1. 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 R01EB

2. 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 ohms
Results for the direct I-V probe are discussed
Next, two decay rigs are described, capable of measuring R in the R= and lower regime
Then MgB2 joints reaching into the  regime are shown
Finally, plans for persistent switch development, and coil segments with persistent joints/switches are described

3. 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 = )
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

4. Test Sequence
Demonstrate joints with R <  (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 =  and lower
May be: W&R and R&W MgB2, multifilament, MRI-like strand

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

6. 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 < 
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

7. Persistent Joints Between Reacted MgB2 Wires
MgB2 Wires Joined

8. 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 
Thus, a decay rig was needed, as well as some initial testing and verification

9. 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

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

11. 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

12. 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

13. 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.

14. 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

15. 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

16. 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

17. 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)

18. 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

19. 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 

20. 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

21. 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 

22. 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 

23. Next Steps R&W style joint
Reduce R value even further – , 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

24. 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

25. Layout for Demonstration coil and persistent switch

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

27. Summary
Superconducting MgB2 joints are being developed, both W&R type and R&W type
Initial screening results showed good results with R < 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  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