Gyrotron Efficiency Research Articles

Experimental 35 GHz Gyrotron Efficiency Enhancement with Magnetic Field Taper in Fundamental Harmonic

Experimental 35 GHz Gyrotron Efficiency Enhancement with Magnetic Field Taper in Fundamental Harmonic

Upgradation of 82.6GHz ECRH system for SST-1 and Aditya-U

In SST-1 and Aditya-U Tokamaks, two ECRH systems (42GHz-500kW and 82.6GHz-200k) are used to carry out various experiments related to ECRH assisted plasma start-up and heating at fundamental and second harmonic. The SST-1 tokamak is operated at 1.5T magnetic field and 42GHz ECRH system has been extensively for plasma start-up and heating. The 82.6GHz ECRH could not be used thoroughly due limitation in power as 200kW power is not enough for breakdown in SST-1 at second harmonic. The original 82.6GHz-200kW ECRH system is being upgraded in terms of ECRH power to carry out experiment at higher power. The 82.6GHz-200kW Gyrotron has been upgraded to 400kW power. The Gyrotron has been tested successfully at factory and delivers 400kW power at 48kV cathode voltage and draw around 20A cathode current, the efficiency of Gyrotron is around 42%. The 82.6GHz Gyrotron will be commissioned at IPR and the total ECRH power 0.9MW (0.5MW at 42GHz and 0.4MW at 82.6GHz) will facilitate to carry out ECR heating experiments simultaneously at fundamental and second harmonic. The paper presents the recent test results of 82.6GHz-400kW Gyrotron and discusses the experimental plan with upgraded ECRH system.

Fabrication and assembly of the gyrotron multi-stage depressed collector prototype at KIT

In this paper details of the fabrication and assembly of the first Multi-stage Depressed Collector (MDC) prototype developed at KIT for megawatt-class gyrotrons are presented. Utilizing the E×B drift concept for electron trajectory separation, the cylindrical Short-Pulse (SP) MDC prototype features a design compatible with applications in different fusion gyrotrons at KIT, for W7-X, ITER, and DEMO. The fabrication process includes inner electrodes out of copper-chromium-zirconium (CuCr1Zr) with a triple helix isolation design, modular vacuum housing consisting of four parts, and additional external coils for electron beam confinement. Detailed assembly procedures are provided for two configurations: the 170 GHz 2 MW coaxial cavity gyrotron and the W7-X upgrade SP gyrotron. The successful manufacturing of the modular collector and a robust design for vacuum tightness are demonstrated. The prototype is now primed for verification in the KIT FULGOR test stand with the W7-X gyrotron for validation of the E×B drift concept and improvement of the gyrotron efficiency. This comprehensive work bridges theoretical concepts with practical implementation, offering insights crucial for refining and advancing MDCs for megawattclass gyrotrons for fusion applications.

Evaluation of emission inhomogeneity of gyrotron scandate cathodes

The efficiency of gyrotrons is significantly affected by the emission inhomogeneity of the cathode. The current-voltage characteristics (CVCs) of the cathode, which can be used for the evaluation of emission inhomogeneity, is a promising method for daily diagnosis of gyrotrons. In this paper, the CVC evaluation method considering a semiconductor emission model is proposed to evaluate the emission inhomogeneity of the scandate cathode whose emission characteristics are different from the traditional M-type cathode. The experiments of a scandate-cathode gyrotron that has been run for two months, as well as a scrapped scandate-cathode gyrotron are carried out on a specially designed data acquisition (DAQ) system. The CVCs obtained in the experiments are fitted by using genetic algorithm. The calculated emission inhomogeneity of the cathode successfully reveals the efficiency change of the gyrotron to be diagnosed. The Miram curves are measured to calculate the Longo model and the practical work function distribution (PWFD) curves considering the semiconductor emission model. These results can also verify the rationality of the proposed evaluation method of scandate cathodes.

Траекторный анализ в коллекторе с многоступенчатой рекуперацией энергии для прототипа гиротрона DEMO. Часть I. Идеализированное распределение магнитного поля

The results of modeling of collector for the gyrotron prototype being developed for the DEMO project are presented. A trajectory analysis in a collector with a 4-stage recovery of the residual beam energy based on the method of spatial separation of electrons in crossed azimuthal magnetic and axial electric fields was performed. In this part of the study, the formation of the azimuthal magnetic field was carried out using a conductor located on the axis of the device. The study was performed for a spent electron beam with a velocity and coordinate distribution of particles close to those obtained in experiments with powerful gyrotrons. Due to a thorough choice of the geometry of the collector sections, a total gyrotron efficiency of more than 80% was achieved, which is close to maximum efficiency with perfect separation of fractions of an electron beam with different energies. The data obtained will be used for development of a toroidal solenoid designed to create an azimuthal magnetic field.

