- Email: [email protected]
Design and fabrication of the KSTAR in-vessel cryo-pump
Fusion Engineering and Design 86 (2011) 1993–1996
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Design and fabrication of the KSTAR in-vessel cryo-pump H.J. Lee a,∗ , Y.M. Park a , Y.B. Chang a , J.H. Kim a , D.K. Lee a , H.L. Yang a , A.S. Bozek b , J.P. Smith b , P.M. Anderson b a b
National Fusion Research Institute, Daejeon, 113 Gwahangno, Yuseong-gu, Yuseong-gu, Daejeon 305-333, Republic of Korea Genertal Atomics, San Diego, CA, United States
a r t i c l e
i n f o
Article history: Available online 6 May 2011 Keywords: In-vessel cryo-pump Divertor pumping Hoop bracket Inconel 718 In-vessel components
a b s t r a c t In-vessel cryo-pump (IVCP) of the Korea Superconducting Tokamak Advanced Research (KSTAR) has been designed, fabricated, and installed in the vacuum vessel for effective particle control by pumping through a divertor gap. For the final engineering design of the IVCP supports to withstand all external forces, a structure analyses were performed for two cases. The first is the thermal stress due to cool-down from room temperature to operating temperature (cryo-panel: 4.4 K, thermal shield: 77 K), and the other is the electro-magnetic stress due to the induced eddy currents during plasma disruptions. When the plasma disrupts, the maximum stress and displacement on the supports were estimated to be 849 MPa and 5.36 mm, respectively. These results were taken into account in the support design. The IVCP system was fabricated in two half-sectors and a pre-assembling test was successfully completed in the factory. Final installation of the IVCP in the vacuum vessel was fulfilled in parallel with a pressurization test (thermal shield: 30 bar, cryo-panel: 10 bar), a helium leak test, and a thermal shock test using liquid nitrogen. As a result, the IVCP system was successfully installed in the vacuum vessel. © 2011 Elsevier B.V. All rights reserved.
1. Introduction
2. Design requirement
Development of the in-vessel cryo-pump (IVCP) for the Korea Superconducting Tokamak Advanced Research (KSTAR) was launched in the middle of 2008 according to in-vessel component upgrade plan to enhance the plasma performance by exhausting plasma impurities. The basic study was performed for optimization of the number of pump and configuration in the vacuum vessel based on the cryo-condensation pump of the DIII-D [1]. Various researches continuously implemented to find the operating requirements and cooling scheme using liquid helium. Finally, the engineering design was terminated in early 2009, which was followed by subsequent fabrication, and installation of the IVCP in the vacuum vessel by early start of 2010. Two IVCPs were installed behind outboard divertor both at top and at bottom sides of the vacuum vessel (r = 2000 mm, z = ±1395 mm) with up-down symmetry, and were supported by the 16 vertical and radial supports in every 22.5◦ . This paper will describe the design feature of the IVCP, as well as several key results of the structure analyses. Moreover, process of the IVCP fabrication and installation in the vacuum vessel will be explained in summary.
The basic configuration of the IVCP is concentric tubes of three layers that contain a cryo-panel, thermal shields (inner and outer thermal shield) and a particle shield [2,3]. The cryo-panel is cooled by two phase liquid helium to 4.44 K with helium inlet pressure of 1.24 bar, and is surrounded by concentric thermal shields that are cooled by liquid nitrogen. The cryo-panel is supported by a conical spring and the concept was adapted to minimize thermal loads due to conduction between the cryo-panel and the thermal shield, as well as to absorb external loads [4]. The thermal shields are finally enclosed by a particle shield that protects the thermal shields and the cryo-panel from incoming energetic particles or radiations. The particle shield was divided into 16 segments, and electrically isolated with the thermal shields by three ceramic buttons. While the vertical support have to be robustly rigid one, the hoop bracket-shaped radial support should absorb the accumulated external load and have flexibility to allow the thermal contraction and expansion due to cool-down and bake-out of vacuum vessel [5,6]. Fig. 1 shows both an elevation view of the in-vessel component to illustrate position of the IVCPs and a three dimensional view of the detailed configuration of the IVCP. Table 1 summarizes general characteristics of the IVCP. The pumping area is about 1.01 m2 to have pumping speed of 25,000 l/s per each pump for deuterium, and the estimated gas throughput is about 20 torr l/s. As shown in Fig. 2, the cryo-panel must be regener-
∗ Corresponding author. Tel.: +82 42 879 5322. E-mail address: [email protected] (H.J. Lee). 0920-3796/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2011.03.058
1994
H.J. Lee et al. / Fusion Engineering and Design 86 (2011) 1993–1996
Fig. 3. The IVCP Modeling for ANSYS analysis.
3. Structure analysis
Fig. 1. The configuration of the KSTAR in-vessel cryo-pump.
Table 1 Requirement of the KSTAR IVCP. General characteristics Number Pumping speed Pulse length Perimeter of the pump Pumping area Helium coolant condition Inlet mass flow Inlet Temperature Pressure Outlet Temperature Pressure Liquid nitrogen condition Inlet mass flow Heat load Helium panel Thermal shield
The IVCP has to be operated in the extremely low temperature and rapidly changing electro-magnetic field. Hence, it should endure the thermal stress and electro-magnetic stress. The structural analysis on the IVCP was carried out for two cases (cool-down and plasma disruption) using ANSYS code. A modeling for analysis is shown in Fig. 3. To expedite completion of the support design, the global analysis was performed for entire body structure of the IVCP, and subsequent detailed analyses on the maximum stress point was employed again to take out the final results. 3.1. Thermal stress
2 EA 25,000 l/s 20/300 s 12.56 m 1.01 m2 6.5 g/s 4.44 K 1.24 bar 4.43 K 1.23 bar 10 g/s 32 W 1325 W
The temperature distribution of the IVCP is shown in Fig. 4. And thermal shrinkages due to cool-down of the IVCP to the operation temperature lead to displacements of the cryo-panel (5.065 mm) and the thermal shield (4.609 mm). Thermal stress caused by a displacement of the cryo-panel can be absorbed by conical springs. However, the stress due to shrinkage of the thermal shields that are supported by fixed radial supports may result in serious damages on the whole structures. Hence, detailed analyses should be carried out to solve the problem. Fig. 5 shows a detailed simulation result of the radial and vertical support in the case of the cool-down. The thermal shrinkage of the thermal shield compresses the radial support, and causes distortion of 3.9 mm with stress of 620 MPa. The distortion imposes another principal loads on the vertical support with maximum stress of 94.2 MPa. 3.2. Stress from electro-magnetic forces
ated between every plasma shots to maintain the steady pumping speed. It should be warmed up to 100 K within 3 min, and cooled down again to the operating temperature within 5 min after holding the 100 K about 15 min during wall conditioning period.
Since the IVCP has toroidally continuous configuration, plasma disruptions will produce rapidly changing magnetic flux that may
Fig. 2. Regeneration method of the IVCP.
Fig. 4. Temperature distribution of the IVCP after cool-down.
H.J. Lee et al. / Fusion Engineering and Design 86 (2011) 1993–1996
1995
40
Fig. 5. Stress on a radial, and on a vertical support caused by cool-down from 300 K to 77 K.
induce enormous eddy currents on the cryo-panel and on the thermal shields. To calculate the eddy current on the pump, one of the worst case scenario was considered as 2 MA plasma current is disrupted at fixed position (r = 1.5 m, z = 0.8 m) in 2 ms after vertical displacement. In this analysis, a coupled circuit equations were solved under assumption of the plasma acting as a single filament with every nearby structures such as vacuum vessel, passive stabilizer, poloidal field (PF) coils, and the cryo-pump. Disruptive plasma current is a driving source of eddy current in coupled circuit equations, which can be described as following Eq. (1). [M] ·
− → dIp dI − → + [R] · I = [Mp ] · dt dt
Maximum vertical force Maximum hoop force
30 25 20 15 10 5 0 -5 0
1
2
3
4
5
Time [msec] Fig. 7. Electro-magnetic force on the cryo-pump when the plasma disrupt.
(1)
where, M represents a mutual inductance matrix of the vacuum vessel with a passive stabilizer, PF coils, and cryo-pump components. R and I are a diagonal resistance matrix and eddy current of the structures described above, respectively. On right hand side, Mp represents a diagonal mutual inductance matrix in plasma filament and in the components in left hand side. As shown in Fig. 6, calculated maximum eddy current is 9.727 kA in the outer thermal shield, and the induced current is almost 2 kA on the other components. To calculate of the electro-magnetic forces, two sets of the PF coil current were chosen among 225 scenarios for the case of upward drift of the plasma. One of the two sets may generate the maximum hoop force and the other set can produce a maximum vertical force. Fig. 7 shows calculated electro-magnetic forces on the IVCP. The induced maximum electro-magnetic force is 36.4 kN and 19.4 kN for the case of maximum vertical force and the maximum hoop force, respectively. Final analyses on the support structure utilized the electromagnetic forces with thermal stress together, and Fig. 8 shows a
10
Without Passive Cryo-panal (A) : 2082 A Outer Thermal Shield (A) : 9727 A Inner Thermal Shield (A) : 1359 A
8
Eddy current (A)
Electro-magnetic force [kN]
35
Fig. 8. Stress contour at the radial and the vertical support for the case of disruption event.
detail analysis result of the supports at the maximum stress point. The maximum displacement due to the electro-magnetic force is 5.356 mm with maximum stress of 849 MPa at the radial support, while displacement and maximum stress of the vertical support is 0.092 mm and 129 MPa, respectively. Table 2 summarizes the results of the detailed structure analyses. The thermal stress is continuously imposed on the pump during the operation, and the stress from electro-magnetic forces abruptly superpose on the thermal stress when the plasma disruption. Therefore the maximum superimposed stress is 849 MPa. In accordance with analyses results, the material of radial support was selected as Inconel 718, while that of vertical support was chosen as stainless steel SS 316L. Because the tensile strength of Inconel 718 is 1240 MPa and that of stainless steel 316L is 480 MPa, these materials are satisfying the requirements for the supports of IVCP. 4. Fabrication and assembling of the IVCP
6
The IVCP has been fabricated by bending and annealing on tubes in two sectors (180◦ ) in order to reduce welding risks. Prior to start of IVCP fabrication, real-sized mock-up for a 180◦ seg-
4 2
Table 2 Results of the structural analysis for cool-down and plasma disruption cases.
0
Component
0
1
2
3
4
5
Time (msec) Fig. 6. Eddy current on the IVCP during disruption event (2MA plasma for 2 s after vertical displacement at r = 1.5 m, z = 0.8 m).
Radial support Vertical support
Case
Stress Displacement Stress Displacement
Cool-down
Plasma disruption
620 MPa 3.9 mm 94 MPa 0.067 mm
849 MPa 5.356 mm 129 MPa 0.092 mm
1996
H.J. Lee et al. / Fusion Engineering and Design 86 (2011) 1993–1996
Fig. 10. IVCP installed in the vacuum vessel. Fig. 9. Pre-assembling in the shop on the machined plat.
