Iec 61788 1 2006

untitled NORME INTERNATIONALE CEI IEC INTERNATIONAL STANDARD 61788 1 Deuxième édition Second edition 2006 11 Supraconductivité – Partie 1 Mesure du courant critique – Courant critique continu de supra[.]

Matériau du mandrin de mesurage

Le mandrin de mesurage doit être constitué d’un matériau isolant ou d’un matériau conducteur non ferromagnétique recouvert ou non d'une couche isolante

La déformation de traction à la température de mesurage, produite par la contraction thermique différentielle du spécimen et du mandrin de mesurage, ne doit pas excéder 0,2 %

Des matériaux de mandrin appropriés sont recommandés dans l'Annexe A Il est permis d'utiliser n'importe lequel d'entre eux

NOTE 1 L'exception C.2.3 remplace cette phrase pour les spécimens Cu/Cu-Ni/Nb-Ti

When a conductive material lacks an insulating layer, the leakage current in the core must be less than 0.2% of the total current when the specimen is under I c conditions (refer to sections 9.5 and A.3.1).

NOTE 2 L'exception C.2.4 remplace cette phrase pour les spécimens Cu/Cu-Ni/Nb-Ti.

Construction du mandrin

Le diamètre du mandrin doit être supérieur à 24 mm et compatible avec la limite de déformation de flexion (voir 7.2)

De préférence, le mandrin doit comporter un sillon hélicọdal autour duquel le spécimen doit être enroulé L'angle de pas du sillon doit être inférieur à 7°

If the specimen is not wrapped around a helical groove, it is essential to adhere to the conditions related to the pitch angle This wrapping method may render the specimen's support unsuitable and lead to greater variations in its pitch angle (see 7.2).

LICENSED TO MECON Limited - RANCHI/BANGALORE FOR INTERNAL USE AT THIS LOCATION ONLY, SUPPLIED BY BOOK SUPPLY BUREAU.

Measuring very large direct currents at low voltages can pose hazards, particularly if the leads are accidentally shorted with other conductors like tools or transfer lines, potentially releasing significant energy and causing arcs or burns Therefore, it is crucial to isolate and protect current leads to prevent shorting incidents.

Superconducting magnets, often utilized for background magnetic fields, can store significant energy, leading to substantial current or voltage pulses This stored energy can also release large amounts of thermal energy into cryogenic systems, potentially resulting in rapid boil-off or even explosive scenarios.

Under rapid boil-off conditions, cryogens can create oxygen-deficient environments, necessitating additional ventilation Cryogenic liquids are crucial for cooling superconductors to achieve the superconducting state Direct skin contact with cold liquid transfer lines, storage dewars, or spilled cryogens can result in immediate freezing injuries Improper use of liquid helium storage dewars may lead to the freezing of air or water in pressure vent lines, potentially causing over-pressurization and failure, despite safety devices The use of liquid hydrogen is neither recommended nor necessary for these measurements Adhering to safety precautions when handling cryogenic liquids is essential.

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 I c

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°

When winding the specimen without a helical groove, it is essential to adhere to the specified pitch angle conditions This method may lead to insufficient support for the specimen and increased variability in the pitch angle.

The angle between the specimen axis (the section between the voltage taps) and the magnetic field must be (90 ± 7)° This angle should be determined with a combined standard uncertainty not exceeding 1°.

The current contact and the measuring mandrel must form a rigid assembly to prevent any stress concentration on the specimen in the transition area between the mandrel and the current contact.

Fixation du spécimen

The winding tension and/or a low-temperature adhesive, such as vacuum silicone grease or Apiezon® 2 grease, should be used to securely attach the specimen to the measuring mandrel, minimizing its movement When employing a low-temperature adhesive, it is essential to apply a minimal amount, and any excess adhesive must be removed from the outer surface of the specimen after it has been mounted.

L'adéquation du montage du spécimen doit être attestée par l'obtention de la reproductibilité du courant critique spécifié

Le spécimen ne doit pas être fixé au mandrin par soudage entre les contacts de courant

Montage du spécimen

Le spécimen d'essai ne doit pas comporter de jointure ni d'épissure

La superficie de section S du spécimen doit être déterminée dans le plan transversal à l'axe du conducteur avec une incertitude type combinée ne dépassant pas 2,5 %

Le fil doit être enroulé sous forme d’une petite bobine de manière inductive L'enroulement du spécimen doit empêcher toute torsade supplémentaire du spécimen

For a wire with a rectangular cross-section, the specimen must be wound into a coil so that the applied magnetic field is parallel to the widest face of the specimen.

