Joseph V. Minervini

HTS twisted stacked-tape cable conductor

The feasibility of high field magnet applications of the twisted stacked-tape cabling method with 2G YBCO tapes has been investigated. An analysis of torsional twist strains of a thin HTS tape has been carried out taking into account the internal shortening compressive strains accompanied with the lengthening tensile strains due to the torsional twist. The model is benchmarked against experimental tests using YBCO tapes. The critical current degradation and current distribution of a four-tape conductor was evaluated by taking account of the twist strain, the self-field and the termination resistances. The critical current degradation for the tested YBCO cables can be explained by the perpendicular self-field effect. It is shown that the critical current of a twisted stacked-tape conductor with a four-tape cable does not degrade with a twist pitch length as short as 120 mm. Current distribution among tapes and hysteresis losses are also investigated. A compact joint termination method for a 2G YBCO tape cable has been developed. The twisted stacked-tape conductor method may be an attractive means for the fabrication of highly compact, high current cables from multiple flat HTS tapes.

Progress of the ITER central solenoid model coil programme

The worlds largest pulsed superconducting coil was successfully tested by charging up to 13 T and 46 kA with a stored energy of 640 MJ. The ITER central solenoid (CS) model coil and CS insert coil were developed and fabricated through an international collaboration, and their cooldown and charging tests were successfully carried out by international test and operation teams. In pulsed charging tests, where the original goal was 0.4 T/s up to 13 T, the CS model coil and the CS insert coil achieved ramp rates to 13 T of 0.6 T/s and 1.2 T/s, respectively. In addition, the CS insert coil was charged and discharged 10 003 times in the 13 T background field of the CS model coil and no degradation of the operational temperature margin directly coming from this cyclic operation was observed. These test results fulfilled all the goals of CS model coil development by confirming the validity of the engineering design and demonstrating that the ITER coils can now be constructed with confidence.

Prospects for pilot plants based on the tokamak, spherical tokamak and stellarator

A potentially attractive next-step towards fusion commercialization is a pilot plant, i.e. a device ultimately capable of small net electricity production in as compact a facility as possible and in a configuration scalable to a full-size power plant. A key capability for a pilot-plant programme is the production of high neutron fluence enabling fusion nuclear science and technology (FNST) research. It is found that for physics and technology assumptions between those assumed for ITER and nth-of-a-kind fusion power plant, it is possible to provide FNST-relevant neutron wall loading in pilot devices. Thus, it may be possible to utilize a single facility to perform FNST research utilizing reactor-relevant plasma, blanket, coil and auxiliary systems and maintenance schemes while also targeting net electricity production. In this paper three configurations for a pilot plant are considered: the advanced tokamak, spherical tokamak and compact stellarator. A range of configuration issues is considered including: radial build and blanket design, magnet systems, maintenance schemes, tritium consumption and self-sufficiency, physics scenarios and a brief assessment of research needs for the configurations.

Conductor Characterization of YBCO Twisted Stacked-Tape Cables

Cable performances of a flat high temperature superconductor (HTS) tape cable fabricated by twisted stacked-tape cable (TSTC) method were experimentally evaluated under various bending diameters at 77 and 4.2 K in self-field. The straight conductor was made with 32 YBCO tapes and twisted using the TSTC method. The 32-tape YBCO TSTC straight conductor was soldered, and then the critical currents were tested at various bending diameters between 1 m and 140 mm. The critical current at a bending diameter of 140 mm was about 6% lower than the straight cable. The same cable with a bending diameter of 250 mm was charged to 10 kA in one second in liquid helium. A new magnet winding technique, stacked-tape twist-wind (STTW), of YBCO tapes using the TSTC concept has been developed, and a 2.3-m, 50-tape YBCO tape coil fabricated with this method was tested. Its critical current was 4.0 kA at 100 μV/m at 19.7 T in liquid helium. It was observed that for an electromagnetic Lorentz force of 50 kN/m the soldered YBCO cable started degrading. A curved saddle coil winding using a STTW technique was demonstrated.

