Calorimetric Measurements Of AC Loss Research Articles

Simultaneous Magnetic Shielding and Magnetization Loss Measurements of YBCO Cylinders at Variable Temperatures Under Cryogenic Helium Gas Circulation

A novel experimental setup for simultaneous measurements of magnetic shielding characteristics and magnetization ac loss measurements is described. The measurement system consists of a controllable helium circulation system that enables cooling of the samples to temperatures as low as 15 K and simultaneously allows calorimetric ac loss measurements. The simultaneous measurement of shielding characteristics and ac losses permits estimation of losses in realistic operating conditions of high-temperature superconducting magnetic shields. It is shown that if the extent of losses is high compared with the cooling efficiency, the magnetic shields could quench. The data on magnetic shielding efficacy along with the corresponding magnetization losses are presented.

Interstrand Contact Resistance and Magnetization of ${\hbox {Nb}}_{3}{\hbox {Sn}}$ Rutherford Cables With Cores of Different Materials and Widths

Rutherford cables with cores of E-glass and S-glass woven tape and types AISI-316 and AISI-304 stainless steel (SS) ribbon were subjected to calorimetric AC loss measurement in transverse magnetic fields of amplitude 400 mT and frequencies of up to 90 mHz applied in the face-on (FO) and edge-on (EO) orientations. The results yielded the effective interstrand con- tact resistances (ICR), Reff (to be defined), and corresponding estimates of the FO coupling magnetizations generated by fields ramping at the LHC-specified 6.5 mT/s. Detailed analysis of the results indicated: 1) a cable with a full-width 316-SS core and a satisfactory Reff value would be suitable for magnet winding; 2) a cable with an off-center partial-width core had a very low Reff and an excessively large coupling magnetization; 3) The cables with glass-tape cores although characterized by large Reffs and very small coupling magnetizations turned out to have mostly uncoupled strands with intermittent points of low resistance contact. In order to achieve the target ICR values, the level of glass insulation insert should be reduced and cable compaction adjusted.

Calorimetric method of ac loss measurement in a rotating magnetic field

A method is described for calorimetric ac-loss measurements of high-T(c) superconductors (HTS) at 80 K. It is based on a technique used at 4.2 K for conventional superconducting wires that allows an easy loss measurement in parallel or perpendicular external field orientation. This paper focuses on ac loss measurement setup and calibration in a rotating magnetic field. This experimental setup is to demonstrate measuring loss using a temperature rise method under the influence of a rotating magnetic field. The slight temperature increase of the sample in an ac-field is used as a measure of losses. The aim is to simulate the loss in rotating machines using HTS. This is a unique technique to measure total ac loss in HTS at power frequencies. The sample is mounted on to a cold finger extended from a liquid nitrogen heat exchanger (HEX). The thermal insulation between the HEX and sample is provided by a material of low thermal conductivity, and low eddy current heating sample holder in vacuum vessel. A temperature sensor and noninductive heater have been incorporated in the sample holder allowing a rapid sample change. The main part of the data is obtained in the calorimetric measurement is used for calibration. The focus is on the accuracy and calibrations required to predict the actual ac losses in HTS. This setup has the advantage of being able to measure the total ac loss under the influence of a continuous moving field as experienced by any rotating machines.

AC-loss considerations of a pulse SMES for an accelerator

In particle accelerators quasi-DC superconducting magnets are used to keep particles in desired tracks. The needed rapid field variations of these high energy magnets require large energy bursts. If these bursts are taken from and fed back to the utility grid, its voltage is distorted and the quality of the electricity degrades. In addition, these bursts may decrease operation life time of generators and extra arrangements may be required by the electricity producers. Thus, an energy storage is an essential component for a cost-effective particle accelerator. Flywheels, capacitors and superconducting magnetic energy storage (SMES) are possible options for these relatively large and high power energy storages. Here we concentrate on AC-loss of a pulse SMES aiming to demonstrate the feasibility of NbTi SMES in a particle accelerator. The designing of a SMES requires highly reliable AC-loss simulations. In this paper, calorimetric AC-loss measurements of a NbTi magnet have been carried out to consider conductor's suitability in a pulse SMES. In addition, the measured results are compared with AC-loss simulations.

