C. Scientific and technological Programme
C.6. Contributions to Wendelstein 7-X
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Introduction
The Max-Planck-Institute of Plasma Physics in Garching is currently constructing the Wendelstein 7-X stellarator at Greifswald – with contributions from Research Centres Jülich and Karlsruhe. The stellarator concept is regarded as an attractive candidate for a future fusion reactor due to its specific potential for continuous operation. Wendelstein 7-X is a large stellarator, which has been optimised according to the quasi-symmetry principle. It consists of superconducting coils and is intended to provide plasma discharges of 30 seconds duration at a heating power of 10 MW. The aim is to demonstrate the basic suitability of the chosen concept for magnetic confinement with long pulses.
According to its specific expertise, Research Centre Jülich has taken over comprehensive work packages for the construction of the Wendelstein 7-X stellarator. Above all this includes work for the design and manufacture of components of the superconducting coils, of the leads and electrical connections, supporting work in welding technology, strength calculations as well as diagnostics development. Due to the strong concentration on the actual construction, major emphasis has been put on engineering and welding technology tasks; the diagnostics development is also ongoing but it is mainly concentrated on two tasks; other activities are pursued in the frame of the institute’s R&D programme. The work described in the following was carried out in the year under review.
Engineering
The stellarator is the most promising alternative to the tokamak because of its inherent stationary plasma operation. The prospect of stationarity opens new possibilities to investigate reactor-relevant physics issues. However, it also requires additional solutions for the accompanying technical prob-lems which are related to the superconducting field coils, the durability and cooling of wall ele-ments as well as the control and data analysis of diagnostics. FZJ participates in the design and con-struction of diagnostics for Wendelstein 7-X by taking over essential work packages. During opera-tions, FZJ will also participate in the scientific analysing of the experimental results.
The technical department of IPP Jülich has built TEXTOR including all upgrades. The most recent upgrade is the Dynamic Ergodic Divertor (DED) with high current coils (15 kA) located on the high field side inside the vacuum vessel, including the coil support structure and all components for elec-trical coil interconnections.
Fig. 1: Superconductor bus bars for a single Wendelstein 7-X module.
Based on this experience the following tasks for the Wendelstein 7-X project have been performed by the technical department of our institute.
Superconducting Bus-System
The coils are interconnected by superconducting bus bars (Fig. 1). The technical specifications are the basis for the design, construction, qualification, manufacturing and assembly of the buses and their appropriate supports.
A new bus topology was developed to avoid collisions with other parts and to facilitate the assem-bly. To check the geometry of the bent buses and to examine the buses assembly, a 1:1 model was manufactured and assembled.
Fig. 2: Superconductor insulation.
For the qualification of the insulation (Fig. 2) and the fabrication process different samples have been fabricated and examined as follows:
• High voltage insulation tests at 13 kV including measurements of the Paschen firmness,
• thermal tests under cryo-temperatures at 77 K,
• high pressure tests after mechanical bending to simulate quench situations, and
• check of vacuum compatibilities of the materials and methods used.
For series production of the 125 buses a production line has been installed. The production steps are:
(1) Straightening of the superconductor on a rolling machine,
(2) rounding on a special turning lathe, required to facilitate 3-dimensional bending, (3) bending on a 3-D-bending machine (Fig. 3),
(4) check of geometry on the 1:1 model (Fig. 4),
(5) electrical insulation and conductive lacquer coat applied by hand,
(6) vacuum and high voltage test at several pressure steps (Paschen test) inside bellow tube, and (7) transportation in bundle of 6 buses to Greifswald.
Fig. 3: Bending of a superconductor. Fig. 4: Check of geometry on the 1:1 model.
One of the most critical points to be solved was the handling of the up to 13 m long 3-dimensional shaped bus bars. For weight compensation of the buses during bending and installation helium filled balloons are used (Fig. 3). To perform these tasks an appropriate hall space was made available and prepared according to the requirements for manufacturing and testing of the high voltage insulated bus bars. Test of production with dummy conductors has been finished and series production will start soon.
Bus support structure
The design of the support structure is based on different adjustable sub-modules which are able to compensate fabrication tolerances in all directions and to facilitate the assembly on site (Fig. 5).
The bus support structure is mounted to the magnetic coils or to the central ring (Fig. 6). These root points move due to bending of components caused by the high magnetic forces and due to differ-ences of thermal shrinkage during cool down (bus: aluminium and epoxy; coils and ring: stainless steel).
These effects together with the magnetic loads acting on the buses are taken into account during several iterations of support design and stress calculations.
Fig. 5: Adjustable bus support. Fig. 6: Support structure on coil and Z-ring.
Joints
Approximately 230 low-resistance joints are required for electrical and hydraulic interconnections between superconductors at the coil terminals and between five adjacent modules. Based on a con-ceptual design for a pressure of 30 bar and a current of 18 kA, demountable joints for 200 bar and 20 kA have been redesigned (Fig. 7). After design review three joints have been manufactured and tested under pressure. Resistance tests at 4 K are in preparation and after delivering of material the manufacturing of 230 joints including inner clamping parts will be started at the FZJ central work-shop.
