design and qualification of the ams-02 flight...

DESIGN AND QUALIFICATION OF THE AMS-02 FLIGHT CRYOCOOLERS

Kimberly Shirey∗1, Stuart Banks1, Rob Boyle1 and Reuven Unger2

1NASA Goddard Space Flight Center Code 552 – Cryogenics and Fluids Branch Greenbelt, MD 20771, USA 2Sunpower, Inc. 182 Mill Street Athens, OH 45701, USA

ABSTRACT

Four commercial Sunpower M87N Stirling-cycle cryocoolers will be used to

extend the lifetime of the Alpha Magnetic Spectrometer-02 (AMS-02) experiment. The

cryocoolers will be mounted to the AMS-02 vacuum case using a structure that will

thermally and mechanically decouple the cryocooler from the vacuum case. This paper

discusses modifications of the Sunpower M87N cryocooler to make it acceptable for

space flight applications and suitable for use on AMS-02. Details of the flight model

qualification test program are presented.

AMS-02 is a state-of-the-art particle physics detector containing a large superfluid

helium-cooled superconducting magnet. Highly sensitive detector plates inside the

magnet measure a particle’s speed, mass, charge, and direction. The AMS-02

experiment, which will be flown as an attached payload on the International Space

Station, will study the properties and origin of cosmic particles and nuclei including

antimatter and dark matter.

∗ Corresponding author, fax number (301) 286-1637, email address [email protected]

1Copyright © 2005 by Sunpower, Inc. Space Cryogenics Workshop, 25-26 August 2005, Colorado Springs, Colorado

99

mailto:[email protected]

Two engineering model cryocoolers have been under test at NASA Goddard since

November 2001. Qualification testing of the engineering model cryocooler bracket

assembly including random vibration and thermal vacuum testing was completed at the

end of April 2005. The flight cryocoolers were received in December 2003. Acceptance

testing of the flight cryocooler bracket assemblies began in May 2005.

KEYWORDS

Stirling (E), Space Cryogenics (F)

AMS-02 INTRODUCTION

AMS-02 is an experiment that will search space for the presence of dark matter,

strange matter and antimatter. A large superconducting magnet cooled with superfluid

helium will bend the path of cosmic particles through a number of highly sensitive

detectors, which then measure the particle’s speed, mass, charge, and direction. The

experiment is currently scheduled to launch no earlier than June 2008, subject to changes

in the shuttle manifest. AMS-02 will be installed on the International Space Station (ISS)

for a minimum three-year mission.

The magnet, superfluid helium tank, layers of super-insulation and 4 vapor-cooled

shields are suspended within a toroidal vacuum case. The vacuum case is machined out

of aluminum with two large support rings on the top and bottom of the outer cylinder.

In an effort to extend the life of the stored cryogen, four Sunpower M87N

Stirling-cycle cryocoolers will be used to cool the outermost vapor cooled shield. To

minimize thermal gradients on the vapor cooled shield, two cryocoolers will be mounted


to ports on the upper vacuum case support ring and the remaining two cryocoolers will be

mounted to the lower vacuum case support ring. The baseline performance requirement

for the four cryocoolers is a total of 9.4 watts of heat lift at 60 K with 400 watts of input

power. Figure 1 shows a view of two of the cryocooler port locations on the AMS-02

vacuum case. The remaining two cryocooler port locations are 180 degrees from its pair

on either support ring.

Figure 1. Cryocooler port locations on AMS-02 vacuum case.

The cryocooler mounting brackets will provide a hermetic seal to the vacuum case

and will thermally and mechanically decouple the cryocooler from the vacuum case. In

order to allow force attenuation using a passive, tuned spring-mass balancer system, the

mount is required to be compliant in the cryocooler thrust axis.

The cold tip of the cryocoolers will be connected via flexible straps to the outer

vapor cooled shield of the dewar. Each strap will span a distance of approximately 100


mm and allow for relative motions no more than 12 mm between the cold tip and the

vapor-cooled shield. Motion of the strap is expected during: launch, vacuum pump

down, magnet cool down, magnet charging and discharging, and in the case of a quench.

Each cryocooler will reject heat to two propylene loop heat pipes, sunk to a direct

condensing zenith octagonal radiator. One quadrant of the radiator will be dedicated to

each cryocooler. The thermal control system will provide a nominal cryocooler operating

temperature between 0°C and +10°C. Survival heaters will be implemented to maintain a

minimum non-operating temperature of -40°C and to assure a minimum turn on

temperature of -10°C. The maximum allowable operating temperature is 40°C.

