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Outgassing of plasma facing antenna front for lower hybrid wave launcher
Fusion Engineering and Design 49 – 50 (2000) 269 – 273 www.elsevier.com/locate/fusengdes
Outgassing of plasma facing antenna front for lower hybrid wave launcher Sunao Maebara a,*, Marc Goniche b, Fabienne Kazarian b, Masami Seki a, Yoshitaka Ikeda a, Tsuyoshi Imai a, Philippe Bibet b, Philippe Froissard b, Guy Rey b a
Naka Fusion Research Establishment, Japan Atomic Energy Research Institute, Naka-machi, Naka-gun, Ibaraki-ken, 311 -0193 Japan b De´partment de Recherches sur la Fusion Controle´e, Association EURATOM-CEA, CEA-Cadarache, F-13108 Saint Paul-lez-Durance Cedex, France
Abstract A 3.7 GHz mock-up antenna module using carbon fiber composite (CFC) was fabricated and tested for the development of a heat-resistive front of the lower hybrid current drive (LHCD) antenna. This module has four waveguides and a water cooling channel, the length is 206 mm. The CFC surface was coated with a thin titanium layer and was plated with copper in order to reduce RF losses, to bond rods and septum plates and to assemble them with cooling channel. The RF losses and the outgassing rates of this CFC module at high RF power were measured during long pulses. When the injected power varies between 30 and 100 kW, the RF losses measured by calorimetery, were found to be in the range of 1.0–1.2%. It is found that this experimental value is 2.5 – 3.0 times higher than the theoretical value of pure copper. Stationary operation of the CFC module with water cooling is performed at the RF power density of 45 MW m − 2 during 1000 s. The outgassing rates from the CFC module are in the range of 0.931.3×10 − 6 Pam − 3 s − 1 m − 2 at the module temperature of 120°C, it is low enough for an antenna material. No significant bonding defects occurred during the steady-state operation. It is assessed that a CFC module is an attractive candidate for a heat-resistive front of LHCD antenna. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Outgassing rates; CFC module; Lower hybrid current drive (LHCD) antenna
1. Introduction For the next step machine, the lower hybrid current drive (LHCD) antenna is inevitably * Corresponding author. Tel.: +81-29-2825102; fax: + 8129-2825102. E-mail address: [email protected] (S. Maebara).
placed at the same position as the first wall in order to have a good coupling of the wave. The tip of the antenna will be subjected to erosion and quite large heat fluxes. In present day experiments the antenna waveguides are metallic. The metal is copper-coated stainless steel on JET and JT-60U, an alloy of copper and zirconium on Tore Supra, Al2O3-dispersion strengthened copper (DSC) on
0920-3796/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 0 - 3 7 9 6 ( 0 0 ) 0 0 3 7 5 - 6
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TdeV. Sputtering and erosion may lead to an undesirable contamination of the plasma with metallic impurities. The mouth of the lower hybrid antenna has to be hardened with a low Z material to reduce contamination of the plasma by impurities. A low Z refractory material is required and carbon fiber composite (CFC) is a good candidate. A possible solution is to fabricate the entire multi-junction from a CFC. In this case, the inside surface of this multi-junction has to be Cu-plated in order to reduce the RF losses. Because of the difference of thermal expansion coefficients between copper and CFC, the Cu-plating on CFC material is a key of the fabrication technique, heat-resistive test is first needed to check Cu adherence and bonding quality. Second, an RF test using long pulses at high power is needed for the measurement of the RF properties, the withstand voltage and the outgassing rate. For the first point, an 8 GHz mock-up module using CFC was fabricated by JAERI [1], the heat-resistive test was done on JAERI Electron Beam Irradiation Stand (JEBIS). For irradiation heat fluxes up to 3.2 MW m − 2 for 2 min., no peeling of the Cu-plating and no bonding defects were observed. This heat load of 3.2 MW m − 2 is about 13 times higher than the steady-state average heat load on the first wall of ITER. For the second point, 3.7 GHz module which has two waveguides without cooling channel, was fabricated with the same techniques as used for the 8
Fig. 1. Schematic drawing of four-waveguides CFC module with cooling channel.
