Very rarely is a research area as dependent on 3D-measurement technology in the set-up of its major experiments as fusion plasma physics. When one realises how highly complex these systems are, and how deeply industrial 3D laser measurement technology, and in particular the mobile variants – highly-developed laser trackers, scanning and photogrammetry systems – have become interwoven with this research area in the meantime, one can imagine what the systems are now capable of achieving.
One thing is clear: without their sisters-in spirit – 3D CAD software – the Wendelstein 7-X fusion research system that is close to completion in Greifswald could not have even been designed, let alone constructed, without laser trackers. But the goal is (almost) worth all the effort: power generation on the basis of nuclear fusion, as has taken place in the Sun for millions of years, and will also be realised here on Earth in the not too distant future.
Theory and practice
The Wendelstein 7-X fusion experiment that is being constructed in the Max Planck Institute in Greifswald is based on the Stellarator principle. This special design, characterised in particular through the irregular and highly complex, three-dimensional shape of its coils, places special demands on the development, design and construction of this machine. Wendelstein 7-X will not actually create any fusion energy, but, within a few years, should provide the proof that the Stellarator principle is suitable for use in a power plant. In a fusion power plant, deuterium and tritium nuclei are fused together to form helium. The fusion of only 86 grams of this mixture results in an amount of available energy equal to the combustion of 1,000 tons of coal.
This is the theory. In practice, the problems on the path to this become much more concrete – and a large portion of them can only be solved with high precision 3D measurement technology, paired with a healthy dose of metrological inspiration and an unwavering passion for metrology.
The plasma vessel, which is surrounded by hot plasma at up to 100 million degrees, is reminiscent of a twisted, half-deflated bicycle inner tube of approx. 11 m at the large diameter and approx 1,5 m at the small diameter. The highly complex, three-dimensional contour of the plasma vessel orientates itself to the geometry of the toroidal magnetic field generated on the Wendelstein 7-X. The field lines, which are comparable to the bars of the cage of a wild animal, form a cage that makes it impossible for the plasma particles to escape, and thereby retains the plasma inside the magnetic field, while preventing any contact with the wall of the vessel and the resultant cooling of the plasma.
The manufacturing and assembly of the steel tube of the plasma vessel, consisting of five almost identical modules, confronts engineers and scientists with serious challenges. How can we determine the extent to which the actual vessel, made from 17-mm-thick special steel, deviates from the model created in the CAD?
During the manufacture of the individual plasma vessel modules, the latter were accessible from the outside, and their contours could therefore be checked from the outside at the manufacturer’s site using a laser tracker. On assembling the individual modules to form the complete vessel in the experiment hall of the Wendelstein 7-X, access from the outside is no longer possible, and both the contour of the vessel and, up to recently, the reference system that will be required later has had to be determined from the inside with the help of photogrammetry.
Photogrammetric measurements are associated with a high time outlay, however.
With the introduction of highly compact laser trackers that are not much larger than a shoebox and weigh less than 10 kg, it is now also possible to bring these devices into the plasma vessel through the narrow access opening of approx 800 x 400 mm cross-section, and to set them up there. Thanks to the large opening angle of 360° horizontal and approx. ± 70° vertical, it is easy to achieve a slight orientation of the device, even in the narrow vessel. In comparison to photogrammetry, the use of a laser tracker saves process time, and provides equivalent, or even somewhat more accurate measurement results.
A subsequent scan of the entire surface of the plasma vessel, based on the reference points measured with the laser tracker, provides a highly accurate As-Built model of the plasma vessel that will serve as the basis for the design of various elements that will later be installed in the plasma vessel.
Mobility is everything
The compactness of today’s laser trackers also offers an invaluable asset in another area, however, when it comes to dealing with the upcoming measurement tasks. In the assembly and alignment of ports, for example. On the Wendelstein 7-X, ports connect the plasma vessel with the outer experiment envelope, the outer vessel. Supply lines (pipes and cables) are led into the interior of the plasma vessel from outside through these, and heat up the plasma with the help of microwave and particle beams, or measure the temperature and density of the hot plasma.
A total of around 250 of such ports are installed on the Wendelstein 7-X. Half of these ports consist of two pipes of virtually identical thickness, one connected to the plasma vessel and the other with the outer vessel. Both pipes connect to a flexible, stainless steel bellows. During the welding of the outer pipe to the outer vessel, which can be controlled quite easily with a laser tracker positioned anywhere in the line of sight of the port, the control of the connection of the inner pipe with the plasma vessel is a great challenge. In order to be able to measure the pipe to be welded onto the plasma vessel, a laser tracker must be able to look into the approx. 2-metre-long port from the outside. It must also be positioned directly in the axis of the port. A small laser tracker that can be flexibly mounted in its position is a decisive advantage here. The laser tracker is fitted to the support with appliances that have been specially designed for this purpose. Only in this way can measurements be carried out in the inside, and the pipe position be determined, which is what really matters. And the measurement system, which is normally mounted vertically on a stand, must be able to achieve its accuracy, even in the most oblique installation position.
