Rainer Wesche

High performance new MgB2 superconducting hollow wires

MgB2 hollow wires have been produced with a new technique which uses a conventional wire manufacturing process but is applied to composite billets containing the elemental B and Mg precursors in an appropriate shape. The technique has been applied to the manufacture of both monofilamentary and multifilamentary wires of several tens meters length. The superconducting transport properties of the MgB2 hollow wires have been measured in magnetic field and in the temperature range from 4.2 to 30 K. Promising results are obtained, which indicate the possibility of application of these wires as superconductors in the temperature range of 15-30 K and at medium-high values of magnetic field.MgB2 hollow wires have been produced with a new technique which uses a conventional wire manufacturing process but is applied to composite billets containing the elemental B and Mg precursors in an appropriate shape. The technique has been applied to the manufacture of both monofilamentary and multifilamentary wires of several tens of metres in length. The superconducting transport properties of the MgB2 hollow wires have been measured in a magnetic field and in the temperature range from 4.2 to 30 K. Promising results are obtained, which indicate the possibility of the application of these wires as superconductors in the temperature range of 15–30 K and at medium-high values of magnetic field.

Results of a New Generation of ITER TF Conductor Samples in SULTAN

A new generation of ITER TF conductor samples has been assembled and tested in SULTAN in 2007 following a common procedure agreed among the ITER parties. The test results of six SULTAN samples, made of twelve conductor sections manufactured in Europe, Japan, Korea and Russia, are reported here. The conductor layout reflects the ITER TF conductor design, with minor differences for the Nb3Sn strand characteristics, void fraction and twist pitch. The object of the test is a straight comparison with the ITER requirement of 5.7 K current sharing temperature at 68 kA current and 11.3 T field. A broad range of behavior is observed.

Test results of the ITER PF insert conductor short sample in SULTAN

A short sample of the NbTi cable-in-conduit conductor (CICC) manufactured for the ITER PF insert coil has been tested in the SULTAN facility at CRPP. The short sample consists of two paired conductor sections, identical except for the sub-cable and outer wraps, which have been removed from one of the sections before jacketing. The test program for conductor and joint includes DC performance, cyclic load and AC loss, with a large number of voltage taps and Hall sensors for current distribution. At high operating current, the DC behavior is well below expectations, with temperature margin lower than specified in the ITER design criteria. The conductor without wraps has higher tolerance to current unbalance. The joint resistance is by far higher than targeted.

Design of superconducting current leads

Abstract The thermal behaviour of superconducting current leads operating between 4.2 and 293 K has been studied. For operating current densities above 10 000 A cm −2 , textured Bi-2223/Ag tapes are an alternative to Bi-2212 bulk material. Results for these two cases will be compared. The room temperature refrigerator power required to cool a superconducting current lead has been calculated for different cooling concepts, taking into consideration the normal conducting copper part. Use of high T c superconductors has the potential to reduce the required room temperature refrigerator power to one-fifth of that consumed by a conventional current lead. The effects of complete coolant loss have been simulated numerically. To avoid burn-out the current has to be switched off, i.e. the supplied superconducting magnet has to be discharged. The maximum possible operating current densities in the superconducting part of the current lead have been calculated assuming an exponential decay of the current with a time constant τ = 10 s and allowing a maximum peak temperature of 400 K in the current lead. The operating current density should not exceed 500 A cm −2 for the Bi-2212 bulk material, whereas values above 10 000 A cm −2 are possible for Bi-2223/Ag tapes. In both cases it has been assumed that the current starts to decay when a voltage of 0.1 V has been developed across the superconducting part of the current lead.

Test Results of Two European ITER TF Conductor Samples in SULTAN

Four conductor lengths were prepared according to the ITER TF conductor design and assembled into two SULTAN samples. The four lengths are not fully identical, with variations of the strand supplier, void fraction and twist pitch. Lower void fractions improve the strand support and increased twist pitches also lower the strand contact pressure but both tend to increase the AC loss and the lower void fraction also increases the pressure drop so that the mass flow rate in the strand bundle area of the cable is reduced. The assembly procedure of the two samples is described including the destructive investigation on a short conductor section to assess a possible perturbation of the cable-to-jacket slippage during the termination preparation. Based on the DC performance and AC loss results from the test in SULTAN, the impact of the void fraction and twist pitch variations is discussed in view of freezing the ITER conductor design and large series manufacture. A comparison with the former generation of conductors, using similar strands but based on the ITER Model Coil layout, is also carried out. The ITER specifications, in terms of current sharing temperature, are fulfilled by both samples, with outstanding results for the conductor with longer twist pitches.

