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Thermal behavior of the LHCD launchers in Tore Supra
Fusion Engineering and Design 82 (2007) 658–661
Thermal behavior of the LHCD launchers in Tore Supra C. Portafaix a,∗ , P. Bibet a , J.H. Belo b , A. Bou´e a , M. Chantant a , L. Delpech a , A. Ekedahl a , J.P. Gaston a , M. Goniche a a
b
Association Euratom-CEA, CEA/DSM/DRFC, CEA-Cadarache, 13108 Saint-Paul-lez-Durance, France Centro de Fus˜ao Nuclear, Associa¸ca˜ o Euratom-IST, Instituto Superior T´ecnico, 1049-001 Lisboa, Portugal
Received 13 July 2006; received in revised form 23 May 2007; accepted 24 May 2007
Abstract An actively cooled lower hybrid current drive (LHCD) launcher has been installed in 1999 in the Tore Supra tokamak. During the shots, the temperature of the antenna front part is measured with infrared cameras and on the back with thermocouples. The energy removed by the cooling water loop is also recorded. The infrared analysis and the calorimetric balance sheet for plasma indicates that the temperature increase and the absorbed energy are higher than expected. The thermal measurements have been compared to finite elements calculations with the Cast 3M code taking into account the RF losses, the plasma radiated heat flux and an additional heat flux on the antenna and guard limiter surface in front of the plasma. This additional source is most likely attributed to the interaction of fast ions. © 2007 Elsevier B.V. All rights reserved. Keywords: Lower hybrid current drive launcher; Thermal measurement; Thermal calculation
1. Introduction An actively cooled lower hybrid current drive (LHCD) launcher has been designed with the goal of injecting 4 MW during 1000 s at 3.7 GHz in the Tore Supra tokamak [1]. This antenna has permitted to inject two thirds of a power of 3 MW during ∗ Corresponding author: Tel.: +33 4 42 25 46 09; fax: +33 4 42 25 49 90. E-mail address: [email protected] (C. Portafaix).
0920-3796/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2007.05.074
6 min allowing to reach a world record energy of 1.08 GJ in the plasma. During the shots, the temperature of the antenna front part is measured with infrared cameras and on the back with thermocouples. The energy removed by the cooling water loop is also recorded. In order to understand the discrepancy between the measurements and the expected heat load, the thermal measurements have been compared with 2D and 3D finite elements calculations with the Cast 3M code taking into account the RF losses, the plasma radiated heat
C. Portafaix et al. / Fusion Engineering and Design 82 (2007) 658–661
659
flux and an additional heat flux on the antenna and guard limiter surface in front of the plasma. 2. General description This LHCD launcher has been designed in order to support: • plasma radiated flux of 0.15 MW m−2 • convected power flux of 10 MW m−2 on its guard limiter • RF losses • electromagnetic forces due to plasma disruptions. To achieve this objective, the multijunction in front of the plasma is composed of two rows of eight modules made of copper and stainless steel plates linked by explosive bonding and electrons beam stainless steel welds (Figs. 1 and 2) to withstand the electromagnetic forces. The plates are also brazed to ensure a good electrical contact. To allow for steady state operation, all the components are water cooled with 30 bars, 150 ◦ C demineralised water.
Fig. 2. Front view of a module (cross-section).
3. Thermal measurements During the shots, the temperature of the antenna front part is measured with infrared cameras and on the back with thermocouples. The energy removed by the cooling water loop is also recorded. The infrared analysis and the calorimetric balance sheet for plasma discharges either with LHCD (1–4.5 MW), or with LHCD combined with ion cyclotron resonance heating (ICRH) in the range 2–8 MW indicates that the temperature increase and the absorbed energy are higher than expected taking into account all the heat sources and especially the RF losses [2].
4. Thermal calculations
Fig. 1. Front view of the TS LHCD launcher.
The RF losses have been calculated with the HFSS finite element code [3]. For an incident power per module of 350 kW and a power reflection coefficient of 30% at the plasma–antenna interface, the RF losses are in the range of 2.5 to 5 kW m−2 . The thermal measurements have been compared to 2D and 3D finite elements calculations with the Cast 3M code [4] taking into account the RF losses, the
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C. Portafaix et al. / Fusion Engineering and Design 82 (2007) 658–661
Fig. 3. Thermocouple 5 temperature versus time (pulse 32299).
