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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.

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For the ECRH heating system of the stellarator Wendelstein 7-X, two remote steering antennas are developed and manufactured. The principle of remote steering antennas is based on the imaging characteristics of corrugated rectangular waveguides, which is well understood and can accurately be simulated. Several details, however, require deeper investigation. The antenna needs a miter-bend and a 24 mm gap. The positions of these elements need to be chosen carefully to reduce losses and stray radiation. The antennas are manufactured from copper by electroforming. This allows to integrate all components, including the corrugated inner walls and the cooling channels, in one vacuum-tight piece. This paper reviews the design process of the remote steering antennas for W7-X as well as technological issues and experimental results from test pieces.

During the first operational phase (OP 1.1) of Wendelstein 7-X (W7-X) electron cyclotron resonance heating (ECRH) was the exclusive heating method and provided plasma start-up, wall conditioning, heating and current drive. Six gyrotrons were commissioned for OP1.1 and used in parallel for plasma operation with a power of up to 4.3 MW. During standard X2-heating the spatially localized power deposition with high power density allowed controlling the radial profiles of the electron temperature and the rotational transform. Even though W7-X was not fully equipped with first wall tiles and operated with a graphite limiter instead of a divertor, electron densities of n e > 3·1019 m-3 could be achieved at electron temperatures of several keV and ion temperatures above 2 keV. These plasma parameters allowed the first demonstration of a multipath O2-heating scenario, which is envisaged for safe operation near the X-cutoff-density of 1.2·1020 m-3 after full commissioning of the ECRH system in the next operation phase OP1.2.

Electron cyclotron resonance heating (ECRH) is the main heating mechanism in the Wendelstein 7-X stellarator (W7-X). W7-X is equipped with five absolutely calibrated sniffer probes that are installed in each of the five modules of the device. The sniffer probes monitor energy flux of unabsorbed ECRH radiation in the device and interlocks are fed with the sniffer probe signals. The stray radiation level in the device changes significantly during the start-up phase: Plasma is a strong microwave absorber and during its formation the stray radiation level in sniffer probes reduces by more than 95%. In this paper, we discuss the influence of neutral gas pressure and gyrotron power on plasma breakdown processes.

The polarization of the directed ECRH power has to be matched to the plasma boundary with respect to the magnetic field at the density gradient region close to the last closed flux surface (LCFS). This is achieved by means of grooved mirrors, which provide the required polarization and which are part of the matching optics unit (MOU) of the gyrotrons. The RF radiation from the gyrotrons has to pass typically 16 mirrors in a complex three-dimensional arrangement in order to reach the plasma. The paper discusses the modeling of the ECRH transmission in order to find the required polarizer adjustment for each possible injection angle and plasma wave type (O- or X-mode). This includes the calculation of the polarization state on the plasma boundary, the back-propagation through the transmission line up to the MOU and finally the calculation of the corresponding angles of both polarizers.

Parasitic beam tunnel oscillations have been hampering the series production of gyrotrons for W7-X. This problem has now been overcome thanks to the introduction of a specially corrugated beam tunnel. Two gyrotrons equipped with the new beam tunnel have fully passed the acceptance tests. Despite excellent power capability, the expected efficiency has not yet been achieved, possibly due to the presence of parasitic oscillations suspected to be dynamic after-cavity-oscillations (ACI's) or due to insufficient electron beam quality. Both theoretical and experimental investigations on these topics are ongoing. On previous W7-X gyrotrons collector fatigue has been observed, not (yet) leading to any failures so far. The plastic deformation occurring on the collector has now been eliminated due to the strict use (on all gyrotrons) of a sweeping method which combines the conventional 7 Hz solenoid sweeping technique with a 50 Hz transverse-field sweep system. Starting in 2013, the gyrotron test facility at KIT will be enhanced, chiefly with a new 10 MW DC modulator, capable of testing gyrotrons up to 4 MW CW output power with multi-stage-depressed collectors.

Wendelstein 7-X, a superconducting optimized stellarator built in Greifswald/Germany, started its first plasmas with the last closed flux surface (LCFS) defined by 5 uncooled graphite limiters in December 2015. At the end of the 10 weeks long experimental campaign (OP1.1) more than 20 independent diagnostic systems were in operation, allowing detailed studies of many interesting plasma phenomena. For example, fast neutral gas manometers supported by video cameras (including one fast-frame camera with frame rates of tens of kHz) as well as visible cameras with different interference filters, with field of views covering all ten half-modules of the stellarator, discovered a MARFE-like radiation zone on the inboard side of machine module 4. This structure is presumably triggered by an inadvertent plasma-wall interaction in module 4 resulting in a high impurity influx that terminates some discharges by radiation cooling. The main plasma parameters achieved in OP1.1 exceeded predicted values in discharges of a length reaching 6 s. Although OP1.1 is characterized by short pulses, many of the diagnostics are already designed for quasi-steady state operation of 30 min discharges heated at 10 MW of ECRH. An overview of diagnostic performance for OP1.1 is given, including some highlights from the physics campaigns.

