METHOD FOR EVALUATING THE ELECTRICAL PROPERTIES OF A HTS SUPERCONDUCTOR

Abstract

A measurement current (i) is injected into an active part (4) of an HTS superconductor. The active part is cooled, but not reservoirs (1, 2) from/to which the superconductor is wound. Only a fraction of the active part is exposed to a magnetic field for testing the electrical properties of the superconductor. Buffer devices (20a, 20b) prevent current sharing from outside the active part. The measurement current is injected where the residual magnetic field is at least 3 times lower than the magnetic field for testing, and/or the local critical current at the current injection locations is at least three times higher than the critical current at the magnetic field for testing. The electrical properties, e.g. the critical current, are tested by determining an integral of a voltage drop (U) across the active part, e.g. between two voltage pick-up elements (15a, 15b), as a function of measurement time ().

Claims

1. A method for evaluating electrical properties of a high temperature superconductor(HTS), comprising: exposing the HTS superconductor to a cryogenic environment of a temperature T.sub.env, passing the HTS superconductor through a characterization zone while applying a magnetic field to the HTS superconductor in the characterization zone, wherein the characterization zone comprises a central region and two peripheral regions through which the HTS superconductor passes, and supplying the HTS superconductor with a measuring current (i) by two current exchange elements, wherein each of the current exchange elements contacts the HTS superconductor in a respective one of the peripheral regions, wherein the HTS superconductor is continuously translated from a first reservoir through the characterization zone to a second reservoir, wherein the HTS superconductor is exposed to the cryogenic environment only in a cryogenic zone, wherein the cryogenic zone includes the characterization zone and wherein the first and second reservoirs are located outside the cryogenic zone, wherein the HTS superconductor passes two buffer zones where two decoupling sections are formed in the HTS superconductor, with a first decoupling section established between the first reservoir and the characterization zone, and a second decoupling section established between the characterization zone and the second reservoir, wherein the magnetic field is selected such that a maximum magnetic flux density at the HTS superconductor in the central region is B.sub.centr, with B.sub.centr1.5 Tesla, and a maximum magnetic flux density at the HTS superconductor at the current exchange elements is B.sub.peri, such that, at the temperature T.sub.env of the HTS superconductor established by the cryogenic environment in the characterization zone, for a critical current I.sub.centr.sup.crit of the HTS superconductor in the central region and a critical current I.sub.peri.sup.crit of the HTS superconductor at the current exchange elements, the following applies: I.sub.peri.sup.crit3*I.sub.centr.sup.crit, and wherein in each buffer zone the HTS superconductor is treated such that the HTS superconductor becomes normally conducting within the decoupling section, or a local critical current I.sub.buffer.sup.crit of the HTS superconductor in the decoupling section is reached with I.sub.buffer.sup.crit1/50*I.sub.centrc.sup.crit.

2. A method according to claim 1, wherein the HTS superconductor is a HTS superconductor tape.

3. A method according to claim 1, wherein B.sub.centr and B.sub.peri are selected such that I.sub.peri.sup.crit5*I.sub.centr.sup.crit.

4. A method according to claim 3, wherein B.sub.centr and B.sub.peri are selected such that I.sub.peri.sup.crit20*I.sub.centr.sup.crit.

5. A method according to claim 1, wheein B.sub.centr and B.sub.peri are selected with B.sub.peri0.3*B.sub.centr.

6. A method according to claim 1, wheein B.sub.centr and B.sub.peri are selected with B.sub.peri0.05*B.sub.centr.

7. A method according to claim 1, wherein the first decoupling section of the HTS superconductor is established between a last guiding element of the HTS superconductor fed from the first reservoir and a first current exchange element of the current exchange elements, and the second decoupling section of the HTS superconductor is established between a second current exchange element of the current exchange elements and a first guiding element of the HTS superconductor fed to the second reservoir.

8. A method according to claim 1, wherein, in the buffer zones the HTS superconductor undergoes at least one of: a) an active heating to a temperature T.sub.buffer, with T.sub.buffer>T.sub.env, and b) exposure to a jamming magnetic field that suppresses the local critical current I.sub.buffer.sup.crit.

9. A method according to claim 1, the current exchange elements are positioned such that a local direction of the magnetic field at the current exchange elements is opposite in direction to the magnetic field in the central region at the HTS superconductor.

10. A method according to claim 1, wherein a voltage drop at least across the central region of the HTS superconductor is monitored using two voltage pick-up elements.

11. A method according to claim 10, wherein the voltage pick-up elements contact the HTS superconductor between the current exchange elements.

12. The method according to claim 10, wherein, for the voltage drop V.sub.drop, 0.5 VV.sub.drop2 V.

13. The method according to claim 12, wherein, for the voltage drop V.sub.drop, 1 VV.sub.drop<1 V.

14. A method according to claim 10, wherein the voltage pick-up elements are connected to electrical wires, and at least a part of at least one of the electrical wires is guided within the characterization zone together with the HTS superconductor through a cleavage of a magnetic field generation device.

15. A method according to claim 10, wherein a probing voltage drop is determined as a function of time () or location (x) on the HTS superconductor, wherein for determining the probing voltage drop, the voltage drop is repeatedly read out at an identical respective magnetic field strength and at an identical respective measurement current value, or the voltage drop is integrated during repeated congeneric cycles of sweeps of the measuring current (i) or the magnetic field until an identical measurement current value or an identical magnetic field strength has been reached in each case, wherein a first derivative with respect to the time () or the location (x) of the probing voltage drop is determined, and wherein a transport of a defect of the HTS superconductor through the characterization zone during the continuous translation is established by identifying a maximum of the first derivative of the probing voltage drop followed by a minimum of the first derivative of the probing voltage drop.

16. A method according to claim 1, wherein the evaluation of the electrical properties of the HTS superconductor further comprises: a) applying a constant magnetic field in the characterization zone, applying a sweep of the measuring current (i), and monitoring a voltage drop at least across the central region along the HTS superconductor, or b) applying a constant magnetic field in the characterization zone, and regulating and monitoring the measuring current (i) such that a constant voltage drop at least across the central region along the HTS superconductor is obtained, or c) applying a sweep of the magnetic field in the characterization zone, applying a constant measuring current (i), and monitoring a voltage drop at least across the central region along the HTS superconductor.

17. A method according to claim 16, wherein, for a cycle duration CD of at least one of the sweep of the measuring current (i) and the sweep of the magnetic field, or for a cycle duration CD of regulating the measuring current (i) to re-establish the constant voltage drop: 0.5 msCD100 ms.

18. A method according to claim 1, wherein the evaluation of the electrical properties includes evaluating the critical current I.sub.centr.sup.crit of the HTS superconductor at the temperature T.sub.env established by the cryogenic environment and at the maximum magnetic flux density B.sub.centr.

19. A method according to claim 1, wherein B.sub.centr is selected with B.sub.centr6 Tesla, and T.sub.env is selected with T.sub.env24 K.

20. A method according to claim 19, wherein B.sub.centr4 Tesla and T.sub.env77 K.

21. A method according to claim 18, wherein the evaluation of the electrical properties further comprises estimating a high field low temperature critical current I.sub.HFLT.sup.crit for the HTS superconductor at a magnetic flux density B.sub.high and at a temperature T.sub.low based on I.sub.centr.sup.crit, with B.sub.centr6 Tesla, and B.sub.high3*B.sub.centr, and further with T.sub.env24 K, and T.sub.low<4.2 K.

22. A method according to claim 21, wherein the evaluation of the electrical properties further comprises estimating a high field low temperature critical current I.sub.HFLT.sup.crit for the HTS superconductor at a magnetic flux density B.sub.high and at a temperature T.sub.low based on I.sub.centr.sup.crit, with B.sub.high10 Tesla.

23. A method according to claim 1, further comprising shaping the magnetic field using a ferromagnetic screening.

24. An apparatus for evaluating the electrical properties of a HTS superconductor tape, and configured to perform the method claimed in claim 1.

25. A method for measuring electrical properties of a high temperature superconductor (HTS), comprising: continuously translating a HTS superconductor from a first reservoir through a first buffer zone, a characterization zone and a second buffer zone to a second reservoir, while translating the HTS superconductor, exposing the HTS superconductor to a cryogenic environment of a temperature Tenv only in a cryogenic zone, wherein the cryogenic zone includes the characterization zone, and the characterization zone comprises a central region and two peripheral regions through which the HTS superconductor passes, and wherein the first reservoir and the second reservoir are located outside the cryogenic zone, while translating the HTS superconductor, supplying the HTS superconductor with a measuring current (i) with two current exchange elements, wherein each of the current exchange elements contacts the HTS superconductor in a respective one of the peripheral regions, and wherein respective decoupling sections are formed in the HTS superconductor when the HTS superconductor passes each of the two buffer zones, with a first of the decoupling sections being established between the first reservoir and the characterization zone, and a second of the decoupling sections being established between the characterization zone and the second reservoir, and while translating the HTS superconductor, applying a magnetic field to the HTS superconductor in the characterization zone, wherein the magnetic field is set such that a maximum magnetic flux density at the HTS superconductor in the central region is B.sub.centr, with B.sub.centr1.5 Tesla, and a maximum magnetic flux density at the HTS superconductor at the current exchange elements is B.sub.peri, and such that, at the temperature T.sub.env of the HTS superconductor established by the cryogenic environment in the characterization zone, for a critical current I.sub.centr.sup.crit of the HTS superconductor in the central region and a critical current I.sub.peri.sup.crit of the HTS superconductor at the current exchange elements: I.sub.peri.sup.crit3*I.sub.centr.sup.crit, and while translating the HTS superconductor, treating the HTS superconductor in each of the buffer zones such that either: the HTS superconductor becomes normally conducting within the decoupling section, or a local critical current I.sub.buffer.sup.crit of the HTS superconductor in the decoupling section is reached with I.sub.buffer.sup.crit1/50*I.sub.centr.sup.crit.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0086] The invention is shown in the drawing.

[0087] FIG. 1 shows a schematic cross-section of an exemplary embodiment of an inventive apparatus for conducting a variant of the inventive method;

[0088] FIG. 2A illustrates a characterization zone, used in a variant of the inventive method, with an electrical line of a voltage pick-up element running through the magnetic field generation device;

[0089] FIG. 2B shows a schematic diagram for a magnetic field distribution in the characterization zone, for use with the inventive method;

[0090] FIG. 3 illustrates a characterization zone, used in another variant of the inventive method, with two electrical lines of voltage pick-up elements running through a respective half of the magnetic field generation device;

[0091] FIG. 4 shows a schematic diagram of the course of a measurement for use in the inventive method, of i-sweep type;

[0092] FIG. 5 shows a schematic diagram of the course of a measurement for use in the inventive method, of B-sweep type;

[0093] FIG. 6 shows a schematic diagram of the course of a measurement for use in the inventive method, of constant voltage type;

[0094] FIG. 7 shows a schematic diagram of the first derivative of the probing voltage drop d.Math.(t)/d as a function of time t when a defect passes the characterization zone.

DETAILED DESCRIPTION

[0095] FIG. 1 shows in a schematic cross-section an inventive apparatus 100 for carrying out the inventive method by way of example.

[0096] The apparatus 100 comprises a first reservoir 1 and a second reservoir 2 for a tape type HTS superconductor 3. The HTS superconductor 3 is guided from the first reservoir 1 over a (last first side) guiding element 9 of deflection pulley type through a characterization zone 4 where electric properties of the HTS superconductor 3 are measured, further over a (first second side) guiding element 10 of deflection pulley type and onto the second reservoir 2. The first and second reservoir 1, 2 are designed here with reels, wherein the second reservoir 2 resp. its reel comprises a reel drive 5, here with an electric motor. The first and second reservoir 1, 2 and guiding elements 9, 10 are arranged on a respective support or supports (not shown in detail here, for simplicity), preferably on a common outer base frame. The guiding element 10 comprises a tension sensor (not shown in detail).

[0097] The characterization zone 4 is located within a cryogenic zone 6, which is formed by the interior of a cryostat, here comprising a cryovessel 7 and a thermally insulating cover 8. The interior of the cryostat is here at least partially filled with liquid nitrogen (LN2) (not shown here for simplicity). The cryostat is connected to a cryocooler (not shown here for simplicity) to cool its interior and to save LN2 here; alternatively, LN2 can be added more often.

[0098] Accordingly, the HTS superconductor 3 is locally exposed to a cryogenic environment 11, namely when passing the cryogenic zone 6, but for the most part, the HTS superconductor 3 is exposed to ambient conditions (room temperature of about 20 C.), in particular at the first and second reservoirs 1, 2. The cryogenic environment 11 has a temperature Tenv of here 77 K, which is assumed by the HTS superconductor 3 within the cryogenic zone 6.

[0099] On top of the cover 8 of the cryostat, there is arranged a base plate 29 with a number of feedthroughs for accessing the equipment inside the cryostat. In particular, there are a number of pendants 28 which suspend an inner base frame 14 (lower basis) located in the interior of the cryostat. The pendants 28 are typically made of thin stainless steel which provides reduction of cryo loss. Further, there is a LN2 nozzle for re-filling of the cryostat with LN2 and a level sensor 26 extending through the cover 8 of the cryostat.

[0100] The inner base frame 14 carries a magnetic field generation device 17, here constructed as a normally conducting electromagnet, having a cleavage 18 through which the HTS superconductor 3 passes. Electrical lines (current leads) 19 provide the magnetic field generation device 17 with operating current; the electric lines 19 pass through the cover 8. The magnetic field generation device 17 generates a magnetic field, both in a central region 23 (intended for the measurement) and in peripheral regions 24a, 24b (as an unintended stray field) in the characterization zone 4. The magnetic field distribution is set with the help of a ferromagnetic screening 25, which encompasses the magnetic field generation device 17 here. The generated magnetic field is strongest at a magnetic center within the central region 23 (compare also FIG. 2A. There the HTS superconductor 3 is exposed to the maximum flux density B.sub.centr, here with B.sub.centr=2.5 Tesla.

[0101] Further, the inner base frame 14 carries a first current exchange element 12a and a second current exchange element 12b, which are designed as deflection rollers here. The current exchange elements 12a, 12b are connected to electric lines (current leads) or wires 13a, 13b which extend straight upwards, passing through the cover 8. The current exchange elements 12a, 12b contact the HTS superconductor 3 in the peripheral regions 24a, 24b for applying a measurement current. Here, the maximum magnetic flux density is B.sub.peri. In the example illustrated, B.sub.peri=0.25 Tesla.

[0102] Further, the inner base frame 14 carries a first voltage pick-up element 15a and a second voltage pick-up element 15b, which are also designed as deflection rollers here. The voltage pick-up elements 15a, 15b are connected to electrical lines (voltage leads) or wires 16a, 16b. In the example shown, electrical line 16a is guided in parallel to the HTS superconductor 3 outside magnetic field generation device 17, and above the second voltage pick-up element 15b, the electrical lines 16a, 16b are twisted and pass through the cover 8. The voltage pick-up elements 15a, 15b are located, with respect to the motion of the tape, between the current exchange elements 12a, 12b.

[0103] Introduction of the HTS superconductor 3 into the characterization zone 4 (and here also the cryogenic zone 6) is done via buffer devices 20a, 20b. The buffer devices 20a, 20b are here designed with electrical tape heaters, which establish buffer zones 21a, 21b inside of which the HTS superconductor 3 is heated, here to a temperature Tbuffer above ambient temperature, in the example shown to a temperature of about 50 C. The buffer zones 21a, 21b cause respective decoupling sections 22a, 22b of the HTS superconductor 3, where the HTS superconductor is safely normally conducting (and not superconducting), and therefore electric current transfer is minimized. The decoupling sections 22a, 22b here have lengths (in tape direction) of about 50 cm.

[0104] In the course of evaluating the electrical properties of the HTS superconductor 3, the HTS superconductor 3 is exposed to an electrical measurement current via the current exchange elements 12a, 12b and exposed to a magnetic field up to Bcentr in the central region 23. The voltage drop V.sub.drop across the central region 23 is measured via the voltage pick-up elements 15a, 15b. Thanks to the low value of B.sub.peri as compared to B.sub.centr, the (smallest) local critical current I.sub.centr.sup.crit in the central region 23 is here more than 5 times smaller than the (highest) local critical current I.sub.peri.sup.crit in the peripheral regions 24a, 24b, i.e. I.sub.centr.sup.crit5*I.sub.peri.sup.crit. The measurement current, the voltage drop and the magnetic field generated by the magnetic field generation device 17 and here also the LN2 level are controlled and/or monitored via an electronic controller device 27, which is shown as a single unit here, but which may also comprise a number of subunits such as magnet current control box, a measurement current distribution box, a voltage signal box and a LN2 distribution box.

