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1 Development of Remountable High Temperature Superconducting Magnet Prof. Hidetoshi Hashizume Tohoku University

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2 1. Concept of remountable HTS magnet Magnet can be mounted and demounted iteratively. Magnet can be fabricated in short parts. It is easy to access inner structural components. Making the fabrication easy Making maintenance easy Remountable Superconducting Magnet High Temperature Superconductor HTS can be used in relatively high temperature. Specific heat: Large (c T 3 in cryogenic environment.) HTS is robust against heat generation at jointing parts. Long cable is NOT necessary. High Performance / Low Cost Cable Remountable HTS Magnet Spherical Tokamak FFHR

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3 2. Mechanical joint of HTS tape Mechanical joint of HTS cable must be developed. Development of remountable HTS magnet Just Jointed, Mechanically HTS Tape Butt Joint J Joint // ab-plane Current Flowing Direction Lap Joint J Joint // c-axis Current Flowing Direction HTS Tape Just Jointed, Mechanically The but joint method is suitable for remountable HTS magnet.

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4 3. Fundamental study on butt joint of HTS tape 3-1. Experiment of butt joint of HTS tape Current-Resistance Characteristic There exists dependence of joint resistance on current. Joint Resistance: 3.6 (at 60A) ResultExperimental Apparatus Voltage Tap BSCCO 2223 Tape Vinyl Chloride Board (I C : 67A, w: 4.0mm, t: 0.26mm) Joint Region Vinyl Chloride Board Contact force : Thermal contraction 30mm Jointing Resistance is almost inversely proportional to the contact area.

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5 3. Fundamental study on butt joint of HTS tape 3-2. Performance analysis in different filament location Composition of HTS tape Superconducting Filament Ag (Stabilizer) Ag Alloy Several tens of superconducting filament Ag (Stabilizer) + Cross Section Ag (Stabilizer) Superconducting Filament Ideal Case Real Case Same Different Ag Superconductor Contact Surface Filament Location in Butt Jointing Electromagnetic field analysis is performed to evaluate the influence of filament location BSCCO 2223 Tape

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6 3. Fundamental study on butt joint of HTS tape 3-2. Performance analysis in different filament location Analytical model and numerical scheme Ag Gap Superconductor Ag Gap Superconductor Model 1Model 2 2-D FEM Current Vector Potential Method Critical State Model : Bean Model Governing Equation : Faradays Low Current Distribution Joule heating loss J = 44.8% of J C to avoid quenching Gap Region : Assumed Material to Treat Contact Resistance Gap Resistance m (230 in butt jointing of one-layer HTS tape)

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7 3. Fundamental study on butt joint of HTS tape 3-2. Performance analysis in different filament location Analytical result JCJC Ag SC Gap JCJC Ag SC Gap Model 2 Model 1 Current distribution in SC region agrees with critical state model Current expands near the gap region SC filament location does not affect the butt jointing performance ModelJoule Heat (W) Model 1 Model Joule heating loss Contact condition is dominant Increase of contact point is effective to reduce joint resistance for example with metal-plating.

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8 3. Fundamental study on butt joint of HTS tape 3-3. Dependence of joint resistance on current Assumption of flat contact surface The dependence can not be obtained No Temperature Increase No Flux Flow State ( No decrease of J C ) Result Dependence of resistance on current Flux flow Heat generation Flat contact surface Jointing region Electromagnetic Field Analysis Thermal Analysis Joule Heating Loss Jointing Resistance Model 3 Superconductor AgGap(Ag) Gap(SC) Gap Resistance: m (Jointing Resistance at 5A) Thermal Conductivity Specific Heat of Assumed Material substituted by those of materials besides the assumed material

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9 Decrease of J C High J Expression of decrease of J C Depth: 0.2mm Depth: 0.4mm Current density becomes larger than critical current density Decrease of current flowing areaDecrease of critical current density = 3. Fundamental study on butt joint of HTS tape 3-3. Dependence of joint resistance on current Assumption of notch and/or degradation Degradation of superconducting filament Notched contact surface

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10 3. Fundamental study on butt joint of HTS tape 3-3. Dependence of joint resistance on current Analytical result Dependence of jointing resistance on the transport current notched contact surface Depth = 0.4mm Reason of the dependence Degradation of SC filament Actual depth Protection of joint surface is important for example with metal-plating.

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11 4. Study on butt joint of laminated HTS cable 4-1. Test cable and experimental set-up Rod 10-Layered BSCCO 2223 Cable Voltage Tap Joint Region Coolant: Liquid Nitrogen Silver Ratio Critical Current A (77 K) HTS Cable Specification of BSCCO 2223 Tape Critical Current: 400A Joint Load V Loading Area: Unit: mm 10-Layered BSCCO 2223 Cable Low Temperature Solder Contact Surface 30, 40, 45, 50, mm 2mm

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12 4. Study on butt joint of laminated HTS cable 4-2. Stress-resistance characteristic Enlarge Joint resistance becomes almost constant (160 ~ 200MPa) begins to increase slightly (200MPa~ ) Improvement of contact condition degradation of HTS cable > As compressive stress increases There exists optimum stress

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13 4. Study on butt joint of laminated HTS cable 4-3. Current-resistance characteristic Current-resistance characteristic in Dry Joint Thermal Quench ICIC Current- R characteristic Joint Resistance 4 Temperature increase disappears - No decrease of critical current - No change of resistance gradient Joint resistance can be reduced small enough to prevent quench occurrence due to temperature increase caused by heat generation below 500A

