Patent Application Publication

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Nov. 3, 2022 Sheet 4 of 4 US 2022/0349628 Al

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US 2022/0349628 Al

unless there is sufficient lateral thermal conduction. Alter-
natively, because the displacer gap is dynamic, the local gap
dimensions can vary during the operating cycle, causing
local oscillating flow to not exactly reverse motion, again
leading to unregenerated flow and cooling loss.

[0028] While it is well documented that, in large diameter
regenerators, net circulation can lead to significant degra-
dation in coolers, we were concerned that even in small
diameter regenerators, some level of circulation can exist,
causing measurable cooling loss. In multi-stage coolers,
circulation can be generated in the regenerators near the
junctions between stages, where the phasing of the flows
between the upper stage regenerator, lower stage regenera-
tor, and buffer tube will necessarily lead to circulation in the
regenerators if the upper and lower stage regenerators are
in-line with each other because flows entering the regenera-
tor can come from either the adjacent regenerator or flow
channel from the buffer tube. To mitigate this, some embodi-
ments locate the second stage regenerator such that the
warm entrance is in the connecting channel between the first
stage regenerator and first stage pulse tube rather than
directly in-line with the first stage regenerator.

[0029] Thus, the flow entering the cold end of the first
stage regenerator always comes from the channel. This
arrangement was not workable in this embodiment for the
junction between the second stage and third stage regenera-
tors because the second and third stage regenerator packing
access was through an attachment flange for the third stage
regenerator which was directly in line with the second stage
regenerator. However, the small diameters of the second and
third stages likely mitigated any effects of circulation.
[0030] For the JT system 160, the very low cooling
capacity requirement allowed a design driven by ease of
fabrication rather than by thermodynamics. The stage nomi-
nally operates between a high pressure of 200 kPa and a low
pressure of 3.2 kPa, with a flow rate of 0.6 mg/s. The
thermodynamic power required to recompress the gas is
only a few watts, which is such a small fraction of the total
power budget that a highly thermodynamically efficient
design was not required.

[0031] All four counterflow heat exchangers 110, 112,
114, 116 are simple tube-in-tube designs, with high-pressure
gas flowing in the narrow annular space between the two
tubes and the low-pressure gas flowing through the inner
tube This configuration simplifies fabrication, plus has a
large internal surface area on the high-pressure return side to
allow condensable contaminants to freeze out without plug-
ging the flow passage.

[0032] The expansion impedance 120 was a 2 m long, 50
jun inner-diameter stainless steel capillary.

[0033] At the cold ends of the warmer three counterflow
heat exchangers 110, 112, 114, a small heat exchanger
consisting of fine mesh copper screen diffusion bonded to a
copper body was used to heat sink the incoming gas to the
pulse tube. The 1.7 K coldstage 116 also had a small copper
screen mesh heat exchanger for thermal contact to the load
(see FIG. 2B).

[0034] To mitigate particulate contamination, sintered
stainless steel particulate filters 104 were inserted in the
high-pressure line prior to entering the cryostat, at the end of
the heat exchanger 132 at 80 K (not shown) and just
upstream of the JT expansion impedance (not shown). In
addition, a small capsule of activated charcoal (not shown)

Nov. 3, 2022

was placed in the high-pressure stream on the 80 K stage to
adsorb any condensable contaminants.

[0035] To expedite cooldown of the JT coldstage from
room temperature, a heat switch 140 consisting of a small
bar that clamped down on a small copper tab attached to the
JT coldhead was used. The clamp was heat-sunk to the pulse
tube 10 K stage 136 and was actuated by pulling on a fine
stainless steel wire attached to the bar (not shown). The far
end of the wire was fed through a bellows feedthrough
through the room temperature vacuum flange to allow
actuation of the heat switch.

[0036] In the 2.2 K embodiment of FIG. 2A, both the inlet
and outlet of the JT expansion capillary 120 were housed in
the same copper block 202 and therefore at the same
temperature. In the 1.7 K embodiment of FIG. 2B, the warm
and cold ends of the capillary were separated, and only the
cold end was housed in copper block 204. The result was a
lower minimum base temperature of 1.7 K in comparison to
the 2.2 K base temperature of the FIG. 2A embodiment.
[0037] The JT compressor is a scroll compressor, selected
because of the ability to achieve low suction pressures,
compact size, and low per-unit cost. Because of the very low
flow rates and low thermodynamic compression power, the
IT loop did not require a high degree of optimization, so the
counterflow heat exchangers were designed for ease of
fabrication. They were modeled using a simple NTU analy-
sis using temperature-averaged fluid properties, along with
a separate calculation of thermal conduction down the heat
exchanger tubes. There was considerable leeway in the
design, allowing heat exchanger lengths to vary by a factor
of two without significantly affecting performance. The
main consideration was minimizing the demand on the JT
compressor, so the system was designed for the lowest
reasonable high-side pressure to minimize stresses on bear-
ings and back-leakage through seals. Final flow and pressure
requirements for the compressor were determined from test
data.

