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Strain response of superconducting magnets during excitation and quench training based on polymer-FBG and cryogenic strain gauge measurements

Background

Qiang Hu 1,2, Mingzhi Guan 2, Xingzhe Wang 1, Beimin Wu 1,2 1. Key Laboratory of Mechanics on Western Disaster and Environment, Ministry of Education, College of Civil Engineering and Mechanics, Lanzhou University, Lanzhou 730000 2. Institute of Modern Physics of Chinese Academy of Science, Lanzhou 730000, China

Presented at 25th International Conference on Magnet Technology, 2017 August. 27 – September. 1, The Netherlands, Amsterdam; Session: Mechanical Behaviour; Program I.D. number: Tue-Af-Po2.09-14

Intense magnetic field and current of superconducting magnet can induce intense and nonuniform stress in coils that may initiate the quench during operation, it is

essential to study that mechanical behavior of superconducting coil and support structure as the magnet is energized. Currently, the conventional resistance strain gauge(RSG)

and optical fiber Bragg gratings (FBGs) were usually used to investigate strain induced by Lorentz force or cooling, or detect a quenching for a LTS or HTS. Unfortunately, FBGs

with hard package or bare FBGs both have very fragile mechanical performance at cryogenic temperature, so it’s necessary to improve mechanical property of FBGs at

cryogenic environment, then, to develop the double strain measurements system(FBGs + RSG) monitoring the strains of the on-line superconducting magnets.

NbTi/Cu racetrack superconducting magnet

Polymer-FBGs

Resistance strain gauges(RSG)

RSG(KFL series, 4mm× 2.7mm, 120 Ω , KYOWA produced) with half wheatstone compensation bridge circuit. It’s a conventional method to detect strain of superconducting magnet.

We studied the strain response of two superconducting magnets with different LTS materials and configurations based on homemade polymer-FBGs and conventional RSG strain measurement methods during excitation and quench, and numerical analysis of the strain fields was also carried out in the two LTS magnets based on FEM.

These measurements show that polymer-FBGs strain sensors embedded within the magnet structure exhibit better stability and reproducibility at cryogenic environment(4.2K) and intense magnetic field, and also reveal more attractions like smaller size, immunity to electromagnetic interferences, non circuits compensation and sensing performance of internal strain measurments.

The quench feature can be also detected with observations of abrupt strain pulse when a quench occurs based on polymer-FBG and conventional RSG measure methods, and the abrupt strain pulse was detected by polymer-FBGs was earlier than conventional RSG, and exerts better reliability and stability than conventional RSG during quench occurs.

The superconducting magnet is charged from 0A to 230A until the coils spontaneous quenched, the central magnetic field of 3.25T was achieved .

The strain increases to a value of about 1510με with the transport current reaches to 230A until a quench occurs.

The strain results of Polymer-FBGs agree well with conventional RSG, and the histories of the strain and applied current are almost synchronous.

Hybrid superconducting solenoid(NbTi/Cu+Nb3Sn/Cu)

The FBGs with polymer coating can enhance mechanical properties at cryogenic environment(4.2K).

The wavelength shift showed good linear dependence on the applied strain, and the strain sensitivity of the polymer-FBGs is temperature independent.

Maximum central magnetic field is 3.25T. MM01 , TM01 and SM01 are Hall sensor, temperature sensor and strain sensor, respectively.

Magnet real structure and strain sensors attachment

Magnet structure and locations of sensors

Polymer-FBGs and RSG as strain sensors were embedded within the overbanding structure and attached onto the surface of coil.

Hybrid superconducting solenoid(NbTi/Cu+Nb3Sn/Cu)

The hybrid superconducting magnet with field up to 10T was designed and fabricated, consisting of Nb3Sn/Cu and NbTi/Cu concentric solenoids.

Four distributed polymer-FBGs labeled SM01~SM04 were attached on the surface of Nb3Sn/Cu solenoid to detect hoop strain of the central Nb3Sn/Cu solenoid.

