- Email: [email protected]
Assembly and test of the W7-X demo-cryostat
Fusion Engineering and Design 56 – 57 (2001) 861– 866 www.elsevier.com/locate/fusengdes
Assembly and test of the W7-X demo-cryostat F. Schauer *, H. Bau, I. Bojko, R. Brockmann, J.-H. Feist, B. Hein, M. Pieger-Frey, H. Pirsch, J. Sapper, B. Sombach, J. Stadlbauer, O. Volzke, I. Wald, M. Wanner Max-Planck-Institut fu¨r Plasmaphysik, Euratom Association, Teilinstitut Greifswald, Wendelsteinstr. 1, D-17491 Greifswald, Germany
Abstract An overview is given on the status of the demo-cryostat project for the WENDELSTEIN 7-X stellarator. Construction and assembly of the prototype are finished, and the test period is near completion. The intention of this project was to get experience with design and construction of W7-X-components, as well as with assembly of this complex system. The goal is now practically achieved, and it could be demonstrated that the W7-X cryostat can be built with reasonable effort. Many of the solutions found can be adopted directly for W7-X, or are starting points for further improvements. A short description is given of the cryostat, its assembly, and of the most important tests which were performed so far. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Demo-cryostat; WENDELSTEIN 7-X; Plasma; Half-module
1. Introduction The demo-cryostat (DC) is a full size prototype of a WENDELSTEIN 7-X (W7-X) torus sector extending toroidally over 47° without counting the side covers (Fig. 1). It includes a 36°-halfmodule. A half-module is the basic W7-X-unit which recurs 10 times around the torus, arranged alternately upside-down. The DC is built up from the plasma and outer vessels (PV and OV, respectively), six non-planar and two planar coil models, support structure, ports, thermal and electrical insulation, cryogenic piping, PV and port heating * Corresponding author. Tel.: +49-3834-882-715; fax: + 49-3834-882-709. E-mail address: [email protected] (F. Schauer).
system, and instrumentation [1,2]. Manufacturer were Balcke-Duerr Energietechnik as main contractor, and Linde as sub-contractor for the cryogenic components.
2. Main components In addition to earlier papers [1,2], a short description of the actually built main components, and their relevance for the finally chosen W7-X design is given in the following.
2.1. Plasma 6essel The main dimensions can be roughly taken from Fig. 1. The vessel wall thickness is generally
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F. Schauer et al. / Fusion Engineering and Design 56–57 (2001) 861–866
17 mm, with some reinforcements up to 25 mm around the largest port and suspension points. It consists of DIN 1.4311 stainless steel which is an option for the W7-X vessel, whose wall thickness will also be 17 mm [3]. The PV suspension system is specific for this prototype only. The vessel is hanging via two adjustable rods from the outer vessel, a third one beyond the module separation was used during assembly only. The adjustable horizontal and vertical support is very massive for balancing the PV torque forces resulting from differential pressures. In W7-X these forces will be balanced by the closed torus. The horizontal and vertical supports will be independent too, but the latter ones will be provided by feet.
2.2. Outer 6essel Shape and main dimensions can also be gathered from Fig. 1, the wall thickness is 20 mm. The material is ordinary carbon steel. In W7-X it will be stainless steel with a wall thickness of 25 mm to reduce deformations under vacuum load to B3 mm.
2.3. Ports Twenty-six ports of representative shapes and sizes were built. All are equipped with bellows for compensation movements due to PV heating and adjustments. Twenty three of the ports are round with inside diameters reaching from 80 to 350 mm. Two ports are oval having inside diameters of 650/300 and 690/390 mm, respectively. The largest port is rectangular with varying cross section, the smallest being 800/400 mm at the inside. The port shape is indicated in Fig. 1. All round port flanges are provided with standard CF metallic seals. For the rectangular and one of the oval ports silver wire O-rings are used, and the second oval port is sealed with VATSEAL® which is a flat copper gasket coated by a silver layer. The non-round ports have backup Viton O-ring seals with the possibility to evacuate the intermediate space. The W7-X ports will be built and installed basically the same way. However, the oval and rectangular ones will be sealed by Helicoflex®-Orings which are well-tried at IPP.
