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Integration of in-vessel diagnostic sensors in ITER
Fusion Engineering and Design 56 – 57 (2001) 883– 888 www.elsevier.com/locate/fusengdes
Integration of in-vessel diagnostic sensors in ITER C.I. Walker a,*, A. Costley b, L. DeKock a, K. Ebisawa b, G. Janeschitz a, A. Malaquias a, M. Yamada a, G. Vayakis b, S. Yamamoto a b
a ITER Joint Central Team, Boltzmannstr 2, 85748, Garching, Germany ITER Joint Central Team, Naka-machi, Naka-gun, Ibaraki-ken 311 -0193, Japan
Abstract Integration of diagnostic electrical sensors and their signal transmission in ITER is a combination of the process of physical location within a limited space and complex topology, while accounting for effects on the transmitted signals from the particular local nuclear environment [1]. All components of the signal transmission chain are important. Diagnostic sensors are seen in all parts of the ITER vacuum vessel and transmission lines must cross many functional zones, notably the blanket and the vacuum/pressure boundaries. Effects on the signal emanate from conductor variations (radiation and thermally induced conductivity drift, thermo-electric effects, e-m loading, contact resistance), insulator variations (reduction of voltage stand-off, resistivity degradation, material deposition) and interference (e-m noise, microphony, current and voltage leakage). The particular nuclear aspects to be addressed are the high first wall photon flux, the high fast neutron and gamma fluxes, and high neutron fluence. Designs have been developed for the integration of sensors and their connecting wiring within the vacuum vessel, shielding blanket, divertor cassettes and port structures. These are presented here, considering first the radiation effects, then the integration of the diagnostic sensors, the transmission routes and finally the transmission hardware. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Diagnostic; Transmission; ITER; Gamma fluxes
1. Introduction It is essential for the diagnostic system, and ultimately for the control of the plasma, to optimise the design of the position, support, cooling, electrical connection and insulation of diagnostic sensors and their cables. Many of the design problems (e.g. magnetic noise interference, baked
* Corresponding author. Tel.: +49-89-3299-4408; fax: + 49-89-3299-4165. E-mail address: [email protected] (C.I. Walker).
vacuum compatibility) are common to diagnostic systems in all plasma devices. Others, such as remote handling, gamma noise and remote signal processing, have already been addressed in the last generation of Tokamaks with moderate neutron fluxes (JET, TFTR). For ITER the increased hostility of the radiation environment (neutron wall loading up to 1 MW/m2, fluence 0.5 MWa/ m2) affects diagnostic performance, lower leakage resistance, raised conductor resistivity, increased noise, longer integration of this noise, nuclear heating, variable spring rates, etc. and have to be considered.
0920-3796/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 0 - 3 7 9 6 ( 0 1 ) 0 0 3 7 1 - 4
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C.I. Walker et al. / Fusion Engineering and Design 56–57 (2001) 883–888
Although the number of diagnostic systems ( 50) is similar to most previous devices [2], the quantity of sensing elements has increased because of the increased plasma size (k95 a= 3.4m) while spatial resolution requirements have remained fixed. The need for the diagnostics to perform a plasma control role demands a higher signal reliability and repeatability pulse to pulse [3]. The integrity of the vacuum boundaries has become more important now they are also confinement barriers, for loss of tritiated dust with activated corrosion products and of toxic beryllium and tungsten dust. The requirement for remote replacement and maintenance by tele-manipulation introduces additional connections in transmission systems [4]. 2. Radiation effects on sensors and transmission elements Radiation effects on diagnostic materials are most serious near the plasma [5]. RIED (radiation induced electrical degradation) give a problem of voltage stand-off for a few, high-voltage diagnostics. RIC (radiation induced conductivity) is a serious problem in insulators, inducing signal leakage and interference pick-up. Near the first wall, many insulator materials are no longer suitable The preferred electrical insulator is alumina (Al2O3) with a resistivity reduced, by RIC, to within an order of magnitude of the required level. RIEMF (radiation induced emf) [6] gives concern about the stability of any low level signals that are integrated over the length of the discharge. Nuclear heating increases temperatures, and thus resistivity, during the pulse, and mechanical radiation damage affects mechanical spring rates and possibly contact resistance. Helium production, leading to swelling, affects thin substrate sensors (such as strain gauges and bolometers). 3. In-vessel sensors The diagnostics with in-vessel sensors that are considered here are shown in Table 1, with some of their particular attributes and problems that are discussed later.
