- Email: [email protected]
Status of the ITER Ion Cyclotron H&CD system
Fusion Engineering and Design 88 (2013) 517–520
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Status of the ITER Ion Cyclotron H&CD system P. Lamalle a,∗ , B. Beaumont a , F. Kazarian a , T. Gassmann a , G. Agarici e , P. Ajesh f , T. Alonzo b , B. Arambhadiya a , A. Argouarch j , R. Bamber h , G. Berger-By j , J.-M. Bernard j , C. Brun j , S. Carpentier a , F. Clairet j , L. Colas j , X. Courtois j , A. Davis h , C. Dechelle j , L. Doceul j , P. Dumortier i , F. Durodié i , F. Ferlay j , M. Firdaouss j , E. Fredd g , J.-C. Giacalone j , R. Goulding g , N. Greenough g , D. Grine i , D. Hancock h , J.V.S. Hari f , J. Hillairet j , J. Hosea g , S. Huygen i , J. Jacquinot a , J. Jacquot j , A.S. Kaye m , D. Keller j , V. Kyrytsya i , D. Lockley h , F. Louche i , H. Machchhar f , E. Manon c , N. Mantel h , R. Martin e , M. McCarthy g , A. Messiaen i , L. Meunier e , D. Milanesio a , M. Missirlian j , K. Mohan f , A. Mukherjee f , M. Nightingale h , D. Patadia f , A.M. Patel f , G. Perrollaz d , B. Peters g , R. Pitts a , M. Porton h , K. Rajnish f , D. Rasmussen g , D. Rathi a , R. Sanabria g , R. Sartori e , M. Shannon h , A. Simonetto k , R. Singh f , G. Suthar f , D. Swain g , P. Thomas a , P. Tigwell h , R.G. Trivedi f , M. Vervier i , M. Vrancken i , D. Wilson h , K. Winkler l a
ITER Organization, Route de Vinon sur Verdon, 13115 St Paul-lez-Durance, France Solution F, Allée du Verdon, 13770 Venelles, France c Assystem Engineering, ZAC Saint Martin, 84120 Pertuis, France d Ametra, Z.I. Les Cabassols, 13770 Venelles, France e Fusion for Energy, Carrer Josep Pla 2, Torres Diagonal Litoral Edificio B3, 08019 Barcelona, Spain f ITER India, Institute for Plasma Research, Bhat, Gandhinagar 382424, Gujarat, India g ITER US, 1055 Commerce Park, PO Box 2008, MS-6483, Oak Ridge, TN 37831-6483, United States h EURATOM/CCFE Association, Culham Science Centre, Abingdon OX14 3DB, UK i LPP/ERM-KMS, Association EURATOM-Belgian State, Brussels, Belgium j CEA Cadarache, IRFM, F-13108 St-Paul-lez-Durance, France k Associazione EURATOM-ENEA-CNR, 20125 Milano, Italy l IPP-MPI, EURATOM-Assoziation, Garching, Germany m Consultant b
h i g h l i g h t s
We summarize the progress and outstanding issues in the development of the ITER Ion Cyclotron Heating and Current Drive (IC H&CD) system. The system is designed to robustly couple 20 MW in quasi-CW operation for a broad range of plasma scenarios, and is upgradeable to up to 40 MW. The design is rendered challenging by the wide spectrum of requirements and interface constraints to which it is subject. R&D is ongoing to validate key antenna components, and to qualify the radio-frequency (RF) sources and the transmission and matching components. Intensive numerical modeling and experimental studies on antenna mock-ups have been conducted to validate and optimize the RF design.
a r t i c l e
i n f o
Article history: Available online 13 March 2013 Keywords: ITER Plasma heating Ion cyclotron
a b s t r a c t The ongoing design of the ITER Ion Cyclotron Heating and Current Drive system (20 MW, 40–55 MHz) is rendered challenging by the wide spectrum of requirements and interface constraints to which it is subject, several of which are conflicting and/or still in a high state of flux. These requirements include operation over a broad range of plasma scenarios and magnetic fields (which prompts usage of wideband phased antenna arrays), high radio-frequency (RF) power density at the first wall (and associated operation close to voltage and current limits), resilience to ELM-induced load variations, intense thermal and mechanical loads, long pulse operation, high system availability, efficient nuclear shielding, high density of antenna services, remote-handling ability, tight installation tolerances, and nuclear safety function as tritium confinement barrier. R&D activities are ongoing or in preparation to validate critical
∗ Corresponding author. Tel.: +33 4 4217 6439. E-mail address: [email protected] (P. Lamalle). 0920-3796/$ – see front matter © 2013 ITER Organization. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.fusengdes.2012.11.027
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P. Lamalle et al. / Fusion Engineering and Design 88 (2013) 517–520
antenna components (plasma-facing Faraday screen, RF sliding contacts, RF vacuum windows), as well as to qualify the RF power sources and the transmission and matching components. Intensive numerical modeling and experimental studies on antenna mock-ups have been conducted to validate and optimize the RF design. The paper highlights progress and outstanding issues for the various system components. © 2013 ITER Organization. Published by Elsevier B.V. All rights reserved.