Fig. 1. Horizontal cross section of the demo-cryostat.
F. Schauer et al. / Fusion Engineering and Design 56–57 (2001) 861–866
2.4. Thermal insulation and heat radiation shields Fig. 1 shows the insulation system enclosing the cryo-space. The multilayer-insulation (ML) is situated at the ‘warm’ sides of the shields only. One layer consists of a 10 mm Al-foil, and two 72 mm layers of glass fleece as spacer. The ML surfaces and edges are covered by glass silk fabric. Gaps between MLs of different components are filled with glass wool (CRYOLITE®) which reduces radiation leaks and compensates for relative movements. The 80 K-shields are made of copper sheet with— except at the port shields — rectangular copper cooling tubes soldered onto the ‘cold’ side. The PV and OV shields are connected in series, and the port shields are cooled by heat conduction from the PV and OV shields. Design losses are 4 W/m2 at 80 K, and 0.1 W/m2 at 5 K. The W7-X thermal insulation will be simplified mainly to reduce the assembly time, without much reduction of the insulation quality. The PV insulation is assembled from pre-fabricated panels of about 0.8×1.7 m2 each. They consist of 90 reflecting layers mounted onto the 2 mm copper shield by sewing along the panel edges. The large layer number reduces losses within the compressed zone. Half of the ML packet thickness is staggered for overlapping with the adjacent panels. The copper shield is attached to the plasma vessel via low-loss stainless steel springs which allow flexibility for thermal contraction compensation. The cooling pipes of the individual panels are connected via flexible S-shaped tubes. Four toroidal panel rows are arranged poloidally around the plasma vessel. The panels within a row, and also the rows are connected in series. The insulation thickness lies between 30 and 50 mm. The outer vessel insulation is built up the same way as the PV insulation. There are eight series connected panel rows around the OV poloidal circumference. The insulation of the domes for the dummy conductor connections, and the insulation of the OV foot casings for the cold structure feet are part of the OV insulation.
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Twenty layers of ML-insulation are wound directly onto the ports which are inserted into the previously mounted shield tubes within the cryostat. These port shields consist of two conduction cooled co-axial tubes each which are sliding against each other and are attached to the PV and OV shields, respectively. The separately cooled 80 and 5 K side shields simulate thermally a continuous torus, Fig. 1. The outer 80 K shield is again covered with a 90 layer ML-insulation. Both side shield pairs consist of 1 mm copper sheet on stainless steel support structures.
2.5. Coil models The coil models are covered by 1 mm copper sheet for cooling by one Cu-tube each around the coil circumference. This cooling is representative for the real W7-X coil housings [4]. In W7-X, steel tubes are attached to the housings and their copper covers. At the coil terminal openings, Al-alloy tubes like the real coil cable jackets are inserted. These short tubes, bent to U-shape, represent the W7-X conductor double layers. Their ends, protruding from the coil models, are connected analogously to the real W7-X-conductors, but via hollow joint casings only. The weight of each DC dummy coil assembly is :500 kg. Due to development progress, the geometry of the W7-X coil terminations and the bus system will be different, and another design of the joints will be used.
2.6. Support structure The support structure is covered to a bigger part by 1 mm copper sheet with soldered-on rectangular copper tubes. The copper sheets are attached to the stainless steel by spot-welding. The weight of the support structure is roughly 4 tons. The massive W7-X central structure will have different cooling and foot arrangements. The cooling tubes will be stainless steel, no copper shield will be applied.
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2.7. Cooling loops There are four independent cooling loops: circuit No. 1 is the coil and support loop. All coils are cooled in parallel. Valves within the return lines are set in the warm state for equal flow resistances. The coil support structure is cooled in series with the exhaust He from the coil models. Circuit No. 2 is for series cooling the 5 K side shields. The main shields of the PV, OV, and the ports are cooled serially by circuit No. 3. Circuit No. 4 cools both 80 K side shields serially. Both 80 K circuits are operated during cool-down and for loss measurements with helium. For further cool-down of the 5 K-loops they are switched to LN2-operation. The DC cryo-system is relevant for W7-X with regard mainly to the housing and radiation shield cooling. The dummy conductor cooling system of the DC was important to get experience in handling these components under limited space conditions, but is not relevant with regard to cooling. In W7-X the conductor cooling conditions are completely different, and experience on that was gained from the demo-coil project [5].
