Engineering Sciences Reports, Kyushu University
Vol.31,No.2,pp.1-4, SEP.2009

九 州 大 学 大 学 院 総 合 理 工 学 報 告第 31 巻 第 2 号 1-4 頁 平成 21 年 9 月

AlN ceramic thermal guide for cooling a SQUID and its
effect on thermal magnetic noise

Mohamad Javad ESHRAGHI*1,† Ichiro SASADA *1 Jin Mok KIM *2
and Yong Ho LEE*2
E-mail of corresponding author: javad4@asem.kyushu-u.ac.jp†(Received July 31, 2009)
We evaluated the thermal conductivity of AlN ceramic plates as a potential material of a thermal guide for cooling SQUID in cryocooler cooled SQUID systems. We found that the AlN ceramic
plate does not show a good thermal conductivity at very low temperature. We tried to enhance its
thermal conductivity by attaching a lot of thin insulated copper wires in parallel with each other tocover a back plane of the plate. With this enhancement we succeeded to operate a double relaxation
oscillation SQUID (DROS) first-order gradiometer. Comparison of measurement results on white
noise level between two cases where DROS was attached to a copper plate thermal guide and whereit was attached to a modified AlN thermal guide shows almost 5 time reduction in noise floor withthe latter plate.
Key words: Thermal magnetic noise, Magnetic measurements, SQUIDs

1.

Introduction

Superconducting quantum interference devices (SQUIDs) are used to measure magnetocardiogram (MCG) and magnetoencephalogram (MEG), which are used to diagnose heartdiseases and map brain functions. One of prerequisites for the operation of SQUIDs is thelow temperature sources that usually obtainedwith liquid helium. Closed cycle cryocoolers
are potential low temperature sources for
SQUID systems. If use of such cryocoolers forcooling SQUIDs becomes possible, costs forliquid helium and its handling will be eliminated. The most critical issue for use of such
systems for cooling SQUIDs is an impact of thecooler noise on the operation of the SQUID
system [1,2]. Among the several types of cryocoolers available, the pulse tube cryocooler(PT) is more stable than the other types such
as Gifford-McMahon (GM) cryocooler [3], in
respect of physical vibrations and magnetic
noise, because, unlike the GM cryocooler, thePT cryocooler has no moving part in the cold
stage. Therefore we used a two-stage PT cryocooler in our investigation.
In cryocooled LTS-SQUID systems, coppersensor holder could be used as a thermal guide,
but white noise level due to the thermal magnetic noise is high [4]. Therefore finding insu*1 Department of Applied Science for Electronics andMaterials, Interdisciplinary Graduate School ofEngineering Sciences.
*2 Korean Research Institute of Standards and Sciences.

lating materials with high thermal conductivity at very low temperatures is of great importance. AlN ceramic is an insulating material that shows good thermal conductivity at
the room temperature. Its thermal conductivity at the room temperature is nearly half ofthat of the copper. We examined the thermal
conductivity of AlN ceramic as a potential
material for the thermal guide in a cryocooler
cooled SQUID systems. We found out that theAlN doesn't show very good thermal conductivity at very low temperature, therefore byimplementing thin copper wires on one surfaceof the AlN thermal guide, we tried to enhance
its thermal conductivity and succeeded to operate it. Finally we compared the noise floor ofthe DROS attached to AlN thermal guide andcopper thermal guide and show that by using
AlN the noise floor could be reduced by a factorof 5.

2.

Experimental Setup

For examining the thermal conductivity ofAlN, an AlN thin rectangular plate was prepared with dimensions of 3 mm in thickness,
12 cm in length and 4 cm in width. A thickL-shape copper plate attachment was used tofix the AlN thermal guide to the second coldhead of our two-stage pulse tube cryocooler.

2AlN ceramic thermal guide for cooling a SQUID and its effect on thermal magnetic noiseof the temperature sensors were acquired automatically with a DAQ board every 16
seconds. The experimental setup is shown
schematically in Fig. 1(a). Fig. 1(b) is the photograph of the AlN ceramic plate attached tothe L-shaped copper at Pulse Tube cold head.
The SQUID used in our experiment wasdouble relaxation oscillation SQUID (DROS).
The pickup coil was a first-order planar gradiometer with two sets of square coils connected in series, and the baseline was 40 mm[10].

3. Result and Discussion(a)Results of the temperature measurement
are shown in Figs. 2(a). In Fig. 2(a) the temperature difference between the colder end
which is connected to the L-shaped copper attachment and the free end of the AlN thermal
guide is plotted for the cold head temperature(solid line). As it is visible in this graph thefree end of the AlN thermal guide in the bestcase is about 2 K warmer than the other end

(a)

(b)
Fig.1

(a) Schematic of the experimental setup including PT cold head, L-shape copper stage, AlN
sample and temperature sensors. (b) Photograph of the experimental setup.