Magnetic pick-up coils [7] on the vacuum vessel wall will withstand 105 discharges. With integration times well over 1000 s, spurious e-m noise pick-up and the generation of a RIEMF are design problems for the equilibrium coils. Redundancy in this diagnostic is given by the incorporation of extra poloidal sets of coils and by sets of hard-wired or jointless coils. Toroidal flux loops and saddles are attached to the vessel behind the blanket. The micro-fission chambers, similar to those used for flux measurements inside fission power reactors, are mounted on the vacuum vessel, behind blanket modules. The bolometry system requires 340 lines of sight to acquire tomographic images. 70 minicameras, each providing several lines of sight, are located in port plugs on the vessel wall and on divertor cassettes. Because of the dimensional stability of substrate and thin film circuits bolometers require replacement after 10,000 discharges. The capacitive pick-up (microphony) of the many cables (23 cores, 5 screens and outer earth, 35 mm diameter) is a particular problem. Many Langmuir probes are located on divertor cassette side plates. They are small (10× 10×50 mm), electrically insulated, CFC electrodes protruding into the plasma and are thus exposed to high heat fluxes, particle damage, and dust. They run at up to 250 V and are susceptible to RIED which occurs at electrical field strengths exceeding 70 V/mm. Hot cathode ionisation gauges measure neutral pressure in the divertor cassette. The restraint of the cable for the high current filament (520 A) requires special attention. Photo-electric detectors, for UV and soft Xrays, in ports and on the vessel wall, are only conceptually designed at present. Noise in these systems that produce only nA signals is a significant problem to be solved. There are to be a number of engineering sensors, such as thermocouples, limit switches and strain gauges. These do not need to see the plasma and are well shielded (10 − 2 wall loading), but they must be bakeable, operate at high and varying temperature and, receive a large integrated radiation damage (B0.1 dpa), retain their spring rate etc.
C.I. Walker et al. / Fusion Engineering and Design 56–57 (2001) 883–888
4. Sensor location Removable structures in ITER are the preferred sites for diagnostic sensors which are fitted in modules in 11 upper port plugs, 7 equatorial port plugs and 15 divertor cassettes. (Fig. 1). Sensors are also required on the vacuum vessel wall. During the remote maintenance of diagnostic modules in these structures, sensor wiring is made with a demountable connectors, typically with 500 cable cores, fed by 5 vacuum feedthroughs (Fig. 2). In the divertor, 10– 20 mm thick side-plates are bolted to some cassettes. These are made complete with pre-wired sensors and wiring loom, allowing relatively simple replacement of diagnostic components (Fig. 3). Sometimes heavy wiring demands are seen, for instance where Bolometers and Langmuir probes are installed ( 300 cores). Signals are marshalled at the elec-
885
trical connector on the outboard end of the cassette. The side plate can incorporate water cooling, but the connector, with 0.04 W/cm3, is passively cooled. For diagnostics on the vacuum vessel wall, locations are provided as integral components of the vessel inner wall and, outboard, on top of the blanket manifold. Several sensors can be mounted in small (160 mm diameter) diagnostic plugs (Fig. 4) which receive 0.8 W/cc and are conductioncooled. Remote replacement is necessary because of radiation damage, copper, for example receives up to 0.25 dpa. There are over 100 diagnostic sockets at 13 toroidal positions. Further sensors are welded directly to the vessel wall (e.g. micro-fission chambers, magnetic loops). Most sensors and cables are shielded by the blanket although bolometers and photoelectric sensors view through the 20 mm gap between blanket modules.
Table 1 Summary of Diagnostic Sensors Diagnostic
Location
Fast Response Coils
Vs
Eq
Up
Div
Equilibrium Coils Jointless Equilibrium Coils jXB Sensor Diamagnetic Loop Voltage & Saddle Loops Halo Current Rogowskis Bolometer, core and divertor Photo-electric, UV & X-ray Langmuir probes Micro-fission Chambers Pressure Gauges Thermocouples Strain Gauges
Vs Vw
Eq
Up
Div
Vs Vw Vw
Eq
Vw
Particular radiation effects
Particular design problems
Heating, RIC, radiation RIEMF Heating, RIC, radiation He Heat on gaps Heat on gaps
Fragile, exposed insulator, e-m noise
Required miniaturisation Perfect poloidal plane Position accuracy, ports
He, radiation
Microphony, large cable, many LOS, ECH
Signal drift integration, thermal stability, connector noise Life
Div
Vsv
Eq
Up
Vsv
Eq
Up
Div
nA signals, e-m noise Div
Radiation, REID
250 V supply, debris, erosion
Div Div Div
REID
20 A supply, Cold junction, compensation e-m noise
Vw
Vw
Eq Eq
Up Up
He
Vacuum Vessel Sensors: Vw, on wall; Vs, in sockets; Vsw, in sockets viewing the plasma;.Eq, Equatorial port plug sensors; Up, Upper port plug sensors; Div, Divertor cassette sensors.