Cabling Method for High Current Conductors Made of HTS Tapes

A small-scale test of a twisted stacked-tape conductor made of coated YBCO tapes was performed using four-tape cable. The critical current degradation and current distribution of this four-tape conductor was evaluated by taking account the twist-strain, the self-field and the termination resistance. The critical current degradation for the tested YBCO tape may be explained by the perpendicular self-field effect solo. The critical currents of the twisted stacked-tape conductor with four-tape cable have been confirmed not to degrade up to 120 mm twist pitch length. This type of conductor design is proposed to make it possible to fabricate highly compact, high current cables from multiple flat HTS tapes.

Effect of conduit material on CICC performance under high cycling loads

Recent International Thermonuclear Experimental Reactor (ITER) Model Coils and tests on Nb/sub 3/Sn Cable in Conduit Conductors (CICC) showed a significant and unexpected increase in the broadness of the transition to the normal state, resulting in degradation of superconducting properties. To investigate these phenomena, two CICC samples were built with identical 144 strand cables but different conduit materials. One sample had titanium conduit with low coefficient of thermal expansion, the other had stainless steel conduit. The purpose of this experiment was to study changes in strand properties in the cable (critical current, current sharing temperature, n-value), the effects of cycling and high electromagnetic load, and the effect of the conduit on the CICC performance.

TORSION STRAIN EFFECTS ON CRITICAL CURRENTS OF HTS SUPERCONDUCTING TAPES

A torsional twist strain effect on the critical current of a thin HTS tape has been found to be well described by a longitudinal strain model taking into account the internal shortening compressive strains accompanied with the tensile longitudinal strains due to a torsional twist. The critical current of a twisted tape is given by the integration of the critical current densities corresponding to the strain distribution over the tape cross‐section using axial strain data of the tape. The model is supported with experimental results of YBCO and BSCCO‐2223 tapes. It has been also found that torsional twisting effects on the critical currents of a tape composing of the conventional lapped‐tape cable and the twisted stacked‐tape cable are described by the same equation as that of a twisted single tape.

Pure bending strains of Nb3Sn wires

This paper presents in detail recent pure bending strain investigations of ITER Nb3Sn wires including a pure bending device development, bending effects on the critical currents and their model analyses. The pure bending test device allows applying a uniform bending over a large range of bending strain on a strand sample under a large electromagnetic Lorentz force. Pure bending characteristics of five different Nb3Sn wires including three internal tin and two bronze wires were tested and evaluated with a newly developed integrated model that accounts for neutral axis shift, current transfer length, mechanical filament breakage and uniaxial strain release due to the applied bending load.

Resistance of Demountable Mechanical Lap Joints for a High Temperature Superconducting Cable Connector

Experiments conducted to determine the resistance of demountable HTS lap joints with variations in compressive stress are reported here. A joint fixture was constructed that enabled the simultaneous testing of multiple joints of different configurations. The joint fixture was installed in a loading facility, comprising a pneumatic press mounted in a vacuum bell jar and connected to a GM cryocooler. The resistance across each joint was measured by means of voltage taps over a range of compressive joint forces. The test results demonstrated that joint resistances less than 2 muOmega were possible with only light joint loading.

Investigation of twisted stacked-tape cable conductor

A cable fabrication method of a twisted stacked tape conductor for HTS YBCO tapes has been developed and tested. A 2 m long, 32–tape conductor of 4 mm width YBCO was fabricated with a 200 mm twist pitch. The measured critical current of the straight cable agreed with the expected values estimated from the self-magnetic field. The 2 m long cable wound on a surface of 0.5 m diameter circle did not show any degradations. It has been confirmed through tests with single tape and cable tests that it is possible to develop a YBCO mutliple-tape stacked cable with a 200 mm twist pitch and to make a coil with an innermost turn of at least 0.5 m diameter. A joint method for multi-tape YBCO cable using BSCCO tapes has been developed and operated at 2.2 kA. AC losses of a twisted stacked YBCO tape cable have also been analyzed. The twisted stacked-tape cabling method for YBCO tapes will be very useful for high-current, high-field magnets for various applications.