U.S. patent application number 14/843711 was filed with the patent office on 2017-03-02 for compact adiabatic demagnetization refrigeration stage with integral gas-gap heat switch.
The applicant listed for this patent is U.S.A. as represented by the Administrator of the National Aeronautics and Space Administration, U.S.A. as represented by the Administrator of the National Aeronautics and Space Administration. Invention is credited to PETER J. SHIRRON.
Application Number | 20170059214 14/843711 |
Document ID | / |
Family ID | 58103818 |
Filed Date | 2017-03-02 |
United States Patent
Application |
20170059214 |
Kind Code |
A1 |
SHIRRON; PETER J. |
March 2, 2017 |
COMPACT ADIABATIC DEMAGNETIZATION REFRIGERATION STAGE WITH INTEGRAL
GAS-GAP HEAT SWITCH
Abstract
An adiabatic demagnetization refrigeration stage includes a salt
pill, a magnet surrounding the salt pill, and a gas-gap heat switch
interposed between the salt pill and the magnet. A method of
operating an adiabatic demagnetization refrigeration stage includes
using a magnet surrounding a salt pill to apply an increasing
magnetic field to the salt pill, producing a gas to activate a gas
gap heat switch interposed between the magnet and the salt pill to
provide a path for heat flow from the salt pill through the magnet
to a heat sink, and decreasing the magnetic field applied to the
salt pill while adsorbing the gas to de-activate the gas gap heat
switch to cool the salt pill to a lower temperature and cool an
object attached to a cold tip extending from the salt pill.
Inventors: |
SHIRRON; PETER J.; (SILVER
SPRING, MD) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
U.S.A. as represented by the Administrator of the National
Aeronautics and Space Administration |
Washington |
DC |
US |
|
|
Family ID: |
58103818 |
Appl. No.: |
14/843711 |
Filed: |
September 2, 2015 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F25B 21/00 20130101;
Y02B 30/00 20130101; F25B 2321/0023 20130101; Y02B 30/66
20130101 |
International
Class: |
F25B 21/00 20060101
F25B021/00 |
Goverment Interests
INVENTION BY GOVERNMENT EMPLOYEE(S) ONLY
[0001] The invention described herein was made by one or more
employees of the United States Government, and may be manufactured
and used by or for the Government for governmental purposes without
the payment of any royalties thereon or therefor.
Claims
1. An adiabatic demagnetization refrigeration stage comprising: a
salt pill; a magnet surrounding the salt pill; and a gas-gap heat
switch interposed between the salt pill and the magnet.
2. The refrigeration stage of claim 1, wherein the salt pill
comprises a paramagnetic material refrigerant in a helium
atmosphere enclosed in a copper container.
3. The refrigeration stage of claim 1, comprising a port in fluid
communication with both the gas gap heat switch and a gas source
for providing a gas and charge pressure to the gas gap heat
switch.
4. The refrigeration stage of claim 1, comprising a passive gas gap
heat switch.
5. The refrigeration stage of claim 4, comprising a getter
thermally coupled to the salt pill and in fluid communication with
the gas-gap heat switch.
6. The refrigeration stage of claim 1, comprising an active gas gap
heat switch.
7. The refrigeration stage of claim 6, comprising a getter in fluid
communication with the gas-gap heat switch and having an
independent temperature control.
8. The refrigeration stage of claim 1, wherein the gas gap heat
switch comprises an outer surface of the salt pill substantially
concentric with an inner surface of the magnet and a gas confined
there between.
9. The refrigeration stage of claim 1, wherein the gas gap heat
switch comprises radially extending fins of an outer surface of the
salt pill interleaved with radially extending fins of an inner
surface of the magnet and a gas confined there between.
10. The refrigeration stage of claim 1, comprising a structure
supporting the salt pill within the surrounding magnet.
11. The refrigeration stage of claim 10, wherein the structure
comprises a standoff positioned around a bellows attached to a
first end of the salt pill to constrain axial and lateral
motion.
12. The refrigeration stage of claim 10, further comprising a hub
circumscribing a second end of the salt pill and a plurality of
stays connecting the hub and a concentric cylindrical sleeve.
13. A method of operating an adiabatic demagnetization
refrigeration stage comprising: using a magnet surrounding a salt
pill to apply an increasing magnetic field to the salt pill;
producing a gas to activate a gas gap heat switch interposed
between the magnet and the salt pill to provide a path for heat
flow from the salt pill through the magnet to a heat sink; and
decreasing the magnetic field applied to the salt pill while
adsorbing the gas to de-activate the gas gap heat switch to cool
the salt pill to a lower temperature and cool an object attached to
a cold tip extending from the salt pill.