Even in science, nothing can be achieved without precise knowledge of (one’s own) position; and, particularly in the case of 3D measurement, this is of decisive importance. The laser-based measurement system can be easily moved from one position to the other in the experiment hall thanks to its compactness, and its position must be known down to a few tens of a millimetre for every measurement on the fusion system. It is thereby crucial that the measurements are made within the global reference system. This takes place with the help of so-called reference marks that are applied to the four walls of the 30 x 32 x 20 metre experiment hall.
These are laser tracker targets, mostly nests for corner-cube reflectors, and the laser tracker should be able to see 6 to 10 targets in every measurement position – which is frequently a problem due to the limited sight lines through the complex overall construction. In order to solve this task, a total of 128 targets are applied to the interior walls of the hall by the people at Greifswald, and their positions are determined with an accuracy of better than one tenth of a millimetre in relation to the global machine coordinate system. In many areas of the industry, such as the automobile industry, this type of referencing has been commonplace for a long time – although the effort involved is by no means comparable to the efforts here.
Reduction of the measurement time
When changing locations, small and compact measurement systems no longer have to be moved using a crane, and thereby also no longer have to be disconnected from the power supply during the changeover. A small, non-interruptible power supply is sufficient to maintain the power, and can be moved together with the measurement system. The necessary warm-up time of the device after reconnection to the power network is eliminated, and the unit can be used again immediately. This saves time, not only for the measurement team, but also when supporting the teams of fitters and welders.
In the last few years, the build-up of the fusion system has moved well ahead with regard to the principal components, and the outer shell has become almost completely closed. The assembly of the Wendelstein 7-X is now concentrated on the installation of the fittings within the plasma vessel. Until recently, photogrammetric systems in combination with jointed measurement arms were used in the interior of the plasma vessel. Today, however, it appears that that the measurement during the installation in the interior of the plasma vessel can also be solved through small, compact laser trackers in combination with jointed measurement arms. While the jointed measurement arm accompanies the installation of the components locally, within the volume of a sphere with a radius of approx. 1 m, the laser tracker creates the preconditions for the orientation of the jointed measurement arm to the master coordinate system. In order to do this, a reference point system is set up in the plasma vessel that is dense enough to be able to orientate the jointed measurement arm to at least 6 points in any set-up position, with more than 300 points being required for the complete plasma vessel.
The determination of the coordinates of these reference points in the hall coordinate system is carried out with the help of mobile laser trackers. Thanks to their mobility, it is now possible to measure the network of reference points in the interior of the plasma vessel for the first time. Through ports, or in other words the opening of the plasma vessel to the outside world (the experiment hall), the coordinates of the points inside the vessel are then determined at a few locations in the hall coordinate system. This is also only possible with highly compact, mobile laser trackers, as these can be mounted directly on the flanges of the ports, and are thereby also able to look through the ports into the plasma vessel, as well as measuring the points of the hall reference system.
In the meantime, these laser tracker systems (for example, the Radian laser tracker from API) also have an integrated camera that can be used to help with the rapid orientation of the measurement systems, and can also be used, for example, for the documentation of the measurements (video or photo).
Quod erat demonstrandum
In mid 2012, it became apparent that an important goal could be achieved in connection with the set-up of the Wendelstein 7-X research fusion system: the proof that the production and installation of such an ambitious scientific project is possible, even under conditions similar to those in industry.
The completion of Wendelstein 7-X is now imminent. There only remains the proof of an optimised plasma confinement and the controlled continuous operation of the plasma. By 2015, we will know whether the Stellarator in Greifswald can do what is expected of it: provide the experimental proof that this type of installation is suitable for a fusion power plant and that the process of nuclear fusion, as it has taken place for millions of years in the Sun, can also be implemented on the Earth in order to create energy.
The fact that the fossil fuel resources of the Earth are quickly becoming depleted is now known by virtually everyone, and the phasing-out of the once promising field of nuclear energy has already been definitively agreed in many countries, including Germany. Future nuclear fusion reactors on the basis of the technology being developed in Greifswald could close this energy gap.l
The Max Planck Institute for Plasma Physics www.ipp.mpg.de
Automated Precision Europe: