In this paper, we present the results of the experimental study of the current-carrying capacity for two high-temperature superconducting samples made of either one 12-mm bare REBCO tape or a soldered stack of five such tapes on current growth rates of up to 350 kA/s. For samples in the form of rings with junctions, the transformer method of current injection was used. Experiments at 77 K showed that the currents of thermal equilibrium with the coolant achieved in the samples very weakly depend on the rate of change in the current and are close to critical ones. At 4.2 K, single mechanical defects of the superconducting layer appeared in the samples with a significant decrease in the local critical current. Current charging at different rates were accompanied by local transitions to the normal state caused by Joule heating of the silver coating when the current bypassed a defect with a resistance of ~1 μΩ at 4.2 K. Repeated measurements at 77 K showed that the resistance of these defects increased to 4 μΩ. This resistance determined the achievable currents at low induction voltages in the samples (low rates of current input). However, when passing through 200 kA/s, the currents reached during repeated measurements at 77 K practically equaled the currents of the first measurements at 77 K before the appearance of defects in the samples in liquid helium.

The current-carrying capacity of second-generation commercial high-temperature superconducting REBCO tapes with a width of 12 mm without a stabilizing copper coating (high-temperature superconductor (HTSC) tapes) in liquid helium has been experimentally investigated at current injection rates up to 350 kA/s. In the experiments, tape samples in the form of rings with junctions are placed in the working volume of a superconducting magnet at 4.2 K. The current in the samples is induced by the transformer method. With an increase in the rate of current injection in the ring-shaped samples, single mechanical defects of the superconducting layer appear in the region of junctions, which causes premature transitions of the samples to the normal state. As a result, the maximum attained currents in the ring samples turned out to be up to 50% lower than the critical currents of short HTSC tape samples at 4.2 K. In conclusion, the analysis of the experimental results is given. Possible explanations for the observed effect are given. In addition, the thermomagnetic stability of two single pancake coils comprised of five isolated turns wound from REBCO tapes supplied from different manufacturers is investigated. The jumps in the magnetic flux are not detected up to an external magnetic field variation rate of 1.7 T/s.

Экспериментально исследована токонесущая способность коммерческих высокотемпературных сверхпроводящих REBCO-лент второго поколения шириной 12 mm без стабилизирующего медного покрытия (ВТСП лент) в жидком гелии при скоростях ввода тока до 350 kA/s. В экспериментах образцы лент в виде колец со спаями размещались в рабочем объеме сверхпроводящего магнита при 4.2 K. Ток в образцах индуцировался трансформаторным способом. В процессе повышения скорости ввода тока в образцах-кольцах было обнаружено появление одиночных механических дефектов сверхпроводящего слоя в области спаев, вызывающее преждевременные переходы образцов в нормальное состояние. В результате максимально достигнутые токи в образцах-кольцах оказались до 50% ниже критических токов коротких образцов ВТСП лент при 4.2 K. В заключении приводится анализ результатов эксперимента. Даны возможные объяснения наблюдаемого эффекта. Кроме этого, исследована термомагнитная стабильность двух одинарных галет из пяти изолированных витков, намотанных из REBCO-лент разных производителей. Скачки магнитного потока не были обнаружены вплоть до скоростей изменения внешнего магнитного поля 1.7 T/s. Ключевые слова: сверхпроводимость, высокотемпературные сверхпроводники, ВТСП ленты второго поколения, REBCO-ленты, критический ток, термомагнитная стабильность, скачок магнитного потока.

Here we present the comparative experimental study of the stability of the superconducting state in 4 mm YBCO tapes with copper lamination against local heat disturbances at 77 K. The samples are either directly cooled by immersing a bare YBCO tape into a liquid nitrogen pool or operate in nearly-adiabatic conditions when the tape is covered by a 0.6 mm layer of Kapton insulation. Main quench characteristics, i.e. minimum quench energies (MQEs) and normal zone propagation (NZP) velocities for both samples are measured and compared Minimum NZP currents are determined by a low ohmic resistor technique eligible for obtaining V - I curves with a negative differential resistance. The region of transport currents satisfying the stationary stability criterion is found for the different cooling conditions. Finally, we use the critical temperature margin as a universal scaling parameter to compare the MQEs obtained in this work for YBCO tapes at 77 K with those taken from literature for low-temperature superconductors in vacuum at 4.2 K, as well as for MgB2 wires cooled with a cryocooler down to 20 K.

