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In recent years, the after cavity interaction (ACI) in high-power gyrotrons operating in the 100–200 GHz range gained special attention as an influence factor on overall efficiency. While investigations concentrated on ACI as a stationary effect until now, recent simulations show that undesired interactions in the output waveguide region (uptaper) can also result in additional parasitic oscillations. In this paper, such nonstationary, dynamic processes are investigated in first simulations and results are described using the self-consistent time-dependent Karlsruhe Institute of Technology multimode code. In this paper, results are generated along with the assumption of constant Bz(z)= B0 (maximum magnetic field) as well as with the implementation of the adiabatic approximation in the case of varying magnetic field for determination of the axial velocity of each particle. The simulations show weak parasitic/spurious oscillations in the uptaper of the gyrotron cavity that may be attributed to the occurrence of dynamic ACI. [ABSTRACT FROM PUBLISHER]

To control different types of magnetohydrodynamic instabilities in the fusion experiment ITER, various amplitude modulation scenarios for the gyrotrons of the electron cyclotron system are required. This paper focuses on the impact of different modulation scenarios on the power loading on the collector wall of the 2-MW EU gyrotron. In addition, first investigations of the transient thermal behavior of the collector structure are performed. [ABSTRACT FROM AUTHOR]

In this paper, the design of a 10-kW/28-GHz gyrotron is presented. The main characteristic of the new gyrotron is the emitter ring, which is assembled from twelve individually supplied emitters based on controlled-porosity reservoir (CPR) technology. The main goal is to evaluate the use of CPR emitters for gyrotron applications. In addition, the azimuthally segmented emitter ring could be used for the generation of controlled nonuniform electron beams. In this way, it is planned to experimentally study the effect of nonuniform emission on the gyrotron operation. In this context, the effect of nonuniform emission on the beam quality is numerically investigated using the 3-D, electrostatic, parallel-code Ariadne, while the effect of the degraded beam on mode coupling and stability of the wave-particle interaction in the cavity is numerically studied using the nonstationary, self-consistent multimode cavity code Euridice. [ABSTRACT FROM AUTHOR]

A 1-MW, continuous wave, 170-GHz, TE32,9-mode gyrotron for use in International Thermonuclear Experimental Reactor (ITER) is under development within the European Gyrotron Consortium. A quasi-optical mode converter is employed in the gyrotron to transform the high-order cavity mode into a fundamental Gaussian wave beam. The quasi-optical mode converter contains a launcher and a mirror system. The launcher is numerically optimized to provide Gaussian mode content of 98.43% at the launcher aperture. The mirror system consists of three mirrors. The first mirror is a quasi-elliptical mirror, the second and third mirrors are beam-shaping mirrors, which are used to change the beam parameters, such as the beam waist and the position of the focusing plane. The field distribution in the mode converter has been analyzed. The simulation results show that the fundamental Gaussian mode content of the wave beam is 98.6% at the window plane. A first numerical estimation of the stray radiation generated by the mode converter is 1.75%, to be verified in future measurements. The proper synthesis of the quasi-optical mode converter has been verified by comparison of the simulation results from TWLDO with results obtained using the commercial 3-D full-wave vector analysis SURF3D code. [ABSTRACT FROM AUTHOR]

Bremsstrahlung X-rays produced at the collector can be used for definition of electron energy distributions in gyrotrons and maybe in other high-power microwave devices. Such a possibility underwent early testing at the Karlsruhe Institute of Technology on two gyrotrons. Electron energy distributions calculated on the basis of X-ray spectra measured near the gyrotrons operated at different regimes have plausible shapes and demonstrate good correlation with expected characteristics of the electron beam, thus confirming the viability of the proposed diagnostic technique. [ABSTRACT FROM AUTHOR]

Gyrotron oscillators are millimeter wave sources, capable of reaching megawatt power levels. Such high RF power is required for electron cyclotron resonance heating and current drive systems for current and future nuclear fusion facilities. With total heating system powers in the range up to about 100 MW, these installations call for unit powers above 1 M W, to reduce cost and complexity of the complete heating system. In this paper, a mode selection process for a 4-MW 170-GHz coaxial-cavity gyrotron is presented and stable operating parameters are elaborated. The employed formalism, based on normalized variables, suggests one mode, namely the TE{-52,31}, which is sufficiently separated from its competitors and supports an advanced two-beam quasi-optical mode converter refid="ref1"/. Through the utilization of time-dependent and self-consistent approaches, a coaxial cavity is optimized and stable single-mode operation at 4.35 MW of generated output power with an interaction efficiency of 33% is predicted. This paper discusses the mode selection process under consideration of realistic technical limitations, optimization of the cavity for stable operation with a pure output mode, and finally thermo-mechanical calculations on cavity cooling and surface temperature. [ABSTRACT FROM PUBLISHER]

