Instrumentation & Measurement Magazine 24-9 - 33

Mechanical Strain Measurements
in High-Field Low-Temperature
Superconducting Magnets
Keziban Kandemir, Michael Guinchard, Laura Bianchi, and Sylvain Mugnier
O
ptical fiber sensors based on Fiber Bragg Grating
(FBG) technology are used to monitor the mechanical
behavior of the magnet's coils from its assembly
at room temperature to its powering at cryogenic temperatures.
The development of this instrumentation required several years
of research and development effort to validate the precision and
accuracy of FBGs based measurements in cryogenic conditions.
FBGs are bonded on the coils in different locations along with
the longitudinal and azimuthal directions that allow studying
the average stress levels. The stainless-steel shells of the magnets
and each extremity of the four rods are equipped with
resistive strain gauges (RSG). This paper provides a description
of the instrumentation used to monitor the strain profile of superconducting
magnets. Strain measurements during magnet
assembly, cool-down, and powering are presented to confirm
the agreement between electrical and optical sensors.
Introduction to Optical Strain Sensors
For the High Luminosity upgrade of the Large Hadron Collider
(HL-LHC) and for the future high-energy accelerator projects,
a sustained research and development effort is required to implement
advanced technologies for the development of a new
generation of superconducting magnets [1], high-energy proton
beam dumps, powerful particle physics detectors and cryogenic
radio-frequency cavities. During the research and developments
phases, strain monitoring of prototypes is paramount to validate
Finite Element Analysis (FEA) and confirm the mechanical
response of complex structures in harsh environments such as
cryogenic temperatures, high magnetic and electric fields, high
radiation level or the vicinity of high energy proton beams. The
instrumentation conventionally used to monitor the strain profile
is based on resistive strain gauges (RSG) with compensation of
temperature and magnetic field. The compensation in harsh conditions
requires a large number of wires, which might be reduced
by using optical strain sensors, based on Fiber Bragg Grating
(FBG) [2]. As for electrical strain gauges in the past, several test
campaigns were carried to validate the optical measurement
technique suitable for CERN environmental conditions, both in
terms of precision (repeatability and reproducibility) and accuracy.
This paper reports the last development in the mechanical
December 2021
measurement techniques based on optical fibers and presents
an overview of the measurements performed in prototype superconducting
magnets for the High Luminosity upgrade of the
Large Hadron Collider based on this advanced technique.
Fiber Bragg Grating Sensing Principle
The optical fibers can be used to measure strains through
the FBG technology. Optical fibers are mostly made from
thin flexible silica glass and are non-metallic. They transmit
signals through pulses of light and thus, are immune to electromagnetic
fields. The optical signals are not perturbated by
electrical noise in the environment. Optical fibers are therefore
an interesting type of sensor to monitor the strain profile of superconducting
magnets. Single-mode FBG is particularly used
as optical fiber sensor due to the single mode propagation with
low propagation loss. Within a short section of the optical fiber,
the refractive index of the core is periodically modulated
such as the FBG reflects a specific wavelength λB
called Bragg
wavelength and transmits all other wavelengths. The coupled-mode
theory describes the spectral response of an FBG
technology. In this approximation, the forward propagating
core modes couple to the phase-matched counter-propagating
core modes [3]. For most sensing applications, only the fundamental
core mode LP01
is involved [4]. The Bragg wavelength is
obtained following the phase-matching condition:
2
 n eff
B
where neff is the effective refractive index of the LP01
(1)
mode and,
Λ is the period of the FBG.
After applying a relative change in the Bragg wavelength,
Magne et al. [4] proposed a formula for the Bragg wavelength
shift depending on temperature and strain. The Bragg wavelength
λB
shifts with strain ε and temperature T according to:
  
 pT  
BB 1
 e    (2)
where pe is the photo elastic coefficient of the optical fiber, α is
the coefficient of thermal expansion and, ξ is the thermo-optic
coefficient or change of refractive index with temperature.
IEEE Instrumentation & Measurement Magazine
1094-6969/21/$25.00©2021IEEE
33

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