2.8. Instrumentation There are in total 49 Rh– Fe surface temperature sensors attached to the 5 K-surfaces, and 17 Rh – Fe sensors within He-loops. Eighty Kelvin shield temperatures are measured by 28 thermocouples type E, and the PV and port walls are monitored by another 28 thermocouples type K. Further sensors are differential pressure indicators within loop No. 1, and vacuum indicators at the top and bottom of the OV. Plasma vessel movements and deformations are measured by six inductive displacement pickups at either PV side. Nine electrical contacts indicate minimal distances between the PV shield and the coils. Nine strain gauges are attached to the PV surface, and another eight to the PV suspension rods. All sensors are connected to a PC-controlled data acquisition system. On both vessels, the coils, and the structure, numerous reflecting position points for optical survey were installed and partly dismantled dur-
ing assembly. Such points were also used for deformation measurements of the OV.
2.9. Plasma 6essel heating system For baking experiments up to 150 °C, the PV is equipped with 13 poloidal electrical heaters plus one on each side lid. For baking the ports, heaters were attached to the port outsides by different means. All port bellows are thermally coupled to the heaters via copper tapes. During W7-X cryostat design it was decided to use water for baking as well as cooling the PV and ports. Therefore, the electrical DC heaters have limited relevance for W7-X.
2.10. Electrical insulation One of the aims of the DC project was to get experience in the application of the 12 kV electrical insulation to the W7-X cable joints and coil connection system under realistic conditions. The DC dummy conductor insulation consists of three layers of flexible fibre glass tube and two layers of Kapton in between, impregnated with epoxy at ambient conditions. For the cable joints, a new type of gas-permeable insulation [6] was applied which is relatively easy to assemble and disassemble in case of a joint failure. Sample tests proved voltage strengths of 30 kV for the conductor insulation, and \20 kV for the joint insulation under worst case Paschen minimum conditions. Voltage breaks for conductor cooling pipes were also developed. They were simply machined from fibreglass-epoxy tubes. All voltage breaks were temperature shocked with LN2 and He leak tested near 80 K without rejects. One was also successfully leak-checked in LHe. The voltage breaks were used also as transitions between the Al-alloy tubes attached to the joints, and the stainless steel He cooling pipes.
3. Final assembly The demo-cryostat was delivered to IPP Garching in pre-assembled parts. The main unit was the ‘inner part’ consisting of the thermally insulated
F. Schauer et al. / Fusion Engineering and Design 56–57 (2001) 861–866
plasma vessel, the coil models, and the support structure. The separate outer vessel components were two horizontally divided half-shells and the end caps. The half-shells were reinforced by additional steel sheet structures for keeping the proper shapes which were removed later. The ports were also delivered separately. At IPP, the inner part was finished by inserting and connecting the dummy conductors into the coils and connecting them, and by completing the cryogenic piping and instrumentation. Concurrently, the outer vessel half shells were thermally insulated. To this end both parts were situated such that the open side was on top for facilitating the work. However, insulating the numerous domes and the foot casings turned out to be very time consuming. Then the inner part assembly was lifted into the lower half-shell, and the cryo-feet were installed. Part of the lower ports were inserted. After connecting both half-shells, the PV side lids, the side shields, the final plasma vessel supports, the upper ports, and the OV side caps were installed. The cryogenic piping, including valves and instrumentation, was completed, and the Oring sealed dome covers were attached. After lifting the whole assembly onto the final supports, the last ports underneath could be installed. Finally, the cooling loops were connected to the distribution box via transfer lines.
4. Tests First the plasma vessel and cold mass supports were checked and adjusted by movements simulating baking and thermal contraction, respectively, at W7-X. Next the plasma and outer vessel deformations under different vacuum load conditions were measured. The plasma vessel results were everywhere B5 mm as specified. The OV measurements were generally B3 mm directly at the shell, as specified, with a few acceptable excursions up to 6 mm. At the end caps and domes generally values up to 7 mm were reached (not specified). Then a 1.5 bar pressure test was performed on the plasma vessel with the OV being evacuated. The PV deformations remained within the linear range.