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Fig. 3. Sensors on the Side Plate of a Divertor Cassette.
Fig. 1. In-vessel diagnostic sensor positions and signal transmission routes.
5. Transmission hardware There will be approximately 150 km of cable in the vessel ( 50 km for vessel mounted sensors, 25 km in the 11 upper port plugs, 15 km in the 7 equatorial port plugs, 8 km in diagnostic cassettes and 21 km in divertor ports). Cabling is predominantly of Mineral Insulated (MI) cable, with copper core, alumina insulant and stainless steel sheathing. This can been supplied in lengths up to 350 m. For special applications, free wires with a glass fibre insulant and braided in stainless steel armour are used, sometimes configured as multi-quad-cored cable. The
Fig. 2. A typical equatorial port plug showing the structure with shielded diagnostic modules removed for replacement.
specific problem of microphony in the bolometer wiring is solved by crimping the cabling loom. There are several designs of leak-tight MI termination available. The one chosen is particularly suited for feeding directly into connectors, minimising the number of cable joints. All wires are twisted in pairs to cancel inductive pick-up. The possibility of using MI cable with twisted-pair cores is being investigated. The sheath of the cable is grounded, deliberately and frequently, by the loom casing, to guard against spurious voltages that can induce destructively large currents. The in-vessel sensor cabling is marshalled in specially constructed cable looms and conduits. Cables, loomed within an outer jacket, are laid in a poloidal conduit, up to 12.5 m long, running behind the blanket modules. The loom casing is welded to the conduit to increase the thermal contact to the cooled vessel. These cables are
Fig. 4. Vacuum Vessel Socket and Diagnostic Plug (160 mm diameter). The RH features of the plug are removed to show the assembly of (1) a fast pick-up coil, (2) 5 soft X-ray tdetectors, (3) an equilibrium coil and (4) the electrical plug.
C.I. Walker et al. / Fusion Engineering and Design 56–57 (2001) 883–888
brought out along the upper port to feedthroughs. Some toroidal wiring is required, such as voltage and saddle loops. Special connectors are designed for these that allow the reinstatement after the replacement of a vessel sector. Port plugs and interspace flanges carry electrical wiring looms and feedthroughs. Connectors 6ersus Hard-wiring — Failures have been recorded on existing machines in the regions of cable joints, either in the main core, in the termination insulation or in the contact. Most are seen where there are high disruption-induced voltages. Sliding or micro-welded contact resistance varies during disruptions, which affects continuity. It has been noted that where contacts have been purposely welded these failures have been effectively eliminated. Currents induced in the sheaths of coils and wiring can result in forces and movement during disruptions. At joints, simple crimped connections are suspect because thermal cycling relaxes the crimp and irradiation can aggravate this. Cable crimps will thus be augmented by welding and there will be a minimum number of connections. Electromagnetic pick-up is a general failure seen in many initial diagnostic installations. It is solved by strict attention to the design of the connector body, screen and grounding. In order to prevent loss or interruption of continuity, a fused connector located near the sensor, with contact blades ‘resistance’ welded in-situ, is particularly important in the magnetic pick-up circuits. This type of connector has been used successfully on JET. A similar connector has been designed for ITER that is breakable without jeopardising the remaining wiring, allowing at least one replacement. There is also a case for some continuously wired, jointless, magnetic pickup coils. These increase signal reliability at the expense of maintainability. An MI cable, 90 m long, is made into a coil and twisted lead with the first connector well outside the vacuum vessel. This is installed early in the assembly of the vacuum vessel, by-passing all removable vacuum flanges. A mixture of transmission systems, with and without connectors, is kept to guard against common mode failures in coils in any one sector or coils of any particular construction.
