# CHATS on Applied Superconductivity 2017

Asia/Tokyo
Institute for Materials Research, Tohoku University

#### Institute for Materials Research, Tohoku University

2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan
Description

### News:

• 2017/07/16   The Accomodation page was updated.
• 2017/07/19   The Registration page was updated.
• 2017/07/21   The deadline for abstract submission was extended until 4th August
• 2017/10/12   The Local Information page was updated.
• 2017/11/27   Information of the paper submission was updated.
• 2017/12/6     Time table was updated
• 2017/12/16   The paper submission due date was extended to ** Jan. 31  2018 **
• 2018/2/2  The paper submission due date was extended to ** Feb. 16 2018 **

### Objective and topics:

CHATS-AS 2017 will be the twelfth in a series of international workshops that have followed the advancements in the state-of-the-art of modeling of superconducting systems, from cables to magnets, power generation and transmission devices, fusion, high-energy physics, and other applications.

The focus in CHATS-AS is on novel principles and tools in engineering design and analysis of superconducting systems. The Workshop also aims at building bridges between analysis and experiments, in order to validate the sophisticated analysis tools available with proper data sets.

The CHATS-AS 2017 Workshop welcomes contributions fitting this scope, with a specific interest in the following topics:

• Quench simulation and analysis for all classes of LTS and HTS magnets (e.g. accelerator, fusion, MRI/NMR and other applications) and devices (e.g. power cables).
• Quench detection and protection of HTS magnets: novel concepts, models and experiments.
• Electro-mechanical modeling of Nb3Sn and HTS magnets. Engineering tools for performance extrapolation in the 20 T range.
• Multi-scale modeling of cables and magnets made of LTS and HTS materials, and their relation to the strand or short-sample performance.
• Simulation and optimization of large-scale superconducting and cryogenic systems, covering the combination of thermo-hydraulics, AC loss, current distribution, field quality, and operating margin calculations.
• Modeling in support of the design and development of cryocoolers, and conduction-cooled superconducting devices.

This workshop is co-sponsored by Institute for Material Research, Tohoku University.

### Important dates:

 May 29, 2017 Abstract submission page open August 4, 2017 Deadline for abstract submission August 11, 2017 Notification of abstract acceptance November 10, 2017 Registration deadline February  16, 2018 Close for manuscript submission

Date Masamune, famous samurai

Sendai Pageant of Starlight

Registration
Registration
Contact
• Sunday, 10 December
• 09:30 10:20
Registration Auditorium (Institute for Materials Research)

### Auditorium

#### Institute for Materials Research

• 10:20 10:30
Opening welcome Auditorium (Institute for Materials Research)

### Auditorium

#### Institute for Materials Research

Convener: Dr Daisuke Miyagi (Tohoku University)
• 10:30 12:30
Session I: HTS magnet design and mechanics Auditorium (Institute for Materials Research)

### Auditorium

#### Institute for Materials Research

Convener: Prof. Zhenan Jiang (Victoria University of Wellington)
• 10:30
Basic development studies for mineral insulated superconducting magnet using REBCO coated conductors. 30m

The future particle physics experiment will require a 20 T-class high field magnet operated under radiation environment more than 200 MGy. High Energy Accelerator Research Organization (KEK) has been performing basic development studies of the radiation-resistant high field magnet using REBCO coted conductors. A goal of this research program is an experimental proof of the small-scale magnet system constructed in mineral insulated indirect cooling coils and radiation-resistant refrigerator to establish technology for a radiation-resistant high field magnet. Trial surface treatment with ceramics to commercial REBCO coated conductors and magnet materials, neutron irradiation of the REBCO conductors, and trial manufacture of the radiation-resistant pulse tube cryocooler are currently in progress. In this contribution, development status of the radiation-resistant superconducting magnet and results of the irradiation test of REBCO conductors will be presented.

Speaker: Masami Iio (High Energy Accelerator Research Organization (KEK))
• 11:00
Forced Flow Cooling of High Field, REBCO-Based, Fusion Magnets Using Supercritical Hydrogen and Helium 30m

We are studying the use of REBCO High Temperature Superconductor (HTS) tape as the superconductor for the Toroidal Field (TF) magnets for a highly compact, high field fusion reactor. Very high operating currents are desirable for use in this application due to the large volumes and very high stored magnetic energy. We envision using stacked tapes of REBCO cooled by forced flow of cryogenic fluid to maintain stable operation at magnetic fields as high as 21 T and at operating temperatures in the range of 20-30 K. The main source of heating in the conductors is from neutron radiation emitted by the burning plasma. The highest heat flux is in the coil turns closest to the plasma and falls off exponentially radially through the winding pack. Several cryogenic fluids are being considered for the magnet cooling including hydrogen, helium, and neon. We describe advantages and disadvantages of each fluid, and give results of thermal-fluid analysis and computations to determine the performance limits of a conceptual design for a high field TF magnet system.

