J.Yue, D. Doll, X. Peng, M.Rindfleisch, Mike Tomsic Demonstration of Conduction Cooled MgB2 and Nb3Sn Coil Segments for Magnet Applications Hyun Sung Kim, Chris Kovacs, Milan Majoros, Mike Sumption, E.W. Collings Center for Superconducting and Magnetic Materials, MSE, The Ohio State University J.Yue, D. Doll, X. Peng, M.Rindfleisch, Mike Tomsic Hyper Tech Research Funded by a National Institute of Biomedical Imaging and Bioengineering grant R01 grant, R01EB018363, and also an NIH bridge grant

Motivation MRI systems based on MgB2 are of significant interest as one way to develop liquid-cryogen-free MRI systems MgB2’s Tc of 39 K invites conduction cooling, and lower cost makes it an economically viable The low-Tc-like coherence length allows for persistent joints Intermediate Tc helpfully increases the minimum quench energy while not suppressing the normal zone velocity too much Conductor performance is at the level needed for MRI application, and conductor designs appropriate for MRIs can be made -- the need is for developing conduction cooled coil technology

Outline Conductor/Coil specification Instrumentation Facility and Cool down Ic measurement vs T Discussion and Conclusions If Time Nb3Sn Coil work Initial Persistent joint development

Conductor Name 3364 In-situ 36 filament Nb barriers # Mono Barrier Mono sheath Multi sheath Central fil(s) powder material Mg:B dia (mm) % powder 36 Nb Cu Monel MgB2_2%C 1:2 0.84 18.1 Name 3364 In-situ 36 filament Nb barriers Cu interfilamentay matrix Monel outer matrix SMI-2% C doped S-glass insulated Reacted at 675/90 min under Ar Optical Micrograph of MgB2 conductor cross section, shown at wire OD = 0.84 mm.

Coil Design Specifics Coil material Former : 101 OFE copper All fasteners : 316 stainless steel Lead Insulator : Garolite G-10 or ceramic standoff Lead connector : Alloy 101 OFE Copper Coil Parameters Winding pack OD : 482.2 mm Winding Pack ID : 457.2 mm (18”) Winding Pack Height : 19.685 mm (0.775”) WP cross sectional area : 246.06 mm2 No. Turns : 225 Total Conductor L 328.8 m (1078.7’) Turns/layer : 19 (approx.) No. Layers : 12 Final coil preparation Solder for leads : Pb-Sn (40/60) Epoxy CTD Resistance measurements (after winding, after epoxy) L1 to L2 41.9 Ohms After winding L1 to former OL L2 to former After CTD 528 Epoxy

FEM Simulation Ic(B) curve of a typical strand with the calculated load line of the coil Magnetic field map at the coil critical current. Coil critical current [A] Max. on-axis field at coil critical current [T] Max. field in the winding 273 0.14 1.43 200 0.10 1.05

Experimental Set up (2) Two stage GM cryocoolers (2) Cold heads attached to the copper ring Copper ring and coil are thermally connected with high purity copper strips 4 BSCCO leads for each current tap Heater attached in the copper ring to control the temperature of coil Data acquisition system: Nanovoltmeters (Keithley), Thermometers (Lakeshore), Data acquisition (Labview) Copper Ring MgB2 Coil Copper Strips

Voltage Taps Critical Current Measurement: I-V measurement Resistance @RT [Ω] Estimated length [cm] CTI – C1 0.00249 1.94 C1 – C2 0.135 105.6 C2 – C3 13.9 10849 C3 – C4 7.07 5522 C4 – C5 7.12 5559 C5 – C6 7.08 5526 C6 –C7 6.81 5313 C7 – CTO 0.00478 3.73 Whole coil resistance: 42.12 Ω, Total conductor length: 328.8 m Critical Current Measurement: I-V measurement

Location/Description Temperature Sensors   Location/Description CTI 1 1st Type-E Thermo. On “IN” current tap CTI 2 2nd Type-E Thermo. On “IN” current tap CTO 1 1st Type-E Thermo. On “OUT” current tap CTO 2 2nd Type-E Thermo. On “OUT” current tap C1 1st Type-E Thermo. On the top surface of coil C2 2nd Type-E Thermo. On the top surface of coil CS Type-E Thermo. On the side surface of coil CL1 1st Type-E Thermo. On BSCCO current lead CL2 2nd Type-E Thermo. On BSCCO current lead C1_cernox CERNOX. On the top surface of coil CR_cernox CERNOX. On the top surface of cold ring CTI_cernox CERNOX. On “IN” current tap CTO_cernox CERNOX. On “OUT” current tap

