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Second VAMAS a.c. loss measurement intercomparison: magnetization measurement of low-frequency (hysteretic) a.c. loss in NbTi multifilamentary strands
Cqogenics 37 (1997) 49-60 0 1996 Elwvier Science Limited Printed in Great Britain. All rights reserved 00 I I -2215/97/$I7.00
PII:SOOll-2275(96)00103-S ELSEVIER
Second VAMAS intercomparison: measurement of a.c. loss in NbTi
a.c. loss measurement magnetization low-frequency (hysteretic) multifilamentary strands
E.W. Collings, M.D. Sumption,
K. Itoh*,
The Ohio State University, Columbus, OH, USA* *National Research Institute for Metals, Tsukuba, tTokai University, Hiratsuka, Japan
H. Wada*
and K. Tachikawat
Japan
The results of the 2”d VAMAS measurement intercomparison program on low-frequency (hysteretic) a.c. loss are presented and discussed. Two sets of multifilamentary NbTi strands (Set No. 1: copper matrix, fil. diams 0.5, 1, 3, and 12 Frn; Set No. 2: cupronickel matrix, fil. diams 0.4, 0.5, and 1 pm) were subjected to interlaboratory testing. In an initial series of tests, samples in various forms (e.g. wire bundles, coils) were measured mostly by vibrating-sampleand SQUID magnetometry. Considerable scatter was noted especially in the small-filament-diameter a.c.-loss data. In a study of measurement accuracy, a supplementary series of tests compared the results of VSM measurement of a given pair of copper-matrix samples. In the light of all the results, factors contributing to a.c. loss error are discussed and recommendations are made concerning the specification of future a.c.-loss measurement intercomparisons. 0 1996 Elsevier Science Limited Keywords:
a.c. loss; magnetization
measurement;
The goals of the Versailles Project on Advanced Materials and Standards (VAMAS) and its measurement intercomparison programs have been outlined by Tachikawa’. Under the VAMAS intercomparison programs, measurements are carried out in numerous laboratories using various techniques. The results are compared and the technical problems analyzed with a view towards making recommendations for the standardization of such measurements. The results of the first VAMAS a.c. loss intercomparison program were reported in the proceedings of three successive ICMC conferences-in 19891m6, 19917, and 19928. Partially based on the outcome of that program a second one, designated 2”d VAMAS a.c. loss measurement intercomparison was initiated. Subworking Group-2 (SWG-2) of this program was dedicated to the magnetic measurement of hysteresis loss in slowly varying ap+lied fields. The initiation of the second a.c. loss program was announced by Wada and Itoh who subsequently reported on the Japanese SWG-2 results’, and went on to summarize the results of some of the intercomparison tests”. This paper represents the first
*Research performed while E.W.C. telle Memorial Institute, Columbus,
and M.D.S. OH, USA
were
with
Bat-
NbTi multifilamentary
strands
complete report on the findings of the 2”d VAMAS loss measurement intercomparison.
a.c.
General guidelines to the a.c. loss measurement intercomparison SWG-2 Sample specifications The VAMAS Technical Working Party Office (National Research Institute for Metals, Tsukuba, Japan) supplied the participating groups with two sets, or series, of multifilamentary NbTi strand (Series H: copper matrix, nominal fil. diams 0.5, 1, 3, and 12 pm; Series I: cupronickel matrix, nominal fil. diams 0.4, 0.5, and 1 pm) from which they were to prepare their own magnetization samples. Recommended sample geometries were: (a) a coil, (b) a square bundle (dense square pack), and (c) a flat bundle (single linear row of strands), although other geometries (e.g. cylindrical bundle) were admissible. To be specified by the individual groups were: (a) the numbers of layers and turns per layer, and the inner and outer diameters of the coil; (b) the numbers of rows and strands per row and length of the square bundle; (c) the number and length of the strands in the flat bundle. It was recommended that bundles should be prepared
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Second
VAMAS a.c. loss measurement
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from equal length pieces of strand, cut perpendicular to the strand axis and metallographically polished to eliminate the possibility of interfilamentary connections at the ends. The strand pieces were to be insulated from each other (in fact the strands had already been varnished by the manufacturer). It was recommended that any coil sample receive no more than 2% bending strain during winding around a form.
E. W. Callings et al. Table 1
Participating laboratories
Institution
Code
Atominstitut der dsterreichischen Universitaten,
Batelle Memorial OH, USA
specifications
C
CISE, Milan, Italy Hitachi Research Laboratory, Japan
Magnetic field The external field was to be applied perpendicular to the strand axis and oscillated at amplitude ABl2 (field-sweep span, AB) about background fields, B,. The standard background field was to be B. = 0, and the associated results are reported here. The measurement group could decide on the form of the field oscillation (linear ramps or triangular waveforms were usually selected). It was recommended that for AB < 1 T, AB should be incremented in steps of 0.1 T, and for AB > 1 T the increments should be an integral number of tesla. The field uniformity over the sample volume should be better than 1%. Nonuniformities between 1% and 5% should be specified; deviations greater than 5% were not allowed. The field sweep rate should be sufficiently slow as to eliminate the possibility of significant error due to eddy current loss. Otherwise the experiments should be performed over a range of values of dB/dt and the results backextrapolated to zero dB/dt. Measuring technique It was recommended that the a.c. loss measurements be performed primarily using SQUID magnetometers or the vibrating sample type of magnetometer (VSM). Hysteretic a.c. loss-including proximityeffect (PE) interfilamentary coupling loss-was to be obtained by integrating the magnetic hysteresis loop. Temperature The temperature of measurement was required to be 4.2 K; otherwise the results should be corrected to that temperature. Details of the correction should be provided. Calibrations (a) Magnetic Moment-The magnetic moment was to be calibrated against reference samples. Niobium foil was supplied in order to provide a materialindependent magnetization throughout its Meissner regime. The saturation moment of pure Ni was also recommended as a reference, although other standard reference materials were implicitly admitted provided they also were specified. (b) Magnetic Field-No recommendations were issued regarding magnetic field calibration. Secondary SWG-2
program
within intercomparison
Participating groups were also invited to carry out a secondary research program to study the effects on the measured moment of: (a) variation of the number of wires in the bundle, (b) variation of the sample position relative to those of the pickup coils. Participants were also invited to explore the question of field homogeneity. A list of program participants is given in Table 1; some experimental details are provided in Table 2.
50
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B
Institute, Columbus,
C.R.E. ENEA, Frascati (Rome),
Measurement
A
Vienna, Austria
Italy
D
Hitachi,
E
National Research Institute for Metals, Tsukuba, Japan
F
Tokai University,
G
Toshiba
Technische Austria Teledyne USA
Hiratsuka, Japan
R&D Center, Kawasaki, Japan Universitlt
H, h
J, L h
Wien, Vienna,
K
Wah Chang Albany, Albany,
Vacuumschmelze Germany
Supplementary
L
GmbH, Hanau,
intercomparison
program
In order to separate errors associated with individual sample preparation from measurement error a supplementary intercomparison program was offered. In it, two pre-prepared samples containing specified volumes of NbTi, were circulated for VSM measurement. Groups B, J, and L elected to participate in the supplementary program. Although small in number, the results made an important contribution to the error analysis.
Sample
materials
and experimental
results
Strand materials Two series of multifilamentary double-stacked strands were manufactured by a commercial vendor. The Series-H strands used as a matrix oxygen-free seamless copper pipe (Japanese JIS Code 11351OClOl lT-H) with a starting RRR of 350-420. The Series-I strands used as matrix material commercial ‘cupronickel’, an alloy of nominal composition 10 wt% Ni plus 0.7-0.8 wt% Mn. In this program, and in several publications stemming from it, the commercial cupronickel matrix alloy has been referred to just as ‘Cu lO%/Ni’ ’ ’ or ‘CuNi’ 12-i4. However, it will be shown in Appendix A that the presence of the 0.7-0.8 wt% Mn played a very important role in controlling the magnetic properties of the I-series strands. The strand manufacturing sequences are summarized in Figure I. It is important to note that cold extrusion was employed; according to the manufacturer this was intended to obviate the need for providing the NbTi filaments with a Nb coating. The strand specifications are listed in Table 3, and SEM micrographs are presented in Figures 2a and 2b. Experimental
results
Magnetization, M, was measured as the applied field was cycled between B,,, and B,,i, (such that B,,, - B,i, = AB) about a background or bias field B,. Although a few groups used bias fields of 0 and 1 T, only the B. = 0 T results are reported here. The hysteretic loss per cycle was obtained
Second Table 2
Measurement
VAMAS
a.c. loss measurement
intercomparison:
