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Simulation of the ITER Poloidal Field Coil Insert DC performance with a new model
Fusion Engineering and Design 84 (2009) 1912–1915
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Simulation of the ITER Poloidal Field Coil Insert DC performance with a new model E.P.A. van Lanen ∗ , A. Nijhuis University of Twente, Faculty of Science and Technology, P.O. Box 217, 7500 AE Enschede, The Netherlands1
a r t i c l e
i n f o
Article history: Available online 4 March 2009 Keywords: Superconductors Non-uniform currents Joint Cable model NbTi Model coil Poloidal Field Coil
a b s t r a c t The Poloidal Field (PF) Coil Insert is made from a NbTi cable in conduit conductor and has been subjected to tests in the Central Solenoid Model Coil facility at JAEA in Japan. For the interpretation of the voltage tap signals from these tests, we adapted the JackPot model – which was used previously to analyse short sample tests – to simulate also the model coil experiments. A key ingredient of JackPot is that the local magnetic field on the superconducting strands and the inter-strand contact resistances all depend on the “trajectories” of the strands within the cable. These trajectories are precisely calculated, ensuring a realistic distribution of magnetic field- and contact resistance values. The results of the model calculations show that the applied joints are most likely responsible for the poor performance of short samples of similar PF conductors in earlier experimental tests. The model predicts that the influence of the joints is significantly less pronounced for the Poloidal Field Coil Insert. © 2009 Elsevier B.V. All rights reserved.
1. Introduction For the performance analysis of the Poloidal Field (PF) coils of ITER, a full-scale test has recently been carried out on a PF Coil Insert (PFCI, see [1] for details on its design) in the Central Solenoid Model Coil (CSMC) facility at JAEA in Naka, Japan [2]. The data reduction of tests on short samples of the same PF conductor, carried out earlier in the SULTAN facility at CRPP in Villigen, Switzerland, gave scope for different interpretations on its performance. A likely explanation for the cause of these differences was found to be a non-uniform current distribution in the joints [3,4], and it was reasonable to expect that the PFCI would have performed likewise as similar joint design was applied in both the PFCI and the PF short sample. Recently tested short sample Nb3 Sn toroidal field conductors showed comparable performance, but the newly developed numerical model “JackPot” explains this behaviour [5]. An adaptation to this model made it fit for simulations on the PFCI experiment as well. The present paper describes these modifications and discusses the simulation results. 2. The model The model simulates the cable’s electrical behaviour, i.e. the current distribution among the strands in the joints, and the voltage
∗ Corresponding author. Tel.: +31 534894839. E-mail address: [email protected] (E.P.A. van Lanen). 1 http://lt.tnw.utwente.nl/. 0920-3796/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2009.01.094
signals that were measured on the outer surface of the sample’s steel jacket (hence the name JackPot, an abbreviation for jacket potential). The novelty of this model is that it makes explicit use of the precisely calculated trajectory coordinates of all strands to evaluate key parameters such as the magnetic field profile on the strands, the spatial interstrand contacts, and the contacts between strands and the joint’s copper sleeve. The latter two distributions are multiplied with resistivity parameters to obtain interstrand resistances (with the parameter ss ) and strand-to-joint resistances (with the parameter sj ), respectively. The parameters ss and sj are fixed by matching the results from simulations with interstrand [6] and joint measurements [7] to obtain a realistic model for the PFCI. Table 1 gives a summary of this matching procedure. The resistances between strands from different petals (final stage sub-cable bundles) are considerably higher than between strands from the same petal, due to the presence of stainless steel wraps around these bundles [6]. Earlier simulations revealed that their contribution to the results is negligible. Therefore, they have not been included in the simulation to save computation time. As a consequence, the model assumes that strands from different petals are only connected through the copper sleeves of the cable terminations. Up till now, JackPot was used to simulate ITER TF conductor samples, with their specific cabling patterns and Nb3 Sn strands, either in SULTAN samples or in longer length conductors with a homogeneous background field (simulating the behaviour in an ITER TF coil). The conversion to a PFCI model required, among others, the implementation of NbTi strand properties and a revised calculation for the magnetic field profiles on the strands.