GHz mock-up module. In order to compare the RF properties, withstand voltage and outgassing rates, a reference module made of DSC was also fabricated. These two components were tested at the lower hybrid test bed facility of Cadarache in the frame work of the collaboration between JAERI and CEA [2–4]. After a high power injection up to 200 MW m − 2 and a long pulse operation up to 1000 s providing a wide thermal cycling from 100–200°C to 400–500°C, no significant peeling of the Cu layer occurred. For the baking conditions of this test (300°C/15 h), it was concluded that the outgassing is about seven times larger than that of DSC module which was built with the same geometry. As a next step, a fourwaveguides CFC module with a water cooling channel is fabricated and tested. This paper describes the RF power tests of this four-waveguides water-cooled CFC module in details.
2. The four-waveguide module and experimental set-up The septum plates of the module are made of 2 mm thick CFC plates and the top and bottom spacers are also in CFC. The CFC is a two-dimensional structure for which the fibers are woven at an intersection angle of 15° in order to increase the bending strength of the material. The carbon fiber diameter is 10 mm and the bundle number is 3000. Each plate and rod is coated with titanium by physical vapor deposition. The thickness of titanium layer is 5 8 mm. Copper was electrochemically plated on the titanium layer, the thickness is 2030 mm. Plates and rods are assembled by diffusion bonding method at the temperature of 920°C and an isostatic pressure of 20 MPa. At the top and bottom of the module, copper water cooling channels are assembled with silver brazing. All cooling pipes are connected in a series. The schematic drawing of the CFC module is shown in Fig. 1. The length is 206 mm and the inner dimension of the secondary waveguides is 7.0× 72.0 mm. A choke flange is used at each end to connect the module to the connection waveguides. The choke length is 49.95 mm (lg/2) and the nominal gap length is 3.0 mm.
S. Maebara et al. / Fusion Engineering and Design 49–50 (2000) 269–273
271
Fig. 2. Experimental set-up for the high RF power test of four-waveguides CFC module with cooling channel.
The high power test were performed at the Cadarache LH test bed facility. The experimental set-up is sketched in Fig. 2. The total length under vacuum is 1603 mm. Each window is connected to a bellow for the vacuum tightness. Inside the bellow, a standard waveguide is in contact with the window. Between this waveguide and the test module, a tapered waveguide (from 36 × 76 to 34× 72 mm) is connected. This tapered waveguide (L= 200 and 300 mm), made of coppercoated stainless steel plates, is vacuum tight. Two pumping pipes are connected in order to have an efficient pumping of the inside and outside of the module. The total effective pumping speed is estimated to be 0.0189 m3 s − 1 for a total evacuated volume V= 6.2 ×10 − 3 m3. An ion gauge allows to measure the pressure outside of the module. Two thermocouples are connected to the module. These thermocouples allow to control the temperature of the module at different locations (midplane and corner) of the same transverse section. One thermocouple is also installed on each tapered waveguide. The three connection waveguides are water-cooled in series with the transmission line. The flow rate could be varied from 0.2 to 2 l min − 1.
3. High RF power test After RF conditioning with 10 ms pulse length in every 100 ms, breakdown-free shots could be obtained up to 100 kW. RF losses of the CFC module are measured with high RF power in the range of 30–95 kW (15–47 MW m − 2). The inlet and outlet water temperature were recorded by K-type thermocouples. Low flow rates (0.25–1.4 l min − 1) were used in order to have a significant temperature increase (DT \10°C) and a good accuracy in the power calculation. Uncertainty on flow rate is estimated to be 0.05 l min − 1. Stationary temperature at the outlet is obtained after 300 s of power injection. Temperature of the module is in the range of 70–130°C. When the injected power varies between 30 and 100 kW, the RF losses measured by calorimetry varies between 0.35 and 1.1 kW, indicating the range of 1.0–1.2% in the whole range of forward RF power. Taking into account the electrical resistance of copper at 110°C (2.3× 10 − 8 V.m), the RF loss is expected to be 1.9% m − 1, and therefore 0.39% for the 206 mm long CFC module. The experimental value is therefore 2.5–3.0 times higher than the theoretical value of pure copper. This discrepancy may be
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partly due to the roughness of copper layer but diffusion of titanium, which has a rather high electrical resistance (55×10 − 8 V.m at 110°C), into the thin copper layer during diffusion bonding process may also account for the enhancement of RF losses. Using water cooling, stationary operation with RF injection time of 1000 s is performed at the high power density of 47 MW m − 2 (Fig. 3). When steady-state operation is established ( 300 s), the temperature difference between the module and the connection waveguides is quite low (30°C) and the total surface is considered as the outgassing surface (0.335 m2). The outgassing rate is obtained by closing the pump valve for 10–20 s Fig. 4. Temperature dependence of the outgassing rates for two-waveguides CFC module, four-waveguide CFC module with cooling channel and DSC module.
