U.S. patent application number 11/073728 was filed with the patent office on 2005-09-15 for superconducting magnet system with pulse tube cooler.
This patent application is currently assigned to Bruker BioSpin GmbH. Invention is credited to Roth, Gerhard.
Application Number | 20050198974 11/073728 |
Document ID | / |
Family ID | 34485682 |
Filed Date | 2005-09-15 |
United States Patent
Application |
20050198974 |
Kind Code |
A1 |
Roth, Gerhard |
September 15, 2005 |
Superconducting magnet system with pulse tube cooler
Abstract
A superconducting magnet system with an operating temperature
T.sub.1<3K which is disposed in a first helium tank (4) of a
cryostat (1), wherein a second helium tank (2) is provided which is
connected to the first helium tank (4) and contains liquid helium
at an operating temperature T.sub.2>3K, wherein a cooling means
is provided in the first helium tank (4) which generates an
operating temperature T.sub.1<3K in that first helium tank (4)
is characterized in that the cooling means is the cold end (19) of
a pulse tube cooler (11) whose warm end (10) is disposed outside of
the cryostat (1). The inventive magnet system minimizes the helium
consumption thereby providing continuous measuring operation.
Inventors: |
Roth, Gerhard;
(Rheinstetten, DE) |
Correspondence
Address: |
KOHLER SCHMID MOEBUS
RUPPMANNSTRASSE 27
D-70565 STUTTGART
DE
|
Assignee: |
Bruker BioSpin GmbH,
Rheinstetten
DE
|
Family ID: |
34485682 |
Appl. No.: |
11/073728 |
Filed: |
March 8, 2005 |
Current U.S.
Class: |
62/51.1 |
Current CPC
Class: |
F25B 9/10 20130101; F25B
2309/003 20130101; F25B 9/145 20130101; H01F 6/04 20130101; F25D
19/006 20130101; F25D 19/00 20130101 |
Class at
Publication: |
062/051.1 |
International
Class: |
F25B 019/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 13, 2004 |
DE |
10 2004 012 452.3 |
Claims
I claim:
1. A magnet system having a superconducting magnet disposed in a
cryostat, the system comprising: a first helium tank disposed in
the cryostat, the superconducting magnet being disposed in said
first helium tank, said first helium tank containing liquid helium
at an operating temperature T.sub.1<3K; a second helium tank
disposed in the cryostat above and in liquid helium communication
with said first helium tank, said second helium tank containing
liquid helium at an operating temperature T.sub.2>3K; and a
pulse tube cooler, said pulse tube cooler having a cold end
disposed in said first helium tank to generate said operating
temperature T.sub.1<3K, said pulse tube cooler having a warm end
disposed outside of the cryostat, below said first helium tank.
2. The magnet system of claim 1, wherein the magnet system is
structured and dimensioned for magnetic resonance measurements.
3. The magnet system of claim 1, wherein said pulse tube cooler has
several stages.
4. The magnet system of claim 1, wherein said pulse tube cooler has
two stages.
5. The magnet system of claim 3, wherein a stage of said pulse tube
cooler upstream of said cold end is thermally conductingly
connected to a radiation shield disposed in the cryostat.
6. The magnet system of claim 5, wherein said radiation surrounds
said first and said second helium tanks to replace a liquid
nitrogen tank in the cryostat.
7. The magnet system of claim 1, further comprising a thermal
barrier disposed in the cryostat between said first and said second
helium tanks, said thermal barrier having a residual conductivity
to further cool helium in said second helium tank and minimize
consumption of liquid helium therein.
8. The magnet system of claim 1, wherein said second helium tank
has a heater to adjust a pressure in said second helium tank
through heating.
9. The magnet system of claim 1, wherein said pulse tube cooler
contains a regenerator material substance having a phase
transition.
10. The magnet system of claim 9, wherein said phase transition is
a magnetic phase transition.
11. The magnet system of claim 10, wherein said the regenerator
material is magnetically shielded in the cryostat.
12. The magnet system of claim 1, wherein said pulse tube cooler
contains helium as a regenerator material.
13. The magnet system of claim 1, wherein a section of said pulse
tube cooler which contains a regenerator is disposed at a location
in the cryostat having a minimum magnetic field during
operation.
14. The magnet system of claim 1, wherein the cryostat and said
pulse tube cooler are designed and dimensioned such that helium
must not be refilled into the cryostat during operation of the
magnet system.
15. The magnet system of claim 1, wherein the magnet system
comprises a main field magnet coil and an active shielding coil
which is disposed coaxially with respect to and radially outside of
said main field magnet coil, wherein a common axis of said main
field and said active shielding coils is vertical and said cold end
of said pulse tube cooler is disposed between said main field
magnet coil and said shielding coil.
16. The magnet system of claim 1, wherein said cryostat has a valve
for filling-in helium, said valve being connected to at least one
of said first and said second helium tanks.