887
Multiple Connector Types— Regardless of the above arguments, connectors will be unavoidable in ITER for certain applications. At the in-vessel sockets there is a need for connecting up to 24 cables which are in a relatively exposed position. At the vacuum feedthroughs there is a requirement to marshal up to 120 wires at one, protected, position. In the divertor, 300 contacts must be made, reasonably well protected but within a confined space. It therefore is necessary to develop different connector principles in order to be able to optimise the connector at any particular location. So far the pinned and the edge contact connectors have been studied. Pinned connectors, with nickel or steel MI terminations pins and roll-spring Chromel sockets (ITT type), as used successfully by JET in their divertor wiring, have been taken as the reference for the ITER multi-way connectors. Helical spring loaded contacts are also considered and undergoing radiation testing. Standardised, selfguiding, RH plugs have been designed with 12, 24, 80 or 120 connections. Edge connectors for the divertor cassette are geometrically similar to standard circuit board connections and made of ITER acceptable materials. The size is kept to a minimum (300 contacts within a 200 mm cube) because of space restrictions and heating considerations. Designs use double-sided blades of stainless steel, plasma sprayed with insulator and electro-deposited or metallized with 80 contact conductors. In the plug housing there are up to 6 blades, each flexibly mounted and provided with self-alignment features. The method of locking the contacts, attaching and restraining the wires and the impact of this on flexibility and installation remain to be designed. The sockets of this connector incorporate the more fragile sprung contacts and these will be mounted on the cassette. The plug, considered to be relatively robust, is attached to the port cabling. Electrical feedthroughs— All in-vessel signals require the transmission through the two vacuum boundaries. Approximately 180 feedthroughs (120-way) are needed on each. ITER electrical feedthroughs are made of MI cables, each with individual vacuum-tight terminations, vacuum
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brazed into a stainless steel bulkhead, giving a high cable density (over 40 cores in 35mm diameter). The required leak rate of each feedthrough B 5.0× 10 − 11 Pa (He) m3/s and all other operational requirements are met by this design. The feedthrough can be baked to 350 °C, easily withstands a differential pressure of \ 0.5 MPa, is sufficiently robust for harsher than normal handling, and requires no maintenance. The voltage and current rating can be chosen from the range of MI cable available. At installation, the feedthrough assemblies are welded into tubes on the main vacuum flanges. The terminations are marshalled in the same way as the pins of an electrical connector.
6. Conclusion Designs exist for the integration of in-vessel diagnostic sensors and their transmission lines in all regions of ITER. Past experience has been heeded and designs have been adapted for the particular arrangements and conditions in ITER. Several components, such as MI cable, bolometer substrates and electrical connectors, are under radiation testing [8]. More design work and R&D is required on the details of in-vessel connectors, particularly on multi-pin coaxial connectors and modular wiring looms, and on the installation strategy where it affects the details of the signal transmission elements and ITER components.
Acknowledgements The authors would like to acknowledge the work
on the designs of many of the elements of the in-vessel sensor integration by the RF Diagnostic Dedicated Design Group at the Kurchatov Institute, Moscow, under the direction of V. Zaveraiev. This report is an account of work undertaken within the framework of the ITER EDA Agreement. The views and opinions expressed herein do not necessarily reflect those of the Parties to the ITER EDA Agreement, the IAEA or any agency thereof. Dissemination of the information in these papers is governed by the applicable terms of the ITER EDA Agreement.
References [1] G. Janeschitz, C. Walker, et al., Integration of Diagnostics into the ITER machine, 17th IAEA, Fusion Energy Conference, Yokohama, Japan. October 1998. [2] A.E. Costley, et al., Overview of the ITER Diagnostic System, Diagnostics for Experimental Thermonuclear Fusion Reactors 2, Plenum Press, New York, 1998, p. 41 et seq. [3] K. Ebisawa et al., Plasma Diagnostics for ITER-FEAT, 13th Top. Conf. on High Temperature Plasma Diagnostics, Tucson, June 2000, to be published in Review of Scientific Instruments. [4] C.I.Walker et al., Nuclear Aspects of Diagnostics in RTO/ RC ITER, ISFNT Rome (1999). [5] Yamamoto, S. et al., Irradiation Tests on ITER Diagnostic Components, Ref. 2, p. 269. [6] V.M. Chernov et al., Investigation of RIEMF Nature in Magnetic Sensors for ITER, E-06 this Symposium. [7] L. deKock, et al., Design of the Magnetic Diagnostic for ITER, Rev. Scient. Instr. 70 (1999) 452. [8] T. Nishitani et al., Neutron Irradiation Tests on Diagnostics Components at JAERI, E-05 this Symposium.