Fig. 7
Superconductor
: Joint for 20 kA and 200 bar connecting two superconductors.
Stress analysis
1. The structural analyses of the numerous options of the NSEs (narrow support elements) have been performed with FE models. Based on the stress-strain state of the NSE elements the allow-able compression for each type of NSE has been specified (Fig. 8).
2. The bus-bar supports should satisfy the following requirements:
• Should be stiff enough to withstand electromagnetic forces for different operational regimes,
• should be flexible enough to tolerate movements of the bus supports attached to the coils and central ring for different operational regimes, and
• should allow relative movements of the neighbouring buses during the cooling down proc-ess caused by their temperature difference up to 50 K.
alf of the bus system, feeding and interconnecting the coils of the 5th module, the final numerical iteration was started. For the FE modelling this means a replacement of the
sim-3. ress-strain state in the
struc-ture was analyzed by means of FE modelling. (Fig.9).
For the top h
plified support models with the actual design supports models.
The design changes in the joint structure were modelled and the st
Fig. 9
Fig. 8: NSE (narrow support element) FE calculations. : Joint FE calculation.
The tasks have been performed in the framework of co-operation between the Max-Planck-Institute of Plasma Physics (IPP) and Forschungszentrum Jülich (FZJ).
Welding Tests for Wendelstein 7-X
For the construction of the fusion experiment Wendelstein 7-X the ZAT (FZJ Central Institute of Technology) develops the technology for welding the pressure supports between the magnet coils.
Fig. 10 shows the coil assembly of a half module; 10 of these form the whole experiment. In the picture the supports are marked in red and green.
Fig. 10: Coil assembly of a half module of Wendelstein 7-X.
The special difficulty is that according to the original plans the supports should be connected to the coils by screws. However, due to the calculations performed the screws had to be replaced by welds. From the technological point it is extremely disadvantageous that the connections on the coils have already been prepared correspondingly (fig. 11). The pressure supports have to be welded on both sides with the coil cases and these welds have to be up 25 mm thick because of the ex-tremely high magnet forces. The weldings have to be performed on the completely mounted half module under considerably difficult access conditions. The already applied cooling structures (see fig. 11) and the isolation of the plasma vessel must not be influenced in the slightest. The most dif-ficult demand is, however, that the welding disto
to influence the position of the coils and the magnet field so that the plasma position will be in
ac-cordance with calcula ogy is a key tool for
a successful start of Wendelstein 7-X. The present technology does not allow to simulate the weld-g distortion mathematically with complex prefabricated parts. In the course of the year, altoweld-gether
rtion has to account only up to 1 mm in order not tions. Therefore, the development of the welding technol
in
6 welding attempts were executed in order to determine and to optimise the occurring welding dis-tortion. For all attempts the geometrical dates between the welds have been acquired on a 3D CNC-machine. The necessary thickness of the weld for a sufficient power transmission was established by IPP Greifswald. In the course of the attempts, the welds were optimised in such a way that the occurring welding distortion is as low as possible and also symmetrical. Beside, the welding distor-tion value was also set to a high weld quality since this has an influence on the power transmission of the weld. The welding distortion tests were executed under original conditions with restricted accessibility as shown in fig. 12.
Fig. 11: Junction area on the coils (about 280 x 160 mm). Fig. 12: Welded lateral support with restricted accessibility.
These welding tests guarantee that the weld quality can also be executed on a high level at the ex-periment. In the course of the attempts miscellaneous cuts of the welds were prepared in order to prove that a good weld quality is possible despite the difficult boundary conditions. In fig. 13 dif-ferent cuts from the welds are shown. The manufacture of the welds puts high demands on the dex-terity of the executing welder. In February 2005 two welders of IPP Greifswald were trained at FZJ in order to be able to pass on their experiences and skills obtained during the test welds. This
ex-ce exchange was carried out in the framework of a two-week course. All executed welding sts were not executed with the original material but with a very similar one. The original mate al was used for the last attempt, and it turned out that the determined technical welding data are
trans-fe e occu
gated furthermore during and after the welding. In fig. 14 one can s t-up with e temperature sensors.
perien
te ri
rable to the original materials. During this attempt th rring temperature course was investi-ee the experimental se
th
Fig. 13: Welding cuts for quality control.
Fig. 14: Welding set-up with temperature sensors.
The geometrical and thermo-graphic data were made available to the Italian company LTC that should develop a simulation program for the prediction of the Verzugsverhalten of the individual coils.
