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A 4 K Pulse Tube Cryocooler with
Large Cooling Capacity
C. Wang
Cryomech, Inc.
Syracuse, NY, USA 13211
ABSTRACT
We are developing a large cooling capacity pulse tube cryocooler to be used for
applications such as cryogen-free dry dilution refrigerators and large superconducting magnets,
etc. The pulse tube cryocooler with standard design, model PT420S, is driven by a compressor,
Cryomech model CP1114, with an input power of ~11.5 kW. After optimizing the PT420S
design, it provides, simultaneously, a cooling capacity of 2.01 W at 4.2 K on the 2nd stage and
52.6 W at 45 K on the 1st stage. By modifying the first stage regenerator and flow controls, a
large cooling capacity of 83 W at 45 K and 1.85 W at 4.2K could be provided. A low vibration
design with a remote motor, Model PT420RM, can provide 1.76 W at 4.2K and 53W at 45 K.
INTRODUCTION
4 K pulse tube cryocoolers have opened many challenging applications which require low
vibration, high reliability and long mean time between maintenance. For example, they cool
superconducting magnets and SQUIDs; pre-cool dilution refrigerators, He3 adsorption coolers
and adiabatic de-magnetization refrigerators for sub-Kelvin cooling1,2
. Some 4 K pulse tube
cryocoolers have been used for laboratory scale helium liquefiers and reliquefiers to re-liquefy
the boil off from open liquid helium cryostats3
. Many Cryomech 4 K pulse tube cryocoolers
have operated continuously for more than 50,000 hours in the field without cold head
maintenance.
Currently, the largest 4 K pulse tube cryocooler can provide 1.5 W at 4.2 K. Some large
superconducting magnets have to use multiple 4 K pulse cryocoolers4
. Furthermore, some
applications of dry dilution refrigerator demand a large 4 K pulse tube cryocooler to increase
cooling capacity at a temperature of ~10 mK. Some laboratories also require helium liquefiers
with higher liquefaction rate.
In response to these market demands, Cryomech is developing a larger 4 K pulse tube
cryocooler to provide a cooling capacity of ~2 W at 4.2 K. This paper presents some results of
the development.
#96
299Cryocoolers 19, edited by S.D. Miller and R.G. Ross, Jr.©¶International Cryocooler Conference, Inc., Boulder, CO, 2016
2
Figure 1. Large 4 K pulse tube cryocoolers. (a) standard design, model PT420S; (b) remote motor
design, model PT420RM.
SYSTEM DESIGN
We are developing the 4 K pulse tube cryocooler with two types of rotary valve integration.
1. Standard design with the rotary valve/motor assembly integrated on the warm end of the pulse
tube expander, See figure 1 (a), Model PT420S; 2. Remote motor design with the rotary
valve/motor assembly separated from the pulse tube expander, see figure 1 (b), Model
PT420RM. The standard design of the 4 K pulse tube cryocooler had been described in
reference [1].
In the remote motor design, the rotary valve/motor assembly is connected to the pulse tube
expander through a 61 cm long stainless steel flexible line. An electrical isolator is installed on
the flexible line to isolate electrical noise generated by the compressor and cold head motor. Two
reservoir volumes are connected to the pulse tube expander through one meter long stainless
steel flexible lines. The remote motor model can be used for extremely vibration-sensitive
applications.
Both the PT420S and PT420RM are driven by the same compressor, Cryomech model
CP1114, with an input power of ~11.5 kW.
Performance Optimization
The development of the larger 4 K pulse tube cryocooler was started with the standard cold
head model PT420S by modifying an existing 4 K pulse tube cryocooler, model PT415, which
provides 1.5 W at 4.2 K. The new system is driven by a larger compressor model, CP1114.