The cryocoolers will be powered from either of the ISS 124 V DC buses (main

and auxiliary). The electronics must provide the capability of being powered from either

bus and must maintain galvanic isolation between the two buses. The drive electronics

must be capable of supplying 150 W to each cooler. Due to power limitations imposed

by the ISS, the nominal input power will be limited to 100 W. A lower limit of 60 W is

required to insure proper floatation of the gas bearings. A modulated pulse duration drive

was selected for use on AMS-02 for its simplicity, high efficiency, and simple interfacing

to the 120V DC bus.

AMS-02 will be the first space flight mission that will have Stirling-cycle

cryocoolers operating within a substantial steady-state magnetic field. The cryocoolers

will be mounted in locations with a magnetic gradient over the entire length of each

cryocooler and fields as high as 925 Gauss perpendicular to the cryocooler axis and 400

Gauss along the cryocooler axis. Magnetic compatibility testing [1,2,3] conducted on the


engineering model cryocoolers showed the coolers were capable of operating in the

AMS-02 magnetic field with no performance degradation.

SUNPOWER M87N CRYOCOOLER

The Sunpower M87N cryocooler is a modified M87 with enhancements targeted

to better suit the AMS-02 program. The M87, shown in figure 2, is a single free-piston,

integral Stirling-cycle cryocooler designed for high volume manufacturing [4]. The M87

was designed to provide 7.5 watts of cooling at 77 K with 150 watts input power while

operating at a reject temperature of 35°C. This cryocooler has a design lifetime of

>40,000 hours.

Figure 2. Schematic configuration of a linear free piston integral cooler

The M87’s intended use is as an Oxygen liquefier at a patient’s home. Cooler

orientation during operation is vertical with the cold end down. The piston is driven by a

moving-magnet linear motor. The M87 is a true free-piston machine in which the piston

is not axially constrained by mechanical means. Such a configuration simplifies the

mechanical arrangement, which manifests itself in ease of assembly and reduction of

potential side loads. When at rest, the piston can be anywhere along its cylinder. The


controller [5] drives the piston to the axial center position through a start-up sequence,

prior to applying AC to the motor. The voltage applied to the linear motor controls the

piston amplitude. To simplify the electronic driver/controller, the free piston feature was

changed in the M87N design through the use of spring-magnet arrangements [6] that hold

the piston about the center position in any orientation. The permanent magnet segments

are arranged to form a cylinder on a structure of non-magnetic material [7].

Dynamic centering is achieved through a pneumatic network of passages formed

by the piston and cylinder, which allows the working gas to flow between compression

and bounce spaces at predefined locations. The combination of the static magnetic

centering and the pneumatic dynamic centering makes the M87N a true free-piston

machine. A pressure oscillation generated in the compression space drives the displacer

through its rod, which extends through the piston and into the bounce space. Where the

piston relies on the gas spring in the compression space for its resonance, the displacer is

attached to a planar spring through a compliant member [8]. The displacer, containing

the random fiber regenerator, shuttles the gas in-between the cold end and the warm end

heat exchanger. That heat exchanger is made in a form of individual rings that are

thermally shrunk into place in combination with the pressure vessel structure. This

configuration and method of the heat exchanger’s construction [9], results in a cost

effective and manufacturable design.

The gas bearing systems of the commercial cooler were redesigned for the M87N

to provide better performance regardless of orientation. The gas bearings systems are

used to radially center the piston and displacer and thus prevent contact of the moving

parts. The gas bearings are formed [10] into and by the very parts of the piston and


displacer through machining operations, which are highly repeatable, cost effective and

eliminate the need for manual adjustments of the gas bearings.

Cooler vibrations generated by the piston and displacer are countered by a passive

(tuned spring-mass) balancer system. The AMS-02 M87N cryocoolers were

tested/qualified for a 250-hour period before delivery to NASA Goddard and performed

to specification. Figure 3 shows the average measured performance of the six coolers

with a 35oC reject temperature and 150W of input power.

6

7

8

9

10

11

12

13

60 70 80 90 100 110

Average Performance of 6 M87N Final Qalification Test

Lift

(W)

Cold End Temperature (K)

Figure 3. Performance curve of M87N cryocoolers with a 35oC reject

temperature and 150W of input power

CRYOCOOLER QUALIFICATION PROGRAM INTRODUCTION

The AMS-02 project purchased both a standard Sunpower M87 (EM#1) and a

modified M87N (EM#2) for the two engineering models. The engineering models have


been under test at NASA Goddard since November 2001. EM#1 has accumulated just

over 16,000 hours of runtime and EM#2 has accumulated just under 14,000 hours.

