# Presentation: 410908

## Observations on Critical-Current Measurements of Marginally Stable Strands

**Loren Goodrich and Ted Stauffer**

**Viewpoint:**
- Different challenges in building a magnet and making *I*c measurements
- Space is limited for *I*c measurements -> short length + short contacts
- Also -> shorter distances for current transfer -> more dissipation
- Temperature margin effectively zero for *I*c measurements
**Intro:**
- Sample holder following Dan Dietderich’s (LBL) design
- Good experimental platform, but still have early quenches 9-14 T

## Stability in Critical-Current Measurements

**Intrinsic factors: (completely determined by properties of wire)**
- • current and current density		• amount and resistivity of stabilizer
- • effective filament diameter		• flux jumps
- • twist pitch				• design of cross section
- • inhomogeneity			• temperature margin
- • surface to volume ratio  		• heat capacity
- • *E*-*I* characteristic (***n*****-value**)
**Extrinsic factors:** **(heating, cooling, shunting, and luck)**
- • contact resistance			• sample and cryostat motion
- • current transfer			• current-ramp rate
- • *I*c inhomogeneity due to variations in wire strain along the test fixture
- • sample damage			• current and field ripple and spikes
- • magnetic field profile			• wetted perimeter contacts, sample
- • active sample length 		• helium circulation
- • shunting conditions 		 	• soldered along length
- • luck of relative *I*c between *V* taps

## New conductors are pushing the limits of thermal stability -> marginally stable wires

- As suggested by Emanuela Barzi (Fermi), we made measurements on a Nb-Ti wire
- - tested current limits of our Nb3Sn holder to over 2000 A
-  looked at quench limits to electric field
- Made measurements on latest Nb3Sn wire from Oxford
-  tested “twin” specimens
-  extrinsic modifications ->effect on quench limits to electric field

## Ic

***E*****-*****I***** curves + Quench limits of Nb-Ti ****Supercon sample**

- Can get to **5 to 10 times *****E*****c** of 10 μV/m (0.1 μV/cm) up to 2000 A!
- 1600 A is pushing the limits of current leads -> low duty cycle

*I*c

- wire dia. 0.95 mm
- non-Cu *J*c = 2648
- A/mm2 at 5 T
- 47 % Cu fraction
- 50 cm V taps

## n-value of Nb-Ti Supercon sample

***n*****-value of Nb-Ti ****Supercon sample**

*n*-value ~ 61 near 10 μV/m and 5 T
- low duty cycle for >1600 A (<2.5 T) limits accuracy of *n*-value

*n*-value
- determined using
- 3 adjacent points
- along *E*-*I* curve

## Norm. (at 10 μV/m) n-value-E curves on Nb-Ti

**Norm. (at 10 μV/m)***** n*****-value*****-E***** curves on Nb-Ti**

## Comparison of Ic for Nb-Ti on diff. holders

-  inside coil dia. 3.07 cm
-  sample pitch  8 turns/inch

**Comparison of***** I*****c**** for Nb-Ti on diff. holders**

- High-current Nb-Ti holder
-  inside coil dia. 6.22 cm
-  sample pitch  4 turns/inch
-  current limit ~2600 A

|  | Nb-Ti holder | Nb3Sn holder |  |
| --- | --- | --- | --- |
| field, T | Ic, A | Ic, A | del Ic |
| 2.000 | 1783 | 1795 | 0.7% |
| 2.500 | 1606 | 1607 | 0.1% |
| 3.000 | 1457 | 1451 | -0.5% |
| 4.001 | 1204 | 1194 | -0.9% |
| 5.002 | 986 | 974 | -1.2% |
| 6.000 | 781 | 769 | -1.5% |
| 7.001 | 584 | 572 | -2.0% |
| 8.001 | 392 | 381 | -2.8% |
| 9.001 | 213 | 203 | -4.6% |

- Differences likely due to
-  self-field effect
-  bending strain

## Twin Nb3Sn Oxford samples

***I*****c**** diff. ~0.34 %!**
**We are encouraged**

-  wire dia. 0.7 mm
-  non-Cu *J*c = 2922 A/mm2 at 12 T and 4.2 K
-  42 % Cu fraction
-  50 cm V taps

| Field (T) | 1st sample | 2nd sample | % diff. |
| --- | --- | --- | --- |
| 9.000 | 1077.5 |  |  |
| 10.002 | 917.5 |  |  |
| 11.002 | 775.2 | 776.5 | 0.18 |
| 12.001 | 650.7 | 652.3 | 0.24 |
| 13.001 | 540.9 | 542.7 | 0.34 |
| 14.000 | 443.9 | 445.4 | 0.34 |

- First thermal cycle for both samples

## Slight increase in Nb3Sn Ic with each thermal cycle

**Slight increase in Nb****3****Sn *****I******c***** with each thermal cycle**

*Y*-axis relative to 1st *I*c measured at each field
- Ref.: *IEEE Trans. On Appl. Superconductivity*, Vol **7**, No. 2, pp 1508-1511 (June 1997).