Speaker: Dr Joseph Minervini (Massachusetts Institute of Technology)
• 11:30
Electromagnetic and thermal modeling of small coil made from HTS round cable 30m

Placing of tapes in helical manner on a round core represents one of possible ways to accomplish high-current cables from the 2nd generation of high-temperature conductors. We follow the concept similar to that of the Conductor On Round Core (CORC) however with a metallic tube utilized as the core allowing a coolant to flow through the central channel of such Conductor On Round Tube (CORT).
We present the results of experiments and modeling of the coil with 140 mm inner diameter wound from the hand-made CORT 4 meters long. This cable was used to wind the coil with 8 turns on 14 cm diameter. First the coil was tested in liquid nitrogen bath. Afterwards the layers of commercial aerogel and polyurethane foam were applied to provide a vacuum-less thermal insulation and a simple circuit arranged for its cooling by the flow of 77.3 K cold liquid nitrogen at DC tests.
Electromagnetic modeling based on the Jc(B, ) data has been performed assuming two limiting cases of current sharing between tapes. Comparison with experimental data obtained in nitrogen bath indicated that the tapes did not experience any damage during cabling nor coil winding, and also pointed to certain amount of DC current migration between tapes in the cable.
Numerical modeling for the regime of flow cooling revealed an imperfection in cable manufacturing: in order to reach the temperature distribution observed in the experiment we had to introduce an additional thermal resistance between the copper tube and the tapes. This indicates that either the surfaces of tube and tapes were not clean enough or that the tapes did not adhere perfectly to the cylindrical surface of cooling tube.

Speaker: Fedor Gömöry (Slovak Academy of Sciences)
• 12:00
High-field magnets wound from CORC® cables or wires 30m

Advanced Conductor Technologies has been developing Conductor on Round Core (CORC®) cables and wires wound from REBCO coated conductors for use in high-field magnets. An overview of the current status of the CORC® cables and wires will be presented, including details about the application of CORC® cables and wires in different types of magnets. Large magnets for fusion require multiple CORC® cables to be bundled into a 6-around-1 cable in conduit configuration (CICC) where large transverse stresses act on the cables. The cable design and test of a 80 kA CORC®-CICC in SULTAN will be discussed.
Four programs are currently ongoing in which CORC® wires will be used to develop high-field insert magnets that will be tested in various background fields within the near future. The first program is developing CORC® wires for canted-cosine-theta (CCT) accelerator magnets that ultimately will generate 5 T in a 15 T background field. The second program is focused on developing a CORC® wire insert solenoid that would about 3 T in a 14 T background field and is scheduled for completion in early 2018. The third program aims to develop CORC® wire racetrack coils that would be tested in FRESCA-2 at CERN. Finally, a fourth program is developing a common coil magnets wound from CORC® cables. We’ll provide an overview of the status of each of these magnet programs, starting with modeling of the magnet layout, stresses and performance, their conductor winding, and of the magnet construction and test preparation.

Speaker: Danko van der Laan (Advanced Conductor Technologies)
• 12:30 14:00
Lunch 1h 30m
• 14:00 15:30
Session II: HTS elecromagnetics and HTS quench Auditorium (Institute for Materials Research)

### Auditorium

#### Institute for Materials Research

Convener: Fedor Gömöry (Slovak Academy of Sciences)
• 14:00
Calculations and measurements of electromagnetic characteristics of striated and copper-plated coated conductors and their coils 30m

The striation of coated conductors is attracting interests as a means of the reduction of shielding-current-induced fields (SCIFs) in magnets made with coated conductors. We focus on striated and copper-plated coated conductors, in which plated copper allows current sharing between filaments. We characterized their electromagnetic characteristics at three steps, and, through these steps, we validated our numerical model by comparison between calculations and measurements.
At the first step, we measured and calculated coupling time constants of short pieces of the conductors with various lengths and their stacks and determined the transverse resistance between filaments.
At the second step, single pancake coils wound with striated and not-striated coated conductors were exposed cusp magnetic fields, and we compared the measured and calculated SCIFs in the pancake coils wound with the two types of coated conductors.
At the third step, we compared the decay of SCIFs in a layer-wound solenoid coil and a stack of pancake coils.
Through these studies, we the validity of our numerical model was confirmed and examined the effect of striation to reduce SCIFs.

Speaker: Naoyuki Amemiya (Kyoto University)
• 14:30
Numerical simulation on current distribution in multifilamentary HTS tape 30m

The research and development on the applications of REBCO superconducting coil to the high field magnets for NMR, MRI, accelerator and so on are in progress. In these application, the magnets need to create highly homogeneous and temporally stable field. However, the screening currents lead to the serious problem in REBCO magnets for the applications required very high field quality. Then, multifilamentary REBCO tapes are expected to reduce the screening current induced magnetic field in the REBCO coil. The multifilamentary REBCO tape is plated the copper for mechanical strength and thermal stability. And, the coupling currents are induced between filaments in REBCO tape during excitation. In REBCO coil using copper-plated mutifilamentary REBCO tape, the transverse resistivity between filaments in REBCO tape is important factor for thinking about the behavior of magnetic field and current distribution. In previous study, we developed three-dimensional numerical simulation code using finite element method (FEM) and fast multipole method (FMM) to calculate the spatial and temporal behavior of screening current distribution in REBCO coil. In this study, we focused on the transverse resistivity between filaments in REBCO tape and investigated the current distribution in the mutifilamentary REBCO tape by using our developed simulation code. We discuss on the effects of transverse resistivity of mutifilamentary REBCO tape on the diffusion process of screening current and the spatial and temporal behavior of the magnetic field.