Effect of Core Width, Placement, and Condition on Calorimetrically Measured AC Loss and Interstrand Contact Resistance of Stainless-Steel-Cored $\hbox{Nb}_{3}\hbox{Sn}$ Rutherford Cables

Calorimetric measurements of AC loss were made on Nb3Sn Rutherford cables furnished with cores of widths 4.5, 5.2, and 10.8 mm in the face-on (FO) and edge-on (EO) orientations of the applied AC field. The resulting FO-measured effective interstrand contact resistances, Rperp,e,f,f., were compared with that of the uncored cable after which it was noted that although the uncored Rperp,e,f,f. agreed well with those of previously measured cables, the cored values -0.3 to 8 muOmega were lower than expectation. The relatively low -Rperp,e,f,f. s are attributed to (i) drifting of the narrower cores to one edge of the cable (ii) shrinkage of the effective width of the widest core as a result of severe texturization. After correcting for these deficiencies the projected Rperp,e,f,f. for the widest-core cable moved closer to the range of values expected for a full-width-core Nb3Sn cable.

AC loss of Nb/sub 3/Sn-based Rutherford cables with internally and externally added Cu

Calorimetric measurements of AC loss and hence interstrand contact resistances (ICR) were measured on various stabilized (Cu-added) Nb/sub 3/Sn Rutherford cables. The cable variations formed two sets; externally added Cu and internally added Cu (mixed strand cables). In the first set, Cu strips were diffusion bonded onto a pre-existing cable. Two distinct variants were made, in the first a Cu strip was added to the "upper" and "lower" cable surfaces and then diffusion bonded during a reaction heat treatment (RHT) of 180 h/650/spl deg/C under a "cold-applied pressure" of 20 MPa. The second was formed by diffusion bonding a helically wrapped Cu strip along the cable. A control cable with no strip was included in this set. The second set consisted of a cable with approximately 1/3 of the strands replaced with pure Cu wires during the cable winding, and here also a control cable was fabricated for comparison. Calorimetric measurements of AC loss were made on all cables at 4.2 K in a transverse sinusoidal field of amplitude 400 mT, frequency 5 to 90 mHz, applied both parallel and perpendicular to the face of the cable. The loss data were interpreted in terms of an effective interstrand contact resistance, R/sub /spl perp/,eff/.

AC loss in cored, stabrite-coated, superconducting cables in response to external compaction and variation of core thickness and width

AC loss due to coupling currents in a Rutherford cable can be modified by changing the interstrand contact resistance (ICR) by: adjusting the level of native oxidation of the strand, coating it, or by inserting a ribbon-like core into the cable itself. With regard to cored cables further effective-ICR adjustments can be achieved by changing: (i) the degree of compaction during manufacture, (ii) the thickness of the core at fixed overall thickness, and (iii) the width of the core. We report on the results of magnetic and calorimetric AC loss measurements on stainless-steel-cored stabrite cables which had been: (i) externally compacted by rolling to thicknesses of 0%, 6%, 9%, and 11% below the standard thickness, (ii) internally compacted by being furnished with cores of thicknesses 1 mil ( 25 μm ) and 2 mil ( 50 μm ), and (ii) furnished with cores whose widths, w core, were about 20%, 50%, 75% and 100% of the maximum available width. The measurements were made in applied AC fields that were directed perpendicular (face-on, FO) and parallel (edge-on, EO), respectively, to the broad cables faces. It was noted that: (i) the introduction of the core dramatically reduced the FO loss, (ii) core-induced compaction moderately increased the side-by-side ICR at higher curing temperatures and hence increased the EO loss, (iii) increases in either external or internal compaction still further increased the losses, particularly those in the FO direction (which of course had already been strongly core-suppressed). The loss results were interpreted in terms of the FO-measured effective ICRs, R ⊥,eff, which were found to decrease from 198 to 37 μΩ during the compaction of a 170 °C-heat treated (“cured”, HT) cored cable and from 38 to 10 μΩ in the case of a 200 °C HT one. With increase in the w core of a 170 °C HT cable R ⊥,eff increased from 2.5 to 123 μΩ , passing through 15 μΩ at a w core of 57% w core,max. The results show that there is an opportunity for achieving the LHC-target R ⊥,eff of 15±5 μΩ either by compacting the standard cored cable ( T f=200 °C) by 4–7% or by reducing the width of the core ( T f =170 ° C ) to within 57±7% of w max. We prefer the latter route since it exposes more of the strands to crossover-type current sharing.