5. Installation in the vacuum vessel ment of the IVCP was fabricated to test feasibilities of fabrication process, assembly and installation sequences in the vacuum vessel. In the fabrication of the IVCP, magnetic permeability for all of the components was kept below 1.03 both at cryogenic temperature and at room temperature. The cryo-panel and thermal shield, of which diameter is about 12 m, were made of Inconel 625, and surface of the cryo-panel was electro-polished to 0.1 m of thickness to enhance the reflectivity for minimizing the heat loads. It is supported by conical springs that are installed at every 22.5◦ points. The flow pattern of liquid helium in the cryo-panel is single pass, and the 3/4 inch tube was inserted to enhance the stability of the helium flow, and to improve the efficiency of heat transfer. The inserted tube was machined to make slits with 0.2 mm of width in 20 mm of spacing distance using laser cutting method. The inner and outer thermal shields surround the cryo-panel to reduce the heat loads, and were designed in order that energetic particles have to bounce off the thermal shield at least twice before absorption at the cryo-panel. The thermal shields were annealed in the air for 30 min at 940 ◦ C to form an oxide layer with a high emissivity and to remove of the residual stress. The outer thermal shield is coated with copper strips by flame-spray method to enhance azimuthally thermal conductance. However, the copper strips were toroidally discrete to minimize the electro-magnetic forces on the pump. The cooling tubes were welded on the thermal shields using a series of 25 mm-long plug welds with 50 mm-space between every welding points. The particle shield was made of stainless steel SS316L, and divided into 16 sectors to prevent excessive eddy current on the shield. It was electrically isolated with the thermal shields by three-ceramic buttons. Gap between adjacent sectors was 3 mm for mechanical stability during machine cool-down and baking process of the vacuum vessel. After all components were completed in welding, helium leak test, which was preceded by pressurizing test, has been fulfilled to validate the leak tightness. After leak test described earlier was completed, final pressurizing test and leak test were performed again after thermal shock test using liquid nitrogen. As shown in Fig. 9, the IVCP was pre-assembled on a machined plate. Thermal shocks in three times were applied on each sector with liquid nitrogen for final helium leak test before its transportation to the KSTAR site.
After all of the sub-components were tested in helium leak in the main experimental hall, two 180◦ sectors were sequentially installed through a median vacuum vessel port. The feed-through piping lines for liquid helium (and liquid nitrogen) were also installed at top and bottom side of the cryostat. Finally, each two sectors and feed line are assembled, and welded to each other in the vacuum vessel. Fig. 10 shows the IVCP system installed in the vacuum vessel. Every welding point that was welded on site has been finally tested in helium leak test to make a confidence of vacuum tightness. As a result, we could not find any detectable leak in the whole IVCP system under sensitivity of the helium leak detector showed less than 5.0 × 10−10 mbar l/s. 6. Conclusions Since start of researches on the basic concept for KSTAR IVCP in late 2008, two sets of IVCP was successfully designed, fabricated, and installed. Estimated pumping speed through a divertor gap is almost 25,000 l/s per each IVCP, which will be crucially dedicated to divertor pumping experiments for particle control. Several combinations of test confirm that the IVCP has been satisfactorily fabricated and installed without any severe problems for its own performance in early 2010. The IVCP system will be operated from 2011 after fabrication of the refrigerant supply and distribution system. References [1] K.H. Im, J.H. Han, et al., Edge plasma analysis for KSTAR divertor design, in: 17th IEEE/NPSS Symposium on, Fusion Engineering, 2, 1997, pp. 756–759. [2] J.P. Smith, et al., The design and fabrication of a toroidally continuous cryocondensation pump for the DIII-D advanced divertor, in: Proceedings, 14th IEEE/NPSS Symposium on, Fusion Engineering, 2, 1991, pp. 1230–1232. [3] J.P. Smith, et al., Installation and initial operation of the DIII-D advanced divertor cryocondensation pump, in: 14th IEEE/NPSS Symposium on, Fusion Engineering, 2, 1993, pp. 1043–1046. [4] E.E. Reis, et al., Design and analysis of the cryopump for the DIII-D advanced divertor, in: Conference: 17, Symposium on Fusion Technology, Rome, Italy, 1992, pp. 14–18. [5] A.S. Bozek, et al., Engineering design of cryocondensation pumps for the DIIID radiative divertor program, in: SOFE 95. Seeking a New Energy Era., 16th IEEE/NPSS Symposium on, Fusion Engineering, 2, 1995, pp. 898–901. [6] E.E. Reis, et al., Design and analysis of the cryopump for the DIII-D upper divertor, in: 18th Symposium on Fusion Engineering, 1999, pp. 519–522.