E.P.A. van Lanen, A. Nijhuis / Fusion Engineering and Design 84 (2009) 1912–1915
1913
Table 1 Results of resistivity parameter matching. is the relative standard deviation from the resistance value. Measured
Joint [7] (sample #2) Cable [6] Rc (interstrand) Rc -ib (inter-bundle)
Simulated
R []
[%]
R []
[%]
0.025 12.1 4.2
5.7 – –
0.025 12.0 4.9
6.2 18.0 4.2
Fig. 3. PFCI current and temperature evolution in Test Run #44.
Fig. 1. Illustration of the self-field profile (arrows) in the cable’s cross-section somewhere halfway the main winding. The ‘radial’ and ‘axial’ directions refer to those of the coil, and the current flows out of the plane.
The basics of the JackPot model are described in [5]. Essentially, JackPot solves an electrical network with discrete parallel and series resistances that simulate the distributed interstrand resistance and the resistance between the strands and the copper sleeves in the joints. The electrical properties of the superconductors are approximated with a power law, whose parameters are taken from measurements on strands [8]. The scaling law for NbTi strands and their parameters are taken from [8,9] and the dependence of the critical current on the angle between field and strand from [10,11]. The background field of the CSMC was taken from [12]. For the calculation of the magnetic field profile on strands, the conductor is divided along its length in steps of one tenth of the conductor’s shortest twist pitch. In each step, the cross-sectional self-field is determined on eight points around the conductor’s surface with the aid of the Biot-Savard law, where the current is assumed to flow through a straight line along the centre of the conductor (see Fig. 1). The results from points on the inner and outer radius of the PFCI are in agreement with calculations performed by others [13]. For points on strands inside the cable, the field is calculated by interpolating the magnitude and angle of the field in the eight points on the surface. This is considerably faster than applying the Biot-Savard law along the trajectory of each strand. The procedure has been validated by comparing its results with the results from a simulation of a similar coil in finite element software (COMSOL MultiphysicsTM ), with a homogeneous current distribution in the
conductor’s volume. Fig. 1 shows the self-field profile with arrows on arbitrary points inside the conductor’s cross-section. To construct the electrical network, the cable is again divided into a number of sections. A larger number of sections increase the accuracy of the calculated current redistribution in the cable. However, the computational capacity of a pc limits the number of sections which can be solved in an acceptable time. Therefore, the model solves the problem with a smaller than actual number of strands, and adjusts the values for the interstrand- and the strandto-sleeve resistances to arrive at the same results as with all the strands. There is, of course, a limit to this strand reduction to avoid an unacceptable deviation in the results. The model configuration described in this work uses 240 strands and 25 cable sections over the conductor length of 44 m in the case of the PFCI, and 54 cm long joints. The spacing between the intersections is smaller where a larger spatial change is expected in the voltage and current profiles, i.e. near the joints. After solving the system, the results are interpolated to get the results at the locations of the voltage taps (Vtaps, see Fig. 2). The cabling pattern and the twist pitch sequence of a PF cable are 3 mm × 4 mm × 4 mm × 5 mm × 6 mm superconducting strands and 45 mm × 85 mm × 125 mm × 160 mm × 410 mm, respectively. The model further assumes a homogeneous temperature everywhere in the sample. 3. Electric field Test Run #44 is used to compare the performance of the model with the measurement results. This is a critical current test at 7.3 K, and with a background field of 3.57 T. Since the temperature sensor exhibits a scatter of several tens of mK (see Fig. 3), the simulation of this test is carried out twice with different temperature: 7.30 K and 7.34 K. Fig. 4 shows that at a PFCI current of 25 kA, most of the measured results are between the results from the two simulations. Fig. 5 demonstrates that eventually at higher currents, the measured electric field between some Vtap pairs increases beyond the results from the simulation with the highest temperature. In this example, the temperature between Vtap pairs V06 and V07 is
Fig. 2. Schematic representation of the voltage taps and temperature sensor on the PFCI. Helium flows from the right (bottom joint) to the left (intermediate joint).
