangular lattice configuration have been analyzed. On introducing plasma in the background, the photonic bands of PMPC are shifted towards higher normalized frequencies. This shift is more for lower bands and increases with plasma frequency. The cut-off frequency was observed for both TE and TM polarizations in PMPC and showed strong dependence on r/a ratio as well as plasma frequency. Photonic bandgaps of PMPC may be tuned by controlling plasma parameters, giving opportunity for utilizing these PMPC structures for various applications such as fine-tuning cavities for enhanced light-matter interaction, plasmonic waveguides, and Gyrotron cavities.

Analysis of Plasma-Loaded Noncorrugated and Triangular Corrugated Coaxial Cavity

A field analysis of plasma-loaded noncorrugated coaxial cavity and plasma-loaded triangular corrugated cavities for megawatt-class gyrotrons has been undertaken. The presence of magnetic field in plasma changes its characteristics to an anisotropic media. In plasma waveguide, three families of modes can exist: plasma modes (space charge modes), cyclotron modes, and waveguide EH and HE modes. The modes present in the plasma are hybrid modes with all six nonzero field components. The presence of the low-density plasma in a gyrotron cavity leads to the coupling of these families of modes and causes a shift in the eigenvalue of the operating (desired) mode. In this paper, the dispersion relation for a plasma-loaded noncorrugated and triangular corrugated coaxial cavity has been derived. For the gyrotron case, where the cyclotron frequency of the plasma is approximately equal to the operating frequency and the operating frequency is near mode cutoff frequency in the cavity, the desired HE mode couples with the cyclotron mode. This coupling leads to shift in the eigenvalue of the HE mode.

Direct Voltage Depression Calculation of Arbitrary Electron Beams in Misaligned Coaxial Gyrotron Cavities

Coaxial-cavity gyrotrons for electron cyclotron heating in plasma experiments for nuclear fusion can operate with very high-order modes, having reduced mode competition and decreased voltage depression compared with hollow-cavity tubes. However, since exact alignment of coaxial insert and cavity wall can only be ensured up to a certain precision, the effects of misalignment must be properly understood. In this paper, an efficient method is presented to determine the voltage depression on beam electrons for arbitrary misalignment between cavity wall and insert, and for a beam with arbitrary shape and density distribution. The method has been verified using a 3-D code, and it can be generalized to some other geometries.

Investigation on Mode Competition Between the Fundamental and Second-Harmonic Modes in the Complex Gyrotron

Stable operation in a single mode is an important goal of high-power gyrotron. Both multimoding and switching into unwanted modes can lead to lower efficiency. Based on the multimode, multifrequency, and time-dependent fixed field theory, a numerical code has been written for simulating the mode competition in the complex cavity gyrotron operating at the second harmonic, which is confirmed to be correct by comparison with the self-consistent simulation. With the help of the code, the mode competition in the designed complex gyrotron with the frequency of 0.42 THz is simulated by tracking several competition modes without considering ohmic losses, whose influence is roughly estimated. Through mode competition simulations, it is verified that the complex gyrotron with gradually tapered cavity has a good capability to suppress the mode competition, especially the competition from the fundamental modes. Meanwhile, the simulations show that the single mode oscillation of the desired mode TE17.4 at 150-kW level can be expected when the optimal operating condition is that the beam voltage is 50 kV, the beam current is 6 A, and the applied magnetic field is 7.98 T.