Review of high power density superconducting generators: Present state and prospects for incorporating YBCO windings

With the first tokamak designed for full nuclear operation now well into final assembly (ITER), and a major new research tokamak starting commissioning (JT60SA), nuclear fusion is becoming a mainstream potential energy source for the future. A critical part of the viability of magnetic confinement for fusion is superconductor technology. The experience gained and lessons learned in the application of this technology to ITER and JT60SA, together with new and improved superconducting materials, is opening multiple routes to commercial fusion reactors. The objective of this roadmap is, through a series of short articles, to outline some of these routes and the materials/technologies that go with them.

Superconductors for fusion: A roadmap

The unique features of HTS offer opportunities and challenges to a number of applications. In this paper we focus on NMR and MRI magnets and present a brief description of some of the NMR/MRI magnet programs at FBML that are currently on-going and expected to begin in the near future, specifically a 1 GHz NMR magnet comprising an LTS magnet and an HTS insert wound with BSCCO-2223 tape, an NMR magnet assembled from YBCO annuli, and an MRI “demonstration” magnet wound with MgB2 wire. Two conclusions are presented for HTS as applied to NMR/MRI magnets: (1) impact of HTS on NMR/MRI magnet technology is great, because HTS offers many opportunities and challenges, some of which are described here; and (2) impact of NMR/MRI magnets on HTS manufacturers is mixed—for high-field NMR magnet, it is small because the required amount of HTS in these magnets is negligible compared with that in electric power applications; for “low-field” NMR magnets, it may be potentially large if MgB2 replaces Nb–Ti and Nb3Sn; and for MRI magnets, it is potentially huge if MgB2 replaces Nb–Ti and Nb3Sn.

HTS and NMR/MRI magnets : Unique features, opportunities, and challenges

The quench behavior of conduction-cooled Y Ba2Cu3O7?? (YBCO) coated conductor pancake coils is reported. Two coils, one stabilized with copper and one with brass, are wound with 25?m of conductor and instrumented with a heater and a large number of voltage taps and thermocouples. The critical current, minimum quench energy (MQE) and two-dimensional normal zone propagation velocity (NZPV) are measured as a function of I/Ic, where I is the transport current and Ic is the critical current. Although the non-uniform temperature and self-field distributions within the coils result in a non-uniform Ic, the heater is able to induce quenches with energies above the MQE and both longitudinal and transverse propagation velocities are measured. In both coils, the longitudinal NZPV (10?40?mm?s ? 1) is about one order of magnitude larger than the transverse NZPV (1?2?mm?s ? 1). Moreover, a comparison between the Cu-stabilized coil and a short, straight Cu-stabilized sample shows that the one-dimensional longitudinal propagation in the short sample is significantly faster than the longitudinal propagation in the coil. This is due to transverse heat conduction (transverse propagation) which reduces the temperature gradients in the coil but also slows down the longitudinal propagation. Thus, designing a quench detection system based upon data from one-dimensional experiments may result in an unintended level of risk.

http://iopscience.iop.org/article/10.1088/0953-2048/23/6/065021/pdf

Quench behavior of conduction-cooled Y Ba2Cu3O7−δ coated conductor pancake coils stabilized with brass or copper

YBa2Cu3O7-δ coated conductors have very slow normal-zone propagation velocity, which renders quench detection and protection very difficult. To develop effective quench detection methods, it is paramount to study the underlying behavior that drives quench propagation at the micrometer-scale level. Toward this end, numerical mixed-dimensional models, composed of multiple high-aspect-ratio thin layers, are developed. The high-aspect-ratio modeling issues are tackled by approximating the thin layers either as a 2-D surface or as an analytical contact resistance interior boundary condition, which also acts as a coupling bridge between the 2-D and 3-D behaviors. The tape models take into account the thermal and electrical physics of each layer in actual conductor dimensions and are implemented using commercial finite-element analysis software. In the first part of this two-part paper, the mixed-dimensional models are introduced and then computationally and experimentally validated. Validations are gauged by comparisons in normal-zone propagation velocity and in the time-dependent voltage and temperature profiles. Results show that the mixed-dimensional models can not only effectively address the high-aspect-ratio modeling issues of thin films but also accurately and efficiently reproduce physical quench phenomena in a coated conductor.

Three-Dimensional Micrometer-Scale Modeling of Quenching in High-Aspect-Ratio $\hbox{YBa}_{2}\hbox{Cu}_{3}\hbox{O}_{7 - \delta}$ Coated Conductor Tapes—Part I: Model Development and Validation

The paper presents results, experiment and simulation, of quench/recovery study of coated YBCO "composite" test samples, cooled by boiling liquid nitrogen and subjected to an over-current pulse. To operate stably and be protected from damage under adverse operating conditions of a real device, the YBCO tape must be a "composite", loaded with normal metal of a sufficient thickness that significantly increases the thickness of the original YBCO tape.

Stability and quench protection of coated YBCO "Composite" tape

In this study of stability of YBCO composite, we investigate quench/recovery behavior of YBCO test samples, bare and copper-laminated, by subjecting each test sample, immersed in a bath of liquid nitrogen boiling at 77.3 K, to a transport current pulse superimposed to a baseline DC current of 90-95% the critical current. The current pulse has an amplitude up to /spl sim/4.5 times the critical current and a duration of 300 ms. This paper presents both experimental and simulation results showing that composite YBCO tape that incorporates a copper lamina stabilizes and to a degree protects the conductor against large over-current pulses.

Stability of bare and copper-laminated YBCO samples: experimental & simulation results

In this paper we report results of two series of experiments, one performed on a double-pancake coil wound with Bi2223/Ag tape and the other on coated YBCO samples to demonstrate that an acoustic emission (AE) technique is a promising diagnostic tool to detect the presence of a "hot spot" in HTS devices. The AE technique may be used as complement to standard voltage technique to facilitate early detection of the presence of a "hot spot" in HTS devices and thus help to protect the HTS devices from permanent damage.

Detection of 'hot spots' in HTS coils and test samples with acoustic emission signals

In this paper we report thermal and magnetic responses of a magnetized YBCO disk cooled by a cryogenic design/operation concept that incorporates solid nitrogen in the system. The YBCO disk was "field-cooled" at 88 K and then operated in the range 8-60 K. During each cooling sequence, the system is thermally coupled to a cryocooler; then thermally decoupled from the cryocooler at 8 K and left decoupled until it reaches 60 K. As an application of this cryogenic design/operation concept, we describe a 0.5-T/80 cm bore magnet wound with MgB/sub 2/ presently being developed at FBML.

Thermal and magnetic responses of a solid nitrogen/magnetized YBCO disk system undergoing temperature cycles in the range of 8-60 K

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Papers

Stability and quench protection of coated YBCO "Composite" tape

Stability of bare and copper-laminated YBCO samples: experimental & simulation results

Detection of 'hot spots' in HTS coils and test samples with acoustic emission signals

Thermal and magnetic responses of a solid nitrogen/magnetized YBCO disk system undergoing temperature cycles in the range of 8-60 K