Indian Journal of Pure & Applied Physics
Vol. 46, May 2008, pp. 334-338
Cryogen-free low temperature and high magnetic field apparatus
S D Kaushik, Anil K Singh, D Srikala & S Patnaik
School of Physical Sciences, Jawaharlal Nehru University, New Delhi 110 067
Received 12 December 2007; accepted 15 January 2008
The importance of low temperature and high magnetic field measurements in pure and applied science research cannot
be overstated. Traditionally these experiments have been carried out by evaporation of liquefied helium. This is a costly
proposition, especially in our country, where maintaining liquid helium plants and the recovery lines has become persistent
predicament. In this paper, the possibility of an alternative cost-effective technology based on two stage Gifford-McMahon
closed cycle cryocoolers which is most ideally suited for small groups of researchers has been presented. The principle of
operation and instrumentation details of a recently installed single compressor 1.6 K, 8 tesla cryocooler has been described.
Keywords: Liquid helium, Gifford-McMahon cycle, Regenerator, Cryocooler
1 Introduction
Temperature and magnetic field are the most
crucial variables for the entire spectrum of pure
science research. Particularly in material science and
condensed matter physics, all characterization
techniques are primarily centered on these facilities.
Studies over the widest range of temperature and
magnetic field therefore, hold the key to discovering
new science and functional materials. While
measurements above the room temperature can be
attained by controlled heating, most low temperature
experiments are carried out using a variety of gases,
prominently nitrogen and helium, in their liquid form.
The standard method is first to liquefy the gas by
mechanical means, then to transfer it to the
measurement set-up, and then to vapourize it over a
small volume. The latent heat of vaporization during
this process is obtained from the liquid itself leading
to further lowering of temperature of the liquid phase
of the gas. The time honored procedure involves
liquid helium (He24), that remains a gas at
atmospheric pressure down to 4.2 K. By evaporating
helium at low pressure, temperature down to 1.5 K
can be easily achieved. For generating high magnetic
field too, liquid helium is indispensable. Almost all
high field magnets routinely used in research
laboratories and industry are made up of Nb based
superconductors (e.g. Nb-Ti, Nb3Sn). These materials
need cooling down to 4.2 K to achieve
superconducting state and sustain high currents in
solenoidal geometry for up to 18 tesla field.
Therefore, the production, supply and storage of
liquid helium are the most critical infrastructure
requirement for a quality condensed matter program.
Current situation however has been discouraging
on two counts. For over ten years now American
Physical Society has been predicting severe shortage
of helium gas by early 21st century. The cost of
liquefier (with reasonable throughput and longevity)
and its maintenance have skyrocketed in the recent
past making it next to impossible for small groups to
conduct low temperature measurements. Fortunately,
with the advent of high wattage closed cycle
refrigerators, the dependence on costly liquid helium
to carry out quality low temperature experiments is a
thing of the past. In this paper, we describe the
principle of operation and successful installation of an
entirely liquid cryogen-free low temperature high
magnetic facility at the School of Physical Sciences,
Jawaharlal Nehru University, New Delhi.
2 Principle of Closed Cycle Cryocoolers
The first patent for industrial production of liquid
nitrogen by liquefaction of atmospheric gas was filed
by Carl Linde (dated July 9, 1895). The invention was
based on Joule-Thomson effect which states that any
real gas undergoes a change in temperature while
traversing from a high pressure region to low pressure
region under isoenthalpic condition. The change in
temperature, whether increase or decrease, is
determined by change in internal energy of the gas
when the average separation between the molecules of
the gas increases under expansion2. Almost 65 years
later, Kohler and Jonkers at the Philips company,
were the first to implement a closed cycle based air
liquefier, formally known as the Philips-Sterling
cycle. The cooling was achieved by free expansion of
gas rather than the throttling process of J-T effect.
KAUSHIK et al.: CRYOGEN-FREE LOW TEMPERATURE
This was based on compression of gas followed by
transfer through a regenerator onto a space where the
gas is expanded and cooled before returning to
uncompressed state. The regenerator acts as a heat
exchanger as well as a thermal seal between the warm
and cold ends. The ‘Kohler’ design involved out of
phase movement of two pistons with helium gas as
the working substance3. The Gifford-McMahon (GM)
cycle4,5, on which most of the present day sub-4 K
cryogen-free refrigerating systems are based, is a
straight forward modification of the Philips-Sterling
method.
