EN 61788-2:2007

SLOVENSKI STANDARD
01-januar-2008
1DGRPHãþD
SIST EN 61788-2:2001
6XSHUSUHYRGQRVWGHO0HULWYHNULWLþQHJDWRND(QRVPHUQLNULWLþQLWRNSUL
VXSHUSUHYRGQLNLKL]1E6QNRPSR]LWD,(&
Superconductivity - Part 2: Critical current measurement - DC critical current of Nb3Sn
composite superconductors
Supraleitfähigkeit - Teil 2: Messen des kritischen Stromes Kritischer Strom (Gleichstrom)
von Nb3Sn-Verbundsupraleitern
Supraconductivité - Partie 2: Mesure du courant critique - Courant critique continu des
supraconducteurs composites Nb3Sn
Ta slovenski standard je istoveten z: EN 61788-2:2007
ICS:
17.220.20 0HUMHQMHHOHNWULþQLKLQ Measurement of electrical
PDJQHWQLKYHOLþLQ and magnetic quantities
29.050 Superprevodnost in prevodni Superconductivity and
materiali conducting materials
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD
EN 61788-2
NORME EUROPÉENNE
January 2007
EUROPÄISCHE NORM
ICS 17.220; 29.050 Supersedes EN 61788-2:1999

English version
Superconductivity
Part 2: Critical current measurement -
DC critical current of Nb Sn composite superconductors
(IEC 61788-2:2006)
Supraconductivité  Supraleitfähigkeit
Partie 2: Mesure du courant critique - Teil 2: Messen des kritischen Stromes -
Courant critique continu des Kritischer Strom (Gleichstrom)
supraconducteurs composites Nb Sn von Nb Sn-Verbundsupraleitern
3 3
(CEI 61788-2:2006) (IEC 61788-2:2006)

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.

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 61788-2:2007 E
Foreword
The text of document 90/195/FDIS, future edition 2 of IEC 61788-2, prepared by IEC TC 90,
Superconductivity, was submitted to the IEC-CENELEC parallel vote and was approved by CENELEC as
EN 61788-2 on 2006-12-01.
This European Standard supersedes EN 61788-2:1999.
Modifications made to EN 61788-2:1999 are mostly wording that essentially includes no technical
changes and an addition of a new annex (normative Annex D) in which the specifications in the
one-mandrel method are described.
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 61788-2:2006 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 61788-1 NOTE  Harmonized as EN 61788-1:2007 (not modified).
__________
- 3 - EN 61788-2:2007
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 2000 International Electrotechnical Vocabulary - -
(IEV)
Part 815: Superconductivity
NORME CEI
INTERNATIONALE
IEC
61788-2
INTERNATIONAL
Deuxième édition
STANDARD
Second edition
2006-11
Supraconductivité –
Partie 2:
Mesure du courant critique –
Courant critique continu des
supraconducteurs composites Nb Sn
Superconductivity –
Part 2:
Critical current measurement –
DC critical current of Nb Sn
composite superconductors
© IEC 2006 Droits de reproduction réservés ⎯ Copyright - all rights reserved
Aucune partie de cette publication ne peut être reproduite ni No part of this publication may be reproduced or utilized in any
utilisée sous quelque forme que ce soit et par aucun procédé, form or by any means, electronic or mechanical, including
électronique ou mécanique, y compris la photocopie et les photocopying and microfilm, without permission in writing from
microfilms, sans l'accord écrit de l'éditeur. the publisher.
International Electrotechnical Commission, 3, rue de Varembé, PO Box 131, CH-1211 Geneva 20, Switzerland
Telephone: +41 22 919 02 11 Telefax: +41 22 919 03 00 E-mail: inmail@iec.ch Web: www.iec.ch
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МеждународнаяЭлектротехническаяКомиссия
Pour prix, voir catalogue en vigueur
For price, see current catalogue

