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Progress of ITER equatorial electron cyclotron launcher design for physics optimization and toward final design
Fusion Engineering and Design 86 (2011) 982–986
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Progress of ITER equatorial electron cyclotron launcher design for physics optimization and toward final design K. Takahashi a,∗ , K. Kajiwara a , Y. Okazaki a , Y. Oda a , K. Sakamoto a , T. Omori b , M. Henderson b a b
Japan Atomic Energy Agency, Naka, Ibaraki 311-0193, Japan ITER Organization, CS90 046, 13067 St. Paul lez Durance Cedex, France
a r t i c l e
i n f o
Article history: Available online 4 March 2011 Keywords: Equatorial EC launcher Quasi-optical Counter-ECCD Poloidal tilt Nuclear shielding capability
a b s t r a c t The design of the equatorial electron cyclotron (EC) launcher has advanced toward a more reliable and manufacturable final design. The modification to quasi-optical layout of the millimeter wave transmission line leads to the launcher design being reliable and more realistic toward the final design and the cost reduction. It is proposed that one of three beam rows of the launcher is flipped so that the EC power enables to contribute the counter-EC current drive based on the ITER physics requirement. In addition, a poloidal beam tilt angle of 5◦ has been introduced in the top and bottom beam row so that all beams can access from on axis to near mid-radius. These design modifications render the EC system more flexible to adapt the needs of advanced ITER physics experiments. It is preliminarily estimated that the nuclear shielding capability of the present modified design satisfies the neutron flux criterion, but the shut down dose rate of ␥-ray is slightly higher than the criterion. © 2011 Elsevier B.V. All rights reserved.
1. Introduction In the ITER, an electron cyclotron heating and current drive (EC H&CD) is an effective tool to attain steady state and high performance plasma operation and suppression of MHD instabilities such as neoclassical tearing modes (NTMs) and sawteeth [1–3]. In order to attain these physics tasks, the ITER EC H&CD system consisted of twenty-four 170 GHz, 1 MW gyrotrons [4–6], the power supply system to operate the gyrotrons [7], twenty-four transmission lines with the length of 100–150 m [8,9] and one equatorial port and four upper port launchers [10–12] is required. These system components are provided by five domestic agencies (DAs), Japan, Europe, Russia, the US and India and installed in collaboration with the ITER Organization. It should be noted that the EU DA has had the program to develop a 2 MW gyrotron and four 2 MW gyrotron will be delivered, instead of eight 1 MW gyrotron if the program is successfully done. The ITER equatorial EC launcher is required to inject 170 GHz, 20 MW millimeter (mm) wave beams into the plasma. The equatorial launcher (EL) should also be capable of the toroidal steering of the incident beam (20◦ ≤ T ≤ 40◦ ). The top view of the EL is shown in Fig. 1. The twenty-four waveguide transmission lines arrive from the right (as shown in Fig. 1) in six rows of four waveguide lines. Two miter bends are used to regroup the lines in three bundles
∗ Corresponding author. Tel.: +81 29 270 7562; fax: +81 29 270 7569. E-mail address: [email protected] (K. Takahashi). 0920-3796/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2011.02.032
of 8 lines. The beams are projected from the waveguide bundles and propagate in frees space via two mirrors: a fixed-focusing and a rotatable-flat mirror forming the quasi-optical arrangement. The rotatable mirros provide the toroidal steering to control the power deposition across the plasma core. The introduction of the quasi optical (QO) section is a modification from the reference design, which had an additional miter bend in-vessel along with the rotatable mirror [10]. The QO modification offers a potential cost reduction and design simplification. The QO propagation results in a large beam on the mirror, reducing the thermal loading and the structural design [11]. The above access range is compatible with the physics objectives of the EL, which includes on/off axis co-current drive. The coupling of heating and current drive might lead to the excess peaking of the current profile, which is un-favorable in hybrid and advanced physics scenarios, where the central current profile should be flat or even hollow. Dedicated physics analysis [13] has demonstrated that balancing co- and counter-current drive will provide pure central heating. Counter-ECCD is achieved by flipping on the beam assembly so that the toroidal injection direction of the beam row becomes opposite. In addition, it is also proposed that the top and the bottom beam row are favorably tilted by 5◦ down and up, respectively. Then, these beams are possibly deposited closer to the plasma core region. The objective of this paper is to summarize the engineering analysis associated with the above mentioned modifications, which aim at improved functionality as well as increased the reliability ad the manufacturability and the physics optimization is presented.