1. System functional requirements and ensuing general layout The ITER Ion Cyclotron Heating and Current Drive (IC H&CD) system is designed to couple 20 MW of radio-frequency (RF) power in quasi-CW operation (pulses up to 3600 s), for a variety of ITER plasma scenarios and a broad range of magnetic fields including nominal (5.3 T) and half-nominal (2.65 T). The system must provide robust coupling in the presence of ELMs, and be designed to perform wall conditioning at low power between main plasma shots. Several antenna components and a few transmission line components form part of the ITER tritium confinement systems and have a nuclear safety function. To meet the physics and operations requirements, broadband (40–55 MHz) phased antenna arrays will be installed in two equatorial ports, each antenna being designed for a 20 MW capability (capped by a 45 kV limit on the system). Using two antennas strongly reduces risks to system performance associated with (1) very large uncertainties on ITER edge density profiles, hence on coupling; (2) RF voltage standoff (reduced risk of arcing, yielding enhanced reliability); (3) RF current (i.e. power dissipation) limit in CW operations; and (4) RF sheath dissipation per antenna (reduced associated heat loads). This will allow delivering nominal power in a wide range of plasma scenarios. Having two antennas increases the versatility of the system, e.g. by allowing dual frequency operation, and increases its availability in case of repair operations. The second antenna is also a step in the direction of a future upgrade. Up to 40 MW could be coupled at a later stage without in-vessel intervention by increasing the generator power and tailoring the plasma target. The extent of this upgrade depends on the plasma loading range that is achieved in operation, to be assessed after acquisition of sufficient experimental information. The layout of the system is illustrated in Fig. 1. The main features and the status of its subsystems are summarized in the following sections. 2. Antenna port plugs Each one of the two antennas (Fig. 2) comprises an array of 6 poloidal by 4 toroidal short radiating straps. The 24 straps are connected in 8 poloidal triplets. The radiated power spectrum is adjustable by control of toroidal phase differences between antenna voltages or currents, and by control of the voltage or current ratios between columns of straps. Each antenna port plug is composed of a support structure, the RF structure (antenna radiating box, straps, Faraday shield (FS), in-vessel vacuum transmission lines, vacuum windows), a grounding system, neutron shielding structures and diagnostics. The RF performance prediction and optimization [1] is based on a reference 15 MA inductive H-mode burning plasma scenario. It uses a highly conservative density profile derived from the lowest ITER SOL density specifications, as described in [2]. (This is a much more demanding configuration than in earlier studies, see e.g. [3].) In this case predictions indicate that 20–25 MW could be coupled with two antennas in the baseline second harmonic tritium heating scenario (53 MHz), depending on the array toroidal phasing (see [1]). At the lower end of the frequency range power would be limited to 10–22 MW (depending on phasing) by the maximum line
voltage. It is worth noting that the reference plasma equilibrium can be brought several cm closer to the antenna without exceeding the first wall (FW) design heat loads. The maximum allowable shift still needs to be checked. A 4 cm shift restores the ability to deliver nominal power at all frequencies and phasings. In the case of the high ITER SOL density specification (unshifted equilibrium), nominal power delivery would take place with very comfortable margins (maximum system voltages in the 20 kV range, to be compared with the 45 kV design limit), which would be highly beneficial to reliability and availability. RF modeling of the antenna with its in-vessel surroundings has shown the importance of adequate RF grounding of the plug to the port extension [4]. A mechanical implementation of deployable contacts compatible with plug installation and removal has been developed [5]. Modeling has also revealed that voids between nearby blanket modules and vacuum vessel may influence the RF characteristics [6]. The evolution of the thermal and mechanical design is reported in [7]. Besides implementing the RF design, it has strived to accommodate the many requirements and interface constraints to which the plugs are subject. Given the sensitivity of antenna coupling to the distance to the fast wave cutoff, the antenna reference radial position is recessed by just 1 cm behind the FW, where it is only partially protected from plasma convective fluxes (see Fig. 3) by the surrounding FW elements. The FS bar design is therefore akin to the FW, a Be–Cu–CuCrZr–stainless steel sandwich actively cooled by a single internal channel. FS bar mock-ups, without and with beryllium layer, are being manufactured under F4E management. Their thermal behavior will be validated by means of high power density fatigue tests (up to 4.5 MW/m2 ). Manufacturing and high heat flux testing of a FS prototype made of several bars and their attachments is also in preparation. To achieve the best trade-off between plasma convective heat fluxes and RF coupling (and provide an additional means to compensate fabrication and installation tolerances), each of the 4 antenna front modules will be radially adjustable (during shutdown) by −1 to +2 cm around the reference. Key outstanding technical issues are: - Validation of a nuclear shielding solution for the gap between port plugs and tokamak port extension, without putting RF performance at risk. - Design of the Faraday screen (FS) bar attachment and remotehandling replaceability of FS modules during maintenance. - Validation of high current (2 kA peak) RF contacts. A test facility at IRFM has recently been upgraded [8] to perform this task in ITER-relevant vacuum and temperature conditions. - Detailed definition and approval of the qualification process of the brazed alumina RF vacuum windows, which are safety-important components. The RF window R&D (still in preparation) includes irradiation of samples of six grades of alumina to determine those least prone to degradation of RF dielectric loss and thermal conductivity under neutron flux. The thermal and mechanical properties of irradiated samples of the two most suitable grades will then be measured in detail, including sub-critical crack growth and brazing tests. RF and mechanical tests of full-size window prototypes will validate the final design.
