The development of persistent joints for MgB2-based conductors M.D. Sumption, C. Kovacs, F. Wan, M. Majoros, E.W. Collings Center for Superconducting and Magnetic Materials, MSE, The Ohio State University D. Doll, C.J. Thong, D. Panik, M.A. Rindfleisch, M.J. Tomsic Hyper Tech Research Funded by an NIH bridge grant, and also an NIH NIH, National Institute of Biomedical Imaging and Bioengineering grant, under R01EB018363.

Outline Joint development – Hyper Tech is developing both superconducting solder type MgB2 persistent joints and MgB2-MgB2 persistent joints (for both W&R and R&W applications) OSU is developing the test process and performing the test of the MgB2 persistent joints First, direct I-V measurements are used which can probe down to R value of 10-10 ohms Results for the direct I-V probe are discussed Next, two decay rigs are described, capable of measuring R in the R= 10-12 and lower regime Then MgB2 joints reaching into the 10-12  regime are shown Finally, plans for persistent switch development, and coil segments with persistent joints/switches are described

Place of this effort in Larger goals Overall Goals Develop MgB2 joints which are ready for both W&R coils as well as R&W coils (R = 10-12-10-15 ) Develop persistent switch Integrate into sets of coil segments and test in OSU’s large cryocooled test cryostat Demonstrate an operating set of field coils with persistence for a whole body MRI (image guided system) Key step enabling commercial MRI applications

Test Sequence Demonstrate joints with R < 10-10  (measured by direct I-V). May be: (i) SC solder, R&W MgB2, W&R MgB2 Develop test rig and develop MgB2 joints with R = 10-12  and lower May be: W&R and R&W MgB2, multifilament, MRI-like strand

MgB2-MgB2 joint (example results) Screening test Simple I-V measurement Good to 10-10 

Summary of screening measurements of Persistent Joints Two styles of joints Superconducting solder type Direct MgB2-MgB2 In both cases, used already reacted wire Preliminary testing using direct I-V, R < 10-10  Next Step -- Decay Testing rig and samples 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

Persistent Joints Between Reacted MgB2 Wires MgB2 Wires Joined

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  Thus, a decay rig was needed, as well as some initial testing and verification

Horizontal test arrangement for decay measurements – machine drawing MgB2 joint Hall probe inside, below gap level 100 A current leads NbTi primary coil SS holder assembly MgB2 secondary Gap in primary for secondary insertion NbTi primary coil

Actual Horizontal Test rig for decay measurements – with test NbTi joint mounted

NbTi/SC solder joint: test of Persistent joint apparatus Blue curve at right shows current in the NbTi primary. Red curve indicates the field reading by the Hall sensor. Protocol (by Regime): Use NbTi coil to generate Bcoil (I = 20 A) II. Increase the Bext to 3 T (pushes joint > SC) III. Drop Bext to 0. IV. Turn off NbTi coil rapidly. Note: Only the field in step IV indicates the field generated by the test (here NbTi) joint. Persistent regime, flat, very small decay

Expansion of decay region NbTi test joint Results for a NbTi persistent joint (at the time of stage III, IV in overview result). No interruption: current I primary was excited and de-excited, but no B field up and down ramp Field Interrupt: Field was cycled up and down while current was on Superconducting solder type. Shows initial time of 1 h run

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). Some trapping induced, but current decay is rapid Superconducting solder type.

Next Steps Test Device works Need to develop a lower R MgB2 joint In the meantime the MRI magnet design (image guided) wire becomes clear, optimizes with a larger wire Thus -- Need to test a joint made with a larger OD MgB2 wire Will need a modified holder which can handle larger OD wire

MRI Subscale and Fullscale wire Round wire OD = 0.95 mm 18 filament Outer Monel 32 % Cu SC = 12% Jc = 2 x 105 A/cm2, 5 T, 4 K Small MRI wire Ic = 155 A at 5 T, 4 K (0.95 mm OD) Large MRI wire Ic = 320 A at 5 T, 4 K Larger aspected wire Wire area = 1.46 mm2

Larger Strand, vertical Persistent Joint Test Assembly NbTi Primary Current injection Primary MgB2 secondary former Cu can MgB2 secondary Support rod Used large MRI wire for joint

MgB2 persistent joints under investigation Large diameter wire for demonstration magnet require new holder Investigating MgB2-MgB2 joint (not solder) Pursue first joint on unreacted wires (W&R style) Second joints on reacted wires (R&W style)

Step 1: Ramp external field, Bext up to reduce MgB2 Ic Protocol A: With Primary Step 2: Ramp up current in primary (Bext + Bp now present) Step 3: Ramp down external field, Bext to increase MgB2 Ic (at Bext = 0, Bp present) Step 4: Ramp down current in primary Secondary will attempt to retain Bp, observe decay with hall probe 40-turn NbTi Primary Primary Bp Ip Ip Bext 1.75-turn MgB2 Secondary Is Ips Secondary Bsec Hall-sensor centered in both primary and secondary SC-Joint

W&R Style- Zero Field Persistent joint results I Protocol Ramp to 4 T 50 A into Primary Ramp to zero field Turn off current Observe decay  is changing with time but reaches 3 x 105 sec R = 5 x 10-12 

Protocol B: Without Primary Step 1: Ramp external field, Bext up to reduce MgB2 Ic Step 2: Ramp down external field, Bext to increase MgB2 Ic and trap some of the previous Bext (at Bext = 0, Bext present) -- observe decay with hall probe. Baseline set by field in absence of joint Bext 1.75-turn MgB2 Secondary Is Ips Secondary Bsec Hall-sensor centered in both primary and secondary SC-Joint

Second run – zero field long time Protocol Ramp to 4 T Ramp to zero field Observe decay Tau = 3.7 x 105 sec R = 4 x 10-12 

W&R style Joint -- Persistent Current vs B Protocol Ramp to 4 T Ramp to zero field Observe decay B = 3 T  = 2.5 x 105 sec R = 3.8 x 10-11 

Next Steps R&W style joint Reduce R value even further – 10-15 , increase current performance and explore temperature performance Persistent Switch Coil segment (parallel development) Coil Segment Testing (basic operation, quench) Two Coil segment with SC joint and Persistent switch Demonstration of magnet system for image guided MRI system

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

Layout for Demonstration coil and persistent switch

New Cro-conduction system Coils up to 1.5 m OD T down to 4 K 3 W at 4 K 700 A capacity

Summary Superconducting MgB2 joints are being developed, both W&R type and R&W type Initial screening results showed good results with R < 10-10 ohm, indicate that it was time to move to decay measurements The design of horizontal and vertical PJ decay rig were shown Direct MgB2-MgB2 joints using multifilamentary MgB2 of MRI-relevant design were demonstrated, with R in the 10-12  regime HTR/OSU are moving at an accelerated pace to develop useful MgB2 PJ, and are planning their test in a larger device Testing and development is in full swing, moving now to larger conductor joints for image guided system