Diagnostics
In the course of 2005, the construction of the VUV spectrometer system HEXOS (High Efficiency XUV Overview Spectrometer) for Wendelstein 7-X has been completed, together with the labora-tory light sources and all peripheral equipment such as mechanical stands, vacuum systems, detec-tors, cameras and data acquisition which are needed to perform the laboratory testing of the spec-trometers. The spectrometers have been assembled and aligned at the Horiba Jobin-Yvon laborato-es with participation of employelaborato-es from FZJ and many tlaborato-est spectra were record d in order to heck and improve the alignment. The completed laboratory setup in shown in figure 15.
sing the DC hollow cathode plasma as a light source, spectra from different working gases have
t point-like light source at 1 m distance from the spectrometer) the spectral resolution is in greement with the expected instrumental width (0.13 nm FWHM) which is mostly defined by the etector properties (open MCP detector). The agreement of the actual focal plane with the detector moving the light source perpendicularly to the optical axis, thereby iteratively im-roving the alignment by readjusting the detector position.
ri e
c U
been measured with the HEXOS 3 and HEXOS 4 spectrometers. As an example, we show the HEXOS 3 spectra from Helium and Neon (fig. 16). It was found that for this case of illumination (almos
a d
was checked by p
The alignment of the HEXOS 1 spectrometer was tested using a pinch discharge (by AIXUV GmbH, Aachen) burning in argon. Using again the case of almost point-like illumination, the spec-trometer allows the clear separation of the two components of the Ar IX doublet around 4.9 nm, which are 0.045 nm apart, see fig. 17.
Fig. 15: Laboratory setup of the HEXOS 3/4 spectrometer together with a DC hollow cathode light source.
Fig. 16: HEXOS 3 spectra from a hollow cathode discharge in Helium and Neon.
Fig. 17: HEXOS 1 spectrum from a pinch discharge operated in Argon.
A high energetic hydrogen beam for diagnostic applications is foreseen on Wendelstein 7-X for the measurement of ion temperatures and impurity density profiles using charge exchange recombi-nation spectroscopy. A diagnostic injector will be developed, the beam of which provides an equivalent current of more than 5 A at 60 keV energy. During the whole duration of injection (i.e.
10 s) the beam properties (divergence < 0.5º, particles with full energy > 70 %) should be main-tained. The pulse duration shall also cover the phases with additional neutral particle heating and will – with a pulse frequency of at least 0.5/min – also allow measurements during very long dis-charges. An additional beam modulation of 100 Hz will help to discriminate active and passive sig-nal intensities and to improve the quality of the data.
The timetable for the development of the diagnostic beam was adapted to the schedule for construc-tion of Wendelstein 7-X. The MPG-IPP placed an order with Budker Institute of Nuclear Physics (BINP) in Novosibirsk, Russia, for the development and fabrication of the whole diagnostic beam until 2012.
The R&D work on optimisation of the grid structure for higher current densities and lower beam divergence of the ion optics was continued. The same ion optic can be applied at the two different plasma sources (RF or arc), whic
lied for the dia m at TEXTOR
here prototypes of an RF and an arc produced plasma source are compared under realistic
condi-seen for the Wendelstein 7-X diagnostics could be continued. Possible laser beam paths were de-h are designed alternatively. Tde-he decision wde-hicde-h type will be ap-gnostic beam depends on their performance on an existing syste
p w tions.
Due to budget and personnel constraints (see annual report 2004) only a part of the activities
fore-veloped for the laser-induced fluorescence diagnostic (LIF) in close collaboration with the engi-neering team in Greifswald. The detection limit permitting, an LIF system should allow to measure velocity distributions of specific light impurities and the time evolution of their density in the diver-tor region of Wendelstein 7-X. Shifted to a low level, however not cancelled, were a window lock system, the high-resolution X-ray imaging spectrometer, the thermal He-beam diagnostics and the Target Tile Manipulator. The two latter devices should be fitted later into the divertor modules ithout too much additional effort but now have to cope with a different divertor target plate design
ly resolving spectrometers. At FZJ the passive spectroscopy of molecules and line pro-le measurements has been considerably improved by the use of an Echelpro-le spectrometer, which
e UV range to the red spectral region. It has a comparable resolution, however, it contains aps in the red wavelength range because of the circular structure of the image intensifier. A com-w
in order to reduce the manufacturing costs for the divertor.
The spectroscopic methods for the investigation of particle release from divertor and wall will also require high
fi
allows the simultaneous recording of a wavelength range between 375 and 700 nm with a high reso-lution of λ/Δλ = 20000. This system has also been installed at JET for the campaigns C11, C13 and C14, where the set-up automatically recorded spectra of about 300 discharges. A similar instrument is in use at Greifswald and was shipped to Jülich for comparison with the Echelle spectrometer at FZJ. Due to an additional image intensifier the Greifswald instrument is capable to measure well from th
g
bination of the advantages of both instruments seems to be a reasonable solution for such a spectro-scopic device on Wendelstein 7-X. Figure 18 displays two UV spectra taken in front a tungsten lim-iter which display some important lines for the interpretation of the measured photon fluxes in terms of atomic tungsten fluxes.
Fig. 18: emission spectra in front of a W-limiter in TEXTOR taken with the Greifswald Echelle spectrometer.
The wall material in thermonuclear fusion devices is subject to intense fluxes of charged and neu-ral plasma particles and radiatio
t n. An aim of these activities is to develop a material for a low-Z all coating of the SS-wall panels. Earlier tests with B4C in TEXTOR have shown that arcs develop hich may lead to an enhanced release of particles. In order to reduce the arcing, tests with Si-doped B4C have been planned and will be carried out in 2006.
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C. SCIENTIFIC AND TECHNOLOGICAL PROGRAMME