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A suite of diagnostics is proposed to characterize microwave plasma dissociation of CO2: laser scattering, Fourier transform infrared spectroscopy, and passive emission imaging. It provides a comprehensive performance characterization as is illustrated on the basis of experiments in a 2.45 GHz, 1 kW microwave reactor with tangential gas injection. For example, two operating regimes are identified as function of pressure: the diffuse and constricted plasma mode. Their occurrence is explained by evaluation of microwave propagation, which changes with the electron-heavy particle collision frequency ve−h. In the diffuse mode, gas temperatures of 1500–3500 K are determined. The measured conversion degree, specific energy input, and temperature are summarized in a two-temperature thermal model, which is solved to obtain the gas temperature at the periphery of the reactor and the size of the hot zone. Solutions are found with edge temperatures of hundreds of K, and hot zone fractions which agree with the measured behavior. The agreement shows that non-thermal processes play only a marginal role in the measured parameter space of the diffuse discharge. In the constricted mode, the radial plasma size is independent of power. A skin depth equal to the plasma size corresponds to electron densities of 1018–1019 m−3. Temperatures in the central filament are in the range 3000–5000 K. Both discharge modes are up to 50% energy efficient in CO production. Rayleigh signals increase in the afterglow, hinting at rapid gas cooling assuming that the gas composition remains unchanged.

A basic requirement for ITER equipment to meet is a high level of reliability, because ITER operation time is precious and radioactive operation leaves limited scope for repair. In order to reduce the risk of failure during ITER operation an effective risk mitigation strategy is necessary. This paper presents such strategy for the ITER electron cyclotron upper port launcher (ECUPL). A preliminary ECUPL risk analysis identifies possible failure modes. A probabilistic risk assessment quantifies the risk of failure using a 4 x 4 impact-likelihood matrix. Impact is quantified through technical, cost and schedule elements. Likelihood depends on the risk mitigation strategy adopted. A cost benefit analysis determines the most cost effective risk mitigation strategy. An essential element in risk mitigation is the testing of equipment prior to installation on the ITER machine. This paper argues the need for low- and highpower millimetre wave tests carried out on the fully assembled ECUPL It presents a conceptual design for a dedicated on-site test bed that can handle 2 of 8 microwave beams at 2 MW long pulse operation. (C) 2010 Elsevier B.V. All rights reserved.

The millimeter-wave system of the remote steering launcher at the upper port level is composed of beam-lines that are rated for 2-MW continuous-wave operation at 170 GHz. In each beamline, a torus window is located between the entrance to the in-vessel square corrugated waveguide and the steerable mirrors in the launcher back end. In the reference design, the maximum steering angle of 12 deg imposes a 27-mm off-center beam shift to the window disk center, which in turn leads to asymmetrical heating of the window. This raises particular concerns of enhanced thermomechanical stresses in the window and in the metallic window cuffs. In order to qualify the optical, mechanical, and thermohydraulic design, high-power short-pulse and thermohydraulic tests were performed using a prototype chemical vapor deposition diamond torus window developed and manufactured at Forschungszentrum Karlsruhe. It was proven that arcing did not occur even under maximum millimeter-wave power levels available (up to 0.53 MW) and that the millimeter-wave beam profile was fully maintained. A test facility allowed thermohydraulic studies of the window cooling system with parameters characteristic for component cooling water loops at ITER (p w =1.0 MPa, T w = 40°C).

In the course of the development of a remote steering ECRH upper port launcher for ITER, it became clear that a modification could be introduced in the conceptual design in order to solve a number of structural weaknesses. Up to that point, all conceptual layouts were based on each remote steering beamline having a single front mirror placed in front of the square waveguide to aim the beam towards its resonance surface in the plasma. By placing an additional mirror per line inside the front shield of the upper port plug - effectively creating a dogleg routing - a number of structural issues were solved. This modification allows for a decrease of the heat load on the front mirrors and a shift downwards of the launching point. Additionally, through correct placement and focusing of the mirrors, the front shield penetration could be reduced by a factor of 4 and the cut in the blanket module below the upper port could be reduced significantly, while the level of overall performance could be increased as well. In order to visualise this new concept accurately, a more detailed design of the beam propagation was required. Through concerted effort within our institute, two different approaches were made to come to this new design; further advancements of the 3D-model and an Excel-based 2D simulation. This dual approach, together with beam tracing calculations done by affiliated institutes have indicated that the dogleg can prove to be a reliable design for a RS upper port launcher.