P. Lamalle et al. / Fusion Engineering and Design 88 (2013) 517–520
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Fig. 1. ITER IC H&CD system layout: the RF sources and HVPS are located in the RF building, the matching units in the Assembly hall, and the pre-matching units in the tokamak building port cells. The antennas are two equatorial port plugs.
Ongoing studies of the antenna plug assembly and remote handling maintenance are reported in [9,10]. 3. Transmission and matching systems The transmission lines and matching systems [11] use 12 , 50 coaxial lines rated for CW transmission of up to 2.5 MW per line at VSWR ≤ 2. These lines require active water cooling of the external conductors. They will be filled with dielectric gas at 3 bar to improve voltage standoff. This gas will be circulated between inner and outer conductor to cool the inner conductor. Coaxial switches will allow connection of any RF source to a high power load for commissioning, testing and maintenance. The matching systems use two-stub tuners, in conjunction with the ‘ELM dump’ scheme based on hybrid splitters. This RF circuit arrangement provides high resilience to variations of the antenna resistive loading, such as provoked by edge-localized modes (ELMs), or transitions from L- to H-confinement mode. Decoupling networks will connect adjacent circuits in order to cancel dominant mutual reactive coupling between antenna straps and facilitate matching. They will also allow equalizing antenna strap currents and line powers in various radiated power spectrum configurations [12,13]. A compromise must be reached between the antenna performance optimization, which tends to yield high
Fig. 2. Cut isometric view of an IC antenna port plug, showing the main internal components and the rear transition frame supporting various services.
mutual coupling between straps, and a feasible design of the decoupling network. The matching and decoupling systems have been relocated from the tokamak building port cells to the Assembly hall. This strongly improves the accessibility and maintainability of their adjustable components (vacuum capacitors and stubs), which are feedback controlled in real time during operations. The IC H&CD port cells now house pre-matching units made of phase shifters and stubs which perform a first reduction of the line VSWR. These components only require adjustment when the operating frequency is modified and are not adjusted during high power operations. Direct water cooling of the inner conductor will be used in the high VSWR lines (13 , 20 ) between pre-matching units and antennas. A qualification programme for the transmission and matching components is under way at US ITER [11]: an RF resonant ring is under commissioning to test components at the ITER specifications,
Fig. 3. Convective plasma heat flux onto the FS during 7.5 MA startup/rampdown plasma scenario. The uniform grey areas are shadowed by surrounding blanket elements (not shown), or by other bars. Peak flux: 0.7 MW/m2 (bar front faces); 3.2 MW/m2 (lateral faces). Max. flux on a bar: 7.6 kW; total flux 107 kW. Refer to the online version of the paper for a color version.
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and the manufacturing of first straight sections and elbow prototypes has now started. Numerical and experimental matching studies using a scaled mock-up of the ITER antenna are reported in [12–14]. 4. RF sources The nine RF sources (four per antenna plus one spare) are specified to deliver either 2.5 MW in CW operation on a VSWR = 2 load in the band 35–65 MHz, or 3 MW CW on VSWR = 1.5 in the antenna design band (in both cases for any phase of the reflection coefficient). The sources will use two parallel amplifier chains with a combining circuit. A /4 combiner was chosen to save space. Dynamic control of the anode voltage is required to adjust power tubes working conditions as a function of the phase and amplitude of reflection coefficient changes. Each amplifier chain is made of three stages: pre-driver, driver and final stage. The combined specifications of high power and high VSWR are challenging. The CW aspect of the operation further constrains the design as efficient cooling is required for all components [15]. The broad frequency range associated with accurate instantaneous bandwidth (±1 MHz at 1 dB point) requires specific designs for the tube input and output cavities. Candidate high power tubes are available from industrial suppliers but have never been used in conditions matching the ITER specifications. The development of the RF sources is under responsibility of ITER India (II), who has placed two R&D contracts in June 2012 with Thales Electron Devices and Continental Electronics (USA), for driver and final stage. Tubes and cavities for these stages will be procured and integrated in a full amplifier chain developed by II. Tests under ITER specifications will validate each design. The power specification of a single chain (∼1.65 MW on VSWR = 1.5) exceeds half of the full RF source power since the non-ideal 3 dB characteristics of the combiner over the full band as well as the RF losses must be taken into account. In parallel with this tube qualification, integration work is ongoing in India as the total space allocated for each RF source must not exceed 3.6 m × 9 m × 5 m. Component layout should be as compact as in transmitters developed for telecommunications. Besides the amplifier chains and the combiner, the local control unit made of 4 cubicles, water cooling pipes carrying a flow rate in excess of 32.5 kg/s and power supply cables providing around 6 MW will have to be accommodated in the allocated volume. 5. High voltage power supplies (HVPS) The HVPS are based on Pulsed Step Modulator (PSM) design, with a large number of low-voltage (<1 kV) modules stacked in series, which can be switched on/off individually for fine regulation of the output voltage. The IC H&CD HVPS includes a total of 18 identical PSM-based HVPS units. Each unit will supply both driver and final stage of one amplifier chain of an RF source. The PSM principle allows tapping a connection at intermediate voltage in order to supply the driver stage at lower voltage, whilst the final stage receives the full unit voltage. During high power operation, the driver stage is operated at a constant voltage, typically in the 17–18 kV range, while the final stage can be operated at up to 27 kV, for a maximum output power of more than 3 MW for the two stages [16]. The April 2012 preliminary design review allowed validating the main design choices of II before proceeding to the procurement of a prototype unit. The design includes 48 PSM modules, with a rated voltage of 740 V, supplied with two transformers, having respectively 30 and 18 secondary outputs. The output voltage is filtered with a 1 mH inductor. This configuration was modeled to
Fig. 4. HVPS module rack internal layout.