E. W. Callings et al.
method and sample form
Code
T (K)
Technique/calibration standard
Sample form
No. of wires
Wire length, cm
d B/d t, mT/s
A B C D E F G H h J#i jj K
4.2 4.2 4.6 4.2 4.5 4.2 4.3 4.2 4.2 4.2 4.2 4.22
SQUID:Pt,Cu,AI VSM:Ni(=T) SQUID:Pd IND%Nb SQUID:Nb VSM:Ni (2T) VSM:Ni SQUID:Pd IND:Nb VSM:Ni VSM:Ni VSM:Ni (IT)
single layer flat” cylindrical bundle single wire l-3 layer coils single wire square bundle square bundle l-layer coil 5-layer coil single layer flat” square bundle helical coil
6 19-25 1 1 1 36” 36 1 1 6 25 1
0.310-0.356 0.637-0.785 0.505-0.600 256.7-279.9 0.534-0.571 0.464-0.472 0.5 9.98-30.03 7891-7937 0.208-0.357 0.327 12.66
Ob 0” 0” 20 Ob 1.25-2.5 0.67-1.67 0.23-5.0 0” 0.18-1.0 0.18-1.0 Ob
“H, to flat “Negligibly small dB/dt for SQUID magnetometer “Corrected to zero dBldt, see below dlnductive method “35 wires in one case
(or other step and hold type loops)
NbTi rod -----
NbTi rod ---
monocore
- _-
“CuNi” pipe
---II
CuNi” pipe
monocore
First Stack --f Hl: 931 cores
First Stack 931 cores
assembling c
1
drau(ing
/drawing
]
Second Stack H2: 7x93 1 = 6,5 17 cores r&T” H3: 61x931 = 56.791 cores
pipe
H4: 361x931 = 336,091 cores) +I
11: 61%?:&: cores h&j-12: 361x931 = 336.091 cores 13: 931x931 = 8661761 cores)
“CuNi” pipe
1 strand Hl only
1
twiscfing
1 L,=8.9mm
final
drawing
L, = 8.9 mm
0.51 mrna
0.51 mm@
5
insulation
(a) Figure 1
Manufacturing
(b)
Fabrication of the H-Series Strands
steps for (a) the Cu-matrix
strands (Series-H)
from the area of the M-B hysteresis loop and converted to a per-cyclic loss per unit volume (Q, J/m’) based on the measured or deduced volume of NbTi present in each sample. The experimental results were expressed as log-lot plots of Q versus AB as shown in Figure 3 for the seven
Fabrication of the I-Series Strands
and (b) the cupronickel-matrix
strands (Series-l)
samples H 1-H4 and 11-13. It can be readily seen from these figures that the spread in the data increased as the filament diameter decreased. Also from the observation that data from a given group tended to be either uniformly higher or uniformly lower than ‘average’ we deduce that the domi-
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Second Table 3
VAMAS a.c. loss measurement
intercomparison:
Strand specifications
Sample code
Specific mass NbTi, m,, mg/cm
Filament No., N
Measured filament diam.“, dr, Km
Nominal filament diam.“, d,, wrn
Filament volume variance,
HI H2 H3 H4
5.731 3.188 3.947 4.638
931 6517 56791 33609 1
11.34 3.20 1.20 0.54
11.8 3.4 1.3 0.54
8.11 12.5 16.7 0
II I2 I3
3.854 4.356 4.725
56791 33609 1 866761
1.19 0.52 0.34
1.2 0.54 0.35
1.7 7.7 5.9
“Takina 6.097 a/cm3 for the densitv of NbTi (assumina bSuppied by TWP office “By image analysis
Discussion of the results of the primary intercomparison program The results of the primary intercomparison program (Table 2) are summarized in Figure 3. The as-reported data exhibit scatter ranging from 11% (12) to 27% (H4). These errors, which derive from numerous sources, may be broadly characterized as: (i) external errors, including temperature, NbTi volume ratio, and NbTi density; (ii) experimental errors, including ramp rate effects (eddy current coupling), data point density, and calibration; (iii) sample induced errors, which are mainly demagnetization-, shape-, and proximity-effects. Some errors may of course belong to more than one category. All of them are considered below. Sources of errors Temperature Several groups made magnetization measurements at temperatures significantly higher than 4.2 K. As shown in Table 2 temperatures as high as 4.6 K were encountered. The submitted data were not corrected to 4.2 K. In one case the variations were so large that the data could not be included in this report. NbTi volume and density NbTi volume determination was a significant source of error. The primary methods of volume determination (not all were reported) were: (i) calculation based on the supplied nominal filament diameters; (ii) in-house measurements by the etch-and-weigh procedure. The latter is expected to yield very small errors provided the NbTi density is accurately known and the presence of filament/matrix barriers or external coatings are properly accounted for. In the present cold-extruded strands no filament/matrix barriers were used; the manufacturer had, however, applied an insulating varnish to the external surface. The results of the two groups that had reported using the etch-and-weigh method (B and K) differed by about 5%. This surprisingly large discrepancy cannot be fully accounted for in terms of a combination of gravimetric
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Filament space”, d,, Frn
dJdr
2.14 0.54 0.19 0.10
0.19 0.17 0.16 0.19
% IL 0.24 f 0.08 * 0.09 * 0.05
Nb-46.5 Ti)
nant errors are systematic rather than random. As discussed below, the principal systematic errors, which increase as the filament diameter decreases are: (i) a disparity between the nominal filament diameter, used by some groups, and measured diameters used by others; and (ii) a samplelength-dependent proximity effect (PE) contribution to the loss.
52
E. W. Callings et al.
uncertainties (which both groups reported to be much less than 1%) and density differences ( 1.7% deriving from accepted densities of 6.200 g/cm”, Group K, and 6.097 g/cm3, Group B). When based on the starting diameter of the Nb rods and the overall area reduction ratio, filament diameter, and hence NbTi volume, is underestimated, since during cold drawing the Cu/SC ratio decreases with strand diameter.‘” The gravimetrically determined NbTi volume relies on: (i) the application of suitable etching-and-weighing procedures; (ii) an accurate knowledge of the alloy’s density. Some uncertainty is still associated with both of these factors, for which reason further work on Cu/SC ratio measurement procedures and NbTi density determination is recommended. Eddy current coupling It is well known that the energy loss per cycle due to eddy currents in a multifilamentary composite is proportional to the frequency of oscillation of the applied field, hence the ramp rate dB/dt in the case of a swept field16. For a given sample the actual constant of proportionality will depend on the sample length17. For this reason a sweep-rate correction determined on one sample of strand cannot be simply applied to another of different length; thus corrections have to be made on a group-bygroup basis. The sweep rates used by the various groups are listed in Table 2. Group B has estimated that for its samples (about 6 mm in length) the uncorrected error due to eddy currents is about 0.3% per mT/s. Even so that group took data over a range of values of dBldt and back-extrapolated the results to zero dB/dt (see Measurement Specifications, above). Similar corrections were also made by some other groups, as indicated by superscript ‘c’ in Table 2. The fastest uncorrected sweep rate was 20 mT/s (-6% error), but most were less than 5 mT/s (= 1.5%). Data point density The number of data points per complete hysteresis loop varied by roughly an order of magnitude from group to group depending on the measuring technique. SQUID magnetometry was associated with the lowest data point density (of order 100 points per loop) which nevertheless was sufficient to cover even sample H4’s PE-enhanced magnetization peak with adequate resolution. We conclude that data-point-density errors were not significant. Calibration ommended,
As indicated in Table 2, and as initially rectwo classes of calibration sample were
Second
VAMAS
a.c. loss measurement
intercomparison:
Figure 2 (a) Scanning electron micrographs of strands Hl-H4 and II-13 all taken at the same original bar in HI indicates 100 pm; filament numbers are also indicated
Cryogenics
E. W. Callings
magnification
1997 Volume
et al.
of 230 x. The
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VAMAS a.c. loss measurement
Figure 2 (b) Scanning scales are individually
54
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micrographs
1997 Volume
intercomparison:
of strands
37, Number
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Hl-H4
E. W. Callings et al.
and 11-13 taken
at various
magnifications
of 2000-6000
x. The length
Second
VAMAS
a.c. loss measurement
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E. W, Callings
et
al.
ld
lo’
ld
ld
10’
100 1CP I
10-l “‘.”
10’
100 .
. . ..‘.I
. . ..‘.‘I
I
10’
10’
100
‘.....I
. “‘...I
lo“
.
T
10’
16
Applied Field Span, AJ3, tcsla
Applied Field Spaa. AB, tcsla Figure 3 Primary intercomparison program results: hysteresis AB, (tesla) for the Cu-matrix (Series-H) and the cupronickel-matrix are the laboratory codes of Tables 1 and 2
loss,
Oh, (per cycle, per unit NbTi volume) versus (Series-l) types of multifilamentary NbTi strand.
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Data-point letters
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VAMAS a.c. loss measurement
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employed: ( 1) pure Nb, to be used to generate a materialindependent Meissner magnetization; (2) a metal, usually Ni, with a known magnetization. The disadvantage of Nb as a calibration standard is that it must also be associated with an accurately known magnetic field. On the other hand the magnetization of Ni is independent of the field strength provided that it is sufficiently high (several tesla) to achieve saturation. If a sufficiently high field is not available an extrapolation procedure leading to a ‘calibration magnetization’, MC,,, based on the known functional form of M,,(B) can be employed. Group B calibrated its apparatus using a short cylinder of Ni that was 2 mm long and 2 mm in diameter. The near-saturation portion of the M-B curve was fit to the expression M,,, = M,,,( l-alB-hlB’) with M,,, = 58.57 emu/g. The use of a calibration sample implies ‘measurementby-substitution’. This method relies on an insensitivity of the measurement to sample size and shape within the range of interest. These effects, which may be more important in VSM rather than SQUID measurement, were not generally investigated. However, Group B after measuring the specific magnetizations of a series of Ni samples with lengths of up to 14 mm, noted that for a typical VAMAS sample (H3, 7.07 mm in length) a correction of +1.26% needed to be applied. Group B applied sample-length corrections to all of their data. Demagnetization and shape effects In a secondary program, the research groups were invited to study the effect of variation of the number of wires in the bundle. The only participant in this secondary program was Group J who compared the magnetizations of a single flat layer (6 strands) with that of a square bundle (5 x 5 = 25 strands). The 2.7% difference between the results is close to the expected level of random error and therefore unassignable to either demagnetization or shape effects. In fact, bundle-size demagnetization effects are not to be expected. If, following common practice, it is appropriate to treat an uncoupled multifilamentary strand as a bundle of noninteracting filaments, the same approach should be valid for a cluster of such strands. Proximity effects (PE) Studies of PE-coupled multifilamentary strands in general’x-‘n and VAMAS strands in particular” have shown that the PE component of magnetization is both sample-length dependent and twist-pitch (L,) dependent. A length-independent effect can be obtained only when an untwisted strand is sufficiently long or a twisted one longer than a few twist pitches. For the present strands L, = 0.9 cm; sample lengths were mostly less than this. Thus when measurable PE is present, as it is in samples H3, H4, and I3 (with relative magnitudes H3 < I3 < H4), a length dependence should in principle be encountered in most of those samples as prepared for this study. If uncorrected, a sufficiently large PE magnetization would introduce a systematic error into the results. Unfortunately no single group performed a full study of the sample-length dependence of magnetization. Although Group A measured the length dependence of loss of most of its samples they stopped short of H4 which had the strongest PE. Nevertheless as part of a detailed analysis of overall errors (see below) it was noted that PE introduced a discrepancy as large as 85% between the magnetizations of the longest and shortest samples of H4 as measured by different groups.
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E. W. Callings et al. Random errors In addition to the above systematic errors, there are of course random experimental errors. Of those responding to the question, most groups reported total errors of between 2 and 3%. Three participants (including the two who employed inductive techniques) estimated their errors to be between 5 and 10%.