Integration of in-vessel diagnostic sensors in ITER C.I. Walker a,*, A. Costley b, L. DeKock a, K. Ebisawa b, G. Janeschitz a, A. Malaquias a, M. Yamada a, G. Vayakis b, S. Yamamoto a b
a ITER Joint Central Team, Boltzmannstr 2, 85748, Garching, Germany ITER Joint Central Team, Naka-machi, Naka-gun, Ibaraki-ken 311 -0193, Japan
Abstract Integration of diagnostic electrical sensors and their signal transmission in ITER is a combination of the process of physical location within a limited space and complex topology, while accounting for effects on the transmitted signals from the particular local nuclear environment [1]. All components of the signal transmission chain are important. Diagnostic sensors are seen in all parts of the ITER vacuum vessel and transmission lines must cross many functional zones, notably the blanket and the vacuum/pressure boundaries. Effects on the signal emanate from conductor variations (radiation and thermally induced conductivity drift, thermo-electric effects, e-m loading, contact resistance), insulator variations (reduction of voltage stand-off, resistivity degradation, material deposition) and interference (e-m noise, microphony, current and voltage leakage). The particular nuclear aspects to be addressed are the high first wall photon flux, the high fast neutron and gamma fluxes, and high neutron fluence. Designs have been developed for the integration of sensors and their connecting wiring within the vacuum vessel, shielding blanket, divertor cassettes and port structures. These are presented here, considering first the radiation effects, then the integration of the diagnostic sensors, the transmission routes and finally the transmission hardware. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Diagnostic; Transmission; ITER; Gamma fluxes
1. Introduction It is essential for the diagnostic system, and ultimately for the control of the plasma, to optimise the design of the position, support, cooling, electrical connection and insulation of diagnostic sensors and their cables. Many of the design problems (e.g. magnetic noise interference, baked
* Corresponding author. Tel.: +49-89-3299-4408; fax: + 49-89-3299-4165. E-mail address: [email protected] (C.I. Walker).
vacuum compatibility) are common to diagnostic systems in all plasma devices. Others, such as remote handling, gamma noise and remote signal processing, have already been addressed in the last generation of Tokamaks with moderate neutron fluxes (JET, TFTR). For ITER the increased hostility of the radiation environment (neutron wall loading up to 1 MW/m2, fluence 0.5 MWa/ m2) affects diagnostic performance, lower leakage resistance, raised conductor resistivity, increased noise, longer integration of this noise, nuclear heating, variable spring rates, etc. and have to be considered.
0920-3796/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 0 - 3 7 9 6 ( 0 1 ) 0 0 3 7 1 - 4
884
C.I. Walker et al. / Fusion Engineering and Design 56–57 (2001) 883–888
Although the number of diagnostic systems ( 50) is similar to most previous devices [2], the quantity of sensing elements has increased because of the increased plasma size (k95 a= 3.4m) while spatial resolution requirements have remained fixed. The need for the diagnostics to perform a plasma control role demands a higher signal reliability and repeatability pulse to pulse [3]. The integrity of the vacuum boundaries has become more important now they are also confinement barriers, for loss of tritiated dust with activated corrosion products and of toxic beryllium and tungsten dust. The requirement for remote replacement and maintenance by tele-manipulation introduces additional connections in transmission systems [4]. 2. Radiation effects on sensors and transmission elements Radiation effects on diagnostic materials are most serious near the plasma [5]. RIED (radiation induced electrical degradation) give a problem of voltage stand-off for a few, high-voltage diagnostics. RIC (radiation induced conductivity) is a serious problem in insulators, inducing signal leakage and interference pick-up. Near the first wall, many insulator materials are no longer suitable The preferred electrical insulator is alumina (Al2O3) with a resistivity reduced, by RIC, to within an order of magnitude of the required level. RIEMF (radiation induced emf) [6] gives concern about the stability of any low level signals that are integrated over the length of the discharge. Nuclear heating increases temperatures, and thus resistivity, during the pulse, and mechanical radiation damage affects mechanical spring rates and possibly contact resistance. Helium production, leading to swelling, affects thin substrate sensors (such as strain gauges and bolometers). 3. In-vessel sensors The diagnostics with in-vessel sensors that are considered here are shown in Table 1, with some of their particular attributes and problems that are discussed later.