K. Takahashi et al. / Fusion Engineering and Design 86 (2011) 982–986
Fig. 1. Top view of the ITER equatorial EC launcher. Quasi-optical layout design.
In Section 2, the mm-wave design modifications of the EL based on the QO arrangement and the requirement related to the physics optimization are described. The preliminary result of the nuclear analysis of the EL and the related design issues are presented in Section 3, followed by a conclusion in Section 4. 2. Millimeter wave design modification The proposal to introduce a QO section in the EL was motivated to simplify the optical system of the in-vessel components. This proposal has been realized by replacing a set of eight miter bends by a single fixed mirror. Note that the miter bends are more costly, require complicated cooling circuits and generates higher order modes that contribute to stray radiation in the EL. The mirror shape and waveguide configuration are optimized to attain as high transmission efficiency of the QO region as possible using the optical design code called ZEMAX [14]. The optimum configuration maintained a transmission efficiency of mm wave propagation of 99.5% and maintained the BSM opening size. In this arrangement, the surface shape of the first mirror is curved and that of the steering mirror is kept flat. The optimum focusing mirror curvature is 2.78 m in the vertical and 10 m in the horizontal. The full scale EL mockup shown in Fig. 2 has been fabricated, based on the design. The mirrors are shown in forefront and the ex-vessel miter bends are behind the closure plate. At this moment, it is difficult to remove them remotely and the design issue has to be resolved in future. Fig. 3 shows a comparison between the calculated and low power experimental results of the beams field pattern of the beam at the mirrors and the location of 2 m after the outlet of the BSM opening and the comparison between the experiment and the calculation. The lack of variation between the calculated and measured profiles demonstrates that the beams propagate through the EL body with-
983
out perturbation from the launcher structure, thus demonstrating the reliability of the DO beam propagation design (up to low power transmission). It has been requested by the ITER Organization change in the EL design requirement through a Project Change Request (PCR) in 2009 that some power of mm wave beams from the EL is injected to the opposite direction. This wave power is expected to contribute the counter-ECCD for the desired current profile control and the pure central heating of plasma. The wide steering capability providing both the co- and counter direction of mm wave injection is hardly possible because large stress is yielded on the flexible cooling tube of the movable mirror. The design modification, which the central beam row is flipped, is therefore proposed as shown in Fig. 4. This modification has no significant design issue since the layout of the mirrors, the waveguide components and some blanket shield modules (BSMs) are just flipped. There are the minor design changes of the internal shield, the closure plate, the support flame for BSMs, but no impact on the engineering design and the cost increase of the fabrication. Moreover, no impact on the mm wave propagation is expected. The above PCR also requested a tilting of the poloidal injection angle on both top and the bottom beam rows. As a countermeasure for this design change request, the position of the top and bottom fixed focusing mirrors are shifted upward or downward by 50 mm, respectively, and then the mirrors are titled (4.6◦ ) so that beams are reflected toward the plasma core region. As the movable mirror is rotated from 20◦ to 40◦ in the toroidal direction, the poloidal angle of beams is also changed from 5◦ to 7◦ . This variation is due to the rotation axis of the mirror not being in a horizontal plane. The variation in poloidal tilt angle results in degradation of transmission efficiency of up to 7% keeping the same BSM opening as shown in Fig. 5. This indicates that some beams hit the inner wall of the opening. In reality, most of the power incident on the side wall will be reflected toward plasma and however, this beam power would not contribute to localized plasma heating or current drive as expected. In order to prevent or minimize the degradation of the transmission efficiency, the opening size can either be totally enlarged or the some region of BSM is cut out. The “cut-out” case in Fig. 5 assumes a wedge shape cut out with 0 cm removed on the inner side and 5 cm removed from the plasma side of the BSM. Applying this cut-out to both the top and the bottom openings recover to 98.7%. This modification does not affect the lifetime of the launcher and however, the impurity contamination on the inner surface of the BSM opening or the mirror surface may be increased. This contamination might cause the incremental surface heat load on them. The heat load on the steering mirror is one of the key issues on the EL design. The peak heat load at 1 MW transmission is 2.68 MW/m2 in the design modification including the poloidal tilt.
