EN 61788-1:2007

2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.Superconductivity - Part 1: Critical current measurement - DC critical current of Nb-Ti composite superconductorsSupraconductivité Partie 1: Mesure du courant critique - Courant critique continu de supraconducteurs en composite Nb-TiSupraleitfähigkeit Teil 1: Messen des kritischen Stromes - Kritischer Strom (Gleichstrom) von Nb-Ti VerbundsupraleiternTa slovenski standard je istoveten z:EN 61788-1:2007SIST EN 61788-1:2008en,fr,de29.050Superprevodnost in prevodni materialiSuperconductivity and conducting materials17.220.20Measurement of electrical and magnetic quantitiesICS:SIST EN 61788-1:20011DGRPHãþDSLOVENSKI
STANDARDSIST EN 61788-1:200801-januar-2008

EUROPEAN STANDARD EN 61788-1 NORME EUROPÉENNE
EUROPÄISCHE NORM January 2007
CENELEC European Committee for Electrotechnical Standardization Comité Européen de Normalisation Electrotechnique Europäisches Komitee für Elektrotechnische Normung
Central Secretariat: rue de Stassart 35, B - 1050 Brussels
© 2007 CENELEC -
All rights of exploitation in any form and by any means reserved worldwide for CENELEC members.
Ref. No. EN 61788-1:2007 E
ICS 17.220; 29.050 Supersedes EN 61788-1:1998
English version
Superconductivity
Part 1: Critical current measurement -
DC critical current of Nb-Ti composite superconductors (IEC 61788-1:2006)
Supraconductivité
Partie 1: Mesure du courant critique -
Courant critique continu de supraconducteurs en composite Nb-Ti
(CEI 61788-1:2006)
Supraleitfähigkeit
Teil 1: Messen des kritischen Stromes -
Kritischer Strom (Gleichstrom) von Nb-Ti Verbundsupraleitern (IEC 61788-1:2006)
This European Standard was approved by CENELEC on 2006-12-01. CENELEC members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European Standard the status of a national standard without any alteration.
Up-to-date lists and bibliographical references concerning such national standards may be obtained on application to the Central Secretariat or to any CENELEC member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by translation under the responsibility of a CENELEC member into its own language and notified to the Central Secretariat has the same status as the official versions.
CENELEC members are the national electrotechnical committees of Austria, Belgium, Cyprus, the Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, the Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland and the United Kingdom.

Foreword The text of document 90/196/FDIS, future edition 2 of IEC 61788-1, prepared by IEC TC 90, Superconductivity, was submitted to the IEC-CENELEC parallel vote and was approved by CENELEC as EN 61788-1 on 2006-12-01. This European Standard supersedes EN 61788-1:1998. It includes the following significant technical changes with respect to EN 61788-1:1998: – the addition of normative Annex C and informative Annex D; – accuracy and precision statements were converted to uncertainty statements; – the magnetic field uniformity statement was tightened from ± 2 % to be less than the larger of 0,5 % or 0,02 T. The following dates were fixed: – latest date by which the EN has to be implemented
at national level by publication of an identical
national standard or by endorsement
(dop)
2007-09-01 – latest date by which the national standards conflicting
with the EN have to be withdrawn
(dow)
2009-12-01 Annex ZA has been added by CENELEC. __________ Endorsement notice The text of the International Standard IEC 61788-1:2006 was approved by CENELEC as a European Standard without any modification. In the official version, for Bibliography, the following note has to be added for the standard indicated: IEC 61788-5 NOTE
Harmonized as EN 61788-5:2001 (not modified). __________