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Design and fabrication of the KSTAR in-vessel cryo-pump H.J. Lee a,∗ , Y.M. Park a , Y.B. Chang a , J.H. Kim a , D.K. Lee a , H.L. Yang a , A.S. Bozek b , J.P. Smith b , P.M. Anderson b a b
National Fusion Research Institute, Daejeon, 113 Gwahangno, Yuseong-gu, Yuseong-gu, Daejeon 305-333, Republic of Korea Genertal Atomics, San Diego, CA, United States
a r t i c l e
i n f o
Article history: Available online 6 May 2011 Keywords: In-vessel cryo-pump Divertor pumping Hoop bracket Inconel 718 In-vessel components
a b s t r a c t In-vessel cryo-pump (IVCP) of the Korea Superconducting Tokamak Advanced Research (KSTAR) has been designed, fabricated, and installed in the vacuum vessel for effective particle control by pumping through a divertor gap. For the final engineering design of the IVCP supports to withstand all external forces, a structure analyses were performed for two cases. The first is the thermal stress due to cool-down from room temperature to operating temperature (cryo-panel: 4.4 K, thermal shield: 77 K), and the other is the electro-magnetic stress due to the induced eddy currents during plasma disruptions. When the plasma disrupts, the maximum stress and displacement on the supports were estimated to be 849 MPa and 5.36 mm, respectively. These results were taken into account in the support design. The IVCP system was fabricated in two half-sectors and a pre-assembling test was successfully completed in the factory. Final installation of the IVCP in the vacuum vessel was fulfilled in parallel with a pressurization test (thermal shield: 30 bar, cryo-panel: 10 bar), a helium leak test, and a thermal shock test using liquid nitrogen. As a result, the IVCP system was successfully installed in the vacuum vessel. © 2011 Elsevier B.V. All rights reserved.
1. Introduction
2. Design requirement
Development of the in-vessel cryo-pump (IVCP) for the Korea Superconducting Tokamak Advanced Research (KSTAR) was launched in the middle of 2008 according to in-vessel component upgrade plan to enhance the plasma performance by exhausting plasma impurities. The basic study was performed for optimization of the number of pump and configuration in the vacuum vessel based on the cryo-condensation pump of the DIII-D [1]. Various researches continuously implemented to find the operating requirements and cooling scheme using liquid helium. Finally, the engineering design was terminated in early 2009, which was followed by subsequent fabrication, and installation of the IVCP in the vacuum vessel by early start of 2010. Two IVCPs were installed behind outboard divertor both at top and at bottom sides of the vacuum vessel (r = 2000 mm, z = ±1395 mm) with up-down symmetry, and were supported by the 16 vertical and radial supports in every 22.5◦ . This paper will describe the design feature of the IVCP, as well as several key results of the structure analyses. Moreover, process of the IVCP fabrication and installation in the vacuum vessel will be explained in summary.
The basic configuration of the IVCP is concentric tubes of three layers that contain a cryo-panel, thermal shields (inner and outer thermal shield) and a particle shield [2,3]. The cryo-panel is cooled by two phase liquid helium to 4.44 K with helium inlet pressure of 1.24 bar, and is surrounded by concentric thermal shields that are cooled by liquid nitrogen. The cryo-panel is supported by a conical spring and the concept was adapted to minimize thermal loads due to conduction between the cryo-panel and the thermal shield, as well as to absorb external loads [4]. The thermal shields are finally enclosed by a particle shield that protects the thermal shields and the cryo-panel from incoming energetic particles or radiations. The particle shield was divided into 16 segments, and electrically isolated with the thermal shields by three ceramic buttons. While the vertical support have to be robustly rigid one, the hoop bracket-shaped radial support should absorb the accumulated external load and have flexibility to allow the thermal contraction and expansion due to cool-down and bake-out of vacuum vessel [5,6]. Fig. 1 shows both an elevation view of the in-vessel component to illustrate position of the IVCPs and a three dimensional view of the detailed configuration of the IVCP. Table 1 summarizes general characteristics of the IVCP. The pumping area is about 1.01 m2 to have pumping speed of 25,000 l/s per each pump for deuterium, and the estimated gas throughput is about 20 torr l/s. As shown in Fig. 2, the cryo-panel must be regener-
∗ Corresponding author. Tel.: +82 42 879 5322. E-mail address: [email protected] (H.J. Lee). 0920-3796/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2011.03.058
1994
H.J. Lee et al. / Fusion Engineering and Design 86 (2011) 1993–1996
Fig. 3. The IVCP Modeling for ANSYS analysis.