PERFORMANCE AND DISCUSSION
GM & GM-TYPE PULSE TUBE COOLER DEVELOPMENTS 300
3
1.1 1.2 1.3 1.4 1.5 1.6
25
30
35
40
45
50
55
Fir
st sta
ge
co
olin
g ca
pa
city (W
)
Operating frequency (Hz)
1st stage
2nd stage
Se
co
n sta
ge
co
olin
g ca
pa
city (W
)
1.50
1.55
1.60
1.65
1.70
1.75
1.80
1.85
1.90
Figure 2. Performance optimization at different operating frequencies
The performance of the 1st
stage at 45 K and 2nd
stage at 4.2 K has been tested at several
operating frequencies. The results are given in Figure 2. The 2nd
stage provides the best
performance of 1.77 W at 4.2 K with a frequency of 1.3 Hz. It also can also provide 1.73 W at
4.2 K with a 1.4 Hz operating frequency. All existing 4 K pulse tube cryocoolers at Cryomech
operate at 1.4 Hz. Considering production compatibility, an operating frequency of 1.4 Hz was
chosen for the PT420. The following improvements are all obtained at the frequency of 1.4 Hz.
Table 1 lists the performance improvements seen with a few modifications. Using new heat
exchangers for the 1st
and 2nd
stage, the performance of the PT420S was improved to have 1.88
W on the 2nd
stage and 41.4 W on the 1st
stage. The highest 2nd
stage capacity of 2.01 W at 4.2 K
was achieved by increasing the sizes of both 1st
and 2nd
stage pulse tube volumes. Modifying the
1st
stage regenerator gives the best 1st
stage performance of 83W at 45 K. We obtained two
results for the best 1st
stage performance or the best 2nd stage performance with the No.3 design
or No. 4 design.
Figure 3 shows the cooling capacity map for the best 2nd
stage performance implementing
the No. 3 design. It achieves bottom temperatures of 31.5 K on the 1st
stage and 2.83 K on the
2nd
stage. It can simultaneously provide 52.6 W at 45 K and 2.01 W at 4.2 K with an input
power of 11.42 kW.
Table 1. Performance improvements of various designs
No. 1, original PT415; No. 2, New 1st and 2
nd stage heat exchanger; No. 3, increase both 1
st and 2
nd
stage pulse tube volume; No. 4, new 1st stage regenerator.
Cooling performance No.1 design No. 2 design No. 3 design No. 4 design
1st stage at 45 K 40.6 W 41.4 W 52.6 W 83.0 W
2nd stage at 4.2 K 1.73 W 1.88 W 2.01 W 1.85 W
4 K PT CRYOCOOLER WITH LARGE COOLING CAPACITY 301
4
26 28 30 32 34 36 38 40 42 44 46 48 50
2.4
2.6
2.8
3.0
3.2
3.4
3.6
3.8
4.0
4.2
4.4
4.6
4.8
Se
co
nd
sta
ge
te
mp
era
tu
re
(K
)
First stage temperature (K)
2.0 W on 2nd stage
0 W on 2nd stage
0 W on 1st stage
52.6 W on 1st stage
Figure 3. Cooling capacity map for the best 2nd stage cooling capacity
24 26 28 30 32 34 36 38 40 42 44 46 48 502.4
2.6
2.8
3.0
3.2
3.4
3.6
3.8
4.0
4.2
4.4
4.6
4.8
5.0
Second stage tem
perature (K
)
First stage temperature (K)
1.85 W on 2nd stage
0 W on 2nd stage
0 W on 1st stage
83 W on 1st stage
Figure 4. Cooling capacity map with a new regenerator
Figure 4 shows the cooling capacity map for the best 1st
stage performance with No. 4
design. It has a lower bottom temperature of 28.3 K for the 1st
stage and 2.65 K for the 2nd
stage.
The No. 4 design achieved lower temperatures on both stages than the No.3 design. The
cryocooler can simultaneously provide 83 W at 45 K and 1.85 W with an input power of
11.47 kW.
Figure 5 shows a cool-down time of the PT420S with the No.4 design. The cool-down
speed is measured with a 60 W heat load on the 1st
stage and a 1.8 W load on the 2nd
stage. It
takes 133 min for the 1st
stage to reach a bottom temperature of 38.7 K and 105 min for the 2nd
stage to reach a bottom temperature of 4.15 K.
.