- 3 - EN 61788-1:2007 Annex ZA
(normative)
Normative references to international publications with their corresponding European publications
The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.
NOTE
When an international publication has been modified by common modifications, indicated by (mod), the relevant EN/HD applies.
Publication Year Title EN/HD Year
IEC 60050-815 -1) International Electrotechnical Vocabulary (IEV)
Part 815: Superconductivity - -
1) Undated reference.
NORME INTERNATIONALECEIIEC INTERNATIONAL STANDARD 61788-1Deuxième éditionSecond edition2006-11 Supraconductivité – Partie 1: Mesure du courant critique – Courant critique continu de supraconducteurs
en composite Nb-Ti
Superconductivity – Part 1: Critical current measurement – DC critical current of Nb-Ti composite superconductors Pour prix, voir catalogue en vigueur For price, see current catalogue© IEC 2006
Droits de reproduction réservés
⎯
Copyright - all rights reserved Aucune partie de cette publication ne peut être reproduite ni utilisée sous quelque forme que ce soit et par aucun procédé, électronique ou mécanique, y compris la photocopie et les microfilms, sans l'accord écrit de l'éditeur. No part of this publication may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying and microfilm, without permission in writing from the publisher. International Electrotechnical Commission,
3, rue de Varembé, PO Box 131, CH-1211 Geneva 20, SwitzerlandTelephone: +41 22 919 02 11 Telefax: +41 22 919 03 00 E-mail: inmail@iec.ch
Web: www.iec.ch CODE PRIX PRICE CODE U Commission Electrotechnique InternationaleInternational Electrotechnical Commission

61788-1 © IEC:2006 – 3 – CONTENTS FOREWORD.5 INTRODUCTION.9
1 Scope.11 2 Normative references.11 3 Terms and definitions.11 4 Principle.15 5 Requirements.15 6 Apparatus.17 6.1 Measurement mandrel material.17 6.2 Mandrel construction.17 7 Specimen preparation.19 7.1 Specimen bonding.19 7.2 Specimen mounting.19 8 Measurement procedure.21 9 Uncertainty of the test method.23 9.1 Critical current.23 9.2 Temperature.23 9.3 Magnetic field.23 9.4 Specimen and mandrel support structure.25 9.5 Specimen protection.25 10 Calculation of results.25 10.1 Critical current criteria.25 10.2 n-value (optional calculation, refer to A.7.2).27 11 Test report.29 11.1 Identification of test specimen.29 11.2 Report of Ic values.29 11.3 Report of test conditions.29
Annex A (informative)
Additional information relating to the standard.31 Annex B (informative)
Self-field effect.47 Annex C (normative)
Test method for Cu/Cu-Ni/Nb-Ti composite superconductors.51 Annex D (informative)
Guidance for estimating winding tensile force.53
Bibliography.57
Figure 1 – Intrinsic U-I characteristic.27 Figure 2 – U-I characteristic with a current transfer component.27 Figure A.1 – Instrumentation of specimen with a null voltage tap pair.45
Table D.1 – Typical values of E at room temperature for various materials.55

61788-1 © IEC:2006 – 5 – INTERNATIONAL ELECTROTECHNICAL COMMISSION ___________
SUPERCONDUCTIVITY –
Part 1: Critical current measurement – DC critical current of Nb-Ti composite superconductors
FOREWORD 1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising all national electrotechnical committees (IEC National Committees). The object of IEC is to promote international co-operation on all questions concerning standardization in the electrical and electronic fields. To this end and in addition to other activities, IEC publishes International Standards, Technical Specifications, Technical Reports, Publicly Available Specifications (PAS) and Guides (hereafter referred to as “IEC Publication(s)”). Their preparation is entrusted to technical committees; any IEC National Committee interested in the subject dealt with may participate in this preparatory work. International, governmental and non-governmental organizations liaising with the IEC also participate in this preparation. IEC collaborates closely with the International Organization for Standardization (ISO) in accordance with conditions determined by agreement between the two organizations. 2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international consensus of opinion on the relevant subjects since each technical committee has representation from all interested IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any misinterpretation by any end user. 4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications transparently to the maximum extent possible in their national and regional publications. Any divergence between any IEC Publication and the corresponding national or regional publication shall be clearly indicated in the latter. 5) IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for any equipment declared to be in conformity with an IEC Publication. 6) All users should ensure that they have the latest edition of this publication. 7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and members of its technical committees and IEC National Committees for any personal injury, property damage or other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC Publications.
8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is indispensable for the correct application of this publication. 9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of patent rights. IEC shall not be held responsible for identifying any or all such patent rights. International Standard IEC 61788-1 has been prepared by IEC technical committee 90: Superconductivity. This second edition cancels and replaces the first edition published in 1998. It constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous edition: – the addition of normative Annex C and informative Annex D;
– accuracy and precision statements were converted to uncertainty statements;
– the magnetic field uniformity statement was tightened from ±2 % to be less than the larger of 0,5% or 0,02 T.