In this paper, the effect of the ITER stray magnetic field (SMF) on the operation of the European gyrotron has been studied. The SMFs include the magnetic field generated by neighboring gyrotrons, the ITER tokamak, and also the field perturbations caused by the ferromagnetic structural materials used in the ECRH building.The 3-D self-consistent electrostatic code ARIADNE has been used for the evaluation of the SMF effect on the electron beam in the cavity and the collector region. It is shown that the beam parameters at the cavity of the gyrotron are not significantly affected by the SMF. On the other hand, the magnetic field lines in the collector region are significantly deformed in the presence of the SMF. A part of the electron beam is guided to the inappropriately cooled part of the collector surface, while the electron beam power is distributed in a significantly smaller area of the collector walls. In addition, the use of an advanced transverse sweeping system is proposed which would allow to compensate the SMF and to improve the collector operation. [ABSTRACT FROM PUBLISHER]

10 MW, 140 GHz ECH system is currently under construction for the stellarator W7-X. The RF power will be provided by 10 gyrotrons. A European collaboration has been established to develop and build 9 (out of 10) tubes each with an output power of 1 MW for continuous wave (CW) operation. This contribution reports on recent results with the series gyrotrons. [ABSTRACT FROM AUTHOR]

A 2-MW continuous-wave (CW) 170-GHz coaxialcavity gyrotron for electron cyclotron heating and current drive in the International Thermonuclear Experimental Reactor (ITER) is under development within the European Gyrotron Consortium (EGYC1), a cooperation between European research institutions. To support the development of the industrial prototype of a CW gyrotron, a short-pulse tube (preprototype) is used at KIT Karlsruhe (former FZK) for experimental verification of the design of critical components, like the electron gun, beam tunnel, cavity, and quasi-optical RF output coupler. Significant progress has been achieved recently. In particular, RF output power of up to 2.2 MW with 30% output efficiency has been obtained in single-mode operation at 170 GHz. Furthermore, a new RF output system has been designed, with an efficient conversion of the generated RF power into a Gaussian RF output beam. The results have been successful, yielding a Gaussian mode content ~96%. [ABSTRACT FROM AUTHOR]

Abstract--A high-efficiency quasi-optical launcher for coaxial-cavity gyrotrons has been investigated with the mode-matching analysis method. The launcher designed for the TE34,19 mode at 170 GHz is analyzed from 118 to 218 GHz with 67 modes. This investigation is a preliminary study of a launcher toward a step-tunable 2-MW coaxial-cavity gyrotron. Twenty-four modes with fundamental Gaussian mode content (FGMC) that is higher than 90% have been found. At last, nine modes of interest are selected for SURF3D simulations to investigate the transmission properties of millimeter-wave beams through the output window. The results reveal that the beams have deviations of their center points at the RF window within only 2.1 mm, and the FGMCs range from 92.67% to 96.67%. This result indicates that even the present launcher with specific deformations designed for only one mode would be capable of a step-tunable operation. [ABSTRACT FROM AUTHOR]

Megawatt gyrotrons are found to suffer from various parasitic oscillations, in particular, RF oscillations in the beam tunnel prior to the desired interaction zone (the cavity). This paper describes the experimental results from a gyrotron experiment which was dedicated to investigate parasitic oscillations in the beam tunnel and to verify improved beam-tunnel structures. A system for improved spectral measurements and a new analysis method are presented. The results verify theoretical predictions on the parasitic oscillations, and in effect validate the corresponding improved beam-tunnel structure. In addition, other types of parasitic oscillations were observed and explained. [ABSTRACT FROM AUTHOR]

A numerical method for the analysis of the fields in highly oversized waveguides is proposed in this paper. This method allows the simulation of the fields on waveguide walls with arbitrary surface deformations in the case that the waveguide is highly oversized, and the wall deformations are shallow and smooth. Combined with the analysis method, an algorithm has been developed for synthesizing the waveguide wall to provide a desired field distribution. As an example, a 309.6-mm-long wave- guide launcher has been designed for a 170-GHz coaxial-cavity gyrotron to transform the TE34,19 cavity mode to a fundamental Gaussian distribution. An efficiency of transformation to the desired fundamental Gaussian mode of 96.3% has been obtained at the launcher aperture, whereas the transformation efficiency is just 86% using a conventional dimpled-wall launcher with a length of 660 mm. [ABSTRACT FROM AUTHOR]

Abstract: Parasitic beam tunnel oscillations have been discovered on some of the series production gyrotrons for W7-X and also on the coaxial pre-prototype gyrotron for ITER. Solutions to remedy these problems have resulted in a modified beam tunnel design, technologically close to the existing beam tunnel. The new design has successfully been tested on both the coaxial and also the f-step-tunable gyrotrons and has subsequently been implemented on one of the W7-X series-production-tubes presently undergoing factory acceptance tests in Karlsruhe. The ECRH test loads at KIT are operated under normal atmospheric conditions. Several loads have eventually failed in 1MW long pulse experiments and KIT has therefore started to design its own loads. The first KIT-load is based on a fixed conical mirror and an aluminum cylinder coated with a lossy material for increased absorption. The new load has so far successfully been used during the acceptance tests of two 1-MW CW gyrotrons. Nevertheless a new load based on pure (uncoated) stainless steel absorbers is being developed as a backup solution for the ongoing high priority gyrotron testing. A superconducting magnet capable of rapid field changes between 4.15 and 5.67T for frequency step-tunable gyrotrons has been procured, has demonstrated a (static) field of 7.2T and its capability of rapid field-changes. [Copyright &y& Elsevier]