C. Scientific and technological Programme
C.5. Contributions to ITER
tçäÑÖ~åÖ=_áÉä=EfmmF w.biel@fz-juelich.de
Introduction
International fusion research has shown in the past years that the physics principles for igniting magnetically confined fusion plasmas are known. On the way to an economical continuous opera-tion of a fusion power plant, the next step is now to demonstrate the scientific and technological feasibility of fusion energy production. It is therefore planned to design, construct and operate a large tokamak experiment in global cooperation which will for the first time achieve a plasma op-eration with dominant α-particle heating ("burning experiment"): ITER. The key targets of ITER are to obtain a tenfold power gain and a burning time of about eight minutes per plasma pulse at a fusion power of 500 MW. The ITER results will be decisive for the design of the first demonstra-tion power plant DEMO. In the course of 2005, the ITER site was decided (Cadarache, France) and consequently the worldwide efforts on ITER development projects were reinforced.
The fusion research programme of FZJ is geared to the strategy of the European research pro-gramme (Association EURATOM-FZJ and European Fusion Development Agreement, EFDA), in which the realisation of ITER and ITER-supporting research play a central role. The work at FZJ on ITER relevant issues is furthermore embedded into the TEC collaboration (Trilateral Euregio Clus-ter), in which FZJ and the partners FOM (Netherlands) and ERM/KMS (Belgium) are working to-gether. The TEC research activities for ITER comprise the fields of a) first-wall materials, b) plasma-wall interaction, c) diagnostics and d) heating and current drive.
This section of the report presents the TEC contributions to specific ITER development tasks per-formed in 2005. The ITER relevant physics issues as well as the work related to the development and irradiation testing of materials for plasma facing components are described within other sec-tions.
This report is structured as follows:
1) ITER diagnostic systems
• ITER divertor VUV spectroscopy
• Charge Exchange Recombination Spectroscopy (CXRS)
• Synchrotron radiation diagnostic for detection of runaway electrons 2) Material properties
• Testing of first mirrors
3) Heating systems
• Electron Cyclotron Resonance Heating (ECRH)
• Ion Cyclotron Resonance Heating (ICRH) 4) Modelling
• EIRENE simulations of the divertor target heat load
Diagnostics
In 2005 the TEC has continued to work on the development of ITER diagnostics with a view to take over the leading responsibility for the construction of one major ITER diagnostic system (preferably CXRS) together with the corresponding port plug. Furthermore, IPP Jülich has coordinated a broad initiative of seven EURATOM associations for a proposal on a training scheme on Engineering of Optical Diagnostics for ITER, thereby strongly supporting the process of forming consortia of asso-ciations being ready to take over diagnostic procurement packages.
Apart from specific work on ITER diagnostics under EFDA contracts, TEC physicists are actively involved in the ITPA (International Tokamak Physics Activity) Topical Group on Diagnostics that coordinates the worldwide voluntary physics in the field of ITER diagnostics. Specifically, Tony Donné is acting as chair of this group, while Manfred von Hellermann is chair of the Specialists Working Group on Beam-Aided Spectroscopy that resides under the wings of the above Topical Group. Furthermore, Andrey Litnovsky is co-chairman in the ITPA first mirror working group. In 2005 work has been conducted on the following diagnostic systems for ITER: ITER divertor troscopy, charge-exchange recombination spectroscopy (CXRS) and synchrotron emission spec-troscopy for runaway electron diagnosis.
For the development of ITER divertor spectroscopy, FZJ has been working on a task to assess the needs for VUV spectroscopy in the ITER divertor region to characterise heavy impurities (e.g.
tungsten) and to develop an optical design for the corresponding instrumentation. Reviewing meas-ured tungsten spectra from TEXTOR as well as modelled tungsten spectra (by O’Mullane, ADAS group), it was found that the wavelength range from 12 nm to 48 nm is most appropriate to monitor the tungsten content of the divertor plasma based on ionisation stages from W5+ up to W16+. In order to properly resolve the W spectra from the variety of spectral lines from other particle species, a two-channel VUV spectrometer design was developed based on toroidal holographic diffraction gratings. The obtained performance data of the proposed spectrometer channels are summarised in table 1.
The étendue of the instruments has been chosen equally to have comparable incoming intensities in the overlap regions. Grating designs are in accordance with manufacturing capabilities for toroidal holographic gratings (Jobin-Yvon, France).