Magnetic pick-up coils [7] on the vacuum vessel wall will withstand 105 discharges. With integration times well over 1000 s, spurious e-m noise pick-up and the generation of a RIEMF are design problems for the equilibrium coils. Redundancy in this diagnostic is given by the incorporation of extra poloidal sets of coils and by sets of hard-wired or jointless coils. Toroidal flux loops and saddles are attached to the vessel behind the blanket. The micro-fission chambers, similar to those used for flux measurements inside fission power reactors, are mounted on the vacuum vessel, behind blanket modules. The bolometry system requires 340 lines of sight to acquire tomographic images. 70 minicameras, each providing several lines of sight, are located in port plugs on the vessel wall and on divertor cassettes. Because of the dimensional stability of substrate and thin film circuits bolometers require replacement after 10,000 discharges. The capacitive pick-up (microphony) of the many cables (23 cores, 5 screens and outer earth, 35 mm diameter) is a particular problem. Many Langmuir probes are located on divertor cassette side plates. They are small (10× 10×50 mm), electrically insulated, CFC electrodes protruding into the plasma and are thus exposed to high heat fluxes, particle damage, and dust. They run at up to 250 V and are susceptible to RIED which occurs at electrical field strengths exceeding 70 V/mm. Hot cathode ionisation gauges measure neutral pressure in the divertor cassette. The restraint of the cable for the high current filament (520 A) requires special attention. Photo-electric detectors, for UV and soft Xrays, in ports and on the vessel wall, are only conceptually designed at present. Noise in these systems that produce only nA signals is a significant problem to be solved. There are to be a number of engineering sensors, such as thermocouples, limit switches and strain gauges. These do not need to see the plasma and are well shielded (10 − 2 wall loading), but they must be bakeable, operate at high and varying temperature and, receive a large integrated radiation damage (B0.1 dpa), retain their spring rate etc.
C.I. Walker et al. / Fusion Engineering and Design 56–57 (2001) 883–888
4. Sensor location Removable structures in ITER are the preferred sites for diagnostic sensors which are fitted in modules in 11 upper port plugs, 7 equatorial port plugs and 15 divertor cassettes. (Fig. 1). Sensors are also required on the vacuum vessel wall. During the remote maintenance of diagnostic modules in these structures, sensor wiring is made with a demountable connectors, typically with 500 cable cores, fed by 5 vacuum feedthroughs (Fig. 2). In the divertor, 10– 20 mm thick side-plates are bolted to some cassettes. These are made complete with pre-wired sensors and wiring loom, allowing relatively simple replacement of diagnostic components (Fig. 3). Sometimes heavy wiring demands are seen, for instance where Bolometers and Langmuir probes are installed ( 300 cores). Signals are marshalled at the elec-
885
trical connector on the outboard end of the cassette. The side plate can incorporate water cooling, but the connector, with 0.04 W/cm3, is passively cooled. For diagnostics on the vacuum vessel wall, locations are provided as integral components of the vessel inner wall and, outboard, on top of the blanket manifold. Several sensors can be mounted in small (160 mm diameter) diagnostic plugs (Fig. 4) which receive 0.8 W/cc and are conductioncooled. Remote replacement is necessary because of radiation damage, copper, for example receives up to 0.25 dpa. There are over 100 diagnostic sockets at 13 toroidal positions. Further sensors are welded directly to the vessel wall (e.g. micro-fission chambers, magnetic loops). Most sensors and cables are shielded by the blanket although bolometers and photoelectric sensors view through the 20 mm gap between blanket modules.
Table 1 Summary of Diagnostic Sensors Diagnostic
Location
Fast Response Coils
Vs
Eq
Up
Div
Equilibrium Coils Jointless Equilibrium Coils jXB Sensor Diamagnetic Loop Voltage & Saddle Loops Halo Current Rogowskis Bolometer, core and divertor Photo-electric, UV & X-ray Langmuir probes Micro-fission Chambers Pressure Gauges Thermocouples Strain Gauges
Vs Vw
Eq
Up
Div
Vs Vw Vw
Eq
Vw
Particular radiation effects
Particular design problems
Heating, RIC, radiation RIEMF Heating, RIC, radiation He Heat on gaps Heat on gaps
Fragile, exposed insulator, e-m noise
Required miniaturisation Perfect poloidal plane Position accuracy, ports
He, radiation
Microphony, large cable, many LOS, ECH
Signal drift integration, thermal stability, connector noise Life
Div
Vsv
Eq
Up
Vsv
Eq
Up
Div
nA signals, e-m noise Div
Radiation, REID
250 V supply, debris, erosion
Div Div Div
REID
20 A supply, Cold junction, compensation e-m noise
Vw
Vw
Eq Eq
Up Up
He
Vacuum Vessel Sensors: Vw, on wall; Vs, in sockets; Vsw, in sockets viewing the plasma;.Eq, Equatorial port plug sensors; Up, Upper port plug sensors; Div, Divertor cassette sensors.
