Note: Descriptions are shown in the official language in which they were submitted.
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"Method and apparatus for operation of a cryogenic device in a gaseous
environment"
Cross-Reference to Related Applications
The present application claims priority from Provisional Patent Application No
2004903688 filed on 5 July 2004, the content of which is incorporated herein
by
reference.
Field of the Invention
The present invention relates to the operation of a cryogenic device in a
gaseous
environment, and more particularly relates to a method and device for
providing a
gaseous environment at a temperature equal or close to liquid coolant
temperature.
Description of the Prior Art
In the past, cryogenic cooling of cryogenic devices has been provided by
immersing the cryogenic device in a liquid coolant such as liquid nitrogen or
liquid
helium, thus maintaining the temperature of the cryogenic device at or below
the
boiling temperature of the liquid coolant. The use of liquid nitrogen provides
for
cryogenic operation at or below 77.3K, while the use of liquid helium provides
for
cryogenic operation at or below 4.2K.
Recently, cryogenic devices have been designed which rely on movement of the
device for operation. Such a device is set out in International Patent
Publication No.
WO 2004/015435 by CSIRO and Tilbrook, the content of which is incorporated
herein
by reference, which teaches rotation of one or more SQUIDs or superconducting
field
sensors in order to obtain information about a magnetic field. SQUIDs and
superconducting field sensors must be maintained below the critical
temperature Tc of
the superconducting material in order to achieve proper superconducting
operation.
However, should such a moving cryogenic device be immersed in liquid coolant,
significant turbulence will be generated within the fluid, leading to
acoustic, magnetic
and electrical noise. Further, mechanical stress will be placed on the often
delicate
device by viscous drag and/or mechanical vibrations.
Throughout this specification the word "comprise", or variations such as
"comprises" or "comprising", will be understood to imply the inclusion of a
stated
element, integer or step, or group of elements, integers or steps, but not the
exclusion of
any other element, integer or step, or group of elements, integers or steps.
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2
Any discussion of documents, acts, materials, devices, articles or the like
which
has been included in the present specification is solely for the purpose of
providing a
context for the present invention. It is not to be taken as an admission that
any or all of
these matters form part of the prior art base or were common general knowledge
in the
field relevant to the present invention as it existed before the priority date
of each claim
of this application.
Summary of the Invention
According to a first aspect the present invention is an apparatus for
providing a
cryogenic gaseous environment, the apparatus comprising:
a chamber for containing the cryogenic gaseous environment and for excluding
external liquid coolant;
a liquid inlet for selectively flooding the chamber with liquid coolant; and
a chamber gas port for selectively permitting egress of gas from the chamber
during liquid flooding of the chamber, and for selectively containing gas
within the
chamber.
According to a second aspect, the present invention is a method of providing a
cryogenic gaseous environment, the method comprising:
flooding a chamber with liquid coolant; and
causing cryogenic gas to occupy the chamber and displace liquid coolant from
the chamber.
The chamber gas port may comprise a gas injection port for purging the
chamber with gas to evacuate liquid from the chamber. The gas injection port
may
itself permit egress of gas during liquid flooding of the chamber.
Additionally or
alternatively, the chamber may comprise a gas outflow port for permitting
egress of gas
from the chamber during liquid flooding of the chamber.
The chamber gas port may comprise a gas vent having open and closed
positions, such that the gas vent when open allows egress of gas from the
chamber
during liquid flooding of the chamber, and such that the gas vent when closed
contains
gas within the chamber.
Accordingly, in a third aspect, the present invention is . an apparatus for
providing a gaseous environment for operation of a cryogenic device, the
apparatus
comprising:
a chamber for housing the cryogenic device;
a port in the chamber allowing the chamber to be flooded by liquid coolant;
and
a gas vent for allowing escape of gas from the chamber;
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wherein the chamber is configured such that, when the gas vent is closed, gas
boiled off liquid coolant within the chamber will accumulate in the chamber
and force
liquid coolant out of the port.
According to a fourth aspect the present invention provides a method for
providing a gaseous environment for operation of a cryogenic device;
comprising:
flooding a chamber with liquid coolant; and
causing gas boiled off the liquid coolant to accumulate in the chamber, such
that
liquid coolant is forced out of the chamber.
The present invention provides for the chamber to be flooded with liquid
coolant, followed by evacuation of the liquid coolant while maintaining the
interior of
the chamber at cryogenic temperatures. Flooding of the chamber is of value in
order to
provide for rapid and thorough cooling of the interior and contents of the
chamber.