[0105] FIG. 2A illustrates another example of an inventive apparatus 100 for carrying out the inventive method. For simplicity, only the interior of the cryostat is shown, and only the major differences with respect to FIG. 1 are explained.

[0106] In the example shown, the HTS superconductor 3 is guided by current exchange elements 12a, 12b and voltage pick-up elements 15a, 15b through the cleavage 18 of the magnetic field generation device 17. In addition, also the electric line 16a, which is connected to the first voltage pick-up element 15a, is guided through the cleavage 18, in close vicinity and parallel to the HTS superconductor 3. On the right side of the magnetic field generation device 17, the electric line 16a is guided to the top, in close vicinity of and in parallel with (or alternatively twisted with) the electric line 16b, which is connected to the second voltage pick-up element 15b. With this arrangement, antenna effects of the electrical lines 16a, 16b are minimized.

[0107] Further, the field lines 30 of a typical magnetic field generated by the magnetic field generation device 17 are shown. Field lines 30 in the central region 23 have a high density, in particular near the magnetic center 31, where the magnetic field strength is highest (and B.sub.centr acts on the HTS superconductor 3, typically in a direction perpendicular to the plane/flat side of the tape). In contrast, in the peripheral regions 24a, 24b, where the current exchange elements 12a, 12b contact the HTS superconductor 3, the field lines 30 have a relatively low density. The maximum magnetic field acting on the HTS superconductor there (with corresponding B.sub.peri acting on the HTS superconductor 3 at the current exchange elements 12a, 12b, typically in a direction having some inclination to the plane/flat side of the tape) is far below B.sub.centr in terms of absolute value.

[0108] In the example shown, the magnetic field strength simply falls off monotonically away from the magnetic center 31. However, in practice, the magnetic field often changes its sign several times when retreating from the magnetic center 31. Such a behavior may be promoted by appropriate magnet design or ferromagnetic screening design. In this case, it is advantageous to place the current exchange elements 12a, 12b or the respective peripheral regions 24a, 24b in areas where the sign of the magnetic field is opposite to the sign at the magnetic center 31. In this way, a higher magnetic field may be applied to the HTS superconductor 3 for measurement purposes, while at the same time keeping the critical currents I.sub.peri.sup.crit in the peripheral regions 24a, 24b high. FIG. 2B illustrates such a magnetic field (compare axis to the top, illustrating the magnetic flux density B, as a function of location {circumflex over (x)}), and the peripheral regions 24a, 24b and central region 23. The magnetic center is here at {circumflex over (x)}=0, with direction {circumflex over (x)} extending along the cleavage of the magnetic field generation device, along with the HTS superconductor.

[0109] FIG. 3 illustrates an alternative design of an inventive apparatus 100 for carrying out the inventive method, similar to the apparatus shown in FIG. 2A. Only the major differences are explained.

[0110] In the embodiment shown, the electrical lines 16a, 16b connected to the voltage pick-up elements 15a, 15b both are guided through the cleavage 18 of the magnetic field generation device 17, but only to its center. At its center, the electrical lines 16a, 16b are guided to the top in close vicinity and in parallel to each other (or alternatively twisted) through the magnetic field generation device 17. This design has a higher symmetry as compared to the design of FIG. 2A, but requires a feedthrough of lines 16a, 16b through the upper part of the magnetic field generation device (magnet).

[0111] FIG. 4 illustrates a procedure for measuring the electrical properties of a HTS superconductor as a function of time t (illustrated to the right), in an i-sweep variant. The measurement may be performed, for example, on an apparatus as shown in FIG. 1. Note that the time t correlates with the location x via the (continuous) translation speed v of the HTS superconductor, with x=v*.

[0112] In this setup, the measurement current i (compare full line) applied via the current exchange contacts is varied in a saw-tooth like manner as a function of time . Over a period (cycle duration) of here 10 ms, the measurement current i() is increased linearly from zero to about 3 A here and back to zero. The magnetic field is kept constant. The voltage U between the voltage pick-up elements is measured as a function of time, see voltage U() (compare dashed line). The example illustrates here three measurement cycles (see indices j=1, 2, 3).