Cooldown Voltage signals Temperature signals

I – V curves: 20 K, 0.1 A/s A bit of coil heating

I – V curves: 20 K, 2A/s Less coil heating

I – V curves: 20 K, 5A/s Very little coil heating

Temp @ Current Tap IN (∆T) [K] Temp @ Current Tap OUT (∆T) [K] Ic vs T Temp Test # Ramping Rate Ic [A] Temp @ Coil (∆T) [K] Temp @ Current Tap IN (∆T) [K] Temp @ Current Tap OUT (∆T) [K] 30 K # 14 2 23.3 29.4 ~ 29.4 (0.0) 29.9 ~29.9 (0.0) 30.6 ~ 30.6 (0.0) # 15 0.1 17.6 29.6 ~ 9.9 (0.3) 30.1 ~ 30.4 (0.3) 30.8 ~ 31.0 (0.2) 27 K # 16 50.5 27.1 ~ 27.1 (0) 27.3 ~ 27.4 (0.1) 27.7 ~ 27.8 (0.1) # 17 50.1 27.3 ~ 27.8 (0.5) 27.9 ~28.2 (0.3) 22.5 K # 20 5 116 22.5 ~ 22.7 (0.2) 23.0 ~ 24.0 (1.0) 23.5 ~ 23.7 (0.2) # 18 116.5 22.6 ~ 22.8 (0.2) 22.6 ~ 24.2 (1.6) 23.1 ~ 23.9 (0.8) # 19 111.7 22.5 ~ 23.1 (0.6) 22.9 ~ 25.3 (2.4) 23.4 ~ 24.9 (1.5) 20 K # 23 152.4 20.0 ~ 20.5 (0.5) 20.6 ~ 23.1 (2.5) 21.0 ~ 22.3 (1.3) # 24 20.1 ~ 20.6 (0.5) 21.0 ~ 24.0 (3) 21.4 ~ 22.9 (1.5) # 26 140.6 20.1 ~ 21.4 (1.3) 20.5 ~ 24.2 (3.7) 20.9 ~ 23.1 (2.2) 17 K # 27 187.0 17.1 ~ 18.1 (1.0) 18.0 ~22.3 (4.3) 18.6 ~ 20.9 (2.3) # 28 183.8 17.4 ~ 18.6 (1.2) 16.7 ~ 22.6 (5.9) 16.9 ~ 20.7 (3.8) # 29 173.6 17.2 ~ 18.9 (1.7) 16.9 ~ 23.3 (6.4) 17.3 ~ 21.6 (4.3) 15 K # 34 204.6 15.1 ~ 16.8 (1.7) 16.6 ~ 22.7 (6.1) 17.3 ~ 20.9 (3.6) # 35 196.7 15.1 ~ 17.2 (2.1) 16.6 ~ 23.5 (6.9) 17.2 ~ 21.7 (4.5) # 36 191.7 16.5 ~ 23.5 (7.0) 17.2 ~ 21.4 (4.2) A key aspect -- to test the end transitions, thus no preferentially cooling Thus some current tap heating at lower T (where I higher) and for slower ramps. However, we could observe that the coil, as well as the current in and out junctions, were good. The coil transitioned by quench, and no resistive baseline was seen.

Summary An MgB2-based react and wind coil was made and tested as part of a technology development effort for MgB2-based, cryogen-free, MRI. A 36 filament MgB2 conductor was used to wind a small MRI- segment-like coil using a react and wind protocol. The coil was 457 mm ID and used a total length of conductor of 330 m. The coil was then epoxy impregnated, instrumented, and tested. After initial cool down the coil Ic was measured as a function of temperature. Both the coil itself and the terminations were seen to perform well -- A load line was generated – full analysis awaits 15 K measurement of this particular strand. The coil transitioned by quench, and no resistive baseline was seen. A coil Ic of 200 A was seen at 15 K.

Nb3Sn FEM Simulation_Magnetic Ic(B) curve of the strand measured at 4.2 K with the calculated load line of the coil Magnetic field map at the coil critical current. Coil critical current [A] Max. on-axis filed at coil critical current [T] Max. field in the winding 871 0.73 6.818 Physical parameters of the coil at 4.2 K

Nb3Sn FEM Simulation_Temp, Strain. Coil temperature distribution. No current in the coil. Maximum temperature in the winding = 4.2209 K. Volumetric strain distribution in the winding cross-section.

Other Coil Measurements – Future Reporting MgB2 Coil Nb3Sn Coil

New 1.7 M cryostat to be in place this fall! 8 in port for instrumentation Feedthroughs. About 8 – 18pin connectors will fit on one port and there’s 4 of these. Radiation shield for feedthrough (4) Swinging door cover to block shine but allow current leads to be pulled through Through-bolts; should be tapped holes 3 inch clearance w/o MLI New 1.7 M cryostat to be in place this fall!

Development of Persistent Joints Two styles of joints Superconducting solder type Direct MgB2-MgB2 In both cases, used already reacted wire Preliminary testing to date using direct I-V, R < 10-10  Decay Testing rig and samples in preparation for increased R sensitivity and decay test Working now to improve performance and measure to high sensitivity Superconducting Solder Type 4 K MgB2-MgB2Type 4 K

Moving from screening test to decay test While further improvement in critical current of joints is still in process, it is important to measure R at values below 10-10 ohm Thus, a decay rig was needed, as well as some initial testing and verification

New test arrangement for decay measurements – machine drawing

Test rig for decay measurements – with test NbTi joint mounted

Expansion of decay region NbTi test joint Results for a NbTi persistent joint (at the time of stage III, IV in overview result). Superconducting solder type.

Test of first persistent decay MgB2 sample Results for a MgB2 persistent joint (at the time of step III  IV along the lines of NbTi overview figure). Superconducting solder type.

Conclusions (II) Various MgB2 and Nb3Sn coil segments are being tested by conduction cooling For joints Initial screening results show good results R < 10-10 ohm, indicate that it is time to move to decay measurements The design of a new PJ decay rig is shown Initial tests with NbTi and MgB2 joints are shown, and the results where described Joint Testing and development (HTR-OSU) is in full swing, moving now to larger conductor joints for image guided system