Magnet structure and locations of sensors Magnet real structure

NbTi/Cu racetrack superconducting magnet

0 500 1000 1500 2000 2500 3000

0

40

80

120

160

200

240

280 Current

I(A

)

time(s)

0

200

400

600

800

1000

1200

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2000

Polymer-FBGs

RSG0.03A/s

0.05A/s0.05A/s

0.3

A/s

str

ain

(m

icro

str

ain

)

0 500 1000 1500 2000 25000

50

100

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200

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central magnetic field current

I(A

)

Time(s)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

B0(T

)

0 20 40 60 80 100 120 140 160 180 200 220

0

200

400

600

800

1000

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1400

simulation RSG

Polymer-FBGs

Str

ain

(mic

rostr

ian

)

I(A)

Current and central magnetic field during excitation and quench

Strain response of the superconducting magnet during excitation and quench

Strain versus the excitation current of the superconducting magnet

0 200 400 600 8000

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40

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central magnetic field

current

I(A

)

Time(s)

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B0(T

)

0 200 400 600 800 10000

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SM01

SM02

SM03

0.1A/s

0.22A/s

Ho

op

str

ain

(mic

ros

tra

in)

Time(s)

ho

op

str

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(mic

rostr

ain

)

time(s)

371

355366

0 10 20 30 40 50 60 70 80 90 100 110 120

0

25

50

75

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275

SM01

SM02

SM03

simulation

Ho

op

str

ain

(mic

rostr

ain

)

I(A)Current and central magnetic field

during excitation and quench The hoop strain of the superconducting solenoid during excitation and quench

Strain response during excitation, pre-quench and post-quench

Hoop strain versus loading and unloading transport current

The spontaneous quench occurs at magnetic filed of 10T, the corresponding excitation current reaches to 122A.

Hoop strain of each location is almost the same, and the maximum strain is 371με, 366με, 355με, respectively. Then, hoop strain abruptly reduces to about 0 after quench occurs.

It’s indicated that the Polymer-FBGs have good stability and reproducibility at cryogenic environment(4.2K) and intense magnetic field.

The small oscillations on the strains are observed by using embedded polymer-FBGs with low acquisition rate at different locations during a pre-quenching.

The results of hoop strain are almost same at the different locations of the circumference of the solenoid , and they are resonable agree with the FEM simulation during excitation. Since the turn-to-turn friction was restrained during unloading transport current in the SC coils, the hoop strain during unloading transport current is larger than during loading transport current.

This paper presents an approach to monitoring the strain response of two different LTS magnets during excitation and quench training using homemade polymer-FBGs which can enhance mechanical properties of FBGs at cryogenic environment(4.2K). For the purpose of comparison, conventional resistance strain gauges(RSG) were also used to monitor their electromagnetic strains, the steady-state strains of the superconducting magnets were also valuated by means of a FEM.

Background Objective

396 398 400 402 404

1040

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16000

Polymer-FBGs

RSG

S

tra

in(m

icro

str

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)

Time(s)

quench

130

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220 current

I(A

)

4.00

4.25

4.50

4.75

5.00

5.25 temperature

Te

mp

era

ture

(K)

In comparison with the results of Polymer-FBGs and RSG have better agreements with the numerical results.

Polymer-FBGs exhibits an abrupt strain pulse before of quench occurs, but the abrupt strain pulse of RSG almost simultaneously occurs with quench, and exhibits an undesired outcome that appeared much larger abrupt strain pulse after quench.

Polymer-FBGs as a strain sensor also able to detect the quench of superconducting coil the same as conventional RSG, and have better reliability and stability.

Strain response during excitation, pre-quench and post-quench

0 100 200 300 400 5000

100

200

300

400

500

Wavele

ng

th s

hif

t(p

m)

Strain(microstrain)

303K

283K

263K

243K

223K

203K

183K

163K

143K

123K

103K

77K

4.2K

Polymer- optical fiber Bragg gratings(Polymer-FBGs) Resistance strain gauges(RSG)

Wireless strain acquisition with communication protocol IEEE802.15.4 and maximum transmission rate 250Kbps in air was used to connect the resistance strain gauge(RSG).

Processes of homemade polymer-FBGs

Wavelength shift of the polymer-FBGs versus applied strain

Interrogation system (Optical System200 )

A polymer-FBGs was attached on a copper plate, then, Tensile load was applied to the copper plate at different constant temperature.

Experiment calibration for polymer-FBGs strain sensors:

Wireless strain acquisition

Session: Mechanical Behaviour; Tue-Af-Po2.09-14