61788-2 © IEC:2006 – 3 –
CONTENTS
FOREWORD.7
INTRODUCTION.11

1 Scope.13
2 Normative references .13
3 Terms and definitions .15
4 Principle .17
5 Requirements .17
6 Apparatus.19
6.1 Reaction mandrel material.19
6.2 Reaction mandrel construction .19
6.3 Measurement mandrel material .21
6.4 Measurement mandrel construction .21
6.5 Measurement set up.21
7 Specimen preparation.21
7.1 Specimen mounting for reaction heat treatment.21
7.2 Reaction heat treatment.23
7.3 Specimen mounting for measurement.23
7.4 Specimen bonding.23
8 Measurement procedure.25
9 Precision and accuracy of the test method.27
9.1 Critical current.27
9.2 Temperature.27
9.3 Magnetic field.27
9.4 Specimen support structure.27
9.5 Specimen protection.27
10 Calculation of results .29
10.1 Critical current criteria .29
10.2 n-value (optional calculation, refer to A.7.2) .31
11 Test report.31
11.1 Identification of test specimen .31
11.2 Report of I values .31
c
11.3 Report of test conditions.33

Annex A (informative) Additional information relating to Clauses 1 to 10 .35
Annex B (informative) Strain effect of Nb Sn conductors .59
Annex C (informative) Self-field effect.63
Annex D (normative) One-mandrel method .67

Bibliography.73

61788-2 © IEC:2006 – 5 –
Figure 1 – Intrinsic U-I characteristic .29
Figure 2 – U-I characteristic with a current transfer component.29
Figure A.1 – Instrumentation of specimen with a null voltage tap pair .45
Figure B.1 – Uniaxial (tensile) strain dependence of critical current for a typical Nb Sn
composite wire shown with various magnetic fields [7] .61

Table A.1 – Thermal contraction data of Nb Sn superconductor and selected materials .57
61788-2 © IEC:2006 – 7 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
___________
SUPERCONDUCTIVITY –
Part 2: Critical current measurement –
DC critical current of Nb Sn 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 61788-2 has been prepared by IEC technical committee 90:
Superconductivity.
This second edition cancels and replaces the first edition published in 1999. Modifications
made to the second edition are mostly wording that essentially includes no technical changes
and an addition of a new annex (normative Annex D) in which the specifications in the one-
mandrel method are described.
61788-2 © IEC:2006 – 9 –
The text of this standard is based on the following documents:
FDIS Report on voting
90/195/FDIS 90/199/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.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
A list of all parts of the IEC 61788 series, 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.
61788-2 © IEC:2006 – 11 –
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 the
present standard 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.
The test method covered in this standard is based on that for the determination of the critical
)
current of Cu/Nb-Ti composite superconductors (IEC 61788-1[2] and the VAMAS (Versailles
project on advanced materials and standards) prestandardization work on the critical current
of Nb Sn composite superconductors. The critical current of Nb Sn superconductors is known
3 3
to be highly sensitive to mechanical strain compared to Cu/Nb-Ti superconductors. Hence,
some modifications are made on the test procedures which may affect the strain state of a
test specimen. See Annex B for the background to these modifications.
—————————
)
Figures in square brackets refer to the Bibliography.

