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Thermal and mechanical analysis of ITER-relevant LHCD antenna elements
Fusion Engineering and Design 86 (2011) 810–814
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Thermal and mechanical analysis of ITER-relevant LHCD antenna elements L. Marfisi a,∗ , M. Goniche a , C. Hamlyn-Harris b , J. Hillairet a , J.F. Artaud a , Y.S. Bae c , J. Belo d , G. Berger-By a , J.M. Bernard a , Ph. Cara a , A. Cardinali e , C. Castaldo e , S. Ceccuzzi e , R. Cesario e , J. Decker a , L. Delpech a , A. Ekedahl a , J. Garcia a , P. Garibaldi a , D. Guilhem a , G.T. Hoang a , H. Jia f , Q.Y. Huang f , F. Imbeaux a , F. Kazarian b , S.H. Kim a , Y. Lausenaz a , R. Maggiora g , R. Magne a , S. Meschino h , D. Milanesio g , F. Mirizzi e , W. Namkung i , L. Pajewski h , L. Panaccione h , Y. Peysson a , A. Saille a , G. Schettini h , M. Schneider a , P.K. Sharma a,1 , A.A. Tuccillo e , O. Tudisco e , G. Vecchi h , R. Villari e , K. Vulliez a , Y. Wu f , Q. Zeng f a
CEA, IRFM, F-13108 Saint-Paul-lez-Durance, France ITER Organization, CS 90 046, 13067 Sain-Paul-Les-Durance, France c National Fusion Research Institute, Daejeon, Republic of Korea d Associac¸ao Euratom-IST, Centro de Fusao Nuclear, Lisboa, Portugal e Associazione Euratom-ENEA sulla Fusione, CR Frascati, Roma, Italy f .Institute of Plasma Physics, CAS, Hefei, Anhui, China g Politecnico di Torino, Dipartimento di Elettronica, Torino, Italy h Roma Tre University, Roma, Italy i Pohang Accelerator Laboratory, Pohang Univ. of Science and Technology, Pohang, Republic of Korea b
a r t i c l e
i n f o
Article history: Available online 12 February 2011 Keywords: RF heating Lower hybrid Passive-active RF windows Front face
a b s t r a c t A 20 MW Lower Hybrid Current Drive system using an antenna based on the Passive-Active Multijunction (PAM) concept is envisaged on ITER. This paper gives an overview of the mechanical analysis, modeling and design carried out on two major elements of the antenna: the grill front face, and the RF feed-through or windows. The front face will have to withstand high heat and fast neutrons fluxes directly from the plasma. It will be actively cooled and present a beryllium coating upon ITER requirement. The RF window being a critical safety importance class component (SIC) because of its tritium confinement function, two of them will be put in series on each line to achieve a double barrier. A design of a water cooled 5 GHz CW RF “pillbox” window capable of sustaining 500 kW of transmitted power is proposed. Both studies allow to move forward, and focus on critical issues, such as manufacturing processes and R&D associated programs including tests of mock-ups. © 2011 Elsevier B.V. All rights reserved.
1. Introduction A 20 MW 5 GHz Lower Hybrid Current Drive system using an antenna based on the Passive Active Multijunction (PAM) concept is envisaged on ITER to assist the current ramp-up and for steady state operation [1]. Originally the PAM concept was developed at CEA IRFM [2,3], it was first tested on FTU, and a fully featured design is currently tested on Tore Supra [4,5]. In this geometry, the LH grill is composed of a succession of passive and active waveguides in toroidal direction, allowing a denser cooling system on the front face, as well as a sturdier structure than a Full Active Multijunction.
∗ Corresponding author. Tel.: +33 4 42 25 61 52. E-mail address: laurent.marfi[email protected] (L. Marfisi). 1 Permanent address: Institute for Plasma Research, Bhat, Gandhinagar, Gujarat, India. 0920-3796/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2011.01.025
In 2001, a first stage conceptual design of a PAM LH antenna for ITER was proposed [6]. In the framework of an EFDA task that ended in March 2010, a new design of two key components, the front face and the RF windows, has been carried out. The antenna front face is the launching area for the RF wave and the plasma facing element of the antenna. As such, it is required to be coated with beryllium, like the entire ITER wall, in order to avoid the plasma contamination with high Z materials. It is exposed to high heat loads (mainly radiation) and particle flux (neutrons, neutrals and ions) escaping the plasma. The RF windows are located on the antenna rear flange. They realize a separation between the vacuum vessel and the gaspressurized RF transmission lines, thus constituting the first tritium barrier. The window is heated by the RF losses occurring in the ceramic disk while transmitting 500 kW CW of RF power, and the resulting thermal stresses are superimposed with the residual stresses due to the manufacturing process.