14. The method of claim 13, comprising regulating the magnetic
field to maintain the salt pill at the lower temperature.
15. The method of claim 13, comprising providing the gas and a
charge pressure to the gas gap heat switch using a port in fluid
communication with both the gap and a gas source.
16. The method of claim 13, comprising producing the gas using a
getter thermally coupled to the salt pill and in fluid
communication with the gas-gap heat switch.
17. The method of claim 13, comprising producing the gas using a
getter in fluid communication with the gas-gap heat switch and
having an independent temperature control.
18. The method of claim 13, comprising providing an increased
surface area for heat transfer through the gas gap heat switch by
interleaving radially extending fins of an outer surface of the
salt pill with radially extending fins of an inner surface of the
magnet.
19. The method of claim 13, comprising supporting the salt pill
within the surrounding magnet using a standoff positioned around a
bellows attached to a first end of the salt pill to constrain axial
and lateral motion.
20. The method of claim 13, comprising supporting the salt pill
within the surrounding magnet using a hub circumscribing a second
end of the salt pill and a plurality of stays connecting the hub
and a concentric cylindrical sleeve.
Description
ORIGIN OF THE INVENTION
Background
[0002] The disclosed embodiments generally relate to refrigeration
and more particularly to an adiabatic demagnetization refrigeration
system.
[0003] Adiabatic demagnetization is a very robust technique for
cryogenic cooling, easily producing very low temperatures of
approximately 100 milliKelvin and below from relatively high heat
sink temperatures, for example, approximately 1-5 Kelvin. Other
cooling techniques include dilution refrigeration, helium-3 and
helium-4 refrigerators, as well as some more exotic superfluid
stirling and Superconductor Insulator Normal metal Insulator
Superconductor (SINIS) coolers for producing cooling over some part
of these ranges, however, Adiabatic Demagnetization Refrigeration
(ADR) covers a wider temperature range, works well in zero-gravity
and on the ground, is more efficient, and--having no moving
parts--is more reliable.
[0004] ADR utilizes the magnetocaloric effect, which is a
phenomenon in which certain materials warm or cool as they are
exposed to increasing or decreasing magnetic fields. In
paramagnetic materials, the effect originates in the interaction of
an external magnetic field with the magnetic moment of unpaired
outer-shell electrons of the paramagnetic material, and results in
the entropy of the material having a strong dependence on magnetic
field and temperature. An ADR stage produces cooling (or heating)
by the interaction of a magnetic field with the magnetic spins in a
paramagnetic salt. Magnetizing the salt produces heating, and
demagnetizing the salt produces cooling.
[0005] A typical ADR stage may include: a capsule of a solid
paramagnetic material refrigerant, commonly referred to as a salt
pill, an electromagnet, a cold tip, a heat sink and a heat switch.
The salt pill may be suspended within the electromagnet without
making direct contact using a suspension system providing
structural support with minimal thermal conductance. In
conventional implementations, the heat switch is external to the
magnet and salt pill assembly and interposed between the heat sink
and salt pill. A thermal attachment to the heat sink and salt pill
is generally implemented with a thermal strap on one or both
ends.
[0006] The refrigeration cycle generally includes the following
steps: First, the salt pill starts at zero field and is magnetized,
causing it to warm up. Second, when its temperature exceeds that of
the heat sink, the heat switch is powered into the state. Third,
the salt pill continues to be magnetized, generating heat which
flows to the heat sink. This continues until full field is reached,
which is strong enough to significantly align the spins and
suppress the entropy of the salt. In some embodiments, full field
may be in a range of approximately 1-4 Tesla. Fourth, at full
magnetic field, the heat switch is deactivated to thermally isolate
the salt from the heat sink. Fifth, the salt is demagnetized to
cool it to the desired operating temperature. In general, the salt
will then be receiving heat from components parts. The heat is
absorbed and operating temperature is maintained by slowly
demagnetizing the salt at a controlled rate. Heat can continue to
be absorbed until the magnetic field is reduced to zero, at which
point the ADR has run out of cooling capacity.
