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The DEMO-relevant gyrotron research at Karlsruhe Institute of Technology is driven by the European concept for a demonstration fusion reactor (EU DEMO). This paper reports on the recent results of the theoretical and experimental studies towards the development of gyrotrons fulfilling the DEMO needs.

The status of the THALES 140 GHz 1 MW CW industrial gyrotron program for the W7-X stellarator is discussed in this paper. The industrial design and performances arc briefly discussed with a focus on the tube reliability and endurance.

Karlsruhe Institute of Technology (KIT) is progressing in the development of a multi-megawatt class gyrotron for fusion plasma applications. After the successful experimental verification of the performance of the modular KIT 170 GHz, 2 MW short-pulse coaxial-cavity pre-prototype gyrotron with pulse lengths up to a few ms, the key gyrotron components have been upgraded by adding suitable cooling systems, keeping their previously validated scientific design. The new gyrotron construction allows the operation with pulses up to ~100 ms, however further increase of the pulse length is planned, after the installation of a diamond window and an improved collector. The basic upgrade of the individual components, the final assembly and the first experimental results are presented.

The first part of the experimental validation of the EU 1 MW, 170 GHz conventional (hollow-cavity) gyrotron for ITER was successfully completed at KIT. During this period two prototype tubes: (1) short-pulse (SP) gyrotron and (2) industrial continuous-wave (CW) gyrotron have been experimentally tested. The achieved RF output power was in range of 1 MW (SP) and 0.8 MW (CW) with a reasonable efficiency at pulse length up to 10 ms and 180 s, respectively. In order to study the possibility of further optimization of the gyrotron performance, additional experiments with the modified SP gyrotron setup are planned at KIT. The achieved results will be presented in this paper.

Karlsruhe Institute of Technology (KIT) is doing research and development in the field of high power gyrotrons for W7-X, ITER, and, towards a future DEMOnstration fusion power plant. While the initial installation of the ECRH system at W7-X is finished and operating very successfully, KIT is still involved in the development of gyrotrons for ITER. This work is coordinated by F4E and ongoing in frame of EGYC. Moreover, as part of EUROfusion, KIT is investing in the research for advanced fusion gyrotrons of future EU DEMO. The target is to develop gyrotrons which will fulfil the need for RF sources operating at a frequency above 200 GHz, an RF output power above 2 MW and a total efficiency above 60 %. Aditionally, multi-purpose/multi-frequency operation and frequency step-tunability are requested. That requires the research and development on advanced gyrotrons, test environments and control techniques. This contribution provides a view over ongoing and planned activities in the field of gyrotron development at KIT.

ECH&CD in future fusion reactors such as the European DEMOnstration power plant may require the availability of gyrotrons with operating frequency significantly above 200 GHz, RF output power of 2 MW, and total efficiency above 60 %. Depending on the technology for RF beam steering into the plasma, fast frequency tuning in steps of around 2 – 3 GHz might be required for plasma stability control. “Multi-purpose” operation at multiples of the λ/2-resonance frequency of the synthetic diamond gyrotron RF output window, hence in leaps of about 30 – 40 GHz might be considered for plasma start-up, heating and current drive. The combination of all those requirements challenges present-day technological limits for gyrotrons.

A quasi-optical (q.o.) mode converter has been designed for the MW-level dual-frequency gyrotron which is under development for the upgrade of the electron cyclotron (EC) system of the TCV tokamak. The gyrotron design is essentially based on modifications of the W7-X gyrotron [1]. The launcher of the output coupler has been synthesized to adapt the geometry of the quasi-elliptical mirror with same size as in the W7-X gyrotron for both operating modes (TE17,5 @ 85GHz, TE26,7 @ 126GHz). The simulation results show that high conversion efficiencies and low stray radiations in the q.o. system can be achieved.