Apiezon-N grease was used between AlN
thermal guide and the L-shaped copper attachment. The temperature of the L-shaped
copper attachment, which was attached to thecold head, was measured with a Pt-Co temperature sensor. The temperature of the free
end of the AlN thermal guide was measured
with a germanium temperature sensor which
was attached to the AlN thermal guide withbolt and Apiezon-N grease. Output from both(b)Fig.2(a) Temperature difference between the cold endof the AlN thermal guide and its free end (solidline). (b) same quantity for copper wire decorated AlN thermal guide (dotted line).

平成 21 年

九 州 大 学 大 学 院 総 合 理 工 学 報 告where the temperature of the cold end is 3.5 K,
whereas when our cryocooler cold head reaches 3.5 K, the AlN thermal guide’s warm end
cannot reach the desired temperature for operating LTS SQUIDs which is 4.2K.
In order to enhance the cooling capability ofthe AlN thermal guide, 30 percent of one sideof the AlN thermal guide was covered with abundle of 0.1 mm thick insulated copper wires
placed in a single layer. Because adjacentcopper wires do not have electrical contact,electrons in the wires fluctuate only one dimensionally, hence, based on the Ref. [4,5] thethermal magnetic noise may be much lower
than that observed with a thick copper thermal
guide used as a SQUID holder. Photograph ofboth sides of the AlN ceramic plate is shown inFig. 3. The temperature of the free end of theAlN thermal guide with the copper wire decoration has successfully reached 4 K about 1
hour after the cold head had maintained
at 3.75 K. The result of examination of thethermal conductivity of this AlN thermal guideis shown in Figs. 2(b).
For operating DROS inside the cryostat, interfering high frequency current noises conducted through flexible hoses from an inverter
driving the valve motor to the cryostat should
be reduced. The detailed arrangement that weused for reducing the high frequency noise isdescribed elsewhere [6]. The two stage pulse
tube cryocooler contains magnetic regenerative
materials such as Er3Ni and HoCu2 in its
second stage regenerator section. Use of thesematerials is necessary for reaching the liquid
helium temperature (4 K) in such cryocoolers.
In our previous research, we found out that thetemperature variation of these materials
caused by helium gas excursion in the presenceof background magnetic field produce low fre-Fig.3 AlN ceramic plate with copper wire attached to
one side.

第 31 巻第2号

3

quency magnetic interference to the SQUID
attached to cold head [1]. We used superconducting lead shielding over the section containing magnetic regenerative materials andreduced the low frequency interference to theSQUID by a factor of 100. The cryostat was set
in a cylindrical magnetic shield developed in
our laboratory whose transverse shielding
factor for a low-frequency time-varying magnetic field was as large as 40,000 when themagnetic shaking enhancement was on, whereas it was 140 along its axis [11]. After employing aforementioned methods for reducing
high frequency EMI and low frequency magnetic interference, we measured the magnetic
field by the DROS first-order gradiometercooled by the pulse tube cryocooler attached toa copper thermal guide and on wire decorated
AlN thermal guide. Results of these measurements are shown in Figs. 4 and 5.
Fig. 4 is the magnetic field spectrum while
cryocooler is operating. From this graph it
could be understood that by using AlN holder
instead of copper holder the noise floor at 200
Hz reduces from about 500 fT/√Hz to less than
100 fT/√Hz. For evaluating the effect of using
AlN thermal guide on noise floor at lower frequencies we measured the magnetic field byturning off the cryocooler. After turning off thecryocooler the SQUID could be operated forfew minutes. To see the system noise without
interference of cryocooler noise we quit thecryocooler. Result of this measurement isshown in Fig. 5 for the SQUID attached to AlN
thermal guide and copper thermal guide.
One can see that using AlN reduces noisefrom 1 pT/√Hz to less than 300 fT√Hz at 10 Hz.
Noise floor at 200 Hz with copper thermal
guide is 500 fT√Hz, while it is less than 100fT/√Hz with AlN thermal guide. Roughly
speaking implementing AlN results in almostFig.4 Noise floor comparison between DROS attached
to a copper thermal guide (dashed line) and tothat of AlN (solid line) while cryocooler operating.

4

AlN ceramic thermal guide for cooling a SQUID and its effect on thermal magnetic noise

DROS gradiometer mounted on the AlN ceramic plate enhanced with copper wires becomes almost 5 times smaller than that of aDROS mounted on a copper plate.
References

Fig.5 Noise floor comparison between DROS attached
to a copper thermal guide (dashed line) and tothat of AlN (solid line) while cryocooler isturned off.

