- Email: [email protected]
Re-design of ITER Glow Discharge Cleaning system based on a fixed electrode concept
G Model
ARTICLE IN PRESS
FUSION-7281; No. of Pages 5
Fusion Engineering and Design xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Re-design of ITER Glow Discharge Cleaning system based on a fixed electrode concept Y. Yang a,∗ , S. Maruyama a , G. Kiss a , M. O’Connor a , Y. Zhang b , R.A. Pitts a , M. Shimada a , T. Fang c , Y. Wang b , M. Wang b , Y. Pan b , B. Li b , L. Li b a
ITER Organization, Route de Vinon sur Verdon, 13115 St Paul Lez Durance, France Southwestern Institute of Physics, P.O. Box 432, Chengdu, Sichuan 610041, PR China c ITER China Domestic Agency, Ministry of Science and Technology, PR China b
h i g h l i g h t s • • • • •
This paper summarizes the approved new design of ITER GDC. It is based on the fixed electrode design instead of the previous movable concept. Estimates were made on the glow current density. R&D topics on initiation, steady state and heat load were presented. Other relevant considerations were listed in an exhaustive manner.
a r t i c l e
i n f o
Article history: Received 6 September 2013 Received in revised form 30 January 2014 Accepted 4 February 2014 Available online xxx Keywords: ITER Glow Discharge Cleaning Fixed anode Concept design
a b s t r a c t A new design of ITER Glow Discharge Cleaning (GDC) system based on a fixed electrode concept replaces the previous design which was based on a movable electrode integrated with the ITER In-Vessel-ViewingSystem. Recently the conceptual design of the GDC system was reviewed successfully on the functions, safety, operation and maintenance. The design proposed was checked against the requirements and found to be feasible. This paper gives an overall description of the requirements from physics and operation viewpoints and introduces the design at the conceptual level. Main R&D activities are listed and summarized. Further detailed studies are to be performed in the following design stage. © 2014 Published by Elsevier B.V.
1. Introduction ITER needs a reliable wall conditioning method to obtain a clean first wall for plasma breakdown after a venting. Direct Current (DC) Glow Discharge Cleaning (GDC) is chosen as a baseline wall conditioning technique to reduce and control impurity and hydrogenic fuel out-gassing from plasma-facing components. Previously the design was based on a movable electrode integrated with the ITER In-Vessel-Viewing-System (IVVS) because it was expected that GDC would be needed during DT full power operation to close the penetrations for IVVS [1,2]. This design required flexible section in the cooling circuit to accommodate the moving of
∗ Corresponding author. E-mail address: [email protected] (Y. Yang).
the electrodes among different working positions. The only choice meeting the requirements from the limited available space and vacuum viewpoints is metal hose. However, the risk analysis showed that there would be high risk of water leaking due to its moving at the minimum dynamic bending radius of the pipe (limited by the space) and the consequence of water leaking would be severe for ITER. This design was discarded. A basic model including the updated designs of the GDC, blankets, divertor and vacuum vessel (VV) is used for a nuclear analysis [3] on the displacements per atom in steel and helium concentration in steel in vicinity of the electrode path and the nuclear heat on the components on the divertor cooling pipes and superconducting coils. It is concluded that the nuclear shielding of the electrodes is not mandatory for the components behind. Besides, estimation is made on the extra thermal load on the components behind without the GDC electrode acting as shield against the thermal radiation
http://dx.doi.org/10.1016/j.fusengdes.2014.02.021 0920-3796/© 2014 Published by Elsevier B.V.
Please cite this article in press as: Y. Yang, et al., Re-design of ITER Glow Discharge Cleaning system based on a fixed electrode concept, Fusion Eng. Des. (2014), http://dx.doi.org/10.1016/j.fusengdes.2014.02.021
G Model FUSION-7281; No. of Pages 5
ARTICLE IN PRESS
2
Y. Yang et al. / Fusion Engineering and Design xxx (2014) xxx–xxx
Fig. 1. Bird-view of the GDC layout in the upper (left) and equatorial (right) port levels. GDC positions are highlighted. (Note: in-vacuum first wall surface is partially shown).
from the plasma. It turns out to be very low. For example, a rough estimate for the divertor cooling pipe shows an increase of a few hundred watts [4]. Consequently, GDC is decoupled from the IVVS and is based on a fixed electrode concept. For maintainability reasons each GDC anode will be integrated in port plugs at the upper or equatorial port level and the GDC front surface will be flush with the plasma facing surface of the port plug. Having taken into account all available locations, GDC layout is decided to provide toroidal uniform coverage to the maximum extent possible in both layers. There will be three GDC electrodes in the upper port level (in ports 3, 8 and 14, as shown in the left side of Fig. 1) and four GDC electrodes in the equatorial port level (in ports 3, 8, 12 and 17, as shown in the right side of Fig. 1). Because of the staged installation strategy of ITER most port plugs, if not all, will not be in position for ITER’s First Plasma (FP) operation. In this stage, Temporary Electrodes (TE) with a simple design will be used and fixed on the VV near those ports allocated for GDC. The electrodes integrated in port plugs are called Permanent Electrodes (PE). For the FP operation, the plasma facing surface is much bigger because the blankets and divertor will not be installed. Therefore, besides the aforementioned seven electrodes at the equatorial and upper port levels, there will be three additional TEs in the lower port level, near the ports 3, 9 and 15. 2. System requirements According to the ITER Project Requirements, a high level technical baseline document, the ITER GDC system shall provide the capability to perform GDC with an average first-wall current density greater than 0.1 Am−2 indefinitely with the VV and all in-vessel components at their nominal pre-pulse operating temperatures and at their nominal baking temperature. 2.1. Glow current and current density Deriving from the surface to be cleaned and the number of electrodes, the glow current of one PE and one TE is set to be 30 A and 40 A, respectively, at maximum. It is an empirical scaling result based on the published data of the tokamak devices (Table 1) [5]. To simplify, it is assumed that the glow current density has a uniform spatial distribution around the anode. Geometrically, it’s assumed that each of the seven PEs is located in the center of its supporting plug first wall (FW) in this study.
