The feasibility of creating high-current leads on the basis of second-generation (2G) high-temperature superconductors (HTSs) was demonstrated. A method for graded soldering of 2G HTS tape stacks is proposed. It allows one to avoid negative effects that are related to an inhomogeneous current sharing between parallel HTS conductors. A prototype of HTS-coated current leads (CLs) was designed, manufactured, and tested; its critical current exceeds 18 kА. Prototypes of HTS CLs for the superconducting magnetic system of the Nuclotron accelerator (a part of the NICA collider) with an operating current of 6 kA were developed, manufactured, and tested in liquid helium (the overcurrent was 9 kA).

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.

Tensile tests of 316LN-IG austenitic stainless steel samples cut out from tubes along their axis are carried out in liquid and gaseous helium below 7K. The tubes were intended for conductor conduits for the toroidal magnet system for ITER. Time dependences of temperature, strain and strain induced magnetization normal to the sample surface are examined with respect to stress. A complicated behavior of the local deformation near slip bands is detected that seems as unloading and shrinking regions adjacent to slip planes. The magneto-elastic effect indicating a negative longitudinal magneto-striction for the initial γ phase and the α phase induced by strain is found. The events without local heat release when jumping strain are explained as a result of the magneto-caloric effect in the regions unloaded in slipping.

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.

The segments of tubes made of austenitic 316LN-IG stainless steel that are cut along the tube axis are subjected to tensile tests in liquid and gaseous helium at temperatures below 7 K. The tubes are intended for the conductor conduits of the toroidal magnetic system of ITER. The time evolution of the strain, the temperature, and the strain-induced magnetization of specimens in the form of the normal component of the magnetic field on their surfaces is studied as a function of the applied load. The behavior of local deformation near slip bands is complicated: the areas near slip planes undergo unloading and shrinking. A magnetoelastic effect is detected; it indicates a negative longitudinal magnetostriction for the initial γ phase and the strain-induced α phase. The cases of absent local heating during strain jumps are explained by the magnetocaloric effect in the areas unloaded during slip.

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.

Using of HTS AC and DC cables in electric power grids allows increasing of the transferred power, losses diminishing, decreasing of exclusion zone areas, the enhancement of the environmental conditions and fire/explosion safety of electric power systems. However, the use of DC superconducting cable lines together with converters brings additional advantages as reduction of losses in cables and suitable lowering of refrigerating plant capacity, as well as the realization of the function of short-circuit currents limitation by means of the appropriate setting of converter equipment. Russian Federal Grid Company and its R&D Center started the construction of the DC HTS power transmission line which includes the cable itself, cryogenic equipment, AC/DC converters, terminals and cable coupling boxes. This line will connect two substations in Saint-Petersburg – 330 kV "Centralnaya" and 220 kV "RP-9". The length of this HTS transmission line will be about 2500 meters. Nowadays are developed all the elements of the line and technologies of the cable manufacturing. Two HTS cable samples, each 30 m length, have been made. This paper describes the results of cables tests.

Experiments were carried out to measure the delamination strength of 2G HTS tapes. During the tests a tensile force was applied to HTS tapes. Experiments were carried out at room and liquid nitrogen temperatures using "Instron 1195" tensile testing machine. The design of the cryogenic tensile testing device is described. Both copper-clad and bare tapes of different designs were tested. Current-voltage curves were measured under the load in order to evaluate the critical current degradation caused by the tensile strength.

Design of current lead boxes and joint boxes for 2.5 km long St. Petersbubrg HTS DC cable project (2.5 kA, 20 kV) is described. Maximum internal pressure of sub-cooled nitrogen is 1.4 MPa. Both current lead boxes and joint boxes are equipped with terminals for temperature and pressure measurements. Current leads of different polarities have independent electrical insulators. Two current lead boxes and one joint box have been preliminary tested. The first test results are reported.

It is well known that the thermal quench current densities j q in high-temperature superconducting (HTS) coils can considerably differ from the “critical” short sample current densities j c ( T , B ) based on a definite voltage level, for instance 1 μV/cm. The ratio j q /j c (lift-factor) can be higher or lower than one depending on coil parameters. The reasons are smooth voltage-current curves and large temperature margins compared to low-temperature superconducting devices. Several approaches for the description of the quench development in HTS devices are known. However, they are either not suitable for windings with nonuniform magnetic field distribution or were developed for a particular HTS magnet in the limited range of its working parameters. In this paper, we analyzed the simple steady state model of an infinitely long solenoid with nonuniform radial distribution of magnetic field and temperature. It allows to evaluate the main thermal and electrical parameters of HTS coils in j j q region as a function of the tape critical properties, n -value of the j - E curve, cooling conditions, and radial thermal conductivity in a broad range of their values.

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.