1914
E.P.A. van Lanen, A. Nijhuis / Fusion Engineering and Design 84 (2009) 1912–1915
Fig. 6. Normalised strand current distribution along the cable at different cable currents. The intermediate joint is on the left side, the bottom terminal joint on the right. Fig. 4. Simulation and measurement results from voltage taps on the main winding of the PFCI. The measured data from V1213 was corrupted and has therefore not been included in this analysis.
assumed the same as T03, located between V08 and V09. A possible explanation for this observation can be that due to helium heating, the temperature downstream of T03 (such as between V06 and 07), is slightly higher than the measured temperature. The simulation already shows that a temperature difference of 40 mK already evokes a difference in electric field of about 10 V/m at this current. Saturation of strands at higher cable currents causes the current to redistribute in the cable. Fig. 6 depicts this process at three different cable currents in the same simulation as above. Already at low current, the cable starts to counteract the current non-uniformity. At higher currents, it is unable to fully impose uniformity along its whole length. Since there is no electrical connection between petals in the cable, current distribution among them takes place only in the copper sleeves in the joints. This appears to be sufficient to impose current uniformity in the centre of the cable. 4. PFCI versus SULTAN In a SULTAN sample, opposed to the PFCI, the influence of the joint on the current distribution is still present when the electric
Fig. 5. Simulated and measured electric field versus cable current between two Vtap pairs. The measured data ends at the point where the cable quenched.
field becomes 10 V/m. This is illustrated in Fig. 7, which compares two simulation results from a SULTAN sample with those from the PFCI (Vtap pair V0607). The first SULTAN simulation is done with the parameters from Table 1, whereas the second is done with current uniformity. In this assessment of the current sharing temperature Tcs , the total current is 20 kA and the SULTAN background field is 5 T. The PFCI background field is adjusted such that the peak field in the PFCI matches to that in SULTAN. All other model parameters (interstrand resistances, strand-to-copper sleeve resistance, etc.) are also the same in SULTAN Simulation 1 and in the PFCI Simulation. The difference between SULTAN Simulation 1 and the measurement suggests that the contact resistances in the measurements are higher than assumed in the model. As a result, the measured Tcs is more than 0.15 K lower compared to a cable with homogeneous current distribution. The simulation with homogeneous current distribution in the PFCI gave the same result as the one presented, and it appears that the SULTAN results with homogeneous current distribution are almost similar to the PFCI result, despite their different magnetic field profile. Fig. 8 shows the results of similar analysis on a double logarithmic scale, for three different background fields and all with a cable current of 45 kA. This time, the value for B, given in Fig. 8, indicates the peak field in both samples. The PFCI results are taken from Vtap pair V0607. At higher cable currents, the difference in performance between the two samples becomes more pronounced.
Fig. 7. Tcs analysis of the PFIS (SULTAN) and the PFCI cables. The measured data is from the right leg of Test Run PFIS/PCD070405.
E.P.A. van Lanen, A. Nijhuis / Fusion Engineering and Design 84 (2009) 1912–1915
1915
since the magnetic field in this region is lower, this significantly moderates the impact on the performance compared to a short sample test. The influence of the non-uniformity near the joints on the stability needs to be further evaluated. Acknowledgement This work is supported by the ITER International Organisation, Cadarache, under contract ITER/CT/07/013. References
Fig. 8. Tcs analysis with different background field.