An accurate and fast simulation tool for the analysis of quasi-optical output couplers in high power gyrotrons is under development. The electric field in the quasi-optical system is described by the electric field integral equation (EFIE) which is numerically solved by a Galerkin-type method of moments (MoM). To accelerate the simulations, a set of first order basis functions for curved triangles and the adaptive cross approximation algorithm (ACA) are used. The quasi-optical mode converters are parametrized by local B-Splines which can approximate launcher walls with perturbations very well. To speed up the calculations for highly oversized geometries, the software implementation supports parallelization on shared- and distributed-memory systems.

The upgrade of the electron cyclotron (EC) system of the TCV tokamak has entered in its realization phase. The MW-class dual-frequency gyrotron design (84 or 126GHz/2s), based on the 140GHz W7-X gyrotron, is finalized and a contract for the manufacturing of two units has been placed with Thales Electron Devices (TED). For maintaining a high level of operational flexibility of the EC-system the integration of the dual-frequency gyrotrons adds a significant complexity in the evacuated HE11 transmission line system connected to the various TCV low-field side and top launchers. Specific aspects of the gyrotrons design and their integration into the EC-system are described. The EC-system upgrade is an essential step of a larger extent upgrade of TCV.

An industrial 1 MW, 170 GHz continuous-wave (CW) gyrotron prototype has been manufactured in 2015 at Thales. The physical design is provided by the European GYrotron Consortium (EGYC) and is supported by the construction and the measurement of a modular short-pulse (SP) prototype. In this presentation, the latest experimental results are discussed, with respect to the previous results acquired for the modular SP prototype gyrotron.

A short-pulse prototype tube has been designed and manufactured at KIT in order to support the development of the European 1MW, 170 GHz, CW gyrotron for ITER. The aim of the tests is the validation of the design of the most critical components, such as electron gun, cavity, beam tunnel and quasi-optical mode converter. First experimental results will be presented in this paper.

The TCV tokamak is presently undergoing major heating upgrades, installing a neutral beam for direct ion heating and increasing the electron cyclotron (EC) power injected in X-mode at the third harmonic (X3) and second harmonic (X2). The EC-system upgrade consists of adding two dual-frequency, 84/126 GHz, gyrotrons for X2 and X3 heating with a 1MW rf-power per gyrotron at both frequencies and 2s pulse length. The design of the dual-frequency gyrotron presented in this paper is based on the 140 GHz W7-X gyrotron and has been carried out within a European collaborative effort. A partial re-design of some internal components (triode-gun, cavity, QO-launcher and collector) allows to obtain powers in excess of 1 MW and a very-high Gaussian content (>97%) at both frequencies without depressed collector.

A numerical method for the synthesis of hybrid-type launchers for high power high order mode gyrotrons has been developed at KIT [1]. In terms of this numerical method, a launcher operating in the TE 32,9 mode at 170 GHz, has been designed and numerically tested. Compared to corresponding harmonically deformed inner wall and mirror-line launchers, the hybrid-type launcher can provide an RF beam with higher Fundamental Gaussian Mode Content (FGMC) at the launcher aperture.

The quasi-optical components of an 1MW TE 32,9 170GHz gyrotron for the ITER experiment have been measured with low power and compared to design simulations and high-power measurements in order to ensure their performance prior to the assembly of the gyrotron.

EU is presently developing a 1 MW, 170 GHz cylindrical cavity gyrotron for ITER. This activity was initiated in 2008 as a risk mitigation measure in parallel with the development of a 2 MW coaxial-cavity gyrotron, and in 2012 became the main line. In the previous year, the scientific designs of the 1 MW gyrotron components, such as the electron gun, beam tunnel, cavity, quasi-optical output coupler and single-stage depressed collector have been finalized. In addition, in collaboration with the European industrial partner Thales Electron Devices (TED), after completion of the industrial design of the technological parts of the gyrotron, the procurement and manufacturing of subassemblies for a 1 MW CW prototype will start in 2014. Its specifications correspond to the ITER requirements.

Europe is making a significant joint effort to develop high power microwave sources (gyrotrons) for nuclear fusion applications. KIT is taking a large part in this. It is involved into the developments for W7-X and ITER, and it is starting research for gyrotrons which, in future, shall operate at multi-megawatt levels at above 200 GHz and shall allow step-frequency tunability. To enable this, KIT is setting up a new gyrotron test facility. The presentation will provide a comprehensive overview on the 2013 status and plans of the gyrotron development at KIT.