Speaker: Hiroshi Ueda (Okayama University)
• 15:00
Design study on the quench protection system for the HTS magnet based on the I-V characteristics measured in the temperature range of 40-83K. 30m

High $T_c$ superconductor (HTS) is one of the candidates to evolve the magnet technology as it is superior to low $T_c$ superconductor (LTS) in terms of the critical current density, critical magnetic field, and critical temperature. Replacement of the LTS magnet with the HTS one is however still challenging because it is quite difficult to predict the quench behavior due to its slow normal zone propagation velocity. In order to collect data so as to design the protection scheme of the HTS-based magnet, we conducted measurements of the current-voltage (IV) curves with a short sample of the REBCO tape at the International Research Center for Nuclear Material Science of Tohoku University. In this experiment, the sample is cooled by means of the conduction-cooling method with a variable temperature insert, and exposed to the magnetic field that the superconducting solenoid generates. In this way, the IV characteristics are measured in the temperature range of 40-83K at various applied fields up to 15T. All the data obtained in this measurement are then implemented to a stand-alone quench simulation to estimate conditions in which the HTS magnet quenches. In this report, we present the quench protection system that is designed using the simulation tool described above.

Speaker: Kento Suzuki (High Energy Accelerator Research Organization)
• 15:30 15:50
Coffee Break 20m
• 15:50 17:50
Session II: HTS electromagnetics and HTS quench Auditorium (Institute for Materials Research)

### Auditorium

#### Institute for Materials Research

Convener: Fedor Gömöry (Slovak Academy of Sciences)
• 15:50
Applicability evaluation of quench detectable hybrid REBCO tape to HTS magnet for fusion reactors 30m

High-temperature superconducting (HTS) magnet is an attractive option for fusion reactors, especially, the LHD-type helical fusion reactor FFHR-d1, which can employ segment-fabrication of huge and complex helical coils. We have proposed both joint-winding of the HTS helical coils wound by connecting half or one-pitch conductor segments and remountable HTS helical coils assembled from demountable coil segments, as primary and advanced options, respectively. For these designs, mechanical joint (demountable joint) of HTS conductors have been developed and an applied current of 100 kA and a joint resistance of ~2 nano-ohms at 20 K, 5.3 T were achieved. The critical current almost agreed with predicted value even with the joint region. Though the cryogenic stability of the HTS conductor is sufficiently high with its high critical temperature and massive copper stabilizer, protection of the magnet system should be well prepared. We are now investigating a new method to detect quench for HTS applications; a quench detectable hybrid REBCO tape with a superconducting wire for quench detection. In this presentation, we report numerical and experimental evaluations of NbTi/YBCO hybrid tape fabricated as a proof-of-principle testing. In addition, we also examine which superconducting material is suitable as a sensor depending on operating conditions and how the quench detection given by this method affects the expected hotspot temperature in the HTS conductor for FFHR-d1.

Speaker: Dr Satoshi Ito (Tohoku University)
• 16:20
Quench Protection for the HTS Magnet in the Helical Fusion Reactor 30m

Design activities of the LHD-type helical fusion reactor FFHR-d1 and -c1 are progressing at NIFS. The high-temperature superconductor (HTS) is a promising option for the helical coil conductors. In addition to high cryogenic stability and low refrigeration power at elevated temperature operations at >20 K, it is proposed that the helical coils of large-diameter and complex-shape be constructed by connecting conductor segments using the advantage of HTS. A proto-type large-current capacity HTS conductor sample was fabricated and achieved 100 kA at 20 K, 5.3 T.
In our previous studies, the hot-spot temperature, defined as the maximum temperature reached during a normal-transition and a subsequent emergency current discharge, has been calculated by a one-dimensional finite element method (FEM). The thermal diffusion along the longitudinal direction of the conductor is the governing equation, while the percolation model is used to describe the superconducting characteristics of the ReBCO HTS tapes. The obtained hot-spot temperatures are 205 K and 456 K at the current density of the helical coils of 25 and 40 A/mm2, respectively, corresponding to the FFHR-d1 and c1 designs. In the case of FFHR-c1, the temperature significantly exceeds the allowable limit. In the present study, the feasibility of applying the non-insulation coil winding concept is examined by numerical calculations, as an alternative and efficient quench protection method, which has been intensively studied for small HTS coil applications. The trade-off between the current and heat transferring capability among conductors and limitation of excitation time should be discussed.