ensure that the design meets the performance requirements and to refine interface parameters. Based on the results, specifications of individual component have been issued. The components have been integrated in a CAD model (Fig. 4) to validate building space allocation and define interface points. The PSM modules and filter inductor will be integrated in a metallic cabinet, so as to minimize EMI and ensure occupational safety. 6. Plant system controller (PSC) Each of the IC H&CD subsystems includes a local controller. A plant system controller manages the overall operation, safety and investment protection. It provides coordination, synchronization and RF power feedback control. It covers conventional control functions, and the dispatching of all interlocks and safety control functions internal and external to the system. Disclaimer The views and opinions expressed herein do not necessarily reflect those of the ITER Organization. References [1] M. Vrancken, et al., RF Optimization of the Port Plug Layout and Performance Assessment of the ITER ICRF Antenna, Fusion Engineering and Design 88 (2013) 940–944. [2] S. Carpentier, R.A. Pitts, P.C. Stangeby, J.D. Elder, A.S. Kukushkin, S. Lisgo, W. Fundamenski, D. Moulton, Journal of Nuclear Materials 415 (2011) S165–S169. [3] P.U. Lamalle, B. Beaumont, T. Gassmann, F. Kazarian, B. Arambhadiya, D. Bora, et al., AIP Conference Proceedings 1187 (2009) 265. [4] P. Dumortier, et al., ITER ICRH Antenna Grounding Options, Fusion Engineering and Design 88 (2013) 922–925. [5] D. Hancock, et al., Design of a mechanically actuated RF grounding system for the ITER ICRH antenna, Fusion Engineering and Design 88 (2004) 2100–2104. [6] F. Louche, et al., Influence of the blanket shield modules geometry on the operation of the ITER ICRF antenna, Fusion Engineering and Design 88 (2013) 926–929. [7] M. Shannon et al., in this issue. [8] A. Argouarch, et al., Steady State Rf Facility For Testing Iter Icrh Rf Contact Component, Fusion Engineering and Design 88 (2013) 1002–1006. [9] J.M. Bernard, et al., CEA contribution to the Iter ICRH antenna design, Fusion Engineering and Design 88 (2013) 950–955. [10] F. Ferlay, et al., First analysis of remote handling maintenance procedure in the hot cell for the ITER ICH&CD antenna - RVTL replacement, Fusion Engineering and Design 88 (2013) 1924–1928. [11] D.A. Rasmussen, D.W. Swain, R.H. Goulding, P.V. Pesavento, B. Peters, E.H. Fredd, J. Hosea, N. Greenough, AIP Conference Proceedings 46 (2011) 53–56. [12] M. Vervier, et al., Technical Optimization of the ITER ICRH Decoupling and Matching System, Fusion Engineering and Design 88 (2013) 1030–1033. [13] A. Messiaen, et al., Influence of the Plasma Profile and the Antenna Geometry on the Matching and Current Distribution Control of the ITER ICRF Antenna Array.Optimization of the decoupling-matching system, Fusion Engineering and Design 88 (2013) 501–506. [14] D. Grine, A. Messiaen, M. Vervier, P. Dumortier, R. Koch, Fusion Engineering and Design 87 (2) (2012) 167–178. [15] F. Kazarian, B. Beaumont, B. Arambhadiya, T. Gassmann, Ph. Lamalle, D. Rathi, et al., Fusion Engineering and Design 86 (2011) 888–891. [16] T. Gassmann, B. Arambhadiya, B. Beaumont, U.K. Baruah, T. Bonicelli, C. Darbos, et al., Fusion Engineering and Design 86 (2011) 884–887.