[0007] In conventional ADR configurations, the heat flow path from
the cold tip to the heat sink involves a number of components whose
design is a compromise between keeping mass and size small, and
achieving a desired thermal conductance. When heat is being
absorbed, it flows from the cold tip into the salt pill via a
thermal bus that runs the length of the salt pill and distributes
the heat to the refrigerant. When the ADR is being recycled, heat
flows back out along the length of the salt pill via this thermal
bus, through a thermal strap, through the heat switch, optionally
through another thermal strap, and to the heat sink. Because all of
the conductors involved (the salt pill, thermal bus, the thermal
straps and the heat switch) tend to be long and narrow, it is
typically very challenging to achieve sufficiently high thermal
conductance for the heat rejection path.
[0008] The size and shape of the magnet and salt pill assembly and
the heat switch generally result in a relatively large volume for
the ADR assembly in comparison to an implementation where the
components are integrated. This is significant for most cryogenic
systems because a larger experiment volume translates to a larger
cryostat volume, with larger heat loads on the liquid cryogens or
mechanical cryocooler. This in turn requires an increased volume of
cryogens, or higher power cryocoolers. A larger cryostat size also
results in larger, bulkier vacuum vessels which become
progressively harder to manipulate.
[0009] The mass of the external heat switch and thermal straps
individually is typically larger than an implementation where they
are combined. In addition, the externally mounted heat switch and
straps are more subject to damage and the externally mounted heat
switch adds to the operational complexity of the system.
[0010] It would be advantageous to provide Adiabatic
Demagnetization Refrigeration assemblies and techniques that
overcome these and other disadvantages.
SUMMARY
[0011] The disclosed embodiments are directed to a unique
arrangement of adiabatic refrigeration components that reduces the
heat flow path to a short, broad flow path with much higher thermal
conductance, resulting in better thermal performance of the ADR and
faster recycle times.
[0012] According to at least one of the disclosed embodiments, an
adiabatic demagnetization refrigeration stage includes a salt pill,
a magnet surrounding the salt pill, and a gas-gap heat switch
interposed between the salt pill and the magnet.
[0013] The salt pill may be constructed of a paramagnetic material
refrigerant in a helium atmosphere enclosed in a copper
container.
[0014] The refrigeration stage may include a port in fluid
communication with both the gas gap heat switch and a gas source
for providing a gas and charge pressure to the gas gap heat
switch.
[0015] The refrigeration stage may include a passive gas gap heat
switch which may further include a getter thermally coupled to the
salt pill and in fluid communication with the gas-gap heat
switch.
[0016] The refrigeration stage may include an active gas gap heat
switch which may further include a getter in fluid communication
with the gas-gap heat switch and having an independent temperature
control.
[0017] The gas gap heat switch may include an outer surface of the
salt pill substantially concentric with an inner surface of the
magnet and a gas confined there between.
[0018] The gas gap heat switch may include radially extending fins
of an outer surface of the salt pill interleaved with radially
extending fins of an inner surface of the magnet and a gas confined
there between.
[0019] The refrigeration stage may include a structure supporting
the salt pill within the surrounding magnet, which may further
include a standoff positioned around a bellows attached to a first
end of the salt pill to constrain axial and lateral motion.
[0020] The refrigeration stage may include a structure supporting
the salt pill within the surrounding magnet, which may further
include a hub circumscribing a second end of the salt pill and a
plurality of stays connecting the hub and a concentric cylindrical
sleeve.
[0021] According to at least one other disclosed embodiment, a
method of operating an adiabatic demagnetization refrigeration
stage includes using a magnet surrounding a salt pill to apply an
increasing magnetic field to the salt pill, producing a gas to
activate a gas gap heat switch interposed between the magnet and
the salt pill to provide a path for heat flow from the salt pill
through the magnet to a heat sink, and decreasing the magnetic
field applied to the salt pill while adsorbing the gas to
de-activate the gas gap heat switch to cool the salt pill to a
lower temperature and cool an object attached to a cold tip
extending from the salt pill.
[0022] The method may include regulating the magnetic field to
maintain the salt pill at the lower temperature.
[0023] The method may also include providing the gas and a charge
pressure to the gas gap heat switch using a port in fluid
communication with both the gap and a gas source.
[0024] The method may further include producing the gas using a
getter thermally coupled to the salt pill and in fluid
communication with the gas-gap heat switch.
[0025] The method may additionally include producing the gas using
a getter in fluid communication with the gas-gap heat switch and
having an independent temperature control.