Development of a Collector with Multistage Recuperation for Gyrotron

A four-stage collector system is developed for the experimental gyrotron at the Peter the Great St. Petersburg Polytechnic University based on spatial separation of electrons with different energies in crossed electric and magnetic fields. The conditions of use of this system for the efficient recuperation of electron residual energy in a spent beam are determined. The numerical simulation of the recuperator and the analysis of the electric and magnetic field distribution in the gyrotron collector region are accomplished. Theoretical estimations and trajectory analysis of the helical electron beam show that the system developed provides the electron residual energy recuperation necessary to achieve total gyrotron efficiency over 70%.

Design and simulation investigations of high-power millimeter-wave gyrotron

ABSTRACTIn this paper, a high power millimeter wave gyrotron cavity was modelled and studied for its beam-wave interaction, beam-misalignment, and thermal issues. The sub-assemblies including the magnetron injection gun (MIG), RF output window, and electron beam collector were investigated. The MIG has produced an annular electron beam with transverse velocity spread less than ∼4%. The Particle–In–Cell simulation predicted an RF output of 470 kW in TE10,4 mode at ∼139.52 GHz for a DC drive of 80 kV, 20 A. The temperature on the inner, outer surfaces of the cavity and the corresponding average radial expansion of the cavity were calculated. Due to the radial expansion of the cavity, the resonant frequency and RF output power has changed to ∼0.514 GHz and ∼50 kW, respectively. Finally, the overall efficiency of the present gyrotron was improved from ∼29% to ∼47% by using a single–stage depressed collector.

GYROTRONES WITH CONE-SHAPED RESONATORS

In this paper we compared the efficiency of the cylindrical, conical, and biconical types of gyrotron resonators. Based on the results of comparing the three studied variants of gyrotron profile, it was concluded that the regular-type profile is the least efficient. This type of a resonator made it possible to achieve the level of efficiency of only 23 %, which can be increased in the regular-waveguide gyrothrons only through several modes or by recovering the electrons on the collector. The medium efficiency option is the biconical profile of the resonator. Its efficiency accounted for 42 %. Through a scientific study we revealed an increase in the efficiency for gyrotrons with conical resonators from 23 to 50 % in the TE01 wave. It is worth mentioning that obtaining such efficiency requires phase grouping of the electrons in an increasing high-frequency field by means of an electromagnetic field with further selection of energy from the electron beam in a strong decaying electromagnetic field. The efficiency of 50 % exceeds significantly that of a gyrotron with a regular cavity profile of ~30 %. The gyrotron efficiency for a waveguide profile with a conical resonator and with recovery on the collector can reach 80 %. To carry out the calculations, the KEDR software package was used, and the optimization of the gyrotron parameters, in particular, was carried out using the GYRO-K software. This software has several advantages over other similar options based on the “PIC” code. GYRO-K makes it possible to obtain a high convergence rate when solving boundary value problems, as well as to solve the problem of optimizing the waveguide profile of gyroresonance devices with an acceptable computational burden. Conical cavity gyrotrons can be widely used in industry to create effective gyrotrons for spectroscopy, diagnostics of various media, and for technological needs.

Design of second harmonic terahertz gyrotron cavity based on double confocal waveguide

Quasi-optical confocal cylindrical waveguide possesses a lot of good characteristics, such as big power capacity and low mode density, which can suppress the mode competition in beam-wave interaction. So quasi-optical waveguide has a great advantage in designing high harmonic terahertz gyrotrons. For the reason that part of electron beams located in a region of weak field intensity play a limited role in beam-wave interactions, the beam-wave interaction is not efficient in confocal cavity. Motivated by enhancing the beam-wave interaction efficiency of quasi-optical gyrotron, we propose a novel terahertz harmonic gyrotron cavity with double confocal waveguide in this paper. The transverse field distribution and the mode spectrum in double confocal waveguide are analyzed and presented. A 330 GHz second harmonic gyrotron with double confocal cavity is designed, theoretically analyzed and simulated by using a particle-in-cell (PIC) code. The results obtained for double confocal cavity are compared with the results for single confocal cavity, and the physical mechanism of beam-wave interaction enhancement in double confocal cavity is discussed. Theoretical results show that the double confocal cavity is able to increase the coupling strength of beam-wave interaction, thus, to improve the output power and the interaction efficiency of quasi-optical gyrotron. The PIC simulation results suggest that a high-order waveguide mode in double confocal cavity can steadily interact with the high harmonic cyclotron mode of electron beam without mode competition. Driven by a 40 kV, 2 A electron beam with a guiding center radius of 1.65 mm and velocity ratio equal to 1.5, output power of 9.9 kW at 328.93 GHz can be generated in the designed double confocal cavity. The beam-wave interaction efficiency increases from 5.3% in single confocal cavity to 12.4% in dual confocal cavity under the same operation parameters. The double confocal cavity has great potential applications in terahertz band. Moreover, this study indicates that the eigen mode in double confocal waveguide is a kind of hybrid mode superimposed by two independent single confocal waveguide modes. This mode characteristic will be beneficial to designing a multifrequency gyrotron oscillator operated in two modes and two cyclotron harmonics, simultaneously, with a single electron beam used, which provides a new possibility to develop the novel terahertz radiation source.