Here we report the first measurements of the power levels of stray radiation in the vacuum vessel of Wendelstein 7-X using absolutely calibrated sniffer probes. The absolute calibration is achieved by using calibrated sources of stray radiation and the implicit measurement of the quality factor of the Wendelstein 7-X empty vacuum vessel. Normalized absolute calibration coefficients agree with the cross-calibration coefficients that are obtained by the direct measurements, indicating that the measured absolute calibration coefficients and stray radiation levels in the vessel are valid. Close to the launcher, the stray radiation in the empty vessel reaches power levels up to 340 kW/m(2) per MW injected beam power. Furthest away from the launcher, i.e., half a toroidal turn, still 90 kW/m(2) per MW injected beam power is measured.

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 1 MW 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 5.67 T for frequency step-tunable gyrotrons has been procured, has demonstrated a (static) field of 7.2 T and its capability of rapid field-changes.

Electron cyclotron resonance heating (ECRH) systems for next-step large fusion devices operate in continuous wave power in the multimegawatt range. The unique feature of narrow and well-localized power deposition assigns a key role to ECRH for different tasks, such as plasma start-up, electron heating, current drive, magnetohydrodynamic (MHD) control and profile shaping. The integration of high-power microwave diplexers in the transmission lines will improve the flexibility and efficiency while simultaneously reducing the complexity of large ECRH systems. They can serve as power or beam combiners, as slow and fast directional switches to toggle the power from continuously operating gyrotrons between two launchers, and as discriminators of low-power electron cyclotron emission (ECE) signals from high-power ECRH using a common transmission line and antenna. Among various design options a resonant diplexer with a narrow resonance was selected for application at ASDEX Upgrade. The design is driven by the specific physics requirements for MHD control experiments and possible use for line-of-sight ECE. The compact, waveguide-compatible design features a feedback-controlled mirror drive for tracking of the resonator to the gyrotron frequency. High-power, long-pulse tests were performed with the 140-GHz ECRH system for the stellarator W7-X. Results on the transmission characteristics, power combination, and stationary and controlled distribution of the input power to two outputs are presented. The qualification for in-line ECE was investigated.

A 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.

Controlled plasma start-up, heating towards thermonuclear fusion temperatures and steady state discharge control in advanced configurations is a major challenge for both, stellarators and tokamaks. Electron Cyclotron Resonance Heating (ECRH) and current drive (ECCD) plays a key role in the steady state operation scenarios of the ITER and JT-60 SA tokamaks as well as for the W7-X stellarator. The physics demands as well as the key technology of the different ECRH-systems, which are similar in frequency and have continuous wave (cw) capability, are presented. Advanced solutions for future ECRH-systems are discussed.

This paper discusses the use of some relatively new quasi-optical structures for configuring high-power gyrotron-based millimeter-wave systems for plasma fusion, microwave technologies, electron accelerators, radar, and communication. Techniques for wave combining and switching, and for wave matching to transmitters, receivers, and antennas are analyzed.

Electron cyclotron resonance heating (ECRH) is the main heating system for W7-X. A 10-MW ECRH plant with continuous wave (cw) capability is under construction to support the W7-X operation, which aims at demonstrating the steady-state capability of stellarators at reactor-relevant plasma parameters. The ECRH system consists often radio-frequency (rf) modules with 1 MW power each at 140 GHz. The rf beams of the individual gyrotrons are transmitted in common to the W7-X torus via open multibeam mirror lines. The losses of individual components of the transmission system were measured with both low- and high-power methods. Integrated full-power, cw measurements of the long-distance transmission losses are reported and compared to theoretical design estimates.