NASA Goddard will qualify four Sunpower M87N cryocoolers for space flight

use and two cryocoolers for flight spares on AMS-02. The flight cryocoolers were

received from Sunpower in December 2003. Upon arrival at Goddard, the cryocoolers

were inspected, leak tested, and put through low temperature testing to identify any

problems the coolers might have with low temperature operation. Three of the flight

cryocoolers have begun thermal performance characterization testing and have

accumulated just under 1,000 hours of operation. The remaining three cryocoolers will

be characterized by the end of Fall 2005, after integration in their flight mounting

brackets. Fabrication, inspection and testing of the flight mounting bracket piece parts is

expected to conclude the beginning of October 2005. After the flight cryocoolers have

been integrated with their flight mounting brackets, the qualification program consists of

the following tests:

• Vacuum leak test • Random vibration test of cooler in a compliant mount • Operation tests

- Characterizing base reaction forces - Measure temperature delta at the heat reject collar - Measure the temperature distribution around the cooler

• Measure thermal conductance of the bracket • Thermal cycling test

LEAK CHECK

A helium leak check was performed on all of the flight cryocoolers. This test

measured the rate of helium loss from the cryocooler and assessed the impact of this rate

on the likely performance of the cooler. The helium gas inside the cryocooler, at a


pressure of roughly 16 bar, is essential to produce the required refrigeration. Leakage

from the cryocooler may escape to the experiment environment, or if the leak is in the

coldfinger area, may enter the vacuum space of the AMS-02 Vacuum Case. Helium lost

to the environment causes a reduction in the cryocooler charge pressure, and a loss in

refrigeration. Helium lost into the Vacuum Case will also produce a higher heat leak into

the vapor cooled shields and helium tank. While a relatively large amount of helium loss

to the environment is tolerable, only a relatively small loss of helium into the Vacuum

Case would be tolerable. This test measured the total rate of helium loss from the

cryocooler, and in some cases specifically measured the leak rate in the coldfinger area.

The results of the leak check are provided in Figure 4.

Figure 4. Results of flight model leak tests

While FM 6 has a leak-tight coldfinger, it has the largest total leak rate of all the

flight cryocoolers at 8.5 x 10-7 sccs. To put this in perspective, the cooler loses a little

less than 1% of its gas per year and would drop from 16 bar to 15 bar over a ten-year

storage period. This cooler would still be operational at this pressure, and could still be a

useful flight candidate.


LOW TEMPERATURE BINDING TEST

All six flight cryocoolers and EM# 2 were put through low temperature testing to

perature operation caused by

therma

e

able 1. All except FM 7

require

Number 20ºC 60 Hz -40ºC 60Hz urrent @

-60ºC 60Hz Current Ratio -60ºC/20ºC

identify any problems the coolers might have with low tem

l contraction of internal components. The test fixture was instrumented with force

transducers to display the motion of the piston on an oscilloscope. The coolers were

tested through a wide temperature range (-60ºC to +20ºC) while capturing the motor

current during startup to look for changes due to binding.

The coolers generally showed an increase in the required starting current as th

operating temperature decreased. The results are shown in T

d less starting current than EM# 2. No indications of rubbing or any abrupt

changes in cooler characteristics were noticed during the test. Some of the coolers

required a change in startup current by a factor of 2 or more.

Table 1: Starting current as a function of temperature

Serial Starting Current @ Starting Current @ Starting C

EM 2 0.157 0.2 0.213 1.4 FM 2 0.073 0.088 0.109 1.5 FM 3 0.075 0.113 0.14 1.9 FM 4 0.045 0.066 0.067 1.5 FM 5 0.091 0.179 0.209 2.3 FM 6 0.037 0.07 0.074 2 FM 7 0.11 0.255 0.277 2.5

QUALIFICATION AND PERFORMANCE VERIFICATION

fter an initial leak check and the low temperature binding test, three of the flight

cryocoolers were prepared for thermal performance characterization. The remaining

three cryocoolers will be characterized by the end of Fall 2005. The coldfingers were

A


instrumented with a heater, to simulate a thermal load, and a Lakeshore silicon diode, and

then wr

in

ber.

by a

-

t, cryocooler body temperature overheat, and loss of vacuum. A

display plemented

the

n

. A

apped with 5 layers of multi-layer insulation. Dallas/Maxim DS18S20 Digital

thermometers were mounted to the cryocoolers’ heat reject and case to monitor

environmental temperatures. Fluid cooled heat exchangers were attached to the

cryocoolers’ heat rejects allowing the reject temperature to be maintained by a laboratory

recirculating chiller. Each cooler was mounted in a fixture that enclosed the coldfinger

a vacuum bonnet allowing operation both in and out of the thermal vacuum cham

The cryocooler mounts were designed to be compliant to allow force attenuation

passive balancer.