The association FOM-Rijnhuizen is developing a plug-in 170 GHz ECW launcher system for use in the upper-ports of ITER, based on the remote steering (RS) principle. The launching system enables up to eight mm-wave beams per port plug to be directed from the feeding waveguides through corrugated square waveguides (≈44 × 44 mm2) inside the port plug to the plasma. A mock-up system has been manufactured, capable of vacuum & high-power short-pulse operation. The transmission line consists of a taper, to launch an HE11 mode in 63.5 mm circular corrugated waveguide. The circular waveguide is used as input for the steering mechanism, housed in a vacuum vessel, which directs the beam under an angle between −12° and 12° into the copper square corrugated waveguide. Antenna patterns from the exit aperture of the square waveguide are scanned using a 3-axis scanner.

A storage scheme for Renewable Energy (RE) based on the plasmolysis of CO2 into CO and O2 has been experimentally investigated, demonstrating high energy efficiency (>50%) combined with high energy density, rapid start-stop and no use of scarce materials. The key parameter controlling energy efficiency has been identified as the reduced electric field. Basic plasma parameters including density and temperature are derived from a simple particle and energy balance model, allowing parameter specification of an upscale 100 kW reactor. With RE powered plasmolysis as the critical element, a CO2 neutral energy system becomes feasible when complemented by effective capture of CO2 at the input and separation of CO from the output gas stream followed by downstream chemical processing into hydrocarbon fuels.

Power-to-gas is a storage technology aiming to convert surplus electricity from renewable energy sources like wind and solar power into gaseous fuels compatible with the current network infrastructure. Results of CO2 dissociation in a vortex-stabilized microwave plasma reactor are presented. The microwave field, residence time, quenching, and vortex configuration were varied to investigate their influence on energy- and conversion efficiency of CO2 dissociation. Significant deterioration of the energy efficiency is observed at forward vortex plasmas upon increasing pressure in the range of 100 mbar towards atmospheric pressure, which is mitigated by using a reverse vortex flow configuration of the plasma reactor. Data from optical emission shows that under all conditions covered by the experiments the gas temperature is in excess of 4000 K, suggesting a predominant thermal dissociation. Different strategies are proposed to enhance energy and conversion efficiencies of plasma-driven dissociation of CO2.

High-power millimetre wave beams employed on ITER for heating and Current drive at the 170 GHz electron cyclotron resonance frequency require agile steering and tight focusing of the beams to suppress neoclassical tearing modes. This paper presents experimental validation of the remote steering (RS) concept of the ITER upper port millimetre wave beam launcher. Remote steering at the entrance of the upper port launcher rather than at the plasma side offers advantages in reliability and maintenance of the mechanically Vulnerable steering system. A one-to-one scale mock-up consisting of a transmission line, mitre bends, remote steering unit, vacuum window, square corrugated waveguide and front mirror simulates the ITER launcher design configuration. Validation is based on low-power heterodyne measurements of the complex amplitude and phase distribution of the steered Gaussian beam. High-power (400 kW) short Pulse (10 ms) operation under vacuum, diagnosed by calorimetry and thermography of the near- and far-field beam patterns, confirms high-power operation, but shows increased power loss attributed to deteriorating input beam quality compared with low-power operation. Polarization measurements show little variation with steering, which is important for effective current drive requiring elliptical polarization for O-mode excitation. Results show that a RS range Of LIP to -12 to +12 can be achieved with acceptable beam quality. These measurements confirm the back-up design of the ITER ECRH&CD launcher with future application for DEMO. (C) 2009 Elsevier B.V. All rights reserved.