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14 Protective effect with silver-plating Reducing resistance Preventing degradation Angle Dependence Normalized resistance increases when joint surface angle is larger than 45. Slippage occurs because shear stress becomes larger than normal stress. 4. Study on butt joint of laminated HTS cable 4-4. Angle dependence and plating effect

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15 4. Study on butt joint of laminated HTS cable 4-5. Analytical evaluation with structural analysis Analytical model (1) 6 loading system are compared Normal stress distribution on joint surface Intensity of stress concentration Buckling with outward force Structural analysis 3D elastic-plastic deformation analysis Model Width w (mm) Radius r (mm) Bottom loading A4.40× B 0 C4.42.0× D E4.43.0× F2.0 × HTS cable w r Top rod Bottom rod Evaluating Bottom loading O Bottom loading

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16 Unit [mm] Schematic view of analytical model Schematic view of analytical model x y z Loaded area Calculating area 20.0 HTS cable Joint surface Analytical object Analytical object 10-layered laminated HTS cable Length 20mm, Width 4.2mm, Thickness 2.0mm Symmetric restraint condition yz-plane u x =0 zx-plane u y =0 1/4 of real object is calculation area Calculation area Calculation area Element Element Bottom plane condition Bottom plane condition Joint surface Joint surface Loading by rod Loading by rod Displacement is given to nodal point of cable. Regarding Assumed cross-section replicated at yz-plane as joint surface. Curvature at rod edge curve-like displacement is given. Assuming that the surface is jointed ideally. Slippage between layers is considered on yz-plane. 20-node rectangular element Model B, D Load is given as in case of Upper plane. Except Model B, D z-directional displacement is constrained. t 4. Study on butt joint of laminated HTS cable 4-5. Analytical evaluation with structural analysis Analytical model (2)

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17 Stress concentration at rod edge affects normal stress distribution on joint surface. Loading from bottom side is effective to obtain uniform stress distribution. Stress distribution in Model B is more uniform than that in Model A The same tendency is obtained in Model C and Model D Stress distribution is concentrated at upper side in Model F t y n Joint surface HTS cable Calculating area Comparing Model A, B, F ( rod =250MPa) Normal stress on joint surface Normal stress on joint surface Influencing the joint resistance Comparison in term of loading direction (A, B) Comparison in term of rod width (A, F) Nonuniform distribution Joint surface is degraded partially even if under the optimum stress 4. Study on butt joint of laminated HTS cable 4-5. Analytical evaluation with structural analysis Evaluation of normal stress distribution on joint surface Model A Model B Model F 150MPa 50MPa

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18 Equivalent stress distribution on zx-plane Model A r =0 Model C r =2 Model E r =3 Evaluating Influence of rod edge curvature on intensity of stress concentration Large curvature Low stress concentration Stress is dispersed at part of rod edge curvature. Having curvature at rod edge is effective to disperse stress concentration Model A, C, E (Loading from upper side) 4. Study on butt joint of laminated HTS cable 4-5. Analytical evaluation with structural analysis Evaluation of intensity of stress concentration Model B r =0 Model D r =2 Dual loading Stress is dispersed Stress is dispersed with the curvature. Stress is dispersed with dual loading. Model D is the best to avoid stress concentration

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19 z-component of stress on zx-plane HTS cable has layered structurebuckling with layer peeling Model A Model B Outward force occurs locally. Outward force can peel layer especially in Model B. Cable supporting area in Model B is smaller than that in Model A There exists possibility of buckling in dual loading Cable is unstable Model A r=0, Top loading Model B r=0, Dual loading 4. Study on butt joint of laminated HTS cable 4-5. Analytical evaluation with structural analysis Evaluation of buckling with outward force

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20 5. Prototype of remountable HTS magnet Layer number Winding number Outer radius Operating temperature Joint number mm 77 K 4 Specification Solenoid type Photograph of prototype Current-resistance characteristic

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21 6. Summary 1. Fundamental study on butt joint of HTS tape - Joint resistance of 3.6 was achieved at 60A. 2. Study on butt joint of laminated HTS cable - Joint resistance was reduced small value enough to prevent thermal quench and decrease of critical current at 77K below 500A. - Silver-plating is effective to improve and protect joint surface condition. 3. Fabrication of prototype of remountable HTS magnet - Performance of prototype of remountable HTS magnet was evaluated in liquid nitrogen environment. - Dependence of joint resistance on current was confirmed. According to numerical analysis, that is caused by degradation of superconducting material near joint surface. Protecting joint surface with metal-plating is important. Feasibility of remountable HTS magnet was demonstrated. - Joint resistance is decided by reduction of contact resistance and degradation of superconducting material with increase of joint stress. - Joint resistance became stable when angle of joint surface was 30 to How to obtain uniform stress distribution and to avoid stress concentration and buckling are indicated by structural analysis.

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22 7. Future works 8-1. Development of independent heat removal system (a) Porosity: 70% Fiber diameter: 90μm(b) Enlarged view Characteristic of metal porous media High thermal conductivity Enlargement of heat transfer area Transport of latent heat with capillary force Heat removal system with metal porous media Experimental set-up in case of waterResult in case of water Coolant Water LN2 Velocity [m/s] Heat transfer coefficient [MW/m 2 /K]

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23 8. Future works 8-2. Design of cooling channel with metal porous media Preventing thermal quench with heat transfer enhancement Introducing cooling technique with metal porous media to remountable HTS magnet Evaluation of rheological characterization and heat transfer characteristic when liquid nitrogen flows thorough metal porous media. Issues Design and fabrication of jointing region with metal porous media Metal Porous Media Insulator HTS Cable Conduit Flow of LN2 Design Proposal

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