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Qualification tests for ITER TF conductors in SULTAN
Fusion Engineering and Design 84 (2009) 205–209
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Qualification tests for ITER TF conductors in SULTAN P. Bruzzone ∗ , B. Stepanov, R. Wesche EPFL, Centre de Recherches en Physique des Plasmas, Association Euratom–Confédération Suisse, CH-5232 Villigen PSI, Switzerland
a r t i c l e
i n f o
Article history: Available online 20 December 2008 Keywords: Nb3 Sn Cable-in-conduit TF conductors ITER SULTAN
a b s t r a c t From February 2007 to May 2008, 18 short length conductor sections have been tested in SULTAN for design verification and manufacturer qualification of the ITER Toroidal Field (TF) conductor. The test program is focussed on the current sharing temperature, Tcs , at the nominal operating conditions, 68 kA current and 11.15 T effective field, which can be fully reproduced in the SULTAN test facility. A broad range of results was observed, with over 2 K difference among the Tcs of the conductors. In average, the results are poorer compared to the potential performance estimated from the strand scaling law. The key parameters to mitigate the degradation are not yet clearly identified. The experimental challenges to test conductors with performance degradation are highlighted, including enhanced instrumentation sets, the application of gas flow calorimetry to sense the current sharing power and the post-processing of voltage data to cancel the transverse potential across the cable. The updated schedule of the tests in SULTAN is presented with the short-term action plan for conductor test. © 2008 Elsevier B.V. All rights reserved.
1. Introduction For design and verification purposes of high current superconductors it is essential to use a reliable correlation between the properties of the individual strand and those of the large cable-inconduit (CIC). In the case of NbTi conductors, which are ductile and insensitive to strain, the only necessary tools are the scaling laws for the critical current density vs. temperature and magnetic field, Jc (B, T), as well as for the transition index, n(J), and a map of the field distribution over the conductor cross section. With the above tools, a performance prediction for the NbTi CIC can be reliably done and the results are always in good agreement with the predictions for short [1] as well as long [2] conductor samples, see Fig. 1. For the Nb3 Sn conductors, which are brittle and strain sensitive, the scaling laws include the longitudinal strain, ε, which is not measurable for a strand inside a CIC, and cover only the reversible range, i.e. assume that the brittle Nb3 Sn filaments never exceed the tiny elastic range despite the large thermo-mechanical and electromagnetic loads on the strand bundle. A popular procedure applied to Nb3 Sn CIC was to use the longitudinal strain ε as a free parameter to fit the results by the strand scaling laws and then retain the “fitting” ε as the thermal strain in the design [3]. However, a single value of strain was not sufficient to fit a broad range of results of an individual conductor, making necessary the introduction in the fit of an “extra”, variable strain [4]. Further on, the evidence of performance degradation upon cyclic
∗ Corresponding author. Tel.: +41 56 310 4363; fax: +41 56 310 3729. E-mail address: [email protected] (P. Bruzzone). 0920-3796/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2008.11.001
loading [5] and the drop of n-index in strands extracted from CIC [6] made clear that the assumption of intact Nb3 Sn filaments was not adequate for the performance prediction of Nb3 Sn CIC.
2. Description of the ITER TF prototype conductors After the ITER Model Coil experience, the design of the TF conductors was adjusted in 2002 [7], but it was only in February 2007 till the first ITER TF conductor sample was tested in SULTAN [8]. In the following 16 months, till May 2008, nine samples, for a total of 18 conductors, were tested in SULTAN. In agreement with ITER, the Domestic Agencies (DA) applied some layout variations, summarized in Table 1. In August 2007, when six samples were already measured, ITER proposed a minor adjustment of the TF conductor specification [9], which was later retained as final specification except in the “Alt1” and “Alt1b” conductors, which substantially deviate from the ITER specification as far as strand diameter and cable pattern. All the conductors listed in Table 1 should be considered “pre-qualification” samples. They are made from strands supplied by nine different companies in five DA’s, four of them using bronze and the others “internal Sn” technique for the Nb3 Sn strand assembly. More detail about the conductor layouts can be found in [10–15]. Six out of nine SULTAN samples were heat treated and assembled at CRPP with identical procedure. The other three samples (JATF1, JATF3 and USTF1) were delivered to CRPP ready for test in SULTAN. The instrumentation of the samples evolved to very high level of complexity [16]. Two samples (TFPRO2 and JATF2) were tested twice, with enhanced instrumentation in the second campaign.