The development of a 2 MW, 170 GHz short-pulse coaxial-cavity gyrotron pre-prototype for electron cyclotron heating and current drive (ECRH&CD) of magnetically confined plasmas in upcoming fusion devices is in progress at Karlsruhe Institute of Technology (KIT). Significant design modifications have been done recently. First experimental tests of a modified setup show two important unwanted effects. The gyrotron operation is limited by problems, which are occurring in the electron beam. In addition, an excitation of low frequency parasitic oscillations has been observed. The origin for those effects is under study.

Summary form only given. Europe is spending joint efforts to develop and to manufacture MW-level gyrotrons for electron cyclotron heating and current drive (ECRH&ECCD) of future plasma experiments. The two most important are the stellarator Wendelstein W7-X at Greifswald and the tokamak ITER at Cadarache. The construction of the 140 GHz, 1 MW CW gyrotrons for the 10 MW ECRH system of W7-X is proceeding well. It is expected that the next series tube will have finished final acceptance tests well in spring 2013. According to plan, production of the gyrotrons for W7-X will be finalized by 2014. This report will summarize the actual status. In parallel to the production and testing of the W7-X gyrotrons, the European GYrotron Consortium (EGYC) is presently developing the EU-1 MW, 170 GHz gyrotron for ITER. That design had been initiated in 2007 already, as a risk mitigation measure during the development of the advanced EU ITER-2 MW coaxial-cavity gyrotron. The basic idea of the EU ITER-1 MW design is to benefit from the experiences made during development and series production of the W7-X gyrotrons and of the experiences gained from the 2 MW coaxial-cavity gyrotron design. Preliminary designs of the cavity and the magnetron injection gun have been presented earlier. During 2012, the scientific design of the EU ITER-1 MW gyrotron components has been finalized. In collaboration with the industrial partner Thales Electron Devices, the industrial design of the technological parts of the gyrotron is being completed. A short pulse prototype gyrotron is under development to support the design of the CW prototype tube. The technological path towards the EU ITER-1MW gyrotron and the final design will be presented.

This paper presents a method for the design of phase correcting mirrors for gyrotrons. A computer code has been developed for vector analysis of electromagnetic fields in the mirror systems. Based on the Katsenelenbaum-Semenov method, the computer code has been written for the synthesis of adapted phase correcting mirrors. As an example, a quasi-optical mode converter including a Vlasov launcher and two phase correcting mirrors has been designed for a 94-GHz TE6,2 mode gyrotron. The simulation results show that the conversion efficiency is 87.19% (including power transmission losses).

Summary form only given. The European Gyrotron Consortium (EGYC) is responsible for developing one set of 170 GHz mm-wave sources, in support of Europe's contribution to ITER1. The original plan of targeting a 2 MW coaxial gyrotron is currently under discussion, in view of essential delays and damages. This paper reports on the latest results and plans with regard to the 2 MW gyrotron prototypes.

ECH & CD is widely used in fusion plasmas. Gyrotrons are applied as power sources as they offer continuous wave operation at MW power level. This contribution reports on the status and plans of major projects of KIT towards high power gyrotrons, i.e. results from series production of the W7-X gyrotron, the development of the EU pre-prototype coaxial-cavity gyrotron, the 2 MW, 170 GHz CW coaxial cavity gyrotron for ITER and a frequency step-tunable gyrotron.

This paper presents a new algorithm for fast calculation of the wave propagation in tapered gyrotron launchers. A scalar diffraction method for tapered launchers with adaptive surface perturbation is presented for the first time. The verification of the algorithm is done by comparison to a commercial 3D full-wave code that solves the Electric Field Integral Equation (EFIE).