Fig. 2. Equatorial EC launcher mock-up.
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K. Takahashi et al. / Fusion Engineering and Design 86 (2011) 982–986
Fig. 3. Results of the low power experiment: 2D field pattern (top) and the comparison between the experiment and the calculation (bottom) in y-direction.
The heat load on the mirror surface depends on the electrical resistivity and the roughness of the surface. Thermal analysis shows that maximum temperature and stress in the mirror are 231 ◦ C at the mirror surface and 148 MPa at the inner surface of the cooling tube, respectively. The initial temperature is 100 ◦ C. The temperature and stresses are compatible with the envisioned manufacturing technique using CuCrZr alloy (260 MPa maximum allowable stress) for the mirror and the cooling tube. In the case of 2 MW transmission per waveguide line, the incident power on the mirror will increase to 1.8 MW. In this case, the maximum temperature is 415 ◦ C at the mirror surface and the maximum stress is 260 MPa at the inner surface of the cooling tube, which is the operating limit of the CuCrZr alloy. The above calculations assumed a 1.7 surface roughness and a thin Be coating on the plasma facing mirrors.
Fig. 4. Illustration of modified launcher design. The counter-ECCD is achievable with this design modification.
3. Design performance of nuclear shielding The EL must have enough nuclear shielding capability to satisfy the following requirements: 1 Neutron fluence at toroidal field coil ≤5 × 1021 n/m2 (En > 0.1 MeV). 2 Neutron fluence at the closure plate ≤1020 n/m2 (En > 0.1 MeV). 3 Dose rate at 10 days after shut down at the closure plate ≤100 Sv/h. It is confirmed that the reference design of the EL has the shielding capability that satisfies the above condition [10]. In the design, the shield around the waveguides placed in the port plug is sufficiently installed and it is the robust shielding structure. On the other hand, the beam duct, where the mirrors are located, is the vacant
Fig. 5. Transmission efficiency of the beam propagation at each location. WG: waveguide outlet, M1: focusing mirror, M2: steering mirror, Fi: BSM inlet, Fo: BSM outlet and P: plasma. The circles correspond to the case of no cut-out of the BSM opening and the diamonds to the case of a wedge shaped cut-out.
K. Takahashi et al. / Fusion Engineering and Design 86 (2011) 982–986
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Fig. 6. MCNP modeling of equatorial EC launcher for nuclear analysis: A-lite 41 model is applied for the ITER vacuum vessel.