To ensure the specimen is properly positioned in the groove, a pulling force must be applied to the wire during winding, and this force should not cause a tensile deformation of the wire greater than 0.1% (refer to Appendix D).

NOTE L'exception C.2.7 remplace cette phrase pour les spécimens Cu/Cu-Ni/Nb-Ti

La déformation en flexion maximale, provoquée lors du montage du spécimen, ne doit pas être supérieure à 3 %

Both ends of the wire must be welded at the current contact point The minimum length of the welded portion at the current contact must be the largest of the following three values:

40 mm, 30 fois le diamètre du fil ou 30 fois l'épaisseur du fil

2) Apiezon ® est l’appellation commerciale d'un produit distribué par M&I Materials Ltd, UK (www.apiezon.com)

Cette information est donnée à l’intention des utilisateurs du présent document et ne signifie nullement que la

CEI approuve ou recommande l’emploi exclusif du produit ainsi désigné Des produits équivalents peuvent être utilisés s'il est démontré qu'ils conduisent aux mêmes résultats

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°

To prevent stress concentration at the junction of the measurement mandrel and the current contact, it is essential to securely fasten the current contact to the mandrel.

Winding tension and/or a low temperature adhesive (such as silicone vacuum grease,

To minimize specimen motion during measurement, Apiezon® vacuum grease or epoxy should be utilized to securely bond the specimen to the measurement mandrel When employing a low-temperature adhesive, it is essential to apply a minimal amount and ensure that any excess adhesive is cleaned off the outer surface of the specimen after mounting.

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

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 must be wound into a small coil inductively, ensuring that no additional twists are introduced into the specimen during the winding process.

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 properly secure the specimen in the groove, a tensile force must be applied to the wire during the winding process, ensuring that the tensile strain does not exceed 0.1%.

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

Both ends of the wire must be securely attached to the current contact using solder The minimum length of the soldered section should be at least 40 mm, or 30 times the wire's diameter, or 30 times the wire's thickness, whichever is greatest.

Apiezon ®, a product offered by M&I Materials Ltd in the UK, can be found at www.apiezon.com This information is provided solely for the convenience of users and does not imply any endorsement.

IEC of the product named Equivalent products may be used if they can be shown to lead to the same results

Le nombre de tours du spécimen soudés sur chaque contact de courant ne doit pas dépasser trois

La distance la plus courte d'un contact de courant à une prise de tension doit être supérieure à 40 mm

Tension leads must be soldered to the specimen To minimize mutual inductance between the specimen's current and the area formed by the specimen and the tension leads, the untwisted section of the leads should be wrapped in the opposite direction around the specimen, as illustrated in Figure A.1.

The distance $ L $ along the specimen between the voltage taps must be measured with a combined standard uncertainty not exceeding 2.5% Additionally, the separation between the voltage taps should be greater than 50 mm.

For the purpose of the test, the specimen and the mandrel must be installed in a testing cryostat, which consists of a liquid helium Dewar vessel, a magnet with its support structure, and the support structure for the specimen.

The specimen must be immersed in liquid helium during the data acquisition phase The temperature of the liquid helium bath should be measured before and after each determination of I_c.

The specimen's current must remain low enough to prevent it from entering a normal state, unless a quench protection circuit or a resistive shunt is employed to safeguard the specimen.

Si on utilise la méthode à vitesse de balayage constante, le temps de variation entre zéro et

When employing the variation and maintenance method, the current sweep speed between the setpoint signals must be lower than the equivalent speed for a current change from zero to I c within 3 seconds.

Le champ magnétique continu doit être appliqué dans la direction de l'axe du mandrin La relation entre le champ magnétique et le courant de l'aimant doit être préalablement mesurée

Courant critique

Le courant critique doit être déterminé à partir d'une caractéristique tension-courant mesurée grâce à une technique à quatre bornes

La source de courant doit délivrer un courant continu dont les variations périodiques et aléatoires maximales sont inférieures à ±2 % en I c , dans la largeur de bande de 10 Hz à

Une résistance standard à quatre bornes avec une incertitude type combinée ne dépassant pas 0,25 % doit être utilisée pour déterminer le courant du spécimen

Un enregistreur ainsi que les préamplificateurs, filtres ou voltmètres requis, ou une combinaison de ces appareils doivent être utilisés pour enregistrer la caractéristique U-I

L'enregistrement produit doit permettre de déterminer U c avec une incertitude type combinée ne dépassant pas 5 % et le courant correspondant avec une incertitude type combinée ne dépassant pas 0,5 %.

