1

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