During such a cooling phase, gas boiled off the liquid coolant is allowed to
exit the
chamber and thus the chamber remains flooded.
In embodiments of the third and fourth aspects of the present invention,
evacuation of the liquid coolant from the chamber can be initiated by closing
the gas
vent of the chamber. When the gas vent is closed, gas boiled off the liquid
coolant will
accumulate within the chamber, and displace the liquid coolant from the
chamber via
the port. That is, the pressure of the gas within the chamber will equal or
exceed
hydrostatic pressure of the liquid coolant in the chamber and thus displace
the liquid
coolant. Once the gas extends to the port, gas will escape out the under port
at a rate
equal to gas accumulating in the chamber, thus providing a quiescent state in
which
devices within the chamber are provided within a gaseous environment at
substantially
liquid coolant temperatures.
In use, the chamber is preferably positioned within a dewar, and is partially
immersed or more preferably substantially immersed within liquid coolant held
in the
dewar, while maintaining a gaseous environment within the chamber. Immersing
the
chamber within a liquid coolant substantially eliminates transmission of heat
to the
chamber, such that the temperature of the gaseous environment within the
chamber will
remain substantially at the boiling temperature of the liquid coolant used.
Heat may of
course be generated within the chamber by operation of the cryogenic
device(s), and/or
by friction of any moving parts required for moving operation of the cryogenic
device(s). The liquid coolant surrounding the chamber will act as a heat sink
for such
heat, as it will be carried away from the device and/or moving parts via
conduction
and/or convection in the gaseous environment and through the chamber walls
and/or
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port to the liquid coolant. Accordingly the chamber walls are preferably
formed of a
heat conductive material.
The port of the _chamber is, in use, preferably positioned at or proximal to a
lower extremity of the chamber, such that the chamber can be substantially
wholly
evacuated when the gas vent is closed. However positioning of the port away
from a
lower extremity of the chamber, in use, providing for partial evacuation of
the chamber,
may suffice in some embodiments. The presence of liquid coolant in a lower
portion of
the chamber may assist in maintaining suitably low temperatures within the
gaseous
environment in the upper part of the chamber. The port may be a hole through a
wall
of the chamber. The port may comprise a valve to enable selective closing or
sealing of
the port.
In preferred embodiments of the invention, the chamber can be sealed in order
to allow control of pressure within the chamber, for instance by use of a
pressure valve.
Such embodiments are advantageous where a device to be operated within the
chamber
has pressure dependent characteristics. Such embodiments may further comprise
a
standpipe having an inlet within the chamber, and an outlet external to the
chamber and
above an external liquid level, for permitting liquid coolant to flow from the
chamber
when under hydrostatic pressure generated by gas within the chamber. The inlet
of the
standpipe is preferably proximal to a lower extremity of the chamber. In such
embodiments, while the standpipe may allow for pressure equalisation between
the
interior and exterior of the chamber, a dewar containing the external liquid
coolant and
the chamber is preferably sealed to nevertheless provide for pressure control
of the
gaseous enviromnent within the chamber.
Brief Description of the Drawings
Examples of the invention will now be described with reference to the
accompanying drawings in which:
Figures 1A to 1D illustrate a dewar and chamber in accordance with an
embodiment of the present invention;
Figure 2 illustrates a chamber, gas vent and drive shaft in accordance with a
second embodiment of the present invention;
Figure 3 illustrates an apparatus for providing a cryogenic gaseous
environment
in accordance with a third embodiment of the present invention;
Figure 4 illustrates an apparatus for providing a cryogenic gaseous
environment
in accordance with a fourth embodiment of the present invention; and
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Figure 5 is a flowchart illustrating the process of cooling and evacuation of
the
chamber of the apparatus of Figure 4.
Detailed Description of the Preferred Embodiments
5 Figures 1A to 1D illustrate a dewar 100 and chamber 120 in accordance with
an
embodiment of the present invention. Figure 1A illustrates the dewar 100 and
chamber
120 in an unused state, to illustrate the under port 122 and gas vent 124 of
chamber
120.
In accordance with the present embodiment of the invention, a cool-down mode
of operation is shown in Figure 1B. In the cool-down mode, liquid coolant 102
is
introduced to dewar 100, and gas vent 124 serving as a chamber gas port is
held open
by valve 126, thus allowing coolant 102 to enter the chamber 120 through the
under
port 122 so as to flood chamber 120. Introduction of coolant 102 to flood
chamber 120
allows the interior and contents of chamber 120 to be rapidly and thoroughly
cooled.