Improved experimental set-up for calorimetric AC loss measurements on HTSs carrying transport currents in applied magnetic fields at variable temperatures

To utilise an high-temperature superconductor (HTS) in an electro-technical device, an optimal operating current and an optimal operating temperature need to be determined. For this purpose the AC losses should be measured under application-like conditions. To perform such measurements we have designed and constructed a calorimetric measuring system which now has been in operation for a few years. During this time the system has gradually been improved. In this paper we describe the system in its present state and discuss the technical improvements in detail. The improvements include the possibility to inject a transport current and to apply a magnetic field in any direction perpendicular to the current path. In addition the measurement of the AC losses is averaged over the sample length. Experimental results of the AC losses due to combinations of transport currents and magnetic fields in different orientations at various temperatures are presented for two BSCCO/Ag tapes.

Coupling loss and contact resistance in cored stabrite cables-influences of compaction and variation of core width

AC loss due to coupling currents in a Rutherford cable can be modified by changing the interstrand contact resistance (ICS) by: adjusting the level of native oxidation of the strand, coating it, or by inserting a ribbon-like core into the cable itself. With regard to cored cables further effective-ICR adjustments can be achieved by changing: (i) the degree of compaction during manufacture; (ii) the thickness of the core at fixed overall thickness; and (iii) the width of the core. The authors report on the results of magnetic and calorimetric AC loss measurements on stainless-steel-cored stabrite cables which had been: (i) externally compacted by rolling to thicknesses of 0, 6, 9, and 11% below the standard thickness; (ii) internally compacted by being furnished with cores of thickness 1 mil (30 /spl mu/m) and 2 mil (50 /spl mu/m); and (iii) furnished with cores whose widths, w/sub core/, were about 20%, 50%, 75% and 100% of the maximum possible width. The measurements were made in applied AC fields that were directed perpendicular (face-on) and parallel (edge-on), respectively to the broad cables faces.

AC loss and contact resistance in copper-stabilized Nb/sub 3/Al Rutherford cables with and without a stainless steel core

Calorimetric measurements of AC loss and hence interstrand contact resistance (ICR), were measured on three samples of Rutherford cable wound with Cu-stabilized jelly-roll type unplated Nb3Al strand. One of the cable types was furnished with a thin core of AISI 316L stainless steel and the other two were both uncored but insulated in different ways. The cables were subjected to a room-temperature-applied uniaxial pressure of 12 MPa that was maintained during the reaction heat treatment (RHT), then vacuum impregnated with CTD 101 epoxy, and repressurized to 100 MPa during AC-loss measurement. The measurements were performed at 4.2 K in a sinusoidal field of amplitude 400 mT at frequencies of 1 to 90 mHz (no DC-bias field) that was applied both perpendicular and parallel to the face of the cable (the face-on, FO, and edge-on, EO, directions, respectively). For the cored cable the FO-measured effective ICR (FO-ICR), was 5.27 . Those for the uncored cables were less than 0.08 . As shown previously for NbTi- and Nb3Sn-based Rutherford cables, the FO-ICR can be significantly increased by the insertion of a core, although in this case it is still below the range recommended for accelerator-magnet use. Post-measurement dissection of one of the cables showed that the impregnating resin had permeated between the strands and coated the core with a thin, insulating layer excepting for some sintered points of contact. In the uncored cables the strands were coated with resin except for the points of interstrand contact. It is suggested that in the latter case this tendency for partial coating leads to a processing-sensitive FO-ICR.

Calorimetric ac-loss measurement of high Tc-tapes at 77 K, a new measuring technique

A method is described for calorimetric ac-loss measurements of high T c-tapes at 77 K. It is based on a technique approved at 4.2 K for conventional superconducting wires which allows a very easy and precise loss measurement in parallel or perpendicular external field orientation. The slight temperature increase of the sample in an ac-field, thermally insulated from the liquid nitrogen bath, is used as a measure of losses. The thermal insulation is provided by a material of low thermal conductivity, no vacuum vessel is used. Simultaneously the sample magnetization can be measured. All necessary sensors are incorporated in the sample holder allowing a rapid sample change. The combination of the calorimetric method at 77 K with the easier magnetization measurement allows to determine the ac-losses in a wide frequency and amplitude range of the external ac-field. The main part of data is obtained by the magnetization measurement while the calorimetric measurement is used for calibration.

