Note: Descriptions are shown in the official language in which they were submitted.
Back~round of the Invention
This invention relates to cryostats for the contain-
ment of very low temperature liquids such as liquid helium
and in particular to cryostats housing superconducting
apparatus such as NMR spectrometer magnets.
Prior art cryostats for containment of superconduct-
ing apparatus, for example, superconducting magnets, have
employPd a helium vessel shaped to exhibit a relatively
small cylindrical volume surrounding the superconducting
magnet, in open communication with a larger volume dispos-
ed immediately above the solanoid. In this geometry the
solenoid is maintained completely submerged in the liquid
helium bath. A sufficient hold time for the liquid heli-
um is provided by the head of liquid helium in the large
volume. This form of helium reservoir exhibits a surface
area to volume ratio substantially higher than the mini-
mum achievable; consequently additional radiation losses
' are introduced which contribute to a higher rate of
I helium boil-off.
Prior axt cryostats have taken the form of nested
¦ chambers which have been internally braced, as for ex-
ample with stainless steel spokes, to withstand mechanical
shock and to maintain minimum clearances between adjacent
nested walls. Stainless steel has been a popular mater-
ial of choice because of its relatively low thermal
, conductivity and its high strength. However, the thermal
i conductivity of such bracing places a limit upon the
thermal isolation which can be achieved between adjacent
surfaces of nested structures.
' 30 Cryostats of the prior art have employed a second-
ary temperature bath to shield the lowest temperature
coolant from ambient temperature. Ordinarily, the
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li~3143
secondary coolant reservoir is itself insulated from
an~ient temperature~ as for example with layers of an
insulating material. In a superconducting magnet with
room temperature access, a relatively large magnet bore
is required by prior art cryostat structures to provide
sufficient space for this insulation. As a result, the
inside diameter of such prior art solenoid is constrained
to span a proportionately larger diameter to accommodate
the additional insulation, whereby a much greater length
of superconducting wire is required for fabrication of the
solenoid.
It is an object of the present invention to achieve
an improved cryostat for the containment of a liquified
gas whereby the loss of such liquified gas from a cryostat
due to the boiling of such liquified gas is minimized.
Accordingly, the present invention provides-in a
cryostat wherein a first liquified gas is contained in a
central vessel and said central vessel is surrounded by
shielding means maintained at the temperature of another
liquified gas of higher boiling point, said cryostat
further comprising a containment vessel at ambient tempera-
ture, said containment vessel comprising a wall portion
having an interior surface spaced from the outer surface
of said shielding means, the improvement comprising: a
radiation shield disposed between the interior surface of
said containment vessel and the outer surface of said
shielding means; mechanical refrigerating means for main-
taining said radiation shield at a temperature substantially
¦ intermediate said shielding means and am~ient temperature
¦ 30 during operation.
In a described embodiment the amount of liquid
helium required for operation of~ a superconducting magnet is
L43
minimized and the interval between replenishments of the
liquid helium is maximized.
In one embodiment of this invention a substantially
spherical central vessel is provided to contain the liquified
gas forming a primary coolant, whereby the ratio of surface
area to volume of such vessel is minimized. The central
vessel may be constructed of aluminum of such thickness
that the thermal gradient introduced by the finite thermal
conductivity of the necessary fill and vent tubes leading
from the central vessel, and still be small enough to permit
operation of superconducting apparatus only partly submerged
in the liquified gas.
The central coolant reservoir may be surrounded
by a radiation shield spaced from the central reservoir and
maintained at a first intermediate temperature through vapor
cooling provided by the boil-off from the liquified gas
contained in the central reservoir.
In a described embodiment, the central'reservoir
and its ~urround'ing radiation shield are enclosed by an
isothermal surface defined by a second surrounding chamber
j and a second reservoir is provided externally above the
second surrounding chamber, and in thermal contact with the
outer surface of the second chamber, whereby the second
chamber and reservoir form an isothermal body maintained at
i the temperature of a second liquified gas comprising the
secondary coolant filling the second reservoir. An outer
radiation shield is provided, enclosing the isothermal
i body comprising the second reservoir and the second chamber
I in contact therewith, such outer radiation shield maintained
¦ 30 at a temperature intermediate the second coolant and ambient
temperature. The outer radiation shield is maintained at
the desired temperature by an auxiliary refrigeration
~ 3~43
facility.