[0038] Test results of the pulse tube:

[0039] FIGS. 3 and 4 show the cooling capacities of the 80
K stage and 25 K stage as a function of temperature. Each
load curve was taken while holding the other two stages at
constant temperature. These plots illustrate that the 80 K
stage handles loads on the order of watts, while the 25 K
stage handles loads on the order of tenths of watts.

[0040] FIG. 5 is a plot showing load curves for the 10 K
stage and compares the performance of the all ErPr regen-
erator and the regenerator with ErPr in the warm half and
ErNi in the cold half, with data for ErPr/ErNi regenerator
combination shown at three different charge pressures. In all
cases the ErPr/ ErNi regenerator outperforms the all-ErPr
regenerator, except at temperature above 10 K with 1.25
MPa charge pressure for the ErPr/ErNi regenerator. The
ErPr/ErNi combination also consistently reached a mini-
mum temperature in the 6.3 K to 6.5 K range while the
minimum temperature of the all ErPr regenerator reached a
minimum temperature slightly below 8 K. These lower
temperatures were expected because of the lower heat
capacity of ErPr compared to ErNi at lower temperatures.
[0041] These data were taken with no applied load to the
upper two stages such that the upper stages ran colder than
the data in Table 6, which resulted in higher 10 K cooling
capacity.

[0042] Testing on the integrated JT-pulse tube coldhead
was conducted open loop using a regulated compressed gas



Patent Application Publication Nov. 3, 2022 Sheet 3 of 4 US 2022/0349628 Al

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US 2022/0349628 Al

COMPACT LOW-POWER CRYO-COOLING
SYSTEMS FOR SUPERCONDUCTING
ELEMENTS

BACKGROUND OF THE INVENTION

[0001] This application incorporates by reference parent
applications 62/621,483, filed Jan. 24, 2018, and 16/256,
736, filed Jul. 24, 2019

[0002] ‘This invention was made with government support
under grant number 70NANB14H095 awarded by NIST.
‘The government has certain rights in the invention

FIELD OF THE INVENTION

[0003] The present invention relates to compact, low-
power cryo-cooling systems for superconducting elements.
In particular, the present invention relates to such systems
having a pulse tube compressor and a Joule-Thomson (JT)
cooler.

[0004] DISCUSSION OF RELATED ART

[0005] Applications for superconductor-based sensors and
electronics have been steadily increasing over the past
several years in diverse areas such as astrophysics/cosmol-
ogy, X-ray spectroscopy, gamma-ray spectroscopy, quantum
information, and photon science. These systems operate at
temperatures ranging from above 4 K to lower than 50 mK.
Although these systems are developed by scientists with
low-temperature expertise, end users typically have minimal
cryogenic experience, and therefore acceptance has been
greatly facilitated by push-button, closed-cycle cryo-cool-
ers. The majority of these systems depend on precooling
with commercial Gifford-McMahon or valved pulse tube
coolers, which were originally developed for applications
requiring relatively large cooling capacities such as shield
cooling in magnetic resonance imagers or cryo-pumping. As
a result, for many, if not most of these systems, the size and
power consumption of the system are orders of magnitude
larger than necessary. For example, many transition edge
sensor (TES) systems are cooled using adiabatic demagne-
tization refrigerators (ADRs) with cooling capacities of
hundreds of nanowatts at temperatures below 100 mK. At
the 4 K reject temperature for the ADR, the heat rejected is
on the order of a milliwatt, yet precoolers with capacities on
the order of a watt with a power draw of several kilowatts
are used.

[0006] While there are advantages for using excessively
large coolers such as rapid cool-down times and flexibility
in cryostat design, some applications of these types of
systems would benefit from cooling systems having reduced
size and power. Such applications include secure quantum
encrypted communication links that utilize superconducting
nanowire single-photon detectors (SNSPD), and supercon-
ducting transition-edge sensor microcalorimeters for elec-
tron microscope microanalysis.