In this paper we present the results of an experimental investigation of single 12 mm REBCO tapes as well as soft-soldered 5-folded REBCO stacks at current ramp rates up to 320 kA/s. Experimental ring-samples made of one or five HTS tapes with joints were manufactured and charged in external magnetic field. The experiments performed at 77 K showed that the current caring capacity of the samples is very weakly affected by transport current ramp rate. No transitions to the normal state were observed in liquid nitrogen at external magnetic field ramp rates up to 8 T/s. The maximum transport currents achieved by the samples were limited by the thermal equilibrium with the coolant. In contrast, at 4.2 K both single and stacked samples demonstrated preliminary quenches caused by the origination of local defects in the superconducting layers of the HTS tapes with resistances of about 1 mu Ohm at 4.2 K. Reexamination of the damaged samples at 77 K showed that at ramp rates higher than 200 kA/s the currents of the thermal equilibrium with the coolant appeared to be the same before and after the defects appearance.

The stability against magnetic flux jumps has experimentally been studied in the external magnetic field for three samples from NbTi composite superconductors, one monofilamentary and two multifilamentary. A comparison between the experiment and theory of thermomagnetic stability of composite superconductors has been carried out. We have determined threshold values of the rates of the external magnetic fields, starting from which heat capacity and conductivity of the normal composite matrix become determining stabilizing factors. For the first time, the increasing dependence of field of first magnetic flux jump on the rate of the rise in the external magnetic field has been experimentally registered in the superconducting wire for MRI. The reason for this effect is the shunting effect of a high pure copper matrix and the low volume fraction of a superconductor in the composite (~10%).

The thermal stability of NbTi Rutherford-type cables was improved with cold spray technology, namely, by depositing high-heat-capacity coatings onto the surface of the superconductors. Two short samples of identical NbTi Rutherford-type cables with different metallic/ceramic coatings were prepared and tested. The spray particles were deposited onto the cables made of eight monofilamentary NbTi/Cu wires via supersonic velocity impact. The temperature of the process is much lower than the melting point of the sprayed material. The doped sample contained 1 vol.% of large-heat-capacity substance (LHCS) Gd 2 O 3 , whereas the comparison sample did not contain any LHCS. The average heat capacity of the doped sample at 4.2 K was increased more than two times. During the tests, both samples at LHe temperature were subjected to short (~1 ms) electromagnetic disturbances in the transverse constant magnetic field. It was found that the minimum quench energies for the sample with Gd 2 O 3 were 1.4-1.5 times higher than that for the comparison cable without LHCS. The described method is very promising for the stabilization of dense massive windings where the direct heat transfer to the LHe is hampered and the superconductor operates in almost adiabatic conditions.

In the case of plasma current interruption in tokamaks, the conductor of toroidial field (TF) coils experiences the action of a pulsed decreasing magnetic field (PDMF) parallel to the conductor’s axis. To estimate the stability of a cable-in-conduit conductor against the PDMF, a new experimental method to study different types of losses is applied. This method exploits a high sensitivity of temperature and gas pressure to input energy in a closed volume. It allows one to measure hysteresis losses with a rather high accuracy (provided that the rate of change of the PDMF is low) and a sum of hysteresis losses and eddy current losses (when the rate of change of the PDMF is high). An experimental setup to measure the transverse (circumferential) resistance and losses has been developed at the National Research Centre Kurchatov Institute. A Russianmade Nb 3 Sn conductor intended for the TF coils of the International Thermonuclear Experimental Reactor is subjected to a PDMF with different amplitudes and characteristic times. The electromagnetic time constant and the transverse resistivity of the conductor are experimentally determined. The maximum temperature of strands under the action of the PDMF is calculated.

In this paper we report our recent research on thermal stabilization of low-temperature superconducting magnets by means of large heat capacity substances (LHCS). Two samples (lengths ∼100 m) of NbTi composite wires with additional internal filaments made from intermetallic compound PrB6 (5.9–7.3 vol.%) were produced and tested. The design of the wires was similar to that of the conventional MRI sc wires, except for their smaller diameter (0.835 mm instead of 1.345 mm). Our final goal was the investigation of the possibility to minimize (or even eliminate completely) the necessity of MRI magnets training before their commissioning. The comparative stability measurements showed a twofold increase of the minimum quench energies (MQEs) of the doped wires against short heat disturbances. The magnetic field corresponding to the first flux jump increased by 50%. In MQE tests, the PrB6 heat capacity was fully utilized over the course of a 1 ms heat pulse. In the thermomagnetic stability measurements, the efficiency of LHCS doping was about 75% due to the fast evolution of the flux jumps.