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Status of the ITER Ion Cyclotron H&CD system P. Lamalle a,∗ , B. Beaumont a , F. Kazarian a , T. Gassmann a , G. Agarici e , P. Ajesh f , T. Alonzo b , B. Arambhadiya a , A. Argouarch j , R. Bamber h , G. Berger-By j , J.-M. Bernard j , C. Brun j , S. Carpentier a , F. Clairet j , L. Colas j , X. Courtois j , A. Davis h , C. Dechelle j , L. Doceul j , P. Dumortier i , F. Durodié i , F. Ferlay j , M. Firdaouss j , E. Fredd g , J.-C. Giacalone j , R. Goulding g , N. Greenough g , D. Grine i , D. Hancock h , J.V.S. Hari f , J. Hillairet j , J. Hosea g , S. Huygen i , J. Jacquinot a , J. Jacquot j , A.S. Kaye m , D. Keller j , V. Kyrytsya i , D. Lockley h , F. Louche i , H. Machchhar f , E. Manon c , N. Mantel h , R. Martin e , M. McCarthy g , A. Messiaen i , L. Meunier e , D. Milanesio a , M. Missirlian j , K. Mohan f , A. Mukherjee f , M. Nightingale h , D. Patadia f , A.M. Patel f , G. Perrollaz d , B. Peters g , R. Pitts a , M. Porton h , K. Rajnish f , D. Rasmussen g , D. Rathi a , R. Sanabria g , R. Sartori e , M. Shannon h , A. Simonetto k , R. Singh f , G. Suthar f , D. Swain g , P. Thomas a , P. Tigwell h , R.G. Trivedi f , M. Vervier i , M. Vrancken i , D. Wilson h , K. Winkler l a
ITER Organization, Route de Vinon sur Verdon, 13115 St Paul-lez-Durance, France Solution F, Allée du Verdon, 13770 Venelles, France c Assystem Engineering, ZAC Saint Martin, 84120 Pertuis, France d Ametra, Z.I. Les Cabassols, 13770 Venelles, France e Fusion for Energy, Carrer Josep Pla 2, Torres Diagonal Litoral Edificio B3, 08019 Barcelona, Spain f ITER India, Institute for Plasma Research, Bhat, Gandhinagar 382424, Gujarat, India g ITER US, 1055 Commerce Park, PO Box 2008, MS-6483, Oak Ridge, TN 37831-6483, United States h EURATOM/CCFE Association, Culham Science Centre, Abingdon OX14 3DB, UK i LPP/ERM-KMS, Association EURATOM-Belgian State, Brussels, Belgium j CEA Cadarache, IRFM, F-13108 St-Paul-lez-Durance, France k Associazione EURATOM-ENEA-CNR, 20125 Milano, Italy l IPP-MPI, EURATOM-Assoziation, Garching, Germany m Consultant b
h i g h l i g h t s
We summarize the progress and outstanding issues in the development of the ITER Ion Cyclotron Heating and Current Drive (IC H&CD) system. The system is designed to robustly couple 20 MW in quasi-CW operation for a broad range of plasma scenarios, and is upgradeable to up to 40 MW. The design is rendered challenging by the wide spectrum of requirements and interface constraints to which it is subject. R&D is ongoing to validate key antenna components, and to qualify the radio-frequency (RF) sources and the transmission and matching components. Intensive numerical modeling and experimental studies on antenna mock-ups have been conducted to validate and optimize the RF design.
a r t i c l e
i n f o
Article history: Available online 13 March 2013 Keywords: ITER Plasma heating Ion cyclotron
a b s t r a c t The ongoing design of the ITER Ion Cyclotron Heating and Current Drive system (20 MW, 40–55 MHz) is rendered challenging by the wide spectrum of requirements and interface constraints to which it is subject, several of which are conflicting and/or still in a high state of flux. These requirements include operation over a broad range of plasma scenarios and magnetic fields (which prompts usage of wideband phased antenna arrays), high radio-frequency (RF) power density at the first wall (and associated operation close to voltage and current limits), resilience to ELM-induced load variations, intense thermal and mechanical loads, long pulse operation, high system availability, efficient nuclear shielding, high density of antenna services, remote-handling ability, tight installation tolerances, and nuclear safety function as tritium confinement barrier. R&D activities are ongoing or in preparation to validate critical
∗ Corresponding author. Tel.: +33 4 4217 6439. E-mail address: [email protected] (P. Lamalle). 0920-3796/$ – see front matter © 2013 ITER Organization. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.fusengdes.2012.11.027
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P. Lamalle et al. / Fusion Engineering and Design 88 (2013) 517–520
antenna components (plasma-facing Faraday screen, RF sliding contacts, RF vacuum windows), as well as to qualify the RF power sources and the transmission and matching components. Intensive numerical modeling and experimental studies on antenna mock-ups have been conducted to validate and optimize the RF design. The paper highlights progress and outstanding issues for the various system components. © 2013 ITER Organization. Published by Elsevier B.V. All rights reserved.