Closed corrugated waveguide transmission lines operating in the HE 11 mode will be used to transmit the powerful radiation from the coaxial-cavity gyrotrons to the ITER tokamak for Electron Cyclotron Resonance Heating (ECRH) and Current Drive (CD). A Match Optics Unit (MOU) has been developed to transform the gyrotron output into a Gaussian mode distribution with optimal parameters to maximize the coupling efficiency of the TEM 00 Gaussian distribution to the HE 11 mode of the corrugated waveguide. The calculation results reveal that the coupling efficiency of the Gaussian-mode like wave beam to the HE 11 mode is approximately 96.3%.

A 2 MW, CW, 170 GHz coaxial-cavity gyrotron for electron cyclotron heating and current drive in the International Thermonuclear Experimental Reactor (ITER) is under development within the European Gyrotron Consortium (EGYC). To support the development of the industrial CW prototype, a short pulse tube (pre-prototype) is used at KIT for experimental verification of the design of critical components, as electron gun, beam tunnel, cavity and quasi-optical (q. o.) RF-output coupler. The paper reports on running activities on the pre-prototype tube developed at KIT.

The paper presents the design of a MOU for the coaxial ITER gyrotron. Corrugated waveguides are used to transmit the high power mm-waves generated by gyrotrons to the plasma for Electron Cyclotron Resonance Heating (ECRH) and Current Drive (CD). The MOU contains two focusing mirrors, which are used to convert the gyrotron output into a Gaussian distribution with optimal parameters to improve the coupling efficiency of the TEM 00 Gaussian distribution to the HE 11 mode of the corrugated waveguide. The calculation results reveal that the coupling efficiency of the Gaussian beam to the HE 11 mode is approximately 96.33%.

In this paper, we present the latest results on an FFT based method for fast calculation in a rippled-wall overmoded circular waveguide. This FFT based method can be considered as a cylindrical wave decomposition.

In this paper, we present the latest results on an FFT based method for fast calculation in a rippled-wall overmoded circular waveguide. These are compared with the results of a high-order discontinuous Galerkin method solving Maxwell's equations.

A 2 MW, CW, 170 GHz coaxial-cavity gyrotron for electron cyclotron heating and current drive in the International Thermonuclear Experimental Reactor (ITER) is under development within the European Gyrotron Consortium (EGYC). To support the development of the industrial CW prototype, a short pulse tube (pre-prototype) is used at KIT (former FZK) for experimental verification of the design of critical components, as electron gun, beam tunnel, cavity and quasi-optical (q. o.) RF-output coupler. The paper summarizes the progress achieved in 2009 and gives an update on running activities in 2010.

A 2MW, CW, 170 GHz coaxial-cavity gyrotron for electron cyclotron heating and current drive in ITER is under development within an European Gyrotron Consortium (EGYC). To support the development of the industrial C W gyrotron prototype, a short pulse tube (pre-prototype) is used at KIT for experimental verification of the design of critical components, as electron gun, beam tunnel, cavity and quasi-optical (q.o.) RF-output coupler.

A quasi-optical (q.o.) mode converter consisting of a launcher with specific surface contour, a quasi-elliptical and two toroidal mirrors developed for a 2MW, 170GHz, TE34,19 mode coaxial-cavity ITER gyrotron has been theoretically and experimentally investigated at Karlsruhe Institute of Technology (KIT). Both the theoretical and experimental results reveal that the Fundamental Gaussian Mode Content (FGMC) of the RF beam is about 96.7% at the window plane. In order to improve the FGMC of the RF beam further more, and then to decrease the stray radiation in the tube, two adapted phase correcting mirrors have been designed instead of the two toroidal mirrors. The simulation results show that with the specific phase correcting mirrors, the FGMC of the RF beam can be enhanced to 99.1% at the output window plane and the stray radiation could be decreased by 0.6% in the tube.

For future nuclear fusion reactors it is desirable to increase the RF power per gyrotron tube used for electron cyclotron resonance heating. Using gyrotrons with higher output power will reduce their overall number and the number of super-conducting magnets required. In this work the feasibility of a 4 MW 170 GHz continuous wave (CW) coaxial-cavity gyrotron is investigated. Starting from a mode selection procedure proper studies of all major gyrotron components have been performed. Designs for the coaxial cavity, the electron gun, the two-beam quasi-optical output coupler and the collector are presented. All layouts are optimized to guarantee high efficiency and consider long-pulse operation effects.

