Described is a proposed cryogenic system used to cool the superconducting magnet for the Space Station based ASTROMAG Particle Astrophysics Facility. This 2-meter diameter superconducting magnet will be cooled using stored helium II. The paper presents a liquid helium storage concept which would permit cryogenic lifetimes of up to 3 years between refills. It is proposed that the superconducting coil be cooled using superfluid helium pumped by the thermomechanical effect. It is also proposed that the storage tank be resupplied with helium in orbit. A method for charging and discharging the magnet with minimum helium loss using split gas-cooled leads is discussed. A proposal to use a Stirling cycle cryocooler to extend the storage life of the cryostat will also be presented

Castles, S.

Green, M. A.

English

Green, M.A.

eScholarship - University of California

DESIGN CONCEPTS FOR THE ASTROMAG CRYOGENIC SYSTEM

NASA Technical Reports Server

  
 
 
N O T I C E 
 
THIS DOCUMENT HAS BEEN REPRODUCED FROM 
MICROFICHE. ALTHOUGH IT IS RECOGNIZED THAT 
CERTAIN PORTIONS ARE ILLEGIBLE, IT IS BEING RELEASED 
IN THE INTEREST OF MAKING AVAILABLE AS MUCH 
INFORMATION AS POSSIBLE 
https://ntrs.nasa.gov/search.jsp?R=19880003678 2020-03-20T09:12:59+00:00Z
A major purpose of the Techni-
cal Information Center iS to provide
the broadest dissemination possi-
ble of information contained in
DOE's Research and Development
departs to business, industry, the
adademic community, and federal,
stag and local governments.
Although a small portion of this
report is not reproducible, it is
being made available to expedite
the availability of information on the
research discussed herein.
1
X
	 LBL-238_2
Lawrence Berkeley Laboratory
UNIVERSITY OF CALIFORNIA
Engineering Division
Presented at the (nogenrc Engineer-mg Conterence.
St Charles, IL. June I-; -l8. ! 1) 8 -'. and to be published
in Ad^arxes in Cnogemc Engineerin g . Vol 33
Design Conrepts for the ASTRONIAG
Cnogenic Ssstem
M -k. Green and S. Castles
June 198-
w	 =^
. w-s_ ^ai yy • ^ —
•4v
s
Prepa-ed `or the U.S. department of Energy tinder Contract CE-ACO3-76SF00098
DISCI-AIMER
This document was prepared as an account of work sponsored
bs the United States Gosernment Neither the United States
Gosernment nor any agency thereof. nor The Regents of the
t'nrsersny of California. nor am of their employees. makes any
warranty. express or rriphed. or assumes any legal liability or
responsibduy for the accurac-, completeness. or usefulness of
any information. apparatus. product. or process disclosed. or
reXesents that its use would not nifringe privately owned rights.
Reference herein to any specific commeraal products process. or
senice M its trade name. trademark. manufacturer. or other-
wise. does not necessarily constitute or imply its endorsement.
recommendation. or favoring by the United States Government
or any agency thereof. or The Regents of the University o f Cah-
fornra. The %sews and opinions of authors expressed herein do
not necessanth sure or reflect those of the United States
Gosernment or am agency thereof or The Regents of the
I nrrersny of California and shall not be used for advertising or
product endorsement purposes.
Lawrence Berkeley Laboratory is an equal opportunity employer 	 4
1DESIGN CONCEPTS FOR THE ASTROMAG CRYOGENIC SYSTEM
LBL--23822
M. A. Green	 DE87 014683
Lawrence Berkel pv Laboratory
University of :,iforni„
Berkeley, Cal.,.,r•nia 94720
S. Castles
NASA/Goddard Space Flight Center
Greenbelt, Maryland
ABSTRACT
This paper describes the proposed cryogenic system used to cool the
superconducting magnet for the Space Station based ASTROMAG Particle
Astrophysics Facility. This 2-meter diameter superconducting magnet
will be cooled using stored helium II. The paper presents a liquid
helium storage concept which would permit cryogenic lifetimes of up to
3 years between refills. It is proposed that the superconducting coil
be cooled using superfluid helium pumped by the thermomechanical
effect. It is also proposed that the storage tank be resupplied with
helium in orbit. A method for charging and discharging the magnet with
minimum helium loss using split gas -cooled leads is discussed. A
proposal to use a stirling cycle cryocooler to extend the storage life
of the cryostat will also be presented.
THE ASTROMAG SUPERCONDUCTING MAGNET