In a described embodiment the primary coolant
(hereafter, liquid helium) is contained in a central
reservoir having a substantially spherical shape. The
central reservoir is formed of aluminum and has a kore
through its centre defined by an aluminum cylindrical wall
welded to the quasi-spherical reservoir. A superconducting
solenoid is disposed within the central reservoir coaxial
with the bore. In the embodiment the surface to volume
ratio is minimized, reducing the area of the central
reservoir for absorption of heat by radiation and permitting
operation of the solenoid to continue when the level of
liquid helium falls substantially below the top of the
solenoid.
A radiation shield surrounding the helium reservoir
is provided to establish a first isothermal surface inter-
mediate the central reservoir and a surrounding second iso-
thermal surface, the latter maintained at the temperature
of a secondary coolant (hereinafter, liquid nitrogen). me
radiation shield is maintained at about 50K through vapor
cooling affected by the helium boil-off escaping up the
fill and vent tubes with which the radiation shield is in
therMal contact.
Surrounding the radiation shield and partially
surrounding the fill and vent tubes, there is provided a
secondary isothermal shell cooled by thermal contact with a
liquid nitrogen reservoir disposed external the shell, above
the region of the central reservoir. In this geometry the
vent and fill tube from the central reservoir (partially
surrounded by a cylindrical portion of the secondary iso-
thermal shell) passes through a greater length of the
secondary coolant reservoir as compared to prior art
~1~ 31~3
cryostats with conse~uently improved thermal isolation
for the liquid helium reservoir.
The thermal isolation from ambient temperature of the
secondary coolant reservoir and associated secondary iso-
thermal shell is further improved in the embodiment by
provision of an outer radiation shield surrounding the
secondary coolant reservoir and associated isothermal
chamber. This outer radiation shield is maintained at a
temperature intermediate the secondary coolant and ambient
temperature by means of an auxiliary refrigeration facility.
An outer hermetically sealed vessel encloses the
outer radiation shield and the interior of the cryostat
permitting the spaces between adjacent nested surfaces to
be evacuated to a pressure of the order of 10 6 torr. In
this way gas conduction and convection mechanisms are
minimized for heat transport to the central reservoir.
The described embodiments virtually eliminate direct
conduction losses between nested structures due to the
mechanical support and internal bracing by replacing the
bracing spokes of the prior art with bracing formed of
polyester cord.
All of the walls forming the nested structure of the
cryostat are constructed of aluminum, save for the fill and
ven-t tubes of both the liquid nitrogen and helium reservoirs.
The aluminum surfaces are subject to a treatment which
reduces radiant emissivity. Consequently, heat transport
by radiation is further reduced between adjacent surfaces.
Embodiments of the invention will now be described,
by way of example, with reference to the accompanying
drawings in which:-
Figure 1 is a schematic of an NMR spectrometer system.
Figure 2 is a top view of the cryostat of the
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3143
preferred embodiment.
Figure 3 i5 a section through the cryostat of
Figure 2.
Figure 4 is a detail of the section of Figure 2.
Figure 5 is a detail in section of the cryostat of
Figure 2.
Figure 6 shows the cold head for thermal linkage to
the refrigerator.
A superconducting NMR spectrometer system employs a
cryostat l having room temperature access to the magnetic
field created within the cryostat 1 in a manner more
explicitly depicted belo~. A probe 5 containing a sample 7
is introduced through bore 3 for analysis. A transmitter 9,
receiver 11 and control unit 13, data processing unit 15
and display means 17 form the complete spectrometer
(exclusi~e of power supplying systems for initiating
persistent currents for the magnet).