## Settling time for voltage, ramp-and-hold (3 s)

**Settling time for voltage, ramp-and-hold (3 s)**

-  Bracket indicates region where specimen *V* has a longer settling time
-  Near onset of *I*c transition (with increasing *I*), can be more than 5 s
-  Other times <0.5 s  *Adv. Cryog. Eng. Mater.* **48**, pp. 1142-1149 (2002)
-  Can increase uncertainty of extrapolations, appear to be current transfer

## Two current acquisition patterns with time

**Two current acquisition patterns with time**

-  Shown with settling time ~1 s and ~0.2 s to take *V*+*I *data
-  Up mode, current increasing with time between set points
-  Down mode, current decreasing with time between set points,
-        two ramp rates shown (~135 A/s and 57 A/s)

- We use both patterns
- The down pattern results
- in shorter settling times
- or *better* *E*-*I* curve using
- the same settling time
- A slower ramp to maximum
- current is sometimes needed
- in down pattern to reach
- same limit

## Ic

**Eight***** E*****-*****I***** curves at 12 T on Nb****3****Sn ****Oxford sample**

- Range of *I*c 0.06 %!
- Only diff. at very low *E*

*I*c

- We wanted to study
- the repeatability with
- diff. acquisition
- patterns and settling
- times
- Typical data: down
- with 1 s

## Ic

**Eight***** n*****-value*****-E***** curves on Nb****3****Sn ****Oxford sample**

*n*-value (10 μV/m)=83 at 12 T
-  with range of 2.3
- Flat over 2 decades of *E*
- Only diff. at very low *E*
*n*-value at 10 T ~112
- 	      12 T ~ 83
- 	      14 T ~ 65
-                  16 T ~ 47

- We are interested
- in what others get,
- maybe we are wrong

*I*c

## Best E-I curves on high-Jc Nb3Sn 

**Best *****E*****-*****I***** curves on high-*****J*****c**** Nb****3****Sn **

-  Quench occurs at higher *E *at higher fields and lower* I*
-  Can get to *E*c at  10 T
-  Lower limits than on Nb-Ti
-  Quench may still be caused by heating

- We are interested
- in what others get,
- maybe we could do
- better

## n-value - E curves on high-Jc Nb3Sn 

***n*****-value - *****E***** curves on high-*****J*****c**** Nb****3****Sn **

-  *n*-values at lower fields (higher *I*) and *E*<1 μV/m artificially high
-  *n*-values above     1 μV/m are flat
-  our measured      *n*-values jumped up when the effective filament diameter jumped up

*n*-value at 10 T ~112
- 	     12 T ~ 83
- 	     14 T ~ 65
-                  16 T ~ 47

## 3 sets of E-I curves on same Nb3Sn specimen

-  wider lines indicate maximum *E* below quench for each
-  only extrinsic factors were changed between runs
-  these results suggest that the curves are limited by extrinsic factors

**not fully understood-too early to draw general conclusions**

**3 sets of***** E*****-*****I***** curves on same Nb****3****Sn specimen**

## Summary

-  “Berkeley” design holder: good experimental platform to conduct research on early quenching problem
-  Can make measurements on Nb-Ti wire on this holder to 2000 A
-  Observed fewer quenching problems with Nb-Ti wire
-  Can get ~same *I*c on twin high-*J*c Nb3Sn samples
-  *E*-*I* curves very repeatable
-  *n*-values very high ->instability  ~112 at 10 T and ~47 at 16 T
-  Quench limited *E* increases with increasing magnetic field
-  Fewer quenching problems ≥14 T
-  Repeat runs suggest that curves are limited by extrinsic factors
-  **Not fully understood-too early to draw general conclusions**
** ****We are interested in what others get, maybe we can do things better**

## Slide 18

| I2 = I1 (E2/E1)^(1/n) |  |  |  | Calculate |
| --- | --- | --- | --- | --- |
| E1 | I1 | n | E2 | I2 |
| 1 | 100 | 20 | 10 | 112.20 |
| 1 | 100 | 30 | 10 | 107.98 |
| 1 | 100 | 40 | 10 | 105.93 |
| 1 | 100 | 50 | 10 | 104.71 |
| 1 | 100 | 60 | 10 | 103.91 |
| 1 | 100 | 80 | 10 | 102.92 |
| 1 | 100 | 85 | 10 | 102.75 |
| 1 | 100 | 100 | 10 | 102.33 |
| 1 | 100 | 200 | 10 | 101.16 |
|  |  | 39.5 |  | 6% |
|  |  | 78 |  | 3% |
|  |  | 116 |  | 2% |
|  |  | 155 |  | 1.5% |
|  |  | 232 |  | 1% |

## Slide 19

## Slide 20