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Table 1 Summary of conductor layout for the 18 TF conductors tested in SULTAN. TFPRO1
Conductor nickname Strand type
EAS1
Strand identity Strand diameter (mm) Strand twist pitch (mm) Cu:non-Cu in strand Cable pattern Central spiral (mm) Pitches (mm) Final cable pitch (mm) Strand Ic at 12 T 4.2 K (A) Strand n- index Av. void fraction (%) Outer diameter (mm) *
TFPRO2 EAS2
Bronze
OST2
17
Spiral 6.9/9.0
KO16
Internal sn
Internal sn
He2539 0.826
6005-K 0.820
17
15
15
10
1
39.6 29.3 42.05
JAI2
Bronze
193.3
KO19
KATRC38c6b7A 0.82
15
1
RFTF1
45/87/ 126/245 520
JAB1
RF33
Internal sn
Bronze
Internal sn
6005-K 0.820
He2539 0.826
10
15
0.97
454
265.4
302.3
247.2
281.9
32.0 27.7
23.6 29.1
32.7 29.6
33.7 29.8
41.45
42.05
42.68
42.66
41/80/ 125/240 820 508
JATF3
JAI1
1
RF31
> 248
239
34.6
29.7 31.7*
n.a. 33.5
30.1 32.9
43.7
43.7
43.9
43.9
32.9
JAD
Base
Alt1
Internal sn
06494-1 LK0003 0.82
NT8404 0.82
NT8401 0.77
15
15
13
14
1.05
1.19
1.07
Alt1b
Internal sn 9484 0.82
9707 0.77
17 1.20
“ITER”
Alt*
80/140/ 45/85/ 178/300 127/254 420
Alt* Perforated pipe 7.5 × 9.5 25/127 78/139/ /254 190/304 457 457
25/127 /304 437
508
245.9
238.8
209
278.4
238.1
36.2 31.8
31.0
43.7
0.96
Opt2
1.08
45/84/ 124/250 453
234.0
JAC
USTF2
Bronze
Spiral 7×9
45/85/ 130/250 450
USTF1
BrP 0.82
1
“ITER” = (2s/c + 1Cu) × 3 × 5 x 5 + core x 6 Spiral Spiral 7×9 6.7 × 9.0
Spiral 6.7 × 9.0 45/85/ 130/250 447
The “Alt” cable pattern is (((6s/c + 1Cu) × 6 + 1Cu) × 5 + 1Cu) × 6.
JATF1
JAB2
7567, 7603 7730 0.815
Spiral 7.0 × 9.1 116/182/ 245/415 520
45/87/ 126/245 486 492
KOTF
7878
0.915
43.45
OST1
Internal sn
NSTT8305 0.813
33.8
JATF2
Spiral 7×9
207.8 27.1
18.5 n.a.
29 42.65
209
43.7
43.8
30.0
36.1 n.a.
43.7
43.8
P. Bruzzone et al. / Fusion Engineering and Design 84 (2009) 205–209
Sample nickname
P. Bruzzone et al. / Fusion Engineering and Design 84 (2009) 205–209
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3. Test results The test program agreed with ITER has been applied to all the samples, with main focus on the 10 V/m current sharing temperature, Tcs , at the TF nominal operating current, I = 68 kA, and effective field, B = 11.15 T (background field BSULTAN = 10.78 T). A number of load cycles (600–1200) and one thermal cycle (warm-up/cooldown) were also included in the test program, along with ac loss tests and dc test at other operating points. Here, only the first and the final Tcs run at TF operating conditions are discussed. The central channel in the conductors is plugged during the sample assembly to force the coolant flow only in the strand bundle area, with the aim to limit the variation of the coolant velocity and temperature, over the cross section. The mass flow rate is initially set at about 2 g/s. A typical Tcs run lasts 1.5 h and is as large as 120 MB. The current is first zeroed by heating the secondary conductor of the superconducting transformer. Then the current is raised to 68 kA at a rate of 150 A/s with intermediate, 5 min long holds at 10, 20, 30 and 40 kA. Then the inlet temperature is increased in steps of 0.1–0.2 K, with 2–3 min hold at each step until a quench is reached. In case the two conductors of a sample are highly unbalanced, an additional heater is switched on the stronger conductor to unbalance the operating temperature and be able to obtain the full V–T characteristic for both conductors. 3.1. Experimental issues The Nb3 Sn conductor degradation shows up as a broadening of the superconducting transition (low n index), with early voltage well below the take-off temperature. As long as the degradation is limited, say n ≥ 10, the slope of the electric field vs. temperature, dE/dT, at the criterion of 10 V/m is larger than 80 V/m K and voltage drift/offset as large as 2 V cause an very small error, <0.05 K, in the assessment of Tcs . When dE/dT ≤ 15 V/m K, the accuracy of the Tcs assessment is heavily affected by voltage drift/offset as small as 1 V. Moreover, in the large Nb3 Sn samples of the last three years, the voltage drift observed at the end of the current ramp, before any raise of temperature has assumed new proportions, with values sometime above the criterion, and prevents a straight assessment of Tcs from the raw data [17,8]. The large voltage drifts do not generate any measurable power, i.e. they are related to potential difference across the cable rather than along the cable. The nature of the voltage drift is a subject of discussion among the experts and is likely correlated with the performance degradation, as no significant voltage drift is observed is conductors with high n-index and high performance. Post-processing of voltage data as well as an enhanced number of voltage taps is used to extract from the raw data the true longitudinal voltage for the assessment of Tcs . An accurate measurement of the current sharing power by gas flow calorimetry has been used to benchmark the voltage post-processing procedure and is now integral part of the Tcs assessment method [8,16].
