S. L. Kruglov

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.

Testing of a low-resistance joint of CICC for Indian tokamak SST-1

In the frame of CICC testing for sc coils of Indian tokamak SST-1 the soldered joint design was developed and tested up to 50 kA in a loop comprising the secondary winding of a sc transformer. The CICC (14.8/spl times/14.8 mm/sup 2/) was manufactured by Hitachi. It consists of 135 (3/spl times/3/spl times/3/spl times/5) Nb-Ti strands each 0.86 mm dia. In the current range from 10 kA to 45 kA the joint resistance remained almost constant (from 3.6 n/spl Omega/ to 4.0 n/spl Omega/) in the bias field of about 2 T. The temperature dependence of CICC quench current during its charging was measured and showed an excellent (98%) coincidence with critical current values of a single strand. The measured temperature increase of CICC was used to evaluate its self-field AC losses at different charging rates (1.3-1.8 kA/s) and then to estimate the transverse resistivity of the cable. Its low value (approximately 8/spl middot/10/sup -10/ /spl Omega//spl middot/m) most likely reflects the fact that the clean surfaces of the strands in the cable are too tightly pressed. This can negatively influence the CICC stability towards pulse electromagnetic disturbances.

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.

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.

A model of a helical high field strength short period undulator

A hybrid (superconducting + soft iron poles and yoke) short model of a helical undulator was designed, built and successfully tested. The parameters of the model were chosen close to the requirements for free electron laser undulators: 10 mm diameter clear bore, 28 mm period, 0.95 T design field. The number of full periods is 6. A NbTi composite multifilamentary strip was used in the winding. During the tests the central magnetic field reached 1.065 T at 1215 A operating current. As far as we are aware, the combination obtained of bore-to-period ratio and magnetic field strength can be considered one of the best achieved practically for helical undulators.

Effect of additives with extremely high specific heat on training of superconducting windings

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).