For the FW on the high field side (numbered 2–6 in the poloidal direction in the right of Fig. 2), when only the glow current from the nearest electrode is taken into account, the glow current density of a tile could be estimated as I/2/l2 , in which I is the glow current from one anode and l is the distance from this tile to the nearest anode. With 30 A on each PE, the glow plasma current density on the high field side tiles ranges from 0.1 to about 0.8 Am−2 (Fig. 3). In the same way, the glow plasma current density on the high field side VV (surface to be covered by aforementioned tiles) is estimated. With 40 A on each TE, it ranges from about 0.15 to about 0.72 Am−2 (Fig. 4). These estimates show that the glow current density would meet the project requirement.
2.2. GDC operation GDC system operation will be performed prior to plasma operation and before venting the VV. For DC GDC the residual magnetic field near the ITER first wall needs to be less than 1 mT during GDC, which is confirmed by the recent simulation [6]. For starting GDC, there is a concern that the partial pressure of water inside the VV could be too high after venting. Therefore, baking should be performed adequately before starting GDC until the partial pressure of water decreases to a certain threshold. During the GDC process full and partial pressure monitoring could be used for the evaluation of the GDC efficiency as the reference for the GDC operation. Table 2 summarizes the main operational parameters of GDC. The breakdown and steady state parameters are empirical data based on tokamak device publications [5]. Operation parameters will be tested and explored during the real ITER operation.
Table 1 GDC electrode layout and glow current. Anode locations for the FP phase
Anode locations after the FP phase Total current in FP phase Total current after FP phase Max. current for each PE Max. current for each TE
Upper ports 3, 8, 14 Equatorial ports 3, 8, 12, 17 Lower ports 3, 9 and 15 Upper ports 3, 8, 14 Equatorial ports 3, 8, 12, 17 ∼400 A ∼200 A 30 A 40 A
Please cite this article in press as: Y. Yang, et al., Re-design of ITER Glow Discharge Cleaning system based on a fixed electrode concept, Fusion Eng. Des. (2014), http://dx.doi.org/10.1016/j.fusengdes.2014.02.021
G Model FUSION-7281; No. of Pages 5
ARTICLE IN PRESS Y. Yang et al. / Fusion Engineering and Design xxx (2014) xxx–xxx
3
Fig. 2. Plane view of tokamak (on the left) and the poloidal cross-section (on the right). On the right picture Be tiles are numbered from 1 to 18 and divertor parts are labeled I, D and O.
Fig. 3. Estimated toroidal distribution of glow current density on the tiles in the high field side with PEs.
3. Anode design and integration The design is still at conceptual design stage, in which the essential parameters are proposed, feasibility is studied and important topics, such as electro-magnetic (EM) load and heat load are raised. 3.1. Environmental conditions Situated in flush with the diagnostic port plug first wall, GDC electrodes are exposed to very harsh environmental conditions, Table 2 Main GDC operation parameters. Gas species
H2 , D2 or He
Max. environmental temperature in FP phasea Max. environmental temperature after the FP phaseb Breakdown pressure Breakdown voltage Steady state glow pressure Maximum gas flow-ratec
∼200 ◦ C ∼240 ◦ C 15 Pa 1.2 kV 0.05–0.5 Pa 50 Pa m3 s−1
a
The VV temperature during baking. The surrounding blanket temperature during water baking. It is expected that GDC will not be carried out during 350 ◦ C gas baking. However, GDC anodes are expected to be at approximately 350 ◦ C during this gas baking. c Restricted by the ITER Torus Cryopump which pumps the torus exhaust during GDC operation. b
Fig. 4. Estimated toroidal distribution of glow current density on the tiles in the high field side with TEs.
such as plasma radiation, nuclear radiation, EM field and radiofrequency (rf). For the EM load, preliminary analysis shows the torque of the present GDC structure is reduced to about one tenth of that of the previous design (movable electrodes at the divertor port level) in the most conservative load case. For the rf load a Faraday shielding is added around the GDC cooling pipes to avoid GDC electrode picking up the electromagnetic radiation during rf heating. Further study on these topics will be performed in the following design stage. The most important concern is the heat load. It is estimated that the heat load on anode during GDC is 10Ie (Ie is the glow current) [5]. During plasma operation the electrode is like a diagnostic port plug first wall. The heat load density due to radiation and charge exchange particles could be 0.35 MW m−2 . The volumetric heat load due to neutron heating is 10 W cm−3 at the front and the decay length is 12 cm. Table 3 summarizes the heat loads based on the conceptual design model of the electrodes.
