Instrumentation & Measurement Magazine 24-9 - 37

regression was performed
to check the relation between
measurements from
optical and electrical strain
gauges. Fig. 3c shows the
agreement between the optical
and electrical sensors.
A very good correlation
between the optical and
electrical strain gauges is
observed in Fig. 3c.
The mirror instrumentation
was realized on three
different short models LTS
magnets. Therefore, a statistical
study was done on
each magnet. Table 1 summarizes
the mean slopes of
the curves comparing the
RSG and FBG sensors from
different powering tests realized
on three different
magnets.
Design of
Optical Fiber
Feedthrough
After the very good
agreement from the mirror
instrumentation, we
concluded that FBG technology
is a non-invasive
and suitable instrumentation
for mechanical strain
measurements during assembly
and powering tests
at cryogenic temperatures.
Therefore, the coils of the
long model LTS magnets
were only bonded with optical
fibers. Long models of
LTS magnets are inserted in
the cryostat for powering
tests at 1.9 K and in ultrahigh
vacuum (UHV) in the
Fig. 6. (a) Temperature profile during cool-down and warm up. (b) Azimuthal strains measurements on four coils at three
different locations along the coils (not compensated with temperature). The gap in the graph is because the magnet was
on stand-by, and strain measurements were not performed.
horizontal test bench [8]. The optical interrogator used for
monitoring strain cannot be placed in cryogenic temperatures.
Thus, a transition connection box is necessary to extract the optical
signals from FBG sensors outside the cryostat.
An optical fiber feedthrough was designed to extract the optical
signals from FBG sensors installed on the coils of the LTS
magnets. The connection box comprises two flanges. Flange
n°1 is equipped with 8 ferrule connectors with angled physical
contact (FC/APC) welded on it. FC/APC connectors minimize
the back reflections thanks to an 8° angle-polish applied to end
faces. Flange n°3 is screwed with eight optical fibers with FC/
December 2021
APC connectors in the extremities, as shown in Fig. 4. The two
flanges are soldered to the two extremities of a cylindrical element,
where the optical connection is optical fibers coated with
stainless steel. This chamber represents the transition between
the liquid He bath at 1.9 K and UHV and an environment at
10−6
mbar. Flange n°3, on the magnet side, is connected to the
sensors and it is exposed to the liquid He bath. Flange n°1 is
connected to the acquisition system and it features a pipe to be
connected to a pump extracting He in case of leaks.
A test was carried out in liquid He bath at 4 K to evaluate the
signal transmission of the full measuring chain at cryogenic
IEEE Instrumentation & Measurement Magazine
37

Instrumentation & Measurement Magazine 24-9

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