During the last years, electron cyclotron heating (ECH) was proven to be one of the most attractive heating schemes for stellarators because it provides net-current-free plasma startup and heating. Both the stellarator Wendelstein 7-X (W7-X), which is under construction at the Max-Planck-Institut fu umlr Plasmaphysik, Greifswald, Germany, and the International Thermonuclear Experimental Reactor (ITER) tokamak, which will be built in Cadarache, France, will be equipped with a strong ECH and current-drive system. Both systems are comparable in frequency and have continuous-wave capability (140 GHz, 10 MW for W7-X and 170 GHz, 24 MW for ITER). The commissioning of the ECH plant for W7-X is well underway; the status of the project and the first integrated full-power test results from two modules are reported and may provide valuable input for the ITER plant. The ten gyrotrons at W7-X will be arranged in two subgroups symmetrically to a central beam duct in the ECH hall. The RF wave of each subgroup will be combined and transmitted by a purely optical multibeam-waveguide (MBWG) transmission line from the gyrotrons to the torus. The combination of five 1 MW gyrotron beams to one beam line with a power of 5 MW reduces the complexity of the system considerably. The single- and MBWG mirrors and the polarizers have been manufactured. Cold tests of a full-size uncooled prototype line delivered an efficiency exceeding 90%. The microwave power will be launched to the plasma through ten chemical-vapor-deposited-diamond barrier windows and in-vessel quasi-optical plug-in launchers, allowing each 1-MW RF beam to be steered independently. The polarization, as well as the poloidal and toroidal launch angles, will be adjusted individually to provide optimum conditions for different heating and current-drive scenarios.

The development of high power gyrotrons for plasma physics research needs properly matched calorimetric loads able to absorb and measure the power, which nowadays is foreseen to be as high as 2 MW during CW operations. To this end IFP/CNR has developed a family of matched loads useful in the mm-wave frequency band for applications ranging from a few ms to CW in pulse length. The different loads in the family, made of an integrating sphere with a partially reflecting coating on the inner wall, are characterized by having the same absorbing geometry for the incoming beam and a different heat removal system for the specific application. Some important advances have been recently achieved from the point of view of the uniformity of power distribution on the absorbing wall and of the load construction. With the high precision achieved in the coating thickness a better control of the heating power distribution is possible by proper shaping of the local reflectivity, in addition to the shaping of the mirror dispersing the input beam. A more sophisticated model describing the power distribution has been developed, taking into account a variable thickness of the absorbing coating, the proper shape of the spreading mirror, the frequency of the incoming radiation and the shape of the input beam. Lower coating thickness is shown to be preferable, for a given local reflectivity, from the point of view of a lower peak temperature and thermal stress. The paper describes a load with variable coating thickness along the meridian of the sphere, showing a uniform power deposition on the inner walls. The cooling pipe is completely electroformed on the spherical copper shell, ensuring the maintenance of the correct curvature of the inner surface and fast heat conduction from the absorbing coating to the water through the thin copper body. For CW use all heated parts of the load must be cooled and this is achieved with 16 electroformed spiral channels. Both short pulse loads (0.1–1 s) and the CW version at 2 MW, 170 GHz, are described in the paper. High power tests on short-pulse loads have been done using a double frequency gyrotron, 105 GHz/600 kW for 0.5 s and 140 GHz/800 kW for 1 s. Also a method for emulating 2 MW conditions while using 1 MW gyrotron has been applied to test the load to be used for the European 2 MW coaxial cavity gyrotron development programme.

The spent electron beam of megawatt-class CW gyrotrons requires magnetic sweeping to keep the specific heat dissipation on the collector surface within technically acceptable limits of about 500 W per cm2. Conventionally this is realised with a solenoid coil placed over the collector, which sweeps the strike point of the electron beam up and down the collector-wall at low frequency (

To combine powers from a set of gyrotrons and to switch the combined wave beam between a number of output channels, various kinds of multiplexers can be used. Especially, narrow-band frequency diplexers in connection with small frequency-shift keying of gyrotrons can be used to switch the millimeter-wave power between two output channels. This technique can, for example, be used for fast beam steering for synchronous stabilization of rotating neoclassical tearing modes in tokamaks. Beam steering can be performed by a multistage multiplexer, provided that phase-controlled sources are available. In the paper, various concepts for fast directional switches as well as their integration into transmission lines are discussed. Calculations and low-power measurements of prototypes are presented. A resonant diplexer experiment is at present being prepared to be tested at high power in the electron cyclotron resonance heating (ECRH) system for Wendelstein 7X (W7-X). Requirements and techniques for frequency control of the gyrotrons are discussed, and the results of preliminary frequency modulation experiments are shown. Finally, future prospects for the application of diplexers in large ECRH systems are discussed.