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Next, evacuation and leak checks were performed. One small leak was found at one port between the plasma and cryostat vacuum spaces. Some leaks were detected at the outer vessel, and one at a dummy conductor joint. Even though the integral leak test after repair was not completely conclusive, it was decided to try a first cool-down without further fixture. During OV evacuation, a first plasma vessel and port baking test was performed for adjusting the heater control system. Some improvements were implemented. The complete plasma vessel baking test is scheduled for the time after the cryogenic tests. With the OV being evacuated, the electrical insulation was tested too. Breakdown occurred at around 1 kV at a defect epoxy joint which had to be made under difficult conditions on site. Since the principal function of the insulation was proved before, and experience with mounting was gained already for W7-X, repair and further voltage tests had no priority and were postponed. Three days after start of the cold tests at about 250 K, it became clear that the leak within circuit No. 1 was too large for a meaningful experiment. At this point it was decided to disconnect this loop and to continue cooling the three shields only. Cool-down proceeded well to 80 K, and stationary conditions at the shields were achieved within 10 days. The coils and structure— together about 9 tons— were cooled mainly by radiative and residual gas heat exchange, and reached stationary temperatures of : 140 K after 16 days. The cryostat vacuum was 4× 10 − 4 mbar. Under these conditions, the losses of the PV, OV and port shields were 800 W or : 4 W/m2, which was well within the specified value of altogether 1000 W. Then the 80 K shield circuits were switched to LN2 operation. The 5 K side shield was cooled further until after 112 days the temperature reached 20 K. Cool-down became then rather slow because only the one circuit with limited He flow was used. Mainly due to radiation cooling, the coil and structure temperatures reached 135 K, and the vacuum came down to 3× 10 − 5 mbar. It was now clear that all shield circuits were tight and the refrigeration system working.
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For warm-up, the refrigerator was just switched off. All component temperatures balanced out and rose smoothly within 2 weeks to 250 K. Then warm-up was speeded up by flooding the cryostat space with dry nitrogen. After another 3 days all temperatures were \ 0 °C. In order to avoid further time-consuming leak searches, the dummy conductor cooling within circuit No.1 was simply bypassed. This can be done without decreasing the meaningfulness of the experiment (see above). After a final integral leak check the demo-cryostat is now ready for the next cool-down experiment scheduled for September and October.
that the W7-X cryostat can be built according to specification.
Acknowledgements The successful execution of this project would not have been possible without the full commitment of the companies Balcke-Duerr Energietechnik and Linde, who were highly creative in finding problem solutions, and who were never discouraged but incited when difficulties arose.
References 5. Summary In the course of the demo-cryostat project a number of problems—some of them were revealed during the work only—had to be solved, and a lot of learning by doing was necessary. Therefore, the project was extremely instructive for the work on W7-X. All the experience gained from the prototype flowed directly into the W7-X design, and will continue to influence the whole project. It was confirmed that stringent and continuous quality control is indispensable for a project requiring so many new technologies. Anyway, even though some of the components and procedures need to be improved, it was demonstrated
[1] A. Brenner et al., Design and manufacture of the demonstration cryostat for the fusion experiment WENDELSTEIN 7-X, Proc. 20th. Symp. on Fusion Technol., Association EURATOM-CEA, 1998, pp. 1725 – 1728. [2] F. Schauer, et al., Demonstration cryostat sector for Wendelstein 7-X, in: Fusion Technology 1994, Elsevier Science B.V, 1995, pp. 941 – 944. [3] J. Simon-Weidner, N. Jaksic, Safety margins of the W7-X plasma vessel for the static case, paper J-15, this conference. [4] G. Krainz, F. Schauer, Geha¨ useku¨ hlung der W7-X Hauptfeldspulen, IPP-Report, Max-Planck-Institut fu¨ r Plasmapysik, IPP 11/1, September 1998. [5] G. Zahn et al., Cryogenic experiences and test results of the Wendelstein 7-X (W7-X) DEMO coil test in TOSKA, ICEC 18, I.I.T. Bombay, India, February 2000, paper OC3.3. [6] F. Schauer, Gas-permeable high voltage insulation, Europ. patent No. 0809852, January 2000.