As already observed in Fig. 7, the results of the SULTAN simulation are strongly affected by current non-uniformity, and causes relatively shallow rising traces in Fig. 8, which becomes steeper only when the current distribution in the cable becomes uniform. In the SULTAN simulation, this happens only when the electric field exceeds 100 V/m, which can rarely be measured, whereas the PFCI exhibits this behaviour already above 0.5 V/m. 5. Conclusions The JackPot model demonstrated that the disturbing influence of the joints on the PFCI test results, as observed in earlier tests on a short sample of a similar PF conductor, is much less pronounced. A good assessment of the coil requires a sufficiently homogeneous current distribution among the strands, and according to the JackPot simulation, this is largely achieved when the average value of the electric field is about 0.5 V/m. Only near the joints, the current distribution remains inhomogeneous far beyond this level, but
[1] C. Sborchia, et al., Design and manufacture of the Poloidal Field Conductor Insert coil, Fusion Eng. Des. 66–68 (2003) 1081–1086. [2] L. Bottura, Test Results from the PF Insert Coil and Implications for the ITER PF System, IEEE ASC, Chicago, USA, 2008. [3] Y. Ilyin, et al., Interpretation of conduit voltage measurement on the polodial field insert sample using the CUDI-CICC numerical code, Cryogenics 46 (2006) 517–529. [4] R. Zanino, et al., Implications of NbTi short-sample test results and analysis for the ITER poloidal field conductor insert (PFCI), IEEE Trans. Appl. Supercond. 16 (2006) 886–889. [5] E.P.A. van Lanen, et al., New model to study the influence of cabling pattern, magnet field profile and joint properties on short sample qualification tests of ITER CICCs, in: Transactions of ICEC22-ICMC2008. [6] Y.A. Ilyin, et al., Effect of cyclic loading and conductor layout on contact resistance of full-size ITER PFCI conductors, IEEE Trans. Appl. Supercond. 15 (2005) 1359–1362. [7] P. Bruzzone, et al., Results of contact resistance distribution in NbTi and Nb3Sn ITER conductor termination, IEEE Trans. Appl. Supercond. 17 (June) (2007) 1378–1381. [8] L. Zani, et al., JC (B,T) characterization of NbTi strands used in ITER PFrelevant insert and full-scale sample, IEEE Trans. Appl. Supercond. 15 (2005) 3506–3509. [9] L. Bottura, A practical fit for the critical surface of NbTi, IEEE Trans. Appl. Supercond. 10 (2000) 1054–1057. [10] A. Godeke, et al., Angular dependence of critical current versus magnetic field in Nb3 Sn wires at 9–15 T, Eucas (1997) 917. [11] T. Schild, et al., Influence of the field orientation on the critical current density of Nb3 Sn strands, IEEE Trans. Appl. Supercond. 7 (1997) 1512–1515. [12] R. Zanino, et al., Preparation of the ITER poloidal field conductor insert (PFCI) test, IEEE Trans. Appl. Supercond. 15 (2005) 1346–1350. [13] L. Bottura, E. Laface, http://sdb-server.cern.ch/mediawiki/index.php/PF Insert Coil.
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Simulation of the ITER Poloidal Field Coil Insert DC performance with a new model E.P.A. van Lanen ∗ , A. Nijhuis University of Twente, Faculty of Science and Technology, P.O. Box 217, 7500 AE Enschede, The Netherlands1
a r t i c l e
i n f o
Article history: Available online 4 March 2009 Keywords: Superconductors Non-uniform currents Joint Cable model NbTi Model coil Poloidal Field Coil
a b s t r a c t The Poloidal Field (PF) Coil Insert is made from a NbTi cable in conduit conductor and has been subjected to tests in the Central Solenoid Model Coil facility at JAEA in Japan. For the interpretation of the voltage tap signals from these tests, we adapted the JackPot model – which was used previously to analyse short sample tests – to simulate also the model coil experiments. A key ingredient of JackPot is that the local magnetic field on the superconducting strands and the inter-strand contact resistances all depend on the “trajectories” of the strands within the cable. These trajectories are precisely calculated, ensuring a realistic distribution of magnetic field- and contact resistance values. The results of the model calculations show that the applied joints are most likely responsible for the poor performance of short samples of similar PF conductors in earlier experimental tests. The model predicts that the influence of the joints is significantly less pronounced for the Poloidal Field Coil Insert. © 2009 Elsevier B.V. All rights reserved.