plasma radiated heat flux and some additional heat flux on the antenna and guard limiter surface in front of the plasma. 2D thermal calculations with the Cast 3M code of a section of a module allow evaluating the temperature far from the plasma where thermocouples are located. For these measurements, the plasma radiation has no influence and only the RF losses have been taken into account. The thermal calculations show that the calculated temperatures are in relatively good accordance with the measurements during the pulse 32299 (Fig. 3). For this pulse, the measured RF incident power per module is 149 kW and the power reflection coefficient is 34% at the plasma–antenna interface. 3D thermal calculations with the Cast 3M code of a module allow evaluating the temperature in front of the plasma. These thermal calculations show that the increase of the temperature due to RF losses (about 2 kW m−2 ) and plasma radiations (about 0.01 MW m−2 ) is too low to explain the temperature measured with infrared cameras in front of the plasma taking into account an copper emissivity of 0.4 (Fig. 4), and thus an additional heat flux (about 1 MW.m−2 ) has to be added on the antenna plasma facing side. This extra heat flux, localized in the bottom left corner on the antenna (Fig. 4), is characteristic of discharges with ICRH. For example, the Figs. 5–7 show the measurements and the calculations during the pulse 34181 (plasma current: 0.6 MA, toroidal field: 3.8 T, line-average density: 2.7 × 1019 m−3 , measured radiated power: 1.45 MW, measured incident RF power per module: 118 kW, power reflection coefficient: 24% at the plasma–antenna interface). The power reflection
Fig. 4. IR measurement at t = 70 s (pulse 34181).
Fig. 5. LHCD and ICRH power injected versus time (pulse 34181).
coefficient measured at the input of the module is 5.7%, but due to the multijunction, the reflection coefficient at the plasma–antenna interface is increased to 24%. The IR measurement (Fig. 6) is localized in the bottom left
Fig. 6. Maximum temperature versus time.
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5. Conclusions The main conclusions of this study are that the calculated temperatures far from the plasma are in relatively good accordance with the thermocouples measurements while in front of the plasma; an additional heat flux of about 1 MW m−2 on the antenna allows explaining the discrepancy between the IR measured and the calculated temperature. This localized heat power source is characteristic of discharges with ICRH and depends on the ICRH power and on the plasma density. This source is most likely attributed to the interaction of fast ions whose orbits are impinging on the limiters and antennas on the low field side of the tokamak, below or at the mid-plane [2].
References Fig. 7. Temperature at t = 70 s for a heat flux = 1.5 MW m−2 (pulse 34181).
corner on the antenna where the temperature is maximum. These thermal calculations allow explaining the IR measurements with a relatively good accuracy.
[1] P. Bibet et al., Proceeding of the 20th SOFT vol. 1 p. 399– 342. [2] A. Ekedahl et al., 17th Int. Conf. on Plasma Surface Interaction, 2006, Hefei, China. [3] HFSS (High Frequency Structure Simulator) by Ansoft Corp., http://www.ansoft.com. [4] Cast 3M, http://www-cast3m.cea.fr.
Thermal behavior of the LHCD launchers in Tore Supra C. Portafaix a,∗ , P. Bibet a , J.H. Belo b , A. Bou´e a , M. Chantant a , L. Delpech a , A. Ekedahl a , J.P. Gaston a , M. Goniche a a
b
Association Euratom-CEA, CEA/DSM/DRFC, CEA-Cadarache, 13108 Saint-Paul-lez-Durance, France Centro de Fus˜ao Nuclear, Associa¸ca˜ o Euratom-IST, Instituto Superior T´ecnico, 1049-001 Lisboa, Portugal
Received 13 July 2006; received in revised form 23 May 2007; accepted 24 May 2007
Abstract An actively cooled lower hybrid current drive (LHCD) launcher has been installed in 1999 in the Tore Supra tokamak. During the shots, the temperature of the antenna front part is measured with infrared cameras and on the back with thermocouples. The energy removed by the cooling water loop is also recorded. The infrared analysis and the calorimetric balance sheet for plasma indicates that the temperature increase and the absorbed energy are higher than expected. The thermal measurements have been compared to finite elements calculations with the Cast 3M code taking into account the RF losses, the plasma radiated heat flux and an additional heat flux on the antenna and guard limiter surface in front of the plasma. This additional source is most likely attributed to the interaction of fast ions. © 2007 Elsevier B.V. All rights reserved. Keywords: Lower hybrid current drive launcher; Thermal measurement; Thermal calculation
1. Introduction An actively cooled lower hybrid current drive (LHCD) launcher has been designed with the goal of injecting 4 MW during 1000 s at 3.7 GHz in the Tore Supra tokamak [1]. This antenna has permitted to inject two thirds of a power of 3 MW during ∗ Corresponding author: Tel.: +33 4 42 25 46 09; fax: +33 4 42 25 49 90. E-mail address: [email protected] (C. Portafaix).