[0026] The method may likewise include providing an increased
surface area for heat transfer through the gas gap heat switch by
interleaving radially extending fins of an outer surface of the
salt pill with radially extending fins of an inner surface of the
magnet.
[0027] The method may similarly include supporting the salt pill
within the surrounding magnet using a standoff positioned around a
bellows attached to a first end of the salt pill to constrain axial
and lateral motion.
[0028] The method may still further include supporting the salt
pill within the surrounding magnet using a hub circumscribing a
second end of the salt pill and a plurality of stays connecting the
hub and a concentric cylindrical sleeve.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] The foregoing aspects and other features of the embodiments
are explained in the following description, taken in connection
with the accompanying drawings, wherein:
[0030] FIG. 1 shows a cross section of an embodiment of an ADR
stage according to the disclosed embodiments;
[0031] FIG. 2 shows a schematic cross section of a magnet;
[0032] FIG. 3 shows an exemplary embodiment of a gas gap heat
switch;
[0033] FIGS. 4A-4C illustrate further embodiments of the gas gap
heat switch;
[0034] FIG. 5 illustrates an exemplary embodiment an ADR stage
having an active gas gap heat switch;
[0035] FIG. 6 shows exemplary structures for supporting a salt
pill;
[0036] FIGS. 7A-7C illustrate an example of a bottom salt pill
suspension; and
[0037] FIGS. 8A-8C show exemplary stays of the bottom
suspension.
DETAILED DESCRIPTION
[0038] The disclosed embodiments are directed to a compact,
self-contained Adiabatic Demagnetization Refrigeration (ADR) stage
that incorporates a gas-gap heat switch into the internal
structure. This results in a reduction in the size, mass and
complexity of the refrigeration stage. The unit may have two
thermal interfaces: a cold tip, and a heat rejection plate that
also serves as the mounting surface. The latter feature allows the
unit to be mounted directly to a heat sink, which could be a liquid
helium tank or a mechanical cryocooler.
[0039] The disclosed embodiments include the ability to vary gas
gap heat switch gas and gas-gap heat switch charge pressure to
selectively change the transition temperature between heat
conducting and non-conducting states. Thus, the gas gap heat switch
may be adjusted to turn on and off at different temperatures. The
internal gas gap heat switch is tuned or controlled so that the
magnetic refrigerant in the salt pill positioned in a core of a
magnet becomes thermally coupled to the heat sink as it is
magnetized and it warms above the heat sink temperature. Continued
magnetization charges the refrigerator as heat flows to the heat
sink. Subsequent demagnetization causes the refrigerant to cool
while the gas gap heat switch is opened and the refrigerant is
thermally decoupled from the heat sink. With proper choice of
refrigerant, the unit can operate at temperatures well below 1
Kelvin, using heat sinks of up to 5 K.
[0040] In embodiments with a passive gas gap heat switch, a simple
power control may be used to ramp the magnet current up and down
with no power control required for the gas gap heat switch. In
embodiments with an active gas gap heat switch, a simple power
control may be used to ramp the magnet current up and down and to
apply or remove power for the active gas gap heat switch.
[0041] The passive and active gas gap heat switches may be
implemented with interleaved fins providing increased surface area
and thermal conductance.
[0042] The disclosed embodiments also include suspension components
that constrain the movement of the salt pill within the magnet and
minimize heat leak at cold temperatures.
[0043] FIG. 1 shows a cross section of an embodiment of an ADR
stage 100 according to the present disclosure. The ADR stage 100
includes a salt pill 130, a magnet 110 with a center cavity 155, a
gas-gap heat switch 115 interposed between the salt pill 130 and
the magnet 110, a cold tip 120 extending from the salt pill 130,
and a bottom plate heat sink mount 125 for mounting to a heat sink
127.
[0044] The salt pill 130 includes a volume of refrigerant 105, for
example, a single crystal or polycrystalline gadolinium compound,
chromium potassium alum, ferric ammonium alum, or any other
suitable refrigerant. The refrigerant 105 may be encapsulated in a
helium gas atmosphere, for example, helium-3 or helium-4, depending
on the temperature range of operation. An enclosure 143 may be used
to encapsulate the refrigerant 105, and in some embodiments, may be
constructed of thermally conductive material such as copper or
other suitable substance. For purposes of this disclosure, the salt
pill 130 refers to the enclosure 143 and the refrigerant 105.