The supplementary program Description
intercomparison
of the supplementary
program
The supplementary intercomparison program (or supplementary ‘round robin’, SRR) was established in an attempt to separate out systematic error (as discussed above) from random experimental error. Groups B, J, and L participated by measuring one sample each of H3 and H4. H3 was in the form of a cylindrical 25-strand bundle, 7.07 mm in length and with a total NbTi volume of 11.442 x lo-” cm3. H4 was a cylindrical 24-strand bundle, 7.85 mm in length and with a total NbTi volume of 14.338 x 10-j cm3. The Group-B-specified NbTi volume was supplied along with the samples, thus eliminating volume-determination as a possible source of error. VSM measurements at 4.2 K were performed by Groups B, J, and L, and the results intercompared. They are presented in Figure 4 where good agreement is seen. We find the scatter in the data, expressed in the form (T&Q), where au is the standard deviation in loss, and (Q) is the average loss, to be 3.16% for H3 and 3.95% for H4. Eddy-current-induced magnetization enhancements at the ramp rates, dB/dt, used in this SRR are small. Correcting for them leaves relative uncertainties, u/(Q), of 2.25% for H3 and 3.25% for H4, levels which are commensurate with estimated random experimental errors (see above). The ramp-rate-corrected results of the three-participant SRR confirm that the random experimental error to be expected in VSM measurement is about 2-3%. They also indicate that the dominant errors in the primary inter-
& ?i5
3 > p
IO4
!s .s 3
k al
z
u”
& 2
3
$
103
0)
s? D
10-1
100
Applied Field Span, m,
tesla
Figure 4 Supplementary intercomparison program results: hysteresis loss, Q,, (per cycle, per unit NbTi volume) versus field-sweep span, AEZ, (tesla) for provided samples of strands H3 (open symbols) and H4 (filled symbols)
Second comparison program (or primary systematic in nature.
VAMAS
a.c. loss measurement
‘round robin’, PRR) are
Comparison of the primary intercomparison results
and supplementary
of errors: levels of error
As pointed out in the preceding discussion, once the respondents were provided with a prepared sample (fixed length, fixed NbTi volume) the scatter in the data dropped to that expected from random experimental error. Most of the error in the PRR was systematic in nature. In an exploration of the principle sources of such error the PRR data were subjected to successive stages of error correction the results of which are summarized in Figure 5. The following analysis is based just on data taken at a AB of 1 T. Zeroth level of correction Expressed as usual in terms of so/(Q) this level of correction represents the scatter in the as-reported data; it ranges from 11% to 27%. I
I
I
I
First level of correction
Second level of correction The second level of correction was based on adjusting the NbTi volumes of the individual samples measured. All data were adjusted to what we believe to be the correct volume of NbTi in each case. The normalization consisted in ensuring not only that the NbTi volume per cm of strand (V&) was the same for each group but that it agreed with the values of V,,lC obtained by Group B. Only one group reported more-or-less directly the values of V&t that it used. The others provided sufficient information (generally in the form of a nominal filament diameter and a simple formula) to enable us to deduce their individual V,,,lJ!s. These were then compared to the corresponding values of VSC/elGroupB. As expected, the variances encountered correlated with the filament-diameter-calculated ‘Volume Variances’ listed in Table 3. The volume corrections were then applied to the previously corrected results to provide the volume-corrected ad(Q) depicted in Figure 5. Volume correction dropped the previous scatter by 0 to 7 percentage points. Third level of correction Even after the above corrections had been performed large errors still existed, particularly in the Q-data for the tinerfilament strands. The filaments particularly in H4 and 13 are strongly coupled by PE which is known to be responsible for a length dependent magnetization component’x~2”. The visual signature of PE coupling is the excess low-field magnetization that can be seen when the magnetization loops for clad (as-received) and bare (Cu-etched) strands are superposed as in Figure 6. Strands H3 and I3 exhibited similar PE signatures. To correct for PE we took advantage of its sample-length
I
I
I
I
I
I
0
1
2
3
4
Correction
E. W. Callings et al.
The first level of correction is achieved by eliminating data taken at temperatures other than 4.2-4.3 K (the reported data ranged in temperature from 4.2 K to 4.6 K). Almost no change in co/(Q) resulted from this.
Since Group L is not represented in the PRR, PRR/SRR error analysis is possible only by using data from Groups B and J. Although the data are scarce, the results are interesting. Within the SRR’s the Groups B and J results (for AB = 1 T) differed by 2.05% (sample H3) and 5.00% (sample H4). For comparison, the percent differences between Groups B and J’s results in the PRR were 23.39% (sample H3) and 84.91% (sample H4). In the SRR, Groups B and J measured the same samples. In the PRR, different samples with different lengths were measured. The lengths of sample H3 were 7.1 mm (Group B) and 3.3 mm (Group J); those of sample H4 were 7.9 mm (Group B) and 3.3 mm (Group J). The difference in lengths coupled with the existence of PE is responsible for the errors. We are led to the conclusion that length-dependent PE is one of the dominant sources of error in the PRR.
General discussion correction
intercomparison:
5
Level
-2
-1
0
Magnetic Figure 5 Error analysis: relative standard deviations (RSD) after successive levels of correction have been applied to 0, data for the seven strand types. PRR, levels O-3; SRR, level 4
Field Strength,
tesla
Figure 6 Proximity-effect enhanced magnetization (-) as compared to bare l---j samples of strand
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57
Second
VAMAS
a.c. loss measurement
intercomparison:
dependence. All groups reported their sample lengths, e. For each strand type it was therefore possible to create straight-line plots of Q versus e based on the data supplied by each responding group. The existence of slope was expected to correspond to the visual occurrence of PE. This it did, but with some exceptions which may have been a result of random- or some unknown source of error. Thus H4 and I3 (the most strongly PE-coupled strands) exhibited both slope and visual PE. On the other hand 12 yielded just a Q(e) slope while H3 only a visual PE. But whatever the slope, zero or otherwise, Q(e) was extrapolated to some arbitrary reference length (e = 1 cm was chosen) and the resulting length-adjusted co/(Q) computed. The levels of scatter, reduced step-by-step in the manner just outlined, drop to about 6% (Figure 5, Station 3). At this point we have exhausted all correction possibilities as we see them. Nevertheless the breadth of scatter is still several percentage points greater than that of the SRR (Figure 5, Station 4). Some possible future round robin in which the primary and supplementary programs are equally well represented may shed some light on this question, but see below.
Summary
and conclusion
In the PRR, the greatest contributor to scatter in the asreceived data (most severe in the finest filament strands) was associated with the sample-length dependent PE. This error was not present in the SRR in which sample length was not a variable. Another significant source of error in the PRR derived from incorrect estimates of the NbTi volumes. Again this error disappeared when pre-prepared samples were circulated for measurement. Scatter in the PRR results decreased from 1 l-27% (for the as-received data) to about 6% as first NbTi volume and then sample length (for PE) received correction. At this point the scatter in the corrected PRR data should have been equal to that for the SRR. That they were not (6% versus 2-3%, respectively) indicates either that other unidentified sources of error were present, or that the intercomparisons were too unevenly represented. Further error analysis, see Appendix 2, favors the latter alternative.
Recommendations for future a.c. loss intercomparisons
Cryogenics
1997 Volume
et al.
diameter, determination; (2) as before, the circulation for measurement of a typical pre-prepared sample. It may in fact be useful to set up in the near future sepatate international programs to deal with: ( 1) matrix/SC-ratio measurement and (2) NbTi-alloy density determination.
Recommendations for standard frequency a.c. loss testing
low-
The present experience suggests that the standard a.c. loss test should be performed under at least the following conditions: (1) Magnetization should be calibrated by an approved method against either Nb (material-independent Meissner effect) or some standard reference material such as pure Ni. Magnetic field strength should be calibrated against a certificated NMR or Hall probe. (2) Thick-filament samples of any reasonable length may be used; but if preliminary measurement indicates the presence of PE the only recourse is to measure samples that are several twist pitches in length. With an ‘axial field’ solenoidal magnet, long helical coils may be used. In the ‘transverse field’ of an electromagnet, special coils21,‘2 will need to be constructed. (3) NbTi mass per unit length should be measured by each group using an approved etch-and-weigh method and reported. The resulting NbTi volume per unit length, and hence filament diameter, should also be reported along with the value of the NbTi density that was used (and which may be supplied to all participants). (4) As already specified in the PRR: ‘the field sweep rate should be sufficiently slow as to eliminate the possibility of significant error due to eddy current loss; otherwise the experiments should be performed over a range of values of dBldt and the results back-extrapolated to zero dBldt’.