Study of the beam-wave interaction of a 0.34-THz confocal waveguide gyro-TWT with confocal beam

In this paper, in order to enhance the efficiency of the confocal gyrotron travelling wave tube (confocal gyro-TWT), the confocal beam is first introduced based on the field distribution of the operating mode in the confocal waveguide. Besides, a confocal gyro-TWT with a confocal beam operating at 0.34 THz is simulated and designed based on linear and nonlinear theories. It is verified that the confocal beam can enhance the interaction efficiency and output power significantly through simulation. In addition, the effects of electron beam parameters on output power are also analyzed in this paper. The simulation results indicate that the confocal gyro-TWT can operate at TE06 stably with an output power of 17.3 kW when the beam voltage is 60 kV, beam current is 2 A, input power is 40 mW, and transverse velocity spread is 5%. The corresponding frequency and interaction efficiency are 0.34 THz and 14.4%, respectively.

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.

Energy Spectra of Electron Beams and the Possibility of Double-Stage Energy Recuperation in a Double-Beam Terahertz Gyrotron

We study theoretically the possibilities of double-stage energy recuperation in a double-beam terahertz gyrotron. Numerical modeling, which was performed for a gyrotron producing radiation with a frequency of 0.78 THz and operating at the second cyclotron-frequency harmonic at the TE8.5 mode, demonstrates the possibility of spatial and energy separation of electron beams. This makes it possible to implement a double-stage recuperation scheme. The found energy spectra of electrons suggest an increase in the gyrotron efficiency by 3.5–4 times.

Overview on recent progress in magnetron injection gun theory and design for high power gyrotrons

The magnetron injection gun (MIG) is one of the most critical subcomponents in gyrotrons. The electron beam, which has the primary role on the gyrotron operation, is generated and configured at this part of the tube. The electron beam properties determine the excitation mode in the cavity, the power of the generated microwaves and the gyrotron efficiency. The operation of MIGs could be influenced by several factors such as trapped electrons, manufacturing tolerances, roughness of the emitter ring, emitter temperature inhomogeneity, electron beam neutralization effect, etc. The influence of many of these factors on the electron beam quality has been systematically investigated during the last years. Several novelties have been proposed in order to limit the influence of these factors on the gyrotron operation. In particular, new design criteria have been proposed for the suppression of electron trapping mechanisms, a new type of the emitter ring has been proposed to minimize the influence of the manufacturing tolerances and edge effects on the beam quality, alternative MIG design approaches have been proposed, etc. An overview of all these works will be presented here.

Possibilities of increase of gyrotron efficiency via frequency locking by external signal

Possibility of efficiency increase of a gyrotron, which frequency is locked by an external monochromatic signal, is studied. Dependencies of orbital efficiency on cyclotron frequency resonance mismatch for different currents of the electron beam are found. It is shown that in multi-mode gyrotron there is a possibility of shifting of frequency-locking zone to the region of parameters corresponding to the optimal efficiency, so efficiency may be significantly increased compared to the free gyrotron.

Towards Advanced Fusion Gyrotrons: 2018 Update on Activities within EUROfusion

During the ongoing pre-concept phase (2014 – 2020) for a possible future European DEMOnstration Fusion Power Plant (DEMO) the activities within EUROfusion WP HCD EC Gyrotron R&amp;D and Advanced Developments are focusing on options for near-term solutions, and, at the same time, on long-term even more advanced options. The near-term target for DEMO is to realize pulsed operation. According to the current baseline it will probably use an EC system operating at 170 GHz and 204 GHz is being assessed, whereas the long-term target aims for steady-state operation and frequencies for current drive up to 240 GHz. Common targets for both are an RF output power per unit of significantly above 1 MW (target: 2 MW) and a total gyrotron efficiency of significantly higher than 60 %. Frequency step-tunability and multi-purpose/multi-frequency operation have to be considered. Those targets shall be achieved by considering the coaxial-cavity gyrotron technology and advanced technologies for key components (e.g. CVD diamond-disk Brewster angle window). Advanced simulation and test tools are complementing the research and developments. Gyrotron development is additionally supported by a significant investment into a new multi-megawatt long-pulse gyrotron test stand which is under final installation at KIT currently.