A Fast Fourier Transform (FFT) based wide range millimeter wave diagnostics for spectral characterization of scattered millimeter waves in plasmas has been successfully brought into operation. The scattered millimeter waves are heterodyne down-converted and directly digitized using a fast analog-digital converter (ADC) and a compact Peripheral Component Interconnect (cPCI) computer. Frequency spectra are obtained by FFT in the time domain of the intermediate frequency signal. The scattered millimeter waves are generated during high power Electron Cyclotron Resonance Heating (ECRH) experiments on the TEXTOR Tokamak and demonstrate the performance of the diagnostics and, in particular, the usability of direct digitizing and Fourier transformation of millimeter wave signals. Major benefit of the new diagnostics is a tunable time and frequency resolution due to post-detection, near-Real-Time processing of the acquired data. This diagnostics has a wider application in astrophysics, earth observation, plasma physics and molecular spectroscopy for the detection and analysis of millimeter wave radiation, providing high-resolution spectra at high temporal resolution and large dynamic range. Such a diagnostics also has the potential to detect Electron Cyclotron Emission (ECE) and to be used in Real-Time ECE feedback control systems. © 2009 IEEE.

Electron cyclotron current drive will become the main scheme on ITER for the stabilization of neoclassical tearing modes (NTMs) and the control of sawtooth oscillations. The effectiveness of this scheme forms the basis for the requirements of the ITER Upper Port Launcher. These requirements include the need for steering the mm-wave beam to the pertinent magnetic flux surfaces where the instability develops. Several concepts have been explored; front steering (FS) where a system of movable mirrors is situated at the plasma-facing end of the launcher; remote steering (RS) where the movable mirror system is situated at the back end of the launcher system. The advantage of the RS concept is that moving parts are placed away from the plasma in a secondary vacuum system, less exposed to plasma neutron flux, thus offering advantages in reliability and safety of the machine. Also, contamination of the beam steering unit by Be and T is less of a problem. However, beam-focusing properties of the RS launcher deteriorate with increased steering angle, which limits the NTM stabilization performance as compared with FS. In this paper, recent improvements in the RS design are presented. The stabilization efficiency calculated reaches values that meet most of the ITER requirements. Improvements include non-spherical front mirrors and a tapered square corrugated waveguide. Rather than steering all mm-wave beams over the entire vertical height range specified, RS launchers with a limited steering range are explored, yielding a performance improvement of 25% with an acceptable mm-wave heat load of 5.7 MW m−2 on the front mirrors. This result leads to a dual launcher concept with one set of beams dedicated to control NTMs at the outward lying q = 2 surface and another set of beams dedicated to control NTMs at the inward lying q = 3/2 surface in combination with sawteeth control at the innermost q = 1 surface.

An electron cyclotron emission (ECE) receiver inside the electron cyclotron resonance heating (ECRH) transmission line has been brought into operation. The ECE is extracted by placing a quartz plate acting as a Fabry-Perot interferometer under an angle inside the electron cyclotron wave (ECW) beam. ECE measurements are obtained during high power ECRH operation. This demonstrates the successful operation of the diagnostic and, in particular, a sufficient suppression of the gyrotron component preventing it from interfering with ECE measurements. When integrated into a feedback system for the control of plasma instabilities this line-of-sight ECE diagnostic removes the need to localize the instabilities in absolute coordinates. (C) 2008 American Institute of Physics. [DOI: 10.1063/1.2976665]