Acknowledgements The authors wish to thank K. Aihara, P. Gilson, J. McKinnell, M. Siddall, W. Steiner, M. Thoner, H.W. Weber, E.S. Yoneda, and S. Zanella for their conscientious and dedicated participation in the 2”d VAMAS a.c. loss measurement intercomparison, SWG-2.
low-frequency References
It has been emphasized that intercomparison scatter results from a combination of systematic and random experimental error. As pointed out above, the systematic error is dominant. Special care must be taken to ensure that an accurate NbTi volume is being used to normalize the a.c. loss data. Precautions must also be taken to eliminate PE-magnetization variation as a source of error. This can be achieved by insisting that strand samples be longer than a few twist pitches (the minimum length in terms of twist pitch needed to guarantee a length-independent PE magnetization still needs to be determined). It is also suggested that all groups participating in any primary intercomparison program should also agree to participate in a supplementary program aimed at isolating systematic error. The accompanying supplementary program should consist of at least the following two components: ( 1) a matrix/SC-ratio, and hence filament
58
E. W. Callings
37, Number
1
Tachikawa, K. The VAMAS Intercomparison in the Area of Superconducting and Cryogenic Structural Materials, Advnnces in Cryogenic Engineering (Materials) ( 1990) 36 1- 10 Callings, E.W., Marken Jr., K.R., Sumption, M.D., Goldfarb, R.B. and Loughran, R.J. AC Loss Measurements of Two Multifilamentary NbTi Composite Strands, Advnncrs in Cryogenic Engineering (Muteri&) ( 1990) 36, 169- 176 Zannella, S., Gilson, P., Ottoboni, V., Ricca, A.M., Ripamonti, G. AC Loss Measurements on NbTi Superconducting Wires for the VAMAS Round Robin Test, Advances in Cryogenic Engineering (Materials) ( 1990) 36 177- 18 1 Roovers, A.J.M., van der Vegt, H.A., Siekman, K.W., Wells, T., van de Khmdert, L.J.M. and Sabrie, J.L. A.C. Loss Measurements on Superconductors Using a (Low-Inductive) Coil Geometry, Advan& in Cryogenic En&wring (Materials) ( 1990) 36 19 I ~ I98 Itoh, K., Wada, H.. Ando, T.. Yoneda, E., Ito. D.. Iwakuma, M., Yamafuji, K., Nagata, A., Watanabe, K., Knbota, Y., Ogasawara, T., Akita, S., Umeda, M., Kimura, Y. and Tachikawa, K. VAMAS Intercomparison of AC Loss Measurement: Japanese Results, Advances in Cryogenic Engineering (Materials) ( 1990) 36 199-206
Second 6
7
9
10
II
13
14
is
16 I7
(Materials)
( 1992) 38 459-468
(1992)
38 767-774
Yasohama, K., Kubota, Y. and Ogasawara, T. A.C. Susceptibility and Filament Coupling of Multifilamentary Superconducting Wires. IEEE Trms. Superconductivity ( 1993) 3 I38- 14 I Pobik, M., Krempask$, L., MajoroS, M., Suchofi, D. and Kirchmayr, H. Anomalous Magnetization Behavior in Fine Filamentary NbTi Superconducting Wires. IEEE Trans. Superconductivity ( 1993) 3 150-152 Krauth, H., Szulczyk, A. and Thiiner, M. Critical Current Density and Magnetization of NbTi and Nb,Sn Fine Filament Superconductors, Proc. 7”’ Int. Workshop on Critical Currents, Alpach. Austria, Jan. 23-27, 1994, to be published Valaris, P., Kreilick, T.S., Gregory, E. and Callings, E.W. The Effects of Processing on the Filament Array in Multifilament SSC Strand, in Supercollider I (Ed M. McAshan) Plenum Press, NY (199O)pp.449-456 Carr Jr., W.J. A.C. Lms and Macroscopic Theory of Suprrconductom Gordon and Breach, NY ( 1983) Carr Jr., W.J. End Effects on the Loss for Short Superconductors,
Ad~wtcus ill Cryogenic En@rerin~ (Motrriuls) ( 1982) 38 58 l-586 I8
Yamafuji, K., Harada, N., Mawatari, Y., Makeo, M., Akune, T., Sakamoto, N., Minra, 0. and Tanaka, Y. Creep Rates of Excess Magnetization due to Interfilamentary Proximity Couplings in Superconducting NbTi Multifilamentary Wires, Supercond. Sci. Tech&
(1992) 5 sll7~sl2O I9
20
21
22
E. W. Callings et al.
Wada, H. and Itoh, K. Towards the International Standardization of Superconducting Materials: Development of Standard Measurement Methods for Critical Current and a.c. Losses, Ciygenics (1992) 32 (ICMC Supplement) 557-564 Itoh, K. and Wada, H. Magnetization of VAMAS NbTi a.c. Loss Test Wires, Proc. ICFA Workshop on U.C. Superconductivity, KEK Proc. 92-14 (Ed K. Tsuchiya) (1992) 149-151 Itoh, K., Wada, H. and Tachikawa, K. 2’ldVAMAS a.c. Loss Intercomparison on NbTi Wires, Proc. 8”’ US-Jnpnn Workshop on HighField Superconductor Materiuls, University of Wisconsin-Madison, March 17-19, 1993 (Ed K. Yamafuji, H. Wada, D.C. Larbalestier and M. Suenaga) ( 1993) 73-77 Yoneda, E.S., Itoh, D., Takano, I., Akita, S., Torii, S., Kumano, T. and Suzuki, E. kA-Class a.c. Superconducting Cable Development, Advnnces in Cryo,genic Engineering (Materials)
12
intercomparison:
Schmidt, C. Anomalous Low Hysteresis Losses in NbTi Superconductors with Very Fine Filaments, Advances in Cryogenic Engineering (Materi&) ( 1990) 36 207-214 Itoh, K., Wada, H. and Tachikawa, K. Results on the First Intercomparison of ax. Loss Measurements, Advnnces in Ctyogenic Engineering
8
VAMAS a.c. loss measurement
Harada, N., Mawatari, Y., Miura, O., Tanaka, Y. and Yamafuji, K. Excess Magnetization due to Interfilamentary Proximity Coupling in Nb-Ti Multifilamentary Wires, Cryogenics (1991) 31 1X3-191 Sumption, M.D. and Callings, E.W. Effect of Twist Pitch, Sample Length, and Field Orientation on the Proximity Effect Enhanced Magnetization of Fine Filamentary Multifilamentary Strands, Advances in Cryogenic Engineering (Mutericds) ( 1992) 38 783-790 Sumption, M.D., Marken Jr., K.R. and Callings, E.W. Enhanced Static Magnetization and Creep in Fine-Filamentary and SSC-Prototype Strands via Helical Cabling Geometry Enhanced Proximity Effects, IEEE Trans. Appl. Superconductivity ( 1993) 3 75 l-756 Sumption, M.D., Pyun, D.S. and Collings, E.W. Transverse and Longitudinal Resistivities in NbTi Multifilamentary Strands with Cu and CuMn Matrices. IEEE Tram. Appl. SuprrcondwtilGty ( 1993) 3
t
t
-0.6
I
-0.4
-0.2
I
I
0
0.2
I
0.4
0.6
Magnetic Field Strength, B, tesla Figure Al M-B hysteresis NbTi) for cupronickel-matrix (-_). Note the emergence filament strand
loops (normalized to unit volume of strands II (-.-.), 12 (---), and 13 of PE magnetization in the finest-
NbTi/CUlONi multifilamentary composite as well as a Al. Magnetization strong paramagnetic tilt”, Figure measurements of strands 12 and 13 taken at 10 K also exhibited this tilt, Figure AZ, indicating that it is a property of the matrix, not of the superconductor. By way of confirmation, bulk samples of the matrix alloy sent to Battelle for measurement showed esssentially the same M-B behavior in either the as-received condition or after a solution heat treatment of 45h/992”C. The M-B curve for a Cu-lS%Ni research alloy is also shown in Figure A2. This line lies quite close to the B-axis, clearly indicating that the I-series matrix cannot possibly be described as a ‘Cu-lO%Ni’ alloy. About 10% of Ni is certainly present-but so also is a significant concentration of Mn according to microprobe analysis of the bulk matrix material performed at Battelle ( 10.3 wt%Ni, 0.78 wt%Mn),
0.61
859-862 23
Sumption, M.D. and Callings, the Low Temperature Magnetic Appl. Phys.
E.W. Influence of Ni Additions on Properties of a Cu-l%Ni Strand, J.
( 1994) 76 7461-7467
Appendix A Mn-induced paramagnetic offset and proximity-effect suppression in the l-series strands
I-I
-2
The matrix material of the I-series wires was commercial cupronickel. It has been frequently described as a ‘Cu lO%Ni’ ‘I or ‘CuNi’.‘Z-14 alloy; but magnetization measurement showed the I-series strands to have unexpectedly low levels of PE coupling for a supposed
Magnetic
Field Strength,
B,
tesla
Figure A2 M-B curves for a pair of l-series strands (I2 and 13) taken at 10 K juxtaposed against 4.2 K curves for Cu-15%Ni and Cu-l%Mn indicating that it is the Mn and not the Ni that is responsible for the l-series’ paramagnetic tilt
Cryogenics
1997 Volume
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59
Second
VAMAS a.c. loss measurement
intercomparison:
E. W. Collings et al. I4 it is Mn, and not Ni, that is responsible for the PE suppression noted in the VAMAS I-series strands.
Appendix B Confidence levels in intercomparison
Correction Level Error analysis: mean error (ME) after successive levFigure Bl els of correction have been applied to 0, data for the seven strand types. Primary intercomparison (PRR), levels O-3; supplementary intercomparison (SRR), level 4
The loss data’s conventional standard deviation, uo, normalized by its mean value (Q) has been used in the main text since quotients of the form ad(Q) are traditional measures of relative error. The relative standard deviation (RSD) ad(Q) certainly represents the precision of a result derived from an ensemble of measurements. It does not, however, represent the accuracy of any subsequent individual measurement. Whereas the RSD decreases as the ensemble number increases, the accuracy of any single subsequent measurement does not change. The essence of an intercomparison is to develop a measure of this accuracy, or reliability, as well as to develop procedures for improving it. Returning to the question of individual measurement accuracy, we recommend the use of a ‘mean error’, ME, defined by ME =
the subsequently received manufacturer’s specifications (911 wt%Ni, 0.2-1.0 wt%Mn), and microprobe analysis of the composite’s matrix (8.65 wt%Ni, 0.76 wt%Mn). Figure A2 confirms the validity of the latter analysis in that the paramagnetic slope exhibited by 12 and I3 is just a little lower than that of a Cu-l%Mn research alloy. Figure A2 and a detailed study of the magnetic properties of a set of binary and ternary research alloys of compol%Mn and lSat%Ni (see and sitions above), l%Mn+lS%Ni (all at%)2’ confirm that Mn is almost entirely responsible for the observed paramagnetic tilt. Furthermore, it is generally recognized by now that 0.5 wt% of Mn dissolved in the Cu matrix is a strong suppressor of PE in multifilamentary composites. Even more effective would be 0.8 wt% of Mn. Thus contrary to earlier reports”-
60
Cryogenics
1997 Volume
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results
C> n(Q)
Since this is derived from the ensemble study, but yet is not a function of the number of participants, it has the property which we seek. We have calculated the ME for the strands as a function of correction level and have plotted the results in Figure BI. The values are somewhat lower than the RSD numbers plotted in Figure 5, but similar in trend. Since the ME is independent of the number of measurements, we might expect to find the levels of ME in the PRR and the SRR to be the same if all external and sample errors had been eliminated. Indeed for the PRR at correction Level 3, (ME) = 3.5% while MEHs = ME,, = 3.37%. The latter compares well to the corresponding errors for the SRR at the same correction level, which are MEH3 = 2.28% and ME,, = 2.83%.