(the four spikes in the pressure chart). This allows to calculate the outgassing rate q by measuring the derivative of the pressure with a rather good accuracy [5] as, in particular, the effective pumping speed is not needed. The outgassing rate q= (V/S)× dP/dt is in the range of 0.93 1.3× 10 − 6 Pam3 sm − 2 at the module temperature of 110°C as seen from Fig. 4. The rates are about five times higher than that of DSC module. With the use of a mass spectrometer, partial pressure measurements during the baking of small samples showed that the main desorbed molecules from CFC are CO, CO2 and hydrocarbons.
4. Conclusion
Fig. 3. Time trajectories of Klystron output power, inlet/outlet water temperature of cooling channel, CFC module temperature and pressure in vacuum tank.
Steady-state operation of a water-cooled CFC antenna module is performed with no significant bonding defects of the CFC plates to the spacers after 1000 s shots at the RF power density of 45 MW m − 2. During the steady-state operation, the outgassing rate of CFC module at 110°C (0.93 1.3× 10 − 6 Pam3 sm − 2) is about five times higher than that of the DSC module, but still low enough to be considered for a LHCD antenna. As far as the copper plating is concerned, the previ-
S. Maebara et al. / Fusion Engineering and Design 49–50 (2000) 269–273
ous uncooled module has shown good adherence of the copper layer up to the high RF power density of 150 MW m − 2. However, for this module, for which the same deposition technique has been used, inspection after test revealed significant peeling of the copper. The peeling area is about 50 cm2, it is about 3% of cooper plating area. The observed limit of RF power transmission (47 MW m − 2) is related to this poor adherence. An alternative copper coating process (plasma spray technique) is currently under development to improve the adherence.
Acknowledgements The authors would like to express their thanks to Mr Yasuo Suzuki, Toshihisa Okumura and Fusao Saito of Toshiba Corporation for their with respect of the design and fabrication of the module. We also would like to express our thanks to all members of the RF facility division and RF heating laboratory at JAERI, and the Lower Hy-
273
brid group and secretaries at CEA-Cadarache for their continuous support. They would also like to acknowledge Drs M. Chatelier, T. Nagashima, S. Matsuda, M. Ohta, G. Tonon, for their encouragement of our collaborative activities.
References [1] S. Maebara, M. Seki, Y. Ikeda, et al., Development of plasma facing component for LHCD antenna, Fusion Eng. Design 39 – 40 (1998) 355 – 361. [2] S. Maebara, K. Kiyono, M. Seki et al., High RF Power Test of a Lower Hybrid Module Mock-up in Carbon Fiber Composite, JAERI Report, JAERI-Research 97-086. [3] M. Goniche et al., High RF Power Test of a Lower Hybrid Module Mock-up in Carbon Fiber Composite, CEA-Report, EUR-CEA-FC-1656. [4] S. Maebara, M. Seki, Y. Ikeda et al., High RF Power Test of a CFC Antenna Module for Lower Hybrid Current Drive, Proc. of 21st Symp. of Fusion Technology in 1998, Marseille, France, pp. 453 – 456. [5] M. Goniche, M. Seki et al., Very long pulses high RF power test of an LH antenna module, CEA Report, EURCEA-FC-1511.