L. Marfisi et al. / Fusion Engineering and Design 86 (2011) 810–814
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The thermal loads have been updated from initial design and thermo-mechanical computations have been carried out for different versions of these components. Simulations were carried out with ANSYS code on the basis of thermal loads obtained with Ansoft HFSS [7]. 2. Antenna front face description and loads A 2D analysis was performed in the plane where geometrical parameters are strongly related to RF properties (see Fig. 1). In the following paragraph, only the results of the design presenting the best performances are given, i.e. a beryllium/copper alloy (Cu–Cr–Zr) tile bonded to a copper/steel 316L(N) multilayer element with a low temperature technique (‘hot isostatic pressing’). The CuCrZr junction aims at achieving an excellent thermal conductivity and a sturdy bonding between the beryllium tile and the rest of the antenna to counter the forces generated on the beryllium tiles during disruptions. The critical design parameters are the water cooling pipes diameter and the beryllium thickness. This design takes the full advantage of the high thermal conductivity of the CuCrZr combined with a short distance between water flow and Be heated tiles. Material properties are extracted from the ITER data base [8]. Effects of irradiations were not taken into account in the present simulations. The plasma radiation flux (which includes the fast neutral flux resulting from charge exchange) is applied with a form factor based on a simplified 2D solid angle to take into account shading effects. It is a conservative value. Flux resulting from fast ions whose trajectory is guided by the magnetic field is applied only to the surface facing the plasma. RF surface losses along the waveguides are also included. Neutron volume heating, which decays along the waveguides with an e-fold decay length Q ∼ 100 mm, is considered as uniform,
Fig. 1. View of part of the antenna mouth. Materials are visible on the left: in grey color Be, in blue steel, in green copper. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
hypothesis that slightly over-estimates the flux in the 50 mm long section considered for these simulations. Table 1 gives the details of the fluxes and power to be exhausted by the cooling system. The powers are rated for one vertical row of active and passive waveguides, with a length of 1.9 m which equates to 1/48 of the whole antenna. The hydraulic circuit made of parallel ∅ = 8 mm pipes is designed to ensure an active cooling with a mean water speed of 4.8 m s−1 , and a mass flow of 0.23 kg s−1 per pipe, for a corresponding heat transfer coefficient of 35.6 kW m−2 K−1 . The overall pressure drop
Fig. 2. Temperature map (in [◦ C]).
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L. Marfisi et al. / Fusion Engineering and Design 86 (2011) 810–814
Table 1 Front face heat loads. Heat source
Flux (MW/m2 )
Surface (m2 )
Total power (kW)
Percentage of total heating
Plasma radiation Fast ions RF losses on Be RF losses on Cu Volume heating
0.5 0.3 0.028 0.013 3 (MW m−3 )
0.043 0.023 0.063 0.053 0.59 (m3 )
21.4 6.8 1.8 0.7 1.8
65.9 21.1 5.4 2.1 5.4
is estimated at 29.6 kPa in each of the 50 front straight pipes over 1.9 m, leaving a good pressure margin for the rest of the antenna, considering the pressure drop of 1 MPa per equatorial Plug allowed in ITER. We also take into account residual stresses assuming that an operation of annealing at 500 ◦ C has to be conducted to conclude the manufacturing. This value will be taken as a reference temperature for residual stress calculation. An in-pipe water pressure of 3 MPa was also taken into account. 3. Antenna analysis An exothermic reaction that produces BeO and hydrogen occurs between beryllium and water [9], this is a concern in case of a leak in the vacuum vessel. The BeO layer produced by this reaction is protective as long as it remains below 650 ◦ C. Attention should be drawn on the fact that a reasonable margin has to be considered on this maximum temperature to take into account the additional heat produced by the reaction itself. The temperature map is reported in Fig. 2 and shows a maximum temperature in beryllium of 494 ◦ C. Mechanical analyses are performed using ITER requirements [10]. The results are reported in Fig. 3 for thermal stresses (maximum in 316L(N) steel at 331 MPa). In-pipe pressure was also simulated and displayed a maximum induced stress value in 316L(N) stainless steel at 91 MPa. Electromagnetic forces due to eddy current were not taken into account. The Tresca stress intensities from thermal loads and from inpipe pressure are added up and compared to three times the allowable primary membrane stress intensity (“Sm” value) of each material. Be and CuCrZr are well below their limit. Stainless steel is closer to its 3Sm value with a total stress of 341 MPa. However, it remains below the criteria at this temperature (3Sm = 450 MPa at 150 ◦ C).