PII:SOOll-2275(96)00103-S ELSEVIER
Second VAMAS intercomparison: measurement of a.c. loss in NbTi
a.c. loss measurement magnetization low-frequency (hysteretic) multifilamentary strands
E.W. Collings, M.D. Sumption,
K. Itoh*,
The Ohio State University, Columbus, OH, USA* *National Research Institute for Metals, Tsukuba, tTokai University, Hiratsuka, Japan
H. Wada*
and K. Tachikawat
Japan
The results of the 2”d VAMAS measurement intercomparison program on low-frequency (hysteretic) a.c. loss are presented and discussed. Two sets of multifilamentary NbTi strands (Set No. 1: copper matrix, fil. diams 0.5, 1, 3, and 12 Frn; Set No. 2: cupronickel matrix, fil. diams 0.4, 0.5, and 1 pm) were subjected to interlaboratory testing. In an initial series of tests, samples in various forms (e.g. wire bundles, coils) were measured mostly by vibrating-sampleand SQUID magnetometry. Considerable scatter was noted especially in the small-filament-diameter a.c.-loss data. In a study of measurement accuracy, a supplementary series of tests compared the results of VSM measurement of a given pair of copper-matrix samples. In the light of all the results, factors contributing to a.c. loss error are discussed and recommendations are made concerning the specification of future a.c.-loss measurement intercomparisons. 0 1996 Elsevier Science Limited Keywords:
a.c. loss; magnetization
measurement;
The goals of the Versailles Project on Advanced Materials and Standards (VAMAS) and its measurement intercomparison programs have been outlined by Tachikawa’. Under the VAMAS intercomparison programs, measurements are carried out in numerous laboratories using various techniques. The results are compared and the technical problems analyzed with a view towards making recommendations for the standardization of such measurements. The results of the first VAMAS a.c. loss intercomparison program were reported in the proceedings of three successive ICMC conferences-in 19891m6, 19917, and 19928. Partially based on the outcome of that program a second one, designated 2”d VAMAS a.c. loss measurement intercomparison was initiated. Subworking Group-2 (SWG-2) of this program was dedicated to the magnetic measurement of hysteresis loss in slowly varying ap+lied fields. The initiation of the second a.c. loss program was announced by Wada and Itoh who subsequently reported on the Japanese SWG-2 results’, and went on to summarize the results of some of the intercomparison tests”. This paper represents the first
*Research performed while E.W.C. telle Memorial Institute, Columbus,
and M.D.S. OH, USA
were
with
Bat-
NbTi multifilamentary
strands
complete report on the findings of the 2”d VAMAS loss measurement intercomparison.
a.c.
General guidelines to the a.c. loss measurement intercomparison SWG-2 Sample specifications The VAMAS Technical Working Party Office (National Research Institute for Metals, Tsukuba, Japan) supplied the participating groups with two sets, or series, of multifilamentary NbTi strand (Series H: copper matrix, nominal fil. diams 0.5, 1, 3, and 12 pm; Series I: cupronickel matrix, nominal fil. diams 0.4, 0.5, and 1 pm) from which they were to prepare their own magnetization samples. Recommended sample geometries were: (a) a coil, (b) a square bundle (dense square pack), and (c) a flat bundle (single linear row of strands), although other geometries (e.g. cylindrical bundle) were admissible. To be specified by the individual groups were: (a) the numbers of layers and turns per layer, and the inner and outer diameters of the coil; (b) the numbers of rows and strands per row and length of the square bundle; (c) the number and length of the strands in the flat bundle. It was recommended that bundles should be prepared
Cryogenics
1997 Volume
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49
Second
VAMAS a.c. loss measurement
intercomparison:
from equal length pieces of strand, cut perpendicular to the strand axis and metallographically polished to eliminate the possibility of interfilamentary connections at the ends. The strand pieces were to be insulated from each other (in fact the strands had already been varnished by the manufacturer). It was recommended that any coil sample receive no more than 2% bending strain during winding around a form.
E. W. Callings et al. Table 1
Participating laboratories
Institution
Code
Atominstitut der dsterreichischen Universitaten,
Batelle Memorial OH, USA
specifications
C
CISE, Milan, Italy Hitachi Research Laboratory, Japan
Magnetic field The external field was to be applied perpendicular to the strand axis and oscillated at amplitude ABl2 (field-sweep span, AB) about background fields, B,. The standard background field was to be B. = 0, and the associated results are reported here. The measurement group could decide on the form of the field oscillation (linear ramps or triangular waveforms were usually selected). It was recommended that for AB < 1 T, AB should be incremented in steps of 0.1 T, and for AB > 1 T the increments should be an integral number of tesla. The field uniformity over the sample volume should be better than 1%. Nonuniformities between 1% and 5% should be specified; deviations greater than 5% were not allowed. The field sweep rate should be sufficiently slow as to eliminate the possibility of significant error due to eddy current loss. Otherwise the experiments should be performed over a range of values of dB/dt and the results backextrapolated to zero dB/dt. Measuring technique It was recommended that the a.c. loss measurements be performed primarily using SQUID magnetometers or the vibrating sample type of magnetometer (VSM). Hysteretic a.c. loss-including proximityeffect (PE) interfilamentary coupling loss-was to be obtained by integrating the magnetic hysteresis loop. Temperature The temperature of measurement was required to be 4.2 K; otherwise the results should be corrected to that temperature. Details of the correction should be provided. Calibrations (a) Magnetic Moment-The magnetic moment was to be calibrated against reference samples. Niobium foil was supplied in order to provide a materialindependent magnetization throughout its Meissner regime. The saturation moment of pure Ni was also recommended as a reference, although other standard reference materials were implicitly admitted provided they also were specified. (b) Magnetic Field-No recommendations were issued regarding magnetic field calibration. Secondary SWG-2
program
within intercomparison
Participating groups were also invited to carry out a secondary research program to study the effects on the measured moment of: (a) variation of the number of wires in the bundle, (b) variation of the sample position relative to those of the pickup coils. Participants were also invited to explore the question of field homogeneity. A list of program participants is given in Table 1; some experimental details are provided in Table 2.
50
Cryogenics
1997 Volume
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B
Institute, Columbus,
C.R.E. ENEA, Frascati (Rome),
Measurement
A
Vienna, Austria
Italy
D
Hitachi,
E
National Research Institute for Metals, Tsukuba, Japan
F
Tokai University,
G
Toshiba
Technische Austria Teledyne USA
Hiratsuka, Japan
R&D Center, Kawasaki, Japan Universitlt
H, h
J, L h
Wien, Vienna,
K
Wah Chang Albany, Albany,
Vacuumschmelze Germany
Supplementary
L
GmbH, Hanau,
intercomparison
program
In order to separate errors associated with individual sample preparation from measurement error a supplementary intercomparison program was offered. In it, two pre-prepared samples containing specified volumes of NbTi, were circulated for VSM measurement. Groups B, J, and L elected to participate in the supplementary program. Although small in number, the results made an important contribution to the error analysis.
Sample
materials
and experimental
results
Strand materials Two series of multifilamentary double-stacked strands were manufactured by a commercial vendor. The Series-H strands used as a matrix oxygen-free seamless copper pipe (Japanese JIS Code 11351OClOl lT-H) with a starting RRR of 350-420. The Series-I strands used as matrix material commercial ‘cupronickel’, an alloy of nominal composition 10 wt% Ni plus 0.7-0.8 wt% Mn. In this program, and in several publications stemming from it, the commercial cupronickel matrix alloy has been referred to just as ‘Cu lO%/Ni’ ’ ’ or ‘CuNi’ 12-i4. However, it will be shown in Appendix A that the presence of the 0.7-0.8 wt% Mn played a very important role in controlling the magnetic properties of the I-series strands. The strand manufacturing sequences are summarized in Figure I. It is important to note that cold extrusion was employed; according to the manufacturer this was intended to obviate the need for providing the NbTi filaments with a Nb coating. The strand specifications are listed in Table 3, and SEM micrographs are presented in Figures 2a and 2b. Experimental
results
Magnetization, M, was measured as the applied field was cycled between B,,, and B,,i, (such that B,,, - B,i, = AB) about a background or bias field B,. Although a few groups used bias fields of 0 and 1 T, only the B. = 0 T results are reported here. The hysteretic loss per cycle was obtained
Second Table 2
Measurement
VAMAS
a.c. loss measurement
intercomparison:
E. W. Callings et al.
method and sample form
Code
T (K)
Technique/calibration standard
Sample form
No. of wires
Wire length, cm
d B/d t, mT/s
A B C D E F G H h J#i jj K
4.2 4.2 4.6 4.2 4.5 4.2 4.3 4.2 4.2 4.2 4.2 4.22
SQUID:Pt,Cu,AI VSM:Ni(=T) SQUID:Pd IND%Nb SQUID:Nb VSM:Ni (2T) VSM:Ni SQUID:Pd IND:Nb VSM:Ni VSM:Ni VSM:Ni (IT)
single layer flat” cylindrical bundle single wire l-3 layer coils single wire square bundle square bundle l-layer coil 5-layer coil single layer flat” square bundle helical coil
6 19-25 1 1 1 36” 36 1 1 6 25 1
0.310-0.356 0.637-0.785 0.505-0.600 256.7-279.9 0.534-0.571 0.464-0.472 0.5 9.98-30.03 7891-7937 0.208-0.357 0.327 12.66
Ob 0” 0” 20 Ob 1.25-2.5 0.67-1.67 0.23-5.0 0” 0.18-1.0 0.18-1.0 Ob
“H, to flat “Negligibly small dB/dt for SQUID magnetometer “Corrected to zero dBldt, see below dlnductive method “35 wires in one case
(or other step and hold type loops)
NbTi rod -----
NbTi rod ---
monocore
- _-
“CuNi” pipe
---II
CuNi” pipe
monocore
First Stack --f Hl: 931 cores
First Stack 931 cores
assembling c
1
drau(ing
/drawing
]
Second Stack H2: 7x93 1 = 6,5 17 cores r&T” H3: 61x931 = 56.791 cores
pipe
H4: 361x931 = 336,091 cores) +I
11: 61%?:&: cores h&j-12: 361x931 = 336.091 cores 13: 931x931 = 8661761 cores)
“CuNi” pipe
1 strand Hl only
1
twiscfing
1 L,=8.9mm
final
drawing
L, = 8.9 mm
0.51 mrna
0.51 mm@
5
insulation
(a) Figure 1
Manufacturing
(b)
Fabrication of the H-Series Strands
steps for (a) the Cu-matrix
strands (Series-H)
from the area of the M-B hysteresis loop and converted to a per-cyclic loss per unit volume (Q, J/m’) based on the measured or deduced volume of NbTi present in each sample. The experimental results were expressed as log-lot plots of Q versus AB as shown in Figure 3 for the seven
Fabrication of the I-Series Strands
and (b) the cupronickel-matrix
strands (Series-l)
samples H 1-H4 and 11-13. It can be readily seen from these figures that the spread in the data increased as the filament diameter decreased. Also from the observation that data from a given group tended to be either uniformly higher or uniformly lower than ‘average’ we deduce that the domi-
Cryogenics
1997 Volume
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51
Second Table 3
VAMAS a.c. loss measurement
intercomparison:
Strand specifications
Sample code
Specific mass NbTi, m,, mg/cm
Filament No., N
Measured filament diam.“, dr, Km
Nominal filament diam.“, d,, wrn
Filament volume variance,
HI H2 H3 H4
5.731 3.188 3.947 4.638
931 6517 56791 33609 1
11.34 3.20 1.20 0.54
11.8 3.4 1.3 0.54
8.11 12.5 16.7 0
II I2 I3
3.854 4.356 4.725
56791 33609 1 866761
1.19 0.52 0.34
1.2 0.54 0.35
1.7 7.7 5.9
“Takina 6.097 a/cm3 for the densitv of NbTi (assumina bSuppied by TWP office “By image analysis
Discussion of the results of the primary intercomparison program The results of the primary intercomparison program (Table 2) are summarized in Figure 3. The as-reported data exhibit scatter ranging from 11% (12) to 27% (H4). These errors, which derive from numerous sources, may be broadly characterized as: (i) external errors, including temperature, NbTi volume ratio, and NbTi density; (ii) experimental errors, including ramp rate effects (eddy current coupling), data point density, and calibration; (iii) sample induced errors, which are mainly demagnetization-, shape-, and proximity-effects. Some errors may of course belong to more than one category. All of them are considered below. Sources of errors Temperature Several groups made magnetization measurements at temperatures significantly higher than 4.2 K. As shown in Table 2 temperatures as high as 4.6 K were encountered. The submitted data were not corrected to 4.2 K. In one case the variations were so large that the data could not be included in this report. NbTi volume and density NbTi volume determination was a significant source of error. The primary methods of volume determination (not all were reported) were: (i) calculation based on the supplied nominal filament diameters; (ii) in-house measurements by the etch-and-weigh procedure. The latter is expected to yield very small errors provided the NbTi density is accurately known and the presence of filament/matrix barriers or external coatings are properly accounted for. In the present cold-extruded strands no filament/matrix barriers were used; the manufacturer had, however, applied an insulating varnish to the external surface. The results of the two groups that had reported using the etch-and-weigh method (B and K) differed by about 5%. This surprisingly large discrepancy cannot be fully accounted for in terms of a combination of gravimetric
Cryogenics
1997 Volume
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1
Filament space”, d,, Frn
dJdr
2.14 0.54 0.19 0.10
0.19 0.17 0.16 0.19
% IL 0.24 f 0.08 * 0.09 * 0.05
Nb-46.5 Ti)
nant errors are systematic rather than random. As discussed below, the principal systematic errors, which increase as the filament diameter decreases are: (i) a disparity between the nominal filament diameter, used by some groups, and measured diameters used by others; and (ii) a samplelength-dependent proximity effect (PE) contribution to the loss.
52
E. W. Callings et al.
uncertainties (which both groups reported to be much less than 1%) and density differences ( 1.7% deriving from accepted densities of 6.200 g/cm”, Group K, and 6.097 g/cm3, Group B). When based on the starting diameter of the Nb rods and the overall area reduction ratio, filament diameter, and hence NbTi volume, is underestimated, since during cold drawing the Cu/SC ratio decreases with strand diameter.‘” The gravimetrically determined NbTi volume relies on: (i) the application of suitable etching-and-weighing procedures; (ii) an accurate knowledge of the alloy’s density. Some uncertainty is still associated with both of these factors, for which reason further work on Cu/SC ratio measurement procedures and NbTi density determination is recommended. Eddy current coupling It is well known that the energy loss per cycle due to eddy currents in a multifilamentary composite is proportional to the frequency of oscillation of the applied field, hence the ramp rate dB/dt in the case of a swept field16. For a given sample the actual constant of proportionality will depend on the sample length17. For this reason a sweep-rate correction determined on one sample of strand cannot be simply applied to another of different length; thus corrections have to be made on a group-bygroup basis. The sweep rates used by the various groups are listed in Table 2. Group B has estimated that for its samples (about 6 mm in length) the uncorrected error due to eddy currents is about 0.3% per mT/s. Even so that group took data over a range of values of dBldt and back-extrapolated the results to zero dB/dt (see Measurement Specifications, above). Similar corrections were also made by some other groups, as indicated by superscript ‘c’ in Table 2. The fastest uncorrected sweep rate was 20 mT/s (-6% error), but most were less than 5 mT/s (= 1.5%). Data point density The number of data points per complete hysteresis loop varied by roughly an order of magnitude from group to group depending on the measuring technique. SQUID magnetometry was associated with the lowest data point density (of order 100 points per loop) which nevertheless was sufficient to cover even sample H4’s PE-enhanced magnetization peak with adequate resolution. We conclude that data-point-density errors were not significant. Calibration ommended,
As indicated in Table 2, and as initially rectwo classes of calibration sample were
Second
VAMAS
a.c. loss measurement
intercomparison:
Figure 2 (a) Scanning electron micrographs of strands Hl-H4 and II-13 all taken at the same original bar in HI indicates 100 pm; filament numbers are also indicated
Cryogenics
E. W. Callings
magnification
1997 Volume
et al.
of 230 x. The
37, Number
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53
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VAMAS a.c. loss measurement
Figure 2 (b) Scanning scales are individually
54
Cryogenics
electron indicated
micrographs
1997 Volume
intercomparison:
of strands
37, Number
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Hl-H4
E. W. Callings et al.
and 11-13 taken
at various
magnifications
of 2000-6000
x. The length
Second
VAMAS
a.c. loss measurement
intercomparison:
E. W, Callings
et
al.
ld
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ld
ld
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Applied Field Span, AJ3, tcsla
Applied Field Spaa. AB, tcsla Figure 3 Primary intercomparison program results: hysteresis AB, (tesla) for the Cu-matrix (Series-H) and the cupronickel-matrix are the laboratory codes of Tables 1 and 2
loss,
Oh, (per cycle, per unit NbTi volume) versus (Series-l) types of multifilamentary NbTi strand.
Cryogenics
1997 Volume
field-sweep
span,
Data-point letters
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VAMAS a.c. loss measurement
intercomparison:
employed: ( 1) pure Nb, to be used to generate a materialindependent Meissner magnetization; (2) a metal, usually Ni, with a known magnetization. The disadvantage of Nb as a calibration standard is that it must also be associated with an accurately known magnetic field. On the other hand the magnetization of Ni is independent of the field strength provided that it is sufficiently high (several tesla) to achieve saturation. If a sufficiently high field is not available an extrapolation procedure leading to a ‘calibration magnetization’, MC,,, based on the known functional form of M,,(B) can be employed. Group B calibrated its apparatus using a short cylinder of Ni that was 2 mm long and 2 mm in diameter. The near-saturation portion of the M-B curve was fit to the expression M,,, = M,,,( l-alB-hlB’) with M,,, = 58.57 emu/g. The use of a calibration sample implies ‘measurementby-substitution’. This method relies on an insensitivity of the measurement to sample size and shape within the range of interest. These effects, which may be more important in VSM rather than SQUID measurement, were not generally investigated. However, Group B after measuring the specific magnetizations of a series of Ni samples with lengths of up to 14 mm, noted that for a typical VAMAS sample (H3, 7.07 mm in length) a correction of +1.26% needed to be applied. Group B applied sample-length corrections to all of their data. Demagnetization and shape effects In a secondary program, the research groups were invited to study the effect of variation of the number of wires in the bundle. The only participant in this secondary program was Group J who compared the magnetizations of a single flat layer (6 strands) with that of a square bundle (5 x 5 = 25 strands). The 2.7% difference between the results is close to the expected level of random error and therefore unassignable to either demagnetization or shape effects. In fact, bundle-size demagnetization effects are not to be expected. If, following common practice, it is appropriate to treat an uncoupled multifilamentary strand as a bundle of noninteracting filaments, the same approach should be valid for a cluster of such strands. Proximity effects (PE) Studies of PE-coupled multifilamentary strands in general’x-‘n and VAMAS strands in particular” have shown that the PE component of magnetization is both sample-length dependent and twist-pitch (L,) dependent. A length-independent effect can be obtained only when an untwisted strand is sufficiently long or a twisted one longer than a few twist pitches. For the present strands L, = 0.9 cm; sample lengths were mostly less than this. Thus when measurable PE is present, as it is in samples H3, H4, and I3 (with relative magnitudes H3 < I3 < H4), a length dependence should in principle be encountered in most of those samples as prepared for this study. If uncorrected, a sufficiently large PE magnetization would introduce a systematic error into the results. Unfortunately no single group performed a full study of the sample-length dependence of magnetization. Although Group A measured the length dependence of loss of most of its samples they stopped short of H4 which had the strongest PE. Nevertheless as part of a detailed analysis of overall errors (see below) it was noted that PE introduced a discrepancy as large as 85% between the magnetizations of the longest and shortest samples of H4 as measured by different groups.