61788-2 © IEC:2006 – 13 –
SUPERCONDUCTIVITY –
Part 2: Critical current measurement –
DC critical current of Nb Sn composite superconductors
1 Scope
This part of IEC 61788 covers a test method for the determination of the d.c. critical current of
Nb Sn composite superconductors which are fabricated by either the bronze process or the
internal tin diffusion process and have a copper/non-copper ratio larger than 0,2.
This method is intended for use with superconductors which have critical currents of less than
1 000 A and n-values larger than 12 under standard test conditions and at magnetic fields of
less than or equal to 0,7 times the upper critical magnetic field. The test specimen is
immersed in a liquid helium bath at a known temperature during testing. The Nb Sn
composite test conductor has a monolithic structure with a total round-cross-sectional area
that is less than 2 mm . The specimen geometry used in this test method is an inductively
coiled specimen. Deviations from this test method which are allowed for routine tests and
other specific restrictions are given in this standard.
Nb Sn conductors with critical currents above 1 000 A or total cross-sectional areas greater
than 2 mm can be measured with the present method with an anticipated reduction in
precision and a more significant self-field effect (see Annex C). 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 precision.
The test method given in this standard should in principle apply to Nb Sn composite wires
fabricated by any other process. This method is also expected to apply to other
superconducting composite wires after some appropriate modifications.
2 Normative references
The following referenced document is 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 60050-815:2000, International Electrotechnical Vocabulary (IEV) – Part 815:
Superconductivity
61788-2 © IEC:2006 – 15 –
3 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 60050-815, some of
which are repeated here for convenience, and the following apply:
3.1
critical current
I
c
maximum direct current that can be regarded as flowing without resistance
NOTE I is a function of magnetic field strength and temperature.
c
[IEV 815-03-01]
3.2
critical current criterion
I criterion
c
criterion to determine the critical current, I , based on the electric field strength, E, or the
c
resistivity, ρ
-13
NOTE E = 10 μV/m or E = 100 μV/m is often used as the electric field strength criterion, and ρ = 10 Ω·m or
-14
ρ = 10 Ω ·m is often used as the resistivity criterion.
[IEV 815-03-02]
3.3
n-value (of a superconductor)
exponent obtained in a specific range of electric field strength or resistivity when the voltage
n
(U) – current (I) curve is approximated by the equation U ∝ I
[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 state
NOTE A term usually applied to superconducting magnets.
[IEV 815-03-11]
3.5
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 the current density, and B is the magnetic flux
density.
[IEV 815-03-16]
NOTE 2 "Coulomb-Lorentz force" is defined in IEV 121-11-20 [1].
3.6
stress effect
strain effect
change in superconducting properties upon application of mechanical, thermal or electro-
magnetic stress to the superconductor
[IEV 815-03-14]
61788-2 © IEC:2006 – 17 –
3.7
bending strain
ε
b
strain in percent arising from pure bending defined as ε = 100r/R, where r is a half of the
b
specimen thickness and R is the bending radius
[IEV 815-08-03]
3.8
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
NOTE In the I measurement, this phenomenon appears typically near the current contacts where the injected
c
current flows along the conductor from periphery to inside until uniform distribution among filaments is
accomplished.
3.9
constant sweep rate method
U-I data acquisition method where a current is swept at a constant rate from zero to a current
above I while frequently and periodically acquiring U-I data
c
3.10
ramp-and-hold method
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 helium 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 (E ) or resistivity
c
criterion (ρ ) is reached. For either E or ρ , there is a corresponding voltage criterion (U ) for
c c c c
a specified voltage tap separation.
5 Requirements
The specimen shall be wound on a cylindrical reaction mandrel with a helical groove and after
reaction, transferred to a measurement mandrel of the same diameter on which the helical
angle is preserved. An alternate one-mandrel method is given in Annex D.
The specimen shall be longer than 430 mm.
The specimen shall be affixed to the measurement mandrel by tightening the specimen and/or
bonding with a low temperature adhesive.
In this test method, the applied magnetic field shall be parallel to the measurement mandrel
axis.
61788-2 © IEC:2006 – 19 –
The target precision of this method is a coefficient of variation (standard deviation divided by
the average of the critical current determinations), that is less than 3 % for the measurement
at 12 T and near 4,2 K.
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 determine the applicability of regulatory limitations prior to use.
Specific precautionary statements are given below.
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 the 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. It is imperative that
safety precautions for handling cryogenic liquids be observed.
6 Apparatus
For the one-mandrel method, continue with Clause D.2.
6.1 Reaction mandrel material
The reaction mandrel shall be made from a heat-resistant material that may or may not have a
treated surface. Suitable reaction mandrel materials are recommended in A.3.1. Any one of
these may be used.
6.2 Reaction mandrel construction
The overall geometry of the reaction mandrel should be matched closely to that of the
measurement mandrel to which the individual specimen is to be transferred.
The reaction mandrel shall have a diameter large enough that the specimen bending strain,
which is introduced into the specimen during winding, is less than 5 %.
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°. The depth of the groove shall be at least half the
wire diameter.
61788-2 © IEC:2006 – 21 –
6.3 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 critical current may inevitably depend on the measurement mandrel material due to the
strain induced by the differential thermal contraction between the specimen and the measurement
mandrel.
The total strain induced in the specimen at the measuring temperature shall be minimized to
be within ±0,03 %. If there is an excess strain due to the differential thermal contraction of the
specimen and the mandrel, the critical current shall be noted to be determined under an
excess strain state by identification of the mandrel material.
Suitable measurement mandrel materials are recommended in A.3.3. Any one of these may
be used.
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
critical current I (see 9.5).
c
6.4 Measurement mandrel construction
The mandrel shall have a helical groove in which the specimen shall be wound.
The diameter of the measurement mandrel, the pitch angle of the helical groove and its depth
and shape shall be close to those of the reaction mandrel.
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 an accuracy of ±2°.
The current contact shall be rigidly fastened to the measurement mandrel to avoid stress
concentration in the region of transition between the mandrel and the current contact.
6.5 Measurement set up
The apparatus to measure the U-I characteristic of a superconductor specimen consists of a
specimen probe, a test cryostat, a magnet system and a U-I measurement system.
The specimen probe, which consists of a specimen, a measurement mandrel, a specimen
support structure, voltage taps, current leads etc. is inserted in the test cryostat filled with
liquid helium. In most cases, the cryostat contains a superconducting solenoid magnet and its
support structure to apply a magnetic field to the specimen. The U-I measurement system
consists of a d.c. current source, a recorder and necessary preamplifiers, filters or voltmeters,
or a combination thereof. A computer-assisted data acquisition system may be also used.
7 Specimen preparation
For the one-mandrel method, continue with Clause D.4.
7.1 Specimen mounting for reaction heat treatment
There shall be no joints or splices in the test specimen.