A 2 MW, CW, 170 GHz coaxial-cavity gyrotron for electron cyclotron heating and current drive in the International Thermonuclear Experimental Reactor (ITER) is under development within an European Gyrotron Consortium (EGYC*). To support the development of the industrial prototype of a CW gyrotron, a short pulse tube (pre-prototype) is used at KIT (former FZK) for experimental verification of the design of critical components, as electron gun, beam tunnel, cavity and quasi-optical (q.o.) RF-output coupler. Significant progress was 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, an excellent quality of the RF output beam with ∼96 % Gaussian mode content has been obtained by using a new quasi-optical RF output system.

A 2MW, CW, 170GHz, TE 34,19 mode coaxial-cavity gyrotron for electron cyclotron heating and current drive in ITER is under development in cooperation between European Research Institutions (EGYC∗). A new method has been used to design a highly efficient quasi-optical (q.o.) mode converter for this gyrotron to transform the cavity mode into a fundamental Gaussian beam. The simulation results and low power measurements reveal that the fundamental Gaussian mode content can be enhanced to 97.7% at the output window plane.

A 2 MW, CW, 170 GHz coaxial cavity gyrotron for electron cyclotron heating and current drive in the International Thermonuclear Experimental Reactor (ITER) is under development in cooperation between European Research Institutions (EGYC∗∗). The performance of critical components, like electron gun, beam tunnel, cavity and quasi-optical (q.o.) RF-output coupler have been studied on a short-pulse coaxial gyrotron (pre-prototype) at the Forschungszentrum Karlsruhe (FZK). Recent experimental results are presented and discussed.

It is desirable to increase the RF power per gyrotron tube for plasma heating in future fusion devices. The realization of gyrotrons with highest possible output power will reduce their overall number and superconducting magnets required for future fusion reactors. In this work the feasibility of a 4 MW 170 GHz coaxial-cavity gyrotron is investigated. A mode selection procedure is presented and proper designs of major gyrotron components are shown. A possible outer wall corrugation of the cavity is discussed and possible improvements are shown. Further electron gun studies are performed using diode- and triode-type magnetron injection guns (MIGs). First layouts for the two-cut launcher are considered.

To support the development of the first industrial prototype of a 2 MW, CW, 170 GHz coaxial cavity gyrotron, a short pulse (≪ few ms) coaxial gyrotron (“pre-prototype”) of modular type is used for experimental studies at FZK. As a consequence of the limitation on the magnetic field value (6.67 T) of the superconducting magnet (SC) at FZK, it became necessary to reduce the operating voltage in the preprototype tube to values below 80 kV, in order to be able to excite the TE 34,19 mode at 170 GHz. This resulted in decrease of the RF output power to 1.5 MW. During the experiments parasitic oscillations at high frequencies around 152 and 160 GHz have been observed at U c ≫74 kV simultaneously with the nominal gyrotron working mode at 170 GHz. In this region with additional parasitic oscillations operation a reduced of the gyrotron efficiency has been measured. There are some indications that the parasitic oscillations are generated inside the beam tunnel near the cavity. To study these parasitic beam tunnel oscillations different designs of the beam tunnel are under consideration. To make the experiments with the pre-prototype tube more relevant for the industrial prototype it is necessary to operate the tube at the nominal magnetic field, namely at 6.86 T. To enhance the magnetic field to the desired value an additional normal conducting coil has been wounded directly on the housing of the gyrotron around the position of the cavity. First experiments with the new beam tunnel at the nominal magnetic field have been already started. In addition, the improvement of the most critical component of the coaxial cavity gyrotron - the quasi-optical RF output system is in progress. Up to now the Gaussian content of the RF output beam is not satisfactory, namely less than 80 %. The main reason of the poor performance of the RF output system is the launcher antenna with its low efficiency of conversion. The new launcher has been designed using a novel optimization method and is currently fabricated together with a suitable mirror system consisting only of toroidal mirrors. The simulated conversion efficiency of the TE 34,19 mode to the desired fundamental Gaussian mode is near 97 %. The system will be experimentally verified first in low power measurements at the beginning of 2009. Achieved results will be presented and discussed.