Wendelstein 7-X (W7-X) is a large optimized stellarator (B=2.5T, V=30m3) aiming at demonstrating the reactor relevance of the optimized stellarators. In 2015 W7-X will begin its first operation phase (OP1.1) with five inertially cooled inboard limiters made of graphite. Assuming the heat loads can be spread out evenly between the limiters, 1 second discharges at 2 MW of ECRH heating power could be run in OP1.1. The expected plasma parameters will be sufficient to demonstrate the readiness of the installed diagnostics and even to run a first physics program. The diagnostics available for this first operation phase, including some special limiter diagnostics, and their capabilities are being presented. A shorter version of this contribution is due to be published in PoS at: 1st EPS conference on Plasma Diagnostics

Using specific features of the magnetic equilibrium in the W7-X stellarator, the ECRH scenarios with combined X2 and X3 modes are discussed. The RF beams for operation with X2 and X3 modes need to be launched from low- and, via the remote steering launcher, high-field-side, respectivaly, in the different crosssections of the device where the maximum and minimum of the magnetic field located. The aim is to explore the possibility of selective heating of the different classes of electrons, passing and trapped, by changing direction of the beam for X3 or switching between the beams for X2 and X3 launched from the different ports. The numerical predictions for this kind of experiments in W7-X are performed by coupled transport and ray tracing codes

For electron cyclotron resonance heating of the stellarator W7-X at IPP Greifswald, a 140 GHz/10 MW cw millimeter wave system is in construction. Two out of 12 launchers will employ a remote-steering design. This paper describes the overall design of the two launchers, and design issues like input coupling structures, manufacturing of corrugated waveguides, optimization of the steering range, integration of vacuum windows, mitrebends and vacuum valves into the launchers, as well as tests of prototype parts.

The stellarator W7-X, which is currently under construction at IPP Greifswald, Germany, will be equipped with a 10 MW ECRH system working at 140 GHz in CW regime. The microwave power will be generated by 10 gyrotrons delivering 1 MW each and will be transmitted from the gyrotron hall to the W7-X stellarator ports via a fully optical system. The status of the construction of the transmission lines and the design of the launchers is reported. Low-power tests of a prototype system at IPF Stuttgart are reviewed. Now, the first two gyrotrons are operating at IPP Greifswald, and high-power long-pulse tests have started. Measurements on transmission performance, behaviour of the water-cooled mirrors under thermal and microwave loads as well as alignment issues, characteristics of directional couplers, calorimetric loads, and other diagnostics are discussed.

Electron cyclotron resonance heating (ECRH) has proven to be one of the most attractive heating schemes for stellarators. Therefore, ECRH was chosen to be the main heating method for the Wendelstein 7-X stellarator (W7-X) now under construction at IPP Greifswald, Germany. A 10 MW ECRH system with continuous wave (CW) possibilities, operating at 140 GHz will be built up to meet the scientific goals of the stellarator. Two prototype gyrotrons with an output power of I MW were developed in collaboration between European research laboratories and European industry (Thales Electron Devices, France). The gyrotrons are equipped with a single-stage depressed collector, an optimised quasi-optical mode converter and a CVD-diamond window. The prototypes have been successfully tested at FZK. With the second one, an output power of 0.89 MW at a pulse duration of 3 min and an output power of 0.54 MW for about 15 min have been obtained. (c) 2005 Elsevier B.V. All rights reserved.

The WENDELSTEIN 7-X stellarator (W7-X) is the next step device in the stellarator line of IPP and is presently under construction at the Greifswald branch institute. The experiment aims at demonstr...

This paper reports the results of the high-power test of a remote steering launcher mock-up at 140 GHz, which were performed at the ECRH installation for the future stellarator W7-X at IPP Greifswald. The mock-up test system consists of a 6.62 m long corrugated square waveguide with a steerable optic at the entrance and various diagnostics at the exit of the waveguide. A straight and a dog-leg version of the launcher were investigated. The high-power tests of the straight setup have been performed with powers up to P0 = 700 kW (typically 500 kW) and pulse lengths of up to 10 seconds. For both polarizations (parallel and perpendicular to the steering plane), no arcing was observed in spite of the fact, that the experiments were performed under ambient atmospheric conditions. After the integration of 2 mitre bends in the setup, arcing limited the usable parameter range. The ohmic loss PΩ of the waveguide was measured via the temperature increase of the waveguide wall, and was used to calibrate the calculated angular dependence of the total ohmic losses of the waveguide. Short-pulse radiation pattern measurements with thermographic recording show high beam quality and confirm the steering range of −12° < < 12°.