Assembly and test of the W7-X demo-cryostat F. Schauer *, H. Bau, I. Bojko, R. Brockmann, J.-H. Feist, B. Hein, M. Pieger-Frey, H. Pirsch, J. Sapper, B. Sombach, J. Stadlbauer, O. Volzke, I. Wald, M. Wanner Max-Planck-Institut fu¨r Plasmaphysik, Euratom Association, Teilinstitut Greifswald, Wendelsteinstr. 1, D-17491 Greifswald, Germany
Abstract An overview is given on the status of the demo-cryostat project for the WENDELSTEIN 7-X stellarator. Construction and assembly of the prototype are finished, and the test period is near completion. The intention of this project was to get experience with design and construction of W7-X-components, as well as with assembly of this complex system. The goal is now practically achieved, and it could be demonstrated that the W7-X cryostat can be built with reasonable effort. Many of the solutions found can be adopted directly for W7-X, or are starting points for further improvements. A short description is given of the cryostat, its assembly, and of the most important tests which were performed so far. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Demo-cryostat; WENDELSTEIN 7-X; Plasma; Half-module
1. Introduction The demo-cryostat (DC) is a full size prototype of a WENDELSTEIN 7-X (W7-X) torus sector extending toroidally over 47° without counting the side covers (Fig. 1). It includes a 36°-halfmodule. A half-module is the basic W7-X-unit which recurs 10 times around the torus, arranged alternately upside-down. The DC is built up from the plasma and outer vessels (PV and OV, respectively), six non-planar and two planar coil models, support structure, ports, thermal and electrical insulation, cryogenic piping, PV and port heating * Corresponding author. Tel.: +49-3834-882-715; fax: + 49-3834-882-709. E-mail address: [email protected] (F. Schauer).
system, and instrumentation [1,2]. Manufacturer were Balcke-Duerr Energietechnik as main contractor, and Linde as sub-contractor for the cryogenic components.
2. Main components In addition to earlier papers [1,2], a short description of the actually built main components, and their relevance for the finally chosen W7-X design is given in the following.
2.1. Plasma 6essel The main dimensions can be roughly taken from Fig. 1. The vessel wall thickness is generally
0920-3796/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 0 - 3 7 9 6 ( 0 1 ) 0 0 3 6 6 - 0
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F. Schauer et al. / Fusion Engineering and Design 56–57 (2001) 861–866
17 mm, with some reinforcements up to 25 mm around the largest port and suspension points. It consists of DIN 1.4311 stainless steel which is an option for the W7-X vessel, whose wall thickness will also be 17 mm [3]. The PV suspension system is specific for this prototype only. The vessel is hanging via two adjustable rods from the outer vessel, a third one beyond the module separation was used during assembly only. The adjustable horizontal and vertical support is very massive for balancing the PV torque forces resulting from differential pressures. In W7-X these forces will be balanced by the closed torus. The horizontal and vertical supports will be independent too, but the latter ones will be provided by feet.
2.2. Outer 6essel Shape and main dimensions can also be gathered from Fig. 1, the wall thickness is 20 mm. The material is ordinary carbon steel. In W7-X it will be stainless steel with a wall thickness of 25 mm to reduce deformations under vacuum load to B3 mm.
2.3. Ports Twenty-six ports of representative shapes and sizes were built. All are equipped with bellows for compensation movements due to PV heating and adjustments. Twenty three of the ports are round with inside diameters reaching from 80 to 350 mm. Two ports are oval having inside diameters of 650/300 and 690/390 mm, respectively. The largest port is rectangular with varying cross section, the smallest being 800/400 mm at the inside. The port shape is indicated in Fig. 1. All round port flanges are provided with standard CF metallic seals. For the rectangular and one of the oval ports silver wire O-rings are used, and the second oval port is sealed with VATSEAL® which is a flat copper gasket coated by a silver layer. The non-round ports have backup Viton O-ring seals with the possibility to evacuate the intermediate space. The W7-X ports will be built and installed basically the same way. However, the oval and rectangular ones will be sealed by Helicoflex®-Orings which are well-tried at IPP.
Fig. 1. Horizontal cross section of the demo-cryostat.