Two cryocooler test stations have been developed; each station allows

autonomous operation of four cryocoolers. Each cryocooler is protected with Goddard

developed laboratory cryocooler shutdown electronics that protect against cold tip

temperature overhea

showing the total number of hours accumulated on the cryocooler is im

on the front panel of the electronics. The electronics can be switched between the

Sunpower drive electronics and an external input. The external input allows driving

cooler with an arbitrary waveform function generator and power amplifier combinatio

that can be used to produce sinusoidal or non-standard waveforms; additionally this input

can be used to drive the coolers with the flight type modulated pulse duration drive

data acquisition program, written in LabVIEW, data logs the motor voltage, current,

power, power factor, cryocooler body temperature and cryocooler cold tip temperature

every minute.


Station 1, as shown in figure 5, allows the coolers to be operated on an optics

bench. Coldfinger vacuum is provided via a manifold and a recirculating chiller allows

cooler operation

between 10ºC and 40ºC. The coolers are initially operated in this station

for 150

Figure 5: Te flight cryocoolers

0 hours and during this time load curves are performed. Station 2, as shown in

figure 6, allows operation in a vacuum chamber and a recirculating chiller allows cooler

operation between -30ºC and 50ºC. Thermal cycling, as well as, low temperature

performance characterizations are conducted in this station.

st station for performing bench top testing of

Figure 6: Test station for performing testing of flight cryocoolers under vacuum


A series of baseline performance curves were generated while testing the three

flight cryocoolers under various heat reject temperatures, power levels and thermal loads.

Periodically, a subset of this characterization will be repeated to check for thermal

performance degradation. We have not seen any thermal performance degradation in any

of the flight cryocoolers nor in either of the engineering model cryocoolers. All three

flight coolers tested to date exceed the AMS-02 thermal performance requirement with an

80% margin.

THER

oler

f

ue to laboratory limitations, not all of the thermal cycling took place under

acuum with the cryocooler running. Non-operational thermal cycling took place at

tal chamber where the cryocooler was cycled through

a tempe e

MAL CYCLING TESTS

Thermal cycling tests were performed on EM#2 to put the cooler through the

extreme temperature ranges that could be experienced on orbit. Operational cryoco

thermal cycling took place under vacuum with the cryocooler powered to the nominal on-

orbit power level of 100 W. The cryocooler was cycled through a temperature range o

-20ºC to +40ºC eight times with a minimum dwell time at the extreme temperatures of

four hours. D

v

ambient pressure in an environmen

rature range of –55ºC to +55ºC eleven times with a minimum dwell time at th

extreme temperatures of four hours. A load line was performed after thermal cycling to

verify there was no performance degradation. Thermal cycling will be performed on all

of the flight cryocoolers during the qualification program.


DRIVE ELECTRONICS

The cryocooler electronics are being developed in a combined effort between

NASA Goddard and ETH-Zurich in Switzerland. ETH is responsible for final design and

production. Goddard provided initial development, design and performance verification

of the drive technique [3] and subsequent testing of the ETH

flight type electronics with

n M87N cooler. The cryocoolers will be powered from either of the ISS 124 V DC

The electronics must provide the capability of being powered

from ei e

,

as

res, such as at the cryocooler cold tip and

therma

y

a

buses (main and auxiliary).

ther bus and must maintain galvanic isolation between the two buses. The driv

electronics must be capable of supplying 150 W to each cooler. A modulated pulse

duration drive has been selected. The modulated pulse duration drive uses only a single

‘on’ pulse per half cycle and clamps the motor the remainder of the cycle. The duration

of the ‘on’ pulse is modulated to control input power. This scheme results in a simple

robust design and exceeds 90% efficiency.