Fig. 1. Excellent match of CIC results of current sharing temperature vs. predicted performance from the strand scaling laws for three full size NbTi conductors tested in SULTAN.
with a thermal strain arbitrarily fixed at ε = −0.65% (see discussion below). Opposite to the NbTi full size CICC of Fig. 1, for 13 Nb3 Sn ITER TF CICC the picture at the beginning of the test campaign, Fig. 2, is much more scattered. The remaining five conductors are not included in the plot because the full set of scaling law parameters is not available to calculate the predicted performance. At least two out of 13 conductors (likely 4 out of 18) show results “better than expected”. In fact, the presence of two conductors well above the diagonal, matching line of Fig. 2 suggests that the retained strain, ε = −0.65%, for the prediction is by far too conservative and a lower “thermal strain”, in the range of −0.5% should be used. The test results at the end of the test campaign, after cyclic and thermal loading, are plotted vs. the estimated potential performance in Fig. 3. In some cases, the conductor performance was not stable at the end of the test campaign and a further degradation
3.2. The potential performance of the Nb3 Sn CICC The “CIC potential performance” is the current sharing temperature when no irreversible degradation happens and a realistic longitudinal thermal strain is applied. For the sake of qualification and acceptance tests, the only criterion is the fulfilment of the target specification, disregarding any consideration of degradation. However, a comparison between measured and potential performance is important for the refinement of the specific design criteria and, in general, for the judgement of the effectiveness of the design. To estimate the potential performance, the strand scaling law provided by the sample suppliers (ITER Domestic Agencies) are used together
Fig. 2. The initial Tcs results of 13 Nb3 Sn ITER TF CICC compared to the potential performance estimated from the respective strand scaling laws with retained ε = −0.65%.
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P. Bruzzone et al. / Fusion Engineering and Design 84 (2009) 205–209
Fig. 3. The Tcs results at the end of the test campaign of 12 Nb3 Sn ITER TF CICC compared to the potential performance with ε = −0.65%.
but other conductors have a drop of Tcs larger than one degree. The distance from the diagonal line to the measured performance in Figs. 2 and 3 is due to the different sensitivity to the transverse load, which in turn depends on the strand layout and other cable parameters. In five cases, a straight comparison can be done among conductors, which are identical in all parameters except the void fraction in the bundle. No clear trend can be found. The lower void fraction is slightly beneficial for EAS and JAI, slightly detrimental for JAB and RF, and hard-to-decide for KO. The “long twist pitches” in the lower cable stages are applied on four conductors (OST2, JAC, JAD, Opt2) with very different results: quite bad for JAC and JAD, very good for OST2 and initially good for Opt2 (which severely degraded during the test). As there are not two conductors fully identical except the twist pitches, a straight comparison is impossible. However, other data from sub-size conductors [18] suggest that a long twist pitch in the first triplet is beneficial when all other parameters are identical. A broad superconducting transition, with power developed well below Tcs , is common to all the under-performing conductors. The range of n-index (which is not perfectly adequate to fit the very broad transitions) is between 4 and 9. With such low n-index, the Tcs at 10 V/m is not even close to the maximum allowable operating temperature for a winding with long conductor lengths exposed to the maximum field. Depending on the actual value of n-index, the maximum, steady state operating temperature must be set 0.5–1 K lower than Tcs . 5. Conclusions
Fig. 4. Summary of the test results for current sharing temperature of the 18 ITER TF conductors at the first and last run of their test campaigns.
is likely to occur. Only 12 conductors are visible in Fig. 3, as one out of 13 conductors reported in Fig. 2 had performance below 5 K at the end of the test campaign. The absolute Tcs performance of the 18 conductors at the beginning and end of the test campaign is gathered in Fig. 4. Whenever applicable, the results in Fig. 4 are obtained by both post-processing of voltage taps signals and power calorimetry. The error bar is related to the reliability of the post-processing procedure: in most cases it is ± 0.1 K and higher in case of very broad transition.
