U.S. patent application number 11/631596 was filed with the patent office on 2007-10-25 for method and apparatus for operation of a cryogenic device in a gaseous environment.
Invention is credited to Rex Anthony Binks.
Application Number | 20070245748 11/631596 |
Document ID | / |
Family ID | 35782402 |
Filed Date | 2007-10-25 |
United States Patent
Application |
20070245748 |
Kind Code |
A1 |
Binks; Rex Anthony |
October 25, 2007 |
Method and Apparatus for Operation of a Cryogenic Device in a
Gaseous Environment
Abstract
Apparatus for providing a cryogenic gaseous environment (300). A
chamber (320) for containing the cryogenic gaseous environment is
immersed in liquid coolant (306) to effectively cool the interior
chamber, during which time gas boiled off the coolant is allowed to
escape. Gas is then either injected into or allowed to accumulate
in the chamber, such that liquid coolant is forced out of the
chamber under hydrostatic pressure, whether through an open under
port (322) of the chamber of through a standpipe (324). The
interior of the chamber then provides a gaseous environment at
cryogenic temperatures.
Inventors: |
Binks; Rex Anthony; (West
Pymble, GB) |
Correspondence
Address: |
KENYON & KENYON LLP
1500 K STREET N.W.
SUITE 700
WASHINGTON
DC
20005
US
|
Family ID: |
35782402 |
Appl. No.: |
11/631596 |
Filed: |
June 28, 2005 |
PCT Filed: |
June 28, 2005 |
PCT NO: |
PCT/AU05/00945 |
371 Date: |
May 8, 2007 |
Current U.S.
Class: |
62/45.1 |
Current CPC
Class: |
F25D 29/001 20130101;
F25D 3/10 20130101 |
Class at
Publication: |
062/045.1 |
International
Class: |
F25D 17/08 20060101
F25D017/08 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 5, 2004 |
AU |
2004903688 |
Claims
1. 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.
2. The apparatus of claim 1 wherein the chamber gas port comprises
a gas injection port for purging the chamber with gas to evacuate
liquid from the chamber.
3. The apparatus of claim 2 wherein the gas injection port permits
egress of gas during liquid flooding of the chamber.
4. The apparatus of claim 1 further comprising a gas outflow port
for permitting egress of gas from the chamber during liquid
flooding of the chamber.
5. The apparatus of claim 1 wherein the chamber gas port comprises
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.
6. (canceled)
7. The apparatus of claim 1, wherein the chamber can be pressure
sealed.
8. The apparatus of claim 7 further comprising a pressure regulator
to regulate pressure within the chamber.
9. The apparatus of claim 1, further comprising a dewar containing
the chamber, the dewar for containing liquid coolant to immerse the
chamber.
10. The apparatus of claim 9, wherein the chamber comprises a
second port allowing liquid exchange between the dewar and the
chamber.
11. The apparatus of claim 10, wherein in use the second port is
positioned proximal to a lower extremity of the chamber.
12. The apparatus of claim 9 wherein the second port can be
selectively sealed.
13. The apparatus of claim 1, further comprising a standpipe having
an inlet within the chamber, and having an outlet external to the
chamber and in use above an external liquid level, for permitting
liquid coolant to flow from the chamber when under hydrostatic
pressure generated by gas within the chamber.
14. The apparatus of claim 13 wherein in use the inlet of the
standpipe is proximal to a lower extremity of the chamber.
15. 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.
16. The method of claim 15 wherein causing cryogenic gas to occupy
the chamber comprises injecting gas into the chamber to evacuate
liquid from the chamber.
17. The method of claim 15 wherein causing cryogenic gas to occupy
the chamber comprises containing within the chamber gas boiled off
the liquid coolant.
18. The method of claim 15 further comprising permitting egress of
gas during the flooding of the chamber.
19. The method of claim 15 further comprising, after causing
cryogenic gas to occupy the chamber, pressure sealing the
chamber.
20. The method of claim 19 further comprising regulating pressure
within the chamber.
21. The method of claim 15, further comprising immersing the
chamber in liquid coolant.
22. The method of claim 15, further comprising allowing liquid
exchange between the interior and exterior of the chamber during
flooding.
23. The method of claim 15 further comprising preventing liquid
from entering the chamber after flooding.
24. The method of claim 15, further comprising permitting liquid to
exit the chamber under hydrostatic pressure after flooding.
25. 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; 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.