1. Introduction For the performance analysis of the Poloidal Field (PF) coils of ITER, a full-scale test has recently been carried out on a PF Coil Insert (PFCI, see [1] for details on its design) in the Central Solenoid Model Coil (CSMC) facility at JAEA in Naka, Japan [2]. The data reduction of tests on short samples of the same PF conductor, carried out earlier in the SULTAN facility at CRPP in Villigen, Switzerland, gave scope for different interpretations on its performance. A likely explanation for the cause of these differences was found to be a non-uniform current distribution in the joints [3,4], and it was reasonable to expect that the PFCI would have performed likewise as similar joint design was applied in both the PFCI and the PF short sample. Recently tested short sample Nb3 Sn toroidal field conductors showed comparable performance, but the newly developed numerical model “JackPot” explains this behaviour [5]. An adaptation to this model made it fit for simulations on the PFCI experiment as well. The present paper describes these modifications and discusses the simulation results. 2. The model The model simulates the cable’s electrical behaviour, i.e. the current distribution among the strands in the joints, and the voltage
∗ Corresponding author. Tel.: +31 534894839. E-mail address: [email protected] (E.P.A. van Lanen). 1 http://lt.tnw.utwente.nl/. 0920-3796/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2009.01.094
signals that were measured on the outer surface of the sample’s steel jacket (hence the name JackPot, an abbreviation for jacket potential). The novelty of this model is that it makes explicit use of the precisely calculated trajectory coordinates of all strands to evaluate key parameters such as the magnetic field profile on the strands, the spatial interstrand contacts, and the contacts between strands and the joint’s copper sleeve. The latter two distributions are multiplied with resistivity parameters to obtain interstrand resistances (with the parameter ss ) and strand-to-joint resistances (with the parameter sj ), respectively. The parameters ss and sj are fixed by matching the results from simulations with interstrand [6] and joint measurements [7] to obtain a realistic model for the PFCI. Table 1 gives a summary of this matching procedure. The resistances between strands from different petals (final stage sub-cable bundles) are considerably higher than between strands from the same petal, due to the presence of stainless steel wraps around these bundles [6]. Earlier simulations revealed that their contribution to the results is negligible. Therefore, they have not been included in the simulation to save computation time. As a consequence, the model assumes that strands from different petals are only connected through the copper sleeves of the cable terminations. Up till now, JackPot was used to simulate ITER TF conductor samples, with their specific cabling patterns and Nb3 Sn strands, either in SULTAN samples or in longer length conductors with a homogeneous background field (simulating the behaviour in an ITER TF coil). The conversion to a PFCI model required, among others, the implementation of NbTi strand properties and a revised calculation for the magnetic field profiles on the strands.
E.P.A. van Lanen, A. Nijhuis / Fusion Engineering and Design 84 (2009) 1912–1915
1913
Table 1 Results of resistivity parameter matching. is the relative standard deviation from the resistance value. Measured
Joint [7] (sample #2) Cable [6] Rc (interstrand) Rc -ib (inter-bundle)
Simulated
R []
[%]
R []
[%]
0.025 12.1 4.2
5.7 – –
0.025 12.0 4.9
6.2 18.0 4.2
Fig. 3. PFCI current and temperature evolution in Test Run #44.
Fig. 1. Illustration of the self-field profile (arrows) in the cable’s cross-section somewhere halfway the main winding. The ‘radial’ and ‘axial’ directions refer to those of the coil, and the current flows out of the plane.
The basics of the JackPot model are described in [5]. Essentially, JackPot solves an electrical network with discrete parallel and series resistances that simulate the distributed interstrand resistance and the resistance between the strands and the copper sleeves in the joints. The electrical properties of the superconductors are approximated with a power law, whose parameters are taken from measurements on strands [8]. The scaling law for NbTi strands and their parameters are taken from [8,9] and the dependence of the critical current on the angle between field and strand from [10,11]. The background field of the CSMC was taken from [12]. For the calculation of the magnetic field profile on strands, the conductor is divided along its length in steps of one tenth of the conductor’s shortest twist pitch. In each step, the cross-sectional self-field is determined on eight points around the conductor’s surface with the aid of the Biot-Savard law, where the current is assumed to flow through a straight line along the centre of the conductor (see Fig. 1). The results from points on the inner and outer radius of the PFCI are in agreement with calculations performed by others [13]. For points on strands inside the cable, the field is calculated by interpolating the magnitude and angle of the field in the eight points on the surface. This is considerably faster than applying the Biot-Savard law along the trajectory of each strand. The procedure has been validated by comparing its results with the results from a simulation of a similar coil in finite element software (COMSOL MultiphysicsTM ), with a homogeneous current distribution in the