Fig. 4. “Pillbox” RF window.
4. RF vacuum windows description and loads The currently analyzed window is of “pillbox” type with a circular ceramic disk. It is a brazed assembly of a beryllium oxide (BeO) ceramic (Thermalox 995 by Brush Ceramics) disk with a copper skirt assembled with a cupronickel 30 housing. Cooling water flows between copper skirt and housing (see Fig. 4). Material properties are obtained from ITER [8] and Brush Ceramics [11]. The following loads and hypothesis were considered for the window simulation: - All components of the window are brazed together in a single vacuum braze cycle. The difference in thermal expansion coefficient between the ceramic and the copper skirt are expected to generate residual stresses. This analysis considers that the brazing was performed at 600 ◦ C, set as our reference temperature. - RF electric currents are generated on the surfaces of the copper skirt, resulting in “RF losses” while in operation. - The dielectric RF losses (the predominant thermal load) are proportional to the square of the electric field in the ceramic. The electric fields were mapped with a 3D HFSS simulation. Inlet temperature and pressure of the cooling circuit are respectively 22 ◦ C and 3 bars. 5. Windows analysis
Fig. 3. Tresca thermal stress intensity (in [Pa]).
The analysis focused on the ceramic disk as it represents the critical part of the window. It was conducted with a multilinear material model of copper to allow plastic deformations of the skirt. The model used for the design is based on an RF optimized version of the window [7]. Fig. 5 gives the stress map of 1/4 (taking advantage of symmetries) of the copper skirt and ceramic. It shows the overall deformations and stress concentration areas. The simulation was performed allowing the copper skirt to deform plastically. As a brittle material, the BeO ceramic is more sensitive to tensile than to compressive stresses. In the case of the Thermalox ceramic, the tensile strength is 124 MPa when the compressive strength is 1550 MPa. Consequently the maximum principal stress was plotted on the ceramic to spot tensile stress concentrations. The micro-cracks present in the structure of ceramics also tend to expand when put under tensile stresses that are below their tensile strength. This phenomenon, often named static fatigue, leads us to the definition of a tensile stress limit of 50 MPa with a failure probability of 0.001 [12].
L. Marfisi et al. / Fusion Engineering and Design 86 (2011) 810–814
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Fig. 7. Pinch suppression in Pa after applying 70 MPa compression.
Fig. 5. Von Mises equivalent stress in Pa. Overall amplified deformation (7.5 times).
These windows will be placed at the rear of the antenna, where we expect neutron fluences of 1020 n m−2 for a lifetime of 107 s. This low value will not cause any degradation of the ceramic nor loss of conductivity [13]. In Fig. 6, high tensile stress concentrations close to the ceramic–copper braze junction can be seen. This is caused by a pinch stress that occurs because the copper retracts more than the beryllium oxide when the assembly cools down after brazing (high difference in expansion coefficient between materials). The stresses reach levels that exceed the limit of 50 MPa. However these stresses can be mitigated applying a compression of 70 MPa at room temperature around the disk edge (Fig. 7). This could be achieved with a metallic ring put around the copper sleeve at the position of the ceramic. The exact amount of pre-compression will be a trade-off between the pinch stresses observed at room temperature and those observed in steady state. Tests on mock ups will be required, as pre-compression is difficult to apply accurately in such cases. Simulation of the stresses induced in the ceramic by the heat loads in steady state was conducted (see Fig. 8). A temperature
Fig. 8. Maximum principal stress in Pa. The model was loaded with thermal data only (no residual stresses). Maximum value: 31 MPa.