Table 3 Max. heat load on one anode in different states. TE during GDC PE during GDC PE during DT full power operation
0.4 kW (with 40 A) 0.3 kW (with 30 A) ∼72 kW
Please cite this article in press as: Y. Yang, et al., Re-design of ITER Glow Discharge Cleaning system based on a fixed electrode concept, Fusion Eng. Des. (2014), http://dx.doi.org/10.1016/j.fusengdes.2014.02.021
G Model FUSION-7281; No. of Pages 5
ARTICLE IN PRESS
4
Y. Yang et al. / Fusion Engineering and Design xxx (2014) xxx–xxx
Fig. 5. GDC PE unit (left), cross-section view (middle) and front view (right).
Consequently, there will be water cooling after the FP phase but not in the FP phase.
3.2. Permanent electrode As shown in the left picture of Fig. 5, the main part of a PE consists of a head, a body and cooling tubes. The head part is embedded in the diagnostic port plug first wall (right picture of Fig. 5). The body penetrates the shielding block but electrically insulated from it (middle picture of Fig. 5). The cooling tubes which are also insulated are connected to the cooling manifolds outside the torus vacuum. The front surface of the head, effective GDC anode surface is extrapolated from the JET experiences. The wall area of JET is about 200 m2 and its total anode area is 0.1 m2 . The total first wall surface of ITER is about 1000 m2 , and the total PE anode area needs to be 0.5 m2 correspondingly. For each PE, the front surface is proposed to be approximately 0.07 m2 . The length of the head and the dimensions of the body depend strongly on the diagnostic systems around. They need to compromise among the weight, volume for cooling channel and the structural integrity. Another importance factor is maintainability. PE will be made of stainless steel, same as the diagnostic FW.
3.3. Temporary electrode A TE is basically a metal block fixed on the VV, but electrically insulated. TE has no active cooling. For the FP phase, corresponding to about twice as much first wall surface, the total anode surface should be 1 m2 . Therefore, each of the ten anodes should have a front surface area of approximately 0.1 m2 . TE will be made of stainless steel as well.
4. Main R&D in the conceptual design stage There are two major different features of the ITER PEs comparing with other tokamaks. Firstly, the front surface of PEs is flush with the port plug FW, which is about 100 mm recessed from the plasma comparing with the blanket first wall. Secondly, the side of the anode head is exposed to the diagnostic first wall at the ground potential just about 20 mm away. The main subjects of the R&D are breakdown, steady state operation and heat load during GDC. Southwest Institute of Physics (SWIP) and IRFM (Institut de Recherche sur la Fusion Magnétique) carry out active studies on these topics [7,8].
4.1. Breakdown Different scenarios and environmental geometries are tested on the test-beds, and it is shown that the breakdown voltage could be much higher than 1.2 kV (2.4 kV for IRFM; and 2.7 kV for SWIP). Both test-beds show that it is possible to initiate glow with an anode recessed from the surrounding surface, although at the higher breakdown voltage (for the same pressure) than the case for an anode in free space. IRFM experiments show that breakdown is possible even when the anode is located 60 mm behind the surrounding surface. Increasing pressure is helpful for having a lower breakdown voltage. The maximum voltage is approximately 2.4 kV. SWIP group instead applied the anode voltage to the set value first and then raise the pressure to breakdown. When breakdown voltage is about 1.9–2.2 kV, its over-current protection threshold, 40 A, is triggered frequently. Therefore, the maximum breakdown voltage in SWIP results is 1.9 kV. Despite that, glow could be initiated even when the anode is located 100 mm behind the neighboring surface. In JET it is demonstrated that it is feasible to run the multianodes in a staggered operation without increasing the torus pressure to the normal breakdown pressure [9]. These R&D works show that glow can be initiated with an environmental geometry like in the present ITER design. 4.2. Steady state Visual observation is performed on the glow brightness during the adjusting of the pressure in SWIP’s experiments. It is observed that the bright glow light extends away from the anode surface further and with the decrease of pressure. Preliminary results of the IRFM discharge uniformity also shows that the decrease of the gas pressure is favorable for increasing the wall fluxes (from ion saturation current). It should be noted also that hollow cathode discharge is observed in SWIP’s experiments when the pressure is higher than 2–3 Pa. Hollow cathode discharge is also observed in IRFM tests. These suggest that it is preferable to run GDC at a lower pressure for better glow uniformity. The bright glow in the gap is rarely observed when the gap is less than 15 mm in SWIP’s experiments. IRFM results show that there is no parasitic plasma in the gaps at about 30 mm. The R&D work shows the glow can be kept stable and uniformity can be improved by tuning the pressure. 4.3. Heat load during GDC SWIP measures heat load with different gas species, pressure, glow current and electrode locations. The results vary too much to indicate a reliable scaling law. It is noted that the heat load