Température

A cryostat must provide the necessary environment for measuring the critical current ($I_c$), with the specimen immersed in liquid helium The liquid helium bath should be maintained at a temperature close to the normal boiling point for the typical atmospheric pressure at the testing site It is assumed that the specimen's temperature matches that of the liquid The liquid's temperature must be reported with a combined standard uncertainty not exceeding 0.01 K, measured using an appropriate pressure or temperature sensor.

La différence entre la température du spécimen et celle du bain doit être minimisée

To convert the pressure observed in the cryostat into a temperature value, the phase diagram of helium must be utilized The pressure measurement should have a sufficiently low uncertainty to achieve the required temperature measurement accuracy When the depth of the liquid helium exceeds 1 meter, it may be necessary to apply corrections to the sensors.

Champ magnétique

Un système d'aimant doit fournir le champ magnétique avec une incertitude type combinée ne dépassant pas 0,5 % ou 0,01 T, selon la valeur la plus élevée

Le champ magnétique, sur la longueur du spécimen entre les contacts de tension, doit avoir une uniformité ne dépassant pas 0,5 % ou 0,02 T, selon la valeur la plus grande

Les variations périodiques et aléatoires maximales du champ magnétique ne doivent pas dépasser ±1 % ou ±0,02 T, suivant la valeur la plus grande

The baseline voltage for the U-I characteristic is determined by the recorded voltage at zero current using the ramp and hold current method, or by the average voltage at approximately 0.1 times the critical current (I c) when employing the constant sweep rate method.

9 Uncertainty of the test method

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 I c , 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

U c with a combined standard uncertainty not to exceed 5 % and the corresponding current with a combined standard uncertainty not to exceed 0,5 %

A cryostat is essential for measuring critical current (Ic), with the specimen immersed in liquid helium The liquid helium bath must be maintained near its normal boiling point at the test site's typical atmospheric pressure It is assumed that the specimen temperature matches that of the liquid The liquid temperature should be reported with a combined standard uncertainty not exceeding 0.01 K, using a pressure sensor or suitable temperature sensor for measurement.

The difference between the specimen temperature and the bath temperature shall be minimized

To convert the observed pressure in the cryostat to a temperature value, the phase diagram of helium is utilized It is essential that the pressure measurement has a sufficiently low uncertainty to achieve the desired accuracy in temperature measurement Additionally, for liquid helium depths exceeding 1 meter, a head correction may be required.

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

Structure de support du spécimen et du mandrin

The support structure must provide adequate support for the specimen and allow for its orientation relative to the magnetic field Proper specimen support is essential for obtaining additional measurements of the critical current with the repeatability outlined in Article 8.

La configuration d'essai du spécimen doit avoir la forme d'une bobine inductive.

Protection du spécimen

If a resistive shunt or a quench protection circuit is used in parallel with the specimen, the current in the shunt or circuit must be less than 0.2% of the total current at I c.

Critères de courant critique

The critical current, I c, should be determined using either an electric field criterion, E c, or a resistivity criterion, ρ c, particularly when the total cross-section of the composite superconductor is preferred for resistivity estimation (refer to Figures 1 and 2).

In the case of an electric field criterion, two critical values of \$I_c\$ must be determined for thresholds of 10 μV/m and 100 μV/m Conversely, for resistivity criteria, two values of \$I_c\$ should be established for resistivity levels of \$10^{-14}\$ Ωm and \$10^{-13}\$ Ωm.

When obtaining an accurate measurement of I c for a criterion of 100 μV/m proves challenging, it is necessary to replace E c with a criterion lower than 100 μV/m Alternatively, measurements using the resistivity criterion are recommended.

Le courant I c doit être déterminé comme étant le courant correspondant au point de la courbe

U-I ó la tension est égale à U c mesurée en fonction de la tension de base (voir Figures 1 et 2):

U c est le critère de tension, en microvolts;

L est la séparation entre prises de tension, en mètres;

E c est le critère de champ électrique, en microvolts/mètres ou, en utilisant un critère de résistivité:

U c , I c et ρc représentent respectivement la tension, le courant et la résistivité correspondants mesurés au point d'intersection d'une ligne droite et de la courbe U-I, comme le montre la Figure 1, et

S représente la superficie de section hors-tout exprimée en mètres carrés

Une ligne droite doit être tracée de la tension de base à la tension moyenne proche de 0,7 I c

Figures 1 and 2 illustrate that the finite slope of this line may be attributed to current transfer For a valid determination of \$I_c\$, the slope of the line must be less than 0.3.