gradient that allowed low tensile stresses in the ceramic was achieved on the RF optimized window, with a maximum value at 31 MPa. 6. Conclusion
Fig. 6. Pinch stress in Pa when applying residual stresses alone (red parts above 50 MPa). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
New designs of the antenna beryllium front face and RF 500 kW window have been studied. The study focuses on the thermal behavior and structural integrity of these two components. According to the analysis, they are expected to withstand ITER thermal loads. In the presented design, the temperature of the front face does not exceed 650 ◦ C. The subsequent thermal stresses do not deteriorate it. The RF window optimized design allows low heat loads in the ceramic and consequently low temperature increase and low stresses. The residual pinch stresses can be suppressed with a pre-compression of the ceramic. However further study should be carried out. The front face needs to be assessed taking into account disruption forces and irradiations effects on the materials. Fatigue should be also analyzed
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L. Marfisi et al. / Fusion Engineering and Design 86 (2011) 810–814
more in details. The windows should be assessed towards cyclic fatigue. Finally, tests on mock ups will be realized in both cases prior manufacturing. Acknowledgments This work was supported by the European Communities under the contract of Association between EURATOM and CEA, and was carried out within the framework of the European Fusion Development Agreement. The views and opinions expressed here in do not necessarily reflect those of the European Commission. References [1] G.T. Hoang, et al., A lower hybrid current drive system for ITER, Nucl. Fusion 49 (2009) 075001. [2] P. Bibet, et al., Conceptual study of a reflector waveguide array for launching lower hybrid waves in reactor grades plasmas, Nucl. Fusion 35 (1995) 1213–1223.
[3] Ph. Bibet, B. Beaumont, J.H. Belo, L. Delpech, A. Ekedahl, G. Granucci, et al., Fusion Eng. Des. 74 (2005) 419–423. [4] A. Ekedahl, et al., Validation of the ITER-relevant passive-active-multijunction LHCD launcher on long pulses in Tore Supra, Nucl. Fusion 50 (2010) 112002. [5] A. Bécoulet, Steady state tokamak operation using lower hybrid current drive, Invited talk SOFT 2010, Porto, Portugal. [6] DDD-2001, ITER Detailed Design Document. [7] J. Hillairet, et al., RF modeling of the ITER-relevant lower hybrid antenna, in: SOFT Conference, Porto, Portugal, 2010. [8] ITER SDC-IC 222RLN document, Structural design criteria for ITER in-vessel components Appendix A: Materials design limit data. [9] K. McCarthy, Beryllium interaction with steam or air in ITER under accident conditions, Idaho National Engineering and Environmental Laboratory (INEEL), Idaho Falls, Idaho, USA. [10] ITER SDC-IC 222RHC document, Structural design criteria for ITER in-vessel components. [11] Brush Ceramic Products, Thermalox 995, Isopressed, Material properties chart. [12] P.F. Bercher, M.K. Ferber, Mechanical reliability of ceramic windows in high frequency microwave heating devices, Journal of Materials Science 19 (1984) 3778–3785. [13] A. Serikov, et al., Nuclear analyses for the ITER ECRH launcher, Nucl. Fusion 48 (2008) 054016.
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Thermal and mechanical analysis of ITER-relevant LHCD antenna elements L. Marfisi a,∗ , M. Goniche a , C. Hamlyn-Harris b , J. Hillairet a , J.F. Artaud a , Y.S. Bae c , J. Belo d , G. Berger-By a , J.M. Bernard a , Ph. Cara a , A. Cardinali e , C. Castaldo e , S. Ceccuzzi e , R. Cesario e , J. Decker a , L. Delpech a , A. Ekedahl a , J. Garcia a , P. Garibaldi a , D. Guilhem a , G.T. Hoang a , H. Jia f , Q.Y. Huang f , F. Imbeaux a , F. Kazarian b , S.H. Kim a , Y. Lausenaz a , R. Maggiora g , R. Magne a , S. Meschino h , D. Milanesio g , F. Mirizzi e , W. Namkung i , L. Pajewski h , L. Panaccione h , Y. Peysson a , A. Saille a , G. Schettini h , M. Schneider a , P.K. Sharma a,1 , A.A. Tuccillo e , O. Tudisco e , G. Vecchi h , R. Villari e , K. Vulliez a , Y. Wu f , Q. Zeng f a