GM & GM-TYPE PULSE TUBE COOLER DEVELOPMENTS 302
5
0 20 40 60 80 100 120 140
-25
0
25
50
75
100
125
150
175
200
225
250
275
300T
em
peratu
re
s (K
)
Cool-down time (min)
1st stage with 60W
2nd stage with 1.8W
80 85 90 95 100 105 110
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
T2
(K
)
100 110 120 130 140 150
35
40
45
50
T
1
(K)
Time (min)
Figure 5. Cool-down curve of the PT420S
Performance of the remote motor model, PT420RM
Based on the improved performance of the PT420S, two remote motor pulse tube
cryocoolers, Model PT420RM, were built for an OEM customer. They have comparable
performance on both 1st
and 2nd
stages. Table 2 lists their performance. The cooling load map of
#1 system is given in figure 6. It has bottom temperatures of 29.0 K on the 1st
stage and 2.73 K
on the 2nd
stage. It provides 53 W at 45 K and 1.76 W at 4.2 K simultaneously with a power
input of 11.34 kW. Comparing the results of the PT420RM with that of the PT420S, the 1st
stage
cooling capacity is significantly reduced from 83 W to 53 W at 45 K. This loss will be
investigated later on.
Table 2. Performance of two PT420-RM systems
Cooling performance #1 system #2 system
1st stage at 45 K 53.0 W 52.6 W
2nd stage at 4.2 K 1.76 W 1.74 W
4 K PT CRYOCOOLER WITH LARGE COOLING CAPACITY 303
6
25 30 35 40 45 50
2.4
2.6
2.8
3.0
3.2
3.4
3.6
3.8
4.0
4.2
4.4
4.6
4.8
Se
co
nd
sta
ge
te
mp
era
tu
re
(K
)
First stage temperature (K)
0W
on 1st stage
53
W on 1st stage
1.76W on 2nd stage
0W on 2nd stage
Figure 6. Cooling load map of the PT420RM
0 25 50 75 100 125 150
-25
0
25
50
75
100
125
150
175
200
225
250
275
300
325
Te
mp
erature
s (K
)
Cool-down time (min)
2nd stage with 1.7W
1st stage with 50W
90 95 100 105 110 115 120 125 130
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
7.5
8.0
T2 (K)
90 100 110 120 130 140 150 160 170
40
45
50
55
60
65
70
T1(K)
Time (min)
Figure 7. Cool-down curve of the PT420RM
Figure 7 shows the cool-down curve of the PT420RM with 50W on the 1st
stage and 1.7 W
on the 2nd
stage. The PT420RM, with lower heat loads on the both stages, has a slower cooling
down rate than the PT420S. It takes 125 min for the 2nd
stage to reach a temperature of 4.14 K
and 160 min for the 1st
stage to reach 44.7 K.
Development of the PT420 pulse tube cryocooler is still ongoing to achieve better
performance and a faster cool-down rate.
CONCLUSION
A high cooling capacity 4 K pulse tube cryocooler, with options of the standard and remote
motor cold heads, is being developed at Cryomech, Inc. So far, this cryocooler can provide
2.01 W at 4.2 K and 52.6 W at 45 K using a standard cold head. Using a low vibration remote
GM & GM-TYPE PULSE TUBE COOLER DEVELOPMENTS 304
7 motor cold head, it can provide 1.76 W at 4.2 K and 53 W at 45 K. Developments are ongoing
to provide a final product with increased performance.
ACKNOWLEDGMENT
Author would like to thank J. Cosco for his help in testing and B. Zerkle for building and
assembling the system.
REFERENCES
1. Wang, C., "Characteristic of 4 K Pulse Tube Cryocooler in Application," Proceeding of ICEC20,
Elsevier Publisher (2005), pp. 265-268.
2. Uhlig, K., ”Dry dilution refrigerator with pulse-tube precooling,” Cryogenics, Vol. 44 (2004), p. 53.
3. Wang, C., "A helium re-liquefier for recovery and liquefying helium vapor from cryostat," Advances
in Cryogenic Engineering, Vol.55A, AIP publisher, Melville, NY (2010), pp. 687-694.
4. Green, M.A. and Wang, S.T., “Tests of four PT415 coolers installed in the drop-in mode,”
Proceedings of ICEC-22, Published by KIASC, Seoul Korea (2008), pp. 105-110.
4 K PT CRYOCOOLER WITH LARGE COOLING CAPACITY 305
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