Boiled coolant from chamber 20 exits as gas through gas vent 124.
Once the interior and contents of chamber 120 are sufficiently cooled, a
chamber evacuation step commences as illustrated in Figure 1C. The temperature
of
chamber 120 may be assessed by monitoring the gas flow through vent 124, and
determining that the interior and contents of chamber 120 are sufficiently
cool once the
gas flow reduces below a threshold rate. To cause evacuation of coolant 102
from
chamber 120, gas vent 124 is closed by use of valve 126. When the gas vent 124
is
closed, gas 104 boiled off coolant 102 accumulates in chamber 120, and
continued
boiling generates sufficient pressure to counteract the hydrostatic pressure
of the
coolant 102 within chamber 120 so as to force coolant 102 out of chamber 120
through
under port 122.
Figure 1D illustrates the quiescent state for operation of one or more
cryogenic
devices within a gaseous environment 104 provided within chamber 120. Valve
126
holds gas vent 124 closed. Liquid coolant 102 is maintained within dewar 100.
Gas
pressure within chamber 120 is equal to the head of liquid outside the chamber
120 and
thus holds liquid coolant out of chamber 120. As chamber 120 is entirely
immersed in
liquid coolant, very little heat is able to enter chamber 120 and thus the
interior and
contents of chamber 120 remain substantially at the boiling temperature of the
liquid
coolant.
It is to be recognised that heat generated within chamber 120 may cause the
temperature within the chamber 120 to rise. Accordingly, it is desirable to
match the
dimensions of chamber 120 closely to the dimensions of a device to be operated
within
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6
chamber 120, such that the conduction of heat from the heat source out of the
chamber
to the heat sink provided by coolant within dewar 100 is made efficient in
order to
maintain suitable cryogenic temperatures within chamber 120. Also for this
reason,
chamber 120 is preferably made of heat conductive material.
Figure 2 illustrates a dewar insert 200 comprising a chamber 220, gas vent 224
serving as a chamber gas port, and drive chain 240, 242 in accordance with a
second
embodiment of the present invention. Such an embodiment provides for operation
of a
moving cryogenic device in a gaseous environment. A superconducting
gradiometer
mounted on a flexible substrate, for example of the type set out in
International Patent
Publication No. WO 2004/015435 or International Patent Publication No. WO
2004/015788 by CSIRO, Tilbrook and Leslie, the content of which is
incorporated
herein by reference, may be mounted on the lower curved portion of rotor
device mount
230, which is driven by lower drive shaft 240. Lower drive shaft 240 is in
turn driven
by upper drive shaft 242. When dewar insert 200 is placed within a dewar
holding
liquid coolant, upper drive shaft 242 and gas vent 224 are immersed in liquid
coolant
and thus conduct little heat to the chamber 220. A stator device mount 232 is
provided
with an under port 222 to enable liquid from a dewar to flood, cool and
evacuate
chamber 220 in the manner described above with reference to Figs 1A to 1D.
As can be seen, a cavity 226 is provided outside under port 222 in order to
create a further gaseous region within cavity 226. Altering the dimensions of
cavity
226 will enable the dewar insert and dewar to be placed on an angle such that
drive
shaft 242 is off-vertical. Such a configuration may be desirable where the
dewar insert
is for use as one of a plurality of rotating gradiorneters having orthogonally
positioned
axes. Such a configuration is set out in Figure 2 of WO 2004/015435, and in
conjunction with which the embodiment of Figure 2 may be applied.
Figure 3 illustrates an apparatus 300 for providing a cryogenic gaseous
environment in accordance with a third embodiment of the present invention.
Apparatus 300 comprises a dewar 302, and a dewar insert 304. Dewar insert 304
comprises a chamber 320, gas vent 324 serving as a chamber gas port and a
drive chain
340, 342. Again, a superconducting gradiometer mounted on a flexible substrate
may
be mounted on the lower curved portion of rotor device mount 330, which is
driven by
lower drive shaft 340. Lower drive shaft 340 is in turn driven by upper drive
shaft 342.
When dewar insert 304 is placed within dewar 302 holding liquid coolant 306,
upper
drive shaft 342 is immersed in liquid coolant and thus conducts little heat to
the
chamber 320. A stator device mount 332 is provided, for example to support a
stationary SQUID to be flux coupled to a rotating gradiometer mounted on rotor
330.