[0007] The smallest capacity, commercially available
closed cycle cooler is a Gifford-McMahon cycle with
100mW of cooling at 4 K, and drawing about 1.5 kW wall
power. This unit is considerably larger and consumes sub-
stantially more power than necessary for such applications.
Its minimum temperature is about 2.5 K, so that lower
temperatures require a second stage of cooling such as
helium-4 sorption refrigerator.

Nov. 3, 2022

SUMMARY OF THE INVENTION

[0008] It is an object of the present invention to provide a
compact, low power cooler for cryogenic systems. One
embodiment consists of a pulse tube compressor and a He
Joule-Thomson (JT) cooler operating at temperatures down
to 2.2 K, precooled to 10 K using a 3-stage linear compres-
sor pulse tube operating at 35 Hz. Although a He JT would
reach temperatures approaching 1 K, He was selected for the
initial development because of the relative high cost and
rarity of He. This cooler embodiment has a room tempera-
ture compressor followed by particulate and condensable
gas filtration. Within the cryostat, four counterflow heat
exchangers precool the incoming high-pressure gas using
the outflowing low-pressure gas. The three warmest heat
exchangers are successively heat sunk to the three stages of
the pulse tube to absorb residual heat from the slight
ineffectiveness of the heat exchangers. The pulse tube cold
head also absorbs loads from instrumentation leads and
radiation loads. The pulse tube stages operate nominally at
80K, 25 K, and 10 K. The entire system—cryo-cooler, drive
and control electronics, and detector instrumentation, fits in
a7U (0.31 m) tall standard electronics rack mount enclosure
approximately 0.61 m long, and does not require water
cooling.

[0009] Another embodiment also has a form factor and
low power draw of a standard rack-mountable electronics
instrument. This embodiment increased the cooling capacity
over the 2.2 K embodiment at 10 K by 35% and reduced the
minimum temperature from 2.2 K to 1.7 K. For this embodi-
ment, the cooler is a pulse tube/Joule-Thomson (PT/JT)
hybrid, with the JT stage achieving 1.4 mW of cooling at 1.7
K, and the three-stage pulse tube providing cooling at 80 K,
25 K, and 10 K.

[0010] The cooler design was a compromise between
thermodynamic efficiency, production costs, and risk. Pre-
ferred embodiments avoid complex. costly components and
fabrication processes typically used in spaceflight coolers,
resulting in thermodynamic efliciencies lower than those
coolers. For example, standard size stainless steel tubing
was used for regenerator and pulse tube walls instead of
machined titanium, which is commonly used in aerospace
coolers. Replacing the stainless-steel tubes with titanium
tubes would result in 1.8 W, 0.07 W, and 0.001 W of
additional cooling on the 80 K, 25 K, and 10 K stages,
respectively.

[0011] The JT stage operates between a high pressure of
0.2 MPa and a low pressure of 1.3 kPa, with a flow rate of
0.6 mg/sec. The thermodynamic power required to recom-
press the gas is only a few watts, which is such a small
fraction of the total power budget that a highly thermody-
namically efficient design is not required.

[0012] Preferred embodiments utilize geometries that
minimize the effects of secondary flows within the pulse
‘tube and regenerator.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] FIG. 1 is a schematic block diagram of a preferred
embodiment of the present cooler system, including a pulse
tube compressor and a JT cooler.

[0014] FIGS. 2A and 2B are schematic block diagrams
illustrating a more detailed view of the JT stage of the cooler
of FIG. 1 for two variations of the embodiment of FIG. 1