Outgassing of plasma facing antenna front for lower hybrid wave launcher Sunao Maebara a,*, Marc Goniche b, Fabienne Kazarian b, Masami Seki a, Yoshitaka Ikeda a, Tsuyoshi Imai a, Philippe Bibet b, Philippe Froissard b, Guy Rey b a
Naka Fusion Research Establishment, Japan Atomic Energy Research Institute, Naka-machi, Naka-gun, Ibaraki-ken, 311 -0193 Japan b De´partment de Recherches sur la Fusion Controle´e, Association EURATOM-CEA, CEA-Cadarache, F-13108 Saint Paul-lez-Durance Cedex, France
Abstract A 3.7 GHz mock-up antenna module using carbon fiber composite (CFC) was fabricated and tested for the development of a heat-resistive front of the lower hybrid current drive (LHCD) antenna. This module has four waveguides and a water cooling channel, the length is 206 mm. The CFC surface was coated with a thin titanium layer and was plated with copper in order to reduce RF losses, to bond rods and septum plates and to assemble them with cooling channel. The RF losses and the outgassing rates of this CFC module at high RF power were measured during long pulses. When the injected power varies between 30 and 100 kW, the RF losses measured by calorimetery, were found to be in the range of 1.0–1.2%. It is found that this experimental value is 2.5 – 3.0 times higher than the theoretical value of pure copper. Stationary operation of the CFC module with water cooling is performed at the RF power density of 45 MW m − 2 during 1000 s. The outgassing rates from the CFC module are in the range of 0.931.3×10 − 6 Pam − 3 s − 1 m − 2 at the module temperature of 120°C, it is low enough for an antenna material. No significant bonding defects occurred during the steady-state operation. It is assessed that a CFC module is an attractive candidate for a heat-resistive front of LHCD antenna. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Outgassing rates; CFC module; Lower hybrid current drive (LHCD) antenna
1. Introduction For the next step machine, the lower hybrid current drive (LHCD) antenna is inevitably * Corresponding author. Tel.: +81-29-2825102; fax: + 8129-2825102. E-mail address: [email protected] (S. Maebara).
placed at the same position as the first wall in order to have a good coupling of the wave. The tip of the antenna will be subjected to erosion and quite large heat fluxes. In present day experiments the antenna waveguides are metallic. The metal is copper-coated stainless steel on JET and JT-60U, an alloy of copper and zirconium on Tore Supra, Al2O3-dispersion strengthened copper (DSC) on
0920-3796/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 0 - 3 7 9 6 ( 0 0 ) 0 0 3 7 5 - 6
270
S. Maebara et al. / Fusion Engineering and Design 49–50 (2000) 269–273
TdeV. Sputtering and erosion may lead to an undesirable contamination of the plasma with metallic impurities. The mouth of the lower hybrid antenna has to be hardened with a low Z material to reduce contamination of the plasma by impurities. A low Z refractory material is required and carbon fiber composite (CFC) is a good candidate. A possible solution is to fabricate the entire multi-junction from a CFC. In this case, the inside surface of this multi-junction has to be Cu-plated in order to reduce the RF losses. Because of the difference of thermal expansion coefficients between copper and CFC, the Cu-plating on CFC material is a key of the fabrication technique, heat-resistive test is first needed to check Cu adherence and bonding quality. Second, an RF test using long pulses at high power is needed for the measurement of the RF properties, the withstand voltage and the outgassing rate. For the first point, an 8 GHz mock-up module using CFC was fabricated by JAERI [1], the heat-resistive test was done on JAERI Electron Beam Irradiation Stand (JEBIS). For irradiation heat fluxes up to 3.2 MW m − 2 for 2 min., no peeling of the Cu-plating and no bonding defects were observed. This heat load of 3.2 MW m − 2 is about 13 times higher than the steady-state average heat load on the first wall of ITER. For the second point, 3.7 GHz module which has two waveguides without cooling channel, was fabricated with the same techniques as used for the 8
Fig. 1. Schematic drawing of four-waveguides CFC module with cooling channel.
