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The introduction of advanced Multistage Depressed Collectors (MDC) to high-power fusion gyrotrons will be the key to increase the gyrotron overall efficiency above 60 % as required for future DEMOnstration power plant. MDCs which base on the E×B drift concept might solve the fundamental issue of handling the secondary electrons. To consider the effect of secondary electrons in a stationary state,...
The key to increase the gyrotron efficiency to a level significantly higher than 50% is the development of a highly efficient collector system for the recovery of the spent electron beam energy. In this context, a significant effort has been dedicated at KIT during the last years in order to design a reliable, robust and efficient multistage collector system. For this purpose several design approaches...
Manufacturing tolerances at the emitter ring region of a high power gyrotron cathode structure can have a dramatic effect on the electron beam quality, and, finally, are determining the gyrotron performance which will be achieved. A sensitivity analysis of manufacturing imperfections of the emitter ring in the gyrotron cathode structure on the electron beam quality has been performed. It has been...
An Inverse Magnetron Injection Gun (IMIG) design is proposed for future conventional-type high-power gyrotrons. The design is deduced from the design for the KIT 2-MW, 170-GHz coaxial-cavity gyrotron prototype which has been published recently. As for the coaxial-cavity gyrotron IMIG, the inverse MIG design for the conventional hollow-type gyrotron is offering the possibility for compact geometry,...
The output power, efficiency and stability of high power CW gyrotrons depend strongly on the quality of the electron beam generated by the magnetron injection gun. To understand deteriorating effects like low frequency oscillations or the generation of an electron beam halo, the KIT in-house beam optics code ESRAY [1] has been extended from a pure 2D electrostatic raytracing code to an advanced quasi-static...
In order to find the upper limit for mode eigenvalues for stable 1 MW-class gyrotron operation, the behavior of a 236 GHz cylindrical (hollow) cavity gyrotron design is theoretically analyzed for operation with even higher order modes (eigenvalues between 100 and 145). In this frame, two independent methods are used to determine the effect of neighboring modes on mode stability and to estimate the...
An inverse Magnetron Injection Gun (IMIG) has been designed for the KIT 2 MW, 170 GHz coaxial cavity gyrotron. The ambition is the possibility for implementation of a larger emitter ring compared to the “conventional” MIGs used in today's fusion gyrotrons. Considering the fundamental beam parameters, in the theoretical analysis an excellent beam quality has been achieved which results in a very low...
A design approach for a multistage depressed collector system for gyrotron is presented here. The new concept based on two steps: (i) the transformation of the cylindrical hollow beam to sheet beam and (ii) the sorting of the spent beam electrons on the electrodes, based on ExB approach, in order to achieve the optimal efficiency. The simulation results predict very high levels of efficiency (more...
Multistage Depressed Collectors (MDCs) have been successfully applied in many kinds of vacuum tubes; however, there are only few discussions about applicable MDCs for high-power gyrotrons, regarding the hollow small-orbit electron beam and relatively strong stray magnetic field in the collector region. A novel concept has been recently proposed, where the electron beam is transformed into one or more...
Fusion gyrotrons are high-power microwave sources which are capable to generate RF power in the MW range at millimeter wave and sub-THz frequencies. Today, typically around 35 % electronic efficiency is achieved in the energy transfer (interaction) from the electron beam to the electromagnetic field. Using a single-stage depressed collector (SDC), the overall efficiency of a gyrotron can be increased...
The Karlsruhe Institute of Technology (KIT) is working on a possible physical design of 2 MW coaxial-cavity gyrotrons for future fusion applications, initially considering a first demonstration fusion power plant (DEMO). One focus of the investigations is how successfully such gyrotrons can be operated at significantly different frequencies. The gyrotron design as presented here is optimized for operation...
In order to evaluate the emission inhomogeneity of a thermionic gyrotron emitter, three mathematical treatments of the current-voltage characteristics (CVC) have been investigated and will be presented in this paper. Results for three different gyrotrons and for different emitter temperatures based on experimental data and mathematical treatment will be considered. The relation between the effective...
Along with high power and high efficiency operation around 240 GHz, the requirements of DEMO gyrotrons include the possibility of multi-frequency operation and fast frequency tunability in 2–3 GHz steps. In this paper, multi-frequency operation of a proposed design for a conventional cavity DEMO gyrotron is presented, including operating parameters at different frequencies. The performance of the...
In this work, the design of an inverse Magnetron Injection Gun (MIG) for the 2 MW / 170 GHz modular coaxial cavity KIT gyrotron is presented. The gun design criteria related to the suppression of the electron trapping mechanisms are considered. In addition, the proposed gun design satisfies the demanded electron beam criteria set at the start of the project.
The KIT two-dimensional beam optics code ESRAY is an efficient tool for the design and optimization of magnetron injection guns and collectors of gyrotrons or comparable vacuum tubes. The applied numerical methods and latest modifications and enhancements will be presented. To achieve more realistic results, a new emitter surface roughness model and the treatment of secondary electrons has been implemented...
In Europe, significant progress in gyrotron research, development and manufacturing has been made in 2014, starting from the successful continuation of the 1 MW, 140 GHz gyrotron production for the stellarator Wendelstein 7-X (W7-X) at Greifswald, Germany and the accelerated development of the EU 1 MW, 170 GHz conventional cavity gyrotron for the ITER tokamak at Cadarache, France. Based on that, a...
The EU CW gyrotrons for ITER [1] will operate at a frequency of 170 GHz with an output power of 1 MW per tube, at an efficiency of approximately 50 %. With this efficiency, 1 MW of power from the electron beam has to be absorbed in the gyrotron collector. To avoid local overheating of the collector structure, the electron beam is swept across the inner surface at low frequencies (5 Hz – 50 Hz). This...
During the first few seconds of longer pulses in high-power gyrotrons, the main mode frequency decreases over a few hundred MHz, until a stable operation is reached. This is due to the combination of two effects, namely the thermo-mechanical expansion of the cavity and a shift in the effective electron energy through ionization of residual gas in the tube. Most investigations focus on the quasi-stationary...
Physical design studies towards DEMO-compatible gyrotrons (frequency above 230 GHz, output power above 1 MW, frequency-tunable) have been started at KIT. Usage of very high-order modes (eigenvalue above 120) is necessary and allows a novel mode-selection strategy. Two different resonator designs for 237.5 GHz gyrotrons are under investigation.
The effect of the emitter surface roughness on the quality of an electron beam emitted by a gyrotron magnetron injection gun (MIG) has been studied. Several different shapes of microstructures (imperfections) have been modeled on the emitter surface. A modified version of the two-dimensional, finite-difference beam optics code ESRAY has been used for the numerical simulations. Results for the different...
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