61788-2 © IEC:2006 – 23 –
When using resistivity criteria for the critical current determination, the total cross-sectional
area S of the specimen shall be determined to a precision of 5 %.
The specimen shall not be wound in a manner that would introduce additional twists into the
specimen.
The specimen shall be located in the groove on the reaction mandrel under almost zero
tension (less than 0,1 % tensile strain) so that location continues to be preserved and the
contact pressure reduced to a minimum to discourage diffusion bonding.
The specimen wire shall be retained on the reaction mandrel by bending ends through small
holes, one at each end of the mandrel, or be retained by some equivalent method.
The specimen shall be cleaned to avoid effects of contamination.
7.2 Reaction heat treatment
Reaction heat treatment shall be carried out according to the manufacturer's specification,
which includes error limits which shall not be exceeded. Temperature variations within the
furnace shall be controlled so as not to exceed those limits.
7.3 Specimen mounting for measurement
After the reaction heat treatment, the ends of the specimen shall be trimmed to suit the
measurement mandrel.
The specimen shall be unscrewed from the reaction mandrel by lightly restraining it and
rotating the mandrel within it.
The specimen shall be immediately screwed onto the measurement mandrel in the same
manner as it was removed from the reaction mandrel. When mounting on the measurement
mandrel, the specimen shall be laid into the groove and one end shall be soldered to the
current contact ring. Starting from the fixed end, the specimen shall be stroked along its entire
length, thus firmly seating the specimen in the groove. The free end shall then be soldered to
the other contact ring.
The minimum length of the soldered part of the current contact shall be greater than the
smaller of 40 mm and 30 wire diameters. No more than three turns of the specimen shall be
soldered to each current contact.
The shortest distance from a current contact to a voltage tap shall be greater than 100 mm.
The voltage taps shall be soldered to the specimen. Minimize the mutual inductance between
the applied 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 along the specimen between the voltage taps, L, shall be measured to an
accuracy of 5 %. This voltage tap separation shall be greater than 150 mm.
7.4 Specimen bonding
Specimen tension and/or a low temperature adhesive (such as silicone vacuum grease or
epoxy) shall be used to bond the specimen to the measurement mandrel to reduce specimen
motion. If specimen tension is used to bond the specimen, then this shall be accomplished
during the specimen mounting for the measurement process (see 7.3).