61788-1 © IEC:2006 – 7 – The text of this standard is based on the following documents: FDIS Report on voting 90/196/FDIS 90/201/RVD
Full information on the voting for the approval of this standard can be found in the report on voting indicated in the above table. A list of all parts of IEC 61788 series, published under the general title Superconductivity, can be found on the IEC website. The committee has decided that the contents of this publication will remain unchanged until the maintenance result date indicated on the IEC web site under "http://webstore.iec.ch" in the data related to the specific publication. At this date, the publication will be
• reconfirmed; • withdrawn; • replaced by a revised edition, or • amended.

61788-1 © IEC:2006 – 9 – INTRODUCTION The critical currents of composite superconductors are used to establish design limits for applications of superconducting wires. The operating conditions of superconductors in these applications determine much of their behaviour, and tests made with the method given in this part of IEC 61788 may be used to provide part of the information needed to determine the suitability of a specific superconductor. Results obtained from this method may also be used for detecting changes in the superconducting properties of a composite superconductor due to processing variables, handling, ageing or other applications or environmental conditions. This method is useful for quality control, acceptance or research testing, if the precautions given in this standard are observed. The critical current of composite superconductors depends on many variables. These variables need to be considered in both the testing and the application of these materials. Test conditions such as magnetic field, temperature and relative orientation of the specimen, current and magnetic field are determined by the particular application. The test configuration may be determined by the particular conductor through certain tolerances. The specific critical current criterion may be determined by the particular application. It may be appropriate to measure a number of test specimens if there are irregularities in testing.

61788-1 © IEC:2006 – 11 – SUPERCONDUCTIVITY –
Part 1: Critical current measurement – DC critical current of Nb-Ti composite superconductors
1 Scope This part of IEC 61788 covers a test method for the determination of the d.c. critical current of either Cu/Nb-Ti composite superconductors that have a copper/superconductor ratio larger than 1 or Cu/Cu-Ni/Nb-Ti wires that have a copper/superconductor ratio larger than 0,9 and a copper alloy (Cu-Ni)/superconductor ratio larger than 0,2, where the diameter of Nb-Ti superconducting filaments is larger than 1 μm. The changes for the Cu/Cu-Ni/Nb-Ti are described in Annex C. The Cu-Ni uses all of the main part of the standard with the exceptions listed in Annex C that replace (and in some cases are counter to) some of the steps in the main text. This method is intended for use with superconductors that have critical currents less than 1 000 A and n-values larger than 12, under standard test conditions and at magnetic fields less than or equal to 0,7 of the upper critical magnetic field. The test specimen is immersed in a liquid helium bath at a known temperature during testing. The test conductor has a monolithic structure with a round or rectangular cross-sectional area that is less than 2 mm2. The specimen geometry used in this test method is an inductively coiled specimen. Deviations from this test method that are allowed for routine tests and other specific restrictions are given in this standard. Test conductors with critical currents above 1 000 A or cross-sectional areas greater than 2 mm2 could be measured with the present method with an anticipated increase in uncertainty and a more significant self-field effect (see Annex B). Other, more specialized, specimen test geometries may be more appropriate for larger conductor testing which have been omitted from this present standard for simplicity and to retain a lower uncertainty.
The test method given in this standard is expected to apply to other superconducting composite wires after some appropriate modifications. 2 Normative references The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies. IEC 60050-815, International Electrotechnical Vocabulary (IEV) – Part 815: Superconductivity
3 Terms and definitions For the purposes of this standard, the terms and definitions given in IEC 60050-815, some of which are repeated here for convenience, and the following apply.