Since megawatt gyrotrons used as high power millimeter-wave sources operate with high-order cavity modes, it is necessary to adapt a quasi-optical (q.o.) mode converter to transform the cavity mode into a fundamental Gaussian wave beam. In general, a q.o. mode converter consists of a highly oversized waveguide launcher and a mirror system. As the stray radiation inside the gyrotron can heat the structure of the tube, it is very important to keep the level of trapped RF power inside the gyrotron low for stable operation. In order to pre-shape the wave beam in the launcher to enhance the Gaussian mode content of the radiated field and to decrease the diffraction losses at the cut of the launcher, the launcher wall surfaces are usually periodically, helically deformed to convert the cavity mode into a specific set of modes to generate a Gaussian-like field distribution. However, for gyrotrons operated in very high-order cavity modes like the TE 34,19 at 170 GHz, due to the ratio of caustic to cavity radius of approximately 0.323, a periodically, helically deformed launcher could just provide a field distribution with a fundamental Gaussian mode content of 86% at the launcher aperture, and the launcher length is 660mm which is too long to be integrated in the gyrotron..

A 2 MW, CW, 170 GHz coaxial cavity gyrotron for electron cyclotron heating and current drive in the International Thermonuclear Experimental Reactor (ITER) is under development in cooperation between European Research institutions. The design of critical components, especially the electron gun, cavity and quasi-optical (q.o.) RF-output system has been studied on a short pulse coaxial gyrotron (pre-prototype) at the Forschungszentrum Karlsruhe (FZK). Recent results of the experimental investigations will be presented and discussed.

A 2MW, 170GHz, TE 34,19 mode coaxial-cavity gyrotron is under development for ITER at Forschungszentrum Karlsruhe (FZK), Germany. A prototype of the quasi-optical (q.o.) mode converter has been designed and tested for this gyrotron to transform the cavity mode into a fundamental Gaussian distribution. High power measurements show that the fundamental Gaussian mode content of the RF wave beam is about 85% at the output window. A new method has been used to improve the performance of the q.o. mode converter. The simulation results reveal that the fundamental Gaussian mode content can be enhanced to 97.7% at the output window plane.

The development of a 2 MW, CW, 170 GHz coaxial cavity gyrotron at Forschungszentrum Karlsruhe (FZK) is in progress in cooperation with several European research institutions. The gyrotron is foreseen as a source for heating, current drive and plasma stabilization in the International Thermonuclear Experimental Reactor (ITER). The design of critical components, especially the electron gun, cavity and quasi-optical RF output system are verified at FZK in tests with a short pulse gyrotron.

A 2 MW, 170 GHz coaxial cavity gyrotron is under development for ITER at Forschungszentrum Karlsruhe (FZK), Germany. A quasi-optical (q.o.) mode converter integrated in the pre-prototype gyrotron is investigated, and measurements revealed that the mode converter could only provide a corrected wave beam with low fundamental Gaussian mode content in the output window plane. In order to improve the performance of the launcher, a numerical method is used to iteratively optimize the launcher wall surface to pre-shape the wave beam with high Gaussian mode content. Simulation results show that a vector power correlation coefficient of 93% of the outgoing wave beam to an ideal Gaussian-like distribution can be achieved at the aperture of the improved launcher. With the new launcher, both the Gaussian mode content and the power transmission of the outgoing wave beam in the window plane are expected to be larger than 95%.