The large spread of results of the 18 ITER TF prototype conductors measured in SULTAN, over 2.5 K in Tcs , is due to the spread of the Jc in the strands supplied by nine companies worldwide and to the different load sensitivity. In most cases, the performance is well below the potential performance estimated for conductors made of non-damaged strands. As the extent of the irreversible degradation is not predictable by scaling laws, the project has retained large engineering margins in the specification, using only a fraction of the potential performance of the Nb3 Sn strands. The strategy of the test in SULTAN of every combination of strand and cable manufacturer (qualification test) is based on the assumption that the extent of degradation is reproducible as long as there is no change in the layout specification and the combination of manufacturers, including each production step. However, some minor variation of degradation (and hence performance) cannot be excluded and must be accounted in the acceptance criterion after the qualification test, to minimize the probability of rejection during the series production. As a last measure to rescue marginal conductors, the increase of the heat removal rate (larger pumping power) in the ITER TF coils may be considered. Acknowledgements
4. Discussion The potential performance in Figs. 2 and 3 is not the same for all conductors. The main driver for the large range of potential performance, from 6.03 to 6.97 K, is the strand, where the Ic at 12 T, 4.2 K ranges from 193 to 302 A, see Table 1. Other reasons are the variation of superconductor cross section due to the different Cu:non-Cu data. The scattering in the measured performance is larger, with a range of 2.09 K at the initial run and about 2.5 K at the end of the test campaign, showing that the progression of degradation is different in the various conductors. The “ranking” also changes, as few conductors have negligible change of Tcs over the test campaign,
The authors would like to thank the ITER DA’s for the friendly collaboration during preparation and testing of the ITER TF conductors. The technical support of the Paul Scherrer Institute (PSI) is greatly acknowledged. References [1] R. Wesche, M. Bagnasco, P. Bruzzone, M. Calvi, F. Cau, R. Herzog, C. Marinucci, B. Stepanov, Results of Conductor Testing in SULTAN: A Review, WAMSDO Workshop, CERN, Geneva, June 2008. [2] D. Bessette, L. Bottura, A. Devred, N. Mitchell, K. Okuno, C. Sborchia, The PFI test group, test results from the pf insert coil and implications for the iter pf system, IEEE Appl. Supercond. 19, in press.
P. Bruzzone et al. / Fusion Engineering and Design 84 (2009) 205–209 [3] D. Ciazynski, J.L. Duchateau, T. Schild, A.M. Fuchs, Test results and analysis of two European full size conductor samples for ITER, IEEE Appl. Supercond. 10 (2000) 1058–1061. [4] N. Mitchell, Summary, assessment and implications of the ITER Model Coil test results, Fusion Eng. Des. 66–68 (2003) 971–993. [5] H. Tsuji, S. Egorov, J. Minervini, N. Martovetsky, K. Okuno, Y. Takahashi, R.J. Thome, ITER R&D: magnets: central solenoid model coil, Fusion Eng. Des. 55 (2002) 153–170. [6] P. Bruzzone, R. Wesche, B. Stepanov, The voltage–current characteristic (n value) of the cable-in-conduit conductors for fusion, IEEE Appl. Supercond. 13 (2003) 1452–1455. [7] D. Bessette, Sensitivity of Nb3 Sn ITER conductor design to selected parameters, IEEE Appl. Supercond. 13 (2003) 1433–1436. [8] P. Bruzzone, B. Stepanov, R. Wesche, E. Salpietro, A. Vostner, K. Okuno, Y. Takahashi, H.C. Kim, K. Kim, A.K. Shikov, V.E. Sytnikov, Results of a new generation of ITER TF conductor samples in SULTAN, IEEE Appl. Supercond. 18 (2008) 459–462. [9] D. Bessette, TF conductor reference design options, in: ITER Conductor Coordination Meeting, August 31st, Philadelphia, 2007. [10] U. Besi Vetrella, A. della Corte, G. De Marzi, A. di Zenobio, L. Muzzi, L. Reccia, S. Turtù, A. Baldini, P. Bruzzone, E. Salpietro, A. Vostner, Manufacturing of the ITER TF full size prototype conductors, IEEE Appl. Supercond. 18 (2008) 1105–1108. [11] H.C. Kim, D.K. Oh, S.H. Park, K. Kim, P. Bruzzone, Development and SULTAN test result of ITER TF conductor samples of Korea, IEEE Appl. Supercond. 18 (2008) 1084–1087.