F. Schauer et al. / Fusion Engineering and Design 56–57 (2001) 861–866
2.4. Thermal insulation and heat radiation shields Fig. 1 shows the insulation system enclosing the cryo-space. The multilayer-insulation (ML) is situated at the ‘warm’ sides of the shields only. One layer consists of a 10 mm Al-foil, and two 72 mm layers of glass fleece as spacer. The ML surfaces and edges are covered by glass silk fabric. Gaps between MLs of different components are filled with glass wool (CRYOLITE®) which reduces radiation leaks and compensates for relative movements. The 80 K-shields are made of copper sheet with— except at the port shields — rectangular copper cooling tubes soldered onto the ‘cold’ side. The PV and OV shields are connected in series, and the port shields are cooled by heat conduction from the PV and OV shields. Design losses are 4 W/m2 at 80 K, and 0.1 W/m2 at 5 K. The W7-X thermal insulation will be simplified mainly to reduce the assembly time, without much reduction of the insulation quality. The PV insulation is assembled from pre-fabricated panels of about 0.8×1.7 m2 each. They consist of 90 reflecting layers mounted onto the 2 mm copper shield by sewing along the panel edges. The large layer number reduces losses within the compressed zone. Half of the ML packet thickness is staggered for overlapping with the adjacent panels. The copper shield is attached to the plasma vessel via low-loss stainless steel springs which allow flexibility for thermal contraction compensation. The cooling pipes of the individual panels are connected via flexible S-shaped tubes. Four toroidal panel rows are arranged poloidally around the plasma vessel. The panels within a row, and also the rows are connected in series. The insulation thickness lies between 30 and 50 mm. The outer vessel insulation is built up the same way as the PV insulation. There are eight series connected panel rows around the OV poloidal circumference. The insulation of the domes for the dummy conductor connections, and the insulation of the OV foot casings for the cold structure feet are part of the OV insulation.
863
Twenty layers of ML-insulation are wound directly onto the ports which are inserted into the previously mounted shield tubes within the cryostat. These port shields consist of two conduction cooled co-axial tubes each which are sliding against each other and are attached to the PV and OV shields, respectively. The separately cooled 80 and 5 K side shields simulate thermally a continuous torus, Fig. 1. The outer 80 K shield is again covered with a 90 layer ML-insulation. Both side shield pairs consist of 1 mm copper sheet on stainless steel support structures.
2.5. Coil models The coil models are covered by 1 mm copper sheet for cooling by one Cu-tube each around the coil circumference. This cooling is representative for the real W7-X coil housings [4]. In W7-X, steel tubes are attached to the housings and their copper covers. At the coil terminal openings, Al-alloy tubes like the real coil cable jackets are inserted. These short tubes, bent to U-shape, represent the W7-X conductor double layers. Their ends, protruding from the coil models, are connected analogously to the real W7-X-conductors, but via hollow joint casings only. The weight of each DC dummy coil assembly is :500 kg. Due to development progress, the geometry of the W7-X coil terminations and the bus system will be different, and another design of the joints will be used.
2.6. Support structure The support structure is covered to a bigger part by 1 mm copper sheet with soldered-on rectangular copper tubes. The copper sheets are attached to the stainless steel by spot-welding. The weight of the support structure is roughly 4 tons. The massive W7-X central structure will have different cooling and foot arrangements. The cooling tubes will be stainless steel, no copper shield will be applied.
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2.7. Cooling loops There are four independent cooling loops: circuit No. 1 is the coil and support loop. All coils are cooled in parallel. Valves within the return lines are set in the warm state for equal flow resistances. The coil support structure is cooled in series with the exhaust He from the coil models. Circuit No. 2 is for series cooling the 5 K side shields. The main shields of the PV, OV, and the ports are cooled serially by circuit No. 3. Circuit No. 4 cools both 80 K side shields serially. Both 80 K circuits are operated during cool-down and for loss measurements with helium. For further cool-down of the 5 K-loops they are switched to LN2-operation. The DC cryo-system is relevant for W7-X with regard mainly to the housing and radiation shield cooling. The dummy conductor cooling system of the DC was important to get experience in handling these components under limited space conditions, but is not relevant with regard to cooling. In W7-X the conductor cooling conditions are completely different, and experience on that was gained from the demo-coil project [5].