61788-2 © IEC:2006 – 25 –
When an adhesive is used, a minimum amount of adhesive shall be applied in the groove
containing the specimen, and the excess shall be removed from the outer surface of the
specimen after the specimen has been mounted.
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.
8 Measurement procedure
The specimen shall be immersed in liquid helium for the data acquisition phase. The
specimen may be cooled slowly in helium vapour and then inserted into the liquid helium bath,
or inserted slowly into the liquid helium bath, or first slowly immersed in liquid nitrogen and
then liquid helium. The specimen shall be cooled from room temperature to liquid helium (or
liquid nitrogen) temperature over a time period of at least 5 min.
The cryostat shall provide the necessary environment for measuring I and the specimen shall
c
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 temperature of the liquid helium bath shall be measured during each determination of I .
c
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 I
c
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
I in 3 s. The current drift during each current set point shall be less than 1 % of I .
c c
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 I .
c
The direction of the current and the applied magnetic field shall result in an inward Lorentz
force over the length of the specimen at least between the voltage taps.
Record the U-I characteristic of the test specimen under test conditions and monotonically
increasing current.
A valid U-I characteristic shall give a repeatable I to a precision of 1 % and the characteristic
c
shall be stable with time for voltages at or below the critical current criterion.
The baseline voltage of the U-I characteristic shall be taken as the recorded voltage at zero
current for the ramp-and-hold method, or the average voltage at approximately 0,1 I for the
c
constant sweep rate method.
61788-2 © IEC:2006 – 27 –
9 Precision and accuracy of the test method
9.1 Critical current
The current source shall provide a d.c. current having a maximum periodic and random
deviation of less than ±2 % at I , within the bandwidth 10 Hz to 10 MHz.
c
A four-terminal standard resistor, with an accuracy of at least 0,5 %, shall be used to
determine the specimen current.
The record of U-I characteristic shall allow the determination of U to a precision of 10 %, the
corresponding current to an accuracy of 1 % and with a precision of 1 %.
9.2 Temperature
The specimen temperature is assumed to be the same as the temperature of the liquid. The
liquid temperature shall be reported to an accuracy of ±0,02 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 be accurate enough to
obtain the required accuracy of the temperature measurement. For liquid helium depths
greater than 1 m, a head correction may be necessary.
9.3 Magnetic field
A magnetic system shall provide the magnetic field to an accuracy better than the larger of
±1 % and ±0,02 T and a precision better than the larger of ±0,5 % and ±0,02 T over the length
of the specimen between the voltage taps.
The magnetic field shall have a uniformity better than the larger of 0,5 % and 0,02 T over the
length of the specimen between the voltage taps.
The maximum periodic and random deviation of the magnetic field shall be less than the
larger of ±1 % and ±0,02 T.
9.4 Specimen support structure
The support structure shall provide 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 a precision of 1 %.
9.5 Specimen protection
If a resistive shunt or a 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 I .
c
61788-2 © IEC:2006 – 29 –
10 Calculation of results
10.1 Critical current criteria
The critical current, I , shall be determined by using an electric field criterion, E , or a
c c
resistivity criterion, ρ , where the total cross-sectional area S of the composite super-
c
conductor is preferred for the estimation of the resistivity (see Figures 1 and 2).