1. System functional requirements and ensuing general layout The ITER Ion Cyclotron Heating and Current Drive (IC H&CD) system is designed to couple 20 MW of radio-frequency (RF) power in quasi-CW operation (pulses up to 3600 s), for a variety of ITER plasma scenarios and a broad range of magnetic fields including nominal (5.3 T) and half-nominal (2.65 T). The system must provide robust coupling in the presence of ELMs, and be designed to perform wall conditioning at low power between main plasma shots. Several antenna components and a few transmission line components form part of the ITER tritium confinement systems and have a nuclear safety function. To meet the physics and operations requirements, broadband (40–55 MHz) phased antenna arrays will be installed in two equatorial ports, each antenna being designed for a 20 MW capability (capped by a 45 kV limit on the system). Using two antennas strongly reduces risks to system performance associated with (1) very large uncertainties on ITER edge density profiles, hence on coupling; (2) RF voltage standoff (reduced risk of arcing, yielding enhanced reliability); (3) RF current (i.e. power dissipation) limit in CW operations; and (4) RF sheath dissipation per antenna (reduced associated heat loads). This will allow delivering nominal power in a wide range of plasma scenarios. Having two antennas increases the versatility of the system, e.g. by allowing dual frequency operation, and increases its availability in case of repair operations. The second antenna is also a step in the direction of a future upgrade. Up to 40 MW could be coupled at a later stage without in-vessel intervention by increasing the generator power and tailoring the plasma target. The extent of this upgrade depends on the plasma loading range that is achieved in operation, to be assessed after acquisition of sufficient experimental information. The layout of the system is illustrated in Fig. 1. The main features and the status of its subsystems are summarized in the following sections. 2. Antenna port plugs Each one of the two antennas (Fig. 2) comprises an array of 6 poloidal by 4 toroidal short radiating straps. The 24 straps are connected in 8 poloidal triplets. The radiated power spectrum is adjustable by control of toroidal phase differences between antenna voltages or currents, and by control of the voltage or current ratios between columns of straps. Each antenna port plug is composed of a support structure, the RF structure (antenna radiating box, straps, Faraday shield (FS), in-vessel vacuum transmission lines, vacuum windows), a grounding system, neutron shielding structures and diagnostics. The RF performance prediction and optimization [1] is based on a reference 15 MA inductive H-mode burning plasma scenario. It uses a highly conservative density profile derived from the lowest ITER SOL density specifications, as described in [2]. (This is a much more demanding configuration than in earlier studies, see e.g. [3].) In this case predictions indicate that 20–25 MW could be coupled with two antennas in the baseline second harmonic tritium heating scenario (53 MHz), depending on the array toroidal phasing (see [1]). At the lower end of the frequency range power would be limited to 10–22 MW (depending on phasing) by the maximum line
voltage. It is worth noting that the reference plasma equilibrium can be brought several cm closer to the antenna without exceeding the first wall (FW) design heat loads. The maximum allowable shift still needs to be checked. A 4 cm shift restores the ability to deliver nominal power at all frequencies and phasings. In the case of the high ITER SOL density specification (unshifted equilibrium), nominal power delivery would take place with very comfortable margins (maximum system voltages in the 20 kV range, to be compared with the 45 kV design limit), which would be highly beneficial to reliability and availability. RF modeling of the antenna with its in-vessel surroundings has shown the importance of adequate RF grounding of the plug to the port extension [4]. A mechanical implementation of deployable contacts compatible with plug installation and removal has been developed [5]. Modeling has also revealed that voids between nearby blanket modules and vacuum vessel may influence the RF characteristics [6]. The evolution of the thermal and mechanical design is reported in [7]. Besides implementing the RF design, it has strived to accommodate the many requirements and interface constraints to which the plugs are subject. Given the sensitivity of antenna coupling to the distance to the fast wave cutoff, the antenna reference radial position is recessed by just 1 cm behind the FW, where it is only partially protected from plasma convective fluxes (see Fig. 3) by the surrounding FW elements. The FS bar design is therefore akin to the FW, a Be–Cu–CuCrZr–stainless steel sandwich actively cooled by a single internal channel. FS bar mock-ups, without and with beryllium layer, are being manufactured under F4E management. Their thermal behavior will be validated by means of high power density fatigue tests (up to 4.5 MW/m2 ). Manufacturing and high heat flux testing of a FS prototype made of several bars and their attachments is also in preparation. To achieve the best trade-off between plasma convective heat fluxes and RF coupling (and provide an additional means to compensate fabrication and installation tolerances), each of the 4 antenna front modules will be radially adjustable (during shutdown) by −1 to +2 cm around the reference. Key outstanding technical issues are: - Validation of a nuclear shielding solution for the gap between port plugs and tokamak port extension, without putting RF performance at risk. - Design of the Faraday screen (FS) bar attachment and remotehandling replaceability of FS modules during maintenance. - Validation of high current (2 kA peak) RF contacts. A test facility at IRFM has recently been upgraded [8] to perform this task in ITER-relevant vacuum and temperature conditions. - Detailed definition and approval of the qualification process of the brazed alumina RF vacuum windows, which are safety-important components. The RF window R&D (still in preparation) includes irradiation of samples of six grades of alumina to determine those least prone to degradation of RF dielectric loss and thermal conductivity under neutron flux. The thermal and mechanical properties of irradiated samples of the two most suitable grades will then be measured in detail, including sub-critical crack growth and brazing tests. RF and mechanical tests of full-size window prototypes will validate the final design.