[0113] For low measurement currents i, the HTS superconductor is practically completely superconducting, and the voltage drop-off is practically zero. However, when the measurement current i approaches the critical current (for the given temperature T.sub.env and B.sub.centr), the voltage U increases in an approximately exponential way. Note that the critical current varies locally due to variations in the structure of the HTS superconductor.

[0114] In the example shown, it is assumed that the superconductor has reached its critical current when the voltage U has reached a critical level U.sub.cr. For each cycle j=1, 2, 3, the time .sub.j is indicated at which U()=U.sub.cr. At this point of time, the momentary measurement current I.sub.cj is determined, which is considered as the local critical current (at the magnetic center in the central region of the HTS superconductor at B.sub.centr and at T.sub.env applied during the measurement). The corresponding location along the tape length is determined as x.sub.j=v*.sub.j. In this way, a table of critical currents I.sub.cj as a function of location x.sub.j can be obtained.

[0115] Alternatively, beginning from the start of each cycle, an integral U()d can be determined. The point of time .sub.j in each cycle j when the integral value reaches a critical value CV is determined, i.e. .sub.j is determined for which .sub.0.sup.jU()d=CV, with .sub.0 being the start of the respective cycle. For illustration, the integral area of .sub.0.sup.r1U()d is illustrated with a hatching. When the area of the hatching has reached CV, the point of time .sub.j has been reached, and the measurement current i(.sub.j) at this point of time can be considered as the critical current I.sub.cj at the respective location x.sub.j=v*.sub.j. Determining .sub.j via the integral U()d is somewhat more complex as compared to a simple limit value U.sub.cr, but thus a higher reliability in determining L.sub.cj resp. I.sub.cent.sup.crit(x) can be obtained.

[0116] Further, a probing voltage drop .Math.(t) may be determined, for example by reading out the voltage drop U() in each cycle j=1, 2, 3 at a fixed after the beginning .sub.0 of said cycle, or alternatively by determining in each cycle the integral .sub.0.sup.0+U()d, i.e. from the beginning .sub.0 of the cycle until has elapsed. The first derivative d.Math.()/d of the probing voltage drop .Math.() may be used for identifying defects in the HTS superconductor passing the characterization zone. In this sense, by smooth and continuous translation of the tape, the time t corresponds to a tape coordinate x in the longitudinal direction. On the other hand, the voltage response U is formed as an integral of elementary voltage drops occurring between voltage pick-up elements as U=x.sup.1.sub.x0.sup.x0+xU(x, B)dx, with x: (longitudinal) tape coordinate/location on the tape, and x.sub.0: integral start position (position of first voltage pick-up element at measurement time), and x: tape length between the voltage pick-up elements. Note that U is here a function of x.sub.0, which in turn is a function of time. The latter integral allows to take into account the entire voltage drop including minor drops occurring in the areas with reduced flux density. In this way, improved precision of characterization may be achieved.

[0117] FIG. 7 shows a typical first derivative d.Math.()/d as a function of time during passing of a defect. When the defect enters the characterization zone, a maximum 71 can be observed, and when the defect leaves the characterization zone, a minimum 72 can be observed. Note that the time correlates with the location x on the HTS superconductor via the translation speed v with x=v*.

[0118] It should be noted that the probing voltage drop .Math.(r) can analogously be investigated in the B-sweep variant discussed below.

[0119] FIG. 5 illustrates an alternative procedure for measuring the electrical properties of a HTS superconductor as a function of time t (illustrated to the right), in a B-sweep variant. The measurement may be performed, for example, on an apparatus as shown in FIG. 1. Note that the time t correlates with the location x via the (continuous) translation speed v of the HTS superconductor, with x=v*.

[0120] Here, the magnetic field applied via the magnetic field generation device is varied in a saw-tooth like manner as a function of time . Over a period (cycle duration) of here 10 ms, the magnetic flux density B() (taken at the magnetic center, so B corresponds to B.sub.centr) is increased linearly from zero to about 3 Tesla and back to zero here (compare full line). The measurement current is kept constant. The voltage U between the voltage pick-up elements is measured as a function of time, see voltage U() (compare dashed line). The example illustrates here three measurement cycles (see indices j=1, 2, 3).