US 2022/0349628 Al

storage bottle as a supply and a vacuum pump vented to
atmosphere on the return. The low side pressure was con-
trolled by adjusting a valve in front of the vacuum pump.
[0043] FIG. 6 shows load curves at high side pressures of
0.2 MPa, 0.15 MPa, 0.125 MPa, and 0.1 MPa. A maximum
of 1.4 mW was produced at the minimum temperature of
about 1.7 K. The cooling capacity increase at lower tem-
peratures is from increased mass flow through the capillary
from higher gas density. Operating at 0.1 MPa does not
provide suflicient cooling to cool down the JT stage from the
initial temperature of around 7 K. If the coldstage was
temperature-regulated slightly above the liquification tem-
perature, no liquid accumulates, which stabilizes the cold
stage temperature. the load on the pulse tube from circulat-
ing 4He would cause the third stage temperature to rise
slightly. However, in all situations the temperature remained
below 7.7 K, indicating that Nb-Ti wire can be used for
electrical leads between the 10 K stage and the JT stage.
[0044] Test data described above show that the design
point cooling powers for the pulse tube require a nominal
150 W of compressor power. We have tested a variety of
commercial off-the-shelf power. Given conversion efficien-
cies of off-the-shelf power electronic modules for driving
the compressor are generally better than 90%, the pulse tube
will require a total power draw of about 165 W. The power
draw by the compressor and drive electronics will be about
50 W. If the power for cooling fans and thermometry/
diagnostic instrumentation is included, power requirements
come in at under 300 W, usually about 250 W. In an
‘embodiment using a smaller, commercially available, pulse
tube compressor to reduce system weight, another 30 W-50
W of compressor power is required.
[0045] While the exemplary preferred embodiments of the
present invention are described herein with particularity,
those skilled in the art will appreciate various changes,
additions, and applications other than those specifically
mentioned, which are within the spirit of this invention. For
‘example, for low heat load applications, the flow impedance
can be increased, reducing the ass flow, which would reduce
the power required for both the JT compressor and the pulse
tube compressor. Conversely, there is substantial upside in
the cooling power capability of the JT stage. The JT imped-
ance can be reduced which would result in higher mass flow
rates and higher cooling powers. The pulse tube has sufli-
cient capacity to accommodate higher JT mass flow rates, so
capacities approaching 5 mW are possible, although this will
require development of a larger capacity JT compressor.
[0046] Lower temperatures using 4He are possible but
require a combination of reworking the JT counterflow heat
exchangers to reduce pressure drop, a JT compressor with
lower suction pressure, and superfluid film creep mitigation.
Direct substitution of 3He results in temperatures below 1 K.
What is claimed is:
1. A compact, low-power cryo-cooler system for cryo-
genic systems comprising:

Nov. 3, 2022

a cryo-cooler compressor for providing a flow of hot
high-pressure gas and receiving a return flow of cool
low-pressure gas;

a series of counterflow heat exchangers configured to cool
hotter incoming gas from the compressor with cooler
gas returning to the compressor;

a pulse tube cooler including a series of closed-cycle
pulse tube cooling stages configured to interact with the
counterflow heat exchangers to pre-cool the high-pres-
sure gas to at least as low as 10 K; and

a Joule-Thomson (JT) cooler configured to cool the
pre-cooled gas to at least as low as 1.7 K, the JT cooler
including an expansion capillary having a warm end
and a cold end, wherein the warm and cold ends of the
expansion capillary are physically separated enough to
achieve suflicient thermal isolation between the warm
and cold ends to achieve the 1.7K;

wherein the cryo-cooler and associated drive and control
electronics fit within a 7U electronics rack and the
cooler and associated drive and control electronics
require less than about 300 W of power.

2. The cryo-cooler of claim 1 wherein the cryo-cooler

requires less than 250 W of power.

3. The cryo-cooler of claim 1 wherein the input pressure
to the JT cooler is between 0.1 MPa and 0.2 MPa and the
output pressure from the JT cooler is between 0.2 MPa and
1.3 kPa and wherein the output pressure is lower than the
input pressure.

4. The cryo-cooler of claim 3 wherein the flow from the
JT cooler is on the order of half a mg/sec.

5. The cryo-cooler of claim 1 having three pulse tube
cooling stages, the three stages pre-cooling the gas to about
50-100K, 20-30K, and 6-10K in turn.

6. The cryo-cooler of claim 5 wherein the three cooling
stages are implemented with a 3-stage linear compressor
pulse tube operating at 35 Hz.

7. The cryo-cooler of claim 1 wherein the gas is 4He.

8. The cryo-cooler of claim 1 wherein the gas is 3He and
the JT cooler is configured to cool the gas to around 1.25 K.

9. The cryo-cooler of claim 1 wherein the pulse tube
stages have regenerators and pulse tube walls comprising
stainless-steel tubing.

10. The cryo-cooler of claim 1 wherein the coldest pulse
tube cooling stage utilizes erbium-nickel spheres.

1. The cryo-cooler of claim 1 wherein the counterflow
heat exchangers are tube-in-tube.

12. The cryo-cooler of claim 1 wherein a JT expansion
element within the JT cooler is comprises a 1 m-3 m long,
40 um-60 um inner-diameter stainless steel capillary

13. The cryo-cooler of claim 1 wherein the cold end of the
expansion capillary is housed in copper block and the warm
end of the expansion capillary is spaced apart from the
copper block.



as) United States
a2) Patent Application Publication co) Pub. No.: US 2022/0349628 Al

Kotsubo et al.