P. Lamalle et al. / Fusion Engineering and Design 88 (2013) 517–520
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Fig. 1. ITER IC H&CD system layout: the RF sources and HVPS are located in the RF building, the matching units in the Assembly hall, and the pre-matching units in the tokamak building port cells. The antennas are two equatorial port plugs.
Ongoing studies of the antenna plug assembly and remote handling maintenance are reported in [9,10]. 3. Transmission and matching systems The transmission lines and matching systems [11] use 12 , 50 coaxial lines rated for CW transmission of up to 2.5 MW per line at VSWR ≤ 2. These lines require active water cooling of the external conductors. They will be filled with dielectric gas at 3 bar to improve voltage standoff. This gas will be circulated between inner and outer conductor to cool the inner conductor. Coaxial switches will allow connection of any RF source to a high power load for commissioning, testing and maintenance. The matching systems use two-stub tuners, in conjunction with the ‘ELM dump’ scheme based on hybrid splitters. This RF circuit arrangement provides high resilience to variations of the antenna resistive loading, such as provoked by edge-localized modes (ELMs), or transitions from L- to H-confinement mode. Decoupling networks will connect adjacent circuits in order to cancel dominant mutual reactive coupling between antenna straps and facilitate matching. They will also allow equalizing antenna strap currents and line powers in various radiated power spectrum configurations [12,13]. A compromise must be reached between the antenna performance optimization, which tends to yield high
Fig. 2. Cut isometric view of an IC antenna port plug, showing the main internal components and the rear transition frame supporting various services.
mutual coupling between straps, and a feasible design of the decoupling network. The matching and decoupling systems have been relocated from the tokamak building port cells to the Assembly hall. This strongly improves the accessibility and maintainability of their adjustable components (vacuum capacitors and stubs), which are feedback controlled in real time during operations. The IC H&CD port cells now house pre-matching units made of phase shifters and stubs which perform a first reduction of the line VSWR. These components only require adjustment when the operating frequency is modified and are not adjusted during high power operations. Direct water cooling of the inner conductor will be used in the high VSWR lines (13 , 20 ) between pre-matching units and antennas. A qualification programme for the transmission and matching components is under way at US ITER [11]: an RF resonant ring is under commissioning to test components at the ITER specifications,
Fig. 3. Convective plasma heat flux onto the FS during 7.5 MA startup/rampdown plasma scenario. The uniform grey areas are shadowed by surrounding blanket elements (not shown), or by other bars. Peak flux: 0.7 MW/m2 (bar front faces); 3.2 MW/m2 (lateral faces). Max. flux on a bar: 7.6 kW; total flux 107 kW. Refer to the online version of the paper for a color version.
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and the manufacturing of first straight sections and elbow prototypes has now started. Numerical and experimental matching studies using a scaled mock-up of the ITER antenna are reported in [12–14]. 4. RF sources The nine RF sources (four per antenna plus one spare) are specified to deliver either 2.5 MW in CW operation on a VSWR = 2 load in the band 35–65 MHz, or 3 MW CW on VSWR = 1.5 in the antenna design band (in both cases for any phase of the reflection coefficient). The sources will use two parallel amplifier chains with a combining circuit. A /4 combiner was chosen to save space. Dynamic control of the anode voltage is required to adjust power tubes working conditions as a function of the phase and amplitude of reflection coefficient changes. Each amplifier chain is made of three stages: pre-driver, driver and final stage. The combined specifications of high power and high VSWR are challenging. The CW aspect of the operation further constrains the design as efficient cooling is required for all components [15]. The broad frequency range associated with accurate instantaneous bandwidth (±1 MHz at 1 dB point) requires specific designs for the tube input and output cavities. Candidate high power tubes are available from industrial suppliers but have never been used in conditions matching the ITER specifications. The development of the RF sources is under responsibility of ITER India (II), who has placed two R&D contracts in June 2012 with Thales Electron Devices and Continental Electronics (USA), for driver and final stage. Tubes and cavities for these stages will be procured and integrated in a full amplifier chain developed by II. Tests under ITER specifications will validate each design. The power specification of a single chain (∼1.65 MW on VSWR = 1.5) exceeds half of the full RF source power since the non-ideal 3 dB characteristics of the combiner over the full band as well as the RF losses must be taken into account. In parallel with this tube qualification, integration work is ongoing in India as the total space allocated for each RF source must not exceed 3.6 m × 9 m × 5 m. Component layout should be as compact as in transmitters developed for telecommunications. Besides the amplifier chains and the combiner, the local control unit made of 4 cubicles, water cooling pipes carrying a flow rate in excess of 32.5 kg/s and power supply cables providing around 6 MW will have to be accommodated in the allocated volume. 5. High voltage power supplies (HVPS) The HVPS are based on Pulsed Step Modulator (PSM) design, with a large number of low-voltage (<1 kV) modules stacked in series, which can be switched on/off individually for fine regulation of the output voltage. The IC H&CD HVPS includes a total of 18 identical PSM-based HVPS units. Each unit will supply both driver and final stage of one amplifier chain of an RF source. The PSM principle allows tapping a connection at intermediate voltage in order to supply the driver stage at lower voltage, whilst the final stage receives the full unit voltage. During high power operation, the driver stage is operated at a constant voltage, typically in the 17–18 kV range, while the final stage can be operated at up to 27 kV, for a maximum output power of more than 3 MW for the two stages [16]. The April 2012 preliminary design review allowed validating the main design choices of II before proceeding to the procurement of a prototype unit. The design includes 48 PSM modules, with a rated voltage of 740 V, supplied with two transformers, having respectively 30 and 18 secondary outputs. The output voltage is filtered with a 1 mH inductor. This configuration was modeled to
Fig. 4. HVPS module rack internal layout.