region in the QO arrangement of the mm wave transmission in the modified EL design. High energy neutrons can easily pass through these space without reducing the flux and it can be presumed that the shielding capability may degrade, compared to the reference design. A Monte Carlo N-Particle (MCNP) model has been developed for the modified EL design with the counter ECCD and beam tilting as shown in Fig. 6. The vacuum vessel model, called A-lite 41 is used. The content ratio of stainless steel (SS316) and water (H2 O) in the BSM is 80/20 and that in port plug is 50/50. The model includes some shield blocks placed the rear of the gap between the port extension and the port plug. The total fusion power assumed is 500 MW and an average neutron fluence at the first wall of 0.3 MW/a/m2 integrated over an operating period of 4600 h. The MCNP model has an open space from the waveguide aperture to the fixed steering and then onward to the steering mirror, while a dense shielding structure is inserted around the waveguide sections that are inside the closure plate. The nuclear analysis report [15] indicates that the effect of the neutron streaming passed through the narrow gap of 20 mm between the port plug and the port is not negligible and it is recommended that some shielding material are stuffed in the gap. Hence, stainless steel block is stuffed at the rear region (56 cm from the back end) of the gap in the analysis model. The preliminary result of the analysis is summarized as follows:
i Fast neutron fluence at coil case around the port: 1.4 × 1020 n/m2 (En > 0.1 MeV). ii Fast neutron fluence at the closure plate: 8.0 × 1018 n/m2 (En > 0.1 MeV). iii Dose rate at 10 days after shut down: 120 Sv/h (at the closure plate).
4. Conclusion The EL mm-wave design has been simplified from the waveguide structure to the QO arrangement so as to increase the reliability and the manufacturability. The low power experiment of the EL mock-up based on the design modification confirmed that the beam integrity is maintained while propagating through the EL structure. As requested by the ITER Organization, the EL design has been modified to include counter-ECCD (by flipping one row) and the steering mirrors have been tilted by 5◦ for improved central access. A modification to the BSM is required in order to avoid degradation of the mm-wave transmission efficiency. This modification might increase the impurity contamination on the inner surface of the BSM opening or the mirror surface, which could cause the incremental surface heat load on them. This concern will have to be investigated. The nuclear analysis for the QO layout design and the preliminary result shows that it satisfies the nuclear shielding criteria except the shut down dose rate. The additional shield is necessary to reduce the dose rate less than the criteria.
Acknowledgments The authors thank the technical staffs, Yu. Ikeda, N. Koabayshi and Y. Okazaki for their support of the experiments, and assistance of design and analysis activities. We would also like to acknowledge Dr. T. Nishitani, Dr. M. Akiba and Dr. H. Takatsu for their encouragement of this work. The nuclear analysis in these design activities was carried out using an adaptation of the A-lite MCNP model which was developed as a collaborative effort between the FDS team of ASIPP China, ENEA Frascati, JAEA Naka, UKAEA and the ITER Organization.
References Here, fluence is the total amount of neutron at the integrated full power operation time. The result satisfies the shielding criteria with the exception of the residual dose rate at 10 days after shut down. As a countermeasure, a SS316 bulk shield of 30 cm in thickness can be installed behind the miter bends [16] for increased shielding potential. The Ref. [16] also shows that the approximately 10 cm of the shield block reduces the neutron flux by the factor of 10 when the SS316 to water ratio is 70/30.
[1] O. Sauter, et al., Steady-state fully noninductive current driven by electron cyclotron waves in a magnetically confined plasma, Phys. Rev. Lett. 84 (15) (2000) 3322. [2] H. Zohm, et al., First experiments on neoclassical tearing mode stability by ECCD in ASDEX Upgrade, Nucl. Fusion 39 (1988) 11. [3] A. Isayama, et al., Long sustainment of quasi-steady-state high bp H mode discharges in JT-60U, Nucl. Fusion 41 (6) (2001) 761. [4] K. Sakamoto, et al., Achievement of robust high-efficiency 1MW oscillation in the hard-self-excitation region by a 170 GHz continuous-wave gyrotron, Nat. Phys. 3 (2007) 411.