The 2 MW coaxial cavity gyrotron for ITER investigated by the EU EFDA team is based on results from a short pulse gyrotron with similar cavity, gun and quasi-optical mode converter. This tube is still under investigation at FZK to understand and overcome possible problems. Limitations on the magnetic field of the FZK gyrotron magnet allow for an operation at reduced parameters only, thus the expected output power at 170 GHz is only 1.5 MW. The problems found in first experiments were low frequency parasitic oscillations in the gun region at about 300 MHz and insufficient Gaussian mode content of the output wave (76%), at no more than 1 MW rf output power. After changing the electron gun the parasitic oscillations vanished, and an rf output power of 1.3 MW at an efficiency of 23% was reached. The efficiency now seems to be limited by another parasitic high frequency oscillation at 160 GHz, which appears simultaneously to the desired working mode and which, at higher energy operating points, enforces multi- moding. This oscillation is believed to be a gyro-backward wave interaction located in the beam tunnel. Experiments with modified beam tunnel are underway, the results will be presented at the conference. To improve the quasi-optical mode converter and to increase the Gaussian mode content of the output wave, respectively, a different design approach for the launcher is under investigation, using arbitrarily shaped wall deformations.First results with 86% Gaussian mode content are promising.

Summary form only given as follows. A TE34.19-mode, 2 MW, CW, 170 GHz coaxial cavity gyrotron is under development for ITER at FZK, Germany. For gyrotrons operated in such a high order mode, it is necessary to adapt a quasi-optical mode converter to transform the cavity mode into a fundamental Gaussian wave beam. A pre-prototype quasi-optical mode converter has been investigated for this gyrotron. Low power measurements revealed a low fundamental Gaussian mode content in the output window plane. Our analyses show that the outgoing wave beam cannot be well pre-shaped with a high Gaussian mode content in a helically deformed launcher with an acceptable length. The Gaussian mode content of the field distribution radiated from the launcher is low and it makes the synthesis and fabrication of the adapted phase correction mirrors very difficult. In a highly oversized waveguide, if the deformations are shallow and smooth, the field remains paraxial. This allows us to calculate the field in the launcher by taking use of the scalar diffraction integral equation. Based on the scalar diffraction integral equation, a computer code has been developed for the analysis of the field on the launcher wall surface with arbitrarily shaped deformations. As an example, the field on the helically corrugated launcher wall surface used in the pre-prototype coaxial cavity gyrotron is calculated with the new code. The simulation results reveal a very high vector power correlation coefficient of 99% of the field in comparison with the field distribution calculated using the coupled mode method. Combined with the computer code for the analysis of the field in launchers, a new method has been developed for the synthesis of the launcher wall surface to achieve a wave beam with high Gaussian mode content. The numerical results show that a vector power correlation coefficient of the field to an ideal Gaussian-like field distribution of 86% is obtained at the aperture of the launcher.

The development of a 2 MW, CW, 170 GHz coaxial cavity gyrotron at Forschungszentrum Karlsruhe (FZK) is in progress in cooperation with several European research institutions. The gyrotron is foreseen as a source for heating, current drive and stabilization of plasmas in the International Thermonuclear Experimental Reactor (ITER). Tests with a first industrial prototype started recently at CRPP Lausanne. The design of critical components, especially the electron gun, cavity and quasi-optical (q.o.) RF-output system has been verified at FZK in tests with a short pulse gyrotron, the so called pre-prototype. Results of the experimental investigation at FZK was presented and discussed.

A design of a quasi-optical mode converter operating at nine modes is described in this paper. The main application for high-power gyrotrons is electron cyclotron resonance heating (ECRH) and plasma stabilization in nuclear fusion devices like tokamaks. The absorption of the microwave in the plasma depends on its frequency and the plasma parameters. Thus having a multi-frequency high-power source would give more flexibility to plasma stabilization. Experimental results with a fast step-tunable gyrotron are also demonstrated. At the ASDEX Upgrade tokamak at IPP in Garching there is an interest in step-tunable gyrotrons operating at frequencies between 105 GHz and 143 GHz. For this purpose a multi-frequency gyrotron is under investigation at the Institute of Applied Physics in Nizhny Novgorad, Russia, in cooperative development with the Forschungszentrum Karlsruhe (FZK). Here we deal with the quasi-optical mode converter of this gyrotron. To enhance the efficiency and minimize the losses due to stray radiation, an optimization of the quasi-optical mode converter for all operating modes is important.