Summary form only given, as follows. Electron cyclotron resonance heating (ECRH) has proven during the last few years to be one of the most attractive heating schemes for stellarators, as it provides net current free plasma start tip and heating. Extensive measurements on stellarators at IPP Garching provide a solid physical and technological basis for ECRH systems. Therefore, ECRH will be the main heating method for the Wendelstein 7-X stellarator (W7-X) now under construction at Greifswald / Germany. A 10 MW ECRH system with continuous wave (CW) possibilities operating at 140 GHz will be built to meet the scientific goals of the stellarator at Greifswald with inherent steady-state capability at reactor relevant plasma parameters. A prototype gyrotron with an output power of 1 MW was developed in collaboration between European research laboratories, and European industries. The gyrotron is equipped with a single stage depressed collector for increasing the efficiency, an optimized quasi-optical mode converter and a CVD-diamond window. The prototype gyrotron has been successfully tested with an output power of 0.89 MW at a pulse duration of three minutes and all output power of 0.54 MW for about 15 minutes. The specified value of Gaussian beam output power exceeding 0.9 MW has been obtained for about 1 minute. Further results will be given.

Basic research on high-power ECRH started 20 years ago at IPP using 28 GHz pulses with 200 kW for 40 ms at the W7-A stellarator. These pilot experiments triggered a strong activity of exploration of the unique capabilities of localized heating and current drive. The achievements in physics were strongly linked with progress in source and transmission line technology. The capability and versatility of ECRH are reviewed using W7-A and W7-AS as example experiments; the latter was shut down on July 31, 2002. The milestone achievements are discussed. Standard heating scenarios such as O-mode and X-mode as well as advanced scenarios like mode-conversion heating via the O–X–B process at different harmonics were investigated and selected results are presented. First experiments with current drive by Bernstein-waves are reported. The physics of wave interaction with stellarator-specific trapped particle populations is discussed. The results from W7-A and W7-AS establish the experimental and technological bases for the 10 MW, CW ECRH system at W7-X, which aims to demonstrate the inherent steady-state capability of stellarators.

For the future stellarator W7-X in Greifswald, a powerful electron cyclotron resonance heating (ECRH) system at 140 GHz with a total RF power of 10 MW CW is under construction. The transmission of the radiation from the gyrotron hall to the machine will be realized via a fully optical system consisting of more than 160 water cooled mirrors. This paper describes the design of the mirrors and presents results of calculations of the thermo–mechanical properties. Experimental verification of the calculations showed that mirror deformation under application of a heat load equivalent to the absorbed power from a 1 MW mm-wave beam is in the tolerable range. Water cooled mirrors of the single-beam type have been used successfully during conditioning of a prototype gyrotron with a mm-wave power of 850 kW and a pulse length of up to 180 s. To test and optimize in-situ measurements of beam parameters and power of the CW ECRH system on W7-X, these mirrors are equipped with different diagnostics.

WENDELSTEIN 7-X (W7-X) shall confirm the favourable plasma properties and the high density and beta limits of the helical advanced stellarator and demonstrate steady-state operation. The magnetic configuration of W7-X is characterized by a set of 50 non-planar and 20 planar superconducting coils. The magnet system will be manufactured and assembled to a precision of a few millimetres and maintain its symmetry during cool-down to cryogenic temperatures. Power supplies allows one to adjust the magnetic field with a precision of a few milli Tesla and safely dump the magnet energy in case of a quench. The plasma vessel gives maximum space for the plasma and is kept symmetrically with respect to the plasma by dedicated means. Steady-state heating is achieved by 10 MW ECRH. Energy and particles are controlled by a continuously working divertor. All plasma-facing surfaces are covered by CFC, graphite or B4C, respectively. The paper reviews the status of the construction and describes details of the design.

Stellarators have the intrinsic property of steady state operation. However, on present-day stellarators the pulse length is usually not only limited due to technical reasons, but also by physical problems. Lack of density control and a subsequent radiation collapse terminate the discharges quite often at high densities. To improve the control of the plasma-wall interaction, the island divertor concept was developed for optimized stellarators. To test this divertor concept on W7-AS, all limiters were removed and replaced by ten divertor modules. In subsequent divertor experiments a promising new plasma operational regime has been discovered which is termed 'high density H-mode' (HDH-mode). During the transition into that regime a clear reduction of ELM-like events and turbulent fluctuations is observed. The HDH-mode combines good energy confinement with very low impurity confinement resulting in low core radiation, but high edge-localized radiation. Consequently, stationary discharges at densities of typically 2 x 10 20 m -3 can be performed within the accessible pulse length of about 1 s. At densities above 3 x 10 20 m -3 a controlled transition from attached to partially detached plasmas is observed. The still edge-localized radiation reaches 90% of the heating power so that the power load onto the divertor target plates is further reduced. At a lower toroidal field of 0.9 T average β-values could be raised from earlier 2% to more than 3% in magnetic field configurations with rather smooth flux surfaces at the plasma boundary. The recently obtained results render excellent prospects for W7-X, the larger superconducting successor experiment of W7-AS.