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[5] A.G. Litvak, et al. Development of Megawatt Gyrotrons for Nuclear Fusion in Russia, Conf. Digest of IRMMW, 2006, p. 8. [6] B. Piosczyk, et al., Towards a 2 MW, CW 170 GHz coaxial cavity gyrotron for ITER, Fusion Eng. Design 66–68 (2003) 481. [7] T. Bonicelli, et al., The high voltage power supply system for the European 2 MW electron cyclotron test facility, in: Proc. 13th European Conference on Power Electronics and Applications, 1–10, 2009. [8] K. Takahashi, et al., Investigation of transmission characteristic in corrugated waveguide transmission lines for fusion application, in: Proc. 8th International Vacuum Electronics Conference, Kitakyushu, Japan, 2007. [9] R.W. Callis, et al., Design and testing of ITER ECH & CD transmission line components, Fusion Eng. Design 84 (2009) 526. [10] K. Takahashi, et al., Design performance of front steering-type electron cyclotron launcher for ITER, Fusion Sci. Technol. 47 (2005) 1.
[11] K. Takahashi, et al., Improved design of an ITER equatorial EC launcher, Nucl. Fusion 48 (2008), 054014. [12] M.A. Henderson, et al., Overview of the ITER EC upper launcher, Nucl. Fusion 48 (2008), 054013. [13] G. Ramponi, et al., Physics analysis of the ITER ECW system for optimized performance, Nucl. Fusion 48 (2008), 054012. [14] K. Kajiwara, et al., Design of a high power millimeter wave launcher for EC H&CD system on ITER, Fusion Eng. Design 84 (2009) 72. [15] ITER Nuclear analysis report, July 2004. [16] T. Hayashi, et al., Neutronics assessment of advanced shield materials using metal hydride and borohydride for fusion reactors, Fusion Eng. Des. 81 (2006) 1285.
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Progress of ITER equatorial electron cyclotron launcher design for physics optimization and toward final design K. Takahashi a,∗ , K. Kajiwara a , Y. Okazaki a , Y. Oda a , K. Sakamoto a , T. Omori b , M. Henderson b a b
Japan Atomic Energy Agency, Naka, Ibaraki 311-0193, Japan ITER Organization, CS90 046, 13067 St. Paul lez Durance Cedex, France
a r t i c l e
i n f o
Article history: Available online 4 March 2011 Keywords: Equatorial EC launcher Quasi-optical Counter-ECCD Poloidal tilt Nuclear shielding capability
a b s t r a c t The design of the equatorial electron cyclotron (EC) launcher has advanced toward a more reliable and manufacturable final design. The modification to quasi-optical layout of the millimeter wave transmission line leads to the launcher design being reliable and more realistic toward the final design and the cost reduction. It is proposed that one of three beam rows of the launcher is flipped so that the EC power enables to contribute the counter-EC current drive based on the ITER physics requirement. In addition, a poloidal beam tilt angle of 5◦ has been introduced in the top and bottom beam row so that all beams can access from on axis to near mid-radius. These design modifications render the EC system more flexible to adapt the needs of advanced ITER physics experiments. It is preliminarily estimated that the nuclear shielding capability of the present modified design satisfies the neutron flux criterion, but the shut down dose rate of ␥-ray is slightly higher than the criterion. © 2011 Elsevier B.V. All rights reserved.