(43) Pub. Date: Nov. 3, 2022

G4)

ay

(72)

(73)

(e29)

(22)

(63)

(60)

COMPACT LOW-POWER CRYO-COOLING
SYSTEMS FOR SUPERCONDUCTING
ELEMENTS

Applicant: The Regents of the University of
Colorado, a body corporate, Denver,
CO (US)

Inventors: Vincent Kotsubo, Superior, CO (US):
Joel Ullom, Boulder, CO (US); Sae
Woo Nam, Boulder, CO (US)

National Institute of Standards and
Technology (NIST), Gaithersburg, MD
(US)

Assignee:

Appl. No.: 17/857,778

Filed: Jul. 5, 2022
Related U.S. Application Data

Continuation-in-part of application No. 16/256,736,
filed on Jan. 24, 2019, now abandoned.

Provisional application No. 62/621,483, filed on Jan.
24, 2018.

Publication Classification

(51) Int. Cl.
F25B 9/02 (2006.01)
F25B 9/14 (2006.01)
F25B 9/10 (2006.01)
(52) US. Cl
CPC eases F25B 9/02 (2013.01); F25B 9/145
(2013.01); F25B 9/10 (2013.01)
(57) ABSTRACT

‘A compact, low power cryo-cooler for cryogenic systems
capable of cooling gas to at least as low as 2.5 K. The
cryo-cooler has a room temperature compressor followed by
filtration, Within the cryostat, four counterflow heat
exchangers precool the incoming high-pressure gas using
the outflowing low-pressure gas. The three warmest heat
exchangers are successively heat sunk to three stages of a
pulse tube to absorb residual heat from the slight ineffec-
tiveness of the heat exchangers. The pulse tube cold head
also absorbs loads from instrumentation leads and radiation
loads. The pulse tube stages operate at around 80 K, 25 K,
and 10 K. The entire system—cryo-cooler, drive and control
electronics, and detector instrumentation, fits in a standard
electronics rack mount enclosure, and requires around 300
W or less of power.

104
406
~ 702

142 +"



US 2022/0349628 Al

[0015] FIG. 3 is a plot showing load curves for the final
stage of the pulse tube compressor.

[0016] FIG. 4 is a plot showing cooling capacities of the
next to final stage of the pulse tube compressor.

[0017] FIG. 5 is a plot showing load curves for the final
stage of the pulse tube compressor for various configura-
tions.

[0018] FIG. 6 is a plot showing load curves for the JT
cooler at various high-side pressures.

DETAILED DESCRIPTION OF THE
INVENTION,

[0019] FIG. 1 is a schematic block diagram of the present
cooler system 100. System 100 is a compact cooler having
a form factor and low power draw of a standard rack-
mountable electronics instrument. The cooler is a pulse
tube/Joule-Thomson (PT/IT) hybrid, with the JT stage 160
achieving 1.4 mW of cooling at 1.7 K or 2.2K (depending on
the configuration, see FIGS. 2A and 2B), and the three-stage
132, 134, 136 pulse tube 138 providing cooling at 80 K
(132), 25 K (134), and 10 K (136),

[0020] System 100 comprises a cryo-cooler compressor
102 providing high-pressure gas out 106 and low-pressure
gas in 108 and includes a filter 104. Pulse tube cooler 130
includes an 80K stage 132, a 25K stage 134, anda 10K stage
136 housed within vacuum shell 150. Also within vacuum
shell 150 are thermal busbar 142, heat exchangers 122, 124,
126, and 118, as well as counterflow heat exchangers 110.
112, 114, and 116. Counterflow heat exchanger 116 and heat
exchanger 118 are part of JT stage 160, as is JT expansion
capillary 120 (see FIGS. 2A and 2B). The counterflow heat
exchangers could be tube-in-tube heat exchangers. Since
one important application of the present invention is the
cooling of quantum encrypted communication links that
utilize superconducting nanowire single-photon detectors
(SNSPD), and superconducting transition-edge sensor
microcalorimeters for electron microscope microanalysis,
the stages of this embodiment were developed with esti-
mated cooling loads as shown in Table 1. Those skilled in the
art will appreciate that specific applications will result in
different cooling load requirements and thus variations in the
cooler design within the spirit of the present invention.