Please cite this article in press as: Y. Yang, et al., Re-design of ITER Glow Discharge Cleaning system based on a fixed electrode concept, Fusion Eng. Des. (2014), http://dx.doi.org/10.1016/j.fusengdes.2014.02.021
G Model FUSION-7281; No. of Pages 5
ARTICLE IN PRESS Y. Yang et al. / Fusion Engineering and Design xxx (2014) xxx–xxx
measured is often higher than the baseline estimation (10Ie), in the same magnitude or even one magnitude higher. ASDEX upgrade GDC study shows that heat load on electrode during GDC could increase dramatically with the decrease of the pressure [10]. This phenomenon is also observed by IRFM team. These suggest that running GDC at a lower pressure evokes the risk of generating higher heat load on the electrodes. This is especially important for TEs which have no active cooling. Finding the key operating parameter for heat load on the anode and mitigation schemes will be at high priority in the following design stage. If increasing the ratio between the anode surface to the first wall surface could decrease the heat load, this is easy to implement for the TE design. “Pulsed GDC” like on AUG and “staggered operation” like on JET would potentially decrease the heat load. 5. Safety consideration ITER GDC is required to be operational during 250 ◦ C baking. In this condition the water temperature is at 240 ± 10 ◦ C and the water pressure is at 4.4 ± 0.4 MPa (gauge pressure). The boiling point of water at 4.1 MPa is 251.8 ◦ C, very close to the upper limit of the water temperature. In case the water temperature reaches the boiling temperature due to extra heat load, there is a risk that vaporization of the water leads to pressure rise inside the cooling channel jeopardizing the integrity of the cooling circuit. In the conceptual design, each anode has a pair of cooling pipes with approximately one inch in diameter. At a flow rate of 1 kg s−1 , the heat load acceptable for the cooling circuit is about 8 kW per anode, much higher than the baseline estimation of 0.3 kW. Besides, as discussed in Section 4.3, reducing heat load is possible by adjusting working pressure and modifying operation scenarios. The commissioning and operation during FP phase will also accumulate necessary experiences for ITER. As a conclusion, the risk of over-pressurization can be avoided by engineering with sufficient margin and manipulating the operation parameters though further R&D and real ITER GDC operation. 6. Summary This paper describes the re-design of ITER GDC system based on the fixed anode concept. The requirements are summarized and latest design and operation are introduced. The R&D studies are performed based on ITER’s special geometry. The conceptual design is shown to be feasible and detailed studies are to be performed in the following design stage.
5
Disclaimer The views and opinions expressed herein do not necessarily reflect those of the ITER Organization Acknowledgements The authors would like to thank: V. Rohde in Max-PlanckInstitut fuer Plasmaphysik, D. Kogut and D. Douai in Institut de Recherche sur la Fusion Magnétique, E. Polunovskiy and A. Kukushkin in ITER Organization for the constructive support and discussion. This paper was prepared as an account of work by or for the ITER Organization. The Members of the Organization are the People’s Republic of China, the European Atomic Energy Community, Republic of India, Japan, Republic of Korea, the Russian Federation, and the United States of America. The views and opinions expressed herein do not necessarily reflect those of the Members or any agency thereof. Dissemination of the information in this paper is governed by the applicable terms of the ITER Joint Implementation Agreement. References [1] Y. Yang, S. Maruyama, R.A. Pitts, M. Shimada, M. Wang, T. Jiang, et al., System requirements and design challenges of the ITER Glow Discharge Cleaning system, in: 18th International Vacuum Congress, Beijing, PR China, August 23–27, 2010, PST1-O-2. [2] Y. Yang, S. Maruyama, G. Kiss, Y. Pan, M. Wang, T. Jiang, et al., Concept design of the ITER Glow Discharge Cleaning System, in: 13th International Workshop on Plasma-Facing Materials and Components for Fusion Applications and 1st International Conference on Fusion Energy Materials Science, Rosenheim, Germany, May 9–13, 2011, P38A. [3] M.J. Loughlin, P. Batistoni, C. Konno, U. Fischer, H. Iida, L. Petrizzi, et al., ITER nuclear analysis strategy and requirements, Fusion Science and Technology 56 (2009) 566–572. [4] A. Kukushkin, Estimate of radiation power load on GDC electrode in shielding position, Internal communication. [5] M. Shimada, S. Putvinski, R.A. Pitts, Glow discharge cleaning on ITER, in: 38th EPS Conference on Plasma Physics, Strasbourg, France, 27 June–1 July, 2011, P5.060. [6] Y. Gribov, Assessment of residual magnetic fields due to remanent magnetization of Ferromagnetic Inserts, Internal communication. [7] Y. Wang, M. Wang, M. Dan, X. Ren, Y. Pan, D. Wang, et al., Preliminary results of Glow Discharge Cleaning test on SWIP test bench, in: 25th Symposium on Fusion Engineering, San Francisco, CA, USA, June 10–14, 2013, TPO-81. [8] D. Kogut, D. Douai, R.A. Pitts, G. Hagelaar, Assessment of the new ITER GDC system performance, in: 40th EPS Conference on Plasma Physics, Espoo, Finland, 1st–5th July, 2013, P2.113. [9] Personal communication with D. Douai. [10] Personal communication with V. Rohde.