GHz mock-up module. In order to compare the RF properties, withstand voltage and outgassing rates, a reference module made of DSC was also fabricated. These two components were tested at the lower hybrid test bed facility of Cadarache in the frame work of the collaboration between JAERI and CEA [2–4]. After a high power injection up to 200 MW m − 2 and a long pulse operation up to 1000 s providing a wide thermal cycling from 100–200°C to 400–500°C, no significant peeling of the Cu layer occurred. For the baking conditions of this test (300°C/15 h), it was concluded that the outgassing is about seven times larger than that of DSC module which was built with the same geometry. As a next step, a fourwaveguides CFC module with a water cooling channel is fabricated and tested. This paper describes the RF power tests of this four-waveguides water-cooled CFC module in details.
2. The four-waveguide module and experimental set-up The septum plates of the module are made of 2 mm thick CFC plates and the top and bottom spacers are also in CFC. The CFC is a two-dimensional structure for which the fibers are woven at an intersection angle of 15° in order to increase the bending strength of the material. The carbon fiber diameter is 10 mm and the bundle number is 3000. Each plate and rod is coated with titanium by physical vapor deposition. The thickness of titanium layer is 5 8 mm. Copper was electrochemically plated on the titanium layer, the thickness is 2030 mm. Plates and rods are assembled by diffusion bonding method at the temperature of 920°C and an isostatic pressure of 20 MPa. At the top and bottom of the module, copper water cooling channels are assembled with silver brazing. All cooling pipes are connected in a series. The schematic drawing of the CFC module is shown in Fig. 1. The length is 206 mm and the inner dimension of the secondary waveguides is 7.0× 72.0 mm. A choke flange is used at each end to connect the module to the connection waveguides. The choke length is 49.95 mm (lg/2) and the nominal gap length is 3.0 mm.
S. Maebara et al. / Fusion Engineering and Design 49–50 (2000) 269–273
271
Fig. 2. Experimental set-up for the high RF power test of four-waveguides CFC module with cooling channel.
The high power test were performed at the Cadarache LH test bed facility. The experimental set-up is sketched in Fig. 2. The total length under vacuum is 1603 mm. Each window is connected to a bellow for the vacuum tightness. Inside the bellow, a standard waveguide is in contact with the window. Between this waveguide and the test module, a tapered waveguide (from 36 × 76 to 34× 72 mm) is connected. This tapered waveguide (L= 200 and 300 mm), made of coppercoated stainless steel plates, is vacuum tight. Two pumping pipes are connected in order to have an efficient pumping of the inside and outside of the module. The total effective pumping speed is estimated to be 0.0189 m3 s − 1 for a total evacuated volume V= 6.2 ×10 − 3 m3. An ion gauge allows to measure the pressure outside of the module. Two thermocouples are connected to the module. These thermocouples allow to control the temperature of the module at different locations (midplane and corner) of the same transverse section. One thermocouple is also installed on each tapered waveguide. The three connection waveguides are water-cooled in series with the transmission line. The flow rate could be varied from 0.2 to 2 l min − 1.
3. High RF power test After RF conditioning with 10 ms pulse length in every 100 ms, breakdown-free shots could be obtained up to 100 kW. RF losses of the CFC module are measured with high RF power in the range of 30–95 kW (15–47 MW m − 2). The inlet and outlet water temperature were recorded by K-type thermocouples. Low flow rates (0.25–1.4 l min − 1) were used in order to have a significant temperature increase (DT \10°C) and a good accuracy in the power calculation. Uncertainty on flow rate is estimated to be 0.05 l min − 1. Stationary temperature at the outlet is obtained after 300 s of power injection. Temperature of the module is in the range of 70–130°C. When the injected power varies between 30 and 100 kW, the RF losses measured by calorimetry varies between 0.35 and 1.1 kW, indicating the range of 1.0–1.2% in the whole range of forward RF power. Taking into account the electrical resistance of copper at 110°C (2.3× 10 − 8 V.m), the RF loss is expected to be 1.9% m − 1, and therefore 0.39% for the 206 mm long CFC module. The experimental value is therefore 2.5–3.0 times higher than the theoretical value of pure copper. This discrepancy may be
272
S. Maebara et al. / Fusion Engineering and Design 49–50 (2000) 269–273
partly due to the roughness of copper layer but diffusion of titanium, which has a rather high electrical resistance (55×10 − 8 V.m at 110°C), into the thin copper layer during diffusion bonding process may also account for the enhancement of RF losses. Using water cooling, stationary operation with RF injection time of 1000 s is performed at the high power density of 47 MW m − 2 (Fig. 3). When steady-state operation is established ( 300 s), the temperature difference between the module and the connection waveguides is quite low (30°C) and the total surface is considered as the outgassing surface (0.335 m2). The outgassing rate is obtained by closing the pump valve for 10–20 s Fig. 4. Temperature dependence of the outgassing rates for two-waveguides CFC module, four-waveguide CFC module with cooling channel and DSC module.