The ASTROMAG experiment requires a magnetic field for charged
particle detection, momentum resolution and energy resolution. The
scientific capabilities of the ASTROMAG experiment depend on the size,
shape and placement of the magnet coils. The coil configuration in turn
stron,ly influences the cost and complexity of the facility. The magnet
and cryogenic design presented in this paper represents a relatively
simple magnet design concept which can be carried to the Space Station
using the shuttle.
The design concept for the strawman magnet configuration for the
ASTROMAG Particle Astrophysics Facility has the following
characteristics:1.2
1)	 The magnet coils are designed as relatively high-current coils
which operate at stored magnetic energies approaching ;?0 MJ. The
coils are designed so that the ratio of stored energy to active
coil mass is about 15 J per gram of active coil mass. About 1/3 of
MASTER
the overall magnet s ystem mass is expected to be active coil mass.
The coils are arranged so that the net magnetic dipole magnet is
zero. The field drops to earth's field about 20 meters from the
center of the magnet.
2) The magnet coils are to be located outside of the helium storage
tank. This permits the physics experiment to be brought close to
the coil package. (The goal is to put the closest detector about
10 cent,meters from the coil package.) In addition, the stored
magnetic energy which is dumped into the coils during a quench can
be decoupled from the liquid helium in the storage tank allowing it
to be cooled back down with helium vapor after it quenches.
3) The strawman design calls for spherical or near spherical tankage
for the liquid helium. This will minimize the tank mass for a
given stored helium volume and tank pressure rating; also, the
surface to volume ratio is favorable. The difficulty with s,rch a
tank is the added complexity of the superinsulation system.
4) The magnet cryostat vessel will have dished, inward facing heads at
the ends to permit the detectors to be brought close to the coil
ends without the weight penalty associates with flat or near flat
heads. The outside surface of the cryostat would be cylindrical
rear the coil and conical over the centea, helium tank. This
increases the room inside the cryostat for cold mass supports,
vapor-cooled shields and intercepts and a possible helium
refrigerator thermal strap for the outermost shield and intercept.
5) The magnet, which will have a persistent switch, will be powered
through vapor-cooled retractable leads. These leads will be part
of the vapor-cooled shield and intercep t_ circuit. This concept
permits one to maximize the charging efficiency of the magnet. The
lead current will be high enough to permit the coil to charge and
discharge it a reasonable rate, and the voltages within the coil
will be acceptably low during a quench. (See Table 1)
Table 1 presents some basic parameters for a high-current density
version of the 2-coil HEAO-type magnet proposed as a strawman
configuration for ASTROMAG. The location of the magnet coils with
respect to the helium storage tank and the outer vessel is shown in
Figure 1.
It is proposed that the magnet coils be cooled from the liquid
helium storage tank by circulating the helium through tubes built into
the coil packages. Helium II from the storage tank at 1.6 to 1.8 K will
be circulated through the coils using a thermomechanical (fountain
effect) pump.
THE MAGNET HELIUM II CRYOGENIC SYSTEM
The helium storage tank will be spherical with attached support
rings. The magnet coil packages are attached to these support rings
which also mate with the cold mass support system. A schematic
representation of the cold mass is shown in Figure 1. As shown, the
coil packages have 8 tubular struts between them to carry the
4.405 x 10 5 N (44.9 metric tons) tensile force between the ,oils.
These struts connect the coils to the helium tank. It is proposed that
the struts be made from low thermal conductivity MP-35 N steel to
thermally isolate the coils from the helium tank. This isolation allows
a thermally efficient recovery fr.)m a magnet quench.
2
r-I,tt,jr-Coll Support	 Worm Support Ring
i\	 Support Strut
	 Superconducting Coll
^ /.
	 Helium Tank
Vacuum Vessel
Space for Persis4rit Switch,
—%-via support Ring	 Gas Cooled Leads, and Cryogenic
Distribution Assembly
0	 ,^	 1
motors
XBL 876-2580