The TEC partners FZJ and FOM have continued to work together on the further development of the ITER CXRS diagnostic system, imbedded into the frame of two joint EFDA tasks. This pro-ject was started with reviewing the status of the existing design of the ITER CXRS system. It was found that the large energy and particle loads from the plasma make it necessary to install the first mirror at a retracted position, which led to the request of shifting the CXRS system from the
fore-seen upper port no. 3 towards the upper port no. 2, in order to preserve viewing angles which pro-vide measurable Doppler shifts compatible with ITER measurement requirements. A CAD design comparing both locations for ITER CXRS is shown in figure 1.
Spectrometer channel no. 1 2
wavelength range / nm 12 – 27 25 – 48
Line width / nm 0.055 0.068
Mean wavelength resolution λ/Δλ, including detector properties
350 530
incidence angle α / degrees -71 -60
mean deviation angle / degrees 135.6 112.3 distance slit – grating LA / mm 440 440 mean distance grating – detector LB / mm 519 513
Étendue / mm2 sr 1.0e-04 1.0e-04
f-number vertical 42 42
f-number horizontal 42 26
Table 1: Summary of geometric and optical data of the ITER VUV divertor spectrometer channels.
Fig. 1: Isoview comparing the possible port 2 and port 3 assembly of the CXRS system including the diagnostic beam (in red).
In order to protect the first mirror from plasma deposition, the installation of a shutter is foreseen, which will be closed during the phases when the diagnostic beam is switched off. Furthermore, it is
proposed that the necessary maintenance and exchange of optics within the CXRS port plug is fa-cilitated by installing each optical element onto a retractable tube, which can be removed towards the back side of the port plug within reasonable efforts. A CAD drawing of this scheme is shown in figure 2.
Fig. 2: Three pipe assembly for mirror tubes of port 2 assembly.
Detailed calculations were performed to simulate the temperature evolution of the first mirror re-sulting from the heat flux from the plasma. It was concluded that the bending of the first mirror and the related misalignment of the optical path due to thermal effects are only small. However, an ele-vated mirror temperature can reduce the deposition rate and thus increase the mirror lifetime sig-nificantly. Therefore, it is proposed that the first mirror is kept at an elevated temperature around 300 °C, using an appropriate heat exchanger system.
An important ingredient for the future working plan on ITER CXRS is the initialisation of pilot ex-periments which can be used as test-beds for key components such as the choice of spectrometers (see Fig. 3) and detectors. From the physics point of view, pilot experiments on present plasma de-vices can address critical issues in measuring capabilities such as the accuracy of magnetic field measurements in terms of absolute field strength and local pitch angles. One example is the use of beam emission spectroscopy in combination with CXRS, as demonstrated on TEXTOR, providing on-line calibration tools for absolute ion densities, which is a key concern for plasmas with ex-tended pulse lengths. For ITER about 1000 seconds are anticipated and a continuous deterioration of optical periscope components is expected during a single pulse. Here again, the expertise on high precision evaluation of beam emission spectra – developed at JET and TEXTOR – will most likely offer a promising approach to maintain the intensity calibration of CXRS data.
Fig. 3: High étendue Echelle spectrometer, developed at TRINITI (Moscow) as a candidate for the ITER CXRS spectrometer system. A first prototype of this kind is being tested in the plot CXRS experiment at
TEXTOR.
As a second important topic, the TEC partner FOM is involved in the development of a synchro-tron radiation diagnostic to detect runaway elecsynchro-trons. During disruptions in ITER, currents of several MA of energetic electrons in the range of 10 to 50 MeV can be generated. These runaway electrons are a potential danger for ITER, as they can deposit their energy localised in the wall structure, with damaging consequences of melting or cracking of materials or even leakages of wa-ter or vacuum systems. Over the last decade TEC has developed a unique tool to study these con-fined multi-MeV electrons by exploiting their synchrotron radiation in the infrared wavelength range. An ITER task has been finalized in 2005 to assess whether the same technique is applicable to ITER, using the wide-angle viewing thermography system. The expected synchrotron radiation spectrum at ITER is analysed and compared with the infrared emission. From this study, the rele-vant wavelength range as well as the minimal detectable runaway current is estimated. Some meth-ods of how to deduce the runaway parameters from a typical set-up have been outlined with special attention to the field of view and ways to discriminate the synchrotron radiation from the back-ground. The main conclusion was that the planned wide angle viewing system for thermography is fully capable of performing these runaway electron measurements as well for runaway electrons above 15 MeV. It is recommended to devote one infrared system especially to runaway detection, in order to be able to do spectral measurements and to determine the runaway energy distribution from that.
