Instrumentation & Measurement Magazine 24-9 - 40

Accurate Displacement
Measurements During the
Powering of High Field
Superconducting Magnets
Óscar Sacristán de Frutos, António Carvalhosa, and Michael Guinchard
S
train gauges have been the technique of choice for
experimental stress analysis in superconducting
magnets since the emergence of this magnet technology.
Since then, important efforts have been taken to prevent
and delimit strain measurement errors caused by magnetic
fields and temperature changes. The maturity and relative
simplicity of this technique has permitted the development of
several force and displacement sensors that are able to work
in the harsh conditions in which superconducting magnets
operate, such as intense magnetic fields and cryogenic temperatures.
This paper describes the methodology followed
to develop and characterize a tailor-made displacement sensor
and presents a successful application of this technology
for the determination of displacements in the High Luminosity-Large
Hadron Collider (HL-LHC) beam screen magnet
quench testing campaign.
Introduction to Strain Measurements in
Superconducting Magnets
Future high energy particle colliders will imperatively work at
increasingly high magnetic fields, pushing the conductors and
magnet structures to unprecedented electromagnetic and electromagnetic
stress. A multidisciplinary research program at
CERN in Geneva, Switzerland is currently focused on the design,
manufacture and test of Nb3Sn subscale dipoles magnets
[1]. Nb3Sn, due to its brittle and strain sensitive characteristics,
requires new approaches for magnet design and fabrication.
The performance of Nb3Sn magnets can be strongly affected
by the mechanical stresses in the windings during magnet
operation. It is therefore more important than ever to fully
understand and monitor the strain in the superconductor
through the whole service life of the magnet: assembly, cool
down and powering. Electrical strain gauges have proven to
be an extremely useful asset in this task since the emergence of
superconducting magnet technology [2]. The efforts taken to
validate the usage of this technique under extreme operative
conditions such as ultra-low temperatures, high electromagnetic
fields and a broad range of mechanical strains make this
technique relatively mature in the present days [3].
40
The lessons learned with the instrumentation of superconducting
magnets have enabled measurements in a number of
other systems working under similar operating conditions [4].
A prime example of such system is the new HL-LHC beamscreen
[5]. The new High Luminosity-Large Hadron Collider
(HL-LHC) Q1 beam screen, shown in Fig. 1, is an octagonallyshaped
stainless-steel pipe whose main functions are ensuring
the beam vacuum and shielding the cold mass from collision
debris and other beam induced heat loads. The beam screen is
placed within the bore of the new superconducting magnets
[2], [3]. On the longitudinal flat surfaces of the beam screen
four tungsten alloy blocks and four cooling tubes are placed
in an alternate way. A magnet quench, a resistive transition of
the superconducting magnet, represents a critical scenario for
the beam screen. Due to the fast decay of the magnetic field,
high intensity Lorentz forces are induced in the beam screen
that can undermine its mechanical integrity. The blocks, also
known as heat absorbers, are needed to intercept the collision
debris and are laid on the octagonal pipe to avoid residual
stress during cool-down [4].
Following a design phase based on a novel multiphysics
FEM approach which considers, amongst other aspects, a fully
coupled solving method in a three-dimensional space [6], an
extensive experimental validation campaign [7] was undertaken
with the aim of obtaining evidence on the mechanical
integrity of the beam-screen under the solicitations induced
by repetitive MQXFS-4b magnet [8] quenches at different currents.
Besides strain measurements on the outer surface of the
beam-screen and the cold bore of the magnet, performed with
strain gauges and optical fibers [9], it was of paramount importance
to determine the expansion of the beam screen as well
as the tilt of the tungsten alloy heat absorber during the CLIQ
[10] phase as a function of the magnetic field times the timederivative
of the magnetic field (B). Such measurements were
undertaken by means of custom-made strain gauge-based displacement
sensors mounted in a self-supported glass fiber
reinforced plastic structure inserted in the beam-screen (Fig.
2). The present work discusses the design and characterization
of magnetic-field insensitive strain gauge-based displacement
IEEE Instrumentation & Measurement Magazine
1094-6969/21/$25.00©2021IEEE
December 2021
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