[0045] A thermally conductive rod extends from a first end 133 of
the salt pill 130 to serve as the ADR's cold tip 120. The thermally
conductive rod 120 may comprise copper or other suitable thermally
conductive material. A second end 137 of the salt pill 130 may
include a cap 135 for filling and sealing in the helium gas, as
well as an attachment 140 for a bottom suspension 145.
[0046] In this exemplary embodiment, the gas gap heat switch 115 is
a passive gas gap heat switch where the transition temperature
between heat conducting and non-conducting states is dependent on
the type of gas and the number of atoms of gas within the gas gap
heat switch 115. A getter 150 may be bonded onto the first end of
the salt pill 130 for effecting the transition between heat
conducting and non-conducting states. In an exemplary embodiment,
the getter of the passive gas gap heat switch may comprise a
quantity of sintered stainless steel powder.
[0047] FIG. 2 shows a schematic cross section of the magnet 110. In
at least one embodiment, the magnet includes one or more conductors
205, such as small diameter NbTi wire, wound onto a mandrel 210.
Other exemplary conductors may include aluminum, Nb3Sn, Nb3Al,
MgB2, or high temperature superconductors such as YBCO. The mandrel
210 includes a center cavity 215 for enclosing the salt pill 130
and the gas-gap heat switch 115. The mandrel 210 is generally
constructed of a heat conductive material, for example, copper.
[0048] Returning to FIG. 1, the salt pill 130 may be suspended
within the center cavity 155 of the magnet 110 to maintain a stable
gap between an outer diameter of the salt pill 130 and an inner
diameter of the magnet 110. The stable gap is used to implement the
gas gap heat switch 115.
[0049] The volume inside the center cavity 155 of the magnet 110 is
hermetically sealed to retain the gas used to implement the gas gap
heat switch. A bellows 160 may be used as a seal between the cold
tip 120 and the magnet mandrel 210. The bellows may be constructed
of stainless steel and one or more solder or braze seals 195 may be
made between the bellows 160 and the cold tip 120. The bellows 160
may also be sealed to a top plate 165 which may be further sealed
to the magnet mandrel 210 by, for example, an indium seal 185. A
bottom plate 170 may also be sealed to the magnet mandrel using an
indium seal 190. The bottom plate 170 may include a central post
175 extending to the heat sink mount 125. The top plate 165 and
bottom plate 170 may be constructed of a suitable thermally
conductive material, for example copper.
[0050] A port 180 may be provided for evacuating the sealed center
cavity 155 and for connecting to a gas source 182, for example,
helium-3 or helium-4, for the gas-gap heat switch 115. The
transition temperature between heat conducting and non-conducting
states is dependent on the type of gas and the number of atoms of
gas within the gas gap heat switch 115, and the port provides the
ability to vary the gas gap heat switch gas and the gas-gap heat
switch charge pressure to selectively change the transition
temperature.
[0051] A magnetic shield 183 may enclose the ADR system to prevent
fringing fields. In at least one embodiment, the magnetic shield
may be made of a ferromagnetic material or any other suitable
magnetic shielding material. In some embodiments, the magnetic
shield may be assembled from a number of component pieces. The
bottom plate 170, central post 175, and heat sink mount 125 are
shaped to provide a thermal path from the magnet 110 to a heat
sink, and for allowing the magnetic shield 183 shield enclose the
magnet 110 to limit fringing fields, for example, while the ADR
stage is at full field.
[0052] FIG. 3 shows a top view of an exemplary implementation of
the gas gap heat switch. In FIG. 3 the gas gap heat switch 300
includes the inner surface 305 of the mandrel 210, the outer
surface 310 of the salt pill 130, a gas 315, such as helium-3,
confined in a gap 320 between the inner surface 305 of the mandrel
210 and the outer surface 310 of the salt pill 130. The getter 150
is shown for reference and may be positioned elsewhere in some
embodiments. In this embodiment, the inner surface 305 of the
mandrel 210 and the outer surface 310 of the salt pill 130 are
substantially round and substantially concentric with each
other.
[0053] FIGS. 4A-4C illustrate other embodiments of the mandrel 405
and the salt pill 410 having an increased surface area for heat
transfer through the gas gap heat switch between the mandrel and
the salt pill. The increased surface area provides an increase in
the thermal conductance of the gas gap heat switch. In this
embodiment, the mandrel 405 as shown in FIG. 4A, and the salt pill
410 as shown in FIG. 4B, both include a splined configuration in
which the mandrel 405 has radial fins 415 and the salt pill 410
also has radial fins 420 positioned to interleave with each other.