CEA, IRFM, F-13108 Saint-Paul-lez-Durance, France ITER Organization, CS 90 046, 13067 Sain-Paul-Les-Durance, France c National Fusion Research Institute, Daejeon, Republic of Korea d Associac¸ao Euratom-IST, Centro de Fusao Nuclear, Lisboa, Portugal e Associazione Euratom-ENEA sulla Fusione, CR Frascati, Roma, Italy f .Institute of Plasma Physics, CAS, Hefei, Anhui, China g Politecnico di Torino, Dipartimento di Elettronica, Torino, Italy h Roma Tre University, Roma, Italy i Pohang Accelerator Laboratory, Pohang Univ. of Science and Technology, Pohang, Republic of Korea b
a r t i c l e
i n f o
Article history: Available online 12 February 2011 Keywords: RF heating Lower hybrid Passive-active RF windows Front face
a b s t r a c t A 20 MW Lower Hybrid Current Drive system using an antenna based on the Passive-Active Multijunction (PAM) concept is envisaged on ITER. This paper gives an overview of the mechanical analysis, modeling and design carried out on two major elements of the antenna: the grill front face, and the RF feed-through or windows. The front face will have to withstand high heat and fast neutrons fluxes directly from the plasma. It will be actively cooled and present a beryllium coating upon ITER requirement. The RF window being a critical safety importance class component (SIC) because of its tritium confinement function, two of them will be put in series on each line to achieve a double barrier. A design of a water cooled 5 GHz CW RF “pillbox” window capable of sustaining 500 kW of transmitted power is proposed. Both studies allow to move forward, and focus on critical issues, such as manufacturing processes and R&D associated programs including tests of mock-ups. © 2011 Elsevier B.V. All rights reserved.
1. Introduction A 20 MW 5 GHz Lower Hybrid Current Drive system using an antenna based on the Passive Active Multijunction (PAM) concept is envisaged on ITER to assist the current ramp-up and for steady state operation [1]. Originally the PAM concept was developed at CEA IRFM [2,3], it was first tested on FTU, and a fully featured design is currently tested on Tore Supra [4,5]. In this geometry, the LH grill is composed of a succession of passive and active waveguides in toroidal direction, allowing a denser cooling system on the front face, as well as a sturdier structure than a Full Active Multijunction.
∗ Corresponding author. Tel.: +33 4 42 25 61 52. E-mail address: laurent.marfi[email protected] (L. Marfisi). 1 Permanent address: Institute for Plasma Research, Bhat, Gandhinagar, Gujarat, India. 0920-3796/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2011.01.025
In 2001, a first stage conceptual design of a PAM LH antenna for ITER was proposed [6]. In the framework of an EFDA task that ended in March 2010, a new design of two key components, the front face and the RF windows, has been carried out. The antenna front face is the launching area for the RF wave and the plasma facing element of the antenna. As such, it is required to be coated with beryllium, like the entire ITER wall, in order to avoid the plasma contamination with high Z materials. It is exposed to high heat loads (mainly radiation) and particle flux (neutrons, neutrals and ions) escaping the plasma. The RF windows are located on the antenna rear flange. They realize a separation between the vacuum vessel and the gaspressurized RF transmission lines, thus constituting the first tritium barrier. The window is heated by the RF losses occurring in the ceramic disk while transmitting 500 kW CW of RF power, and the resulting thermal stresses are superimposed with the residual stresses due to the manufacturing process.