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Chamber 320 further comprises an under port 322 to enable liquid 306 from
dewar 302
to flood, cool and evacuate chamber 320 in the manner described in the
preceding with
reference to Figs 1A to 1D.
Further, a cavity 326 is provided outside under port 322 in order to create a
further gaseous region within cavity 326. Altering the dimensions of cavity
326 will
enable the dewar insert 304 and/or dewar 302 to be placed on an angle such
that drive
shaft 342 is off-vertical. Such a configuration may be desirable where the
dewar insert
304 is for use as one of a plurality of rotating gradiometers having
orthogonally
positioned axes. Such a configuration is set out in Figure 2 of WO
2004/015435, and
in conjunction with which the present embodiment may be applied.
Figure 4 illustrates an apparatus 400 for providing a cryogenic gaseous
environment in accordance with a fourth embodiment of the present invention.
Apparatus 400 comprises a dewar 402 being a glass vacuum flask refill, a
chamber 420,
valve 424 and a drive shaft 440. Apparatus 400 may be housed in a PVC tube
(not
shown), which may be coated on both inside and outside surfaces with silver
paint in
order to effect RF interference shielding, for example where a magnetic field
detection
device is to be operated within chamber 420. A superconducting device may be
mounted on rotor device mount 430, which is driven by drive shaft 440. Drive
shaft
440 may for example be driven by hand or by motor. Dewar 402 holds liquid
coolant
406 immersing chamber 420. Apparatus 400 further comprises a standpipe 428
having
an inlet within chamber 420 and proximal to a lower extremity of chamber 420
allowing liquid coolant within chamber 420 to be drawn down to level 452. The
outlet
of standpipe 428 is external to chamber 420 and above a level 450 to which
liquid 406
initially fills dewar 402.
A valve 462 can be opened and closed, to selectively allow liquid flow into or
out of chamber 420. Valve 464 can be opened to allow gas or liquid to be bled
out of
dewar 402. Valve 466 and pressure regulator 468 allow gas pressure within
chamber
420 to be held at or below a level defined by pressure regulator 468. Burst
disc 470
provides a failure mechanism should pressure within dewar 402 exceed the
bursting
pressure of the burst disc 470.
Stator device mount 432 is provided, for example to support a stationary SQUID
to be flux coupled to a rotating gradiometer mounted on rotor 430. To maximise
flux
coupling, it may be desirable to minimise a gap between the rotor 430 and
stator 432.
In this event, rotor 430 and stator 432 are preferably constructed of
material(s) having
low thermal expansion coefficient(s), such that temperature variations do not
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undesirably affect the physical gap between the rotor 430 and stator 432, for
example
by avoiding contact between rotor 430 and stator 432.
Figure 5 is a flowchart illustrating the process 500 of cooling and evacuation
of
the chamber 420 of the apparatus 400 of Figure 4. At step 502, the process
begins. At
step 504 valves 464, 462 and 424 are opened, and valve 466 is closed. At step
506
liquid coolant, in this instance liquid nitrogen, is injected through valve
424. During
this step, the liquid coolant freely travels between chamber 420 and dewar
402, due to
valve 462 being open. Entry of the liquid nitrogen through valve 424 displaces
the
atmosphere within the chamber 420 and dewar 402, which is allowed to exit
through
valve 464. Liquid nitrogen injection continues until the liquid level is
substantially at
level 450. A sensor (not shown) may be provided within dewar 402 to determine
the
liquid level.
Such flooding of both the chamber 420 and dewar 402 with liquid nitrogen
provides for thorough and effective cooling of all components within the dewar
402
and chamber 420. As temperatures within the dewar 402 and chamber 420 approach
that of the liquid nitrogen, the liquid nitrogen will boil and produce
nitrogen gas, which
is also allowed to exit through valve 464. Liquid nitrogen is preferably
introduced
throughout this stage to maintain the liquid level substantially at level 450.
The flow
rate of gas out of valve 464 during this stage substantially corresponds to a
boiling rate
of liquid nitrogen within the chamber, which in turn is indicative of the
temperature of
the contents of the chamber. Thus monitoring the gas flow rate out of valve
464 can
give an indication of the temperatures of the components within the chamber
420 and
dewar 402.