Please cite this article in press as: Y. Yang, et al., Re-design of ITER Glow Discharge Cleaning system based on a fixed electrode concept, Fusion Eng. Des. (2014), http://dx.doi.org/10.1016/j.fusengdes.2014.02.021
ARTICLE IN PRESS
FUSION-7281; No. of Pages 5
Fusion Engineering and Design xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Re-design of ITER Glow Discharge Cleaning system based on a fixed electrode concept Y. Yang a,∗ , S. Maruyama a , G. Kiss a , M. O’Connor a , Y. Zhang b , R.A. Pitts a , M. Shimada a , T. Fang c , Y. Wang b , M. Wang b , Y. Pan b , B. Li b , L. Li b a
ITER Organization, Route de Vinon sur Verdon, 13115 St Paul Lez Durance, France Southwestern Institute of Physics, P.O. Box 432, Chengdu, Sichuan 610041, PR China c ITER China Domestic Agency, Ministry of Science and Technology, PR China b
h i g h l i g h t s • • • • •
This paper summarizes the approved new design of ITER GDC. It is based on the fixed electrode design instead of the previous movable concept. Estimates were made on the glow current density. R&D topics on initiation, steady state and heat load were presented. Other relevant considerations were listed in an exhaustive manner.
a r t i c l e
i n f o
Article history: Received 6 September 2013 Received in revised form 30 January 2014 Accepted 4 February 2014 Available online xxx Keywords: ITER Glow Discharge Cleaning Fixed anode Concept design
a b s t r a c t A new design of ITER Glow Discharge Cleaning (GDC) system based on a fixed electrode concept replaces the previous design which was based on a movable electrode integrated with the ITER In-Vessel-ViewingSystem. Recently the conceptual design of the GDC system was reviewed successfully on the functions, safety, operation and maintenance. The design proposed was checked against the requirements and found to be feasible. This paper gives an overall description of the requirements from physics and operation viewpoints and introduces the design at the conceptual level. Main R&D activities are listed and summarized. Further detailed studies are to be performed in the following design stage. © 2014 Published by Elsevier B.V.
1. Introduction ITER needs a reliable wall conditioning method to obtain a clean first wall for plasma breakdown after a venting. Direct Current (DC) Glow Discharge Cleaning (GDC) is chosen as a baseline wall conditioning technique to reduce and control impurity and hydrogenic fuel out-gassing from plasma-facing components. Previously the design was based on a movable electrode integrated with the ITER In-Vessel-Viewing-System (IVVS) because it was expected that GDC would be needed during DT full power operation to close the penetrations for IVVS [1,2]. This design required flexible section in the cooling circuit to accommodate the moving of
∗ Corresponding author. E-mail address: [email protected] (Y. Yang).
the electrodes among different working positions. The only choice meeting the requirements from the limited available space and vacuum viewpoints is metal hose. However, the risk analysis showed that there would be high risk of water leaking due to its moving at the minimum dynamic bending radius of the pipe (limited by the space) and the consequence of water leaking would be severe for ITER. This design was discarded. A basic model including the updated designs of the GDC, blankets, divertor and vacuum vessel (VV) is used for a nuclear analysis [3] on the displacements per atom in steel and helium concentration in steel in vicinity of the electrode path and the nuclear heat on the components on the divertor cooling pipes and superconducting coils. It is concluded that the nuclear shielding of the electrodes is not mandatory for the components behind. Besides, estimation is made on the extra thermal load on the components behind without the GDC electrode acting as shield against the thermal radiation
http://dx.doi.org/10.1016/j.fusengdes.2014.02.021 0920-3796/© 2014 Published by Elsevier B.V.
Please cite this article in press as: Y. Yang, et al., Re-design of ITER Glow Discharge Cleaning system based on a fixed electrode concept, Fusion Eng. Des. (2014), http://dx.doi.org/10.1016/j.fusengdes.2014.02.021
G Model FUSION-7281; No. of Pages 5
ARTICLE IN PRESS
2
Y. Yang et al. / Fusion Engineering and Design xxx (2014) xxx–xxx
Fig. 1. Bird-view of the GDC layout in the upper (left) and equatorial (right) port levels. GDC positions are highlighted. (Note: in-vacuum first wall surface is partially shown).
from the plasma. It turns out to be very low. For example, a rough estimate for the divertor cooling pipe shows an increase of a few hundred watts [4]. Consequently, GDC is decoupled from the IVVS and is based on a fixed electrode concept. For maintainability reasons each GDC anode will be integrated in port plugs at the upper or equatorial port level and the GDC front surface will be flush with the plasma facing surface of the port plug. Having taken into account all available locations, GDC layout is decided to provide toroidal uniform coverage to the maximum extent possible in both layers. There will be three GDC electrodes in the upper port level (in ports 3, 8 and 14, as shown in the left side of Fig. 1) and four GDC electrodes in the equatorial port level (in ports 3, 8, 12 and 17, as shown in the right side of Fig. 1). Because of the staged installation strategy of ITER most port plugs, if not all, will not be in position for ITER’s First Plasma (FP) operation. In this stage, Temporary Electrodes (TE) with a simple design will be used and fixed on the VV near those ports allocated for GDC. The electrodes integrated in port plugs are called Permanent Electrodes (PE). For the FP operation, the plasma facing surface is much bigger because the blankets and divertor will not be installed. Therefore, besides the aforementioned seven electrodes at the equatorial and upper port levels, there will be three additional TEs in the lower port level, near the ports 3, 9 and 15. 2. System requirements According to the ITER Project Requirements, a high level technical baseline document, the ITER GDC system shall provide the capability to perform GDC with an average first-wall current density greater than 0.1 Am−2 indefinitely with the VV and all in-vessel components at their nominal pre-pulse operating temperatures and at their nominal baking temperature. 2.1. Glow current and current density Deriving from the surface to be cleaned and the number of electrodes, the glow current of one PE and one TE is set to be 30 A and 40 A, respectively, at maximum. It is an empirical scaling result based on the published data of the tokamak devices (Table 1) [5]. To simplify, it is assumed that the glow current density has a uniform spatial distribution around the anode. Geometrically, it’s assumed that each of the seven PEs is located in the center of its supporting plug first wall (FW) in this study.