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[12] A. Shikov, V. Pantsyrny, A. Vorobieva, S. Sudyev, S. Sergeev, N. Beliakov, N. Kozlenkova, V. Sytnikov, A. Taran, A. Rychagov, P. Bruzzone, Development and results of testing in SULTAN facility of a new RF produced internal tin ITER TF conductor sample, IEEE Appl. Supercond. 18 (2008) 1076–1079. [13] Y. Takahashi, T. Isono, N. Koizumi, Y. Nunoya, K. Matsui, Y. Nabara, T. Hemmi, M. Oshikiri, Y. Uno, K. Okuno, B. Stepanov, P. Bruzzone, Performance of Japanese Nb3 Sn conductors for ITER Toroidal field coils, IEEE Appl. Supercond. 18 (2008) 471–474. [14] N.N. Martovetsky, D.R. Hatfield, J.R. Miller, C.Y. Gung, J.S. Schultz, N. Cheggour, L.F. Goodrich, P. Bruzzone, B. Stepanov, Test results of the first US ITER TF conductors in SULTAN, IEEE Appl. Supercond. 19, in press. [15] K. Matsui, T. Isono, Y. Nunoya, T. Hemmi, Y. Nabara, Y. Okui, M. Oshikiri, N. Koizumi, Y. Takahashi, K. Okuno, B. Stepanov, P. Bruzzone, Test results of Japanese SULTAN sample and conductor procurement in Japan for ITER TF Coils, IEEE Appl. Supercond. 19, in press. [16] P. Bruzzone, B. Stepanov, R. Wesche, Y. Ilyin, R. Herzog, M. Calvi, M. Bagnasco, F. Cau, Methods, accuracy and reliability of ITER conductor tests in SULTAN, IEEE Appl. Supercond. 19, in press. [17] P. Bruzzone, B. Stepanov, R. Wesche, R. Herzog, D. Ciazynski, J.L. Duchateau, B. Renard, L. Zani, M. Bagnasco, L. Savoldi Richard, R. Zanino, A. della Corte, L. Muzzi, S. Turtu, A. di Zenobio, E. Salpietro, A. Vostner, A. Nijhuis, Y. Ilyin, Test results of two ITER TF conductor short samples using high current density Nb3 Sn Strands, IEEE Appl. Supercond. 17 (2007) 1370–1373. [18] P. Bruzzone, B. Stepanov, R. Wesche, A. della Corte, L. Affinito, M. Napolitano, A. Vostner, Test results of a Nb3 Sn cable-in-conduit conductor with variable pitch sequence, IEEE Appl. Supercond. 19, in press.
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Qualification tests for ITER TF conductors in SULTAN P. Bruzzone ∗ , B. Stepanov, R. Wesche EPFL, Centre de Recherches en Physique des Plasmas, Association Euratom–Confédération Suisse, CH-5232 Villigen PSI, Switzerland
a r t i c l e
i n f o
Article history: Available online 20 December 2008 Keywords: Nb3 Sn Cable-in-conduit TF conductors ITER SULTAN
a b s t r a c t From February 2007 to May 2008, 18 short length conductor sections have been tested in SULTAN for design verification and manufacturer qualification of the ITER Toroidal Field (TF) conductor. The test program is focussed on the current sharing temperature, Tcs , at the nominal operating conditions, 68 kA current and 11.15 T effective field, which can be fully reproduced in the SULTAN test facility. A broad range of results was observed, with over 2 K difference among the Tcs of the conductors. In average, the results are poorer compared to the potential performance estimated from the strand scaling law. The key parameters to mitigate the degradation are not yet clearly identified. The experimental challenges to test conductors with performance degradation are highlighted, including enhanced instrumentation sets, the application of gas flow calorimetry to sense the current sharing power and the post-processing of voltage data to cancel the transverse potential across the cable. The updated schedule of the tests in SULTAN is presented with the short-term action plan for conductor test. © 2008 Elsevier B.V. All rights reserved.