Once it is considered that temperatures within the chamber 420 are at an
appropriate level, valve 462 may be closed, at step 508. At step 510, nitrogen
gas is
then pumped into chamber 420 through valve 424. The nitrogen gas is preferably
at a
temperature close to the boiling temperature of nitrogen to avoid the
introduction of
excessive heat into chamber 420. Due to the gas entering through valve 424,
and the
likely production of nitrogen gas from the boiling of liquid nitrogen within
the chamber
420, and due to valve 462 being closed, liquid nitrogen within chamber 420 is
forced
out of chamber 420 through standpipe 428 under hydrostatic pressure, such that
a liquid
level in dewar 402 may rise above level 450, for example to the level shown in
Figure
4. Gas is injected into and accumulated within chamber 420 until a liquid
level in
chamber 420 falls to substantially level 452. Level 452 may be monitored by
positioning a liquid level sensor within chamber 420. Alternatively level 452
may be
configured to be level with a lower extremity of standpipe 428, such that
continued
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9
accumulation of gas within chamber 420 would cause gas to pass up standpipe
428
rather than liquid.
Once the liquid within chamber 420 has fallen substantially to. level 452,
valves
464 and 424 are closed at step 512 to provide a pressure seal of dewar 402 and
chamber
420. Valve 466 is opened, such that a gas pressure within chamber 420 is
regulated by
pressure regulator 468. Maintaining constant gas pressure will improve the
sensitivity
of devices with pressure dependent characteristics which may be operated
within the
gaseous environment of chamber 420. Having achieved the desired cryogenic
gaseous
operating environment within chamber 420, the process ends at step 514. It has
proven
possible to maintain suitable cryogenic conditions within such a gaseous
environment
for around 3 hours.
The device to be operated within the gaseous environment of any one of
chambers 120, 220, 320 or 420 may be a magnetic sensor. In such embodiments,
all
materials of the apparatus 100, 200, 300, 400 are preferably non-magnetic.
Further,
moving parts of the embodiments of Figures 1 to 4 should be self-lubricating
at
cryogenic temperatures, and should generally have matching and/or low
coefficients of
thermal expansion. For example, the dewar insert 200 may comprise a number of
sections each formed from epoxy impregnated woven fibreglass, each section
having
lapped faces to mate with the adjacent section. Such a modular construction is
advantageous in permitting interchanging of sections, for example
interchanging of
chamber section 220 should a different device be used. Nylon screws hold the
sections
together and application of a small amount of silicone grease on the faces
effectively
seals the sections together for the purpose of gas containment.
Each rotor 230, 330, 430 may be formed of machinable ceramic, while the drive
shaft 240, 340, 440 may be a ground Pyrex glass spindle. Referring to Figure
3, the
Pyrex spindle drive shaft 340 runs in a graphite bearing 344 pressed into the
housing of
chamber 320, with a fibreglass driving dog 346 pressed onto the spindle 340 on
the
outer side of the bearing 344. The running faces between the dog 346 and the
bearing
344 govern the vertical clearance of the rotor 330 from the stator 332 and pre-
load can
be applied by a plastic spring between the rotor 330 and the bearing 344. A
thin-walled
cupro-nickel tube 304, carrying a graphite bearing 348 at its upper end and
pressed into
the upper portion of chamber 320 at its lower end, transmits rotation via a
long thin
ground Pyrex glass rod 342 to a sliding coupling 350 which engages the driving
dog
346. In this way, variations in the length of the drive spindle 342 due to
thermal
effects, do not affect the separation of the rotor 330 and stator 332, and
thus do not alter
the tape-to-SQUID separation where such devices are mounted upon the rotor 330
and
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stator 332. In the room-temperature environment at the upper end of spindle
342, a
paddle-wheel type air motor is used to drive the spindle 342 via a single-
stage epicyclic
plastic gearbox. Rotation angle is monitored by an optical shaft encoder
mounted on
the spindle.
5 A patterned superconducting thin-film magnetic shield may be .moun.ted on
the
module immediately below the stator device, for example a SQUID, to attenuate
the
vertical field component seen by the SQUID . The modular mounting allows fine
tilt
and positioning of the shield by means of three differential screws,
adjustable by thin
rods taken out to the room-temperature environment.
10 It will be appreciated by persons skilled in the art that numerous
variations
and/or modifications may be made to the invention as shown in the specific
embodiments without departing from the spirit or scope of the invention as
broadly
described. The present embodiments are, therefore, to be considered in all
respects as
illustrative and not restrictive.