¦ Figure 2 is a top view of the preferred embodiment of
the cryostat 1 of this invention. A bore 3 provides room
temperature access to the magnetic field created by
¦ apparatus within cryostat l as described below.
Turning now to Figure 3, the cryostat 1 contains a
superconducting solenoid assembly 50 within a central
reservoir llO. Reservoir llO contains a primary coolant,
preferably liquid helium, to maintain the superconducting
state of the windings comprising solenoid assembly 50.
Leads from the solenoid windings, collectively denoted 52
terminate in a connector 54 for access to external current
sources introduced in a manner to be described. Additional
circuits comprising persistence switches for controlling
transitions between the normal and superconducting state
for selected windings are not shown. These circuits and
_ 7 _
` 3~
preferred persistence switche~ are further described in,
and form the subject matter of respective United States
Patents Nos. 4,164,777, and 4,173,775 assigned to the
assignees of the present invention. The solenoid
assembly 50 is not within the scope of the present
invention and is ~urther described in United States Patent
No. 4,180,779 assigned to the assignee of the present
invention.
Central coolant reservoir 110 is formed from 0.125"
10 - aluminum to a substantially spherical shape as shown, by
spinning techniques well known in the art. In the
preferred embodiment, reservoir 110 has a coolant capacity
of about 25 litres. Reservoir 110 is further characterized
by a bore formed by cylindrical wall 111, welded-to reservoir
110. Room temperature access is thereby afforded to the
magnetic field of solenoid assembly 50. Reservoir 110 is
isolated from ambient temperature by means of a plurality
of consecutively nested surrounding chambers 112, 114, 116,
and 118 having coaxial bores defined by cylindrical tubes
113, 115, 117, and 119, respectively. The wall thickness
of each of the respective cylindrical coaxial tubes is
determined by the heat load on each and varies from 0.02"
to 0.049". The spaces between chambers 112, 114, 116, and
118 are mutually communicating in a manner described below
and evacuated through pump-out port 120 in exterior chamber
118 to achieve a very low pressure, as for example 10 6 torr
to minimize thermal conduction between adjacent nested
surfaces through gas conductivity and convection.
A secondary coolant reservoir 114', is disposed above
central reservoir 110 and in thermal contact with chamber
114 whereby chamber 114, preferably formed of nominal
0.190" aluminum, comprises a shell at the temperature of
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. . :) -
~1~ 3:1~`3
the secondary coolant, preferably liquid nitrogen.
Returning to Figure 2, two vent and fill tubes 130 and
130' are required for access to the central reservoir.
These are constructed of 5/8" I.D. stainless steel having
a wall thickness of 0.005". Two such vent and fill tubes
130 and 130' appear in Figure 1 and one such structure is
disclosed in greater detail in Figure 3. These vent and
fill structures differ only in that electrical connector 54
is required only for tube 130. Tube 130 (and 130') is
preferably of stainless steel in order to minimize thermal
conductivity from the liquid helium reservoir to the
exterior of the cryostat. Tube 130 is shielded by coaxial
tubes 132, 134, 136, and 138, each of which form part of
the respective nested chambers 112, 114, 116, and 118.
Thermal transfer collar 133 ~and 133', not shown), preferably
of aluminum, serves to transfer heat to the boil-off
helium vapor ~assing through tube 130 (and 130'), thereby
to maintain isothermal shell 112 at a fixed temperature.
Radiation shield 112 is preferably constructed of
aluminum by conventional spinning techniques and defines
an isothermal shell of temperature intermediate the
secondary coolant (liquid nitrogen at 77.4K) and the
primary coolant (liquid helium at 4.2K). For liquid
nitrogen-liquid helium combinations the temperature of the
I radiation shield 112 is optimized at about 50K. Heat is
transferred to the radiation shield principally by
radiation (and by conduction through mechanical bracing
means described below) from the interior of surrounding
shell 114 and heat is transferred from the radiation shield
112 to the helium vapor in the fill and vent tubes 130 and
130'-through aluminum contact collars 133 and 133'
respectively, welded to fill and vent tubes 130 and 130'
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., . ,. .. ,.. ., . - , i
13i43
and to radiation shield 112. Thermal contact between tubes
~ 130 and 1~0' and their respective collars 133 and 133'
occurs at a point where approximately 10 mw. of thermal
power is supplied to the escaping helium vapor from
radiation shield 112.