Speaker: Dr Yoshiro Terazaki (National Institute for Fusion Science)
• 16:50
Quench behaviors of conduction-cooled coated conductors subject to transient and localized thermal disturbances 30m

If we use coated conductors for accelerator magnets, they might be subject to the transient and localized thermal disturbances caused by beam losses. There are various types of beam losses, but, here, we focus on the direct injections of particle beams to superconducting coils as the most severe event. We are studying the responses of coated conductors against such disturbances.
First, we study the modes of thermal disturbances caused by beam injections by using PHITS (Particle and Heavy Ion Transport code System), which was a software developed at JAEA. The compositions of coil, the energy of injected beam, the angle of the beam injection etc. are varied, and the magnitudes and the spatial distributions of deposited energy are studied.
Then, we carry out quench experiments using short pieces of coated conductors. The 20 cm-sample is cooled to 15 – 40 K by using a GM cryocooler. The magnetic field up to 5 T can be applied, and the sample current up to 500 A can be supplied. After initiating quench by a small heater, the voltage near the heater and that across the entire sample are measured. The sample current is shut down after the entire voltage reaches a threshold value (50 mV, for example). This simulates the quench detection and protection. The hot spot temperature is estimated from the voltage near the heater and the temperature dependence of critical current as well as that of the resistance of copper stabilizer. After each quench experiment, the critical current of the sample is measured to see whether the sample degrades or not. The experimental results are compared with the numerical results by a quench simulation code. Using such experimental and numerical results, we discuss the conditions (current, detection time, shutting down time, etc.) required to protect magnets.

This work is supported by the Japan Science and Technology Agency under the Strategic Promotion of Innovative Research and Development Program (S-Innovation Program).

Speaker: Xijie Luo (Kyoto University)
• Monday, 11 December
• 09:00 10:30
Session III: LTS magnet and cryogenics Auditorium (Institute for Materials Research)

### Auditorium

#### Institute for Materials Research

Convener: Reinhard Heller (Karlsruhe Institute of Technology)
• 09:00
Transient Heat Transfer Through the LHC Polyimide Cable Insulation 30m

During the operation of the LHC, the superconducting magnets are exposed to steady-state and transient heat loads. Knowledge of the thermal link between the superconductor and the He bath is essential to evaluate the stability of the superconductor. This work reports experimental data on the transient heat transfer between a stack of MB Rutherford superconducting cables and a He bath in saturated and pressurized conditions. The sample is prepared from machine insulated superconducting cable, instrumented with a Cernox temperature probe. The heat transfer is studied in the temperature range $1.7 {\ } \textrm{K}$ to $2.1 {\ }\textrm{K}$ and a deposited power range in $0.5 {\ }\textrm{mW}/\textrm{cm}^3$ to $5 {\ } \textrm{mW}/\textrm{cm}^3$.
From the experimental data, two parameters are extracted and are discussed. The first parameter is the steady-state temperature difference and the second parameter is the characteristic time for the temperature difference to reach steady-state. From the two parameters conclusions are drawn on the void volume in the sample and on the dominant cooling paths through the polyimide insulation.

Speaker: Tiemo Winkler (UTwente)
• 09:30
Development and test of a dynamic model of the ITER CSMC He refrigerator. Part II: from 80 K to 4 K 30m

The superconducting (SC) magnets used to confine the plasma in nuclear fusion reactors must be first cooled from 300 K down to ~4-5 K, then kept sufficiently below the current sharing temperature during the tokamak pulsed operation, in order to preserve their SC properties. For that purpose, a He refrigerator is used. During the cooldown, it provides the boundary conditions (He temperature THe and mass flow rate dm/dt) at the coil inlet; during operation, the pulsed heat load coming from the magnets must be suitably removed by the refrigerator and smoothed to avoid operation instabilities.
In order to develop an adequate computational model for both scenarios, we consider here as reference refrigerator configuration that of the ITER CSMC facility, at Naka, Japan, used for 15+ years to test the performance of full-scale ITER cable-in-conduit conductors in conditions relevant for real tokamak operation. The Collins cycle operated by the CSMC refrigerator features two stages: the first one, assisted by LN2 precooling, from 300 K to 80 K, the second, based on two isentropic expansions in turbines and an isenthalpic J-T valve, from 80 K to 4 K.
After the recent development of a model for the 300 K --> 80 K stage, here we present a model of the CSMC refrigerator aimed at the 80 K --> 4 K stage. The simulated THe, pressure and dm/dt at relevant locations, as well as controller outputs and valve openings, during a CSMC cooldown turn out to be in good agreement with the measurements.

Speaker: Dr Roberto Bonifetto (Politecnico di Torino)
• 10:00
Improved hydraulic model of cryogenic circuits for fusion magnet application 30m

On the purpose to upgrade the code package of SUPERMAGNET-based hydraulic model, coupled 1-d flows at the multifunction branch is carefully explored not only in the context of hyperbolic PDEs, but also about the numerical stability in coupled solver models of co-simulation method. Comparing with the conventional method in other competitive tools like the ThermoPower package with Openmodelica, a minimalist implementation of momentum-carrying nodal element is introduced with respect to the accuracy issue in the mixing volume under pressurization and momentum transfer. As a remedy for the instability of such a physically-tight flow, an inventive boundary scheme is followed for the new element by means of characteristic decomposition in virtue of the well-known approach for hyperbolic systems. Discussing the stability issue of example models, conformity of the hydraulic systems to the general numerical framework is also investigated as an attempt to improve the coupled solver system of extensive hydraulic modeling using the SUPERMAGNET code package.