61788-1 © IEC:2006 – 13 – 3.1
critical current
Ic maximum direct current that can be regarded as flowing without resistance NOTE Ic is a function of magnetic field strength and temperature. [IEV 815-03-01] 3.2
critical current criterion
Ic criterion criterion to determine the critical current, Ic, based on the electric field strength, E, or the resistivity,
NOTE E = 10 V/m or E = 100 V/m is often used as the electric field strength criterion, and
= 10-13 ·m or
= 10-14 ·m is often used as the resistivity criterion. [IEV 815-03-02, modified] 3.3
n-value (of a superconductor) exponent obtained in a specific range of electric field strength or resistivity when the voltage/current U(I) curve is approximated by the equation U ∝ In [IEV 815-03-10] 3.4
quench uncontrollable and irreversible transition of a superconductor or a superconducting device from the superconducting state to the normal conducting state NOTE A term usually applied to superconducting magnets. [IEV 815-03-11] 3.5
three-component superconducting wire composite superconducting wire composed of a superconducting component and two normal conducting materials
NOTE This term is mostly used for Cu/Cu-Ni/Nb-Ti composite superconductors [IEV 815-04-33] 3.6
Lorentz force (on fluxons) force applied to fluxons by a current NOTE 1 The force per unit volume is given by J x B, where J is a current density, and B is a magnetic flux density. NOTE 2 "Lorentz force" is defined in IEV 121-11-20.[1]1). [IEV 815-03-16] 3.7
current transfer (of composite superconductor) phenomenon that a d.c. current transfers spatially from filament to filament in a composite superconductor, resulting in a voltage generation along the conductor ————————— 1) Figures in square brackets refer to the Bibliography.

61788-1 © IEC:2006 – 15 – NOTE In the Ic measurement, this phenomenon appears typically near the current contacts where the injected current flows along the conductor from periphery to inside until uniform distribution among filaments is accomplished. 3.8
constant sweep rate method a U-I data acquisition method where a current is swept at a constant rate from zero to a current above Ic while frequently and periodically acquiring U-I data 3.9
ramp-and-hold method a U-I data acquisition method where a current is ramped to a number of appropriately distributed points along the U-I curve and held constant at each one of these points while acquiring a number of voltages and current readings 4 Principle The critical current of a composite superconductor is determined from a voltage (U) – current (I) characteristic measured at a certain value of a static applied magnetic field strength (magnetic field) at a specified temperature in a liquid cryogen bath at a constant pressure. To get a U-I characteristic, a direct current is applied to the superconductor specimen and the voltage generated along a section of the specimen is measured. The current is increased from zero and the U-I characteristic generated is recorded. The critical current is determined as the current at which a specific electric field strength (electric field) criterion (Ec) or resistivity criterion (ρc) is reached. For either Ec or ρc, there is a corresponding voltage criterion (Uc) for a specified voltage tap separation. 5 Requirements The critical current of a superconductor shall be measured by applying a direct current (I) to the superconductor specimen and then measuring the voltage (U) generated along a section of the specimen. The current shall be increased from zero and the voltage-current (U-I) characteristic generated and recorded. The specimen shall be affixed to the measurement mandrel with sufficient tension or a low temperature adhesive.
NOTE 1 Exception C.2.1 replaces this sentence for Cu/Cu-Ni/Nb-Ti specimens. The target uncertainty of this method is defined as a coefficient of variation (standard deviation divided by the average of the critical current determinations) that shall not exceed 3 % in an interlaboratory comparison.
NOTE 2 Exception C.2.2 replaces this sentence for Cu/Cu-Ni/Nb-Ti specimens. The use of a common current transfer correction is excluded from this test method. Furthermore, if a current transfer signature is pronounced in the measurement, then the measurement shall be considered invalid. It is the responsibility of the user of this standard to consult and establish appropriate safety and health practices, and to determine the applicability of regulatory limitations prior to use. Specific precautionary statements are given below.