L. Marfisi et al. / Fusion Engineering and Design 86 (2011) 810–814
811
The thermal loads have been updated from initial design and thermo-mechanical computations have been carried out for different versions of these components. Simulations were carried out with ANSYS code on the basis of thermal loads obtained with Ansoft HFSS [7]. 2. Antenna front face description and loads A 2D analysis was performed in the plane where geometrical parameters are strongly related to RF properties (see Fig. 1). In the following paragraph, only the results of the design presenting the best performances are given, i.e. a beryllium/copper alloy (Cu–Cr–Zr) tile bonded to a copper/steel 316L(N) multilayer element with a low temperature technique (‘hot isostatic pressing’). The CuCrZr junction aims at achieving an excellent thermal conductivity and a sturdy bonding between the beryllium tile and the rest of the antenna to counter the forces generated on the beryllium tiles during disruptions. The critical design parameters are the water cooling pipes diameter and the beryllium thickness. This design takes the full advantage of the high thermal conductivity of the CuCrZr combined with a short distance between water flow and Be heated tiles. Material properties are extracted from the ITER data base [8]. Effects of irradiations were not taken into account in the present simulations. The plasma radiation flux (which includes the fast neutral flux resulting from charge exchange) is applied with a form factor based on a simplified 2D solid angle to take into account shading effects. It is a conservative value. Flux resulting from fast ions whose trajectory is guided by the magnetic field is applied only to the surface facing the plasma. RF surface losses along the waveguides are also included. Neutron volume heating, which decays along the waveguides with an e-fold decay length Q ∼ 100 mm, is considered as uniform,
Fig. 1. View of part of the antenna mouth. Materials are visible on the left: in grey color Be, in blue steel, in green copper. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
hypothesis that slightly over-estimates the flux in the 50 mm long section considered for these simulations. Table 1 gives the details of the fluxes and power to be exhausted by the cooling system. The powers are rated for one vertical row of active and passive waveguides, with a length of 1.9 m which equates to 1/48 of the whole antenna. The hydraulic circuit made of parallel ∅ = 8 mm pipes is designed to ensure an active cooling with a mean water speed of 4.8 m s−1 , and a mass flow of 0.23 kg s−1 per pipe, for a corresponding heat transfer coefficient of 35.6 kW m−2 K−1 . The overall pressure drop
Fig. 2. Temperature map (in [◦ C]).
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L. Marfisi et al. / Fusion Engineering and Design 86 (2011) 810–814
Table 1 Front face heat loads. Heat source
Flux (MW/m2 )
Surface (m2 )
Total power (kW)
Percentage of total heating
Plasma radiation Fast ions RF losses on Be RF losses on Cu Volume heating
0.5 0.3 0.028 0.013 3 (MW m−3 )
0.043 0.023 0.063 0.053 0.59 (m3 )
21.4 6.8 1.8 0.7 1.8
65.9 21.1 5.4 2.1 5.4
is estimated at 29.6 kPa in each of the 50 front straight pipes over 1.9 m, leaving a good pressure margin for the rest of the antenna, considering the pressure drop of 1 MPa per equatorial Plug allowed in ITER. We also take into account residual stresses assuming that an operation of annealing at 500 ◦ C has to be conducted to conclude the manufacturing. This value will be taken as a reference temperature for residual stress calculation. An in-pipe water pressure of 3 MPa was also taken into account. 3. Antenna analysis An exothermic reaction that produces BeO and hydrogen occurs between beryllium and water [9], this is a concern in case of a leak in the vacuum vessel. The BeO layer produced by this reaction is protective as long as it remains below 650 ◦ C. Attention should be drawn on the fact that a reasonable margin has to be considered on this maximum temperature to take into account the additional heat produced by the reaction itself. The temperature map is reported in Fig. 2 and shows a maximum temperature in beryllium of 494 ◦ C. Mechanical analyses are performed using ITER requirements [10]. The results are reported in Fig. 3 for thermal stresses (maximum in 316L(N) steel at 331 MPa). In-pipe pressure was also simulated and displayed a maximum induced stress value in 316L(N) stainless steel at 91 MPa. Electromagnetic forces due to eddy current were not taken into account. The Tresca stress intensities from thermal loads and from inpipe pressure are added up and compared to three times the allowable primary membrane stress intensity (“Sm” value) of each material. Be and CuCrZr are well below their limit. Stainless steel is closer to its 3Sm value with a total stress of 341 MPa. However, it remains below the criteria at this temperature (3Sm = 450 MPa at 150 ◦ C).
Fig. 4. “Pillbox” RF window.
4. RF vacuum windows description and loads The currently analyzed window is of “pillbox” type with a circular ceramic disk. It is a brazed assembly of a beryllium oxide (BeO) ceramic (Thermalox 995 by Brush Ceramics) disk with a copper skirt assembled with a cupronickel 30 housing. Cooling water flows between copper skirt and housing (see Fig. 4). Material properties are obtained from ITER [8] and Brush Ceramics [11]. The following loads and hypothesis were considered for the window simulation: - All components of the window are brazed together in a single vacuum braze cycle. The difference in thermal expansion coefficient between the ceramic and the copper skirt are expected to generate residual stresses. This analysis considers that the brazing was performed at 600 ◦ C, set as our reference temperature. - RF electric currents are generated on the surfaces of the copper skirt, resulting in “RF losses” while in operation. - The dielectric RF losses (the predominant thermal load) are proportional to the square of the electric field in the ceramic. The electric fields were mapped with a 3D HFSS simulation. Inlet temperature and pressure of the cooling circuit are respectively 22 ◦ C and 3 bars. 5. Windows analysis
Fig. 3. Tresca thermal stress intensity (in [Pa]).