Speaker: Dr DONGKEUN OH (National Fusion Research Institute)
• 10:30 10:50
Coffee break 20m
• 10:50 11:50
Session III: LTS magnet and cryogenics Auditorium (Institute for Materials Research)

### Auditorium

#### Institute for Materials Research

Convener: Reinhard Heller (Karlsruhe Institute of Technology)
• 10:50
Thermal-hydraulic analysis of low temperature superconducting CFETR Central Solenoid Model Coil 30m

The China Fusion Engineering Test Reactor (CFETR) is the next device in the roadmap for the realization of fusion energy in China, aiming at bridging the gap between the fusion experimental reactor ITER and the demonstration reactor (DEMO).
CFETR will operate in two phases: steady state operation and self-sufficiency will be the two key issues for Phase I, with a fusion power of up to 200 MW, while Phase II will aim at DEMO validation with a fusion power over 1 GW. For saving the cost of construction and meeting both Phase I and Phase II targets with achievable technical solutions, a new design has been made by choosing a larger machine with R = 6.6 m, a = 1.8 m, BT = 6-7 T. Over 1 GW fusion power can be achieved technically and it is easy to go from Phase I to Phase II with the same machine. In order to obtain the maximum magnetic flux of 224 Wb from the CS coil in Phase II, the use of high temperature superconductors (HTS) made of Bi2212 is envisaged.
In order to test the manufacturing and performance of the superconducting coils for the CFETR in relevant operating conditions, a Central Solenoid Model Coil (CSMC) is being designed, as already done for, e.g., ITER, operating at the nominal current of ~48 kA and at the peak magnetic field of ~12 T. It will be composed by two pancake-wound sub-coils: the inner one, in turn divided into inner and outer module, adopting Nb3Sn Cable-in-Conduit Conductors (CICCs), and the outer one, built assembling three NbTi modules in the vertical direction. All the CICCs are cooled by supercritical He and feature two hydraulic paths, namely the bundle and a central pressure relief channel delimited by a spiral.
In this paper, the 4C code is employed to analyze the thermal-hydraulic behavior of the CFETR CSMC in nominal operating conditions, including the effects of the inter-turn and inter-pancake thermal-coupling across the insulation layers. Moreover, the temperature margin of the different CSMC modules during the reference current scenario is computed adopting the design values for the stands and conductor parameters.

Speaker: Dr Roberto Bonifetto (Politecnico di Torino)
• 11:20
Analytical Method for the Prediction of Quench Initiation and Development in Accelerator Magnets 30m

The optimal design of the next generation of accelerator magnets calls
for a high current density in the superconducting coil, which makes
the magnet protection a challenge. Quenches in the high-field magnets
for the High Luminosity LHC Upgrade typically develop within tens of
ms, and the reaction time needs to be comparable, requiring active
firing of heaters or other heat deposition techniques to increase the
quench propagation velocity in the magnet. It is important to have a
very good understanding of the behavior of a magnet during a quench.
Practical scaling laws, and simplified methods, allow quick scans of
design and operation parameters, and swift feedback based on
experimental results once the magnet is in test.

In this paper we describe simplified methods to predict the quench
initiation and development in accelerator magnets using active quench
protection. We use data from the recent Nb3Sn R&D and model magnets
for the High-Luminosity LHC as a benchmark for the method, discussing
expected accuracy and the reasons for deviations.

Speaker: Susana Izquierdo Bermudez (CERN)
• 11:50 12:20
Session IV: LTS cable design Auditorium (Institute for Materials Research)

### Auditorium

#### Institute for Materials Research

• 11:50
Development of design for large scale conductors and coils using MgB2 for Superconducting Magnetic Energy Storage Device. 30m

MgB2 wires have been commercially available and developed its superconducting characteristics continuously in the last decade. This attracts researchers especially in the field of superconducting magnetic energy storage (SMES) coil applications in terms of its relatively high critical temperature, which enables us to use liquid hydrogen for cooling the coils. The MgB2 wires are sensitive to bending strain like Nb3Sn both before/ after heat treatment to produce MgB2 material inside the wires. Due to the sensitivity, to design the large scale conductors and coils for SMES system, the careful investigations of applied strains during manufacturing process are absolutely needed. Regarding the production technique for superconducting coils, the two methods are generally introduced, which are wind-and-react (W&R) and react-and-wind (R&W). The W&R method is that the conductor fabrication by which the wires are twisted with or without mandrel and coil winding process are performed before heat treatment, while the R&W method is that the coils are wound with heat-treated wires and conductors. The fabrication for large-scale conductors and coils have been reported by a few groups, however, the suitable design parameters such as conductor dimension, twist pitch, appropriate margin of bending strain, etc. for getting better performance of the device have not been cleared.
The aim of our investigation is to show the design parameters for conductor and coil fabrication with the strain below the acceptable level and demonstrate the validity of the design through the several tests using test coils cooled by refrigerator.