U = LE
c
U = Iρ L/S
c
U = LE
c c
U = I ρ L/S
c c c
0 0
I I
c c
DC current  I (arbitrary units) DC current  I (arbitrary units)
IEC  628/06 IEC  629/06
Figure 1a) – Application of the electric field criteria Figure 1b) – Application of the resistivity criteria
NOTE The application of the (Figure 1a) electric field and (Figure 1b) resistivity criteria to determine the critical
current is shown above.
Figure 1 – Intrinsic U-I characteristic

U = LE
c
Current transfer line
Current transfer line
U = Iρ L/S
c
U = LE
c c
U = I ρ L/S
c c c
0 0
I
I
c c
DC current  I (arbitrary units) DC current  I (arbitrary units)
IEC  630/06 IEC  631/06
Figure 2a) – Application of the electric field criteria Figure 2b) – Application of the resistivity criteria
NOTE The application of the (Figure 2a) electric field and (Figure 2b) 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 above.
Figure 2 – U-I characteristic with a current transfer component
In the case of electric field criterion, two values of I shall be determined at criteria of 10 µV/m
c
and 100 µV/m. In the other case, two values of I shall be determined at resistivity criteria of
c
–14 –13
10 Ωm and 10 Ωm.
Voltage  U (arbitrary units)
Voltage  U (arbitrary units)
Voltage  U (arbitrary units)
Voltage  U (arbitrary units)
61788-2 © IEC:2006 – 31 –
When it is difficult to measure the I properly at a criterion of 100 μV/m, an E criterion less
c c
than 100 μV/m shall be substituted. Otherwise, the measurements using the resistivity
criterion are recommended.
The I shall be determined as the current corresponding to the point on the U-I curve where
c
the voltage is U measured relative to the baseline voltage (see Figures 1a) and 2a)):
c
U = L E (1)
c c
where
U is the voltage criterion, in microvolts (μV);
c
L is the voltage tap separation, in metres (m);
E is the electric field criterion, in microvolts/metre (μV/m).
c
Or, when using a resistivity criterion:
U = I ρ L/S (2)
c c c
where U , I and ρ are the corresponding voltage, current and resistivity, in microvolts,
c c c
amperes and micro-ohms × metres, respectively, to the intersecting point of a straight line
with the U-I curve as shown in Figures 1b) and 2b), L is the voltage tap separation, in metres,
and S is the total cross-sectional area, in square metres.
A straight line shall be drawn from the baseline voltage to the average voltage near 0,7 I (see
c
Figures 1 and 2). A finite positive slope of this line may be due to current transfer. A valid
determination of I requires that the slope of the line be less than 0,3 U /I , where U and I
c c c c c
–14
are determined at a criterion of 10 μV/m or 10 Ωm.
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 I is determined, or shall be calculated using two I values as determined in 10.1 at
c c
two different criteria.
The range of the criteria used to determine n shall be reported.
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, volume fractions of filaments, copper/non-copper ratio, barriers, copper
stabilizer and other components in the wire, twist pitch and twist direction;
f) manufacturing process technique (bronze, internal tin diffusion process, etc).
11.2 Report of I values
c
The I values, along with their corresponding criteria, shall be reported.
c
61788-2 © IEC:2006 – 33 –
11.3 Report of test conditions
The following test conditions shall be reported:
a) test magnetic field, uniformity of field and accuracy of field;
b) test temperature and accuracy of temperature;
c) number of turns of the tested coil;
d) length between voltage taps and total specimen length;
e) the shortest distance from a current contact to a voltage tap;
f) the shortest distance between current contacts;
g) soldered length of the current contacts;
h) the specimen bonding method, including identification of the bonding material;
i) reaction mandrel and measurement mandrel materials;
j) reaction mandrel and measurement mandrel diameters;
k) depth, shape, pitch, and angle of grooves;
l) reaction heat treatment conditions.

61788-2 © IEC:2006 – 35 –
Annex A
(informative)
Additional information relating to Clauses 1 to 10

A.1 General
There is a large number of variables that have a significant effect on the measured value of
critical current which needs to be brought to the attention of the user. Some of these are
addressed in this informative annex (see also Annex B).
The method described in this standard is not applicable to wires with a copper/non-copper
ratio (i.e. a volume ratio of Cu stabilizer to all other components of wire, including Nb Sn
filaments and diffusion barriers) that is smaller than 0,2 because the observed voltage-current
...