For the FW on the high field side (numbered 2–6 in the poloidal direction in the right of Fig. 2), when only the glow current from the nearest electrode is taken into account, the glow current density of a tile could be estimated as I/2/l2 , in which I is the glow current from one anode and l is the distance from this tile to the nearest anode. With 30 A on each PE, the glow plasma current density on the high field side tiles ranges from 0.1 to about 0.8 Am−2 (Fig. 3). In the same way, the glow plasma current density on the high field side VV (surface to be covered by aforementioned tiles) is estimated. With 40 A on each TE, it ranges from about 0.15 to about 0.72 Am−2 (Fig. 4). These estimates show that the glow current density would meet the project requirement.
2.2. GDC operation GDC system operation will be performed prior to plasma operation and before venting the VV. For DC GDC the residual magnetic field near the ITER first wall needs to be less than 1 mT during GDC, which is confirmed by the recent simulation [6]. For starting GDC, there is a concern that the partial pressure of water inside the VV could be too high after venting. Therefore, baking should be performed adequately before starting GDC until the partial pressure of water decreases to a certain threshold. During the GDC process full and partial pressure monitoring could be used for the evaluation of the GDC efficiency as the reference for the GDC operation. Table 2 summarizes the main operational parameters of GDC. The breakdown and steady state parameters are empirical data based on tokamak device publications [5]. Operation parameters will be tested and explored during the real ITER operation.
Table 1 GDC electrode layout and glow current. Anode locations for the FP phase
Anode locations after the FP phase Total current in FP phase Total current after FP phase Max. current for each PE Max. current for each TE
Upper ports 3, 8, 14 Equatorial ports 3, 8, 12, 17 Lower ports 3, 9 and 15 Upper ports 3, 8, 14 Equatorial ports 3, 8, 12, 17 ∼400 A ∼200 A 30 A 40 A
Please cite this article in press as: Y. Yang, et al., Re-design of ITER Glow Discharge Cleaning system based on a fixed electrode concept, Fusion Eng. Des. (2014), http://dx.doi.org/10.1016/j.fusengdes.2014.02.021
G Model FUSION-7281; No. of Pages 5
ARTICLE IN PRESS Y. Yang et al. / Fusion Engineering and Design xxx (2014) xxx–xxx
3
Fig. 2. Plane view of tokamak (on the left) and the poloidal cross-section (on the right). On the right picture Be tiles are numbered from 1 to 18 and divertor parts are labeled I, D and O.
Fig. 3. Estimated toroidal distribution of glow current density on the tiles in the high field side with PEs.
3. Anode design and integration The design is still at conceptual design stage, in which the essential parameters are proposed, feasibility is studied and important topics, such as electro-magnetic (EM) load and heat load are raised. 3.1. Environmental conditions Situated in flush with the diagnostic port plug first wall, GDC electrodes are exposed to very harsh environmental conditions, Table 2 Main GDC operation parameters. Gas species
H2 , D2 or He
Max. environmental temperature in FP phasea Max. environmental temperature after the FP phaseb Breakdown pressure Breakdown voltage Steady state glow pressure Maximum gas flow-ratec
∼200 ◦ C ∼240 ◦ C 15 Pa 1.2 kV 0.05–0.5 Pa 50 Pa m3 s−1
a
The VV temperature during baking. The surrounding blanket temperature during water baking. It is expected that GDC will not be carried out during 350 ◦ C gas baking. However, GDC anodes are expected to be at approximately 350 ◦ C during this gas baking. c Restricted by the ITER Torus Cryopump which pumps the torus exhaust during GDC operation. b
Fig. 4. Estimated toroidal distribution of glow current density on the tiles in the high field side with TEs.
such as plasma radiation, nuclear radiation, EM field and radiofrequency (rf). For the EM load, preliminary analysis shows the torque of the present GDC structure is reduced to about one tenth of that of the previous design (movable electrodes at the divertor port level) in the most conservative load case. For the rf load a Faraday shielding is added around the GDC cooling pipes to avoid GDC electrode picking up the electromagnetic radiation during rf heating. Further study on these topics will be performed in the following design stage. The most important concern is the heat load. It is estimated that the heat load on anode during GDC is 10Ie (Ie is the glow current) [5]. During plasma operation the electrode is like a diagnostic port plug first wall. The heat load density due to radiation and charge exchange particles could be 0.35 MW m−2 . The volumetric heat load due to neutron heating is 10 W cm−3 at the front and the decay length is 12 cm. Table 3 summarizes the heat loads based on the conceptual design model of the electrodes.