3. Structure analysis
Fig. 1. The configuration of the KSTAR in-vessel cryo-pump.
Table 1 Requirement of the KSTAR IVCP. General characteristics Number Pumping speed Pulse length Perimeter of the pump Pumping area Helium coolant condition Inlet mass flow Inlet Temperature Pressure Outlet Temperature Pressure Liquid nitrogen condition Inlet mass flow Heat load Helium panel Thermal shield
The IVCP has to be operated in the extremely low temperature and rapidly changing electro-magnetic field. Hence, it should endure the thermal stress and electro-magnetic stress. The structural analysis on the IVCP was carried out for two cases (cool-down and plasma disruption) using ANSYS code. A modeling for analysis is shown in Fig. 3. To expedite completion of the support design, the global analysis was performed for entire body structure of the IVCP, and subsequent detailed analyses on the maximum stress point was employed again to take out the final results. 3.1. Thermal stress
2 EA 25,000 l/s 20/300 s 12.56 m 1.01 m2 6.5 g/s 4.44 K 1.24 bar 4.43 K 1.23 bar 10 g/s 32 W 1325 W
The temperature distribution of the IVCP is shown in Fig. 4. And thermal shrinkages due to cool-down of the IVCP to the operation temperature lead to displacements of the cryo-panel (5.065 mm) and the thermal shield (4.609 mm). Thermal stress caused by a displacement of the cryo-panel can be absorbed by conical springs. However, the stress due to shrinkage of the thermal shields that are supported by fixed radial supports may result in serious damages on the whole structures. Hence, detailed analyses should be carried out to solve the problem. Fig. 5 shows a detailed simulation result of the radial and vertical support in the case of the cool-down. The thermal shrinkage of the thermal shield compresses the radial support, and causes distortion of 3.9 mm with stress of 620 MPa. The distortion imposes another principal loads on the vertical support with maximum stress of 94.2 MPa. 3.2. Stress from electro-magnetic forces
ated between every plasma shots to maintain the steady pumping speed. It should be warmed up to 100 K within 3 min, and cooled down again to the operating temperature within 5 min after holding the 100 K about 15 min during wall conditioning period.
Since the IVCP has toroidally continuous configuration, plasma disruptions will produce rapidly changing magnetic flux that may
Fig. 2. Regeneration method of the IVCP.
Fig. 4. Temperature distribution of the IVCP after cool-down.
H.J. Lee et al. / Fusion Engineering and Design 86 (2011) 1993–1996
1995
40
Fig. 5. Stress on a radial, and on a vertical support caused by cool-down from 300 K to 77 K.
induce enormous eddy currents on the cryo-panel and on the thermal shields. To calculate the eddy current on the pump, one of the worst case scenario was considered as 2 MA plasma current is disrupted at fixed position (r = 1.5 m, z = 0.8 m) in 2 ms after vertical displacement. In this analysis, a coupled circuit equations were solved under assumption of the plasma acting as a single filament with every nearby structures such as vacuum vessel, passive stabilizer, poloidal field (PF) coils, and the cryo-pump. Disruptive plasma current is a driving source of eddy current in coupled circuit equations, which can be described as following Eq. (1). [M] ·
− → dIp dI − → + [R] · I = [Mp ] · dt dt
Maximum vertical force Maximum hoop force
30 25 20 15 10 5 0 -5 0
1
2
3
4
5
Time [msec] Fig. 7. Electro-magnetic force on the cryo-pump when the plasma disrupt.