The development of high-power gyrotrons (118 GHz, 140 GHz) in continuous-wave (CW) operation for heating nuclear fusion plasmas has been in progress for several years in a joint collaboration between different European research institutes and industrial partners. The 140-GHz gyrotron being under development for the installation at the W7-X stellarator now under construction at the IPP Greifswald, Germany, operates in the TE/sub 28,8/ mode and is equipped with a diode type magnetron injection electron gun, an improved beam tunnel, a high mode-purity low-Ohmic loss cavity, an optimized nonlinear up-taper, a highly efficient internal quasi-optical mode converter, a single-stage depressed collector and an edge-cooled, single disk CVD-diamond window. RF measurements at pulse duration of a few milliseconds yielded an RF output power of 1.15 MW at a beam current of 40 A and a beam voltage of 84 kV. Depressed collector operation has been possible up to decelerating voltages of 33 kV without any reduction of the output power. Long pulse operation (10 s at 1 MW) was possible without any signs of a limitation caused by the tube. For this output power the efficiency of the tube could be increased from about 30% without to about 50% with depression voltage. The best performance reached so far has produced an energy per pulse as high as 90 MJ (power 0.64 MW, pulse length 140 s) which is the highest value achieved in gyrotrons operating at this frequency and power level. The pulse-length limitations so far are mainly due to the external system.

For electron cyclotron resonance heating of the stellarator W7-X at IPP Greifswald, a 140 GHz/10 MW cw millimeter wave system has been built. Two out of 12 launchers will employ a remote-steering design. This paper describes the overall design of the two launchers, and design issues like input coupling structures, manufacturing of corrugated waveguides, optimization of the steering range, integration of vacuum windows, mitrebends and vacuum valves into the launchers, as well as low power tests of the finished waveguides.

Using specific features of the magnetic equilibrium in the W7-X stellarator, ECRH scenarios with X2 and X3 modes are discussed. The aim is to explore the possibility of selective heating of the different classes of electrons, passing and trapped, by different RF beams with different frequencies, which can be launched from ports located in different crosssections of the device. Perspectives for this kind of experiments in W7-X are estimated numerically by coupling transport and ray tracing codes.

Microwave stray radiation resulting from unabsorbed multiple reflected ECRH / ECCD beams may cause severe heating of microwave absorbing in-vessel components such as gaskets, bellows, windows, ceramics and cable insulations. In view of long-pulse operation of WENDELSTEIN-7X the MIcrowave STray RAdiation Launch facility, MISTRAL, allows to test in-vessel components in the environment of isotropic 140 GHz microwave radiation at power load of up to 50 kW/m2 over 30 min. The results show that both, sufficient microwave shielding measures and cooling of all components are mandatory. If shielding/cooling measures of in-vessel diagnostic components are not efficient enough, the level of stray radiation may be (locally) reduced by dedicated absorbing ceramic coatings on cooled structures.

For the future stellarator W7-X of IPP at Greifswald, a powerful electron cyclotron resonance heating facility is being built which will include 10 gyrotrons at 140 GHz with a CW power of 1 MW each plus two optional gyrotrons at 70 GHz. The millimetre wave power will be transmitted fully optically via two multiple-beam wave guides (MBWG) with a length of about 50 m. This paper presents calculations and low-power measurements of the transmission characteristics of a prototype MBWG, which promises a high transmission efficiency for the ECRH system under construction. Furthermore, tests of prototype water-cooled mirrors are discussed.

Following the approval by EURATOM and the German government WENDELSTEIN 7-X (W7-X) is presently the largest fusion project under construction. W7-X is a helical advanced stellarator (HELIAS) which produces the confining magnet field only by magnet coils that enables steady-state operation. W7-X aims to demonstrate that the HELIAS configuration has the potential for a future power reactor. The successful application of new technologies for manufacturing prototypes and the positive results gained from tests allowed to design the machine in detail and to order major components. The geometry of the non-planar magnet coils has a considerable impact on the design of the machine in particular on the shape of the plasma vessel, positioning of the plasma-facing components and the size and orientation of the ports. The requirement for steady-state operation has consequences for many subsystems of W7-X. The magnet coils need to be superconducting and cooled to liquid helium temperature. Gyrotrons shall continuously provide 10 MW of ECR heating power. The divertor must be cooled to withstand heat loads of up to 10 MW/m 2 . The schedule of W7-X is determined by the delivery dates of the non-planar coils, the plasma vessel and the outer vessel. Start of commissioning and scientific operation is scheduled for spring 2006.