Material Properties
Investigations of various ITER candidate diagnostic mirror materials have been continued in the framework of the High Priority Task of the ITPA TG on Diagnostics. Currently, the research of diagnostic mirrors is a part of the multi-machine ITPA.
First direct comparative test of polycrystalline and single crystal mirrors in a tokamak environment has been made in TEXTOR. Single crystal and polycrystalline molybdenum and tungsten mirrors were exposed in the SOL plasma of TEXTOR for a series of identical repetitive discharges in ero-sion dominated conditions. After the exposure, no significant changes in total reflectivity of mirrors were observed. At the same time, a drastic increase of diffuse reflectivity was measured for poly-crystalline molybdenum mirrors, single crystal mirrors showed no change. The specular reflectivity of single crystal mirrors is significantly higher than that of polycrystalline ones. The most affected wavelength range is 250-1000 nm, while no significant changes of reflectivity were noticed in the range from 1000 to 2000 nm. No or negligible effect of erosion on polarisation characteristics of mirrors was measured. The conclusion drawn from these studies is that the use of single crystal mir-rors is preferable under erosion conditions.
The experiments were carried out as a collaboration programme between IPP Jülich, the University of Basel, Switzerland and the Kurchatov Institute, Russia. Investigations are partly supported by the EFDA grants TW4-TPDS-DIADEV and TW5-TPDS-DIADEV.
The limiter with mirrors after exposure in
the Scrape-off layer of the TEXTOR plasma DiMES Mirror system exposed to the divertor plasma of the DIII-D tokamak (U.S.A.)
Mirrors
Fig 4: First mirror samples used for plasma exposure experiments.
First dedicated tests of diagnostic mirrors in a tokamak divertor plasma were made in the National Fusion Facility DIII-D (General Atomics, San Diego, USA) under the lead of specialists from IPP Jülich. The mirrors were supplied by IPP Jülich, installed in the divertor region and exposed for a series of ELMy H-mode discharges with partially detached plasmas in the divertor. Such a regime of operation is proposed as a working regime for ITER divertor. Two dedicated exposures were made: with mirrors at room temperature and with heated mirrors kept at 80 to 140 °C. A significant deposition of carbon onto the mirror surfaces was detected after exposure of cold mirrors, whereas for the heated mirrors the carbon deposition was effectively suppressed and the degradation of opti-cal mirror properties was essentially slowed down.
This work has been done in the framework of bilateral U.S.-EURATOM Exchange programme.
Heating systems
TEC is playing a leading role in the development of a remote-steerable ECRH launching system for the ITER upper ports. The aim of the system is to inject Electron Cyclotron Waves (ECW) into the ITER plasma in order to stabilise neoclassical tearing modes. Each upper-port launcher consists of six mm-wave lines capable of transmitting high power up to 2 MW at 170 GHz (see Fig. 5). In order to exploit the capability of ECW for localised heating and current drive over a range of plasma radii in ITER, the ECH & CD upper port launcher must have a beam steering capability. By avoiding movable mirrors at the plasma-facing end of the launcher, the reliability of the ECRH sys-tem is largely increased. For this purpose the launcher syssys-tem was developed based on the concept of remote mm-wave beam steering, having a corrugated square waveguide within the launcher, while the steerable optic is then placed outside of the first confinement boundary of the vacuum vessel. In the remote-steering concept, the scan in the vertical plane is achieved by means of rotat-ing a mirror far away from the plasma, launchrotat-ing the mm-wave beam into a square waveguide, re-sulting in the same scanning angles at the end of the waveguide.
The scan range is ± 12 degrees at maximum at the input and output of the square waveguide, result-ing in a scan range of ± 6 degrees to ± 12 degrees in the ITER plasma, dependresult-ing on the focusresult-ing strength of the end mirror.
A team of 6 full-time professionals of the Dutch TEC partner FOM is involved in the design of the upper-port launcher. In 2005 a detailed design of the full ECRH launching system was finalised and all the components as well as a testing system were procured. After the assembly of the system it was fully tested at low power with the very high-sensitivity mm-wave vector-network measuring device at FOM (Nieuwegein, The Netherlands).
After the successful low-power tests, the full system was taken to FZK at Karlsruhe where a coaxial 170 GHz gyrotron was connected to the remote-steering launcher mock-up. A demonstration of the launcher system was achieved, including power handling of all the components and an accurate polarisation control at high power (see Fig. 6).