ensure that the design meets the performance requirements and to refine interface parameters. Based on the results, specifications of individual component have been issued. The components have been integrated in a CAD model (Fig. 4) to validate building space allocation and define interface points. The PSM modules and filter inductor will be integrated in a metallic cabinet, so as to minimize EMI and ensure occupational safety. 6. Plant system controller (PSC) Each of the IC H&CD subsystems includes a local controller. A plant system controller manages the overall operation, safety and investment protection. It provides coordination, synchronization and RF power feedback control. It covers conventional control functions, and the dispatching of all interlocks and safety control functions internal and external to the system. Disclaimer The views and opinions expressed herein do not necessarily reflect those of the ITER Organization. References [1] M. Vrancken, et al., RF Optimization of the Port Plug Layout and Performance Assessment of the ITER ICRF Antenna, Fusion Engineering and Design 88 (2013) 940–944. [2] S. Carpentier, R.A. Pitts, P.C. Stangeby, J.D. Elder, A.S. Kukushkin, S. Lisgo, W. Fundamenski, D. Moulton, Journal of Nuclear Materials 415 (2011) S165–S169. [3] P.U. Lamalle, B. Beaumont, T. Gassmann, F. Kazarian, B. Arambhadiya, D. Bora, et al., AIP Conference Proceedings 1187 (2009) 265. [4] P. Dumortier, et al., ITER ICRH Antenna Grounding Options, Fusion Engineering and Design 88 (2013) 922–925. [5] D. Hancock, et al., Design of a mechanically actuated RF grounding system for the ITER ICRH antenna, Fusion Engineering and Design 88 (2004) 2100–2104. [6] F. Louche, et al., Influence of the blanket shield modules geometry on the operation of the ITER ICRF antenna, Fusion Engineering and Design 88 (2013) 926–929. [7] M. Shannon et al., in this issue. [8] A. Argouarch, et al., Steady State Rf Facility For Testing Iter Icrh Rf Contact Component, Fusion Engineering and Design 88 (2013) 1002–1006. [9] J.M. Bernard, et al., CEA contribution to the Iter ICRH antenna design, Fusion Engineering and Design 88 (2013) 950–955. [10] F. Ferlay, et al., First analysis of remote handling maintenance procedure in the hot cell for the ITER ICH&CD antenna - RVTL replacement, Fusion Engineering and Design 88 (2013) 1924–1928. [11] D.A. Rasmussen, D.W. Swain, R.H. Goulding, P.V. Pesavento, B. Peters, E.H. Fredd, J. Hosea, N. Greenough, AIP Conference Proceedings 46 (2011) 53–56. [12] M. Vervier, et al., Technical Optimization of the ITER ICRH Decoupling and Matching System, Fusion Engineering and Design 88 (2013) 1030–1033. [13] A. Messiaen, et al., Influence of the Plasma Profile and the Antenna Geometry on the Matching and Current Distribution Control of the ITER ICRF Antenna Array.Optimization of the decoupling-matching system, Fusion Engineering and Design 88 (2013) 501–506. [14] D. Grine, A. Messiaen, M. Vervier, P. Dumortier, R. Koch, Fusion Engineering and Design 87 (2) (2012) 167–178. [15] F. Kazarian, B. Beaumont, B. Arambhadiya, T. Gassmann, Ph. Lamalle, D. Rathi, et al., Fusion Engineering and Design 86 (2011) 888–891. [16] T. Gassmann, B. Arambhadiya, B. Beaumont, U.K. Baruah, T. Bonicelli, C. Darbos, et al., Fusion Engineering and Design 86 (2011) 884–887.