As part of the European program for development of a 2 MW, CW, 170 GHz coaxial cavity gyrotron, the experimental studies on a short pulse (~ few ms) gyrotron has been continued. A hypothesis concerning the excitation of observed parasitic low frequency (LF) oscillations around 260 MHz has been proven experimentally. Small modifications of the shape of the coaxial insert resulted in an increase of the starting current for the LF oscillations by about a factor of 3 (to above 40 A) causing a strong reduction of the LF amplitude. The influence of the level of microwave stray radiation on gyrotron performance (range of single mode oscillation and efficiency) has been investigated. To improve the performance of the quasi optical (q.o.) RF output system a new launcher with a different wall corrugation and a new adapted phase correcting mirror has been tested both at low power and in the gyrotron. The results are going to be analyzed and compared with simulations.

A quasi-optical (q.o.) mode converter is under development for a 2 MW, 170 GHz coaxial cavity gyrotron for ITER at Forschungszentrum Karlsruhe (FZK), Germany. Measurements revealed that the pre-prototype q.o. mode converter could only provide a corrected wave beam with low fundamental Gaussian mode content in the output window plane. A new method is used to numerically optimize the launcher wall surface to improve the performance of the q.o. mode converter. Simulation results show that a Gaussian mode content of 95 % of the outgoing wave beam can be achieved at the aperture of the improved launcher. With the new launcher, both the Gaussian mode content and the power transmission of the outgoing wave beam in the window plane are expected to be larger than 95%.

Summary form only given. 140 GHz gyrotrons with a hollow waveguide cavity at 1 MW, CW output power have been developed at FZK Karlsruhe in cooperation with European institutions tor use at the stellarator W7-X at IPP Greifswald, Germany. TE28.8 is the operating mode. The utilized advanced quasi-optical (qo) mode converter has an excellent performance with respect to low amount of stray radiation ( 97%) of the RF output beam. Two prototype tubes have been built and tested successfully. With a first series tube, out of seven, the required specifications have been fully demonstrated. At a pulse length of 30 mm a power of 920 kW has been measured in the Gaussian mode. The corresponding overall RF output efficiency was nearly 45%. The fabrication of the series gyrotrons is in progress. Results and problems with tubes number 2, 3 and 4 are reported and discussed. Based on results obtained on an experimental coaxial cavity gyrotron at 165 GHz at FZK, the development of a 2 MW, CW, 170 GHz coaxial cavity gyrotron for ITER is in progress in a European cooperation. In the meantime a first industrial prototype of the 170 GHz coaxial gyrotron has been fabricated and is ready for operation, which is expected to start in April 2007. In tests with an experimental pre-prototype tube, operated at short (les 5ms) pulses, the design of the main gyrotron components has been confirmed, in general. The mechanism of parasitic low frequencv (LF) oscillations has been identified and the intensity of the LF oscillations has been reduced significantly by performing some minor geometrical modifications on the coaxial insert. The performance of the q.o. RF output svstem is not satisfying mainly with respect to the Gaussian content of the RF output beam. An improved design is in progress. Results are reported and discussed. In addition, work on a high power, multi-frequency gyrotron is going on. A SC magnet which will allow a fast (~sec) variation of the magnetic field has been specified and ordered. The optimization of the q.o. RF output system for a number of different modes is in progress. Results are reported.

A quasi-optical mode converter is used in the TE/sub 34,19/, 2 MW, CW 170 GHz coaxial cavity gyrotron, which is under development at FZK. The cut of the launcher has been optimized taking the tapering angle of the launcher into account. Further on, the amplitude of the launcher corrugations have been optimized for improved focusing in order to reduce the diffraction losses at the cut. In addition, the influence of phase corrections of a non-quadratic mirror on wave properties has been investigated for a real fabricated surface in comparison to the measurement. Results are reported.