Coaxial Gyrotron Research Articles

A model of electron beam neutralization for gyrotron simulations

A numerical model is presented to describe the electron beam neutralization during long-pulse and continuous-wave operation in gyrotrons. This model has been implemented in the electrostatic, self-consistent electron optics code Ariadne. Using this model, the electron beam parameters in the cavity could be determined as a function of the level of neutralization. The electron beam was shown to be partially neutralized in the cavity, and the effect of neutralization on the beam properties was investigated for a cylindrical gyrotron. This study was extended to a coaxial cavity gyrotron, identifying two ion trapping regimes, which only had a minor impact on the beam parameters.

High-Frequency MW-class Coaxial Gyrotron Cavities Operating at the Second Cyclotron Harmonic

High-Frequency MW-class Coaxial Gyrotron Cavities Operating at the Second Cyclotron Harmonic

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.

Design of MW-Class Coaxial Gyrotron Cavities With Mode-Converting Corrugation Operating at the Second Cyclotron Harmonic

Design of MW-Class Coaxial Gyrotron Cavities With Mode-Converting Corrugation Operating at the Second Cyclotron Harmonic

Research on the Conductivity, Starting Current, Mode Competition of a 250 GHz, MW Coaxial Gyrotron With Double-Electron-Beam

Research on the Conductivity, Starting Current, Mode Competition of a 250 GHz, MW Coaxial Gyrotron With Double-Electron-Beam

Influences of distributed losses on start current, quality factor Q and frequency in multi-mode multi-frequency coaxial gyrotrons

The primary problem that gyrotrons encounter is mode competition during their development, especially multi-mode multi-frequency gyrotrons. In this paper, the influence of distributed losses on start current, quality factor Q and resonant frequency were studied. Four types of distributed losses are discussed, which each type can also increase start oscillation current of operating and competing modes. But the amplitudes of increase are different, which makes the start oscillation current of operating and competing modes further separated. By separating start oscillation currents, they suppress competing modes and enhance devices stability. The adverse effect of distributed losses is that they reduced Q value and raised resonant frequency slightly. But they have little effect on other performance of gyrotrons. The start oscillation current of multi-mode multi-frequency coaxial gyrotrons with distributed losses has the ability to suppress mode competition and improve stability of operation. So it is a new method of suppressing modes competition that deserves consideration.

Suppression of the Parasitic Oscillations in anX-Band Relativistic Coaxial Gyrotron Driven by an Intense Beam Current

In 100-MW level relativistic gyrotrons driven by explosive emission cathodes (EECs), significant parasitic oscillations are caused by the high beam current and the large beam velocity spread. In this study, the parasitic oscillations in an X-band coaxial relativistic intense current gyrotron were explored. The oscillations were suppressed in particle-in-cell (PIC) simulations through modifications of design. Lossy ceramics were inserted to suppress the parasitic modes in the beam tunnel. The beam collector was optimized to keep the electron beam from the cyclotron resonance region of the parasitic modes. In the 3-D PIC simulation of the whole tube, the output power of 35 MW and efficiency of 20.6% were achieved under the voltage of 300 kV, the beam current of 567 A, the pitch factor of 1.24, and the velocity spread of 25.6%. A single TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> mode operation was achieved in this relativistic coaxial gyrotron.

Simulation of Multimode Competition of a Coaxial Cavity Gyrotron With Multiple Electron Beams

In this article, the time-dependent multimode competition equations, the dependence of the starting current on the static magnetic field, and the current ratio of a coaxial cavity gyrotron with multiple electron beams are investigated according to the linear and nonlinear theories. By numerical calculation, the mode competition of a 170-GHz coaxial cavity gyrotron is studied when it has one, two, and three electron beams. Researches demonstrate that under the given magnetic field $B$ , electron beam voltage $U$ , and pitch factor ${\alpha }$ , a gyrotron with different electron beam numbers has different startup scenarios, which show that the gyrotron with three electron beams has potential in the selection of mode, the depression of mode competition, and the increase of output power. Results find that the coaxial cavity gyrotron with three electron beams can operate steadily in the ${\text {TE}_{38,18-}}$ mode at 170.39 GHz and it has an output power of 5.02 MW and a beam–wave interaction efficiency of 35.42% when $B=6.82\text{T}$ , $U=88$ kV, $\alpha =1.3$ , $I=158$ A and the three electron beams locate at 11.14, 12.93, and 14.32 mm. Three electron beams in the gyrotron have different beam–wave interaction efficiencies, and the closer the electron beam is to the inner conductor, the higher its beam–wave interaction efficiency is.