Torus window prototypes for electron cyclotron (EC) heating and current drive (H&CD) systems are developed for different launcher concepts in accordance with the specific transmission requirements. For ITER, single-disk windows are designed for 2 MW power transmission at fixed frequency (170 GHz) in two variants. For the RS launcher, the remote steering unit is placed close to the torus window in the launcher back-end which calls for a large window aperture (95 mm) to avoid beam vignetting at the extreme steering angles of ±12°. For the FS launcher with the steering mechanism placed in the front shield of the launcher, the aperture and the disk thickness are reduced as the torus window is integrated into a cylindrical waveguide configuration with an inner diameter of 63.5 mm. The reduced absorbed power and cooling paths allow to consider indirect cooling as an alternative to edge cooling which eliminates the risk of tritium contact to the cooling water in case of crack formation in the diamond disk. For the multifrequency torus window at ASDEX Upgrade (aperture: 80 mm), a double-disk configuration is realised with the disk separation of 5 mm, fine tuneable over ±1 mm. Prototypes of the high power windows were manufactured. First results of the current thermo-hydraulic and of the mm-wave performance tests support the feasibility of the concepts. For the ITER torus windows, the tool development is described which provides on-site replacement by automated cutting and welding tools.

At the TEXTOR tokamak a 140 GHz, 800 kW, 10 s gyrotron is employed for studies on ECW heating and ECCD. A key program is the suppression of tearing modes, which is aided by the unique facility at TEXTOR to program magnetic structures during a discharge (DED). A scheme is under development which aims to detect the perturbations on the electron cyclotron emission caused by these instabilities via the same line of sight as is used by the ECRH beam, and to use this information for feedback control of the instability. A brief overview of TEXTOR and relevant island parameters is given as well as a description of the upgraded ECRH system. This is followed by a description of the design of the transmission line coupler and receiver for island detection which constitutes the main topic of the paper. The final section sketches the feedback controller that processes the receiver signals, and controls the moveable launcher and the gyrotron power.

The design of the mm-wave system for the ITER upper ports is being carried out with the aim to inject Electron Cyclotron Waves (ECW) in the plasma in order to stabilize neoclassical tearing modes (NTM). Four upper ports are equipped with six mm-wave lines, each capable of transmitting up to 2 MW at 170 GHz. The required beam-steering capabilities es in our approach are achieved by Remote Steering (RS). New design efforts are presented that improve the current-drive capability of the RS set-up substantially by doubling the square waveguide length to around 9 m.

The association FOM-Rijnhuizen, together with EFDA ECHULA group, develops a plug-in 170 GHz ECW launcher system, based on the remote steering (RS) principle. At this principle the steerable mirror is placed before the input of a Square Corrugated Waveguide (SCW), with no plasma facing moving parts [1]. A high-power testing program was performed on a mockup, in cooperation with FZK, to validate the relative power-loss, steering range, beam quality and polarization. Recently the design has evolved in using a longer SCW, of about 8 to 10 meter, to improve the physics performance to guarantee NTM stabilization in the important ITER scenarios.

The Euratom association FOM-Rijnhuizen is developing a plug-in 170 GHz ECW launcher system for use in the upper-ports of ITER, based on the remote steering (RS) principle, designed to stabilise the neoclassical tearing modes (NTM) at the q=3/2 and q=2/1 surfaces by inducing off-axis current drive. The launching system enables up to eight mm-wave beams per port plug to be directed from the feeding waveguides through corrugated square waveguides (/spl ap/ 44/spl times/44 mm/sup 2/) inside the port plug to the plasma. A mock-up system has been manufactured, capable of vacuum & high-power short-pulse operation. The transmission line consists of a taper, to launch an HE/sub 11/ mode in 63.5 mm circular corrugated waveguide. (The taper will be replaced by a gyrotron source in high power testing). The circular waveguide is used as input for the steering mechanism, housed in a vacuum vessel, which directs the beam under an angle between -12/spl deg/ and 12/spl deg/ through the CVD diamond window into the copper square corrugated waveguide. Antenna patterns from the exit aperture of the square waveguide are scanned using a 3-axis scanner. Low-power measurements (Takahashi et al., 2003) are a preparatory phase for high-power tests, currently prepared in FZK. In the low-power measurements, waveguide performance will be inferred from antenna pattern measurements. Of interest are slight differences in beam parameters (angle, waist etc.) relative to the input beam.