Several years ago at Kurchatov Institute the R&D program on the new type of superconducting magnets (SM) doped with large heat capacity substances (LHCS) in order to improve their stability was started. We began from an "external" doping by the LHCS powder mixed with epoxy resin using the wet-winding process. Later on at Bochvar Institute the methods to introduce LHCS inside superconducting wires (both NbTi and Nb3Sn based) were developed. The comparative tests of LHCS doped wires and model windings with LHCS have shown positive results regarding a considerable increase of critical energies, improving of thermomagnetic stability and training behavior. The state-of-the-art of these methods is reviewed and their perspectives are discussed. (C) 2011 Elsevier Ltd. All rights reserved.

During the past decade we have been carrying out R&D aimed at increasing the stability of low temperature superconducting magnets. The main idea is to enhance the superconductor temperature margin by means of introduction into the winding of several volume per cent of specific substances with enormously large heat capacities (LHCS) at liquid helium temperatures. Two doping techniques have been developed at the Kurchatov and Bochvar institutes: the ‘external’ one—introduction of a dopant into the epoxy compound—and the ‘internal’ one—the introduction of doping filaments directly into NbTi and Nb3Sn wires. Up to now, our experiments with LHCS internal doping have been carried out on short samples only. In this paper we report the first experimental and numerical investigation of the stability of small-scale internally doped coils made of NbTi wires of a new type, with Gd2O3 ceramic filaments. The coil wound from doped wire and an identical control coil without any doping were subjected to electromagnetic pulses with 1.4–7.4 ms duration. Minimum quench energies for the doped coil turned out to be about 80% larger than those for the undoped one. The gain is especially pronounced in the range of large transport currents (∼0.9Ic). Comparative effectiveness analyses of the two (external and internal) LHCS doping techniques are also presented.

The Conductor of the toroidal field (TF) coils in Tokamaks are exposed to high amplitude (B m ≥ 2 T) pulse magnetic field (PMF) parallel to conductor axis during plasma disruption. A new experimental method for losses investigation in superconducting cable-in-conduit conductor (CICC) was used in this work. The method is based on a high sensibility of gas pressure in a closed volume to the energy input and allows measuring hysteresis losses with sufficient precision (when the decay rate of PMF is small) and the sum of hysteresis and eddy current losses (when the decay rate of PMF is high). Eddy current losses are rather higher than the coupling ones in CICC when PMF is parallel to the conductor axis. Therefore short samples (length is smaller than the last twist pitch) can be used for such investigations. The test facility for investigating the losses was developed and manufacturing at the “Kurchatov Institute”. The sample of the ITER TF conductor was exposed to PMF with different amplitude and electromagnetic time constant. As result of the experimental investigation the electromagnetic time constant and the resistivity of the bundle were determined. The maximum temperature of the strands during PMF exposure was calculated numerically.

It is demonstrated that the thermomagnetic stability of composite superconductors can be considerably increased by introducing into them a small quantity of a material with an extremely high specific heat at low temperatures. Measurements show that the criterion of "adiabatic" stability for a (Nb3Sn + 7 vol. % PrB6) wire is 70% higher than for a reference Nb3Sn wire (at 4.2 K, the specific heat of the doped sample is seven times higher than that of the reference sample). For a (NbTi + 5vol. % Gd2O2S) sample, the specific heat of which at 4.2 K is nine times higher than that of a reference NbTi wire, this increase in stability is as small as 10% (because the characteristic thermal time in the transverse direction is much longer than the time of the magnetic flux jump development).

The effect of internal doping of a NbTi composite wire by a large-heat-capacity substance (Gd 2 O 2 S ceramics) on the critical currents and stability against short thermal disturbances (with a typical time on the order of 1 ms) is studied experimentally and theoretically. The composite wire studied in this work is similar in design to conductors used in the international thermonuclear experimental reactor (ITER). The additive introduced into the wire in an amount of 5 vol % raises its specific heat ninefold at 4.2 K. It is found that the critical current of the (NbTi + Gd 2 O 2 S) wire increases by 8–11% in comparison with a reference NbTi wire depending on the external magnetic field varying between 5 and 7 T. Although the potential of high specific heat is not utilized completely, the critical thermal energies of the doped wires are three to four times higher than those of the undoped (reference) wires at near-critical currents.