The interleaved radial fins 415, 420 provide an increased surface
area, in some embodiments by as much as a factor of three depending
on the dimensions of the radial fins. FIG. 4C shows a top view of
the mandrel 405 and the salt pill 410 assembled together with the
interleaved radial fins 415, 420. In at least one embodiment,
fasteners 425 are radially spaced around the mandrel 405 for
fastening the top plate 165 to the mandrel 405. In some
embodiments, the gas gap heat switch 430 may be implemented with an
odd number of fins which is one less than twice the number of
fasteners 425. This number of fins allows the salt pill 410 to be
rotated through different radial fastening positions to find the
best spacing for the fins.
[0054] FIG. 5 illustrates an embodiment of an ADR stage 100 having
an active gas gap heat switch 515. In this embodiment, the getter
505 may be external to the center cavity 155 and includes getter
material 510, for example charcoal, and an independent heating
mechanism 520, for example a resistive heater. In this exemplary
embodiment, port 180 may be used for evacuating the sealed center
cavity 155 and filling it with gas, for example, helium-3 or
helium-4, for the gas-gap heat switch 115. The transition
temperature between heat conducting and non-conducting states is
controlled by applying or removing power from the heating mechanism
520, providing a selectable temperature range where the heat
conducting and non-conducting states occur.
[0055] As shown in FIG. 6, the disclosed embodiments include
structures for supporting or suspending the salt pill 130 within
the magnet 110. At the first end 133 of the salt pill 130, a top
suspension 600 may include a standoff 605 and bellows 160. The
bellows 160 provides a rotational constraint on salt pill motion,
at least by way of the one or more solder or braze seals 195
between the bellows 160 and the cold tip 120. The standoff 605,
which may be constructed of a temperature stable material, for
example, a polyimide-based plastic such as Vespel.RTM., may be
positioned around the bellows 160 and attached to the magnetic
shield 183 as well as the top plate 165. With the bellows 160 and
standoff 605, axial motion, as well as motion in the two lateral
degrees of freedom is constrained.
[0056] FIGS. 7A-7C illustrate an example of the bottom suspension
145 utilized at the second end 137 of the salt pill 130. In this
embodiment, the bottom suspension 145 includes a wheel and hub type
suspension including a wheel 610 and a hub 615 connected by a
number of stays 620. While the bottom suspension 145 is shown as
having a wheel and hub configuration, it should be understood that
any suitable suspension may be used so long as the position of the
salt pill 130 is controlled within the center cavity 155. As shown
in FIG. 6, the wheel portion may be implemented by a cylindrical
protrusion 610 integral to the bottom plate 170. In other
embodiments, the wheel portion may be separately constructed and
may be made of other materials so long as the wheel portion is
capable of functioning as described herein.
[0057] Turning to FIG. 7A, the wheel portion of the bottom
suspension 145 may include cylindrical sleeve 710. Cylindrical
sleeve 710 may include a plurality of through holes 725 which may
extend radially through the sleeve and may be equally spaced around
the perimeter of the sleeve 710. The cylindrical sleeve 710 may
also include a groove 730 around its outside circumference in which
the through holes may be located. An exemplary hub 715 is shown in
FIG. 7B. Hub 715 includes a groove 735 around its outside
circumference and has an inner diameter 740 sized to receive the
bottom suspension attachment 140 of the salt pill 130. FIG. 7C
shows an exemplary embodiment of the bottom suspension 145 showing
the position of the cylindrical sleeve 710, hub 715, and stays 720
as assembled together.
[0058] As shown in FIGS. 8A-8C, the stays may be formed by
individual loops 820 A, 820 B, 820 C positioned around the
circumference of the cylindrical sleeve 710 in the groove 730,
threaded through one of the holes 725 and positioned in the groove
735 around the outside circumference of the hub 715. The loops 820
A, 820 B, 820 C have lengths such that the loops in tension,
securely center the hub 715 concentrically with the cylindrical
sleeve 710. In some embodiments, the loops may be constructed of
Kevlar.RTM., however, it should be understood that any other
suitable material may be used. When the salt pill 130 is assembled
within the center cavity 155 of the magnet 110, the bottom
suspension attachment 140 engages the hub to provide constraint in
two degrees of freedom, securing the salt pill 130 within an inner
diameter of the magnet 110.
