3231

Low-noise Cryocooler-cooled Compact SQUID-NDE
System for Carbon-Fiber Composites
Y. Hatsukade1, N. Kasai2, H. Takashima2, A. Sakamaki3, Y. Maruno4,
S. Tanaka1, and A. Ishiyama5
Toyohashi University of Technology1, National Institute of Advanced
Industrial Science and Technology2, Meiji University3, Iwatani Industrial
Gases Corporation4, and Waseda University5
Abstract. A low-noise and compact SQUID-NDE system cooled by acryocooler has been developed for practical NDE of carbon-fiber
composites. In order to suppress the magnetic noise due to a cryocooler, aplanar high-Tc SQUID gradiometer, coaxial pulse tube cryocooler, andSQUID stage separated from the cold head of the cryocooler wereintroduced. There was no increase of magnetic flux noise due to thecryocooler compared to noise by liquid-nitrogen cooling with the same
SQUID. The temperature at the SQUID stage was controlled at 75K±0.05K
for several hours by heaters. The field generator with ferrite cores was
employed for relatively lower electric conductive composites. Detectionsof deep-lying hidden slots in stacked carbon-fiber composite specimens
were demonstrated to show the effectiveness of the system.

1. Introduction
Carbon-fiber composites, such as carbon fiber reinforced plastics (CFRPs) and carbon
fiber reinforced carbon matrix composites (C/Cs), have been employed in many structures,
especially in aerospace structures like noses and wings of aircrafts and space shuttles
because they have lightweight and high fatigue resistance [1][2]. In addition to theseadvantages, C/Cs have heat resistance up to 3000K, too [1][2]. However, conventional
non-destructive evaluation (NDE) techniques, using X-ray, ultrasonic and acoustic
emission, have some difficulties to detect defects in the composites in many cases, due tothe unique properties of the composites [3-5]. For more reliable use of these composites,
development and establishment of effective NDE techniques are the urgent businesses.
On the other hand, it was clarified the NDE technique using superconducting quantum
interference device (SQUID-NDE) is suitable for conductive materials by many researches,
which mainly focused on aluminium materials in aircrafts [6-8], because of theincomparable high magnetic sensitivity of SQUID in the wide frequency range from DC to
1MHz [9]. In recent years, some researches of SQUID-NDE on the carbon-fiber
composites have been started and revealing that SQUID-NDE has possibilities not only todetect small and deep-lying defects in the composites, but also to evaluate damage degreein the defective composites [10-12]. At the present time, SQUID-NDE is an attractive
candidate for the composites and may be unique available technique for C/Cs [11].

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In order to realize on-site NDE measurements on the composites, an easy-handling andpractical NDE system is necessary. For this, we have developed a low-noise and compact
cryocooler-cooled SQUID-NDE system with field generator using ferrite cores. As forcooling a high sensitive SQUID by a cryocooler, there are three well-known problems.
These are the magnetic noise from magnetic materials used in cryocooler, mechanicalvibration due to cryocooler transmitted to the SQUID, and thermal fluctuation at the cold
end that causes drift of Ic of SQUID [13-15]. In order to suppress the noise increase, wehave employed a planar high-Tc SQUID gradiometer, coaxial pulse tube cryocooler (PTC),
and the SQUID-set stage separated from the cold head of the cryocooler. Because thechange of working temperature causes the change of SQUID sensitivity, two heaters havebeen introduced for thermal control. For much lower electric conductivity of thecomposites than metals, we have employed a magnetic field generator using two U-shaped
ferrite cores wounded by wire coils; e.g. electromagnets, to induce enough eddy current
density in the composites [16].
The magnetic flux noise of the cryocooler-cooled system was measured and comparedwith that of liquid-nitrogen-cooled system using the same SQUID. Thermal stability at the
SQUID stage was also investigated. The NDE measurements on deep-lying slots instacked C/Cs board specimens were carried out to demonstrate the defect detection ability
for the carbon-fiber composites.
2. SQUID-NDE system cooled by a cryocooler
Schematic drawing of the developed SQUID-NDE system with cryocooler is shown in
Fig.1 (a). This system is modified from a SQUID-NDE system using liquid nitrogen [11].
The system is composed of a high-Tc SQUID gradiometer and SQUID electronics, lock-in
amplifier and current supplier, two-dimensional scanning stage, the field generator using
electromagnets, and the coaxial PTC with the compressor and the rotary valve.
The field generator composed of a pair of U-shaped ferrite cores with permeability of
2300µ0 and 400 turn coils is shown in Fig.1 (b). The field generator generates ac magnetic
field of 0.5mT at a point 10mm straight away from the core end by current of 100mA. The
frequency dependence of the generated field is roughly flat in the frequency range between
1Hz and 400Hz. Two electromagnets are set at both sides of the SQUID to keep symmetryof the field. Alignments of the electromagnets are carefully adjusted to minimize themagnetic flux from the electromagnets coupled to the SQUID before NDE measurements.
Compressor

