Laser-Based 3D Metrology in Fusion Plasma Physics

Published: Friday, November 2, 2012 - 09:32

Few areas of research are as dependent on 3D measurement technology in the setup of its major experiments as fusion plasma physics. Highly developed laser trackers, as well as portable scanning and photogrammetry systems, have opened up a world of possibilities for improving fusion research processes.

Without laser trackers and 3D CAD software, the Wendelstein 7-X fusion research system, which is close to completion in Greifswald, Germany, could not even have been designed, let alone constructed. But the goal for the Wendelstein 7-X is almost worth all the effort: power generation on the basis of nuclear fusion—the same method of producing energy that has taken place on the sun for millions of years and has the potential to revolutionize energy production here on earth.

Theory and practice

The Wendelstein 7-X fusion experiment underway at the Max Planck Institute in Greifswald is based on the Stellarator principle. This special design, characterized in particular through the irregular and highly complex 3D shape of its coils, places special demands on the development, design, and construction of the Wendelstein 7-X. Although it will not actually create any fusion energy, within a few years it 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 are many and a large portion of them can only be solved with high-precision 3D measurement technology, along with some innovative approaches to measurement and design.

Symmetries: negative

The plasma vessel, which is surrounded by hot plasma (up to 100 million°), resembles a twisted, half-deflated bicycle inner tube with a diameter measuring approximately 11 m at its widest and no less than 1.5 m. The highly complex, 3D contour of the plasma vessel orients itself to the geometry of the toroidal magnetic field generated on the Wendelstein 7-X. The field lines, which are comparable to bars on a cage, form an enclosure that makes it impossible for the plasma particles to escape. The plasma is thereby confined inside the magnetic field, preventing any contact with the vessel wall, which would result in the plasma cooling.

Figure 1: Taking measurements of geometries within the plasma vessel of the Wendelstein 7-X fusion research system

The manufacturing and assembly of the steel tube of the plasma vessel, consisting of five nearly 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. When the individual modules are assembled to form the complete vessel in the experiment hall of the Wendelstein 7-X, access from the outside is no longer possible. The vessel’s contour, and until recently, the required reference system, had to be determined from inside the vessel with the help of photogrammetry. But photogrammetric measurements often require a high time outlay.

Compact laser trackers not much larger than a shoebox can be set up inside the plasma vessel. Thanks to the tracker’s large opening angle of 360° horizontal and approximately 70° vertical, it’s easy to achieve a slight orientation of the device, even in the narrow vessel. Compared to photogrammetry, using a laser tracker saves process time and provides equivalent, and even slightly more accurate, measurements.

A subsequent scan of the plasma vessel’s entire surface, based on the reference points measured with the laser tracker, provides a highly accurate as-built model of the vessel. This model is the basis for designing various elements that will later be installed in the plasma vessel.

Mobility is everything

The portability of today’s compact laser trackers also offers an invaluable asset in another critical area: assembling and aligning the ports. On the Wendelstein 7-X, ports connect the plasma vessel with the outer vessel. Supply lines (e.g., 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 about 250 ports are installed on the Wendelstein 7-X. Half of these ports consist of two pipes virtually identical in thickness, one connected to the plasma vessel and the other with the outer vessel. Both pipes connect to a flexible, stainless steel bellows. The welding of the outer pipe to the outer vessel can be controlled quite easily with a laser tracker positioned anywhere in the line of sight of the port; however, controlling the connection of the inner pipe with the plasma vessel is a great challenge.

Figure 2: An API Radian laser tracker is mounted directly on a support in an inclined position on the reactor’s outer housing. Only in this position can measurements also be taken on the inside.

To measure the pipe, which will be welded onto the plasma vessel, a laser tracker must be able to look into a port, approximately 2 m long, from the outside. It must also be positioned directly in the axis of the port. A small laser tracker that can be flexibly mounted into position is a decisive advantage here. The laser tracker is fitted to the support with specially built accessories. Only in this way can measurements be taken on the inside and determine the pipe position. And the measurement system, which is normally mounted vertically on a stand, must be able to achieve accuracy, even in the most oblique installation position.

Figure 3: If the supports for taking measurements from the outside are located in a favorable position, the laser tracker can also be mounted outside the outer vessel on a 5 m-high tripod. You can access the interior only through these supports.

References requested

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 critical importance. Thanks to its compact size, the laser-based measurement system can easily be moved to different locations in the experiment hall. Its position must be known down to a few tenths of a millimeter 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 m × 32 m × 20 m 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. To address 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 millimeter in relation to the global machine coordinate system.

In many areas of industry, such as automotive, this type of referencing has been commonplace for a long time—although the effort involved is by no means comparable to the efforts here.

Reducing 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 reconnecting 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.

During 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 construction of the Wendelstein 7-X now moves to 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 measurements required during the installation in the plasma vessel’s interior can also be made using small, compact laser trackers in combination with jointed measurement arms.

Figure 4: On the Wendelstein 7-X fusions reactor in Greifswald stands Torsten Bräuer, the head of measurement technology, Max-Planck Institute for Plasmaphysik, Germany

While the jointed measurement arm accompanies the installation of the components locally, within the volume of a sphere with a radius of approximately one meter, the laser tracker creates the preconditions orientating the jointed measurement arm to the master coordinate system. To do this, a reference point system is set up in the plasma vessel that is dense enough to orient the jointed measurement arm to at least six points in any setup position; more than 300 points are required for the complete plasma vessel.

Determining 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 the opening of the plasma vessel to (the experiment hall, the coordinates of the points inside the vessel are then determined at a few locations in the hall coordinate system. Again, this is only possible with highly compact, mobile laser trackers because these can be mounted directly on the ports’ flanges, and are thereby also able to look through the ports into the plasma vessel as well as measure the points of the hall reference system.

In the meantime, these laser tracker systems, including the Radian laser interferometer tracker from API, also have an integrated camera that can be used to help with the rapid orientation of the measurement systems and for measurement documentation in video or photography.

In mid-2012, it became apparent that an important goal could be achieved in connection with the setup 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 optimized 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 on our sun for millions of years, can also be implemented on earth to create energy.

The fact that earth’s fossil fuel resources 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 on in many countries, including Germany. Future nuclear fusion reactors on the basis of the technology being developed in Greifswald could close this energy gap.

About The Author

Max Planck Institute for Plasma Physics

The Max Planck Institute for Plasma Physics (IPP) in Garching and Greifswald carries out research on the principles of a fusion power plant. In a similar way to the Sun, energy will be gained from the fusion of light atomic nuclei. In the two parts of the institute, nine scientific departments are researching the confinement of hydrogen plasma at high temperatures within magnetic fields and developing systems for heating and measuring plasma. They are also involved in magnetic field techniques, data collection and processing, plasma theory, material research and plasma-wall interactions, as well as system studies on fusion.