Figure 1. A schematic representation of the ASTROMAG cryostat showing the
coils, the helium tank, the support struts and the vacuum
vessel.
she ASTROMAG spherical cryogen tank has an outside diameter of
^.2 meters. The thickness of the tank t±ould normally be determined by
the magnet stored energy. If the magnet coils are thermally isolated
from the helium tank, the helium tank thickness is determined by the
1 atm external pressure which is applied during leak testing. The
helium tank can contain 4,600 liters of helium (almost 700 kg of helium
at i.8 K) with an additional 15 percent of the tank volume as ullage.
3
Table 1. Strawman Magnet Parameters
Number of coils
Coil outside diameter
Coil separation
Number turns
Design current
Peak field in coil
Coupling between coil and secondary
Magnet inductance
Magnet stored energy
Current density in s/c plus matrix
EJ 2 limit
Magnet coil cold mass
Magnet active cold mass
Stored energy to active cold mass ratio
Overall cold mass (including helium)
Overall magnet mass
2
2.0 m
2.2 m
6400
754 A
6.25 T
> 0.92
66.9 H
19.0 MJ
2.985 x 108 A m-2
1.69 x 1024 J A 2 m-4
1500 kg
1300 kg
14.6 J g-1
2650 kg
4000 kg
The largest single heat leak into the helium vessel is thrcigh the
cold mass support system. The heat leak into the 'elium vessel through
an ideal cold mass support system (where there is enough room for the
support rods and their end terminations) is proportional to the cold
mass, the design resonant frequency squared and the integrated
temperature function r ( his defined as the integral of thermal
conductivity with temperature from the lowest intercept temperature to
the helium vessel temperature). The heat leak through the support
straps into the vessel is inversely propo rtional to the elastic "dulus
cf the support strap in the direction of Force. The support rod
material of choice is one with the lowest value of r divided by the
elastic modulus. Oriented fiberglass epoxy is among the best materials
which can be used for cold mass supports. It is proposed that this
material be used in the form of tension straps with 2 thermal intercept
points on them. The support strap configuration shown in Figure 1
consists of 32 straps arranged so that the magnet cryostat is
self-centering during cooldown. The major design problem with support
systems is minimizing deflections at the tank and on the outside support
ring as the support system is loaded. The deflections are kept small so
that a support system resonant frequency above 35 Hz is obtainee.
Figure 2 is a schematic diagram of the ASTROMAG cryogenic system.
Cold valves, cold burst discs and crossover pluming associated with
ground operations and shuttle safety requirements have been omitted to
provide a clear exposition of the cryogenic system associated with the
magnets and magnet leads. Figure 2 shows one of the 2 methods being
investigated for circulating helium through the coils and the persistent
switch using a thermomechanical effect pump. 3 , 4
 Figure 2 also shows
how the boil-off gas is used to cool the retractable gas-cooled 750 4
electrical leads, the shields and heat intercepts on the cold mass
supports. The shield-intercept flow design for the ASTROMAG magnet
cryostat is similar to the 2-shield HEAO long-life cryostat built and
tested during the early 1970's. 5
 This cryostat demonstrated that
helium could be stored within a tank with a magnet for up to 2 years.
The HEAD design intercepted heat from the 100 A current leads on
the pipe leading from the tank to the first shield. The ASTROMAG
cryostat desi gn calls for enhanced heat transfer leads split into
4
2 parts. 6 The warm end can be disconnected from the cold end. The
cold end will be permanently connected to the magnet, helium vessel and
persistent switch. After the gas leaves the cold part of the
retractable lead, it passes through the 2 gas-cooled shields and support
intercepts to the warm retractable part of the gas-cooled lead system.
This configuration will result in increased efficiency during charging
and discharging because shields and intercepts will be cooled while
there is increased flow through the leads. As a result, the boil-off
from the helium tank will be reduced for a considerable time after the
magnet has been charged or discharged.
V7
I 1 ----- -- -- --	
----I	 ----1	 ^ 00TEI1 INTERCEPT
+	 I	 I	 I
I
i	 f
00 L TER
va
	