U c /I c quand U c et I c sont déterminés avec un critère de 10 μV/m ou 10 –14 Ωm

9.4 Specimen and mandrel support structure

The support structure must adequately hold the specimen and align it with the magnetic field It is considered sufficient if it enables further measurements of critical current with the repeatability specified in Clause 8.

The test configuration of the specimen shall be an inductive coil

When utilizing a resistive shunt or quench protection circuit in parallel with the specimen, it is essential that the current flowing through the shunt or circuit remains below 0.2% of the total current at I c.

The critical current, $ I_c $, can be assessed using either an electric field criterion, $ E_c $, or a resistivity criterion, $ \rho_c $, with the total cross-section of the composite superconductor being favored for resistivity estimation (refer to Figures 1 and 2).

In the case of an electric field criterion, two values of I c shall be determined at criteria of

10 μV/m and 100 μV/m In the other case, two values of I c shall be determined at resistivity criteria of 10 –14 Ωm and 10 –13 Ωm

When it is difficult to measure the I c properly at a criterion of 100 μV/m, an E c criterion less than 100 μV/m must be substituted Otherwise, the measurements using the resistivity criterion are recommended

The I c shall be determined as the current corresponding to the point on the U-I curve where the voltage is U c measured relative to the baseline voltage (see Figures 1 and 2):

U c is the voltage criterion, in microvolts;

L is the voltage tap separation, in metres;

E c is the electric field criterion, in microvolts/metre or, when using a resistivity criterion:

U c , I c 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 I c

A finite slope in the line may indicate current transfer To accurately determine the critical current (\$I_c\$), the slope must be less than \$0.3 \frac{U_c}{I_c}\$, with \$U_c\$ and \$I_c\$ assessed at a criterion of \$10 \, \mu V/m\$ or \$10^{-14} \, \Omega m\$.

Valeur n (calcul facultatif, se reporter à A.7.2)

The value $ n $ should be calculated as the slope of the log $ U $ versus log $ I $ plot in the region where $ I_c $ is determined, or it can be calculated using two values of $ I_c $ identified in section 10.1 with two different criteria.

L'étendue des critères utilisés pour déterminer n doit être mentionnée

NOTE La figure montre l'application du champ électrique et des critères de résistivité dans la détermination du courant critique

The figure illustrates the application of the electric field and resistivity criteria in determining the critical current on a U-I characteristic curve, featuring a current transfer component depicted as a linear region traversed by a low current.

Figure 2 – Caractéristique U-I avec une composante de transfert de courant

10.2 n -value (optional calculation, refer to A.7.2)

The n-value is determined by calculating the slope of the log U versus log I plot in the region where the critical current (I c) is identified, or by using two I c values obtained under different criteria as specified in section 10.1.

The range of the criteria used to determine n shall be reported

NOTE The application of the electric field and resistivity criteria to determine the critical current is shown

The application of electric field and resistivity criteria is essential for determining the critical current in a U-I characteristic, where a linear region is observed at low current levels.

Figure 2 – U-I characteristic with a current transfer component

Identification du spécimen d'essai

The test specimen should ideally be identified by the following characteristics: a) the manufacturer's name; b) classification and/or symbol; c) lot number; d) raw materials and their chemical composition; e) shape and surface area of the wire cross-section, number of filaments, filament diameter, twist pitch, and copper/superconductor ratio.

Compte rendu des valeurs I c

Les valeurs I c doivent être mentionnées, de même que les critères correspondants.

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

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 I c

NOTE 2 Exception C.2.4 replaces this sentence for Cu/Cu-Ni/Nb-Ti specimens.

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°

When winding the specimen without a helical groove, it is essential to adhere to the specified pitch angle conditions This method may lead to insufficient support for the specimen and increased variability in the pitch angle.

The angle between the specimen axis (the section between the voltage taps) and the magnetic field must be equal to (90 ± 7)° This angle should be determined with a combined standard uncertainty not exceeding 1°.