Speaker: TSUYOSHI YAGAI (Sophia University)
• 12:20 13:50
Lunch 1h 30m
• 13:50 14:20
Session IV: LTS cable design Auditorium (Institute for Materials Research)

### Auditorium

#### Institute for Materials Research

• 13:50
Calculation of interstrand coupling losses in superconducting Rutherford cables with a continuum model 30m

Rutherford cables for particle accelerator magnets can be subjected to time-varying magnetic fields during the typical accelerator operating cycle, which in turn induce coupling currents flowing in the loops formed at the contacts between different strands. A proper analysis of the losses generated by these currents must be carried out for the design of the cryogenic system.
The models reported in the literature exhibit some limits related to computational burden when applied to the analysis of interstrand coupling currents for real cable geometries. To solve this problem, a continuum model with non-uniform contact conductances between the strands was developed. The model allows one to attain the required level of detail in the description of the interstrand currents. The model is validated by comparison with analytical results available in the literature for simplified case studies with uniform magnetic field applied orthogonal to the cable. Examples of current and loss distributions are presented in the paper, to prove the potential of the model to analyze Rutherford cables of any configuration. Finally, a study regarding the proper choice of the boundary conditions of the problem is presented, which opens the path towards new experimental investigations on the actual current distribution and losses in real long Rutherford cables for accelerator magnets.

Speaker: Dr Marco Breschi (University of Bologna)
• 14:20 14:40
Coffee break 20m
• 14:40 16:10
Lab tour 1h 30m
• 19:00 21:00
Social Dinner 2h Hana

#### Hana

• Tuesday, 12 December
• 09:00 10:30
Session V: Thermo-hydraulics and cryogenics Auditorium (Institute for Materials Research)

### Auditorium

#### Institute for Materials Research

Convener: Prof. Arend Nijhuis (University of Twente)
• 09:00
Basic Study on Effect of a Heat Pipe on Cooling Characteristic Improvement in a Tri-axial HTS Cable 30m

The high Tc superconducting (HTS) tri-axial cable fits long-distance power transmission rather than the 3-in-One superconducting cable, since the tri-axial HTS cable has low ac loss and large cooling channel, and the tape length used for a cable is much shorter. We investigated a suitable cable structure of HTS tri-axial cable with counter cooling by liquid nitrogen for long-distance power transmission using the numerical analysis which considered the heat transfer from the cable outside and heat generation caused by to AC loss inside the cable. However, based on our numerical results, the length of the tri-axial cable can be operated in one cooling station is not sufficient for the submarine cable such as a power facilities in the offshore wind power system.
In this study, we proposed a new cooling method in order to achieve the further longer distance of the HTS cable. We analyzed the thermal and fluid characteristics of the tri-axial HTS cable using heat pipe to investigate the application effect of the heat pipe and the design guidelines of the tri-axial HTS cable using the heat pipe suitable for long distance cable.

Speaker: Dr Daisuke Miyagi (Tohoku University)
• 09:30
Thermal-hydraulic analysis of the HTS DEMO TF coil 30m

Abstract
The new HTS design concept for the DEMO toroidal field (TF) coil, based on cross-conductor (CroCo) strands, has been proposed by KIT. The coil is layer wound and consists of 12 layers with different jacket radial thickness grades. The present work is focused on the thermal-hydraulic analysis of the conductors’ design, which includes: 1) hydraulic analysis—calculation of the mass flow rates in each conductor at operating conditions during the dwell time; 2) heat removal analysis aimed at the assessment of the temperature margin at the expected nuclear heat load during the plasma burn; and 3) estimation of the maximum hot spot temperature and pressure in each conductor during quench. The analysis is performed using simplified models and the THEA code, and is aimed at verification if the proposed design fulfills the acceptance criteria.
Acknowledgement
This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014-2018 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission. This scientific work was also partly supported by Polish Ministry of Science and Higher Education within the framework of the scientific financial resources in the year 2017 allocated for the realization of the international co-financed project.

Speaker: Monika Lewandowska (West Pomeranian University of Technology, Szczecin)
• 10:00
Assessment of the performance of a 20 kA REBCO current lead 30m

Most of the large fusion devices like W7-X, JT-60SA and ITER use or will use High Temperature Superconductor (HTS) current leads (CL). In all cases the HTS material Bi-2223 embedded in a AgAu matrix is used. In the meantime, laboratories worldwide are investigating the design of a high current HTS CL composed of REBCO. The Karlsruhe Institute of Technology (KIT) has developed a 20 kA HTS current lead based on the JT-60SA CL design but replacing the Bi-2223 tapes by brass-stabilized REBCO tapes, poloidally distributing 24 REBCO stacks on a stainless steel support. The REBCO CL has been recently tested in the CuLTKa facility at KIT to investigate the steady state and transient performance in comparison to the Bi-2223 CL and to demonstrate the applicability of REBCO in CL for future devices.
The thermal-hydraulic CURLEAD code developed at KIT, recently updated in a collaboration between KIT and Politecnico di Torino and validated in both steady state and transient (pulsed) operation, was applied predictively, i.e. before the tests were performed, to support the test strategy of the REBCO CL. For instance, the prediction of the optimal HX mass flow rate to get to the desired target temperature was used to define the set point for the steady state tests both with and without current, while transient simulations were used to define the number of current pulses needed to reach periodic operation conditions for given current waveforms. A preliminary comparison between predictions and experimental results will also be presented, with particular reference to the GHe mass flow rate, temperature profile, and heat load at the cold end.