Analysis of Coaxial Gyrotron Cavity With a Misaligned Insert

The coaxial cavity can rarefy the mode spectrum and restrain the mode competition, which is beneficial to the mode selection of high-frequency and high-power gyrotron. However, the misaligned insert would result in frequency shift and a decrease in interaction efficiency. Besides, the ohmic loss on the cavity walls would become uneven, which deteriorates the long pulse operation of the gyrotron. In this article, the effects of the insert structural parameters on the characteristics of the cavity, the beam-wave interaction, and the ohmic loss distribution on the cavity walls in a coaxial cavity with a small misalignment of insert are discussed. An optimal design of coaxial resonator is presented to reduce the influence of a slightly misaligned insert on the output characteristics of the resonator.

Triode magnetron injection gun for the KIT 2 MW 170 GHz coaxial cavity gyrotron

Considering the recent understanding of the physics of electron trapping mechanisms taking place in the magnetron injection gun (MIG) region of gyrotrons and the sensitivity of the emitter ring manufacturing tolerances on the electron beam quality, a MIG has been designed and manufactured for the 2 MW, 170 GHz coaxial cavity gyrotron developed at Karlsruhe Institute of Technology. The new MIG has the following novelties: (i) the design satisfies the criteria for the suppression of the electron trapping mechanisms, (ii) a new type of emitter ring is used for the suppression of the influence of the manufacturing tolerances and misalignments on the quality of the generated electron beam, and (iii) the design was optimized to generate a good beam quality in a wide variety of magnetic field profiles to increase the flexibility. An additional important feature of the new triode MIG design is the possibility to operate with only two power supplies by using a special start-up scenario. The first experimental results of the coaxial cavity gyrotron with the new MIG are presented.

Effect of Insert Misalignment on a Triangular Corrugated Coaxial Cavity Gyrotron

In this paper, the field analysis of a triangular corrugated coaxial cavity with misaligned insert is carried out for a megawatt-class sub-terahertz wave gyrotron. In misalignment analysis, both parallel displacement of the insert axis and tilting of the insert axis with the outer resonator axis are considered. Graff's addition theorem is used for deriving the eigenvalue equation of the coaxial cavity with such a misaligned insert. Mathematical formulations are carried out to include the effect of insert misalignment on the beam coupling coefficient and ohmic wall loading of the outer cavity and insert of the coaxial cavity gyrotron. The effect of structural misalignment of the insert on the operation of 2-MW, 220-GHz coaxial cavity gyrotron is analyzed. It is shown that insert misalignment reduces RF output power as well as increases ohmic wall loading of the insert beyond the prescribed cooling limit. Compared to tilting of the insert, parallel displacement of the insert causes more deterioration in the gyrotron operation. Comparative studies are also performed for a coaxial cavity with rectangular slots on the insert.

Multiphysics Modeling of Insert Cooling System for a 170-GHz, 2-MW Long-Pulse Coaxial-Cavity Gyrotron

High-power, high-frequency gyrotrons are the only promising sources for the electron cyclotron resonance heating and current drive (ECRH&CD) in the present thermonuclear fusion plasma experiments and in future power plants. Compared to the hollow-cavity gyrotron design, the coaxial cavity gyrotron design facilitates improved mode competition control, which eventually increases the output power per tube. At KIT-IHM, the successful operation of a 170-GHz gyrotron has been demonstrated in the short-pulse regime, and the ongoing research activities are aiming to upgrade the existing coaxial cavity gyrotron from short-pulse (1–10 ms) to long-pulse (~ 1s) operation. In this paper, with the help of multiphysics simulations, the performance of the insert cooling system is verified for the continuous wave (CW) operation and the operating limits of the insert cooling systems are determined. The perfectly aligned coaxial Glidcop insert will be able to withstand operating conditions leading to a maximal heat flux of about 0.39 kW/cm2. The influence of insert misalignment on insert loading is also studied systematically in this paper.

Triode Type Coaxial Inverse Magnetron Injection Gun for 2-MW, 240-GHz Gyrotron

A triode type coaxial inverse magnetron injection gun (CIMIG) is designed using the 2-D and 3-D beam trajectory simulation codes for 240-GHz, 2-MW coaxial gyrotron. In IMIG, the modulating anode is adjusted at the center of the emitter ring and it provides compact size and better control on the beam instability and beam halo. As per the beam-wave interaction simulations, the CIMIG is designed to deliver the 6.5-MW (100 kV, 65 A) helical electron beam of 8.72-mm guiding center radius at the cavity center. First, the synthesis of CIMIG is performed to estimate a range of electrical and geometrical parameters based on the tradeoff equations. The Gaussian type of magnetic field profile is optimized with the peak magnetic field value of 10 T at cavity center and the beam compression ratio of 35.11. The pitch factor of 1.306 and the transverse velocity spread of 2.74% are realized in 2-D trajectory code electron GUN. The design results obtained in 2-D simulations are validated in 3-D particle trajectory simulator CST-Particle Studio. In 3-D optimization, the gun geometry and the electrical parameters are kept similar to 2-D design. The pitch factor of 1.285 and transverse velocity spread of 3.01% are obtained in 3-D design which shows good agreement with the 2-D design.