2.8. Instrumentation There are in total 49 Rh– Fe surface temperature sensors attached to the 5 K-surfaces, and 17 Rh – Fe sensors within He-loops. Eighty Kelvin shield temperatures are measured by 28 thermocouples type E, and the PV and port walls are monitored by another 28 thermocouples type K. Further sensors are differential pressure indicators within loop No. 1, and vacuum indicators at the top and bottom of the OV. Plasma vessel movements and deformations are measured by six inductive displacement pickups at either PV side. Nine electrical contacts indicate minimal distances between the PV shield and the coils. Nine strain gauges are attached to the PV surface, and another eight to the PV suspension rods. All sensors are connected to a PC-controlled data acquisition system. On both vessels, the coils, and the structure, numerous reflecting position points for optical survey were installed and partly dismantled dur-
ing assembly. Such points were also used for deformation measurements of the OV.
2.9. Plasma 6essel heating system For baking experiments up to 150 °C, the PV is equipped with 13 poloidal electrical heaters plus one on each side lid. For baking the ports, heaters were attached to the port outsides by different means. All port bellows are thermally coupled to the heaters via copper tapes. During W7-X cryostat design it was decided to use water for baking as well as cooling the PV and ports. Therefore, the electrical DC heaters have limited relevance for W7-X.
2.10. Electrical insulation One of the aims of the DC project was to get experience in the application of the 12 kV electrical insulation to the W7-X cable joints and coil connection system under realistic conditions. The DC dummy conductor insulation consists of three layers of flexible fibre glass tube and two layers of Kapton in between, impregnated with epoxy at ambient conditions. For the cable joints, a new type of gas-permeable insulation [6] was applied which is relatively easy to assemble and disassemble in case of a joint failure. Sample tests proved voltage strengths of 30 kV for the conductor insulation, and \20 kV for the joint insulation under worst case Paschen minimum conditions. Voltage breaks for conductor cooling pipes were also developed. They were simply machined from fibreglass-epoxy tubes. All voltage breaks were temperature shocked with LN2 and He leak tested near 80 K without rejects. One was also successfully leak-checked in LHe. The voltage breaks were used also as transitions between the Al-alloy tubes attached to the joints, and the stainless steel He cooling pipes.
3. Final assembly The demo-cryostat was delivered to IPP Garching in pre-assembled parts. The main unit was the ‘inner part’ consisting of the thermally insulated
F. Schauer et al. / Fusion Engineering and Design 56–57 (2001) 861–866
plasma vessel, the coil models, and the support structure. The separate outer vessel components were two horizontally divided half-shells and the end caps. The half-shells were reinforced by additional steel sheet structures for keeping the proper shapes which were removed later. The ports were also delivered separately. At IPP, the inner part was finished by inserting and connecting the dummy conductors into the coils and connecting them, and by completing the cryogenic piping and instrumentation. Concurrently, the outer vessel half shells were thermally insulated. To this end both parts were situated such that the open side was on top for facilitating the work. However, insulating the numerous domes and the foot casings turned out to be very time consuming. Then the inner part assembly was lifted into the lower half-shell, and the cryo-feet were installed. Part of the lower ports were inserted. After connecting both half-shells, the PV side lids, the side shields, the final plasma vessel supports, the upper ports, and the OV side caps were installed. The cryogenic piping, including valves and instrumentation, was completed, and the Oring sealed dome covers were attached. After lifting the whole assembly onto the final supports, the last ports underneath could be installed. Finally, the cooling loops were connected to the distribution box via transfer lines.
4. Tests First the plasma vessel and cold mass supports were checked and adjusted by movements simulating baking and thermal contraction, respectively, at W7-X. Next the plasma and outer vessel deformations under different vacuum load conditions were measured. The plasma vessel results were everywhere B5 mm as specified. The OV measurements were generally B3 mm directly at the shell, as specified, with a few acceptable excursions up to 6 mm. At the end caps and domes generally values up to 7 mm were reached (not specified). Then a 1.5 bar pressure test was performed on the plasma vessel with the OV being evacuated. The PV deformations remained within the linear range.