(1)
where, M represents a mutual inductance matrix of the vacuum vessel with a passive stabilizer, PF coils, and cryo-pump components. R and I are a diagonal resistance matrix and eddy current of the structures described above, respectively. On right hand side, Mp represents a diagonal mutual inductance matrix in plasma filament and in the components in left hand side. As shown in Fig. 6, calculated maximum eddy current is 9.727 kA in the outer thermal shield, and the induced current is almost 2 kA on the other components. To calculate of the electro-magnetic forces, two sets of the PF coil current were chosen among 225 scenarios for the case of upward drift of the plasma. One of the two sets may generate the maximum hoop force and the other set can produce a maximum vertical force. Fig. 7 shows calculated electro-magnetic forces on the IVCP. The induced maximum electro-magnetic force is 36.4 kN and 19.4 kN for the case of maximum vertical force and the maximum hoop force, respectively. Final analyses on the support structure utilized the electromagnetic forces with thermal stress together, and Fig. 8 shows a
10
Without Passive Cryo-panal (A) : 2082 A Outer Thermal Shield (A) : 9727 A Inner Thermal Shield (A) : 1359 A
8
Eddy current (A)
Electro-magnetic force [kN]
35
Fig. 8. Stress contour at the radial and the vertical support for the case of disruption event.
detail analysis result of the supports at the maximum stress point. The maximum displacement due to the electro-magnetic force is 5.356 mm with maximum stress of 849 MPa at the radial support, while displacement and maximum stress of the vertical support is 0.092 mm and 129 MPa, respectively. Table 2 summarizes the results of the detailed structure analyses. The thermal stress is continuously imposed on the pump during the operation, and the stress from electro-magnetic forces abruptly superpose on the thermal stress when the plasma disruption. Therefore the maximum superimposed stress is 849 MPa. In accordance with analyses results, the material of radial support was selected as Inconel 718, while that of vertical support was chosen as stainless steel SS 316L. Because the tensile strength of Inconel 718 is 1240 MPa and that of stainless steel 316L is 480 MPa, these materials are satisfying the requirements for the supports of IVCP. 4. Fabrication and assembling of the IVCP
6
The IVCP has been fabricated by bending and annealing on tubes in two sectors (180◦ ) in order to reduce welding risks. Prior to start of IVCP fabrication, real-sized mock-up for a 180◦ seg-
4 2
Table 2 Results of the structural analysis for cool-down and plasma disruption cases.
0
Component
0
1
2
3
4
5
Time (msec) Fig. 6. Eddy current on the IVCP during disruption event (2MA plasma for 2 s after vertical displacement at r = 1.5 m, z = 0.8 m).
Radial support Vertical support
Case
Stress Displacement Stress Displacement
Cool-down
Plasma disruption
620 MPa 3.9 mm 94 MPa 0.067 mm
849 MPa 5.356 mm 129 MPa 0.092 mm
1996
H.J. Lee et al. / Fusion Engineering and Design 86 (2011) 1993–1996
Fig. 10. IVCP installed in the vacuum vessel. Fig. 9. Pre-assembling in the shop on the machined plat.