Single stage GM cryocoolers (~30 K), have been
widely used as cryopumps and as radiation shield in
MRI machines for decades. The schematic diagram of
the GM cycle is shown in Fig. 1. The main parts of
the GM apparatus are, a) compressor CP, b) displacer
piston D, c) regenerator R, d) intake valve VI, and e)
exhaust valve VE. The working material could be pure
helium gas. The displacer is cased inside a cylinder
335
and its basic function is to displace a volume of gas
through the thermal regenerator R. The regenerator
maintains a large thermal gradient between the cold
and hot ends of the cylinder and it is made up of
materials with large molar heat capacity. The
displacer is tight fitted to the cylinder through sliding
seals that prevent gas flow through the radial space
between the displacer and the cylinder. Effectively,
the cold (C) and warm (W) volumes can be varied by
the movement of the displacer but the total volume
remains constant throughout the cycle. The opening
and closing of the inlet and outlet valves are
synchronized with the position of the displacer in the
cylinder through a rotary drive mechanism. The four
distinct steps of the GM cycles as shown in Figure 1
are:
(i) With the displacer at the cold end the intake
valve VI is opened and the compressed helium gas
fills the volume W; (ii) the displacer is moved to the
warm end with VI open. In order to keep the pressure
(i)
(ii)
VI
VI
CP
CP
W
VE
VE
R
R
D
D
C
Load
Load
(iv)
(iii)
VI
VI
CP
CP
W
VE
VE
D
R
D
R
C
Load
Load
Fig. 1 — Schematic description of 4 stages of GM cycle, (i) intake valve is opened with exhaust valve closed. The displacer is at the cold
end. High pressure helium gas from the compressor fills the warm end volume W, (ii) displacer is moved to the warm end with intake
valve open and exhaust valve closed Helium gas is forced to move towards cold end through regenerator, (iii) intake valve is closed and
exhaust valve is opened. This allows helium gas at cold end to undergo expansion that leads to cooling, (iv) displacer is brought back to
cold end and the exhaust valve is closed
336
INDIAN J PURE & APPL PHYS, VOL 46, MAY 2008
constant gas slowly flows into the cold volume C
through the regenerator R; (iii) the intake valve is then
closed and the exhaust valve VE is opened forcing the
compressed gas at the chamber C to undergo
expansion and consequent cooling; (iv) the displacer
is moved towards the cold end to drive the remaining
gas in the volume C. The exhaust valve is closed and
the cycle is repeated.
The performance of the GM cryocooler depends to
a great extent on the effectiveness of the regenerator
material. In conventional two stage GM cryocoolers,
usually lead (Pb) is used as regenerator and
temperature down to 6 K can be achieved. However,
the heat capacity of lead is negligible as compared to
pressurized helium below 6 K and therefore, it is
impossible to reach sub liquid helium temperatures
with lead as the regenerator. Using rare earth alloy
GdxEr1−xRh, Yoshimura et al.6 were the first group to
reach 3.3 K by using a 3 stage GM cryocooler6. These
compounds undergo a magnetic ordering transition
below 20 K. Here the displacer has three stages,
which means that it uses three different materials as
regenerators depending on the temperature of the
stage. In the process, Yoshimura et al.6 were able to
produce liquid helium at the rate of 10 cc/h by
condensing helium gas near the cold head. However,
high price of the Rh based materials prevented large
scale commercialization of this technique. Recent
development in regenerator technology has enabled
the use of Er3Ni/Er0.9Yb0.1Ni hybrid regenerator which
is cost effective and can provide cooling power7 up to
1.5 W at 4.2 K. Now even sub 2 K cryocoolers are
available using a variety of magnetic resonators. A
typical design as described by Numazawa et al.6 uses
spherical Pb particles in cupper mess as the first stage
regenerator and a mixture of Pb, HoCu2, Gd2O2S,
GdAlO3, and GdVO4 as the second stage regenerator8.
The industry leader in this technology is the
Sumitomo Heavy Industry (SHI) of Japan. Most of
the cryogen-free high magnetic field systems built by
Janis, Cryogenics, CryoIndustries, Cryomagnet, and
Cryomech are based upon SHI compressors and
displacers.