1. Introduction For design and verification purposes of high current superconductors it is essential to use a reliable correlation between the properties of the individual strand and those of the large cable-inconduit (CIC). In the case of NbTi conductors, which are ductile and insensitive to strain, the only necessary tools are the scaling laws for the critical current density vs. temperature and magnetic field, Jc (B, T), as well as for the transition index, n(J), and a map of the field distribution over the conductor cross section. With the above tools, a performance prediction for the NbTi CIC can be reliably done and the results are always in good agreement with the predictions for short [1] as well as long [2] conductor samples, see Fig. 1. For the Nb3 Sn conductors, which are brittle and strain sensitive, the scaling laws include the longitudinal strain, ε, which is not measurable for a strand inside a CIC, and cover only the reversible range, i.e. assume that the brittle Nb3 Sn filaments never exceed the tiny elastic range despite the large thermo-mechanical and electromagnetic loads on the strand bundle. A popular procedure applied to Nb3 Sn CIC was to use the longitudinal strain ε as a free parameter to fit the results by the strand scaling laws and then retain the “fitting” ε as the thermal strain in the design [3]. However, a single value of strain was not sufficient to fit a broad range of results of an individual conductor, making necessary the introduction in the fit of an “extra”, variable strain [4]. Further on, the evidence of performance degradation upon cyclic
∗ Corresponding author. Tel.: +41 56 310 4363; fax: +41 56 310 3729. E-mail address: [email protected] (P. Bruzzone). 0920-3796/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2008.11.001
loading [5] and the drop of n-index in strands extracted from CIC [6] made clear that the assumption of intact Nb3 Sn filaments was not adequate for the performance prediction of Nb3 Sn CIC.
2. Description of the ITER TF prototype conductors After the ITER Model Coil experience, the design of the TF conductors was adjusted in 2002 [7], but it was only in February 2007 till the first ITER TF conductor sample was tested in SULTAN [8]. In the following 16 months, till May 2008, nine samples, for a total of 18 conductors, were tested in SULTAN. In agreement with ITER, the Domestic Agencies (DA) applied some layout variations, summarized in Table 1. In August 2007, when six samples were already measured, ITER proposed a minor adjustment of the TF conductor specification [9], which was later retained as final specification except in the “Alt1” and “Alt1b” conductors, which substantially deviate from the ITER specification as far as strand diameter and cable pattern. All the conductors listed in Table 1 should be considered “pre-qualification” samples. They are made from strands supplied by nine different companies in five DA’s, four of them using bronze and the others “internal Sn” technique for the Nb3 Sn strand assembly. More detail about the conductor layouts can be found in [10–15]. Six out of nine SULTAN samples were heat treated and assembled at CRPP with identical procedure. The other three samples (JATF1, JATF3 and USTF1) were delivered to CRPP ready for test in SULTAN. The instrumentation of the samples evolved to very high level of complexity [16]. Two samples (TFPRO2 and JATF2) were tested twice, with enhanced instrumentation in the second campaign.
206
Table 1 Summary of conductor layout for the 18 TF conductors tested in SULTAN. TFPRO1
Conductor nickname Strand type
EAS1
Strand identity Strand diameter (mm) Strand twist pitch (mm) Cu:non-Cu in strand Cable pattern Central spiral (mm) Pitches (mm) Final cable pitch (mm) Strand Ic at 12 T 4.2 K (A) Strand n- index Av. void fraction (%) Outer diameter (mm) *
TFPRO2 EAS2
Bronze
OST2
17
Spiral 6.9/9.0
KO16
Internal sn
Internal sn
He2539 0.826
6005-K 0.820
17
15
15
10
1
39.6 29.3 42.05
JAI2
Bronze
193.3
KO19
KATRC38c6b7A 0.82
15
1
RFTF1
45/87/ 126/245 520
JAB1
RF33
Internal sn
Bronze
Internal sn
6005-K 0.820
He2539 0.826
10
15
0.97
454
265.4
302.3
247.2
281.9
32.0 27.7
23.6 29.1
32.7 29.6
33.7 29.8
41.45
42.05
42.68
42.66
41/80/ 125/240 820 508
JATF3
JAI1
1
RF31
> 248
239
34.6
29.7 31.7*
n.a. 33.5
30.1 32.9
43.7
43.7
43.9
43.9
32.9
JAD
Base
Alt1
Internal sn
06494-1 LK0003 0.82
NT8404 0.82
NT8401 0.77
15
15
13
14
1.05
1.19
1.07
Alt1b
Internal sn 9484 0.82
9707 0.77
17 1.20
“ITER”
Alt*
80/140/ 45/85/ 178/300 127/254 420
Alt* Perforated pipe 7.5 × 9.5 25/127 78/139/ /254 190/304 457 457
25/127 /304 437
508
245.9
238.8
209
278.4
238.1
36.2 31.8
31.0
43.7
0.96
Opt2
1.08
45/84/ 124/250 453
234.0
JAC
USTF2
Bronze
Spiral 7×9
45/85/ 130/250 450
USTF1
BrP 0.82
1
“ITER” = (2s/c + 1Cu) × 3 × 5 x 5 + core x 6 Spiral Spiral 7×9 6.7 × 9.0
Spiral 6.7 × 9.0 45/85/ 130/250 447
The “Alt” cable pattern is (((6s/c + 1Cu) × 6 + 1Cu) × 5 + 1Cu) × 6.