61788-1 © IEC:2006 – 17 – Hazards exist in this type of measurement. Very large direct currents with very low voltages do not necessarily provide a direct personal hazard, but accidental shorting of the leads with another conductor, such as tools or transfer lines, can release significant amounts of energy and cause arcs or burns. It is imperative to isolate and protect current leads from shorting. Also the stored energy in superconducting magnets commonly used for the background magnetic field can cause similar large current and/or voltage pulses or deposit large amounts of thermal energy in the cryogenic systems causing rapid boil-off or even explosive conditions. Under rapid boil-off conditions, cryogens can create oxygen-deficient conditions in the immediate area and additional ventilation may be necessary. The use of cryogenic liquids is essential to cool the superconductors to allow transition into the superconducting state. Direct contact of skin with cold liquid transfer lines, storage dewars or apparatus components can cause immediate freezing, as can direct contact with a spilled cryogen. If improperly used, liquid helium storage dewars can freeze air or water in pressure vent lines and cause the dewar to over-pressurize and fail despite the common safety devices. The use of liquid hydrogen is not recommended and not necessary for these measurements. It is imperative that safety precautions for handling cryogenic liquids be observed.
6 Apparatus 6.1 Measurement mandrel material The measurement mandrel shall be made from an insulating material or from a conductive non-ferromagnetic material that is either covered or not covered with an insulating layer.
The tensile strain at the measuring temperature, induced by the differential thermal contraction of the specimen and the measurement mandrel, shall not exceed 0,2 %. Suitable mandrel materials are recommended in Annex A. Any one of these may be used. NOTE 1 Exception C.2.3 replaces this sentence for Cu/Cu-Ni/Nb-Ti specimens. When a conductive material is used without an insulating layer, the leakage current through the mandrel shall be less than 0,2 % of the total current when the specimen current is at Ic (see 9.5 and A.3.1).
NOTE 2 Exception C.2.4 replaces this sentence for Cu/Cu-Ni/Nb-Ti specimens. 6.2 Mandrel construction The diameter of the mandrel shall be larger than 24 mm and consistent with the bending strain limit (see 7.2). Preferably the mandrel shall have a helical groove in which the specimen shall be wound. The pitch angle of the groove shall be less than 7°.
If no helical groove is used to wind the specimen, the same conditions given for the pitch angle shall be met. This approach to winding the specimen could result in inadequate support of the specimen and larger variation in the pitch angle of the specimen (see 7.2).

61788-1 © IEC:2006 – 19 – The angle between the specimen axis (portion between the voltage taps) and the magnetic field shall be (90 ± 7)°. This angle shall be determined with a combined standard uncertainty not to exceed 1°. The current contact shall be rigidly fastened to the measurement mandrel to avoid stress concentration on the specimen in the region of transition between the mandrel and the current contact. 7 Specimen preparation 7.1 Specimen bonding Winding tension and/or a low temperature adhesive (such as silicone vacuum grease, Apiezon®2) vacuum grease or epoxy) shall be used to bond the specimen to the measurement mandrel to reduce specimen motion. When a low-temperature adhesive is used, a minimum shall be applied and the excess adhesive shall be removed from the outer surface of the specimen after the specimen has been mounted.
NOTE 1 Exception C.2.5 replaces this sentence for Cu/Cu-Ni/Nb-Ti specimens. The adequacy of specimen bonding shall be demonstrated by a successful completion of the specified critical current repeatability. Solder shall not be used to bond the specimen to the mandrel between the current contacts. NOTE 2 Exception C.2.6 replaces this sentence for Cu/Cu-Ni/Nb-Ti specimens. 7.2 Specimen mounting There shall be no joints or splices in the test specimen. The cross-sectional area S of the specimen shall be determined in the plane transverse to the axis of the conductor with a combined standard uncertainty not to exceed 2,5 %. The wire shall be wound in the shape of a small coil in an inductive manner. The specimen shall not be wound in a manner that would introduce additional twists into the specimen. For a wire with a rectangular cross-section, the specimen shall be wound in a coil so that the applied magnetic field is parallel to the wide face of the specimen. To ensure that the specimen is well-seated in the groove, a tensile force shall be applied to the wire during winding and this force shall not result in more than 0,1 % tensile strain ( see Annex D) on the wire.
NOTE Exception C.2.7 replaces this sentence for Cu/Cu-Ni/Nb-Ti specimens. The maximum bending strain induced during the mounting of the specimen shall not exceed 3 %. Both ends of the wire shall be fixed to the current contact with solder. The minimum length of the soldered part of the current contact shall be the largest of 40 mm, 30 wire diameters or 30 wire thicknesses. ————————— 2) Apiezon® is the trade name of a product supplied by M&I Materials Ltd., UK (www.apiezon.com).This information is given for the convenience of users of this document and does not constitute an endorsement by IEC of the product named. Equivalent products may be used if they can be shown to lead to the same results.