ETH provided engineering test model electronics to Goddard for test and

evaluation with an M87N cryocooler. The electronics included the H-bridge driver,

well as, signal conditioning and processing electronics to allow the monitoring of critical

parameters, such as voltages, currents and temperatures. The average values of the DC

supply voltage and current, and the RMS values of the cryocooler drive voltage and

current are calculated. Cryogenic temperatu

l strap, are measured using Cernox sensors. Cernox was selected due to its

immunity to large magnetic fields. Non-cryogenic temperatures, such as the coolers bod

and heat reject, are measured using the Dallas/Maxim DS18S20 Digital thermometer.


These provide 9-bit centigrade temperature measurements between -55°C to +125°C and

are accurate to ±0.5°C over the range of -10°C to +85°C. Each DS18S20 has a uniqu

64-bit serial code, which allows multiple DS18S20s to function on the same 1-Wire bu

External interface and autonomous control capabilities are implemented using a Ma

DS80C390. The DS80C390 is a fast 8051-compatible microprocessor with dual CAN

2.0B controllers.

Initial testing of the ETH engineering test model electronics included verifying

temperature, voltage and current measurements, as well as, the ability to drive a

simulated cryocooler load. Once initial verification was completed, the driver was

connected to an M87N, and efficiency measurements and thermal performance

verification was completed.

Two effici

e

s.

xim

encies were defined for evaluation of the pulse-duration electronics:

inpowerfrequencylfundamentaatpowereffSYSTEM

outpowereffDRIVER

____

__

=

=

inpower

_

_

The driver efficiency is simply defined as the ratio of Power-Out to the Power-In.

The second efficiency, system efficiency, is the ratio of power delivered at the

fundamental drive frequency of the cooler to the power into the drive electronics. The

reason for this second efficiency is that only power delivered at the fundamental drive

frequency produces actual cooling work. Power delivered to the cooler at higher

frequencies generates excess heat and reduces the motor efficiency. The system


efficien

ion of

TION AND DELIVERY SCHEDULE

s is

ryocoolers is the end of October 2006.

light

g of the M87N

ryocoolers for AMS-02.

cy is the figure of merit for evaluating the coupled performance of the drive

electronics and the cryocooler.

At the nominal expected flight power level of 100W to 150W, the system

efficiency was (93 to 94)% and the thermal performance of the cooler was nearly

identical to a sinusoidal drive. Based on these positive results, design and product

the flight boards is in progress.

FLIGHT COOLER INTEGRA

The integration of the flight cryocoolers with their flight mounting bracket

anticipated to begin in October 2005; a 6-month test program would ready the

cryocoolers for delivery to the project by April 2006. The project has indicated that the

earliest need date for the flight c

ACKNOWLEDGMENTS

The authors wish to thank Ed Quinn and Steve Smith of Orbital Science and

Renea’ LaRock of NASA GSFC for their support in all of the engineering and f

model testing and integration for AMS-02. The authors also wish to thank the entire

team at Sunpower who was involved in the design, fabrication, and testin

c


REFERENCES

[1] Breon, S.R. et al., “Operation of A Sunpower M87 Cryocooler in a Magnetic Field”, Cryocoolers 12, Kluwer Academic/Plenum Publishers, New York (2003), pp. 761-769.

[2] Mustafi, S. et al., “Qualifying the Sunpower M-87N Cryocooler for operation in the AMS-02 Magnetic Field”, Cryogenics 2004; Volume 44 (Issue 6-8): pages 575-580.

[3] Banks, S. et al., “AMS-02 Cryocooler Baseline Configuration and EM Qualification Program”, Cryogenics 2004; Volume 44 (Issue 6-8): pages 551-557.

[4] Unger, R.Z., “The Advent of Low Cost Cryocoolers”, Cryocoolers 11, Kluwer Academic/Plenum Publishers, New York (2001), pp. 79-86.

[5] US patent 6,199,381. DC Centering of Free Piston Machine. Issued 3-13-2001. [6] US patent 5,148,066. Linear Generator or Motor With Integral Magnetic Spring.

Issued 9-15-1992. [7] US patent 5,642,088. Magnet Support Sleeve for Linear Electromechanical

Transducer. Issued 6-24-1997. [8] US patent 5,525,845. Fluid Bearing with Compliant Linkage for Centering

Reciprocating Bodies. Issued 6-11-1996. [9] US patent 6,446,336. Heat Exchanger and Method of Constructing Same.

Issued 9-10-2002. [10] US patent 6,293,184. Gas Bearing and Method of Making a Gas Bearing for a

Free Piston Machine. Issued 9-25-2001.


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