Two superconducting coil test facilities equipped by Sumitomo SRDK-415D cryocoolers were developed, manufactured and tested. The motivation for their constructing was to make cheaper the testing (and especially training of LTS magnets) by liquid helium (LHe) saving. It is well known that the helium price increases rapidly and this tendency most probably will continue for a long time, as the demand of helium grows faster than its production. The utilization of heat-exchange gas considerably reduces many problems, that arise in the design of completely dry LTS magnets. The goal was to decrease or even completely avoid the consumption of rather expensive liquid helium for testing the laboratory size Nb-Ti and Nb3Sn coils including their training process. Several superconducting magnets were tested by using these facilities. For example, the first facility was successfully used for testing of 13 T, 60kg coil cooled by cryocooler in helium gas (several torr pressure) heat exchange atmosphere. The precooling time was about 45hours. The quench current (240 A at 4.2K) was equal to that reached in the pool boiling LHe cryostat. The second facility with 420mm wide access bore can be used for testing of corresponding size superconducting coils with very modest consumption of liquid helium with its level well below the lower flange of the coil. Each test facility is equipped by 2 pairs of HTS current leads. Design and operational experience of one of them is described.

Various high-temperature superconductor (HTS) devices (transformers, electrical machines, accelerator magnets, power transmission lines, current leads, etc.) need operating currents that many times exceed single HTS tapes current capacity. Conductors made of many transposed HTS tapes wound in parallel onto several mm-diameter flexible formers can be made of modern 2G tapes. Such conductors combine large operating currents, low power losses, and flexibility. Our electrical and mechanical properties measurements of various manufacturers' tapes showed the possibility to make such conductors. The HTS tapes (up to 16) were wound helically onto several mm-diameter flexible formers made of twisted bundle consisted of 3000 stainless steel wires of 0.1 mm diameter. The V-I curves at dc in external magnetic field up to 0.6 T and the V-I curves and ac losses at 50 Hz in self field and in the synchronous background field were measured at different cooling conditions. For all cable samples the current capacities were proportional to the number of tapes.

Extremely stringent requirements, which include the impact toughness at the liquid-helium temperature, are imposed on the material of the conduit tubes for International Thermonuclear Experimental Reactor (ITER) Toroidal Field (TF) conductors. Modified 316LN-IG stainless steel is recommended as the conduit tube material. Steel 316LN-IG tube samples (both full-size samples and sub-sized samples) are subjected to mechanical tests at various stages of the process of conductor production: in the as-recieved state and after compacting, preliminary elongation by 2.5% at room temperature, and annealing at 650°C for 200 h in a pure helium gas atmosphere. The tests are carried out at room, liquid nitrogen, and liquid helium temperatures and satisfy the standards of the American Society of Mechanical Engineers (ASME and ASTM). The results of sub-size and full-size samples testing show that the last one gives more representative results to qualify the weld joints in liquid nitrogen. When the temperature decreases or the strain increases, the magnetization of the samples increases, especially in the weld area. Strain measurements with an extensometer demonstrate that the intracrystal processes occurring at the liquid-helium temperature can lead to a significant change in the local load, up to complete unloading in a deformation zone. Unusual local serrated deformation is observed with an extensometer installed in the weld area during tests in liquid helium: this deformation is the result of compressive jumps opposite to the loading direction.

One of the main problems in multi-element superconductors is to ensure the current distribution between several elements in accordance with their individual critical currents.The experimental investigation of self-field current redistribution between several HTS tapes was performed in order to support the design of a resistive type SFCL. The critical currents at 1 mu V/cm of 12 mm wide tapes SF12100 produced by Super Power were in the range 210-300A at 78 K temperature. The rated rms current of SFCL project is 900 A.The samples were wound onto about 200 mm diameter bobbin and consisted of one or two layers of 3 or 6 connected in parallel HTS tapes. The multi-tape conductor turns were at first charged with DC or AC (50Hz) individually. Then they were mounted coaxially and connected in opposite in order to reduce self-field and inductance values. The dynamic behavior of currents in the tapes was reconstructed from simultaneously measured data of 7 Hall probes and V-A curves of several tapes.In spite the critical currents of individual tapes were far from being equal the total critical current of the samples was close to the sum of tapes critical currents (about 750 A for 3 tapes and 1350 A for 6 tapes). Some recommendations for the improvement of current distribution uniformity are given for the case of SFCL. (C) 2012 Published by Elsevier B.V. Selection and/or peer-review under responsibility of the Guest Editors.

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 set of very tough requirements has been formulated for TF jacket materials with extremely high plasticity at liquid helium temperature. The stainless steel 316LN-IG is recommended to be used for TF jacket tubes. Samples of 316LN-IG tubes (whole tubes and sub-size samples) made of the material from the same electro slag remelt have been tested in different conditions -as received tubes and tubes after prescribed compaction, 2.5% deformation at room temperature and heat treatment at 650 degrees C, 200 hours. The tensile tests were carried out at room, liquid nitrogen and liquid helium temperatures down to 4.2 K, meeting corresponding ASME and ASTM requirements. The low temperature testing devices are described. The tests results for sub-size samples and whole tubes show that the latter tests are considerably more representative and important for butt weld qualification at LHe temperature. It was observed that the ferromagnetic properties of all samples and especially of butt welds increase with lowering the temperature and increasing the degree of deformation. At LHe temperature a non-uniform and highly localized serrated deformations were observed.

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.