61788-1 © IEC:2006 – 21 – No more than three turns of the specimen shall be soldered onto each current contact. The shortest distance from a current contact to a voltage tap shall be greater than 40 mm. The voltage taps shall be soldered to the specimen. Minimize the mutual inductance between the specimen current and the area formed by the specimen and the voltage taps by counterwinding the untwisted section of the voltage taps back along the specimen, as shown in Figure A.1.
The distance L along the specimen between the voltage taps shall be measured with a combined standard uncertainty not to exceed 2,5 %. This voltage tap separation shall be greater than 50 mm. For testing, the specimen and mandrel shall be mounted in a test cryostat consisting of a liquid helium dewar, a magnet and support structure, and a specimen support structure.
8 Measurement procedure
The specimen shall be immersed in liquid helium for the data acquisition phase. The temperature of the liquid helium bath shall be measured before and after each determination of Ic. The specimen current shall be kept low enough so that the specimen does not enter the normal state unless a quench protection circuit or resistive shunt is used to protect the specimen from damage.
When using the constant sweep rate method, the time for the ramp from zero current to Ic shall be more than 10 s.
When using the ramp-and-hold method, the current sweep rate between current set points shall be lower than the equivalent of ramping from zero current to Ic in 3 s. The d.c. magnetic field shall be applied in the direction of the mandrel axis. The relation between the magnetic field and the magnet current shall be measured beforehand. The magnet current shall be measured before each determination of Ic. The applied magnetic field shall be parallel to the wide face and orthogonal to the wire axis of the specimens with rectangular cross-sections.
The direction of the current and the applied magnetic field shall result in an inward Lorentz force over the length of the specimen between the voltage taps.
NOTE Exception C.2.8 replaces this sentence for Cu/Cu-Ni/Nb-Ti specimens. Record the U-I characteristic of the test specimen under test conditions and monotonically increasing current. A valid U-I characteristic shall give an Ic with a standard deviation obtained under repeatability conditions not to exceed 0,5 % and the characteristic shall be stable with time for voltages at or below the critical current criterion.

61788-1 © IEC:2006 – 23 – The baseline voltage of the U-I characteristic shall be taken as the recorded voltage at zero current for the ramp and hold current method, or the average voltage at approximately 0,1 Ic for the constant sweep rate method.
9 Uncertainty of the test method 9.1 Critical current The critical current shall be determined from a voltage-current characteristic measured with a four-terminal technique. The current source shall provide a d.c. current having a maximum periodic and random deviation of less than ±2 % at Ic, within the bandwidth 10 Hz to 10 MHz. A four-terminal standard resistor, with a combined standard uncertainty not to exceed 0,25 %, shall be used to determine the specimen current. A recorder and necessary preamplifiers, filters or voltmeters, or a combination thereof, shall be used to record the U-I characteristic. The resulting record shall allow the determination of Uc with a combined standard uncertainty not to exceed 5 % and the corresponding current with a combined standard uncertainty not to exceed 0,5 %.
9.2 Temperature A cryostat shall provide the necessary environment for measuring Ic and the specimen shall be measured while immersed in liquid helium. The liquid helium bath shall be operated so that the bath temperature is near the normal boiling point for the typical atmospheric pressure of the test site. The specimen temperature is assumed to be the same as the temperature of the liquid. The liquid temperature shall be reported with a combined standard uncertainty not to exceed 0,01 K, measured by means of a pressure sensor or an appropriate temperature sensor. The difference between the specimen temperature and the bath temperature shall be minimized.
For converting the observed pressure in the cryostat into a temperature value, the phase diagram of helium shall be used. The pressure measurement shall have an uncertainty that is low enough to obtain the required uncertainty of the temperature measurement. For liquid helium depths greater than 1 m, a head correction may be necessary.
9.3 Magnetic field A magnet system shall provide the magnetic field with a combined standard uncertainty not to exceed 0,5 % or 0,01 T, whichever is larger. The magnetic field, over the length of the specimen between the voltage contacts, shall have a uniformity not to exceed 0,5 % or 0,02 T, whichever is larger. The maximum periodic and random deviation of the magnetic field shall not exceed ±1 % or ±0,02 T, whichever is larger.