Overview of recent gyrotron R&D towards DEMO within EUROfusion Work Package Heating and Current Drive

Gyrotron R&D within the EUROfusion Work Package Heating and Current Drive (WPHCD) is addressing the challenging requirements posed on gyrotrons by the European concept for a demonstration fusion power plant (EU DEMO). These requirements, as specified within WPHCD, ask for highly reliable and robust long-pulse operation of the gyrotron, delivering 2 MW of microwave power at frequencies above 200 GHz with a high overall efficiency above 60% and the option for fast frequency step-tunability. To meet these targets, which are clearly beyond today’s state-of-the-art, the R&D activities within WPHCD are organized in five main branches: these are the experimental verification of the advanced coaxial gyrotron technology at long pulses, the development of a coaxial gyrotron meeting the EU DEMO requirements, the development of multi-stage depressed collectors for enhanced energy recovery, the development of large broadband diamond windows to allow fast frequency tunability of the gyrotron, and the studies on further innovations and improvement of critical gyrotron components, in view of optimization of performance, reliability, and industrialization. The paper presents the progress made on these activities, the recent results, and the near-term planning. The major recent achievements include (i) the experimental validation of the fabrication of the new, water-cooled components for the longer-pulse coaxial gyrotron at Karlsruhe Institute of Technology, by demonstrating 2.2 MW at 170 GHz with 33% efficiency without depressed collector, (ii) the design of key components for a 2 MW, 170/204 GHz dual-frequency coaxial gyrotron, (iii) the design of a two-stage depressed collector with 77% efficiency, (iv) the first-ever production, in an industrial plasma reactor, of large chemical vapor deposition diamond wafers of 180 mm diameter, (v) the procurement of an advanced electron gun with coated emitter edges, and (vi) advances in the theoretical and numerical modelling for investigating improved concepts for the gyrotron beam tunnel and cavity.

Enhanced operation of the Eu Ec test facility

A Test Facility (TF) has been designed and installed at SPC to allow for the commissioning of the EU gyrotrons developed in view of their integration to the ITER EC system. The first phase of operation of this TF was dedicated to the development of the EU 2 MW coaxial cavity gyrotron [1,2].The EU gyrotron development for ITER has been reoriented since then and is presently advancing a 170 GHz/1MW/CW gyrotron developed within the EGYC consortium and manufactured by THALES. At the same time, the TF has been enhanced to host the FALCON project [3], consisting in the integration of a second 170 GHz/1MW/CW gyrotron delivered by GYCOM. This new experiment is aiming to test the main RF components of the EC Transmission Line (TL) and of the Upper Launcher system to be installed on ITER.This paper describes the technical modifications brought to the existing TF to comply with the requirements of both 1 MW gyrotrons operation. Effort has been put in switching of the auxiliaries from one tower to the other within a short time (<1 week) while maintaining operator safety and equipment protection. A detailed description of the additional implementation and of the strategy followed to share the main auxiliaries (like the HVPS, the cooling system, the control system, etc.) will be presented. Moreover, most of the new equipment, such as the control hardware, have been designed to fulfil ITER site standards. Finally, the preliminary operation results with this reconfigured TF will be presented.

Numerical research on a 4 MW 170 GHz coaxial gyrotron with a double electron beam

In this paper, a coaxial gyrotron with a double electron beam is investigated on the ohmic loss, starting current, mode competition, and beam wave interaction. Its beam-wave interaction equation and dependence of the starting current on the static magnetic field and parameter K (the ratio of the current of an electron beam to the total current of a double electron beam) are given. On this basis, a 4 MW 170 GHz coaxial gyrotron with a double electron beam is studied. By the numerical calculation, the influence of K on the starting current is analyzed under a given static magnetic field and at different guiding center radii of the double electron beam. Studies show that the ability of the gyrotron to suppress the mode competition has a relation with the selection of the guiding center radii of the two electron beams. Then, the investigation of a time-dependent multi-mode competition of the gyrotron is performed, which shows that the gyrotron can stably operate in the TE38,18− mode. The results show that by considering the ohmic loss, the gyrotron can operate at 170.4 GHz with the output power of 4.04 MW and the beam-wave interaction efficiency of 35.47% when B = 6.82 T, U = 88 KV, I = 129 A, and K = 0.4. Each of the two electron beams in the gyrotron has different beam-wave interaction efficiencies. Compared to the coaxial gyrotron with one electron beam, the beam-wave interaction efficiency of each electron beam in the coaxial gyrotron with a double electron beam has a different increase.