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E. W. Callings et al. Random errors In addition to the above systematic errors, there are of course random experimental errors. Of those responding to the question, most groups reported total errors of between 2 and 3%. Three participants (including the two who employed inductive techniques) estimated their errors to be between 5 and 10%.
The supplementary program Description
intercomparison
of the supplementary
program
The supplementary intercomparison program (or supplementary ‘round robin’, SRR) was established in an attempt to separate out systematic error (as discussed above) from random experimental error. Groups B, J, and L participated by measuring one sample each of H3 and H4. H3 was in the form of a cylindrical 25-strand bundle, 7.07 mm in length and with a total NbTi volume of 11.442 x lo-” cm3. H4 was a cylindrical 24-strand bundle, 7.85 mm in length and with a total NbTi volume of 14.338 x 10-j cm3. The Group-B-specified NbTi volume was supplied along with the samples, thus eliminating volume-determination as a possible source of error. VSM measurements at 4.2 K were performed by Groups B, J, and L, and the results intercompared. They are presented in Figure 4 where good agreement is seen. We find the scatter in the data, expressed in the form (T&Q), where au is the standard deviation in loss, and (Q) is the average loss, to be 3.16% for H3 and 3.95% for H4. Eddy-current-induced magnetization enhancements at the ramp rates, dB/dt, used in this SRR are small. Correcting for them leaves relative uncertainties, u/(Q), of 2.25% for H3 and 3.25% for H4, levels which are commensurate with estimated random experimental errors (see above). The ramp-rate-corrected results of the three-participant SRR confirm that the random experimental error to be expected in VSM measurement is about 2-3%. They also indicate that the dominant errors in the primary inter-
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k al
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3
$
103
0)
s? D
10-1
100
Applied Field Span, m,
tesla
Figure 4 Supplementary intercomparison program results: hysteresis loss, Q,, (per cycle, per unit NbTi volume) versus field-sweep span, AEZ, (tesla) for provided samples of strands H3 (open symbols) and H4 (filled symbols)
Second comparison program (or primary systematic in nature.
VAMAS
a.c. loss measurement
‘round robin’, PRR) are
Comparison of the primary intercomparison results
and supplementary
of errors: levels of error
As pointed out in the preceding discussion, once the respondents were provided with a prepared sample (fixed length, fixed NbTi volume) the scatter in the data dropped to that expected from random experimental error. Most of the error in the PRR was systematic in nature. In an exploration of the principle sources of such error the PRR data were subjected to successive stages of error correction the results of which are summarized in Figure 5. The following analysis is based just on data taken at a AB of 1 T. Zeroth level of correction Expressed as usual in terms of so/(Q) this level of correction represents the scatter in the as-reported data; it ranges from 11% to 27%. I
I
I
I
First level of correction
Second level of correction The second level of correction was based on adjusting the NbTi volumes of the individual samples measured. All data were adjusted to what we believe to be the correct volume of NbTi in each case. The normalization consisted in ensuring not only that the NbTi volume per cm of strand (V&) was the same for each group but that it agreed with the values of V,,lC obtained by Group B. Only one group reported more-or-less directly the values of V&t that it used. The others provided sufficient information (generally in the form of a nominal filament diameter and a simple formula) to enable us to deduce their individual V,,,lJ!s. These were then compared to the corresponding values of VSC/elGroupB. As expected, the variances encountered correlated with the filament-diameter-calculated ‘Volume Variances’ listed in Table 3. The volume corrections were then applied to the previously corrected results to provide the volume-corrected ad(Q) depicted in Figure 5. Volume correction dropped the previous scatter by 0 to 7 percentage points. Third level of correction Even after the above corrections had been performed large errors still existed, particularly in the Q-data for the tinerfilament strands. The filaments particularly in H4 and 13 are strongly coupled by PE which is known to be responsible for a length dependent magnetization component’x~2”. The visual signature of PE coupling is the excess low-field magnetization that can be seen when the magnetization loops for clad (as-received) and bare (Cu-etched) strands are superposed as in Figure 6. Strands H3 and I3 exhibited similar PE signatures. To correct for PE we took advantage of its sample-length
I
I
I
I
I
I
0
1
2
3
4
Correction
E. W. Callings et al.
The first level of correction is achieved by eliminating data taken at temperatures other than 4.2-4.3 K (the reported data ranged in temperature from 4.2 K to 4.6 K). Almost no change in co/(Q) resulted from this.
Since Group L is not represented in the PRR, PRR/SRR error analysis is possible only by using data from Groups B and J. Although the data are scarce, the results are interesting. Within the SRR’s the Groups B and J results (for AB = 1 T) differed by 2.05% (sample H3) and 5.00% (sample H4). For comparison, the percent differences between Groups B and J’s results in the PRR were 23.39% (sample H3) and 84.91% (sample H4). In the SRR, Groups B and J measured the same samples. In the PRR, different samples with different lengths were measured. The lengths of sample H3 were 7.1 mm (Group B) and 3.3 mm (Group J); those of sample H4 were 7.9 mm (Group B) and 3.3 mm (Group J). The difference in lengths coupled with the existence of PE is responsible for the errors. We are led to the conclusion that length-dependent PE is one of the dominant sources of error in the PRR.
General discussion correction
intercomparison:
5
Level
-2
-1
0
Magnetic Figure 5 Error analysis: relative standard deviations (RSD) after successive levels of correction have been applied to 0, data for the seven strand types. PRR, levels O-3; SRR, level 4
Field Strength,
tesla
Figure 6 Proximity-effect enhanced magnetization (-) as compared to bare l---j samples of strand
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1997 Volume
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dependence. All groups reported their sample lengths, e. For each strand type it was therefore possible to create straight-line plots of Q versus e based on the data supplied by each responding group. The existence of slope was expected to correspond to the visual occurrence of PE. This it did, but with some exceptions which may have been a result of random- or some unknown source of error. Thus H4 and I3 (the most strongly PE-coupled strands) exhibited both slope and visual PE. On the other hand 12 yielded just a Q(e) slope while H3 only a visual PE. But whatever the slope, zero or otherwise, Q(e) was extrapolated to some arbitrary reference length (e = 1 cm was chosen) and the resulting length-adjusted co/(Q) computed. The levels of scatter, reduced step-by-step in the manner just outlined, drop to about 6% (Figure 5, Station 3). At this point we have exhausted all correction possibilities as we see them. Nevertheless the breadth of scatter is still several percentage points greater than that of the SRR (Figure 5, Station 4). Some possible future round robin in which the primary and supplementary programs are equally well represented may shed some light on this question, but see below.
Summary
and conclusion
In the PRR, the greatest contributor to scatter in the asreceived data (most severe in the finest filament strands) was associated with the sample-length dependent PE. This error was not present in the SRR in which sample length was not a variable. Another significant source of error in the PRR derived from incorrect estimates of the NbTi volumes. Again this error disappeared when pre-prepared samples were circulated for measurement. Scatter in the PRR results decreased from 1 l-27% (for the as-received data) to about 6% as first NbTi volume and then sample length (for PE) received correction. At this point the scatter in the corrected PRR data should have been equal to that for the SRR. That they were not (6% versus 2-3%, respectively) indicates either that other unidentified sources of error were present, or that the intercomparisons were too unevenly represented. Further error analysis, see Appendix 2, favors the latter alternative.
Recommendations for future a.c. loss intercomparisons
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1997 Volume
et al.
diameter, determination; (2) as before, the circulation for measurement of a typical pre-prepared sample. It may in fact be useful to set up in the near future sepatate international programs to deal with: ( 1) matrix/SC-ratio measurement and (2) NbTi-alloy density determination.
Recommendations for standard frequency a.c. loss testing
low-
The present experience suggests that the standard a.c. loss test should be performed under at least the following conditions: (1) Magnetization should be calibrated by an approved method against either Nb (material-independent Meissner effect) or some standard reference material such as pure Ni. Magnetic field strength should be calibrated against a certificated NMR or Hall probe. (2) Thick-filament samples of any reasonable length may be used; but if preliminary measurement indicates the presence of PE the only recourse is to measure samples that are several twist pitches in length. With an ‘axial field’ solenoidal magnet, long helical coils may be used. In the ‘transverse field’ of an electromagnet, special coils21,‘2 will need to be constructed. (3) NbTi mass per unit length should be measured by each group using an approved etch-and-weigh method and reported. The resulting NbTi volume per unit length, and hence filament diameter, should also be reported along with the value of the NbTi density that was used (and which may be supplied to all participants). (4) As already specified in the PRR: ‘the field sweep rate should be sufficiently slow as to eliminate the possibility of significant error due to eddy current loss; otherwise the experiments should be performed over a range of values of dBldt and the results back-extrapolated to zero dBldt’.
Acknowledgements The authors wish to thank K. Aihara, P. Gilson, J. McKinnell, M. Siddall, W. Steiner, M. Thoner, H.W. Weber, E.S. Yoneda, and S. Zanella for their conscientious and dedicated participation in the 2”d VAMAS a.c. loss measurement intercomparison, SWG-2.