61788-1 © IEC:2006 – 25 – 9.4 Specimen and mandrel support structure The support structure shall provide an adequate support for the specimen and the orientation of the specimen with respect to the magnetic field. The specimen support is adequate if it allows additional determinations of critical current with the repeatability described in Clause 8. The test configuration of the specimen shall be an inductive coil 9.5 Specimen protection If a resistive shunt or quench protection circuit is used in parallel with the specimen, then the current through the shunt or the circuit shall be less than 0,2 % of the total current at Ic. 10 Calculation of results 10.1 Critical current criteria The critical current, Ic, shall be determined by using an electric field criterion, Ec, or a resistivity criterion, ρc, where the total cross-section of the composite superconductor is preferred for the estimation of the resistivity (see Figures 1 and 2).
In the case of an electric field criterion, two values of Ic shall be determined at criteria of 10 μV/m and 100 μV/m. In the other case, two values of Ic shall be determined at resistivity criteria of 10–14 Ωm and 10–13 Ωm.
When it is difficult to measure the Ic properly at a criterion of 100 μV/m, an Ec criterion less than 100 μV/m must be substituted. Otherwise, the measurements using the resistivity criterion are recommended.
The Ic shall be determined as the current corresponding to the point on the U-I curve where the voltage is Uc measured relative to the baseline voltage (see Figures 1 and 2):
Uc = L Ec (1) where Uc
is the voltage criterion, in microvolts; L
is the voltage tap separation, in metres; Ec
is the electric field criterion, in microvolts/metre. or, when using a resistivity criterion:
Uc = Ic ρc L/S (2) where
Uc, Ic and ρc
are the corresponding voltage, current and resistivity to the intersecting point of a straight line with the U-I curve as shown in Figure 1, and
S is the overall cross-sectional area in square metres. A straight line shall be drawn from the baseline voltage to the average voltage near 0,7 Ic (see Figures 1 and 2). A finite slope of this line may be due to current transfer. A valid determination of Ic requires that the slope of the line be less than 0,3 Uc/Ic, where Uc and Ic are determined at a criterion of 10 μV/m or 10–14 Ωm.

61788-1 © IEC:2006 – 27 – 10.2 n-value (optional calculation, refer to A.7.2) The n-value shall be calculated as the slope of the plot of log U versus log I in the region where the Ic is determined, or shall be calculated using two Ic values as determined in 10.1 at two different criteria.
The range of the criteria used to determine n shall be reported.
Uc = LEc Uc = IcρcL/S U= IρcL/S U= LEc Voltage (arbitrary units) DC current (arbitrary units) 1 0 0 1 IEC
2067/06
NOTE The application of the electric field and resistivity criteria to determine the critical current is shown. Figure 1 – Intrinsic U-I characteristic
Uc = LEc Uc = IcρcL/S U= IρcL/S U= LEc Voltage (arbitrary units) Current transfer line DC current (arbitrary units) 1 0 0 1 IEC
2068/06
NOTE The application of the electric field and resistivity criteria to determine the critical current on a U-I characteristic, with a current transfer component exhibited as a linear region at low current is shown. Figure 2 – U-I characteristic with a current transfer component

61788-1 © IEC:2006 – 29 – 11 Test report 11.1 Identification of test specimen
The test specimen shall be identified, if possible, by the following: a)
name of the manufacturer of the specimen; b)
classification and/or symbol; c)
lot number; d)
raw materials and their chemical composition; e) shape and area of the cross-section of the wire, number of filaments, diameter of filaments, twist pitch and copper/superconductor ratio. 11.2 Report of Ic values The Ic values, along with their corresponding criteria, shall be reported. 11.3 Report of test conditions The following test conditions shall be reported: a)
test magnetic field and uniformity of field; b)
test temperature; c)
number of turns of the tested coil; d)
technique used to wind the coil; e)
length between voltage taps and total specimen length; f)
shortest distance from a current contact to a voltage tap; g)
shortest distance between current contacts; h)
soldered length of the current contacts; i)
specimen bonding method, including identification of bonding material; j)
mandrel material; k)
mandrel diameter; l)
depth, shape, pitch and angle of grooves.