Radiation shield 112 is nested within surrounding
isothermal shell 114 which is maintained at liquid nitrogen
temperature by welded contact with liquid nitrogen reservoir
114'. The outer surface of isothermal body 114-114' is
itself shielded by outer radiation shield 116 which is
maintained at a temperature intermediate that of liquid
nitrogen and room temperature in a manner described more
fully below.
Hermetically sealed external vessel 118 encloses the
cryostat structure and provides external mechanical and
vacuum integrity.
Baffled apertures 135 and 137 are provided in radiation
shields 112 and 116 as shown. A similar baffled aperture
in shell 114, not visible in the section of Figure 2,
provides communication between all interior spaces of the
nested structure whereby these interior spaces are main-
tained at a common pressure by evacuation through port 120.
The liquid nitrogen reservoir 114' and associated
shell 114 are effectively insulated by cooling outer
radiation shield 116 to a temperature intermediate that of
liquid nitrogen and ambient temperature. Maintaining
radiation shield 116 at preferably 173-183K is accomplished
by providing a heat exchanger discussed below in tube 145
to afect thermal exchange between outer radiation shield
¦ 30 116 and an auxiliary refrigeration facility 140. An
¦ external mechanical refrigerator~ such as the Neslab
¦ Instruments, Inc., Crvocool CC-100, has been found
1~33 43
convenient for this purpose although similar equipment
will undoubtedly serve ~s well. Turning now to Figure 5
the means by which the external refrigerati~s facility is
coupled to the cryostat appears in greater detail.
An access port 142 (Fig. 2) in the top of containment
vessel 118 is hermetically sealed by tube 143 which carries
a well assembly for thermal access to outer radiation
shield 116. An outer wall 144 and an inner conducting
tube 144' for the well is formed of a tubing of low thermal
conductivity, as for example stainless steel, and an
inner contact tube 145 is constrùcted of thermally conductive
tubing which is joined to outer radiation shield 116 in a
one turn coil welded along the periphery of shield 116.
Inner and outer walls 144' and 144 are hermetically sealed
to end plug 144'. Thermal linkage of the refrigerator
i 140 to radiation shield 116 is accomplished by insertioninto cooling tube 145 of cold head 147 shown in greater
detail in ~igure 6.
Cold head 147 comprises a single ended flexible
bellows 148 of nickel plated brass or copper tubing having
j a mounting flange 149 and a coaxial capillary tube 150.
Cold refrigerant from the refrigerator 140 flows through
capillary 150 and returns along-the outside of capillary 150
through aperture in flange 149. The bellows serrations provide
a large cooling area and the void between the bellows serrations
and the inner wall of cooling tube 146 is filled with a 90%
menthanol,lO% water mixture to facilitate heat transfer between
cooling tube 146 and cold head 147.
I Figure 4 is a section through the liquid nitrogen vent
¦ 30 and fill tube assembly 152. A thermally non-conductive
¦ central fill tube 153, preferably of stainless steel tubing,
0.005 inches in wall thickness, supports a thermal gradient
'
3~'~3
between the 77K temperature of liquid nitrogen reservoir
114' and ambient temperature over a distance of about
4 1/4". This tube is shielded by concentric tubes 154
and 155, respectively the nitrogen fill tube shield
portions of outer radiation shield 116 and containment
vessel 118. Aluminum end contact tubes 156, brazed to
central fill tube 153 provide strength and a surface for
welding ~urther to reservoir 114' and outside shield tube
155. A thermally conductive collar 157 contacts the
central nitrogen fill tube 153 at a point along the thermal
gradient where the heat transfer from outer radiation
shield 116 to liquid nitrogen escaping up the central fill
tube 153 is sufficient to maintain the outer radiation
shield 116 at a desired temperature intermediate the
temperature of liquid nitrogen and ambient temperature.