26. (canceled)
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application claims priority from Provisional
Patent Application No 2004903688 filed on 5 Jul. 2004, the content
of which is incorporated herein by reference.
FIELD OF THE INVENTION
[0002] 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
[0003] 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 77.3 K, while the use
of liquid helium provides for cryogenic operation at or below 4.2
K.
[0004] 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 T.sub.c 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.
[0005] 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.
[0006] 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
[0007] According to a first aspect the present invention is an
apparatus for providing a cryogenic gaseous environment, the
apparatus comprising:
[0008] a chamber for containing the cryogenic gaseous environment
and for excluding external liquid coolant,
[0009] a liquid inlet for selectively flooding the chamber with
liquid coolant; and
[0010] 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.
[0011] According to a second aspect, the present invention is a
method of providing a cryogenic gaseous environment, the method
comprising:
[0012] flooding a chamber with liquid coolant; and
[0013] causing cryogenic gas to occupy the chamber and displace
liquid coolant from the chamber.
[0014] 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.
[0015] 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.
[0016] 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:
[0017] a chamber for housing the cryogenic device;
[0018] a port in the chamber allowing the chamber to be flooded by
liquid coolant; and
[0019] a gas vent for allowing escape of gas from the chamber;
[0020] 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.
[0021] According to a fourth aspect the present invention provides
a method for providing a gaseous environment for operation of a
cryogenic device; comprising:
[0022] flooding a chamber with liquid coolant; and
[0023] causing gas boiled off the liquid coolant to accumulate in
the chamber, such that liquid coolant is forced out of the
chamber.
[0024] 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.
[0025] During such a cooling phase, gas boiled off the liquid
coolant is allowed to exit the chamber and thus the chamber remains
flooded.
[0026] 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.
[0027] 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 port to the liquid coolant. Accordingly
the chamber walls are preferably formed of a heat conductive
material.
[0028] 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.
[0029] 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.
[0030] 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
environment within the chamber.
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] Examples of the invention will now be described with
reference to the accompanying drawings in which:
[0032] FIGS. 1A to 1D illustrate a dewar and chamber in accordance
with an embodiment of the present invention;
[0033] FIG. 2 illustrates a chamber, gas vent and drive shaft in
accordance with a second embodiment of the present invention;
[0034] FIG. 3 illustrates an apparatus for providing a cryogenic
gaseous environment in accordance with a third embodiment of the
present invention;
[0035] FIG. 4 illustrates an apparatus for providing a cryogenic
gaseous environment in accordance with a fourth embodiment of the
present invention; and
[0036] FIG. 5 is a flowchart illustrating the process of cooling
and evacuation of the chamber of the apparatus of FIG. 4.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0037] FIGS. 1A to 1D illustrate a dewar 100 and chamber 120 in
accordance with an embodiment of the present invention. FIG. 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.
[0038] In accordance with the present embodiment of the invention,
a cool-down mode of operation is shown in FIG. 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.
[0039] Once the interior and contents of chamber 120 are
sufficiently cooled, a chamber evacuation step commences as
illustrated in FIG. 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.
[0040] FIG. 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.
[0041] 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 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.
[0042] FIG. 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.
[0043] 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 gradiometers
having orthogonally positioned axes. Such a configuration is set
out in FIG. 2 of WO 2004/015435, and in conjunction with which the
embodiment of FIG. 2 may be applied.
[0044] FIG. 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 SQUIID to be flux
coupled to a rotating gradiometer mounted on rotor 330. 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.
[0045] 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 FIG. 2 of WO 2004/015435, and in
conjunction with which the present embodiment may be applied.
[0046] FIG. 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.
[0047] 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.
[0048] 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 undesirably affect the physical
gap between the rotor 430 and stator 432, for example by avoiding
contact between rotor 430 and stator 432.
[0049] FIG. 5 is a flowchart illustrating the process 500 of
cooling and evacuation of the chamber 420 of the apparatus 400 of
FIG. 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.
[0050] 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.
[0051] 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 FIG. 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
accumulation of gas within chamber 420 would cause gas to pass up
standpipe 428 rather than liquid.
[0052] Once the liquid within chamber 42Q has fallen substantially
to level 452, valves 464 and 424 are closed at step 512 to provide
a pressure seal of dewar402 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.
[0053] 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 FIGS. 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
[0054] 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 FIG. 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 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.
[0055] A patterned superconducting thin-film magnetic shield may be
mounted 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.
[0056] 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.
* * * * *