5 times reduction in noise floor compared tothe case with a copper plate. Magnetic noiselevel measured with DROS operated in liquid
helium in a magnetically shielded room (MSR)
at 200 Hz is around 6 fT/√Hz [9]. Therefore thenoise floor in magnetic field measurement by
DROS operated in PT cryocooler is 20 times
higher than that operated in liquid helium in aMSR. One factor could be the existence of metallic parts such as bolts and thermal shield
made of aluminum plate at first stage. Anotherfactor could be the difference between theshielding factor of our cylindrical shield andMSR used in Ref. [9] and also difference between magnetic environments of two experiments. Therefore by exchanging conducting
parts of PT cold head with proper insulatingparts and improving the shield the noise floor
could be reduced further.

4.

Conclusion

We observed that the temperature of the free
end of the AlN thermal guide hardly fell below
7 K even one hour after the cold head hadreached 3.75 K. So we conclude that this material by itself cannot cool LTS SQUID sensors
to operate. The cooling capability of the AlN
thermal guide was enhanced by covering almost 30 percent of one side of it with 0.1 mm
diameter insulating coated copper wires placelongitudinally. The free end of AlN thermal
guide successfully cooled down to 4 K. Therefore this material with the aforementioned
improvement could be used as a thermal guideto cool SQUIDs. DROS first-order planar gradiometer successfully operated by implementing AlN ceramic plate as a thermal guide in
two-stage PT cryocooler. The measurement
results show that replacing copper thermal
guide by modified AlN reduced the noise floor
from 500 fT√Hz to less than 100 fT√Hzat 200Hz. Roughly speaking noise floor of a1) M.J.Eshraghi, I.Sasada, J.M.Kim and Y.H.Lee, Characterization of a low frequency magnetic noise from a
two-stage pulse tube cryocooler, Cryogenics
ly 2009) 49, Number7, 334-339
2) K. Sata, T. Yoshida, S. Fujimoto, S. Miyahara, Y.M.
Kang A cryocooled helmet-shaped MEG measurement
system Supercond. Sci. Technol. (1999) 12, 9599623) R. Hohmann, C. Lienerth, Y. Zhang, H. Bousack, G.
Thummes, C. Heiden, Comparison of Low Noise
CooLing Performance of a Joule-Thomson Cooler anda Pulse-Tube Cooler Using a HT SQUID IEEE Trans.
Appl. Super., (1999) 9, No. 24)] T.Varpula, T.Poutanen, Magnetic field fluctuation
arising from thermal motion of electric charge inconductors, J. Appl. Phys. (June 1984) 55, Number 11,4015-4021
5) T. Yoshida, K,i. Sata, K. Yoshii, Y.M. Kang, A. Suzuki
A cryocooled 61-channel MEG system Applied Superconductivity, (July 1998) 5, Number 7 , 385-392
6) M.J. Eshraghi, M. Cho, J.M. Kim and I. Sasada, Conducted EMI Reduction on the Pulse Tube CryocoolerIEEETRANSACTIONSONMAGNETICS,(Jun 2009) 45, Number 6, 2754-2757
7) S. Fujimoto, K. Kazami, Y. Takada, T. Yoshida, H.
Ogata , H. Kado, Cooling of SQUIDs using aGi_ord-McMahon cryocooler containing magnetic regenerative materials to measure biomagnetism
Cryogenics, (1995) 35, 143-148
8) J.M. Kim, Y.H. Lee, K. Kim, H. Kwon, Y. Park and I.
Sasada, Compact Readout Electronics for 62 Channel
DROS magnetocardiogram System, IEEE Trans. Appl.
Supercond. 2005, 15, Number 2, 644-647
9) C.S. Kang, Y.H. Lee, H. Kwon, J.M.Kim, K. Kim,
S.G.Lee and Y.K. Park, A multichannel double relaxation oscillation SQUID magnetometer system formagnetoencephalogram phys. stat. sol. (a) 201,Number 8, 19561960 (2004)
10) Y.H. Lee, J.M. Kim, K. Kim, H. Kwon, K.K. Yu, I.S.
Kim and Y.K. Park, 64-channel magnetocardiogramsystem based on double relaxation oscillation SQUID
planar gradiometers, Supercond. Sci. Technol. 19(2006) S284S288
11) I. Sasada, T. Kimura, Cylindrical magnetic shield
made of a CFRP amorphous magnetic tape CFRP
layered composite with built-in shaking coil J. Appl.
Phys. (2006) 99, Number 1