The current contact and the measuring mandrel must form a rigid assembly to prevent any stress concentration on the specimen in the transition area between the mandrel and the current contact.

The winding tension and/or a low-temperature adhesive, such as vacuum silicone grease or Apiezon® 2 grease, should be used to securely attach the specimen to the measuring mandrel, minimizing its movement When using a low-temperature adhesive, it is essential to apply a minimal amount, and any excess adhesive must be removed from the outer surface of the specimen after it has been mounted.

L'adéquation du montage du spécimen doit être attestée par l'obtention de la reproductibilité du courant critique spécifié

Le spécimen ne doit pas être fixé au mandrin par soudage entre les contacts de courant

Le spécimen d'essai ne doit pas comporter de jointure ni d'épissure

La superficie de section S du spécimen doit être déterminée dans le plan transversal à l'axe du conducteur avec une incertitude type combinée ne dépassant pas 2,5 %

Le fil doit être enroulé sous forme d’une petite bobine de manière inductive L'enroulement du spécimen doit empêcher toute torsade supplémentaire du spécimen

For a wire with a rectangular cross-section, the specimen should be wound into a coil so that the applied magnetic field is parallel to the widest face of the specimen.

To ensure the specimen is properly positioned in the groove, a pulling force must be applied to the wire during winding, and this force should not cause a tensile deformation of the wire greater than 0.1% (refer to Appendix D).

La déformation en flexion maximale, provoquée lors du montage du spécimen, ne doit pas être supérieure à 3 %

Both ends of the wire must be welded to the current contact The minimum length of the welded portion of the current contact should be the largest of the following three values:

40 mm, 30 fois le diamètre du fil ou 30 fois l'épaisseur du fil

2) Apiezon ® est l’appellation commerciale d'un produit distribué par M&I Materials Ltd, UK (www.apiezon.com)

Cette information est donnée à l’intention des utilisateurs du présent document et ne signifie nullement que la

CEI approuve ou recommande l’emploi exclusif du produit ainsi désigné Des produits équivalents peuvent être utilisés s'il est démontré qu'ils conduisent aux mêmes résultats

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°

To prevent stress concentration at the junction of the mandrel and the current contact, it is essential to securely attach the current contact to the measurement mandrel.

Specimen bonding

Winding tension and/or a low temperature adhesive (such as silicone vacuum grease,

To minimize specimen motion during measurement, it is essential to use Apiezon® vacuum grease or epoxy to securely bond the specimen to the measurement mandrel When employing a low-temperature adhesive, ensure that a minimal amount is applied, and promptly remove any excess adhesive from the outer surface of the specimen after mounting.

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.

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 must be wound into a small coil inductively, ensuring that no additional twists are introduced into the specimen during the winding process.

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 properly secure the specimen in the groove, a tensile force must be applied to the wire during the winding process, ensuring that the tensile strain does not exceed 0.1%.

The maximum bending strain induced during the mounting of the specimen shall not exceed

Both ends of the wire must be securely attached to the current contact using solder The minimum length of the soldered section should be at least 40 mm, or 30 times the diameter or thickness of the wire, whichever is greater.

Apiezon ®, a product offered by M&I Materials Ltd in the UK, is mentioned here for user convenience and does not imply any endorsement For more information, visit www.apiezon.com.

IEC of the product named Equivalent products may be used if they can be shown to lead to the same results

Le nombre de tours du spécimen soudés sur chaque contact de courant ne doit pas dépasser trois

La distance la plus courte d'un contact de courant à une prise de tension doit être supérieure à 40 mm

Tension leads must be soldered to the specimen To minimize mutual inductance between the specimen's current and the area formed by the specimen and the tension leads, the untwisted section of the leads should be wrapped in the opposite direction around the specimen, as illustrated in Figure A.1.

The distance L along the specimen between the voltage taps must be measured with a combined standard uncertainty not exceeding 2.5% Additionally, the separation between the voltage taps should be greater than 50 mm.

For the purpose of the test, the specimen and the mandrel must be installed in a testing cryostat, which consists of a liquid helium Dewar vessel, a magnet, its support structure, and the specimen's support structure.

The specimen must be immersed in liquid helium during the data acquisition phase The temperature of the liquid helium bath should be measured before and after each determination of I c.

The specimen's current must remain low enough to prevent it from entering a normal state, unless a quench protection circuit or a resistive shunt is employed to safeguard the specimen.