61788-1 © IEC:2006 – 31 – Annex A
(informative)
Additional information relating to the standard
A.1 Scope
There are a large number of variables that have a significant effect on the measured value of critical current which need to be brought to the attention of the user. Some of these will be addressed in this informative annex. The method described in this standard is not applicable to wires with a copper/superconductor ratio (i.e. a volume ratio of Cu/Nb-Ti) that is smaller than 1, because the observed voltage-current (U-I) characteristics may not be stable at low magnetic fields. The reason for the restrictions in this test method is to obtain the necessary uncertainty in the final definitive phase of long conductor qualification. This standard requires that the specimen is to be tested while immersed in liquid helium that is near the boiling point of liquid helium at the normal atmospheric pressure of the test site. Testing in liquid helium at temperatures other than near this normal boiling point or testing in a gas or a vacuum is not covered by the scope of this standard. A.2 Requirements The d.c. critical current intended to be determined by the present method is the maximum direct electric current below which a superconductor can be regarded as resistance-less, at least for practical purposes, at a given temperature and magnetic field. Typically, the upper limit of the test magnetic field (0,7 of the upper critical magnetic field) will be 8 T at a temperature near 4,2 K. The minimum total length of the specimen is 210 mm, which represents the sum of the following: – soldered length of current contacts (2 × 40 mm);
– distance between current and voltage contacts (2 × 40 mm); – the minimum voltage tap separation (50 mm).
In the case of routine tests where it is impractical to adhere to these specific restrictions, this standard can be used as a set of general guidelines with an anticipated increase in uncertainty.
For routine tests, a wider range of parameters is accepted, but in definitive intercomparisons and performance verification, restrictions are needed to balance ease of use and resulting target uncertainty.

61788-1 © IEC:2006 – 33 – Measurements on short, straight specimens are considered acceptable practice for routine measurements if the cross-sectional area of the specimen is small in comparison with its length. However, for simplicity, this specimen geometry is omitted. Measurements on non-inductively wound (bifilar) specimens in combination with epoxy specimen bonding are expected to give an uncertainty similar to the target uncertainty of this method. However, for simplicity, this specimen geometry is omitted. For a bifilar specimen geometry, the Lorentz force is away from the measurement mandrel for part of the specimen's length, and silicone vacuum grease or tension is not strong enough to keep the specimen from moving in this case. Measurements on a non-ferromagnetic stainless steel mandrel combined with the use of solder to bond the specimen to the mandrel is considered acceptable practice for routine measurements. It will be difficult to estimate the amount of current shunted through the mandrel in this case, especially if a superconducting solder is used and the measurements are made in low magnetic fields. When a magnetic field direction study is requested on a specimen with a rectangular cross-sectional area, there are two options. All field angles are possible by measuring a straight specimen geometry in a radial access magnet. Two field angles (0° and 90°) are possible by measuring a hairpin specimen geometry and a coiled specimen geometry in a solenoid magnet. Neither the straight nor the hairpin specimen geometry method is covered here. The target uncertainty of the method described in this standard is defined by the results of an interlaboratory comparison. Results from previous interlaboratory comparisons were used in this test method to formulate the tolerances of the many variables that affect the uncertainty of critical current measurements. The target uncertainty, for an interlaboratory comparison, is a coefficient of variation (standard deviation divided by the average of critical current determinations) that is less than 3 %. The coefficient of variation provides additional information on the expected distribution of results from a large number of determinations. However, if there are significant systematic errors, the measurements of two laboratories may differ by two or more times the coefficient of variation. The expected and accepted uncertainty of critical current measurements at magnetic fields in the order of 0,8 times the upper critical field (around 9 T at 4,2 K) will have a higher coefficient of variation due to the increased sensitivity of Ic to magnetic field, temperature, strain and required voltage sensitivity.
It is expected that the uncertainty of the magnetic field in this test method may be the single most significant contributor to the overall uncertainty of the critical current measurement. However, a more restrictive tolerance may not be achievable due to the difficulty in calibrating this parameter. The test method for determining the Ic values of superconducting composite wires excluded from the present test method may be addressed in future documents.

61788-1 © IEC:2006 – 35 – A.3 Apparatus A.3.1 Measurement mandrel material The following materials are recommended for measurement mandrel material. There is, however, no restriction on using other materials as long as they satisfy the criteria mentioned in 6.1. Insulating material:
– fibreglass epoxy composite, with the specimen lying in the plane of the fabric; – fibreglass epoxy composite tube fabricated from a plate stock so that the planes of the fabric are perpendicular to the axis of the tube; – thin-walled rolled fibreglass epoxy composite tube. Conductive non-ferromagnetic material covered with an insulating layer: – non-ferromagnetic copper alloy, such as brass; – non-ferromagnetic stainless steel. Conductive non-ferromagnetic material without an insulating layer: – non-ferromagnetic stainless steel; – Ti-5 mass % Al-2,5 mass % Sn, with the limitation that this material is superconductive at temperatures below 3,7 K. – copper alloys like brass (Cu-Zn) and cupronickel (Cu-Ni). The leakage current through a conduct
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