conductor’s volume. Fig. 1 shows the self-field profile with arrows on arbitrary points inside the conductor’s cross-section. To construct the electrical network, the cable is again divided into a number of sections. A larger number of sections increase the accuracy of the calculated current redistribution in the cable. However, the computational capacity of a pc limits the number of sections which can be solved in an acceptable time. Therefore, the model solves the problem with a smaller than actual number of strands, and adjusts the values for the interstrand- and the strandto-sleeve resistances to arrive at the same results as with all the strands. There is, of course, a limit to this strand reduction to avoid an unacceptable deviation in the results. The model configuration described in this work uses 240 strands and 25 cable sections over the conductor length of 44 m in the case of the PFCI, and 54 cm long joints. The spacing between the intersections is smaller where a larger spatial change is expected in the voltage and current profiles, i.e. near the joints. After solving the system, the results are interpolated to get the results at the locations of the voltage taps (Vtaps, see Fig. 2). The cabling pattern and the twist pitch sequence of a PF cable are 3 mm × 4 mm × 4 mm × 5 mm × 6 mm superconducting strands and 45 mm × 85 mm × 125 mm × 160 mm × 410 mm, respectively. The model further assumes a homogeneous temperature everywhere in the sample. 3. Electric field Test Run #44 is used to compare the performance of the model with the measurement results. This is a critical current test at 7.3 K, and with a background field of 3.57 T. Since the temperature sensor exhibits a scatter of several tens of mK (see Fig. 3), the simulation of this test is carried out twice with different temperature: 7.30 K and 7.34 K. Fig. 4 shows that at a PFCI current of 25 kA, most of the measured results are between the results from the two simulations. Fig. 5 demonstrates that eventually at higher currents, the measured electric field between some Vtap pairs increases beyond the results from the simulation with the highest temperature. In this example, the temperature between Vtap pairs V06 and V07 is
Fig. 2. Schematic representation of the voltage taps and temperature sensor on the PFCI. Helium flows from the right (bottom joint) to the left (intermediate joint).
1914
E.P.A. van Lanen, A. Nijhuis / Fusion Engineering and Design 84 (2009) 1912–1915
Fig. 6. Normalised strand current distribution along the cable at different cable currents. The intermediate joint is on the left side, the bottom terminal joint on the right. Fig. 4. Simulation and measurement results from voltage taps on the main winding of the PFCI. The measured data from V1213 was corrupted and has therefore not been included in this analysis.
assumed the same as T03, located between V08 and V09. A possible explanation for this observation can be that due to helium heating, the temperature downstream of T03 (such as between V06 and 07), is slightly higher than the measured temperature. The simulation already shows that a temperature difference of 40 mK already evokes a difference in electric field of about 10 V/m at this current. Saturation of strands at higher cable currents causes the current to redistribute in the cable. Fig. 6 depicts this process at three different cable currents in the same simulation as above. Already at low current, the cable starts to counteract the current non-uniformity. At higher currents, it is unable to fully impose uniformity along its whole length. Since there is no electrical connection between petals in the cable, current distribution among them takes place only in the copper sleeves in the joints. This appears to be sufficient to impose current uniformity in the centre of the cable. 4. PFCI versus SULTAN In a SULTAN sample, opposed to the PFCI, the influence of the joint on the current distribution is still present when the electric
Fig. 5. Simulated and measured electric field versus cable current between two Vtap pairs. The measured data ends at the point where the cable quenched.
field becomes 10 V/m. This is illustrated in Fig. 7, which compares two simulation results from a SULTAN sample with those from the PFCI (Vtap pair V0607). The first SULTAN simulation is done with the parameters from Table 1, whereas the second is done with current uniformity. In this assessment of the current sharing temperature Tcs , the total current is 20 kA and the SULTAN background field is 5 T. The PFCI background field is adjusted such that the peak field in the PFCI matches to that in SULTAN. All other model parameters (interstrand resistances, strand-to-copper sleeve resistance, etc.) are also the same in SULTAN Simulation 1 and in the PFCI Simulation. The difference between SULTAN Simulation 1 and the measurement suggests that the contact resistances in the measurements are higher than assumed in the model. As a result, the measured Tcs is more than 0.15 K lower compared to a cable with homogeneous current distribution. The simulation with homogeneous current distribution in the PFCI gave the same result as the one presented, and it appears that the SULTAN results with homogeneous current distribution are almost similar to the PFCI result, despite their different magnetic field profile. Fig. 8 shows the results of similar analysis on a double logarithmic scale, for three different background fields and all with a cable current of 45 kA. This time, the value for B, given in Fig. 8, indicates the peak field in both samples. The PFCI results are taken from Vtap pair V0607. At higher cable currents, the difference in performance between the two samples becomes more pronounced.