Si on utilise la méthode à vitesse de balayage constante, le temps de variation entre zéro et

When employing the variation and maintenance method, the current sweep speed between the setpoint signals must be lower than the equivalent speed for a current change from zero to I c within 3 seconds.

Le champ magnétique continu doit être appliqué dans la direction de l'axe du mandrin La relation entre le champ magnétique et le courant de l'aimant doit être préalablement mesurée

The current of the magnet must be measured before each determination of the critical current, I_c The applied magnetic field should be parallel to the widest face and perpendicular to the axis of the cable for specimens with rectangular effective sections.

La direction du courant et le champ magnétique appliqué doivent produire une force de

Lorentz dirigée vers l'intérieur sur la longueur du spécimen, entre les prises de tension

Enregistrer la caractéristique U-I du spécimen d'essai dans les conditions de l'essai, ainsi que l'augmentation monotone du courant

A valid U-I characteristic must generate a current \$I_c\$ with a standard deviation not exceeding 0.5% under repeatable conditions, and the characteristic should remain stable over time with voltages equal to or below the critical current threshold.

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

To minimize mutual inductance between the specimen current and the area formed by the specimen and voltage taps, solder the voltage taps to the specimen and counterwind the untwisted section of the voltage taps back along the specimen, as illustrated 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

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 I c

To prevent damage to the specimen, it is essential to maintain a low specimen current, ensuring that it does not reach the normal state unless a quench protection circuit or resistive shunt is employed.

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 c in 3 s

The d.c magnetic field will be aligned with the mandrel axis, and the relationship between the magnetic field and the magnet current must be established in advance Prior to each measurement of the critical current (I c), the magnet current will be recorded Additionally, the applied magnetic field will be oriented parallel to the wide face and perpendicular to the wire axis of specimens that have 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

Record the U-I characteristic of the test specimen under test conditions and monotonically increasing current

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 I c , 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

U c with a combined standard uncertainty not to exceed 5 % and the corresponding current with a combined standard uncertainty not to exceed 0,5 %.

Temperature

A cryostat is essential for measuring critical current (Ic), with the specimen immersed in liquid helium The liquid helium bath must maintain a temperature close to its normal boiling point at the test site's atmospheric pressure It is assumed that the specimen's temperature matches that of the liquid The liquid temperature should be reported with a combined standard uncertainty not exceeding 0.01 K, using a pressure sensor or suitable temperature sensor for measurement.

The difference between the specimen temperature and the bath temperature shall be minimized

To convert the observed pressure in the cryostat to a temperature value, the phase diagram of helium is utilized It is essential that the pressure measurement has a sufficiently low uncertainty to achieve the desired accuracy in temperature measurement Additionally, for liquid helium depths exceeding 1 meter, a head correction may be required.

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

9.4 Structure de support du spécimen et du mandrin

The support structure must provide adequate support for the specimen and allow for its orientation relative to the magnetic field Proper specimen support is essential for obtaining additional measurements of the critical current with the repeatability outlined in Article 8.

La configuration d'essai du spécimen doit avoir la forme d'une bobine inductive

If a resistive shunt or a quench protection circuit is used in parallel with the specimen, the current in the shunt or circuit must be less than 0.2% of the total current at I c.

The critical current, I c, should be determined using either an electric field criterion, E c, or a resistivity criterion, ρ c, particularly when the total cross-section of the composite superconductor is preferred for resistivity estimation (refer to Figures 1 and 2).

In the case of an electric field criterion, two critical values of \$I_c\$ must be determined for thresholds of 10 μV/m and 100 μV/m Conversely, for resistivity criteria, two values of \$I_c\$ should be established for resistivity levels of \$10^{-14} \, \Omega m\$ and \$10^{-13} \, \Omega m\$.

When obtaining an accurate measurement of I c for a criterion of 100 μV/m proves challenging, it is necessary to replace E c with a criterion lower than 100 μV/m Alternatively, measurements using the resistivity criterion are recommended.

Le courant I c doit être déterminé comme étant le courant correspondant au point de la courbe

U-I ó la tension est égale à U c mesurée en fonction de la tension de base (voir Figures 1 et 2):

U c est le critère de tension, en microvolts;

L est la séparation entre prises de tension, en mètres;

E c est le critère de champ électrique, en microvolts/mètres ou, en utilisant un critère de résistivité:

U c , I c et ρc représentent respectivement la tension, le courant et la résistivité correspondants mesurés au point d'intersection d'une ligne droite et de la courbe U-I, comme le montre la Figure 1, et

S représente la superficie de section hors-tout exprimée en mètres carrés

Une ligne droite doit être tracée de la tension de base à la tension moyenne proche de 0,7 I c

The finite slope of this line may be attributed to current transfer For a valid determination of \$I_c\$, the slope of the line must be less than 0.3.