In similar fashion, helium fill and vent tube 130 (see
. Figure 3) is thermally joined to liquid nitrogen reservoir
114' through heat transfer collar 158 and at a point
along the thermal gradient of tube 130 another thermal
- 20 collar 159 provides a heat transfer path from outer
radiation shield 116 to the vapor escaping up tube 130.
The temperature of the thermal contact point of collar 159
.is selected to be substantially equal to that of collar 157
on the nitrogen fill and vent tube 153. A second helium
fill and vent tube 130' not shown provides another thermal
contact point, the details of which do not differ from
that shown and described above. Thus, in addition to the
cooling provided by refrigerator 140, outer radiation
shield 116 is vapor cooled in exact analogy to the cooling
1 30 of radiation shield 112 as described previously.
¦ The central reservoir 110, radiation shield 112, liquid
nitrogcn reservoir 114 ' and shel l 114, oute r radiation
-- 12 --
- , . .. .. ...
shield 116 and containment vessel 118 are fabricated from
an aluminum alloy, preferably alloy 1100-0. This alloy
is well-~nown and commercially available from several
manufacturers. After the above-listed bodies have been
formed by spinning, the interior adjacent facing surfaces
of the respective bodies are polished and subject to a
surface treatment technique which lowers the emissivity
of these surfaces by 35%. In this manner, heat transport
by radiation to the liquid helium central reservoir is
drastically reduced.
The nested structure of a cryostat such as exhibited
by the present invention requires an internal mechanical
support to maintain the centering of the various shells,
and the coaxial alignments and close tolerances therebetween.
It is important that the coaxial tubes 111, 113, 115, 117
and 119 forming the bore for room temperature access be
; precisely located. It is equally important to constrain
the nested structure during shipment of the apparatus
because the thermal-mechanical specifications of certain
components result in a measure of mechanical fragility. It
is clear that any mechanical constraint linking adjacent
I structures must per~orce result in a conductive path for
¦ heat transport; consequently, a very low thermal conductivity
is essential. Moreover, high strength is essential to provide
i the required mechanical constraints. Braided polyester cord
has been found to be an ideal material for this purpose, not-
withstanding the precision required for alignment of the
~ components of the cryostat.
¦ Returning to Figure 3, it will be perceived that
adjacent members of the nested structures 110, 112, 114 and
114', 116 and 118 are subject to constraints through
polyester cord centering spokes. In the interest of clarity
- 13 -
11~3~4~
only a representative spoke 160 is described in detail.
- The spoke itself is formed of polyester cord, preferably
of braided Dacron. The strength and thermal conductivity
parameter of this material are known and exhibit the
highest known ratio of strength to thermal conductivity.
The polyester material which has been employed in the
preferred embodiment is supplied as #2 Corsair DB by
Rocky Mount Cord Company of Rocky Mount, No. Carolina.
A loop is formed at each end secured to the running length
of the cord by aluminum sleeves 162. One of the loops
formed thereby is affixed to an eye-bolt 164 secured to
one of the adjacent pairs of shells and the other loop
engages a snubbing post 1~6 welded to the other ad~acent
shell. These polyester spokes are disposed at regular
intervals for example 120 about the axis of bore 3.
The representative spacing between adjacent coaxial
bore tubes 111-113, 113-115, 115-117, and 117-119 range
from 0.178" to 0.16" for the widest and most narrowly-
spaced Of the aforementioned bore tube pairs; it is
20 - desired to maintain these bore tubes mounted coaxial with
one another and with solenoid assembly 50 to a precision
substantially better than 0.03". This has been accomplished
with the aforementioned polyester spokes with resulting
additional improvement in the shipping properties of the;
apparatus at room temperature. Stainless steel spokes
properly dimensioned for operating conditions in the liquid
helium-liquid nitrogen temperature range are under
substantial tensile stresses at room temperature. Such
rigid spokes which would exhibit thermal conductance
comparable to the spokes of the present invention are known
to be highly susceptible to failure due to shock and
vibration. In contrast, the polyester tensile loaded
- 14 -
11~314~
spokes of the present invention exhibit a degree of
stretch at room temperature during shipment. The bore
tubes are thereby permitted to touch when subject to
lateral shock and vibration. For shipment purposes, a
mandrel slip fit to the central bore, prevents permanent
deformation of the several coaxial bore tubes in collision.