Refrigerator
Cryocooler

Bypassvalve
Send
RotaryValve& Cryostat
ReturnCore position adjustment tool

Electromagnet

80mm

Ferrite core withinduction coil

Stage

(b)
(a)
Figure 1. SQUID-NDE system with cryocooler. (a) Schematic drawing of the system.
(b) The field generator composed of core position adjustment tool and electromagnets.

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1mm

3.5mm

Figure 2. The planar high-Tc SQUID gradiometer with baseline of 3.5mm.
Table 1. Characteristics of the high-Tc SQUID gradiometer (at 77K by LN2).
SQUID inductance
100pH
Pick-up coil size
Baseline length
Ic

3.5mm x 1mm
3.5mm
60µA

Rn

2.4Ω
8µV

ΔV
Magnetic flux noise

20µφ0/Hz1/2@100Hz (in MRS)

2.1.
High-Tc SQUID gradiometer
We have employed a planar high-Tc SQUID gradiometer fabricated in AIST to reduce theenvironmental magnetic noise and also the magnetic noise due to the mechanical vibrationof cryocooler. Use of the SQUID gradiometer also contributes to construct a compact NDE
system because of no necessity of magnetic shielding. Figure 2 shows the microscopic
photograph of the SQUID gradiometer. SrTiO3 bicrystal substrate with mis-orientation
angle of 30 degree was used. YBa2Cu3O7- δ thin film with thickness of 200nm wasdeposited on the substrate using pulse laser deposition technique. The gradiometericpickup coil, composed of two rectangular coils with size of 3.5mm x 1mm (in the middle ofthe photo), are connected directly to the SQUID ring located at the center of the device.
The modulation coils, which shape looks like the mathematical symbol “∽”, locate inupper and down regions of the pickup coil. One of them is actually used, and the other isdummy to keep symmetry. The characteristics of the SQUID are summarized in Table 1.
2.2. Coaxial pulse tube cryocooler
A coaxial PTC has been developed for the system. Maximum amplitude of the mechanicalvibration of a coaxial PTC is generally smaller than conventional two-axial PTC. In orderto suppress the cryocooler-generated magnetic noise, the cryocooler and cryostat weremade of non-magnetic materials. The coaxial pulse tube and cold head were made ofstainless steel and copper, respectively. The cryostat was made of aluminium, copper andacrylic resin. The photograph of the PTC with cryostat is shown in Fig.3. For
easy-handling and practical use, the cryostat and PTC was designed to be compact (totally
4 kg in weight, 50mm in diameter, and 400mm in height). Because the compressor androtary valve for the PTC generate magnetic noises, they are spatially separated from thecryostat with length of a few meters. The characteristics of the PTC are summarized inTable 2.

3234

400mm

50mm
Figure 3. Coaxial pulse tube cryocooler integrated in the cryostat.
Table 2. Characteristics of the coaxial pulse tube cryocooler
Cooling capacity
8W (60Hz at 77K)
Ultimate temperature

53K (60Hz)

Cooling time (77K)
WeightCompressor power

25min.
4kg
800W

The pressure oscillation frequency of the cryocooler, which is generated by means of
the motor-driven rotary valve, is 2Hz. The temperature of the cold head is primarily
controlled by amount of helium gas using the bypass valve that controls the gas volume
supplied to the cryocooler (See Fig.1 (a)), and secondary controlled by heaters.
2.3. Separation of the SQUID-mounted stage
Although the vibration of the coaxial PTC is small, the vibration of the SQUID cannot stillbe neglected if the SQUID is directory mounted on the cold head of the PTC. For further
reduction of the vibration of the SQUID, we have designed an alternative stage where the
SQUID gradiometer is mounted, separated from the cold head. Figure 4 illustrates
schematically the cross-section view of the bottom part of the cryostat. The SQUID-set
stage, made of copper, is connected to the cold head with flexible fine copper wires. The
copper wires reduce the magnitude of the vibration and conduct the heat from the coldhead to the SQUID-set stage. The stage is fixed through FRP rods to the inner wall of thecryostat. The SQUID is mounted on the stage by using Apiezon grease, and cooled around77K. The distance between the SQUID and the outer bottom of the cryostat is 4mm.
The SQUID-set stage and the copper wires should play the role of thermal capacity andthermal buffer, respectively. Thus, the SQUID on the stage should be less affected by thethermal fluctuation of the cold head than the SQUID directly mounted on the cold head.
One-layer super-insulator films are glued on the inner bottom and wall of the acryl cap to
reduce the heat intrusion from outside. Thermocouples are mounted on the sidewall of thecold head and the SQUID-set stage to monitor the temperatures.