Investigation of tunable gyrotron on a cone-shaped waveguide

The results of calculations of a nonlinear model of a tunable frequency gyrotron on a cone-shaped waveguide and the main wave TE01 are presented. It is shown that the adjustment range can reach 2.8 %. To extend this band, it is necessary to lengthen the cone-shaped part of the waveguide without changing the angle of increase in the radius of the waveguide.The wave efficiency of a waveguide expanding along the axis is 21 % at a working frequency of 10 GHz. To achieve these parameters, it is necessary to divide the gyrotron electromagnet into two parts – the main electromagnet and auxiliary one, which has a limited length and can move along the waveguide. The second magnet can be made in the form of a set of individual electromagnets of limited length the set of electromagnets must fill the entire length of the cone-shaped waveguide. The fulfillment of this condition will allow to move the resonant magnetostatic field along the waveguide by switching the current in the coils of this set of electromagnets, which will exclude the mechanical movement of the auxiliary electromagnet. At a frequency of 200 GHz, the wave efficiency is reduced to 15 %, while the ohmic losses in the walls of the waveguide are 3 % of the power of the electron beam.The dependence of the gyrotron efficiency on the initial angular spread of electron velocities was investigated. It was concluded that the initial angular spread of the electron velocities has very little effect on the efficiency of the tunable gyrotron.The wave efficiency of a waveguide narrowing in length can reach 29 % at a frequency of 200 GHz, ohmic losses in the walls of a copper waveguide amount to 4 % of the power of the electron beam. Calculations have shown that a lamp backward wave gyrotron with a waveguide narrowing along the axis is more efficient than the version of the gyrotron traveling wave tube. However, in both cases, the synchronous value of the magnetostatic field must be displaced along the axis, depending on the required operating frequency, otherwise there occurs either a rearrangement of the electron beam or a return of the energy to the high-frequency field by the electron beam.

Possibility of Effective High-Frequency Generation in Low-Voltage Gyrotrons at the Second Cyclotron Harmonic

Despite the high theoretical values of the electron efficiency of the subterahertz gyrotrons working at cyclotron harmonics at low operating voltages, the achievement of acceptable output power levels in such devices is a significant challenge due to the mode competition, the necessity of producing an electron beam with a high current, high ohmic losses in the walls, and the required high accuracy of cavity manufacturing. To solve these problems, we analyze thoroughly variants of low-voltage gyrotrons with conventional cavities and the recently proposed variant of the sectioned cavity, and present a calculation of the electron-optical system for such cavities. It is shown that by ensuring micrometer accuracy of cavity manufacturing at a low operating voltage (5 kV), it is possible to achieve output efficiencies of up to 5% and a power of up to 100 W at frequencies of about 0.4 THz and higher at the second cyclotron harmonic.

Efficiency of the gyrotron with single and double confocal resonators

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.

Numerical Investigation on Spent Beam Deceleration Schemes for Depressed Collector of a High-Power Gyrotron

Single-stage depressed collector systems are widely used in high-power gyrotrons in order to increase the overall efficiency. The position where the deceleration of the beam takes place influences the collector efficiency. In this paper, several deceleration schemes are numerically investigated for the short-pulse prototype of the 170-GHz, 1-MW European gyrotron for International Thermonuclear Experimental Reactor. The results of this paper are in a qualitative agreement with the results of dedicated experiments. A simple idea is also proposed for the significant increase of the overall gyrotron efficiency without an important technological modification.

Gyrotron multistage depressed collector based on E × B drift concept using azimuthal electric field. I. Basic design

Multistage Depressed Collectors (MDCs) are widely used in vacuum tubes to regain energy from the depleted electron beam. However, the design of an MDC for gyrotrons, especially for those deployed in fusion experiments and future power plants, is not trivial. Since gyrotrons require relatively high magnetic fields, their hollow annular electron beam is magnetically confined in the collector. In such a moderate magnetic field, the MDC concept based on E × B drift is very promising. Several concrete design approaches based on the E × B concept have been proposed. This paper presents a realizable design of a two-stage depressed collector based on the E × B concept. A collector efficiency of 77% is achievable, which will be able to increase the total gyrotron efficiency from currently 50% to more than 60%. Secondary electrons reduce the efficiency only by 1%. Moreover, the collector efficiency is resilient to the change of beam current (i.e., space charge repulsion) and beam misalignment as well as magnetic field perturbations. Therefore, compared to other E × B conceptual designs, this design approach is promising and fairly feasible.