Precise location of the components is facilitated
by the behaviour of the coefficient of expansion of the
spoke material of the present invention in the temperature
range from liquid helium to am~ient. As a result of the
present invention, it has been found that the coefficient
of expansion of the subject material which is normal in
behaviour to about -25C anomalously changes sign and the
material expands as the temperature is further reduced.
A very low net thermal expansion is thereby obtained for
this material.
The cryostat of the preferred embodiment achieves
very substantial improvement over the prior art in
consumption of the coolants. For example, the liquid
helium boil-off rate measured for one prior art cryostat
amounts to 30 cc/hr whereas the preferred embodiment of
the instant invention exhibits a measured boil-off rate
of about 6 cc/hr. m e low boil-off rate conjoined with
; the geometry of the central reservoir 110 yields an
extended mean time between replenishment of liquid helium
of about 120 days, wherein about 20.5 liters of liquid
helium are consumed. A superconducting NMR spectrometer
having a magnet of comparable characteristics requires
~ liquid helium replenishment at intervals of 8 days and
i 30 consumes about 86.4 liters of liquid helium in the same
120 day period.
The extended mean time between filling of the central
~
'~ - 15 -
~1~3~3
reservoir 110 is achieved in part because the central
reservoir 110 has a substantially spherical shape. In
the present invention the central reservoir 110 is
fabricated of aluminum of sufficiently heavy gauge that
the thermal gradient from top to bottom of the central
reservoir ~due to heat conducted down fill and vent tubes
130 and 130' and radiation from shield 112) is so xeduced
that the reservoir 110 is isothermal independent of the
level of liquid helium contained therein. It has been
found that in this reservoir the liquid helium level can
be allowed to drop well below the top of the superconducting
solenoid without adverse effect upon the operation of the
solenoid. The solenoid assembly 50, having a length of
about 10 inches, has been operated satisfactorily with
liquid helium level reduced to about 3 inches in the
reservoir 110 exposing about 7 inches of the solenoid
assembly 50.
For the liquid nitrogen coolant the rate of
- consumption is also reduced and the mean interval between
replenishment extended. The liquid nitrogen boil-off rate
is measured at about 20 cc/hour with the outer radiation
shield cooled to 173-183K. With the liquid nitrogen
reservoir insulated from ambient temperature without
benefit of cooling of the radiation shield, the liquid
nitrogen boil-off rate increases to 80 cc/hr and would
increase to 160 cc/hr without any shield. The outer
radiation shield cooled to the above preferred temperature
reduces the thermal transfer by radiation to the liquid
nitrogen reservoir 114' by approximately 88% in comparison
with an unshielded reservoir. This is a consequence of
the Stefan-Boltzmann radiation law which states that the
energy radiated (or absorbed) in~unit time by an emissive
- 16 -
~3143
body is proportional to the difference in the fourth
powers of the absolute temperatures of the radiating
(absorbing~ body and that o its surroundings.
The cryostat of the present invention has been
described particularly in terms of a liquid nitrogen
shielded liquid helium cooled superconducting magnet for
an NMR spectrometer. The inventive contributions to
cryostat design disclosed herein transcend the specific
application and employment of particular coolants. These
inventive contributions may be directed to cryostats
housing apparatus used for applying a variety of low
temperature phenomena and to other superconductive devices.
Since many changes can be made in the above
construction and many apparently widely differing
embodiments of this invention could be made without
departingfrom the scope thereof, is intended that all
matter contained in the above description and shown in
the encompanying drawing shall be interpreted as illustrative
and not in a limiting sense.
.
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