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C.I. Walker et al. / Fusion Engineering and Design 56–57 (2001) 883–888
Fig. 3. Sensors on the Side Plate of a Divertor Cassette.
Fig. 1. In-vessel diagnostic sensor positions and signal transmission routes.
5. Transmission hardware There will be approximately 150 km of cable in the vessel ( 50 km for vessel mounted sensors, 25 km in the 11 upper port plugs, 15 km in the 7 equatorial port plugs, 8 km in diagnostic cassettes and 21 km in divertor ports). Cabling is predominantly of Mineral Insulated (MI) cable, with copper core, alumina insulant and stainless steel sheathing. This can been supplied in lengths up to 350 m. For special applications, free wires with a glass fibre insulant and braided in stainless steel armour are used, sometimes configured as multi-quad-cored cable. The
Fig. 2. A typical equatorial port plug showing the structure with shielded diagnostic modules removed for replacement.
specific problem of microphony in the bolometer wiring is solved by crimping the cabling loom. There are several designs of leak-tight MI termination available. The one chosen is particularly suited for feeding directly into connectors, minimising the number of cable joints. All wires are twisted in pairs to cancel inductive pick-up. The possibility of using MI cable with twisted-pair cores is being investigated. The sheath of the cable is grounded, deliberately and frequently, by the loom casing, to guard against spurious voltages that can induce destructively large currents. The in-vessel sensor cabling is marshalled in specially constructed cable looms and conduits. Cables, loomed within an outer jacket, are laid in a poloidal conduit, up to 12.5 m long, running behind the blanket modules. The loom casing is welded to the conduit to increase the thermal contact to the cooled vessel. These cables are
Fig. 4. Vacuum Vessel Socket and Diagnostic Plug (160 mm diameter). The RH features of the plug are removed to show the assembly of (1) a fast pick-up coil, (2) 5 soft X-ray tdetectors, (3) an equilibrium coil and (4) the electrical plug.
C.I. Walker et al. / Fusion Engineering and Design 56–57 (2001) 883–888
brought out along the upper port to feedthroughs. Some toroidal wiring is required, such as voltage and saddle loops. Special connectors are designed for these that allow the reinstatement after the replacement of a vessel sector. Port plugs and interspace flanges carry electrical wiring looms and feedthroughs. Connectors 6ersus Hard-wiring — Failures have been recorded on existing machines in the regions of cable joints, either in the main core, in the termination insulation or in the contact. Most are seen where there are high disruption-induced voltages. Sliding or micro-welded contact resistance varies during disruptions, which affects continuity. It has been noted that where contacts have been purposely welded these failures have been effectively eliminated. Currents induced in the sheaths of coils and wiring can result in forces and movement during disruptions. At joints, simple crimped connections are suspect because thermal cycling relaxes the crimp and irradiation can aggravate this. Cable crimps will thus be augmented by welding and there will be a minimum number of connections. Electromagnetic pick-up is a general failure seen in many initial diagnostic installations. It is solved by strict attention to the design of the connector body, screen and grounding. In order to prevent loss or interruption of continuity, a fused connector located near the sensor, with contact blades ‘resistance’ welded in-situ, is particularly important in the magnetic pick-up circuits. This type of connector has been used successfully on JET. A similar connector has been designed for ITER that is breakable without jeopardising the remaining wiring, allowing at least one replacement. There is also a case for some continuously wired, jointless, magnetic pickup coils. These increase signal reliability at the expense of maintainability. An MI cable, 90 m long, is made into a coil and twisted lead with the first connector well outside the vacuum vessel. This is installed early in the assembly of the vacuum vessel, by-passing all removable vacuum flanges. A mixture of transmission systems, with and without connectors, is kept to guard against common mode failures in coils in any one sector or coils of any particular construction.