The development of high power gyrotrons in continuous wave (CW) operation for heating of plasmas used in nuclear fusion research has been in progress for several years in a joint collaboration between different European research institutes and industrial partners. A recent R&D program aims at the development of 140 GHz gyrotrons with an output power of 1 MW in CW operation for the 10 MW ECRH system of the new stellarator plasma physics experiment Wendelstein 7-X at IPP Greifswald, Germany. The work is performed under responsibility of FZK Karlsruhe in collaboration with CRPP Lausanne, IPF Stuttgart, IPP Garching and Greifswald, CEA Cadarache and TED Velizy. The gyrotron operates in the TE28.8 mode and is equipped with a diode type magnetron injection electron gun, an improved beam tunnel, a high-mode purity low-ohmic loss cavity, an optimized non-linear up-taper, a highly efficient internal quasi-optical mode converter, a single-stage depressed collector and an edge-cooled, single disk CVD-diamond window. RF measurements at pulse duration of a few milliseconds yielded an RF output power of 1.15 MW at a beam current of 40 A and a beam voltage of 84 kV. Depressed collector operation has been possible up to decelerating voltages of 33 kV without any reduction of the output power, and an efficiency of 49 % has been achieved. Long pulse operation of the gyrotron was possible with an output power of 1 MW at a pulse length of 10 s without any signs of a limitation caused by the tube. For this output power the efficiency of the tube could be increased from about 30 % without depression voltage to about 50% with depression voltage. At an output power of 640 kW, a pulse length of 140 s could be achieved.

For the future stellarator W7-X of IPP at Greifswald, a powerful electron cyclotron resonance heating (ECRH) facility is under design, which will include ten gyrotrons at 140 GHz with a CW power of 1 MW each plus two optional gyrotrons at 70 GHz. The millimetre wave power will be transmitted via two multi-beam waveguides (MBWG) with a length of about 50 m. This paper shows results for the optimum shape of the mirrors and the mode conversion of the MBWG transmission system. The design which will be used for water-cooled mirrors and cooling structures is discussed. Concepts for the back reflectors, which will be mounted at the inner side of the vacuum vessel of W7-X to redirect the non-absorbed part of the beam back into the plasma are presented. A test facility to test the transmission properties of quasi-optical lines, including a measurement system for ohmic loss of mirrors, a reflectometer to detect possible surface deformations and an alignment control system is described.

Electron Cyclotron Resonance Heating (ECRH) is the main heating method for both the W7-AS stellarator, which is operational, and W7-X, which is under construction. An overview on physics studies on ECRH and EC-current drive (ECCD) in an extended parameter range at W7-AS is presented. Current drive was investigated with an ECRH power of up to 1.3 MW at 140 GHz in a wide range of plasma density and launch angle. The experimental results are compared to predictions from quasilinear theory. Electron temperatures of up to 5.7 keV were measured at low to intermediate densities, which can only be explained by the beneficial effect of positive radial electric fields (‘electron root feature’). The results from W7-AS establish the experimental and technological basis for W7-X, which aims at demonstrating the inherent steady state capabality of stellarators at reactor relevant plasma parameters. The status of the design and construction of the ECRH system for W7-X is presented.

The development of high power CW gyrotrons for ECRH heating of fusion relevant plasmas has been in progress for several years in a joint collaboration between different European research institutes and an industrial partner. Two development are on going, aiming, respectively, towards a 0.51-MW-210-s gyrotron at 118 GHz for the tokamaks TCV of CRPP (2 s pulse length) and Tore Supra of CEA (210 s pulse length), and towards a 1 MW-CW gyrotron at 140 GHz for the stellarator W7-X under construction in Greifswald. Series 118 GHz gyrotrons have been delivered to CRPP and CEA. Long pulse results (15.5 s at 400 kW) as well as considerations on power modulation capabilities of the tube and on long pulse effects are discussed. In a second development program, a 1-MW/CW 140 GHz gyrotron with a CVD diamond window and a single-stage depressed collector has been designed and constructed as a first prototype for the 10-MW ECRH (Elecron Cyclotion Resonance Heating) system of the new stellarator experiment Wendelstein 7-X of IPP Greifswald/Germany. The gyrotron operates in the TE28.8 cavity mode and provides a linearly polarized, TEM0.0 Gaussian RF beam. It is composed of a diode MIG gun, an improved beam tunnel, a high-mode purity low-ohmic loss cavity, an optimized non-linear up-taper, a highly efficient internal quasi-optical mode converter employing an improved launcher together with one quasi-elliptical and two beam shaping reflectors, a large single stage depressed collector at ground potential with a beam sweeping magnet, and a horizontal RF output. (C) 2001 Elsevier Science B.V. All rights reserved.