U c /I c quand U c et I c sont déterminés avec un critère de 10 μV/m ou 10 –14 Ωm

Specimen and mandrel support structure

The support structure must adequately hold the specimen and align it with the magnetic field It is considered sufficient if it enables further measurements of critical current with the repeatability specified in Clause 8.

The test configuration of the specimen shall be an inductive coil

Specimen protection

When utilizing a resistive shunt or quench protection circuit in parallel with the specimen, it is essential that the current flowing through the shunt or circuit remains below 0.2% of the total current at I_c.

Critical current criteria

The critical current, $ I_c $, can be assessed using either an electric field criterion, $ E_c $, or a resistivity criterion, $ \rho_c $ For estimating resistivity, it is recommended to consider the total cross-section of the composite superconductor, as illustrated in Figures 1 and 2.

In the case of an electric field criterion, two values of I c shall be determined at criteria of

10 μV/m and 100 μV/m In the other case, two values of I c shall be determined at resistivity criteria of 10 –14 Ωm and 10 –13 Ωm

When it is difficult to measure the I c properly at a criterion of 100 μV/m, an E c criterion less than 100 μV/m must be substituted Otherwise, the measurements using the resistivity criterion are recommended

The I c shall be determined as the current corresponding to the point on the U-I curve where the voltage is U c measured relative to the baseline voltage (see Figures 1 and 2):

U c is the voltage criterion, in microvolts;

L is the voltage tap separation, in metres;

E c is the electric field criterion, in microvolts/metre or, when using a resistivity criterion:

U c , I c 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 I c

A finite slope in the line may indicate current transfer To accurately determine the critical current (\$I_c\$), the slope must be less than \$0.3 \cdot \frac{U_c}{I_c}\$, with \$U_c\$ and \$I_c\$ assessed at a criterion of \$10 \, \mu V/m\$ or \$10^{-14} \, \Omega m\$.

10.2 Valeur n (calcul facultatif, se reporter à A.7.2)

The value $ n $ should be calculated as the slope of the log $ U $ versus log $ I $ plot in the region where $ I_c $ is determined, or it can be calculated using two values of $ I_c $ identified in section 10.1 with two different criteria.

L'étendue des critères utilisés pour déterminer n doit être mentionnée

NOTE La figure montre l'application du champ électrique et des critères de résistivité dans la détermination du courant critique

The figure illustrates the application of the electric field and resistivity criteria in determining the critical current on a U-I characteristic curve, featuring a current transfer component depicted as a linear region traversed by a low current.

Figure 2 – Caractéristique U-I avec une composante de transfert de courant

n-value (optional calculation, refer to A.7.2)

The n-value is determined by calculating the slope of the log U versus log I plot in the region where the critical current (I c) is identified, or by using two I c values obtained under different criteria as specified in section 10.1.

The range of the criteria used to determine n shall be reported

NOTE The application of the electric field and resistivity criteria to determine the critical current is shown

The application of electric field and resistivity criteria is essential for determining the critical current in a U-I characteristic, where a linear region is observed at low current levels.

Figure 2 – U-I characteristic with a current transfer component

The test specimen should ideally be identified by the following characteristics: a) the manufacturer's name; b) classification and/or symbol; c) lot number; d) raw materials and their chemical composition; e) shape and surface area of the wire cross-section, number of filaments, filament diameter, twist pitch, and copper/superconductor ratio.

Les valeurs I c doivent être mentionnées, de même que les critères correspondants

11.3 Compte rendu des conditions d'essai

The following testing conditions must be specified: a) the magnetic field strength and its uniformity; b) the testing temperature; c) the number of turns in the coil used for the test; d) the technique employed for winding the coil; e) the distance between the voltage taps and the total length of the specimen; f) the shortest distance from a current contact to a voltage tap; g) the shortest distance between current contacts; h) the length of the welds for the current contacts; i) the method of securing the specimen, including the identification of the fastening material; j) the material of the mandrel; k) the diameter of the mandrel; l) the depth, shape, pitch, and angle of the grooves.