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Coaxial pulse
tube
Cold head

CryostatCopperwires

O-ring
FRP rod
Acrylic cap
SQUID-set stage
Thermocouple

4mm
SQUID

Figure 4. Schematic cross-section view of the bottom part of the cryostat.
3. Measurements and Results
3.1. System noise
We evaluated the noise performance of the cryocooler-cooled SQUID-NDE system bymeasuring the magnetic flux noise while cooling the SQUID at 75.3K, and comparing thenoise with that by former SQUID-NDE system using liquid nitrogen and the same SQUID.
Figure 5 shows the both noise spectra in the frequency range between 0.1Hz to 1000Hz.
The bold line and broken line correspond to results by using cryocooler and using liquid
nitrogen, respectively. Each measurement was conducted without magnetic shielding andwith FLL (flux locked loop) operation of the SQUID and dc bias. There is neither increaseof white noise nor peak noises due to the vibration of the cryocooler. Minimum magnetic
flux noise and magnetic filed gradient noise at 100Hz is about 80µφ 0/Hz1/2 and 1.3
nT/m/Hz1/2, respectively. The noise by using cryocooler is a little bit lower than that byusing liquid nitrogen. It should be due to the lower cooling temperature. Figure 5 indicatesthat the magnetic noise due to the vibration has been well suppressed by using the high-Tc
SQUID gradiometer, the coaxial PTC, and the separation of the SQUID-set stage.
We preliminary estimated the allowable amplitude of the vibration for the
SQUID-NDE system. For the estimation, we measured the environmental magnetic field
gradient at the place where the SQUID will be set using a fluxgate magnetic sensor. The
magnitude of the gradient was about 20µT/m. In the case that a SQUID magnetometervibrates with amplitude of l in an environmental magnetic field gradient of dBz/dl, themagnetic noise φn due to the vibration is expressed by the following equation,φn=(dBz/dl) ·l·S

(1)

where S is a pick-up coil area of the SQUID magnetometer. In this system, the directly
coupled type high-Tc SQUID gradiometer is used. The imbalanced area between the
pick-up coils of the gradiometer is about 1/1000 of the single pick-up coil area, 3.5mm x
1mm. We desired the peak noise due to the vibration was smaller than 50µφ0. Then, theallowable amplitude of the vibration is estimated to be shorter than 1.4µm according to theequation (1). The real vibration amplitude at the SQUID-set stage was confirmed by3236

Figure 5. Magnetic flux noise spectra of the cryocooler-cooled system, andliquid-nitrogen-cooled system using the same high-Tc SQUID gradiometer.
measurements with an accelerometer. The amplitude of the vibration along x-axis, y-axis(horizontal) and z-axis (vertical) were all within 0.8 – 1.5µm when the gas line made ofcopper between the cryocooler and the rotary valve was rigidly fixed. The pick-up coil areaof SQUID magnetometer is usually about several mm2 or more. It will cause a peak noiseover 104µφ 0 due to the vibration with the amplitude 1.4µm. Therefore, a SQUID
gradiometer is most effective factor to suppress the magnetic noise due to the vibration.
3.2.
Thermal stability
The thermal stability of the system was investigated by measuring the temperatures at the
SQUID-set stage and the cold head by the thermocouples. Firstly, the thermal drifts at bothwere measured. The temperatures starting from 75K (SQUID-set stage) and 68.2K(coldhead) were recorded for 12 hours without temperature control. During the measurements,
the temperature of the laboratory was controlled at 296K±0.5K by air conditioner. The
temperatures at the SQUID-set stage and cold head increased 0.1K and 0.07K per hour,
respectively. The drift at the SQUID-set stage causes the change of magnetic sensitivity
and increase of magnetic noise due to the change of Ic of the SQUID. For long timemeasurements more than several hours, the drift is required to be suppressed less than 0.1K.
Then, we introduced the temperature control. Considering that the time constant by thebypass valve control is too long, although this control makes less magnetic noise, we
adopted a temperature control with heater.
The temperature control system was composed of two heater sheets (MINCO 9914)
and temperature controller (Lakeshore Model331). Two heater sheets enclosed in indium
sheets were wounded around the SQUID stage (see Fig.6 (a)). The current supplied to eachheater is flowing in the opposite direction to each other, in order to cancel out the magnetic
noises due to the currents. The thermal stability with the temperature control system wasmeasured while stabilizing the SQUID-set stage at 75K. The result is shown in Fig.7.
Starting from 72.5K at the SQUID stage, it took about forty-five minutes to stabilize thetemperature at 75K. After the temperature was stabilized, the temperature fluctuation waswithin ±0.05K for several hours.

