I. A. Kovalev

Model of HTS three-phase saturated core fault current limiter

A small (several hundred watts) model of a three phase saturated core HTS fault current limiter (FCL) was developed and tested. Iron yokes of all three phases were saturated by a single DC HTS coil. The coil comprised a 60 turns single pancake (ID 135 mm), wound after heat treatment from Bi-2223 multifilamentary tape in Ag matrix. The critical current of the pancake in liquid nitrogen was 8 A. The tests have shown that the limiting value of the AC current (at 50 Hz) can be easily adjusted in the range from 8 A to 20 A depending on the value of the DC current in the HTS coil. The optimum value of the latter is 4 A, corresponding to the 8 times increase of the differential resistance in the current limiting mode. The response time is very short (less that 1 ms). Under tests the short-circuiting event was made in one, two and all three phases. The case of short-circuiting of one phase in the three-phase FCL is especially favorable from the standpoint of the voltages induced in HTS coil compared to the one-phase FCL.

The background magnets of the Samsung Superconductor Test Facility (SSTF)

The background magnet system of SSTF (Samsung Superconductor Test Facility) for KSTAR (Korea Superconducting Tokamak Advanced Research) is now under design. The main coil (MC) is split solenoids and the gap can be changed from 0 to 750 mm. The ID of MC is 750 mm. It will be wound using a CICC (cable-in-conduit conductor) designed for the central solenoid of KSTAR. The central field is 8 T at 22.5 kA when the gap is 250 mm. The ramp rate of MC is 3 T/s. A pair of blip coils will simulate (during the discharge) 1 T amplitude and 20 T/s rate electromagnetic disturbances expected from the KSTAR operation. To compensate the inductive interaction between MC and blip coils during the discharge of the blip coils, a pair of cancellation coils is foreseen. Both blip and cancellation coils (BCC) are fed in series and generate 1 T central field at 7 kA and 250 mm gap. The BCC are wound with CICC and cooled internally and externally.

Optimized HTS current leads

The problem of optimizing HTS current leads by varying their cross-section along the length is investigated both experimentally and numerically at 500 A current level. Bi-2223-based HTS multi-filament composite tapes were used with two types of matrices: pure Ag and (Ag+1 at.% Au) alloy. The warm ends of the HTS parts of the current leads were cooled with liquid nitrogen. Very low evaporation rates in the case of Au-doped matrix and rather long time constants to reach thermal equilibrium were observed.

Results of the model coil tests for the cable-in-conduit conductor for SST-1 tokamak

SST-1 tokamak, under fabrication at Institute for Plasma Research (IPR), India, deploys superconducting coils for both toroidal field (TF) and poloidal field (PF) magnets. A NbTi based 135 strands cable-in-conduit conductor (CICC) has been fabricated for this purpose by M/S Hitachi Cable Ltd. (Japan) under specification and supervision of IPR. In order to test the performance of this CICC under SST-1 operating scenarios, a Model Coil (MC) has been designed, fabricated and tested at Kurchatov Institute (KI), Russia using the SST-1 CICC under a collaborative program between IPR and KI. The MC was designed to have the same maximum field-to-current ratio (5 T at 10 kA) as of the SST-1 TF coils. Typical SST-1 disturbances were simulated with a toroidal array of four longitudinal disturbances simulating coils and with two transverse disturbances simulating coils.

Considerable stability increase of Nb3Sn multifilamentary wire internally doped with a large heat capacity substance (PrB6)

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 (~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.

Considerable increase in the thermomagnetic stability of multifilament superconductors internally doped by large-heat-capacity substances

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).

NB3SN AND NbTi MULTIFILAMENTARY WIRES WITH ENHANCED HEAT CAPACITY

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.

The superconducting transformer of the Samsung Superconductor Test Facility (SSTF)

In the frames of designing the SSTF (Samsung Superconductor Test Facility) for the KSTAR (Korea Superconducting Tokamak Advanced Research), the 50 kA transformer charging a CICC (cable-in-conduit conductor) short sample for one second is now under design. The primary winding conductor consists of six NbTi and six stainless steel strands tabled around a low RRR rectangular copper core, which was used by Kurchatov Institute in small SMES (superconducting magnetic energy storage) windings. The secondary winding consists of 24 subcables wrapped around and soldered to a low RRR copper strip. Each subcable consists of six NbTi strands cabled around a copper strand. The strands for primary and secondary windings are 0.85 mm diameter NbTi wires with six micrometer 8910 filaments. Both primary and secondary conductors have large current and temperature margins to ensure a reliable operation of the superconducting transformer. The primary coil is placed in a cylindrical LHe vessel. The four secondary turns are glued to the outer surface of the LHe vessel. The joints between the transformer and the sample are described.

Increasing thermomagnetic stability of composite superconductors with additives of extremely-large-heat-capacity substances

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.

Investigation of considerable stability increase of composite superconductors doped with extremely large heat capacity substances

The influence of doping NbTi-based composite superconductors with extremely large capacity substances (LHCSs) on their stability towards electromagnetic disturbances was investigated both experimentally and numerically. The samples comprised of standard NbTi 0.85 mm diameter wires soft-soldered with copper wires of the same diameter that contained one or several LHCS filaments. Some compounds (CeCu6, HoCu2, CeAl2, PrB6, Gd2O2S) with extremely large heat capacities at 4.2 K were introduced into the copper matrix by the powder-in-tube (PIT) method. The comparison sample (without LHCS doping) was made by soft-soldering of the 0.85 mm diameter NbTi wire with the copper wire without any LHCSs. The samples were placed in a transverse magnetic field and at several values of the transport current were subjected to longitudinal electromagnetic disturbances. The typical disturbance times varied in a broad range: from 50 µs to 1.2 ms. It was found that the critical current densities for the samples with LHCS doping were considerably higher than those for the comparison sample. It was also shown that the gain in stability remains even when the samples were directly liquid helium (LHe) cooled.