Design Studies of a 3-MW, Multifrequency (170/204/236 GHz) DEMO Class Triangular Corrugated Coaxial Cavity Gyrotron

In this paper, the design studies of a triple frequency (170/204/236GHz) coaxial cavity gyrotron are carried out which is suitable for plasma heating application in a commercial fusion demonstration (DEMO) tokamak class prototype reactor. To reduce the spatial and maintenance requirements in the heating system of the DEMO tokamak, the output power of the proposed gyrotron is targeted at ≈3 MW for all the three operating frequencies. The cavity modes are selected by considering various physical and technical constraints of the multifrequency gyrotron. The dimensions of the interaction cavity are optimized for the chosen modes at the desired frequencies through cold-cavity calculations. Mode competition studies are then performed to understand the effect of the parasitic modes on the desired modes around the operating frequencies. A triangular corrugated coaxial insert is considered for reducing the problem of localized heating. Initial optimization of beam parameters and magnetic field is carried out using single-mode calculations for the maximum output efficiency. A triode-type coaxial magnetron injection gun is designed for supporting this multifrequency operation. Startup calculations are carried out with beam space-charge neutralization at all the three operating frequencies. These studies predict that a 3-MW, continuous-wave operation is possible at the desired frequencies of the proposed DEMO class gyrotron.

New trends of gyrotron development at KIT: An overview on recent investigations

Since many years KIT is strongly involved in the development of high power gyrotrons for use in ECRH. KIT is pursuing two development lines: (i) the conventional, hollow cavity gyrotron and (ii) the coaxial cavity gyrotron. KIT is pushing conventional cavity gyrotrons from 1 MW to 1.5 MW in a common project with IPP Greifswald. Coaxial cavity technology having the advantage of higher power capability, in particular at higher frequency, is used for the 2 MW 170 GHz short-pulse prototype. This gyrotron is currently being upgraded to allow pulse extension up to approximately 100 ms and up to 1 s in a second step.For a future DEMOnstration fusion power plant two challenging trends with respect to gyrotron features are recognized: (a) the operating frequency will be above 200 GHz and (b) the requested total efficiency of the gyrotron should be higher than 60%. KIT is addressing these requirements by investigating both gyrotron technologies for their performance at a frequency well above 200 GHz. We started careful analysis of multi-staged-depressed collectors.

Analysis of an actively-cooled coaxial cavity in a 170 GHz 2 MW gyrotron using the multi-physics computational tool MUCCA

Continuous Wave gyrotrons are the key elements for Electron Cyclotron Resonance Heating and Current Drive (ECRH&CD) in present fusion experiments and future fusion reactors. In the frame of the EUROfusion activities, a 170 GHz, 2 MW short-pulse (∼ 1 ms), water-cooled coaxial gyrotron, already tested at Karlsruhe Institute of Technology (KIT), is being upgraded for operation at longer pulses (∼ 100–1000 ms).Here we use the MUlti-physiCs tool for the integrated simulation of the CAvity (MUCCA), recently developed in collaboration between Politecnico di Torino and KIT, to analyze the evolution of the operating condition of the coaxial gyrotron cavity, self-consistently coupling thermal-hydraulic, thermo-mechanical and electro-dynamic models. The main results are presented in terms of evolution of temperature, heat load and deformation of the heated surface of the resonator and of the coaxial insert during the first few seconds of operation. We show that the system evolves towards stable operating conditions (no beam loss), with a peak temperature strongly dependent on the cooling configuration, where a large room for the improvement of the current cavity cooling design is found.

KIT coaxial gyrotron development: from ITER toward DEMO

Karlsruhe Institute of Technology (KIT) is doing research and development in the field of megawatt-class radio frequency (RF) sources (gyrotrons) for the Electron Cyclotron Resonance Heating (ECRH) systems of the International Thermonuclear Experimental Reactor (ITER) and the DEMOnstration Fusion Power Plant that will follow ITER. In the focus is the development and verification of the European coaxial-cavity gyrotron technology which shall lead to gyrotrons operating at an RF output power significantly larger than 1 MW CW and at an operating frequency above 200 GHz. A major step into that direction is the final verification of the European 170 GHz 2 MW coaxial-cavity pre-prototype at longer pulses up to 1 s. It bases on the upgrade of an already existing highly modular short-pulse (ms-range) pre-prototype. That pre-prototype has shown a world record output power of 2.2 MW already. This paper summarizes briefly the already achieved experimental results using the short-pulse pre-prototype and discusses in detail the design and manufacturing process of the upgrade of the pre-prototype toward longer pulses up to 1 s.