Speaker: Dr Reinhard Heller (Karlsruhe Institute of Technology)
• 10:30 10:50
Coffee break 20m
• 10:50 12:20
Session VI: HTS magnet and HTS applications Auditorium (Institute for Materials Research)

### Auditorium

#### Institute for Materials Research

Convener: Prof. Marco Breschi (University of Bologna)
• 10:50
Three-dimensional electromagnetic field analysis model for ac loss calculations of HTS coils in superferric magnets 30m

In order to reduce the electricity consumption of rapid cycling synchrotrons, we are studying the feasibility of superferric magnets using high Tc superconductors (HTS). Because magnets for rapid cycling synchrotrons are required to generate time-dependent magnetic fields, the ac loss reduction of HTS coils as well as the magnetization loss reduction of iron yokes is one of the key issues. In this paper, we focus on the ac losses in HTS coils and report a model for electromagnetic field analysis model to calculate their ac losses.
The ac loss in the HTS coil in a superferric magnet is determined by the combination of the self magnetic field generated by the coil current itself and the externally-applied magnetic field generated by the magnetized iron yoke. In order to calculate the ac loss in the HTS coil, firstly, the external field (magnetic flux density) distribution in the HTS coil is calculated. We calculate it by substituting the magnetic field generated by the HTS coil only from the magnetic field generated by the HTS coil and the iron yoke. Each magnetic field is calculated by using OPERA (TOSCA) considering the nonlinear magnetic characteristic of the iron but not considering the nonlinear conducting characteristic of the superconductor, i.e. assuming the uniform current distribution in HTS tape. Secondly, the dependences of electric field – current density (E–J) characteristic of HTS tape on magnitude and orientation of magnetic flux density and temperature are formulated based on measured data. Finally, three-dimensional electromagnetic field analyses of the coil are carried out using the formulated E–J characteristic and the calculated external magnetic field distribution, and, then, ac losses are calculated from the temporal evolution of electromagnetic field. In order to reduce computational memory and computation time for three-dimensional analyses, we use the hierarchical matrices method.

This work was supported by the Ministry of Education, Culture, Sports, Science and Technology under the Innovative Nuclear Research and Development Program.

Speaker: Yusuke Sogabe (Kyoto University)
• 11:20
Simple analytical formulae to evaluate the irregular field in solenoid coils with high-temperature superconducting tape wires 30m

The irregular field due to the screening current in high-temperature superconducting (HTS) tapes can bring about fatal impacts on the spatial and temporal uniformity of the magnetic field in MRI and NMR magnets using HTS tape wires. Although numerical evaluation is necessary to deal with the irregular field, accurate numerical computation of the irregular field in HTS coils requires huge computation costs and highly specialized technique.

Here we propose a simple theoretical evaluation of the irregular field in solenoid coils using HTS tape wires. Such rough but simple evaluation of the irregular field is very useful, especially in the early stage of the magnet design.

We derive a direct relationship between the irregular field in a coil and the magnetization of the tape wires. Assuming the simple distribution of the magnetization of tape wires in a solenoid coil, we derive analytical formulae to evaluate the irregular field in HTS coils. We have confirmed that our simple formulae can roughly and qualitatively evaluate the irregular field by comparing the accurate numerical results. We also propose a scaling law for the irregular field associated with the dimensions of solenoid coils: the ratio B$_{\rm SC}$/B$_{\rm TC}$ of the screening-current-induced irregular field B$_{\rm SC}$ to the transport-current-induced magnetic field B$_{\rm TC}$ is inversely proportional to the coil size.

We thank stimulating discussion with M. Furuse and K. Kajikawa. This work is based on the results obtained from a project commissioned by the New Energy and Industrial Technology Development Organization (NEDO).

Speaker: Dr Yasunori Mawatari (National Institute of Advanced Industrial Science and Technology (AIST))
• 11:50
New Screening Current Simulations on REBCO Pancake Coils 30m

A screening current is one of serious problems for real applications of REBCO magnets. A screening current deteriorates a magnetic field generated by REBCO magnets, hence it is desired to simulate a screening current-induced magnetic field. Researchers have already proposed a few kinds of simulation methods. For example, a finite element method coupling with thin film approximation method (FEM+TAM) is a powerful method. It gives accurate results, but it takes a long computation time. A circular coil approximation method is very simple. Both the methods need a large-memory computer.
In this presentation, we will propose two different screening current simulation methods. One is very simple, and a large memory is unnecessary. The results are not good accurate, but they give enough solutions in a very short time.
Another proposed method takes into account a thickness effect. As mentioned above, the FEM+TAM produces accurate solutions, but the magnetic field parallel to REBCO tapes is ignored in the TAM. However, Norris said the magnetic field parallel to REBCO tapes strongly affects the current distribution in the REBCO layers. We call it the “thickness effect.” Therefore, we propose a new thin film approximation method to consider the thickness effect.
We will present a few simulation results together with measurement data, and discuss the validity of these screening current simulations.

This work was supported in part by JSPS KAKENHI Grant Number 15KK0192.