An ECRH (electron-cyclotron resonance heating) launching system for the ITER upper ports is being designed. The aim of the system is to inject electron cyclotron waves (ECW) in the ITER plasma in order to stabilize neoclassical tearing modes (NTM). Each of the four upper-port launchers consists of six mm-wave lines capable of transmitting high power up to 2 MW per line at 170 GHz. In order to exploit the capability of ECW for localized heating and current drive over a range of plasma radii in ITER, the ECH&CD upper port launcher must have a beam steering capability. The remote steering (RS) principle has great advantages, because it enables to avoid steerable mirrors with flexible cooling lines at the plasma-facing end of the launcher. The principle consists of a long, corrugated, square waveguide having the steerable optics placed outside of the first confinement boundary of the vacuum vessel. All vulnerable components are far away from the hostile plasma environment. Furthermore, the RS launching system enables to do maintenance on the system during shutdown, without affecting the torus vacuum and the blanket cooling circuits.

The Fusion Free-Electron Maser (FFF) is the prototype of a high-power, tunable source of mm-wave radiation, for use on fusion plasma devices. In previous experiments a net output power of 730 kW at 206 GHz was generated in short pulses. The present experiment has been equipped with a system to recover the charge and energy of the spent electron beam. We present experimental results which show output of mm-wave radiation at constant power level during the full pulse length, as well as single-frequency operation; even though the cavity is highly overmoded; the latter is reached by effective suppression of spurious modes by the feedback system.

An intermediate frequency (IF) band digitizing radiometer system in the 100-200 GHz frequency range has been developed for Tokamak diagnostics and control, and other fields of research which require a high flexibility in frequency resolution combined with a large bandwidth and the retrieval of the full wave information of the mm-wave signals under investigation. The system is based on directly digitizing the IF band after down conversion. The enabling technology consists of a fast multi-giga sample analog to digital converter that has recently become available. Field programmable gate arrays (FPGA) are implemented to accomplish versatile real-time data analysis. A prototype system has been developed and tested and its performance has been compared with conventional electron cyclotron emission (ECE) spectrometer systems. On the TEXTOR Tokamak a proof of principle shows that ECE, together with high power injected and scattered radiation, becomes amenable to measurement by this device. In particular, its capability to measure the phase of coherent signals in the spectrum offers important advantages in diagnostics and control. One case developed in detail employs the FPGA in real-time fast Fourier transform (FFT) and additional signal processing. The major benefit of such a FFT-based system is the real-time trade-off that can be made between frequency and time resolution. For ECE diagnostics this corresponds to a flexible spatial resolution in the plasma, with potential application in smart sensing of plasma instabilities such as the neoclassical tearing mode (NTM) and sawtooth instabilities. The flexible resolution would allow for the measurement of the full mode content of plasma instabilities contained within the system bandwidth. (C) 2011 American Institute of Physics.

An electron cyclotron resonance heating (ECRH) system has been designed for JET in the framework of the JET enhanced performance project (JET-EP) under the European fusion development agreement. Due to financial constraints it has been decided not to implement this project. Nevertheless, the design work conducted from April 2000 to January 2002 shows a number of features that can be relevant in preparation of future ECRH systems, e.g. for ITER. The ECRH system was foreseen to comprise six gyrotrons, 1 MW each, in order to deliver 5 MW into the plasma (Verhoeven A.G.A. et al 2001 The ECRH system for JET 26th Int. Conf. on Infrared and Millimeter Waves (Toulouse, 10–14 September 2001) p 83; Verhoeven A.G.A. et al 2003 The 113 GHz ECRH system for JET Proc. 12th Joint Workshop on ECE and ECRH (13–16 May 2002) ed G. Giruzzi (Aix-en-Provence: World Scientific) pp 511–16). The main aim was to enable the control of neo-classical tearing modes. The paper will concentrate on: the power-supply and modulation system, including series IGBT switches, to enable independent control of each gyrotron and an all-solid-state body power supply to stabilize the gyrotron output power and to enable fast modulations up to 10 kHz and a plug-in launcher that is steerable in both toroidal and poloidal angles and able to handle eight separate mm-wave beams. Four steerable launching mirrors were foreseen to handle two mm-wave beams each. Water cooling of all the mirrors was a particularly ITER-relevant feature.