TABLE 1

m Design

Radiation Precooling Conduction Total ~—=— Target

Stage (mW) (mW) (mW) (mW) mW)
80K 1000-2350 400 150 1550-2900 3000
25K S12 28 7 40-87 100
10K 0.2 2 1 3.2 5

2.0K 0 NIA 0.28 0.28 0.5

[0021] Estimated cooling loads on each of the cooler
stages are presented in Table 1 along with the design goals
The cooling load on the JT stage 160 arises primarily from
low thermal conductance bias/readout coaxial cables, since
SNSPD’s dissipate negligible power and photons are
coupled in through optical fiber. Targeted cooling capacities
included design margin at each stage. The 80 K stage 132
radiation load estimate has considerable uncertainty due to
unknown ambient temperature since the cryo-cooler will

Nov. 3, 2022

generally be enclosed in an equipment rack-mounted box,
and because of the uncertain surface emissivity of the
radiation shield,

[0022] The WSi SNSPD detectors currently require cool-
ing to 1.25 K for maximum quantum efficiency, but this
temperature is difficult to achieve with a 4He JT refrigerator
that utilizes a low power, compact compressor, and gener-
ally temperatures up to 2.5 K are acceptable. This 2.5 K
minimum temperature is easily reachable by various
embodiments of the present invention. Thus, this embodi-
‘ment operates at a slightly higher minimum temperature of
around 1.7 K/2.2 K. As an alternative, 3He does reach 1.25
K at the same pressure that 4He reaches 1.7 K, and can be
used when the lower temperature is especially important.
[0023] Similarly, the 10 K for the precooling temperature
for the JT stage 160 is a lower temperature than the
thermodynamic optimum for this hybrid, but was chosen to
reduce the high-pressure requirement for the JT loop, which
in turn reduces stress on the JT compressor. This was done
because the JT compressor is the largest risk to reliability.
Final test results shown below show that the actual operating
temperature of this stage was below 7.7 K. Again, for other
applications, the operating conditions will be adjusted
slightly.

[0024] The cooler is shown schematically in FIG. 1. The
pulse tube 138 is a three-stage (132, 134, 136), cooled
inertance tube configuration 138, resulting in an easy-to-
integrate coldhead. Because the present invention is
intended to provide a low cost, easily produced cooler, the
mechanical design sacrificed performance to reduce costs.
The most impactful tradeoff was the use of standard sized
stainless-steel tubing for the regenerator and pulse tube
walls, rather than machined titanium, which is commonly
used in aerospace coolers. Replacing the stainless-steel
tubes with titanium tubes would result in 1.8 W, 0.07 W, and
0.001 W of additional cooling on the 80 K, 25 K, and 10 K
stages, respectively.

[0025] The upper two-stage inertance tubes were dual
diameters, whereas the third stage used only a single diam-
eter because modeling indicated minimal improvement in
performance with dual diameters. Regenerators used stan-
dard materials; the first stage used die-punched stainless
steel screens with #150 mesh, 66 um wire diameter in the
warm side and #400 mesh, 25.4 jum wire diameter in the cold
side. The second stage used #400 mesh, 25.4 jm wire
diameter stainless steel screens in the warm side and #400
mesh, 25.4 um wire diameter phosphor bronzes screens,
flattened to produce 0.55 porosity, in the cold side.

[0026] The third stage used high heat capacity micro-
spheres. In the previous embodiments, 100 um diameter
50-50 erbium-praseodymium (ErPr) alloy spheres were
used. The present embodiment replaced the lower half of the
regenerator with 100 um diameter erbium-nickel (ErNi)
spheres to improve cooling capacity below 10 K, and
comparative tests results are presented below.

[0027] A critical mechanical design objective was geom-
etries that minimize the effects of secondary flows within the
pulse tube and regenerator, because these flows can seri-
ously degrade performance. Most well known are turbu-
lence/convection in the pulse tube and streaming in regen-
erators. Somewhat less appreciated are secondary flows in
the displacer gap in Stirling coolers. A net circulation can
flow through the displacer gap and return through the
regenerator partially unregenerated and cause cooling loss



Patent Application Publication Nov. 3,2022 Sheet 1 of 4 US 2022/0349628 Al

104
100
130 08 — ms
150 ~~
440
132 ~
112
438 a
134-4
114
136
446
140 160
142 4 118

Figure 1


Patent Application Publication

106

108

A18

120
160
106 108
Se
118
4 204
q
120
160.

Nov. 3, 2022 Sheet 2 of 4 US 2022/0349628 Al

Figure 2A

Figure 2B