5. Installation in the vacuum vessel ment of the IVCP was fabricated to test feasibilities of fabrication process, assembly and installation sequences in the vacuum vessel. In the fabrication of the IVCP, magnetic permeability for all of the components was kept below 1.03 both at cryogenic temperature and at room temperature. The cryo-panel and thermal shield, of which diameter is about 12 m, were made of Inconel 625, and surface of the cryo-panel was electro-polished to 0.1 m of thickness to enhance the reflectivity for minimizing the heat loads. It is supported by conical springs that are installed at every 22.5◦ points. The flow pattern of liquid helium in the cryo-panel is single pass, and the 3/4 inch tube was inserted to enhance the stability of the helium flow, and to improve the efficiency of heat transfer. The inserted tube was machined to make slits with 0.2 mm of width in 20 mm of spacing distance using laser cutting method. The inner and outer thermal shields surround the cryo-panel to reduce the heat loads, and were designed in order that energetic particles have to bounce off the thermal shield at least twice before absorption at the cryo-panel. The thermal shields were annealed in the air for 30 min at 940 ◦ C to form an oxide layer with a high emissivity and to remove of the residual stress. The outer thermal shield is coated with copper strips by flame-spray method to enhance azimuthally thermal conductance. However, the copper strips were toroidally discrete to minimize the electro-magnetic forces on the pump. The cooling tubes were welded on the thermal shields using a series of 25 mm-long plug welds with 50 mm-space between every welding points. The particle shield was made of stainless steel SS316L, and divided into 16 sectors to prevent excessive eddy current on the shield. It was electrically isolated with the thermal shields by three-ceramic buttons. Gap between adjacent sectors was 3 mm for mechanical stability during machine cool-down and baking process of the vacuum vessel. After all components were completed in welding, helium leak test, which was preceded by pressurizing test, has been fulfilled to validate the leak tightness. After leak test described earlier was completed, final pressurizing test and leak test were performed again after thermal shock test using liquid nitrogen. As shown in Fig. 9, the IVCP was pre-assembled on a machined plate. Thermal shocks in three times were applied on each sector with liquid nitrogen for final helium leak test before its transportation to the KSTAR site.
After all of the sub-components were tested in helium leak in the main experimental hall, two 180◦ sectors were sequentially installed through a median vacuum vessel port. The feed-through piping lines for liquid helium (and liquid nitrogen) were also installed at top and bottom side of the cryostat. Finally, each two sectors and feed line are assembled, and welded to each other in the vacuum vessel. Fig. 10 shows the IVCP system installed in the vacuum vessel. Every welding point that was welded on site has been finally tested in helium leak test to make a confidence of vacuum tightness. As a result, we could not find any detectable leak in the whole IVCP system under sensitivity of the helium leak detector showed less than 5.0 × 10−10 mbar l/s. 6. Conclusions Since start of researches on the basic concept for KSTAR IVCP in late 2008, two sets of IVCP was successfully designed, fabricated, and installed. Estimated pumping speed through a divertor gap is almost 25,000 l/s per each IVCP, which will be crucially dedicated to divertor pumping experiments for particle control. Several combinations of test confirm that the IVCP has been satisfactorily fabricated and installed without any severe problems for its own performance in early 2010. The IVCP system will be operated from 2011 after fabrication of the refrigerant supply and distribution system. References [1] K.H. Im, J.H. Han, et al., Edge plasma analysis for KSTAR divertor design, in: 17th IEEE/NPSS Symposium on, Fusion Engineering, 2, 1997, pp. 756–759. [2] J.P. Smith, et al., The design and fabrication of a toroidally continuous cryocondensation pump for the DIII-D advanced divertor, in: Proceedings, 14th IEEE/NPSS Symposium on, Fusion Engineering, 2, 1991, pp. 1230–1232. [3] J.P. Smith, et al., Installation and initial operation of the DIII-D advanced divertor cryocondensation pump, in: 14th IEEE/NPSS Symposium on, Fusion Engineering, 2, 1993, pp. 1043–1046. [4] E.E. Reis, et al., Design and analysis of the cryopump for the DIII-D advanced divertor, in: Conference: 17, Symposium on Fusion Technology, Rome, Italy, 1992, pp. 14–18. [5] A.S. Bozek, et al., Engineering design of cryocondensation pumps for the DIIID radiative divertor program, in: SOFE 95. Seeking a New Energy Era., 16th IEEE/NPSS Symposium on, Fusion Engineering, 2, 1995, pp. 898–901. [6] E.E. Reis, et al., Design and analysis of the cryopump for the DIII-D upper divertor, in: 18th Symposium on Fusion Engineering, 1999, pp. 519–522.