The analysis focused on the ceramic disk as it represents the critical part of the window. It was conducted with a multilinear material model of copper to allow plastic deformations of the skirt. The model used for the design is based on an RF optimized version of the window [7]. Fig. 5 gives the stress map of 1/4 (taking advantage of symmetries) of the copper skirt and ceramic. It shows the overall deformations and stress concentration areas. The simulation was performed allowing the copper skirt to deform plastically. As a brittle material, the BeO ceramic is more sensitive to tensile than to compressive stresses. In the case of the Thermalox ceramic, the tensile strength is 124 MPa when the compressive strength is 1550 MPa. Consequently the maximum principal stress was plotted on the ceramic to spot tensile stress concentrations. The micro-cracks present in the structure of ceramics also tend to expand when put under tensile stresses that are below their tensile strength. This phenomenon, often named static fatigue, leads us to the definition of a tensile stress limit of 50 MPa with a failure probability of 0.001 [12].
L. Marfisi et al. / Fusion Engineering and Design 86 (2011) 810–814
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Fig. 7. Pinch suppression in Pa after applying 70 MPa compression.
Fig. 5. Von Mises equivalent stress in Pa. Overall amplified deformation (7.5 times).
These windows will be placed at the rear of the antenna, where we expect neutron fluences of 1020 n m−2 for a lifetime of 107 s. This low value will not cause any degradation of the ceramic nor loss of conductivity [13]. In Fig. 6, high tensile stress concentrations close to the ceramic–copper braze junction can be seen. This is caused by a pinch stress that occurs because the copper retracts more than the beryllium oxide when the assembly cools down after brazing (high difference in expansion coefficient between materials). The stresses reach levels that exceed the limit of 50 MPa. However these stresses can be mitigated applying a compression of 70 MPa at room temperature around the disk edge (Fig. 7). This could be achieved with a metallic ring put around the copper sleeve at the position of the ceramic. The exact amount of pre-compression will be a trade-off between the pinch stresses observed at room temperature and those observed in steady state. Tests on mock ups will be required, as pre-compression is difficult to apply accurately in such cases. Simulation of the stresses induced in the ceramic by the heat loads in steady state was conducted (see Fig. 8). A temperature
Fig. 8. Maximum principal stress in Pa. The model was loaded with thermal data only (no residual stresses). Maximum value: 31 MPa.
gradient that allowed low tensile stresses in the ceramic was achieved on the RF optimized window, with a maximum value at 31 MPa. 6. Conclusion
Fig. 6. Pinch stress in Pa when applying residual stresses alone (red parts above 50 MPa). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
New designs of the antenna beryllium front face and RF 500 kW window have been studied. The study focuses on the thermal behavior and structural integrity of these two components. According to the analysis, they are expected to withstand ITER thermal loads. In the presented design, the temperature of the front face does not exceed 650 ◦ C. The subsequent thermal stresses do not deteriorate it. The RF window optimized design allows low heat loads in the ceramic and consequently low temperature increase and low stresses. The residual pinch stresses can be suppressed with a pre-compression of the ceramic. However further study should be carried out. The front face needs to be assessed taking into account disruption forces and irradiations effects on the materials. Fatigue should be also analyzed
814
L. Marfisi et al. / Fusion Engineering and Design 86 (2011) 810–814
more in details. The windows should be assessed towards cyclic fatigue. Finally, tests on mock ups will be realized in both cases prior manufacturing. Acknowledgments This work was supported by the European Communities under the contract of Association between EURATOM and CEA, and was carried out within the framework of the European Fusion Development Agreement. The views and opinions expressed here in do not necessarily reflect those of the European Commission. References [1] G.T. Hoang, et al., A lower hybrid current drive system for ITER, Nucl. Fusion 49 (2009) 075001. [2] P. Bibet, et al., Conceptual study of a reflector waveguide array for launching lower hybrid waves in reactor grades plasmas, Nucl. Fusion 35 (1995) 1213–1223.
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