1. Introduction In the ITER, an electron cyclotron heating and current drive (EC H&CD) is an effective tool to attain steady state and high performance plasma operation and suppression of MHD instabilities such as neoclassical tearing modes (NTMs) and sawteeth [1–3]. In order to attain these physics tasks, the ITER EC H&CD system consisted of twenty-four 170 GHz, 1 MW gyrotrons [4–6], the power supply system to operate the gyrotrons [7], twenty-four transmission lines with the length of 100–150 m [8,9] and one equatorial port and four upper port launchers [10–12] is required. These system components are provided by five domestic agencies (DAs), Japan, Europe, Russia, the US and India and installed in collaboration with the ITER Organization. It should be noted that the EU DA has had the program to develop a 2 MW gyrotron and four 2 MW gyrotron will be delivered, instead of eight 1 MW gyrotron if the program is successfully done. The ITER equatorial EC launcher is required to inject 170 GHz, 20 MW millimeter (mm) wave beams into the plasma. The equatorial launcher (EL) should also be capable of the toroidal steering of the incident beam (20◦ ≤ T ≤ 40◦ ). The top view of the EL is shown in Fig. 1. The twenty-four waveguide transmission lines arrive from the right (as shown in Fig. 1) in six rows of four waveguide lines. Two miter bends are used to regroup the lines in three bundles
∗ Corresponding author. Tel.: +81 29 270 7562; fax: +81 29 270 7569. E-mail address: [email protected] (K. Takahashi). 0920-3796/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2011.02.032
of 8 lines. The beams are projected from the waveguide bundles and propagate in frees space via two mirrors: a fixed-focusing and a rotatable-flat mirror forming the quasi-optical arrangement. The rotatable mirros provide the toroidal steering to control the power deposition across the plasma core. The introduction of the quasi optical (QO) section is a modification from the reference design, which had an additional miter bend in-vessel along with the rotatable mirror [10]. The QO modification offers a potential cost reduction and design simplification. The QO propagation results in a large beam on the mirror, reducing the thermal loading and the structural design [11]. The above access range is compatible with the physics objectives of the EL, which includes on/off axis co-current drive. The coupling of heating and current drive might lead to the excess peaking of the current profile, which is un-favorable in hybrid and advanced physics scenarios, where the central current profile should be flat or even hollow. Dedicated physics analysis [13] has demonstrated that balancing co- and counter-current drive will provide pure central heating. Counter-ECCD is achieved by flipping on the beam assembly so that the toroidal injection direction of the beam row becomes opposite. In addition, it is also proposed that the top and the bottom beam row are favorably tilted by 5◦ down and up, respectively. Then, these beams are possibly deposited closer to the plasma core region. The objective of this paper is to summarize the engineering analysis associated with the above mentioned modifications, which aim at improved functionality as well as increased the reliability ad the manufacturability and the physics optimization is presented.
K. Takahashi et al. / Fusion Engineering and Design 86 (2011) 982–986
Fig. 1. Top view of the ITER equatorial EC launcher. Quasi-optical layout design.
In Section 2, the mm-wave design modifications of the EL based on the QO arrangement and the requirement related to the physics optimization are described. The preliminary result of the nuclear analysis of the EL and the related design issues are presented in Section 3, followed by a conclusion in Section 4. 2. Millimeter wave design modification The proposal to introduce a QO section in the EL was motivated to simplify the optical system of the in-vessel components. This proposal has been realized by replacing a set of eight miter bends by a single fixed mirror. Note that the miter bends are more costly, require complicated cooling circuits and generates higher order modes that contribute to stray radiation in the EL. The mirror shape and waveguide configuration are optimized to attain as high transmission efficiency of the QO region as possible using the optical design code called ZEMAX [14]. The optimum configuration maintained a transmission efficiency of mm wave propagation of 99.5% and maintained the BSM opening size. In this arrangement, the surface shape of the first mirror is curved and that of the steering mirror is kept flat. The optimum focusing mirror curvature is 2.78 m in the vertical and 10 m in the horizontal. The full scale EL mockup shown in Fig. 2 has been fabricated, based on the design. The mirrors are shown in forefront and the ex-vessel miter bends are behind the closure plate. At this moment, it is difficult to remove them remotely and the design issue has to be resolved in future. Fig. 3 shows a comparison between the calculated and low power experimental results of the beams field pattern of the beam at the mirrors and the location of 2 m after the outlet of the BSM opening and the comparison between the experiment and the calculation. The lack of variation between the calculated and measured profiles demonstrates that the beams propagate through the EL body with-