Moreover, in 2005 a full set of critical design issues has been analysed leading to the conclusion that all technical issues could be solved and a very attractive, compact and fully tested and reliable system could be designed. However, its relatively low current-drive efficiency has also led to a re-newed interest in an alternative approach: front steering. Nevertheless, it is still an enormous chal-lenge to handle the rotating mirrors and its moving cooling lines in the vulnerable environment of the ITER plasma.
Fig. 5: Scheme of the remote-steering launching system for ITER. On the right-hand side the incoming 6 waveguides, each one carries up to 2 MW of mm-wave power. This is followed by the steering mechanism to direct the beams into the 4 metres long square corrugated waveguides. At the end (on the left-hand side) the beams come out of the waveguides and reflect on the 2 fixed mirrors in dog-leg configuration, before
entering the plasma.
Fig. 6: High-power measurements of the remote-steering launcher mock-up at the 170 GHz coaxial gyrotron in Karlsruhe. At different steering angles (-12, 0 and +12 degrees) rather good Gaussian beams can be seen.
Within 2005, ERM/KMS has finished the design of the ITER-like ICRH antenna for JET-EP:
the last parts concerned the actuators, seesaw mechanisms, pivot rings and drive rods. The pro-curement activities for these are under way and are near to being delivered. The assembly activities have started late in 2005 and the major difficulty hitherto has been the trial welding and assembly of the pressure vessel containing the actuators of the capacitors. The project team believes that the issues at present have been understood and remedial actions were undertaken to tweak the assembly process. The High Power Prototype (HPP) of the JET-EP antenna built at ORNL has been tested.
These tests have confirmed the design modifications proposed by the project team to improve on the antenna’s thermal capability. They have also confirmed the modified capacitor design. The pro-curement of the Vacuum Matching Capacitors from the firm COMET is now complete.
Before installation the ICRH ITER-like antenna for JET-EP will undergo a test and calibration phase. In order to carry out the necessary tests, specific test bed equipment is being installed for
measurements and calibrations. The operation of the test bed is envisaged to proceed in four phases:
(i) measurement of antenna strap array scattering matrix with water load; (ii) calibration of capaci-tor RF probes; (iii) high power tests per Resonant Double Loop (RDL) inside vacuum chamber; (iv) RF matching studies of assembled antenna with water load.
Further simulations of the complete JET-EP antenna have been performed with the CST Microwave Studio (MWS) code. Two main types of studies have been conducted. On the one hand, a detailed model of the antenna in front of a dielectric substance (εr = 2000), simulating the load by a plasma with a steep edge, has been used to study the actual dependence of loading on the antenna-to-load distance (see fig. 7). The antenna impedance matrix variations have been computed and analysed.
On the other hand, a simplified model (not shown) of the antenna has also been developed. This model realistically represents almost all the geometric characteristics of the actual antenna, except for the poloidal curvature and limiters. The results from this model confirm the predictions of the detailed model and can be used as a fast and reliable predictive tool.
3D MWS and RF circuit models of the different phases of the JET ITER-like ICRH EP antenna test bed and auxiliary RF equipment to operate the test bed have also been developed to assist in refin-ing the test bed programme and to obtain reasonable estimates of signal levels to be expected.
Fig. 7: Front and back view of the MWS model of the JET ITER-like antenna. A vertical central symmetry plane has been considered: only four of the eight ports (in red) are simulated.
On the problem of matching control and arc detection, further analysis and a large number of circuit simulations have been carried out. A dynamic simulation of a possible matching algorithm for the JET-EP array has also been done in Matlab™ (Simulink™). A major problem linked to the use of RDLs operating with low impedance at the junction is the difficulty to detect arcs occurring at cer-tain locations. The conventional arc detection by monitoring the voltage standing wave ratio is not
sufficiently sensitive to discriminate between arcs and ELMs for arcs at or near the junctions and it can be problematic for a series arc in one of the matching capacitors. A meeting organised by the Coordinating Committee on Fast Wave Heating and Current Drive (CCFW) was held at JET re-viewing the various arc detection techniques and their possible application to the JET-EP launcher.
sufficiently sensitive to discriminate between arcs and ELMs for arcs at or near the junctions and it can be problematic for a series arc in one of the matching capacitors. A meeting organised by the Coordinating Committee on Fast Wave Heating and Current Drive (CCFW) was held at JET re-viewing the various arc detection techniques and their possible application to the JET-EP launcher.