NE M S ORAOE	 INN°R LEADS
	 OUTER LE DS
TAN
	
vs
,V	 p
POROUS PLUG
^	 'b	 vt	 II
	
ERSISTENT SWITCH	 INNER INTE^ICEPTCQiI
	 COIL	 I
I	 I	 1
I L___ — _—_ 	 INNER SHIELD _J I
I
MELD	 J
_VACUUM V ESSEL
—0*_ Cryopele VSIve
(_ Cheee VaNs
X11 F'^ Rupture Dist
^l^Mwle for
XBL 876-2577
Figure 2. A schematic diagram of the ASTROMAG cryogenic system. (The
arrows show the flow through the coil driven by a simple
thermomechanical pump. The arrows also show gas flow through
the leads and intercepts. The shields are cooled by
condiction.)
The 750 A gas-cooled leads proposed for th- ASTROMAG magnet can
operate within the vacuum vessel in any orientation. 6 The leads would
be connected to the gas side of the porous ^ p ug. The leads can carry
the full cooldown gas flow with a relatively low pressure drop. The
leads are designed to withstand high temperature so that they can
operate for a long time without gas flow.
I
5
In a 2-shield cryogenic system, the most important part of the
insulation lies outside the outer shield. It is proposed that 100 to
120 layers of multi-layer insulation be used outside the warm shield.
This insulation must be applied very carefully, because it is important
that the heat load to the first shield be minimized. The insulation
between the outer and inner shield can be reduced to 40 or 50 layers.
Inside the inner shield only 2 layers of heavy foil are requir?d outside
the foam layer on the helium tank. The layer o f foam is a safety
feature to reduce the influx of heat to the helium vessel should the
vacuum tank be ruptured. The addition of a third shield cooled h^ , a 50
to 7G K refrigerator has been studied for use on the Space 7tation.
This refrigerated shield could increase the useful life of the ASTROMAG
cryostat from 2 years to 3 1/2 years, thus reducing the need for helium
resupply.
When the 2 shields are used alone, the estimated heat leak into the
ASTROMAG magnet cryostat is 0.24 W (about half is through the cold mass
supports). Helium would be boiled off at the rate of 0.0103 gs-1
(325 kg per year). When the leads are connected and the magnet is
charged, the estimated heat leak goes up to 3.0 W (including a 1.0 W
heat load due to charging the magnet at the rate of 250 A n; -1 ). The
integrated helium usage for 4 magnet charges and discharges per year
(3 hours per charge or discharge) is estimated to be under 3 kg of
helium per year.
The cryogenic system shown in Figure 2 is designed so that the
magnet coils can be cooled down (in the event of a quench) using helium
from the storage tank (about 75 kg of helium is required). The helium
storage tank and magnet coils can be cooled down from room temperature
using liquid helium pumped from a large storage tank V;rough the
ASTROMAG storage tank, the coils, the leads and shields.?
The strawman ASTROMAG magnet will have a vacuum vessel outside of
the superinsulation and shield system. This vacuum vessel permits the
magnet system to be tested on the grot r0d, and it separates the gas which
Right be in the physics detectors from the insulation system. The outer
vacuum vessel has a central support ring which connects to the shuttle.
Conical-cylindrical vessels carry the pressure loading from 1 atm
outside the vessel. The ends of the vacuum vessel consist of
'ightweight dished spherical heads (with a radius of curvature of
3 meters). These heads (dished inward) permit the physics detectors to
be brought to a distance of 10 centimeters from the coil package. The