3 Cryogen-Free System at JNU
The low temperature high magnetic field facility
installed at JNU can cool down to 1.6 K in
temperature and can generate up to 8 tesla magnetic
field without any use of liquid cryogen. With separate
attachments the sample space can be cooled to 300
mK (helium 3) and heated up to 1000 K (encapsulated
oven). More advanced models with hybrid magnets
can produce upto 18 tesla field. These models require
two compressors, one for cooling the magnet and the
other for cooling the sample space.
The system that is installed at JNU is a single
compressor open-ended system purchased from M/s
Cryogenics of UK. It involves a SHI compressor
model CSW 71 in conjunction with a two stage
displacer model SRDK 408D. The compressor is
water cooled and requires continuous supply of
chilled water (~15°C) at the rate 7 lit/min. The 3
phase power requirement is ~ 9 kW. The cooling
powers at the first and second stage cold heads are
34 W @ 40 K and 1 W @ 4.2 K, respectively. The
second stage cooling is shared between the sample
space and the Nb-Ti superconducting magnet
(Tc = 9 K). The first stage is connected to an ultra pure
aluminium radiation shield. The magnet is latched to
the second stage of the compressor. A condensation
pot is attached to the magnet. When low pressure
helium gas from a separate close cycle reservoir is
brought in contact with condensation pot the gas
liquefies and with suitable pumping, the base
temperature of 1.6 K can be achieved. It is to be
emphasized that the process is entirely liquid cryogenfree and during the run over the last two years we
have not spent a single rupee on consumables like
liquid nitrogen or liquid helium or helium gas. Further
since the sample space is always cooled by the helium
vapour, the vibrations due to mechanical movement
of displacer do not interfere with the measurements.
Moreover, for entirely vibration-less environment, as
mandated by STM and point contact measurement
set-up, the advanced pulsed-tube displacer could be
opted for.
The block diagram of the cryogen-free system is
shown in Figure 2. The static and dynamic pressures
in the compressor are 1.7 MPa and 2.5 MPa,
respectively. Compressed helium from the
compressor reaches the cold head via flexible high
pressure hoses. The cold head is housed in a specially
designed cryostat chamber that shields it from outside
with vacuum and layers of radiation shield. The
cryostat also includes a vertical column where the
sample is inserted using a dip-stick (variable
temperature insert or VTI), a superconducting magnet
and a condensation pot. The temperature of the
superconducting magnet always remains ~4 K except
for small variation of the order of 0.5 K during
charging and discharging of the magnet. The radial
sample space is 30 mm and the field homogeneity in
KAUSHIK et al.: CRYOGEN-FREE LOW TEMPERATURE
Temperature Scanner
Keithley - 2700
Temperature controller
Lakeshore-340
337
16 pin Connector for
sample characterization
probes
VTI
VTI pressure
Compressor
Sumitomo
CSW-71
Air lock
valve
Magnet Power Supply
SMS 120C
Needle valve
Cryostat
Helium
reservoir
Stage1
Superconducting
magnet
Stage2
Condensation
pot
Dry
Pump
Sample
holder
Fig. 2 — Block diagram of the GM cycle based cryocooler and cryostat. The primary helium closed cycle comprises of the compressor
connected to the displacer through flexible hoses Separate helium gas close loop consists of a low pressure helium reservoir, a
condensation pot, a needle valve and a dry pump. The position of superconducting magnet attached to the second stage of the cold head is
also shown. The other interfaced instruments such as magnet power supply, Keithley multimeter with scanner, and temperature controller
are also shown
this region is 0.09% over 10 mm. The condensation
pot is connected to a separate helium gas recycle
consisting of a reservoir (~50 liter helium gas at ~3
psi pressure), an air filter and a dry pump. The helium
gas gets liquefied locally at the pot and its vapour
flow (and therefore the temperature) in VTI chamber
is controlled by a needle valve. To reach 1.6 K, a
pressure of 8 mbar is maintained in the VTI chamber.
Controlled heating using 25 W heater near VTI base
dictates the temperature of the helium vapour at the
sample space. Resistive temperature sensors are
placed at various points in the cryostat such as the
first stage, shield, second stage, magnet, condensation
pot, and exchanger exhaust to continuously monitor
the system parameters. The resistance (temperature) at
these points is measured using a Keithley 2700
multimeter with a 10 channel scanner. The
temperature in the VTI and on the sample holder is
measured and controlled by Cernox sensors using a
Lakeshore 340 temperature controller. The Magnet
power supply is cryogenic Model SMS 120 C. The
maximum field of 8 tesla requires current flow of
108 Ampere. The magnet is fitted with a persistent
switch that enables very stable homogeneous field.