Identification of test specimen

The test specimen should be identified by the manufacturer's name, classification or symbol, lot number, raw materials with their chemical composition, as well as the shape and cross-sectional area of the wire, including the number and diameter of filaments, twist pitch, and the copper to superconductor ratio.

Report of I c values

The I c values, along with their corresponding criteria, shall be reported.

Report of test conditions

The test conditions to be reported include the magnetic field and its uniformity, the test temperature, the number of turns in the tested coil, and the winding technique used Additionally, it is essential to document the length between voltage taps and the total specimen length, as well as the shortest distances from current contacts to voltage taps and between current contacts The soldered length of the current contacts, the specimen bonding method along with the identification of the bonding material, the mandrel material, mandrel diameter, and the depth, shape, pitch, and angle of grooves must also be specified.

Informations supplémentaires relatives à la norme

Un grand nombre de variables ayant un effet significatif sur la valeur mesurée du courant critique doivent être portées à l'attention de l'utilisateur La présente annexe informative en abordera plusieurs

The method outlined in this standard is not applicable to wires with a copper/superconductor ratio (i.e., the volume ratio of Cu/Nb-Ti) less than 1, as the observed voltage-current (U-I) characteristics may be unstable under low magnetic fields.

Les restrictions de la présente méthode d'essai ont pour but d'obtenir l'incertitude nécessaire dans la toute dernière phase de qualification des conducteurs longs

Cette norme exige que le spécimen soit éprouvé en immersion dans de l'hélium liquide proche du point d'ébullition de l'hélium liquide à la pression atmosphérique normale du site

Tests conducted in liquid helium at temperatures other than those near its normal boiling point, as well as tests in gas or under vacuum, are not included within the scope of this standard.

The critical current determined by this method is the maximum continuous electric current below which a superconductor can be regarded as having no resistance, at least for practical purposes, for a given temperature and magnetic field.

Généralement, la limite supérieure du champ magnétique d'essai (égale à 0,7 fois le champ magnétique critique le plus élevé) sera 8 T à une température proche de 4,2 K

La longueur totale minimale du spécimen est 210 mm, c'est-à-dire la somme des valeurs suivantes:

– longueur de soudure des contacts de courant (2 × 40 mm);

– distance entre les contacts de courant et les contacts de tension (2 × 40 mm);

– distance minimale séparant les prises de tension (50 mm)

In cases of series testing where adherence to specific restrictions is impractical, this standard can serve as a general guideline, allowing for an increase in uncertainty.

For series testing, a broader range of parameters is acceptable; however, in cross-comparisons and final functionality checks, restrictions are essential to mitigate the impact of usability on the desired target uncertainty.

Additional information relating to the standard

Several key variables significantly influence the measured value of critical current, and it is essential for users to be aware of these factors This informative annex will address some of these important considerations.

This standard is not suitable for wires with a copper/superconductor ratio (volume ratio of Cu/Nb-Ti) less than 1, as the voltage-current (U-I) characteristics may exhibit instability 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

The standard mandates that the specimen must be tested while submerged in liquid helium, which is close to its boiling point at the normal atmospheric pressure of the testing location.

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

The d.c critical current is the maximum direct electric current at which a superconductor can be considered resistance-less for practical applications, depending on the specific temperature and magnetic field conditions.

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)

For routine tests where strict adherence to specific restrictions is impractical, this standard serves as a set of general guidelines, acknowledging a potential 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

Measurements on short and straight specimens are acceptable practices in series measurements, provided that the cross-sectional area of the specimen is small compared to its length However, for the sake of simplicity, this specimen geometry is often overlooked.

It is likely that measurements taken on non-inductive (bifilar) wound specimens, combined with specimens fixed using epoxy resin, will yield an uncertainty similar to that sought by the current method.

Cependant, par souci de simplicité, cette géométrie du spécimen n'est pas prise en compte

In a bifilar specimen geometry, the Lorentz force acts outward from the measuring mandrel along part of the specimen's length, and the silicone vacuum grease or tension is insufficient to prevent the specimen from moving.

Measurements conducted on a non-ferromagnetic stainless steel mandrel, fixed by welding, are acceptable practices for series measurements In this scenario, estimating the current derived from the mandrel becomes challenging, especially if the welding is superconductive and the measurements are taken in low magnetic fields.

Tiêu đề	Critical Current Measurement of Nb-Ti Superconductors
Chuyên ngành	Superconductivity
Thể loại	Standard
Năm xuất bản	2006

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