983
out perturbation from the launcher structure, thus demonstrating the reliability of the DO beam propagation design (up to low power transmission). It has been requested by the ITER Organization change in the EL design requirement through a Project Change Request (PCR) in 2009 that some power of mm wave beams from the EL is injected to the opposite direction. This wave power is expected to contribute the counter-ECCD for the desired current profile control and the pure central heating of plasma. The wide steering capability providing both the co- and counter direction of mm wave injection is hardly possible because large stress is yielded on the flexible cooling tube of the movable mirror. The design modification, which the central beam row is flipped, is therefore proposed as shown in Fig. 4. This modification has no significant design issue since the layout of the mirrors, the waveguide components and some blanket shield modules (BSMs) are just flipped. There are the minor design changes of the internal shield, the closure plate, the support flame for BSMs, but no impact on the engineering design and the cost increase of the fabrication. Moreover, no impact on the mm wave propagation is expected. The above PCR also requested a tilting of the poloidal injection angle on both top and the bottom beam rows. As a countermeasure for this design change request, the position of the top and bottom fixed focusing mirrors are shifted upward or downward by 50 mm, respectively, and then the mirrors are titled (4.6◦ ) so that beams are reflected toward the plasma core region. As the movable mirror is rotated from 20◦ to 40◦ in the toroidal direction, the poloidal angle of beams is also changed from 5◦ to 7◦ . This variation is due to the rotation axis of the mirror not being in a horizontal plane. The variation in poloidal tilt angle results in degradation of transmission efficiency of up to 7% keeping the same BSM opening as shown in Fig. 5. This indicates that some beams hit the inner wall of the opening. In reality, most of the power incident on the side wall will be reflected toward plasma and however, this beam power would not contribute to localized plasma heating or current drive as expected. In order to prevent or minimize the degradation of the transmission efficiency, the opening size can either be totally enlarged or the some region of BSM is cut out. The “cut-out” case in Fig. 5 assumes a wedge shape cut out with 0 cm removed on the inner side and 5 cm removed from the plasma side of the BSM. Applying this cut-out to both the top and the bottom openings recover to 98.7%. This modification does not affect the lifetime of the launcher and however, the impurity contamination on the inner surface of the BSM opening or the mirror surface may be increased. This contamination might cause the incremental surface heat load on them. The heat load on the steering mirror is one of the key issues on the EL design. The peak heat load at 1 MW transmission is 2.68 MW/m2 in the design modification including the poloidal tilt.
Fig. 2. Equatorial EC launcher mock-up.
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K. Takahashi et al. / Fusion Engineering and Design 86 (2011) 982–986
Fig. 3. Results of the low power experiment: 2D field pattern (top) and the comparison between the experiment and the calculation (bottom) in y-direction.
The heat load on the mirror surface depends on the electrical resistivity and the roughness of the surface. Thermal analysis shows that maximum temperature and stress in the mirror are 231 ◦ C at the mirror surface and 148 MPa at the inner surface of the cooling tube, respectively. The initial temperature is 100 ◦ C. The temperature and stresses are compatible with the envisioned manufacturing technique using CuCrZr alloy (260 MPa maximum allowable stress) for the mirror and the cooling tube. In the case of 2 MW transmission per waveguide line, the incident power on the mirror will increase to 1.8 MW. In this case, the maximum temperature is 415 ◦ C at the mirror surface and the maximum stress is 260 MPa at the inner surface of the cooling tube, which is the operating limit of the CuCrZr alloy. The above calculations assumed a 1.7 surface roughness and a thin Be coating on the plasma facing mirrors.
Fig. 4. Illustration of modified launcher design. The counter-ECCD is achievable with this design modification.
3. Design performance of nuclear shielding The EL must have enough nuclear shielding capability to satisfy the following requirements: 1 Neutron fluence at toroidal field coil ≤5 × 1021 n/m2 (En > 0.1 MeV). 2 Neutron fluence at the closure plate ≤1020 n/m2 (En > 0.1 MeV). 3 Dose rate at 10 days after shut down at the closure plate ≤100 Sv/h. It is confirmed that the reference design of the EL has the shielding capability that satisfies the above condition [10]. In the design, the shield around the waveguides placed in the port plug is sufficiently installed and it is the robust shielding structure. On the other hand, the beam duct, where the mirrors are located, is the vacant
Fig. 5. Transmission efficiency of the beam propagation at each location. WG: waveguide outlet, M1: focusing mirror, M2: steering mirror, Fi: BSM inlet, Fo: BSM outlet and P: plasma. The circles correspond to the case of no cut-out of the BSM opening and the diamonds to the case of a wedge shaped cut-out.