(the four spikes in the pressure chart). This allows to calculate the outgassing rate q by measuring the derivative of the pressure with a rather good accuracy [5] as, in particular, the effective pumping speed is not needed. The outgassing rate q= (V/S)× dP/dt is in the range of 0.93 1.3× 10 − 6 Pam3 sm − 2 at the module temperature of 110°C as seen from Fig. 4. The rates are about five times higher than that of DSC module. With the use of a mass spectrometer, partial pressure measurements during the baking of small samples showed that the main desorbed molecules from CFC are CO, CO2 and hydrocarbons.
4. Conclusion
Fig. 3. Time trajectories of Klystron output power, inlet/outlet water temperature of cooling channel, CFC module temperature and pressure in vacuum tank.
Steady-state operation of a water-cooled CFC antenna module is performed with no significant bonding defects of the CFC plates to the spacers after 1000 s shots at the RF power density of 45 MW m − 2. During the steady-state operation, the outgassing rate of CFC module at 110°C (0.93 1.3× 10 − 6 Pam3 sm − 2) is about five times higher than that of the DSC module, but still low enough to be considered for a LHCD antenna. As far as the copper plating is concerned, the previ-
S. Maebara et al. / Fusion Engineering and Design 49–50 (2000) 269–273
ous uncooled module has shown good adherence of the copper layer up to the high RF power density of 150 MW m − 2. However, for this module, for which the same deposition technique has been used, inspection after test revealed significant peeling of the copper. The peeling area is about 50 cm2, it is about 3% of cooper plating area. The observed limit of RF power transmission (47 MW m − 2) is related to this poor adherence. An alternative copper coating process (plasma spray technique) is currently under development to improve the adherence.
Acknowledgements The authors would like to express their thanks to Mr Yasuo Suzuki, Toshihisa Okumura and Fusao Saito of Toshiba Corporation for their with respect of the design and fabrication of the module. We also would like to express our thanks to all members of the RF facility division and RF heating laboratory at JAERI, and the Lower Hy-
273
brid group and secretaries at CEA-Cadarache for their continuous support. They would also like to acknowledge Drs M. Chatelier, T. Nagashima, S. Matsuda, M. Ohta, G. Tonon, for their encouragement of our collaborative activities.
References [1] S. Maebara, M. Seki, Y. Ikeda, et al., Development of plasma facing component for LHCD antenna, Fusion Eng. Design 39 – 40 (1998) 355 – 361. [2] S. Maebara, K. Kiyono, M. Seki et al., High RF Power Test of a Lower Hybrid Module Mock-up in Carbon Fiber Composite, JAERI Report, JAERI-Research 97-086. [3] M. Goniche et al., High RF Power Test of a Lower Hybrid Module Mock-up in Carbon Fiber Composite, CEA-Report, EUR-CEA-FC-1656. [4] S. Maebara, M. Seki, Y. Ikeda et al., High RF Power Test of a CFC Antenna Module for Lower Hybrid Current Drive, Proc. of 21st Symp. of Fusion Technology in 1998, Marseille, France, pp. 453 – 456. [5] M. Goniche, M. Seki et al., Very long pulses high RF power test of an LH antenna module, CEA Report, EURCEA-FC-1511.