Fig. 7. Tcs analysis of the PFIS (SULTAN) and the PFCI cables. The measured data is from the right leg of Test Run PFIS/PCD070405.
E.P.A. van Lanen, A. Nijhuis / Fusion Engineering and Design 84 (2009) 1912–1915
1915
since the magnetic field in this region is lower, this significantly moderates the impact on the performance compared to a short sample test. The influence of the non-uniformity near the joints on the stability needs to be further evaluated. Acknowledgement This work is supported by the ITER International Organisation, Cadarache, under contract ITER/CT/07/013. References
Fig. 8. Tcs analysis with different background field.
As already observed in Fig. 7, the results of the SULTAN simulation are strongly affected by current non-uniformity, and causes relatively shallow rising traces in Fig. 8, which becomes steeper only when the current distribution in the cable becomes uniform. In the SULTAN simulation, this happens only when the electric field exceeds 100 V/m, which can rarely be measured, whereas the PFCI exhibits this behaviour already above 0.5 V/m. 5. Conclusions The JackPot model demonstrated that the disturbing influence of the joints on the PFCI test results, as observed in earlier tests on a short sample of a similar PF conductor, is much less pronounced. A good assessment of the coil requires a sufficiently homogeneous current distribution among the strands, and according to the JackPot simulation, this is largely achieved when the average value of the electric field is about 0.5 V/m. Only near the joints, the current distribution remains inhomogeneous far beyond this level, but
[1] C. Sborchia, et al., Design and manufacture of the Poloidal Field Conductor Insert coil, Fusion Eng. Des. 66–68 (2003) 1081–1086. [2] L. Bottura, Test Results from the PF Insert Coil and Implications for the ITER PF System, IEEE ASC, Chicago, USA, 2008. [3] Y. Ilyin, et al., Interpretation of conduit voltage measurement on the polodial field insert sample using the CUDI-CICC numerical code, Cryogenics 46 (2006) 517–529. [4] R. Zanino, et al., Implications of NbTi short-sample test results and analysis for the ITER poloidal field conductor insert (PFCI), IEEE Trans. Appl. Supercond. 16 (2006) 886–889. [5] E.P.A. van Lanen, et al., New model to study the influence of cabling pattern, magnet field profile and joint properties on short sample qualification tests of ITER CICCs, in: Transactions of ICEC22-ICMC2008. [6] Y.A. Ilyin, et al., Effect of cyclic loading and conductor layout on contact resistance of full-size ITER PFCI conductors, IEEE Trans. Appl. Supercond. 15 (2005) 1359–1362. [7] P. Bruzzone, et al., Results of contact resistance distribution in NbTi and Nb3Sn ITER conductor termination, IEEE Trans. Appl. Supercond. 17 (June) (2007) 1378–1381. [8] L. Zani, et al., JC (B,T) characterization of NbTi strands used in ITER PFrelevant insert and full-scale sample, IEEE Trans. Appl. Supercond. 15 (2005) 3506–3509. [9] L. Bottura, A practical fit for the critical surface of NbTi, IEEE Trans. Appl. Supercond. 10 (2000) 1054–1057. [10] A. Godeke, et al., Angular dependence of critical current versus magnetic field in Nb3 Sn wires at 9–15 T, Eucas (1997) 917. [11] T. Schild, et al., Influence of the field orientation on the critical current density of Nb3 Sn strands, IEEE Trans. Appl. Supercond. 7 (1997) 1512–1515. [12] R. Zanino, et al., Preparation of the ITER poloidal field conductor insert (PFCI) test, IEEE Trans. Appl. Supercond. 15 (2005) 1346–1350. [13] L. Bottura, E. Laface, http://sdb-server.cern.ch/mediawiki/index.php/PF Insert Coil.