Table 3 Max. heat load on one anode in different states. TE during GDC PE during GDC PE during DT full power operation
0.4 kW (with 40 A) 0.3 kW (with 30 A) ∼72 kW
Please cite this article in press as: Y. Yang, et al., Re-design of ITER Glow Discharge Cleaning system based on a fixed electrode concept, Fusion Eng. Des. (2014), http://dx.doi.org/10.1016/j.fusengdes.2014.02.021
G Model FUSION-7281; No. of Pages 5
ARTICLE IN PRESS
4
Y. Yang et al. / Fusion Engineering and Design xxx (2014) xxx–xxx
Fig. 5. GDC PE unit (left), cross-section view (middle) and front view (right).
Consequently, there will be water cooling after the FP phase but not in the FP phase.
3.2. Permanent electrode As shown in the left picture of Fig. 5, the main part of a PE consists of a head, a body and cooling tubes. The head part is embedded in the diagnostic port plug first wall (right picture of Fig. 5). The body penetrates the shielding block but electrically insulated from it (middle picture of Fig. 5). The cooling tubes which are also insulated are connected to the cooling manifolds outside the torus vacuum. The front surface of the head, effective GDC anode surface is extrapolated from the JET experiences. The wall area of JET is about 200 m2 and its total anode area is 0.1 m2 . The total first wall surface of ITER is about 1000 m2 , and the total PE anode area needs to be 0.5 m2 correspondingly. For each PE, the front surface is proposed to be approximately 0.07 m2 . The length of the head and the dimensions of the body depend strongly on the diagnostic systems around. They need to compromise among the weight, volume for cooling channel and the structural integrity. Another importance factor is maintainability. PE will be made of stainless steel, same as the diagnostic FW.
3.3. Temporary electrode A TE is basically a metal block fixed on the VV, but electrically insulated. TE has no active cooling. For the FP phase, corresponding to about twice as much first wall surface, the total anode surface should be 1 m2 . Therefore, each of the ten anodes should have a front surface area of approximately 0.1 m2 . TE will be made of stainless steel as well.
4. Main R&D in the conceptual design stage There are two major different features of the ITER PEs comparing with other tokamaks. Firstly, the front surface of PEs is flush with the port plug FW, which is about 100 mm recessed from the plasma comparing with the blanket first wall. Secondly, the side of the anode head is exposed to the diagnostic first wall at the ground potential just about 20 mm away. The main subjects of the R&D are breakdown, steady state operation and heat load during GDC. Southwest Institute of Physics (SWIP) and IRFM (Institut de Recherche sur la Fusion Magnétique) carry out active studies on these topics [7,8].
4.1. Breakdown Different scenarios and environmental geometries are tested on the test-beds, and it is shown that the breakdown voltage could be much higher than 1.2 kV (2.4 kV for IRFM; and 2.7 kV for SWIP). Both test-beds show that it is possible to initiate glow with an anode recessed from the surrounding surface, although at the higher breakdown voltage (for the same pressure) than the case for an anode in free space. IRFM experiments show that breakdown is possible even when the anode is located 60 mm behind the surrounding surface. Increasing pressure is helpful for having a lower breakdown voltage. The maximum voltage is approximately 2.4 kV. SWIP group instead applied the anode voltage to the set value first and then raise the pressure to breakdown. When breakdown voltage is about 1.9–2.2 kV, its over-current protection threshold, 40 A, is triggered frequently. Therefore, the maximum breakdown voltage in SWIP results is 1.9 kV. Despite that, glow could be initiated even when the anode is located 100 mm behind the neighboring surface. In JET it is demonstrated that it is feasible to run the multianodes in a staggered operation without increasing the torus pressure to the normal breakdown pressure [9]. These R&D works show that glow can be initiated with an environmental geometry like in the present ITER design. 4.2. Steady state Visual observation is performed on the glow brightness during the adjusting of the pressure in SWIP’s experiments. It is observed that the bright glow light extends away from the anode surface further and with the decrease of pressure. Preliminary results of the IRFM discharge uniformity also shows that the decrease of the gas pressure is favorable for increasing the wall fluxes (from ion saturation current). It should be noted also that hollow cathode discharge is observed in SWIP’s experiments when the pressure is higher than 2–3 Pa. Hollow cathode discharge is also observed in IRFM tests. These suggest that it is preferable to run GDC at a lower pressure for better glow uniformity. The bright glow in the gap is rarely observed when the gap is less than 15 mm in SWIP’s experiments. IRFM results show that there is no parasitic plasma in the gaps at about 30 mm. The R&D work shows the glow can be kept stable and uniformity can be improved by tuning the pressure. 4.3. Heat load during GDC SWIP measures heat load with different gas species, pressure, glow current and electrode locations. The results vary too much to indicate a reliable scaling law. It is noted that the heat load
Please cite this article in press as: Y. Yang, et al., Re-design of ITER Glow Discharge Cleaning system based on a fixed electrode concept, Fusion Eng. Des. (2014), http://dx.doi.org/10.1016/j.fusengdes.2014.02.021