0920-3796/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2007.05.074
6 min allowing to reach a world record energy of 1.08 GJ in the plasma. During the shots, the temperature of the antenna front part is measured with infrared cameras and on the back with thermocouples. The energy removed by the cooling water loop is also recorded. In order to understand the discrepancy between the measurements and the expected heat load, the thermal measurements have been compared with 2D and 3D finite elements calculations with the Cast 3M code taking into account the RF losses, the plasma radiated heat
C. Portafaix et al. / Fusion Engineering and Design 82 (2007) 658–661
659
flux and an additional heat flux on the antenna and guard limiter surface in front of the plasma. 2. General description This LHCD launcher has been designed in order to support: • plasma radiated flux of 0.15 MW m−2 • convected power flux of 10 MW m−2 on its guard limiter • RF losses • electromagnetic forces due to plasma disruptions. To achieve this objective, the multijunction in front of the plasma is composed of two rows of eight modules made of copper and stainless steel plates linked by explosive bonding and electrons beam stainless steel welds (Figs. 1 and 2) to withstand the electromagnetic forces. The plates are also brazed to ensure a good electrical contact. To allow for steady state operation, all the components are water cooled with 30 bars, 150 ◦ C demineralised water.
Fig. 2. Front view of a module (cross-section).
3. Thermal measurements During the shots, the temperature of the antenna front part is measured with infrared cameras and on the back with thermocouples. The energy removed by the cooling water loop is also recorded. The infrared analysis and the calorimetric balance sheet for plasma discharges either with LHCD (1–4.5 MW), or with LHCD combined with ion cyclotron resonance heating (ICRH) in the range 2–8 MW indicates that the temperature increase and the absorbed energy are higher than expected taking into account all the heat sources and especially the RF losses [2].
4. Thermal calculations
Fig. 1. Front view of the TS LHCD launcher.
The RF losses have been calculated with the HFSS finite element code [3]. For an incident power per module of 350 kW and a power reflection coefficient of 30% at the plasma–antenna interface, the RF losses are in the range of 2.5 to 5 kW m−2 . The thermal measurements have been compared to 2D and 3D finite elements calculations with the Cast 3M code [4] taking into account the RF losses, the
660
C. Portafaix et al. / Fusion Engineering and Design 82 (2007) 658–661
Fig. 3. Thermocouple 5 temperature versus time (pulse 32299).
plasma radiated heat flux and some additional heat flux on the antenna and guard limiter surface in front of the plasma. 2D thermal calculations with the Cast 3M code of a section of a module allow evaluating the temperature far from the plasma where thermocouples are located. For these measurements, the plasma radiation has no influence and only the RF losses have been taken into account. The thermal calculations show that the calculated temperatures are in relatively good accordance with the measurements during the pulse 32299 (Fig. 3). For this pulse, the measured RF incident power per module is 149 kW and the power reflection coefficient is 34% at the plasma–antenna interface. 3D thermal calculations with the Cast 3M code of a module allow evaluating the temperature in front of the plasma. These thermal calculations show that the increase of the temperature due to RF losses (about 2 kW m−2 ) and plasma radiations (about 0.01 MW m−2 ) is too low to explain the temperature measured with infrared cameras in front of the plasma taking into account an copper emissivity of 0.4 (Fig. 4), and thus an additional heat flux (about 1 MW.m−2 ) has to be added on the antenna plasma facing side. This extra heat flux, localized in the bottom left corner on the antenna (Fig. 4), is characteristic of discharges with ICRH. For example, the Figs. 5–7 show the measurements and the calculations during the pulse 34181 (plasma current: 0.6 MA, toroidal field: 3.8 T, line-average density: 2.7 × 1019 m−3 , measured radiated power: 1.45 MW, measured incident RF power per module: 118 kW, power reflection coefficient: 24% at the plasma–antenna interface). The power reflection
Fig. 4. IR measurement at t = 70 s (pulse 34181).
Fig. 5. LHCD and ICRH power injected versus time (pulse 34181).
coefficient measured at the input of the module is 5.7%, but due to the multijunction, the reflection coefficient at the plasma–antenna interface is increased to 24%. The IR measurement (Fig. 6) is localized in the bottom left
Fig. 6. Maximum temperature versus time.
C. Portafaix et al. / Fusion Engineering and Design 82 (2007) 658–661
661
5. Conclusions The main conclusions of this study are that the calculated temperatures far from the plasma are in relatively good accordance with the thermocouples measurements while in front of the plasma; an additional heat flux of about 1 MW m−2 on the antenna allows explaining the discrepancy between the IR measured and the calculated temperature. This localized heat power source is characteristic of discharges with ICRH and depends on the ICRH power and on the plasma density. This source is most likely attributed to the interaction of fast ions whose orbits are impinging on the limiters and antennas on the low field side of the tokamak, below or at the mid-plane [2].
References Fig. 7. Temperature at t = 70 s for a heat flux = 1.5 MW m−2 (pulse 34181).
corner on the antenna where the temperature is maximum. These thermal calculations allow explaining the IR measurements with a relatively good accuracy.
[1] P. Bibet et al., Proceeding of the 20th SOFT vol. 1 p. 399– 342. [2] A. Ekedahl et al., 17th Int. Conf. on Plasma Surface Interaction, 2006, Hefei, China. [3] HFSS (High Frequency Structure Simulator) by Ansoft Corp., http://www.ansoft.com. [4] Cast 3M, http://www-cast3m.cea.fr.