[0059] The materials and construction techniques of the top
suspension 600 and bottom suspension 145 significantly limit any
parasitic heat leak into the salt pill 130.
[0060] After assembly, the center cavity 155 of the ADR stage is
evacuated and filled with a controlled amount of helium gas, for
example, helium-3 or helium-4. In embodiments utilizing a passive
gas gap heat switch, the transition temperature between heat
conducting and non-conducting states is dependent on the type of
gas and the number of atoms of gas within the gas gap heat switch
115. In embodiments utilizing an active gas gap heat switch, the
transition temperature range where the heat conducting and
non-conducting states occur is dependent on when and how much power
is applied to the heating mechanism 520 (FIG. 5). The target on and
off temperatures are generally at or slightly above the expected
heat sink temperature.
[0061] After the gas gap heat switch is filled with gas, the ADR
stage 100 equilibrates to the heat sink temperature. Current may be
increasingly applied to the magnet 110 causing the salt pill 130 to
increase in temperature. In passive gas gap heat switch
embodiments, the temperature increase of the salt pill causes the
gas gap heat switch 115 to turn fully on by releasing helium gas
from the getter 150 into the center cavity 155 between the salt
pill 130 and magnet 110. In active gas gap heat switch embodiments,
the heating mechanism 520 is used to apply heat to the getter
material 510, releasing helium gas from the getter 505 into the
center cavity 155 between the salt pill 130 and magnet 110. The
helium gas conducts heat from the salt pill 130 to the magnet
mandrel 210, and heat flows through the bottom plate 170 and
central post 175 to the heat sink mount 125 and to the heat sink
127.
[0062] As the magnet 110 is ramped to full field, the salt pill 130
approaches its peak charge and begins to re-equilibrate with the
heat sink. The magnet current can then be ramped down, cooling the
salt pill 130. For embodiments utilizing a passive gas gap heat
switch, the cooling causes the helium gas to re-adsorb onto the
getter 150 from the center cavity 155 and the gas gap heat switch
115 to turn off. In active gas gap heat switch embodiments, the
heating mechanism 520 is disabled, allowing the getter material 510
to cool and re-adsorb the helium gas from the center cavity 155,
also causing the gas gap heat switch 115 to turn off. As the salt
pill 130 cools, the magnetic field may be regulated to achieve a
stable low temperature for operation.
[0063] It should be noted that heat is conducted from the
refrigerant through the thin wall of its copper container. That is,
the heat is conducted through a large surface area across a very
small thickness of copper, so the thermal conductance is
exceedingly large and the heat transfer is very efficient.
[0064] In ADRs, a common design consideration is eddy current
heating, relevant because of the use of copper in some embodiments
of the magnet mandrel 210, 405 and salt pill enclosure 143. For
typical magnet ramp rates, for example, 4 Tesla in 2-5 minutes, the
eddy current heating in the copper salt pill enclosure and the
copper magnet mandrel may each be approximately 100 microwatts,
however, the heat is transferred immediately to the heat sink,
where that amount of heat is generally negligible. The heat
generated in the copper salt pill enclosure on the increasing ramp
at the start of recycling is also dumped directly to the heat sink
which has sufficient capacity to accommodate this. The heat
generated on the decreasing ramp after the gas gap heat switch
turns off must be absorbed by the refrigerant, however, in the
disclosed embodiments, the eddy current heating generated after the
gas gap heat switch turns off represents approximately 1% of the
available cooling capacity, representing an acceptable inefficiency
in the system.
[0065] The disclosed embodiments utilize a gap between the salt
pill and the magnet mandrel to create a gas-gap heat switch. As a
result, sensitive components are located on the interior of the
adiabatic stage and are protected and surrounded by the magnet
windings, the magnetic heat shield, and the top and bottom plates,
thus providing rugged exterior surfaces. Control for the ADR stage
is simplified, requiring simple controls for ramping the magnetic
field up and down for embodiments with passive gas gap heat
switches, and optional controls for applying or removing power from
a getter heating mechanism in embodiments with active gas gap heat
switches. The disclosed embodiments provide the ability to vary the
transition temperature between heat conducting and non-conducting
states by varying refrigerants and gas-gap heat switch charge
pressures, or by varying an amount of power applied to a getter
heating mechanism. The disclose embodiments further provide simple,
robust suspension components that constrain the movement of the
salt pill within the magnet and minimize heat leak at cold
temperatures.
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