K. Takahashi et al. / Fusion Engineering and Design 86 (2011) 982–986
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Fig. 6. MCNP modeling of equatorial EC launcher for nuclear analysis: A-lite 41 model is applied for the ITER vacuum vessel.
region in the QO arrangement of the mm wave transmission in the modified EL design. High energy neutrons can easily pass through these space without reducing the flux and it can be presumed that the shielding capability may degrade, compared to the reference design. A Monte Carlo N-Particle (MCNP) model has been developed for the modified EL design with the counter ECCD and beam tilting as shown in Fig. 6. The vacuum vessel model, called A-lite 41 is used. The content ratio of stainless steel (SS316) and water (H2 O) in the BSM is 80/20 and that in port plug is 50/50. The model includes some shield blocks placed the rear of the gap between the port extension and the port plug. The total fusion power assumed is 500 MW and an average neutron fluence at the first wall of 0.3 MW/a/m2 integrated over an operating period of 4600 h. The MCNP model has an open space from the waveguide aperture to the fixed steering and then onward to the steering mirror, while a dense shielding structure is inserted around the waveguide sections that are inside the closure plate. The nuclear analysis report [15] indicates that the effect of the neutron streaming passed through the narrow gap of 20 mm between the port plug and the port is not negligible and it is recommended that some shielding material are stuffed in the gap. Hence, stainless steel block is stuffed at the rear region (56 cm from the back end) of the gap in the analysis model. The preliminary result of the analysis is summarized as follows:
i Fast neutron fluence at coil case around the port: 1.4 × 1020 n/m2 (En > 0.1 MeV). ii Fast neutron fluence at the closure plate: 8.0 × 1018 n/m2 (En > 0.1 MeV). iii Dose rate at 10 days after shut down: 120 Sv/h (at the closure plate).
4. Conclusion The EL mm-wave design has been simplified from the waveguide structure to the QO arrangement so as to increase the reliability and the manufacturability. The low power experiment of the EL mock-up based on the design modification confirmed that the beam integrity is maintained while propagating through the EL structure. As requested by the ITER Organization, the EL design has been modified to include counter-ECCD (by flipping one row) and the steering mirrors have been tilted by 5◦ for improved central access. A modification to the BSM is required in order to avoid degradation of the mm-wave transmission efficiency. This modification might increase the impurity contamination on the inner surface of the BSM opening or the mirror surface, which could cause the incremental surface heat load on them. This concern will have to be investigated. The nuclear analysis for the QO layout design and the preliminary result shows that it satisfies the nuclear shielding criteria except the shut down dose rate. The additional shield is necessary to reduce the dose rate less than the criteria.
Acknowledgments The authors thank the technical staffs, Yu. Ikeda, N. Koabayshi and Y. Okazaki for their support of the experiments, and assistance of design and analysis activities. We would also like to acknowledge Dr. T. Nishitani, Dr. M. Akiba and Dr. H. Takatsu for their encouragement of this work. The nuclear analysis in these design activities was carried out using an adaptation of the A-lite MCNP model which was developed as a collaborative effort between the FDS team of ASIPP China, ENEA Frascati, JAEA Naka, UKAEA and the ITER Organization.
References Here, fluence is the total amount of neutron at the integrated full power operation time. The result satisfies the shielding criteria with the exception of the residual dose rate at 10 days after shut down. As a countermeasure, a SS316 bulk shield of 30 cm in thickness can be installed behind the miter bends [16] for increased shielding potential. The Ref. [16] also shows that the approximately 10 cm of the shield block reduces the neutron flux by the factor of 10 when the SS316 to water ratio is 70/30.
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