G Model FUSION-7281; No. of Pages 5
ARTICLE IN PRESS Y. Yang et al. / Fusion Engineering and Design xxx (2014) xxx–xxx
measured is often higher than the baseline estimation (10Ie), in the same magnitude or even one magnitude higher. ASDEX upgrade GDC study shows that heat load on electrode during GDC could increase dramatically with the decrease of the pressure [10]. This phenomenon is also observed by IRFM team. These suggest that running GDC at a lower pressure evokes the risk of generating higher heat load on the electrodes. This is especially important for TEs which have no active cooling. Finding the key operating parameter for heat load on the anode and mitigation schemes will be at high priority in the following design stage. If increasing the ratio between the anode surface to the first wall surface could decrease the heat load, this is easy to implement for the TE design. “Pulsed GDC” like on AUG and “staggered operation” like on JET would potentially decrease the heat load. 5. Safety consideration ITER GDC is required to be operational during 250 ◦ C baking. In this condition the water temperature is at 240 ± 10 ◦ C and the water pressure is at 4.4 ± 0.4 MPa (gauge pressure). The boiling point of water at 4.1 MPa is 251.8 ◦ C, very close to the upper limit of the water temperature. In case the water temperature reaches the boiling temperature due to extra heat load, there is a risk that vaporization of the water leads to pressure rise inside the cooling channel jeopardizing the integrity of the cooling circuit. In the conceptual design, each anode has a pair of cooling pipes with approximately one inch in diameter. At a flow rate of 1 kg s−1 , the heat load acceptable for the cooling circuit is about 8 kW per anode, much higher than the baseline estimation of 0.3 kW. Besides, as discussed in Section 4.3, reducing heat load is possible by adjusting working pressure and modifying operation scenarios. The commissioning and operation during FP phase will also accumulate necessary experiences for ITER. As a conclusion, the risk of over-pressurization can be avoided by engineering with sufficient margin and manipulating the operation parameters though further R&D and real ITER GDC operation. 6. Summary This paper describes the re-design of ITER GDC system based on the fixed anode concept. The requirements are summarized and latest design and operation are introduced. The R&D studies are performed based on ITER’s special geometry. The conceptual design is shown to be feasible and detailed studies are to be performed in the following design stage.
5
Disclaimer The views and opinions expressed herein do not necessarily reflect those of the ITER Organization Acknowledgements The authors would like to thank: V. Rohde in Max-PlanckInstitut fuer Plasmaphysik, D. Kogut and D. Douai in Institut de Recherche sur la Fusion Magnétique, E. Polunovskiy and A. Kukushkin in ITER Organization for the constructive support and discussion. This paper was prepared as an account of work by or for the ITER Organization. The Members of the Organization are the People’s Republic of China, the European Atomic Energy Community, Republic of India, Japan, Republic of Korea, the Russian Federation, and the United States of America. The views and opinions expressed herein do not necessarily reflect those of the Members or any agency thereof. Dissemination of the information in this paper is governed by the applicable terms of the ITER Joint Implementation Agreement. References [1] Y. Yang, S. Maruyama, R.A. Pitts, M. Shimada, M. Wang, T. Jiang, et al., System requirements and design challenges of the ITER Glow Discharge Cleaning system, in: 18th International Vacuum Congress, Beijing, PR China, August 23–27, 2010, PST1-O-2. [2] Y. Yang, S. Maruyama, G. Kiss, Y. Pan, M. Wang, T. Jiang, et al., Concept design of the ITER Glow Discharge Cleaning System, in: 13th International Workshop on Plasma-Facing Materials and Components for Fusion Applications and 1st International Conference on Fusion Energy Materials Science, Rosenheim, Germany, May 9–13, 2011, P38A. [3] M.J. Loughlin, P. Batistoni, C. Konno, U. Fischer, H. Iida, L. Petrizzi, et al., ITER nuclear analysis strategy and requirements, Fusion Science and Technology 56 (2009) 566–572. [4] A. Kukushkin, Estimate of radiation power load on GDC electrode in shielding position, Internal communication. [5] M. Shimada, S. Putvinski, R.A. Pitts, Glow discharge cleaning on ITER, in: 38th EPS Conference on Plasma Physics, Strasbourg, France, 27 June–1 July, 2011, P5.060. [6] Y. Gribov, Assessment of residual magnetic fields due to remanent magnetization of Ferromagnetic Inserts, Internal communication. [7] Y. Wang, M. Wang, M. Dan, X. Ren, Y. Pan, D. Wang, et al., Preliminary results of Glow Discharge Cleaning test on SWIP test bench, in: 25th Symposium on Fusion Engineering, San Francisco, CA, USA, June 10–14, 2013, TPO-81. [8] D. Kogut, D. Douai, R.A. Pitts, G. Hagelaar, Assessment of the new ITER GDC system performance, in: 40th EPS Conference on Plasma Physics, Espoo, Finland, 1st–5th July, 2013, P2.113. [9] Personal communication with D. Douai. [10] Personal communication with V. Rohde.
Please cite this article in press as: Y. Yang, et al., Re-design of ITER Glow Discharge Cleaning system based on a fixed electrode concept, Fusion Eng. Des. (2014), http://dx.doi.org/10.1016/j.fusengdes.2014.02.021