JATF1
JAB2
7567, 7603 7730 0.815
Spiral 7.0 × 9.1 116/182/ 245/415 520
45/87/ 126/245 486 492
KOTF
7878
0.915
43.45
OST1
Internal sn
NSTT8305 0.813
33.8
JATF2
Spiral 7×9
207.8 27.1
18.5 n.a.
29 42.65
209
43.7
43.8
30.0
36.1 n.a.
43.7
43.8
P. Bruzzone et al. / Fusion Engineering and Design 84 (2009) 205–209
Sample nickname
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3. Test results The test program agreed with ITER has been applied to all the samples, with main focus on the 10 V/m current sharing temperature, Tcs , at the TF nominal operating current, I = 68 kA, and effective field, B = 11.15 T (background field BSULTAN = 10.78 T). A number of load cycles (600–1200) and one thermal cycle (warm-up/cooldown) were also included in the test program, along with ac loss tests and dc test at other operating points. Here, only the first and the final Tcs run at TF operating conditions are discussed. The central channel in the conductors is plugged during the sample assembly to force the coolant flow only in the strand bundle area, with the aim to limit the variation of the coolant velocity and temperature, over the cross section. The mass flow rate is initially set at about 2 g/s. A typical Tcs run lasts 1.5 h and is as large as 120 MB. The current is first zeroed by heating the secondary conductor of the superconducting transformer. Then the current is raised to 68 kA at a rate of 150 A/s with intermediate, 5 min long holds at 10, 20, 30 and 40 kA. Then the inlet temperature is increased in steps of 0.1–0.2 K, with 2–3 min hold at each step until a quench is reached. In case the two conductors of a sample are highly unbalanced, an additional heater is switched on the stronger conductor to unbalance the operating temperature and be able to obtain the full V–T characteristic for both conductors. 3.1. Experimental issues The Nb3 Sn conductor degradation shows up as a broadening of the superconducting transition (low n index), with early voltage well below the take-off temperature. As long as the degradation is limited, say n ≥ 10, the slope of the electric field vs. temperature, dE/dT, at the criterion of 10 V/m is larger than 80 V/m K and voltage drift/offset as large as 2 V cause an very small error, <0.05 K, in the assessment of Tcs . When dE/dT ≤ 15 V/m K, the accuracy of the Tcs assessment is heavily affected by voltage drift/offset as small as 1 V. Moreover, in the large Nb3 Sn samples of the last three years, the voltage drift observed at the end of the current ramp, before any raise of temperature has assumed new proportions, with values sometime above the criterion, and prevents a straight assessment of Tcs from the raw data [17,8]. The large voltage drifts do not generate any measurable power, i.e. they are related to potential difference across the cable rather than along the cable. The nature of the voltage drift is a subject of discussion among the experts and is likely correlated with the performance degradation, as no significant voltage drift is observed is conductors with high n-index and high performance. Post-processing of voltage data as well as an enhanced number of voltage taps is used to extract from the raw data the true longitudinal voltage for the assessment of Tcs . An accurate measurement of the current sharing power by gas flow calorimetry has been used to benchmark the voltage post-processing procedure and is now integral part of the Tcs assessment method [8,16].
Fig. 1. Excellent match of CIC results of current sharing temperature vs. predicted performance from the strand scaling laws for three full size NbTi conductors tested in SULTAN.