The dependence of the quench currents on the ramp rate was studied for four small NbTi coils. Two pairs of superconducting coils were tested. In one pair the SC 0.85-mm-dia wire with 2970 filaments was used, in the other two coils the SC wire contained 8910 filaments of smaller size. Two coils (with different number of filaments) contained 4.9 vol % of Large-Heat-Capacity Substance (LHCS) in the form of tiny powder evenly distributed over the winding volume, therefore their heat capacity at 4.2 K was an order of magnitude larger than that for coils without LHCS. The LHCS was introduced into the winding in a mixture with epoxy resin (“wet” winding). When the self-magnetic field varied with a rate of ≥5 T/s, premature quenches were observed in the central turns of the undoped coil made of a wire with 2970 filaments. These transitions are likely to be caused by magnetic flux jumps. In the LHCS-doped coil made of the same wire, the quenches took place at currents two to three times higher, since the sample was heated up to a critical temperature because of electrical losses (as confirmed by calculations). Thus, the improved stability of the LHCS-containing coils not only against long-term (0.1–1.0 s) disturbances but also against much shorter (10–100 μs) jumps of the magnetic flux is demonstrated.

Two samples of Nb3Sn multifilamentary wires 0.82 mm diameter were prepared by the bronze method. One of the samples was internally doped with 7 vol% of PrB6, a large heat capacity substance (LHCS), while the other sample did not contain any LHCS and was used for comparison. The influence of LHCS internal doping on the stability toward short (similar to 1 ms) heat disturbances and critical currents in a transverse external magnetic field up to 3 T was investigated both experimentally and computationally. The average heat capacity for the doped sample in the temperature range 4-10 K was three times larger than for the undoped one. For the LHCS-doped sample its critical current was found to be slightly larger than for the comparison sample (6-8% depending on the external field), while its critical energies towards external heat disturbances were five times larger.

Our goal was to investigate the dependence of the stability of superconducting windings with extremely large heat capacity dopants on the thermal disturbance duration. The critical energies of the thermal disturbances were measured for two NbTi coils. One of the coils was doped with 5 vol% of Gd2O2S; the other was practically identical to the first, but without a large heat capacity dopant. Both coils were wound using the 'wet-winding' process. The thermal disturbances were generated with alternate magnetic field pulses localized in the innermost winding turns. The threshold pulse parameters were used to calculate the heat dissipated in the wire. In was found that the critical energy densities reach their adiabatic limit (enthalpy margin averaged across the winding) at 1.4 ms for the comparison coil and at 20 ms for the doped coil. The gain in the energy densities was observed in the whole duration range (0.2-34 ms) and at temperature margins (0.25-1 K), but a meaningful (about 30%) increase of energy densities started from about 0.8 ms characteristic disturbance time.

Experiments on training of thin solenoids containing additives with an extremely high low-temperature heat capacity in an external magnetic field are described. The data obtained for the windings with and without the additives are compared. Gd2O2S ceramics is used as an additive (addition of this ceramics in an amount of 6.4 vol % raises the specific heat of the winding by a factor of 12.5). In the solenoid with the additive, the current at which training starts is 35% higher than in the solenoid without the additive, and the transitional current after 20 current feeds increases by 18%. Tensile stress σ reaches 300 MPa (in terms of a free-turn model).

ITER type multifilamentary superconducting wires (0.82 mm Nb3Sn wire and 0.73 mm NbTi wire) with high heat capacity at LHe temperatures have been manufactured and tested. To increase the heat capacity the corresponding billets were doped with several volume fractions of PrB6 (for Nb3Sn) and Gd2O2S (for NbTi). The volumetric heat capacity of these substances at LHe temperatures is about two orders of magnitude larger than that for the other components of the wires. Fine powders of PrB6 and Gd2O2S were incorporated into the billets by the Powder-in-Tube (PIT) method. For comparison the wires without dopants were produced from the same billets using identical heat treatment for each pair of wires. First comparative tests of the wires have demonstrated a noticeable increase of critical energies in a wide range of transport currents. The ways of further increase of critical energies are considered.

We have studied the thermomagnetic stability (with respect to magnetic flux disturbances) of composite superconductors screened by additives of rare earth compounds possessing extremely high heat capacity at low temperatures. Three tubular composite structures have been manufactured and studied with respect to screening of the central region from variations of an external magnetic field. The effect of large-heat-capacity substances (LHCSs) was evaluated by measuring a jump in the magnetic flux in response to the rate of variation (ramp) of the external magnetic field. It is established that the adiabatic criterion of stability (magnetic-flux jump field) in the sample structures containing LHCSs significantly increases-by 20% for HoCu2 intermetallic compound and 31% for Gd2O2S ceramics-as compared to the control structure free of such additives.