[0121] For low magnetic flux densities B, the HTS superconductor is practically completely superconducting, and the voltage drop-off U is practically zero. However, when the magnetic flux density B approaches the critical density (for the given temperature T.sub.env and measurement current), the voltage U increases in an approximately exponential way. Note that the critical magnetic flux density varies locally due to variations in the structure of the HTS superconductor.

[0122] In the example shown, it is assumed that the superconductor has reached its critical magnetic flux density when the voltage U has reached a critical level Ucr. For each cycle j=1, 2, 3, the time .sub.j is indicated at which U()=U.sub.cr. At this point of time, the momentary magnetic flux density B.sub.cj is determined, which is considered as the local critical magnetic flux density (at the given measurement current and T.sub.env applied during the measurement). The corresponding location along the tape length is determined as x.sub.j=v*.sub.j. In this way, a table of critical magnetic flux densities B.sub.cj as a function of location x.sub.j can be obtained. If desired, the critical magnetic flux density may be converted into a corresponding critical current density by appropriate predefined functions.

[0123] Alternatively, beginning from the start of each cycle, an integral U()d can be determined. The point of time t in each cycle j when the integral value reaches a critical value CV is determined, i.e. .sub.j is determined for which .sub.0.sup.jU()d=CV, with .sub.0 being the start of the respective cycle. For illustration, the integral area of .sub.0U()d is illustrated with a hatching. When the area of the hatching has reached CV, the point of time t has been reached, and the magnetic flux density B(.sub.j) at this point of time can be considered as the critical magnetic flux density B.sub.cj at the respective location x.sub.j=v*.sub.j. Determining .sub.j via the integral U()d is somewhat more complex as compared to a simple limit value U.sub.cr, but thus a higher reliability in determining B.sub.cj resp. I.sub.cent.sup.crit(x) can be obtained.

[0124] It should be noted that I.sub.centr.sup.crit(x) is typically transformed into a critical current I.sub.HFLT.sup.crit present at a magnetic flux density B.sub.high, which is typically about 10 Tesla or more, i.e. much larger than B.sub.centr, and at a temperature T.sub.low, which is typically at 4.2 K or below, i.e. much lower than T.sub.env. For this purpose, predefined translation functions may be used, for example applying a lift factor that expresses a ratio of critical currents measured at different fields and temperatures. This procedure is simplified via a reliable correlation of I.sub.c values at B>5T, 4.2 K, where the correlation follows so called alpha-law, i.e. I.sub.cB.sup.alpha where alpha is a known constant for wide field range.

[0125] However, for identifying a low quality of or a local defect in a HTS superconductor of known type, a comparison of I.sub.centr.sup.crit(x) or B.sub.cj(x.sub.j) with previously measured HTS superconductors (of verified good quality) is in general sufficient.

[0126] FIG. 6 illustrates by way of example another procedure for determining the electrical properties of a HTS superconductor according to the invention. The procedure may be performed, in particular, on an inventive apparatus as illustrated in FIG. 1.

[0127] In this example, the measurement current i is measured as a function of time t during continuous translation of the HTS superconductor. The magnetic flux density is fixed here, and the measurement current at the current exchange elements is controlled such that a voltage drop at the voltage pick-up elements, i.e. across the central region, is kept constant at a predefined value U.sub.target. The control algorithm includes increasing the measurement current i if the momentary voltage U is below U.sub.target, and reducing the measurement current i if the momentary voltage U is above U.sub.target; typically a PD-control (proportional-differential control) is applied. Here, the time for determining the momentary voltage U, comparing it with U.sub.target, deriving an adapted measurement current i such that U will be at U.sub.target again and setting this measurement current i at the current exchange elements defines the cycle duration.

[0128] When the HTS superconductor shows no defects in the section of the HTS superconductor under investigation, the measurement current i typically stays practically constant at a high level 60. Preferably, said high level is close to the critical current I.sub.centr.sup.crit for the given magnetic field strength and temperature T.sub.env. However, if a defect (with low or no superconducting current carrying capacity) exists in this section, the measurement current i drops to a significantly lower level 61.