low-frequency References
It has been emphasized that intercomparison scatter results from a combination of systematic and random experimental error. As pointed out above, the systematic error is dominant. Special care must be taken to ensure that an accurate NbTi volume is being used to normalize the a.c. loss data. Precautions must also be taken to eliminate PE-magnetization variation as a source of error. This can be achieved by insisting that strand samples be longer than a few twist pitches (the minimum length in terms of twist pitch needed to guarantee a length-independent PE magnetization still needs to be determined). It is also suggested that all groups participating in any primary intercomparison program should also agree to participate in a supplementary program aimed at isolating systematic error. The accompanying supplementary program should consist of at least the following two components: ( 1) a matrix/SC-ratio, and hence filament
58
E. W. Callings
37, Number
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Tachikawa, K. The VAMAS Intercomparison in the Area of Superconducting and Cryogenic Structural Materials, Advnnces in Cryogenic Engineering (Materials) ( 1990) 36 1- 10 Callings, E.W., Marken Jr., K.R., Sumption, M.D., Goldfarb, R.B. and Loughran, R.J. AC Loss Measurements of Two Multifilamentary NbTi Composite Strands, Advnncrs in Cryogenic Engineering (Muteri&) ( 1990) 36, 169- 176 Zannella, S., Gilson, P., Ottoboni, V., Ricca, A.M., Ripamonti, G. AC Loss Measurements on NbTi Superconducting Wires for the VAMAS Round Robin Test, Advances in Cryogenic Engineering (Materials) ( 1990) 36 177- 18 1 Roovers, A.J.M., van der Vegt, H.A., Siekman, K.W., Wells, T., van de Khmdert, L.J.M. and Sabrie, J.L. A.C. Loss Measurements on Superconductors Using a (Low-Inductive) Coil Geometry, Advan& in Cryogenic En&wring (Materials) ( 1990) 36 19 I ~ I98 Itoh, K., Wada, H.. Ando, T.. Yoneda, E., Ito. D.. Iwakuma, M., Yamafuji, K., Nagata, A., Watanabe, K., Knbota, Y., Ogasawara, T., Akita, S., Umeda, M., Kimura, Y. and Tachikawa, K. VAMAS Intercomparison of AC Loss Measurement: Japanese Results, Advances in Cryogenic Engineering (Materials) ( 1990) 36 199-206
Second 6
7
9
10
II
13
14
is
16 I7
(Materials)
( 1992) 38 459-468
(1992)
38 767-774
Yasohama, K., Kubota, Y. and Ogasawara, T. A.C. Susceptibility and Filament Coupling of Multifilamentary Superconducting Wires. IEEE Trms. Superconductivity ( 1993) 3 I38- 14 I Pobik, M., Krempask$, L., MajoroS, M., Suchofi, D. and Kirchmayr, H. Anomalous Magnetization Behavior in Fine Filamentary NbTi Superconducting Wires. IEEE Trans. Superconductivity ( 1993) 3 150-152 Krauth, H., Szulczyk, A. and Thiiner, M. Critical Current Density and Magnetization of NbTi and Nb,Sn Fine Filament Superconductors, Proc. 7”’ Int. Workshop on Critical Currents, Alpach. Austria, Jan. 23-27, 1994, to be published Valaris, P., Kreilick, T.S., Gregory, E. and Callings, E.W. The Effects of Processing on the Filament Array in Multifilament SSC Strand, in Supercollider I (Ed M. McAshan) Plenum Press, NY (199O)pp.449-456 Carr Jr., W.J. A.C. Lms and Macroscopic Theory of Suprrconductom Gordon and Breach, NY ( 1983) Carr Jr., W.J. End Effects on the Loss for Short Superconductors,
Ad~wtcus ill Cryogenic En@rerin~ (Motrriuls) ( 1982) 38 58 l-586 I8
Yamafuji, K., Harada, N., Mawatari, Y., Makeo, M., Akune, T., Sakamoto, N., Minra, 0. and Tanaka, Y. Creep Rates of Excess Magnetization due to Interfilamentary Proximity Couplings in Superconducting NbTi Multifilamentary Wires, Supercond. Sci. Tech&
(1992) 5 sll7~sl2O I9
20
21
22
E. W. Callings et al.
Wada, H. and Itoh, K. Towards the International Standardization of Superconducting Materials: Development of Standard Measurement Methods for Critical Current and a.c. Losses, Ciygenics (1992) 32 (ICMC Supplement) 557-564 Itoh, K. and Wada, H. Magnetization of VAMAS NbTi a.c. Loss Test Wires, Proc. ICFA Workshop on U.C. Superconductivity, KEK Proc. 92-14 (Ed K. Tsuchiya) (1992) 149-151 Itoh, K., Wada, H. and Tachikawa, K. 2’ldVAMAS a.c. Loss Intercomparison on NbTi Wires, Proc. 8”’ US-Jnpnn Workshop on HighField Superconductor Materiuls, University of Wisconsin-Madison, March 17-19, 1993 (Ed K. Yamafuji, H. Wada, D.C. Larbalestier and M. Suenaga) ( 1993) 73-77 Yoneda, E.S., Itoh, D., Takano, I., Akita, S., Torii, S., Kumano, T. and Suzuki, E. kA-Class a.c. Superconducting Cable Development, Advnnces in Cryo,genic Engineering (Materials)
12
intercomparison:
Schmidt, C. Anomalous Low Hysteresis Losses in NbTi Superconductors with Very Fine Filaments, Advances in Cryogenic Engineering (Materi&) ( 1990) 36 207-214 Itoh, K., Wada, H. and Tachikawa, K. Results on the First Intercomparison of ax. Loss Measurements, Advnnces in Ctyogenic Engineering
8
VAMAS a.c. loss measurement
Harada, N., Mawatari, Y., Miura, O., Tanaka, Y. and Yamafuji, K. Excess Magnetization due to Interfilamentary Proximity Coupling in Nb-Ti Multifilamentary Wires, Cryogenics (1991) 31 1X3-191 Sumption, M.D. and Callings, E.W. Effect of Twist Pitch, Sample Length, and Field Orientation on the Proximity Effect Enhanced Magnetization of Fine Filamentary Multifilamentary Strands, Advances in Cryogenic Engineering (Mutericds) ( 1992) 38 783-790 Sumption, M.D., Marken Jr., K.R. and Callings, E.W. Enhanced Static Magnetization and Creep in Fine-Filamentary and SSC-Prototype Strands via Helical Cabling Geometry Enhanced Proximity Effects, IEEE Trans. Appl. Superconductivity ( 1993) 3 75 l-756 Sumption, M.D., Pyun, D.S. and Collings, E.W. Transverse and Longitudinal Resistivities in NbTi Multifilamentary Strands with Cu and CuMn Matrices. IEEE Tram. Appl. SuprrcondwtilGty ( 1993) 3
t
t
-0.6
I
-0.4
-0.2
I
I
0
0.2
I
0.4
0.6
Magnetic Field Strength, B, tesla Figure Al M-B hysteresis NbTi) for cupronickel-matrix (-_). Note the emergence filament strand
loops (normalized to unit volume of strands II (-.-.), 12 (---), and 13 of PE magnetization in the finest-
NbTi/CUlONi multifilamentary composite as well as a Al. Magnetization strong paramagnetic tilt”, Figure measurements of strands 12 and 13 taken at 10 K also exhibited this tilt, Figure AZ, indicating that it is a property of the matrix, not of the superconductor. By way of confirmation, bulk samples of the matrix alloy sent to Battelle for measurement showed esssentially the same M-B behavior in either the as-received condition or after a solution heat treatment of 45h/992”C. The M-B curve for a Cu-lS%Ni research alloy is also shown in Figure A2. This line lies quite close to the B-axis, clearly indicating that the I-series matrix cannot possibly be described as a ‘Cu-lO%Ni’ alloy. About 10% of Ni is certainly present-but so also is a significant concentration of Mn according to microprobe analysis of the bulk matrix material performed at Battelle ( 10.3 wt%Ni, 0.78 wt%Mn),
0.61
859-862 23
Sumption, M.D. and Callings, the Low Temperature Magnetic Appl. Phys.
E.W. Influence of Ni Additions on Properties of a Cu-l%Ni Strand, J.
( 1994) 76 7461-7467
Appendix A Mn-induced paramagnetic offset and proximity-effect suppression in the l-series strands
I-I
-2
The matrix material of the I-series wires was commercial cupronickel. It has been frequently described as a ‘Cu lO%Ni’ ‘I or ‘CuNi’.‘Z-14 alloy; but magnetization measurement showed the I-series strands to have unexpectedly low levels of PE coupling for a supposed
Magnetic
Field Strength,
B,
tesla
Figure A2 M-B curves for a pair of l-series strands (I2 and 13) taken at 10 K juxtaposed against 4.2 K curves for Cu-15%Ni and Cu-l%Mn indicating that it is the Mn and not the Ni that is responsible for the l-series’ paramagnetic tilt
Cryogenics
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E. W. Collings et al. I4 it is Mn, and not Ni, that is responsible for the PE suppression noted in the VAMAS I-series strands.
Appendix B Confidence levels in intercomparison
Correction Level Error analysis: mean error (ME) after successive levFigure Bl els of correction have been applied to 0, data for the seven strand types. Primary intercomparison (PRR), levels O-3; supplementary intercomparison (SRR), level 4
The loss data’s conventional standard deviation, uo, normalized by its mean value (Q) has been used in the main text since quotients of the form ad(Q) are traditional measures of relative error. The relative standard deviation (RSD) ad(Q) certainly represents the precision of a result derived from an ensemble of measurements. It does not, however, represent the accuracy of any subsequent individual measurement. Whereas the RSD decreases as the ensemble number increases, the accuracy of any single subsequent measurement does not change. The essence of an intercomparison is to develop a measure of this accuracy, or reliability, as well as to develop procedures for improving it. Returning to the question of individual measurement accuracy, we recommend the use of a ‘mean error’, ME, defined by ME =
the subsequently received manufacturer’s specifications (911 wt%Ni, 0.2-1.0 wt%Mn), and microprobe analysis of the composite’s matrix (8.65 wt%Ni, 0.76 wt%Mn). Figure A2 confirms the validity of the latter analysis in that the paramagnetic slope exhibited by 12 and I3 is just a little lower than that of a Cu-l%Mn research alloy. Figure A2 and a detailed study of the magnetic properties of a set of binary and ternary research alloys of compol%Mn and lSat%Ni (see and sitions above), l%Mn+lS%Ni (all at%)2’ confirm that Mn is almost entirely responsible for the observed paramagnetic tilt. Furthermore, it is generally recognized by now that 0.5 wt% of Mn dissolved in the Cu matrix is a strong suppressor of PE in multifilamentary composites. Even more effective would be 0.8 wt% of Mn. Thus contrary to earlier reports”-
60
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results
C
Since this is derived from the ensemble study, but yet is not a function of the number of participants, it has the property which we seek. We have calculated the ME for the strands as a function of correction level and have plotted the results in Figure BI. The values are somewhat lower than the RSD numbers plotted in Figure 5, but similar in trend. Since the ME is independent of the number of measurements, we might expect to find the levels of ME in the PRR and the SRR to be the same if all external and sample errors had been eliminated. Indeed for the PRR at correction Level 3, (ME) = 3.5% while MEHs = ME,, = 3.37%. The latter compares well to the corresponding errors for the SRR at the same correction level, which are MEH3 = 2.28% and ME,, = 2.83%.