Thermocouple
SQUID-set stage

Heater sheet #1

Heater sheet #2

Temperature at SQUID-set stage (K)

3237

76
75.5
75
74.5
74
73.5
73
72.5
72
0

10

20

30

40

50

60

Time (min.)
(b)

(a)

Figure 6. (a) Photograph of the SQUID-set stage wounded by two heater sheets. The
cryostat is up side down in the photo. (b) Time trace of the temperature at SQUID-set
stage with the temperature control system. The final temperature was set at 75K.
3.3. Application to carbon-fiber composites
The ability of the SQUID-NDE system to detect defects in carbon-fiber composites wasdemonstrated by detecting deep-lying slots in C/Cs specimens. Figure 7 (a) shows the
cross-section of the prepared 4 specimens. All specimens were composed of two C/Cboards with thickness of 10mm, in which one board was stacked on the other. Size of thespecimens was 136mm in width, 240mm in length, and 20mm in total thickness. Eachspecimen had a slot of 20mm in length, 2mm in width and 5mm height at various depths in
the center of the specimen plane. The NDE measurements on the specimens were carriedout by the SQUID-NDE system with stand-off of 7mm, current of 5mA at 310Hz supplied
to the electromagnets, and sampling interval of 2mm. The quadratic-poles-like magnetic
signal due to the slot was detected near above the slot by two-dimensional scanning on the(1) Slot at 0 – 5 mm depth
20 mm

136mm
(2) Slot at 5 – 1 0mm depth

(1)
(2)

(3) Slot at 10 – 15mm depth

(3)
(4)

(4) Slot at 15 – 20mm depth

(a)

(b)

Figure 7. Detection of deep-lying slots in thick carbon-fiber composites by the
cryocooler-cooled SQUID-NDE system. (a) Four stacked C/Cs specimens with slots at
various depths. (b) The slot depth from the surface vs. signal amplitude due to each slot.

3238
specimens. Figure 7 (b) shows the amplitude between the positive peak and negative peakof the quadratic-poles-like signal due to each slot. The deepest slot with depth of 15mm
was successfully detected. According to the relation between the slot depth and the signal
amplitude and the system noise, deeper slot with depth of 30mm should be detectable by
the SQUID-NDE system with current of 100mA. Because the amplitude of the current
used in the measurements was limited by the residual magnetic flux coupled to the SQUID,
more accurate adjustment of the electromagnets will solve this problem.
4. Conclusions
An easy-handling and low-noise SQUID-NDE system cooled by a cryocooler has beendeveloped for practical NDE of carbon-fiber composites. A high-Tc SQUID gradiometer,
coaxial PTC, and separation of the SQUID-set stage were employed to realize a low
magnetic noise, low mechanical vibration and compact system without magnetic shielding.
The minimum magnetic flux noise is about 80µφ0/Hz1/2 at 100Hz dominated mainly by
SQUID itself. We estimated that the magnetic noise due to the vibration in this systemshould be about 50µφ0 with vibration amplitude of 1.4µm. This low noise owes primarily
to the SQUID gradiometer, secondly to lowly suppressed vibration. The temperature
control system with two heater sheets was introduced to stabilize the temperature of the
SQUID stage at 75K with thermal fluctuation of ±0.05K for several hours. The deep-lying
slot below 15mm from the surface in stacked C/Cs specimen was successfully detected by
the SQUID-NDE system. We concluded that this system has a potential for easy-handing
and on-site NDE measurements on the carbon-fiber composite structures.
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