The stellarator Wendelstein 7-X (W7-X) is presently under construction at Greifswald, Germany, and the start of operation is planned in 2006. W7-X is a large `advanced stellarator' of the HELIAS type (R = 5.5?m, a = 0.55?m, B0 = 3?T, five periods, moderate shear and variable rotational transform 5/6 ? ? ? 5/4 at the boundary) with the aims of demonstrating the reactor potential of this stellarator line in steady state operation close to fusion relevant parameters. The capability of stationary operation requires the realization of a superconducting magnet system consisting of 50?modular coils and 20?planar coils, the operation of a 140?GHz ECR CW heat source of 10?MW, the installation of a divertor to handle the power and particle flux, and to limit the impurity fraction to tolerable levels. Additional heating schemes, ICRF and NBI, will be provided for flexible experimentation.

Electron cyclotron resonance heating (ECRH) plays a key role in stellarator research, because it provides net current free plasma start up and heating toward reactor relevant plasma parameters. ECRH was extensively used and investigated in the stellarator experiments at IPP Garching, i.e., the W7-A and the W7-AS stellarators. These experiments provide a solid physics and technological basis for the 10 MW, CW ECRH system, which is under construction for the superconducting next step stellarator W7-X and will become operational in 2005. We briefly describe some of the major stellarator specific physics results on ECRH and electron cyclotron (EC)-current drive from W7-A and W7-AS. The scientific goals and the design of W7-X are outlined together with the demands for the ECRH system, which is the main heating system in the first stage of the experiment. The present status of the ECRH engineering design including the gyrotrons, all auxiliary systems, the transmission line, and the launching system are presented.

Experimental and theoretical investigations of lower hybrid (LH) turbulence in the W7-AS stellarator are presented. The turbulence is excited by an ion beam, which is generated by a weak neutral hydrogen beam injected transversely to the confining magnetic field. The instability is detected by collective Thomson scattering of powerful gyrotron radiation. From the measured density dependence of the frequency it was identified as an LH type of instability. The spectrum is characterized by a narrow bandwidth in spite of the inherently poor radial resolution of the backscattering geometry. The theoretical model of an LH instability driven by a transverse fast ion component under the double resonance condition (coincidence of the LH frequency with a high cyclotron harmonic of the fast ions) is developed. The instability growth rate is derived. The stabilizing effect of high bulk ion temperatures was observed experimentally, in accordance with theoretical modelling. An instability saturation mechanism similar to the well known stochastic ion heating is proposed.

Electron cyclotron resonance heating and electron cyclotron current drive (ECCD) in the W7-X stellarator, which is presently under construction, are investigated by means of a ray-tracing code. Two heating schemes are considered: launching of extraordinary and ordinary waves from the low magnetic field side at the second harmonic of the electron cyclotron frequency. Full power absorption is typically obtained for the extraordinary mode at the second harmonic for the predicted plasma parameters of W7-X. In this case, current drive calculations are also performed to determine the optimum launching conditions. A current drive efficiency, , of the order of is obtained for the scenarios of low-field-side injection under consideration (for a density , and a temperature , corresponding to a normalized efficiency . ECCD can be used to tailor the radial profile of the rotational transform (in order, for example, to compensate the residual bootstrap current, and to avoid low-order rational surfaces inside the plasma). A variational calculation of ECCD, extended to take trapped-particle effects into account, and its straightforward implementation in the ray-tracing code are briefly presented. The results for the driven current are compared with estimates of the residual bootstrap current in the optimized magnetic configuration of W7-X. Significant absorption is found over a wide density range also in the case of ordinary mode heating at the second harmonic, allowing an extension of the operational parameter window to higher densities.

In W7-AS, electron cyclotron resonance heating (ECRH) discharges sustained by three gyrotrons, one resonant on-axis and two off-axis, were investigated by a perturbative square-wave modulation of the on- and the off-axis deposited power, respectively. The absorption profile of the modulated ECRH power as well as the transport coefficients, were determined from the response of the amplitude and phase delay of the driven temperature perturbation measured during a scan in the modulation frequency . To explain the results obtained during perturbative off-axis modulation, the appearance of a convective term (not compatible with the results for on-axis modulation) is required. However, an alternative (and more conservative) explanation is related to the ECRH absorption physics. During off-axis modulation the inwardly propagating heat wave causes a temperature perturbation (of the order of few eV at the plasma axis). As the absorption coefficient depends on temperature, this temperature perturbation will also cause a modulation in the absorption of the (stationary) on-axis power. The phase of this `induced modulation' will depend on the phase of the inwardly propagating heat wave. Simplified modelling confirms that this effect may explain the experimental findings.