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Next, evacuation and leak checks were performed. One small leak was found at one port between the plasma and cryostat vacuum spaces. Some leaks were detected at the outer vessel, and one at a dummy conductor joint. Even though the integral leak test after repair was not completely conclusive, it was decided to try a first cool-down without further fixture. During OV evacuation, a first plasma vessel and port baking test was performed for adjusting the heater control system. Some improvements were implemented. The complete plasma vessel baking test is scheduled for the time after the cryogenic tests. With the OV being evacuated, the electrical insulation was tested too. Breakdown occurred at around 1 kV at a defect epoxy joint which had to be made under difficult conditions on site. Since the principal function of the insulation was proved before, and experience with mounting was gained already for W7-X, repair and further voltage tests had no priority and were postponed. Three days after start of the cold tests at about 250 K, it became clear that the leak within circuit No. 1 was too large for a meaningful experiment. At this point it was decided to disconnect this loop and to continue cooling the three shields only. Cool-down proceeded well to 80 K, and stationary conditions at the shields were achieved within 10 days. The coils and structure— together about 9 tons— were cooled mainly by radiative and residual gas heat exchange, and reached stationary temperatures of : 140 K after 16 days. The cryostat vacuum was 4× 10 − 4 mbar. Under these conditions, the losses of the PV, OV and port shields were 800 W or : 4 W/m2, which was well within the specified value of altogether 1000 W. Then the 80 K shield circuits were switched to LN2 operation. The 5 K side shield was cooled further until after 112 days the temperature reached 20 K. Cool-down became then rather slow because only the one circuit with limited He flow was used. Mainly due to radiation cooling, the coil and structure temperatures reached 135 K, and the vacuum came down to 3× 10 − 5 mbar. It was now clear that all shield circuits were tight and the refrigeration system working.
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For warm-up, the refrigerator was just switched off. All component temperatures balanced out and rose smoothly within 2 weeks to 250 K. Then warm-up was speeded up by flooding the cryostat space with dry nitrogen. After another 3 days all temperatures were \ 0 °C. In order to avoid further time-consuming leak searches, the dummy conductor cooling within circuit No.1 was simply bypassed. This can be done without decreasing the meaningfulness of the experiment (see above). After a final integral leak check the demo-cryostat is now ready for the next cool-down experiment scheduled for September and October.
that the W7-X cryostat can be built according to specification.
Acknowledgements The successful execution of this project would not have been possible without the full commitment of the companies Balcke-Duerr Energietechnik and Linde, who were highly creative in finding problem solutions, and who were never discouraged but incited when difficulties arose.
References 5. Summary In the course of the demo-cryostat project a number of problems—some of them were revealed during the work only—had to be solved, and a lot of learning by doing was necessary. Therefore, the project was extremely instructive for the work on W7-X. All the experience gained from the prototype flowed directly into the W7-X design, and will continue to influence the whole project. It was confirmed that stringent and continuous quality control is indispensable for a project requiring so many new technologies. Anyway, even though some of the components and procedures need to be improved, it was demonstrated
[1] A. Brenner et al., Design and manufacture of the demonstration cryostat for the fusion experiment WENDELSTEIN 7-X, Proc. 20th. Symp. on Fusion Technol., Association EURATOM-CEA, 1998, pp. 1725 – 1728. [2] F. Schauer, et al., Demonstration cryostat sector for Wendelstein 7-X, in: Fusion Technology 1994, Elsevier Science B.V, 1995, pp. 941 – 944. [3] J. Simon-Weidner, N. Jaksic, Safety margins of the W7-X plasma vessel for the static case, paper J-15, this conference. [4] G. Krainz, F. Schauer, Geha¨ useku¨ hlung der W7-X Hauptfeldspulen, IPP-Report, Max-Planck-Institut fu¨ r Plasmapysik, IPP 11/1, September 1998. [5] G. Zahn et al., Cryogenic experiences and test results of the Wendelstein 7-X (W7-X) DEMO coil test in TOSKA, ICEC 18, I.I.T. Bombay, India, February 2000, paper OC3.3. [6] F. Schauer, Gas-permeable high voltage insulation, Europ. patent No. 0809852, January 2000.