with a thermal strain arbitrarily fixed at ε = −0.65% (see discussion below). Opposite to the NbTi full size CICC of Fig. 1, for 13 Nb3 Sn ITER TF CICC the picture at the beginning of the test campaign, Fig. 2, is much more scattered. The remaining five conductors are not included in the plot because the full set of scaling law parameters is not available to calculate the predicted performance. At least two out of 13 conductors (likely 4 out of 18) show results “better than expected”. In fact, the presence of two conductors well above the diagonal, matching line of Fig. 2 suggests that the retained strain, ε = −0.65%, for the prediction is by far too conservative and a lower “thermal strain”, in the range of −0.5% should be used. The test results at the end of the test campaign, after cyclic and thermal loading, are plotted vs. the estimated potential performance in Fig. 3. In some cases, the conductor performance was not stable at the end of the test campaign and a further degradation
3.2. The potential performance of the Nb3 Sn CICC The “CIC potential performance” is the current sharing temperature when no irreversible degradation happens and a realistic longitudinal thermal strain is applied. For the sake of qualification and acceptance tests, the only criterion is the fulfilment of the target specification, disregarding any consideration of degradation. However, a comparison between measured and potential performance is important for the refinement of the specific design criteria and, in general, for the judgement of the effectiveness of the design. To estimate the potential performance, the strand scaling law provided by the sample suppliers (ITER Domestic Agencies) are used together
Fig. 2. The initial Tcs results of 13 Nb3 Sn ITER TF CICC compared to the potential performance estimated from the respective strand scaling laws with retained ε = −0.65%.
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Fig. 3. The Tcs results at the end of the test campaign of 12 Nb3 Sn ITER TF CICC compared to the potential performance with ε = −0.65%.
but other conductors have a drop of Tcs larger than one degree. The distance from the diagonal line to the measured performance in Figs. 2 and 3 is due to the different sensitivity to the transverse load, which in turn depends on the strand layout and other cable parameters. In five cases, a straight comparison can be done among conductors, which are identical in all parameters except the void fraction in the bundle. No clear trend can be found. The lower void fraction is slightly beneficial for EAS and JAI, slightly detrimental for JAB and RF, and hard-to-decide for KO. The “long twist pitches” in the lower cable stages are applied on four conductors (OST2, JAC, JAD, Opt2) with very different results: quite bad for JAC and JAD, very good for OST2 and initially good for Opt2 (which severely degraded during the test). As there are not two conductors fully identical except the twist pitches, a straight comparison is impossible. However, other data from sub-size conductors [18] suggest that a long twist pitch in the first triplet is beneficial when all other parameters are identical. A broad superconducting transition, with power developed well below Tcs , is common to all the under-performing conductors. The range of n-index (which is not perfectly adequate to fit the very broad transitions) is between 4 and 9. With such low n-index, the Tcs at 10 V/m is not even close to the maximum allowable operating temperature for a winding with long conductor lengths exposed to the maximum field. Depending on the actual value of n-index, the maximum, steady state operating temperature must be set 0.5–1 K lower than Tcs . 5. Conclusions
Fig. 4. Summary of the test results for current sharing temperature of the 18 ITER TF conductors at the first and last run of their test campaigns.
is likely to occur. Only 12 conductors are visible in Fig. 3, as one out of 13 conductors reported in Fig. 2 had performance below 5 K at the end of the test campaign. The absolute Tcs performance of the 18 conductors at the beginning and end of the test campaign is gathered in Fig. 4. Whenever applicable, the results in Fig. 4 are obtained by both post-processing of voltage taps signals and power calorimetry. The error bar is related to the reliability of the post-processing procedure: in most cases it is ± 0.1 K and higher in case of very broad transition.
The large spread of results of the 18 ITER TF prototype conductors measured in SULTAN, over 2.5 K in Tcs , is due to the spread of the Jc in the strands supplied by nine companies worldwide and to the different load sensitivity. In most cases, the performance is well below the potential performance estimated for conductors made of non-damaged strands. As the extent of the irreversible degradation is not predictable by scaling laws, the project has retained large engineering margins in the specification, using only a fraction of the potential performance of the Nb3 Sn strands. The strategy of the test in SULTAN of every combination of strand and cable manufacturer (qualification test) is based on the assumption that the extent of degradation is reproducible as long as there is no change in the layout specification and the combination of manufacturers, including each production step. However, some minor variation of degradation (and hence performance) cannot be excluded and must be accounted in the acceptance criterion after the qualification test, to minimize the probability of rejection during the series production. As a last measure to rescue marginal conductors, the increase of the heat removal rate (larger pumping power) in the ITER TF coils may be considered. Acknowledgements
4. Discussion The potential performance in Figs. 2 and 3 is not the same for all conductors. The main driver for the large range of potential performance, from 6.03 to 6.97 K, is the strand, where the Ic at 12 T, 4.2 K ranges from 193 to 302 A, see Table 1. Other reasons are the variation of superconductor cross section due to the different Cu:non-Cu data. The scattering in the measured performance is larger, with a range of 2.09 K at the initial run and about 2.5 K at the end of the test campaign, showing that the progression of degradation is different in the various conductors. The “ranking” also changes, as few conductors have negligible change of Tcs over the test campaign,
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