[0129] In summary, the present invention proposes to evaluate the electrical properties, in particular the critical current, of a HTS superconductor (3), in particular of coated conductor tape type, wherein a measurement current (i) is injected into an active part/characterization zone (4) of the HTS superconductor, wherein the active part is cooled, but not reservoirs (1, 2) of the HTS superconductor from and to which the HTS superconductor is wound continuously, and exposing only a fraction of the active part to a magnetic field for testing the electrical properties. The magnetic field for testing has 1.5 T or more, in particular 2 T or more, and typically is oriented perpendicular to a tape flat side. Protection, e.g. in the form of buffer devices (20a, 20b), is provided against current sharing with respect to outside the active part. Injection of the measurement current is done where the residual magnetic field is at least 3 times lower as compared to the magnetic field for testing and/or the local critical current at the current injection locations is at least three times higher as compared to the critical current at the magnetic field for testing. Preferably, current injection is done where the magnetic field has an opposite sign as compared to the magnetic field of testing. Evaluation of the electrical properties, in particular the evaluation of the critical current, may be done by determining an integral of a voltage drop (U) across the active part or a fraction of the active part, e.g. between two voltage pick-up elements (15a, 15b), as a function of measurement time (t), in particular comparing the integral value to a critical value (CV) and determining the corresponding time when the critical value was reached. The inventive method is less susceptible to measurement errors. This may be very well seen in the case when some local defect that reduces local critical current enters the characterization zone. When this defect reaches the high field zone (central region), it will cause a constant addition to the voltage drop until the defect exits the high field zone. Additionally this defect will contribute to a measured voltage drop when it passes the through the transient zones (peripheral regions) with field gradient. At an evaluation stage these effects may be taken into account using a first time (or coordinate) derivative from the voltage drop or the integral voltage drop. In this regard, it should be noted that the voltage drop or the integral voltage drop should be examined at identical boundary conditions, such as at identical magnetic field strength and measurement current strength, e.g. at identical relative points during measurement cycles, in each case. For example, there is a stepwise increase of voltage drop with a positive peak of first derivative when a local defect enters the characterization zone. Accordingly, there is a stepwise reduction of voltage drop with a negative peak of the first derivative when the defect exits the characterization zone with high magnetic field. These relationships significantly improve the linear resolution, accuracy and stability of the characterization procedure.

LIST OF REFERENCE SIGNS

[0130] 1 first reservoir [0131] 2 second reservoir [0132] 3 HTS superconductor [0133] 4 characterization zone [0134] 5 reel drive [0135] 6 cryogenic zone [0136] 7 cryovessel (cryostat) [0137] 8 cover (cryostat) [0138] 9 guiding element (last on first reservoir side) [0139] 10 guiding element (first on second reservoir side) [0140] 11 cryogenic environment [0141] 12a first current exchange element [0142] 12b second current exchange element [0143] 13a electrical line/wire (current lead) [0144] 13b electrical line/wire (current lead) [0145] 14 inner base frame [0146] 15a first voltage pick-up element [0147] 15b second voltage pick-up element [0148] 16a electrical line/wire (voltage lead) [0149] 16b electrical line/wire (voltage lead) [0150] 17 magnetic field generation device (magnet) [0151] 18 cleavage [0152] 19 current lead (current lead of magnet) [0153] 20a buffer device [0154] 20b buffer device [0155] 21a first buffer zone [0156] 21b second buffer zone [0157] 22a first decoupling section [0158] 22b second decoupling section [0159] 23 central region [0160] 24a peripheral region [0161] 24b peripheral region [0162] 25 ferromagnetic shielding [0163] 26 LN2 nozzle and level sensor [0164] 27 controller device [0165] 28 pendant [0166] 29 base plate [0167] 30 magnetic field lines [0168] 31 magnetic center [0169] 60 high level [0170] 61 lower level [0171] 71 maximum [0172] 72 minimum [0173] 100 apparatus [0174] B magnetic flux density [0175] B.sub.cj determined critical magnetic flux density [0176] i measurement current [0177] I.sub.cj determined critical current in cycle j [0178] j cycle index [0179] time [0180] U voltage [0181] U.sub.cr critical voltage [0182] v translation speed [0183] x location on HTS superconductor [0184] {circumflex over (x)} location with respect to magnetic center