Test Results of a

The performance degradation under electro-magnetic, transverse load has grown to a key issue for the design of Nb3Sn cable-in-conduit conductors (CICC). Beside the tolerance to bending strain of the basic Nb3Sn strand and the void fraction of the CICC, a relevant parameter is thought to be the cable pattern. A sequence of ldquolongrdquo twist pitches in the early stages of a multi-stage cable is credited to mitigate the performance degradation compared to ldquoshortrdquo pitches. To assess quantitatively the effect of long/short pitches maintaining constant all other conductor parameters, a short length of four stages CICC is prepared, where the first half length has long pitches (83/140/192 mm) in the first three cable stages and the second half length has short pitches (34/95/139 mm). The last stage pitch is 213 mm for both lengths. The cable is made of Cr plated copper and Nb3Sn strands with a diameter of 0.81 mm. The conductor is assembled into a SULTAN hairpin sample where the two branches have respectively long and short pitches. The DC performance, AC loss and pressure drop are measured in both conductor sections and compared to former conductors with the same design. The results are reported and the balance of advantages and drawbacks of long vs. short pitches is discussed.

{\rm Nb}_{3}{\rm Sn}

In the last decade, a large number of high current, force flow superconductors have been tested as short length samples in the SULTAN facility. The object of the test ranged over transient stability, thermal-hydraulic behavior, AC losses, joint resistance and proof-of-principle for innovative conductor design. Recently, with the ITER cable-in-conduit conductors (CICC), the basic DC transport properties have been the focus of the SULTAN test. The critical steps of the sample assembly and instrumentation are described, with emphasis on the application of the temperature sensors, verification of the signal treatment chain and calibration. The post-processing and the data reduction are focused on the assessment of the current sharing temperature, T cs: the conventional method of electrical field threshold detection by voltage taps is compared with the current sharing power detection by steady state gas-flow calorimetry. The longitudinal strain state of the conductors is discussed through the results of strain gauges applied on the jacket. Eventually, the value of a certified conductor test is highlighted in the frame of the quality control for the ITER magnets.

Cable-in-Conduit Conductor With Variable Pitch Sequence

Two short length samples have been prepared and tested in SULTAN to benchmark the performance of high current density, advanced Nb3Sn strands in the large cable-in-conduit conductors (CICC) for ITER. The cable pattern and jacket layout were identical to the toroidal field model coil conductor (TFMC), tested in 1999. The four conductor sections used strands from OST, EAS, OKSC and OCSI respectively. The Cu:non-Cu ratio was 1 for three of the new strands, compared to 1.5 in the TFMC strand. The conductors with OST and OKSC strands had one Cu wire for two Nb3Sn strands, as in TFMC. In the EAS and OCSI conductors, all the 1080 strands in the cable were Nb3Sn. A dc test under relevant load conditions and a thermal-hydraulic campaign was carried out in SULTAN. The CICC performance was strongly degraded compared to the strand for all the four conductors. The current sharing temperature at the ITER TF operating conditions (jop = 286 A/mm2, B = 11.15 T) was lower than requested by ITER.

Methods, Accuracy and Reliability of ITER Conductor Tests in SULTAN

Following the outcome of the conceptual design phase the EFDA dipole magnet will be made of rectangular cable-in-conduit conductors (CICC) jacketed in 316LN. In order to optimize the required amount of superconductor two different conductor types are used: a high-field (HF) conductor consisting of 144 strands and a low-field (LF) conductor with 108 strands. A high strand with a critical current density (at 4.2 K and 12 T) and an effective filament diameter of was selected. The first series of conductor prototype specimens was tested in summer 2006 but the conductor performances were lower than expected from the pre-prototype tests of 2005 and not fulfilling the design criteria. The conductor layouts were modified to increase the strand support inside the cable and the revised HF conductor design was qualified successfully end of 2006. A current sharing temperature 6 K was found at the dipole operating conditions (12.8 T, 17 kA) confirming the required temperature margin of more than 1 K. The HF conductor qualification process including the design modifications, analysis of the test results and comparison to the expectations are discussed.

Test Results of Two ITER TF Conductor Short Samples Using High Current Density Nb

The design of a flat cable suitable for future fusion reactors has been carried out. The cable consists of twisted round strands, which are composed of tapes stacked between copper profiles. According to calculations, the strand twist pitch and the cable twist pitch should be at least 1.5 or 2 m long to limit the strain and thus the reduction of the critical current. A 4-m-long strand (Ø6.2 mm) was fabricated on a continuous stacking and soldering line to demonstrate that the scaling up to industrial production is feasible. The strand can carry about 940 A in self-field at 77 K. The critical current evolution under bending strain was measured but it turned out to be rather fragile in “hard” bending direction. Three types of joints between strands were also manufactured and tested at 77 K.