central support ring transmits fo ,ces from the cold mass supports tc the
shuttle.
The projected magnet system cold mass including a pair of 750 kg
coils, a persistent switch, the tank. and 700 kg of helium is 2650 kg.
An aluminum vacuum vessel plus the shields is estimated to have a mass
of 1350 kg. (The use of composite structures could reduce the vacuum
vessel mass further.) 	 i
CRYOGENIC SAFETY
The ASTROMAG cryogenic system will be designed to meet the ^hL-±tle
and Space Station safety requirements. Specifically, the requirements
of Mil Standard 1522 will be satisfied. The major safety requirements
to be met are as follows:
The dewar will be two-failure tolerant. To meet this requirement,
the cryogen tank will incorporate 3 independent, high-flow rate
vent lines.
6
2. An engineering model cryogen tank will be proof pressure tested.
3. Fracture critical parts will be dye penetrant inspected.
4. The cryogen tank and plumbing system will be capable of
withstandi•ig a catastrophic piss of dewar guard vacuum.
The dewars being flown on the Superfluid Helium On Orbit Transfer
Flight Demnnstration 8 meet these requirements and provide heritage for
the design of the ASTROMAG cryogenic system.
SUMMARY
The ASTROMAG cryogenic system for the superconducting magnet has
Ir	 two features not found in ballon magnet cryostats or 'n previous designs
For space-borne cryostats. First, the superconductiny magnet coils are
thermally isolated from the helium tank so that the coil can be reccoled
after a magnet quench. Secondly, circulation through the magnet coil is
a thermomechanical effect pump which has no moving parts, and it should
be very reliable. The projected life for the helium system is over
2 years. This life can be extended to.3 1/2-years by adding a third
shield and intercept which is cooled by a small Stirling cycle
refrigerator.
ACKNOWLEDGMENTS
This research is supported by the Office of Astrophysics, NASA, and
the Office of Basic Energy Science, U.S. Department of Energy, under
Contract Number DE-ACO3-76SF00098.
REFERENCES
1. Interim Report of the ASTROMAG Oefinitiun Team on the Particle
Astrophysics Magnet Facility ASTROMAG (19861.
2. M.A. Green, et al, ASTROMAG: A Superconducting Particle
Astrophysics Magnet Facility for the Space Station, to be published
in the IEEE lransactior,s on Magnetics MAG-23 (1987).
3. M.O. Pirro and S. Castles, "Superfluid Helium Transfer Flight
Demonstration Using the Thermumechanical Effect", Cryogenics 26
(1986), P. 84.
4. A. Hofmann, et al, "Considerations on Magnet Design Based on Forced
Flow of Helium II in Internally Cooled Cables", Intor-Report (1986).
5. W.L. Pope, et al, "Superconducting Magnet and Cryostat for a Space
Application', Advances in Cryogenic Engineering 20 (19)4), p. 47.
f	 6.	 R.G. Smits, et al, "Gas-Cooled Electrical Lead for Use on Forced
Cooled "-uperconducting Magnets", Advances in Cryogenic Engineering
27 (1981).
7. M.A. Green, et al, "The Operation of a Forced Two-Phase Cooling
System on a. Large Superconducting Magnet", Proceedings of ICEC-8,
Genova, p. 72 (1980).
8. M. DiPirro and P. Kittel, "The Superfluid Helium On Orbit Transfer
(SHOOT) Flight Demonstration", to be pub'iished in Advances in
Cryogenic Engineering 33 (1987).
7
it
i
FIL NNE[)
i_ ► i_1I0 ',1'87


Design concepts for the ASTROMAG cryogenic system

Abstract

Similar works

Full text

Available Versions

eScholarship - University of California

NASA Technical Reports Server