Speaker: So Noguchi (Hokkaido University)
• 12:20 13:20
Lunch 1h
• 13:20 15:20
Session VI: HTS magnet and HTS applications Auditorium (Institute for Materials Research)

### Auditorium

#### Institute for Materials Research

Convener: Prof. Marco Breschi (University of Bologna)
• 13:20
A pulse tube cryocooler driven by the superconducting linear compressor at 50 Hz 30m

A Stirling-type pulse tube cryocooler (PTC) with a superconducting linear compressor (SCLC) unit was developed and tested. The compressor was able to produce alternating mass flow and oscillating pressure at low temperature environment near 80 K, and transmitted expansion work efficiently to the cold-end of the pulse tube. In the previous research, the compression efficiency of the compressor made of copper coil turned out to be high. However, the superconducting linear compressor can improve the system efficiency by eliminating the Joule heating loss of the conventional coil. The first attempt of superconducting linear compressor did not attain the expected high efficiency. According to the analytical model results, it is evident that the AC losses excessively occur in the extra copper portion of the superconducting coil structure and these account for the large proportions of the total compressor losses. We, therefore, have focused on trying to reduce extensively the AC losses of the superconducting linear motor to utilize its potential merit. The superconducting coil was modified by replacing the extra copper portion with G10 structure to reduce the AC losses. The PTC system, being assisted by the more efficient linear compressor, generated more cooling power at 20 K than that of our previous system. This research paper reports the improvement process of the thermal performance of the entire PTC system in the aspect of compression efficiency of the SCLC unit.

Speaker: Mr Bokeum Kim (KAIST)
• 13:50
AC loss simulation in a HTS 3-phase 1 MVA transformer using H formulation 30m

One of critical issues for HTS transformer application is AC loss in the HTS windings. Accurate prediction of AC loss is therefore very important for the HTS transformer application. In this work, we present AC loss simulation results employing the H-formulation and homogenization method for a HTS 1 MVA 3-phase transformer demonstrated by Robinson Research Institute in New Zealand. The high voltage (HV) windings are composed of 24 double pancakes per phase wound with 4 mm-wide YBCO wire. Each double pancake coil has 38 ¼ turns. The low voltage (LV) winding for each phase consists of a 20 turn single-layer solenoid wound with 15/5 (15 strands of 5 mm width) Roebel cable. The numerical method was first verified by comparing numerical and experimental AC loss results in two coil assemblies composed of two and six double pancake coils. The numerical AC loss result for the transformer was compared with measured AC loss as well with the numerical result obtained using the minimum magnetic energy variation (MMEV) method. The disagreement between the numerical AC loss result in this work and experimental result as well as the numerical result using MMEV at the rated current is less than 20 %. The same numerical method can be applied to calculate AC loss in larger rating HTS transformers.

Speaker: Zhenan Jiang (Victoria University of Wellington)
• 14:20
Electromechanical analysis of REBCO pancake coils reinforced by an outer-shell structure 30m

We have developed the REBCO pancake coils reinforced by an outer-shell structure, which consists of outer/inner rings and a circular disc, so called “YOROI” structure. In this case, the circular disc reinforces the coil together with the outer ring. The stress distribution in the coil, the outer ring and the circular disc can be calculated analytically if the infinite cylinder structure is assumed. We made a REBCO coil reinforced by the outer-shell made of stainless steel and test in 11 T and 4.2 K. The double pancake REBCO coil is 250 mm in inner and 265 mm in outer diameters. Each pancake is reinforced by 5 mm thick outer and inner rings and 0.5 mm thick cover plates. The operation current was applied up to 360 A in a 11 T and 4.2 K. The maximum induced strain on the coil inner surface measured by strain gauges reached about 0.4% at the operation current of 360 A, which is slightly higher than the strain limit of the REBCO tape. However, no anomalous voltage appeared. The analytical calculation indicates that the maximum stress is reduced due to the reinforce structure. In addition, a thermal compressive stress is also induced from the outer-shell. The analytical calculation result is compared to the experimental result and the FEM calculation.

Speaker: Satoshi Awaji (Tohoku University)
• 14:50
Strand level modeling on AC loss, current distribution and quenching of CICC conductors 30m

Cable-In-Conduit Conductors (CICCs) for fusion magnets are subjected to fast changing magnetic fields during the plasma-operating scenario. In order to anticipate to the limitations of the conductors under the foreseen operating conditions, it is essential to have a quantitative assessment of the stability margin of the magnets. In the last decade ITER has launched a campaign for characterization of several types of NbTi and Nb3Sn CICCs comprising quench tests. The conductors are subjected to a singular sine-wave fast magnetic field pulse and relatively small amplitude with respect to the ITER plasma operation scenario. The Minimum Quench Energy (MQE) tests, performed in the SULTAN facility, were reproduced and analyzed using JackPot-ACDC, an electromagnetic-thermal model for CICCs, developed at the University of Twente and THEA (Thermal, Hydraulic and Electric Analysis of Superconducting Cables). The code JackPot-ACDC is used to model the conductor geometry and to study the electro-magnetic behavior on strand level. The experimental results are used to calibrate and benchmark the simulations.
The analysis of coupling loss and current distribution shows the impact of the magnetic field orientation on the rectangular shape of some recent CICC designs, focused on possible issues on the stability and performance of the conductors.
The results provide a good basis for further investigation of conductor stability and extrapolative scaling for different magnetic field pulses, with lower ramp rate and higher amplitude, more similar to the magnet operation scenario’s.

Speaker: Dr Arend Nijhuis (University of Twente)
• 15:20 16:00
Wrap up session Auditorium (Institute for Materials Research)

### Auditorium

#### Institute for Materials Research

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