Core-suppressed AC loss and strand-moderated contact resistance in a Nb 3Sn Rutherford cable

Calorimetric measurements of AC loss and hence interstrand contact resistance (ICR), have been performed on two types of Rutherford cable wound with unplated Nb 3Sn strand. One of the cable types was furnished with a thin core of AISI 316L stainless steel and the other was left uncored. The cables were subjected to pressures of 5, 10, and 20 MPa, respectively, during reaction heat treatment (RHT), and to 100 MPa during measurement. AC loss was measured at 4.2 K in sinusoidal magnetic fields of amplitudes 200 and 400 mT at frequencies of 5 to 90 mHz both with and without the presence of DC bias fields of up to 1 T. The magnetic fields were applied both parallel and perpendicular to the face of the cable. For the cored cables ICR was relatively high (≈78 μΩ). Loss and contact resistance were relatively independent of RHT pressure within the range measured. The contact resistance of the uncored cables were also RHT-pressure independent, however, the absolute values of ICR were much lower, being about 2.7 μΩ with a 1 T background field, and very much smaller in the absence of a background field. The background-field dependence was shown to be due to the presence of an unreacted Nb barrier which enveloped the strand just below the external Cu stabilizer and thereby provided very low resistance coupling paths at small bias fields. The relative roles of strand and cable designs in the determination of interstrand losses are discussed. It is concluded that a low-loss Nb 3Sn Rutherford cable has been achieved through the use of a metallic-alloy core.

AC loss properties of some Bi:2212/Ag Rutherford cables and a comparison with those of cables wound with NbTi and Nb 3Sn

Calorimetric measurements of AC loss have been performed on Rutherford cables wound with NbTi, Nb 3Sn, and Bi:2212/Ag strands, respectively. For the NbTi cables, various strand coatings had been applied, while for the Nb 3Sn and Bi:2212/Ag cables the strand surfaces were just bare Cu or Ag, respectively. Most of the cables contained resistive cores: ribbons of kapton or titanium (NbTi cables), stainless steel (NbTi and Nb 3Sn cables), and nichrome-80 (Bi:2212/Ag cables). In all cases the cores were found to lead to a strong suppression of the face-on (field normal to the broad cable face) coupling current loss; to such an extent that even the Bi:2212/Ag cables, which would otherwise be severely cross-sintered, evinced acceptable coupling loss. For most of the cables side-by-side interstrand contact resistances have been calculated. An `effective interstrand contact resistance', R ⊥,eff, has been defined, and an expression devised to enable its value in different-size cables to be converted to that of a `standard reference cable', R ⊥,eff.,ref., for the purpose of intercomparison.

Coupling current control in Rutherford cables wound with NbTi, Nb 3Sn, and Bi:2212/Ag

Calorimetric measurements of AC loss have been performed on Rutherford cables wound with NbTi, Nb 3Sn, and Bi:2212/Ag strands, respectively. For the NbTi cables Ni and stabrite coatings had been applied, while for the Nb 3Sn and Bi:2212/Ag cables the strand surfaces were just bare Cu or Ag, respectively. Most of the cables contained resistive cores: ribbons of kapton, or titanium (NbTi), stainless steel (NbTi and Nb 3Sn), and nichrome-80 (Bi:2212/Ag). In all cases the cores were found to lead to a strong suppression of the face-on (field normal to the broad cable face) coupling current loss; to such an extent that even the Bi:2212/Ag cables, which would otherwise be severely cross-sintered, show the possibility of acceptable coupling loss. Effective crossover interstrand contact resistances are calculated and normalized to a standard cable specification (`LHC inner') for the purpose of intercomparison.

Calorimetric ac loss measurements of silver sheathed Bi-2223 superconducting tapes

A new calorimetric method has been developed to measure ac losses in superconductors. Calibration of the losses requires a reference heater of the same size and shape as the superconductor mounted in a symmetric environment. Its main advantage is the direct determination of the total losses. Variations of the method include a null technique and averaging of the main Fourier component to reduce noise effects. The method has been tested on Bi-2223 tapes and gives the expected hysteresis losses as well as a frequency independent loss which can be attributed to flux creep. The measured losses are similar to those obtained by other groups with other methods.

Hysteresis losses of multifilament superconductors in superimposed dc and ac magnetic fields

Abstract Theoretical investigations, magnetization and calorimetric ac loss measurements have been performed on the problem, how the superposition of dc- and ac-magnetic fields influences the hysteresis losses of filamentary superconductors. For a given ac-field amplitude the maximum loss value as a function of the dc field can easily be predicted without knowledge of the superconducting properties.