Since we have purchased the bare system without the
characterization attachments, we have the flexibility
to design them as per our requirements. The various
attachments already implemented at JNU include dc
and ac, resistivity, Hall effect, magneto-resistance,
dielectric constant, and RF penetration depth. We are
currently trying to integrate a thermo-electric power
and an ac susceptibility attachment into the system.
The interfacing software and automation have also
been developed in house using Labview. The
attachments and the sample can be taken out of the
system without warming the system to room
temperature (unlike the Quantum Design PPMS). This
is achieved by an airlock valve.
INDIAN J PURE & APPL PHYS, VOL 46, MAY 2008
338
shown in Fig. 3(b). It is to be noted that MgB2 is an
intermetallic superconductor and its 40 K resistivity
(above transition temperature) even in the dirty limit
is of the order of 30 µΩ cm. Evidently, the
experimental data quality is not compromised because
of the mechanical movement of the piston and due to
the varying heat load during the isofield temperature
scan.
35.0
H//ab
30.0
(a)
Resistivity (µ Ω cm)
25.0
20.0
7T
0T
15.0
4 Conclusion
To conclude, we have discussed the importance of
closed cycle based cryogen-free low temperature and
high magnetic field facilities especially in our country
where maintenance of liquid helium plants has
become a regular problem. We have also discussed
the working principle of such an instrument and
shared our experience on installation and running of
such infrastructure. On the whole we believe that the
technology of 4 K closed cycle cryocoolers is reliable
and with constant improvement, it is going to replace
the classical liquid helium based experiments sooner
than later.
10.0
5.0
0.0
22
24
26
28
30
32
34
36
Temperature (K)
1.0
Normalized resistivity
(b)
0.8
5T
4T
3T
2T
1T
0.6
0.5T
0T
0.4
0.2
0.0
0
1
2
3
4
5
6
7
8
Temperature (K)
Fig. 3 — (a) Magneto-resistance study of ion irradiated MgB2 film
Resistivity is plotted as a function of temperature at different
constant magnetic field from 0 to 7 tesla at 1 tesla steps. Magnetic
field is applied parallel to ab plane of the sample, (b) Magnetoresistance study on NbSe2 flake plotted in y-axis is the normalized
resistivity that was measured in the van-der Pauw configuration.
On applying field beyond 3 tesla, transition temperature of the
sample decreased below 16 K
Figure 3(a) shows the data for resistivity as a
function of temperature with applied magnetic field
ranging from 0 to 7 tesla on an ion irradiated MgB2
thin film9 (superconductor with Tc = 35 K). The field
is applied parallel to the ab plane of the sample.
Similar measurement on NbSe2 (superconductor with
Tc = 7.1 K) flake in the presence of 0 to 5 tesla field is
Acknowledgement
We acknowledge the funding from Department of
Science & Technology, New Delhi, for the cryogenfree low temperature high magnetic field facility at
JNU. SDK and AKS acknowledge the CSIR, and
DSK acknowledges University Grants Commission,
New Delhi, for financial support. We thank Indrajit
Naik and J E Giencke for the samples used in this
study.
References
1 Kaplan Karen H, Physics Today, 60 (2007) 31.
2 Staticstical thermal physics (F Reif, McGraw-Hill), 1985, p
175
3 Experimental techniques in low-temperature physics, G K
White, (Clarendon Press Oxford), 1979 p 12
4 Gifford W E, US patent 2966035, 1960.
5 McMahon H O & Gifford W E, Adv in Cryogenic Engg, 5
(1960) 354.
6 YoshimuraHideto, Nagao Masashi, Inaguchi Takashi, Yamada
Tadatoshi & Iwamoto Masatami, Rev Sci Instrum, 60 (1989)
3533.
7 Merida W R & Barclay J A, Advances in Cryogenic
Engineering, 43 (1998) 1597.
8 Numazawa T, Kamiya K, Satoh T, Nozawa H & Yanagitani T,
IEEE Trans Appl Suporcon, 14 (2004) 1731.
9 Kaushik S D, Patnaik S et al. Physica C, 442 (2006) 73.