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Multiple Connector Types— Regardless of the above arguments, connectors will be unavoidable in ITER for certain applications. At the in-vessel sockets there is a need for connecting up to 24 cables which are in a relatively exposed position. At the vacuum feedthroughs there is a requirement to marshal up to 120 wires at one, protected, position. In the divertor, 300 contacts must be made, reasonably well protected but within a confined space. It therefore is necessary to develop different connector principles in order to be able to optimise the connector at any particular location. So far the pinned and the edge contact connectors have been studied. Pinned connectors, with nickel or steel MI terminations pins and roll-spring Chromel sockets (ITT type), as used successfully by JET in their divertor wiring, have been taken as the reference for the ITER multi-way connectors. Helical spring loaded contacts are also considered and undergoing radiation testing. Standardised, selfguiding, RH plugs have been designed with 12, 24, 80 or 120 connections. Edge connectors for the divertor cassette are geometrically similar to standard circuit board connections and made of ITER acceptable materials. The size is kept to a minimum (300 contacts within a 200 mm cube) because of space restrictions and heating considerations. Designs use double-sided blades of stainless steel, plasma sprayed with insulator and electro-deposited or metallized with 80 contact conductors. In the plug housing there are up to 6 blades, each flexibly mounted and provided with self-alignment features. The method of locking the contacts, attaching and restraining the wires and the impact of this on flexibility and installation remain to be designed. The sockets of this connector incorporate the more fragile sprung contacts and these will be mounted on the cassette. The plug, considered to be relatively robust, is attached to the port cabling. Electrical feedthroughs— All in-vessel signals require the transmission through the two vacuum boundaries. Approximately 180 feedthroughs (120-way) are needed on each. ITER electrical feedthroughs are made of MI cables, each with individual vacuum-tight terminations, vacuum
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brazed into a stainless steel bulkhead, giving a high cable density (over 40 cores in 35mm diameter). The required leak rate of each feedthrough B 5.0× 10 − 11 Pa (He) m3/s and all other operational requirements are met by this design. The feedthrough can be baked to 350 °C, easily withstands a differential pressure of \ 0.5 MPa, is sufficiently robust for harsher than normal handling, and requires no maintenance. The voltage and current rating can be chosen from the range of MI cable available. At installation, the feedthrough assemblies are welded into tubes on the main vacuum flanges. The terminations are marshalled in the same way as the pins of an electrical connector.
6. Conclusion Designs exist for the integration of in-vessel diagnostic sensors and their transmission lines in all regions of ITER. Past experience has been heeded and designs have been adapted for the particular arrangements and conditions in ITER. Several components, such as MI cable, bolometer substrates and electrical connectors, are under radiation testing [8]. More design work and R&D is required on the details of in-vessel connectors, particularly on multi-pin coaxial connectors and modular wiring looms, and on the installation strategy where it affects the details of the signal transmission elements and ITER components.
Acknowledgements The authors would like to acknowledge the work
on the designs of many of the elements of the in-vessel sensor integration by the RF Diagnostic Dedicated Design Group at the Kurchatov Institute, Moscow, under the direction of V. Zaveraiev. This report is an account of work undertaken within the framework of the ITER EDA Agreement. The views and opinions expressed herein do not necessarily reflect those of the Parties to the ITER EDA Agreement, the IAEA or any agency thereof. Dissemination of the information in these papers is governed by the applicable terms of the ITER EDA Agreement.
References [1] G. Janeschitz, C. Walker, et al., Integration of Diagnostics into the ITER machine, 17th IAEA, Fusion Energy Conference, Yokohama, Japan. October 1998. [2] A.E. Costley, et al., Overview of the ITER Diagnostic System, Diagnostics for Experimental Thermonuclear Fusion Reactors 2, Plenum Press, New York, 1998, p. 41 et seq. [3] K. Ebisawa et al., Plasma Diagnostics for ITER-FEAT, 13th Top. Conf. on High Temperature Plasma Diagnostics, Tucson, June 2000, to be published in Review of Scientific Instruments. [4] C.I.Walker et al., Nuclear Aspects of Diagnostics in RTO/ RC ITER, ISFNT Rome (1999). [5] Yamamoto, S. et al., Irradiation Tests on ITER Diagnostic Components, Ref. 2, p. 269. [6] V.M. Chernov et al., Investigation of RIEMF Nature in Magnetic Sensors for ITER, E-06 this Symposium. [7] L. deKock, et al., Design of the Magnetic Diagnostic for ITER, Rev. Scient. Instr. 70 (1999) 452. [8] T. Nishitani et al., Neutron Irradiation Tests on Diagnostics Components at JAERI, E-05 this Symposium.