Description
[0001] This application claims Paris Convention priority of DE 10
2004 012 452.3 filed Mar. 13, 2004 the complete disclosure of which
is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
[0002] The invention concerns a superconducting magnet system which
is disposed in a first helium tank of a cryostat having an
operating temperature T.sub.1<3K, wherein a second helium tank
is disposed above and connected to the first helium tank and
contains liquid helium at an operating temperature T.sub.2>3K,
wherein the first helium tank includes a cooling means which
generates the operating temperature T.sub.1<3K in the first
helium tank, wherein the cooling means is the cold end of a pulse
tube cooler whose warm end is disposed outside of the cryostat.
[0003] A magnet system of this type is known per se from U.S. Pat.
No. 5,220,800.
[0004] Superconducting magnet systems of this type generally
comprise a cryostat with two chambers. A superconducting magnet
coil is disposed in the first chamber and the second chamber serves
as a helium supply and is at atmospheric pressure or slight
overpressure at a temperature of approximately 4.2K. The two
chambers communicate with each other such that helium can flow from
the upper into the lower chamber where it is cooled to a
temperature of considerably less than 4.2K using a further cooling
unit which projects into that first chamber. A radiation shield
reduces the incident radiation energy and is surrounded by a tank
filled with a cryogenic liquid which cools the radiation
shield.
[0005] Additional cooling units are conventionally used to further
cool the helium in the first chamber, which relax the helium to a
low pressure using a needle valve and pump it out of the first
chamber. Pumping the helium out of the first chamber is
disadvantageous, since it is removed from the system and the second
chamber, which communicates with the first chamber, is slowly
emptied thereby necessitating replacement of the helium in the
second chamber at regular intervals.
[0006] In the magnet system according to U.S. Pat. No. 5,220,800
and DE 36 33 313 A1, the further cooling unit which projects into
the first chamber pumps liquid helium out of the first chamber to
cause further cooling of the helium bath in the first chamber as a
result of that expansion.
[0007] Disadvantageously, helium is thereby constantly consumed by
the refrigerator requiring corresponding refilling of helium into
the apparatus. This helium may be supplied in liquid form, thereby
necessitating a corresponding storage capacity. Moreover, helium is
not always available in the amounts needed. Another possibility is
to return the helium which escapes from the apparatus through
liquefaction which, however, requires considerable expense with
regard to equipment. In any case, the helium must be refilled in
conventional magnet systems which necessitates interruption of the
measuring operation and thereby involves substantial expense. For
this reason, it is desirable to reduce the helium consumption of a
magnet arrangement of this type.
[0008] U.S. Pat. No. 6,196,005 B1 discloses a cryostat
configuration having an upper and a lower helium tank. The lower
helium tank is cooled by a pulse tube cooler which passes from
above through the upper and into the lower helium tank such the
cold head of the pulse tube cooler projects into the lower helium
tank. The pulse tube cooler cools the magnet system with a minimal
loss of helium. However, in view of the fact that the pulse tube
cooler passes through the upper helium tank and into the lower
helium tank, the upper helium tank must have a sufficient amount of
space to accommodate the pulse tube cooler, which is consequently
no longer available for the storage of helium. The helium tank must
therefore be larger than would otherwise be necessary in view of
the helium requirements alone.
[0009] It is therefore the underlying purpose of the invention to
propose a superconducting magnet system which is not susceptible to
disturbances, wherein the helium consumption is minimized with
simple means thereby eliminating undesired interruption of the
measuring operation due to frequent refilling of helium. The system
should also exhibit a compact construction.
SUMMARY OF THE INVENTION
[0010] This object is achieved in accordance with the invention in
that the warm end of the pulse tube cooler is disposed below the
first helium tank.
[0011] Pulse tube coolers effect expansion and compression of the
working gas using a shock wave front in a pulse tube. The shock
wave front is thereby controlled by a rotating valve. The pulse
tube is connected to a regenerator which provides heat exchange
between the working gas and the regenerator material. After
compression of the working gas, the gas flows through the
regenerator to relax in the expansion chamber. The gas which is
thereby cooled, absorbs heat from the surroundings of the expansion
chamber thereby cooling those surroundings. Since the rotating
valve need not be disposed in the direct vicinity of the magnet
system, the pulse tube cooler is a smoothly running, low-wear
cooling means which avoids moving parts in the low-temperature
region.
[0012] Since the second helium tank is disposed above the first
helium tank, it thereby serves a hydrostatic function to keep the
first helium tank at atmospheric pressure.
[0013] As in the conventional means, the first helium tank contains
a cooling means to cool the helium located therein. In contrast to
the conventional systems, the inventive magnet system does not
discharge helium from the helium tank, since the cooling means is a
pulse tube cooler. The pulse tube cooler has its own, closed cycle.
For this reason, no helium escapes into the atmosphere nor is the
helium heated which would require renewed liquefaction of the gas
and large amounts of energy and significant equipment expense. The
helium consumption is minimized through the inventive magnet system
thereby permitting continuous measuring operation.
[0014] Moreover, in accordance with the inventive magnet system,
the warm end of the pulse tube cooler is disposed below the first
helium tank. The configuration thereby permits use of a shorter
pulse tube to decrease the overall height of the apparatus.
[0015] The invention realizes an evaporation-free superconducting
magnet system, wherein the helium in the first helium tank is
cooled via a cooling means in the form of a pulse tube cooler which
is independent of the helium in the helium tank. The helium in the
helium tank is not consumed during operation of the pulse tube
cooler, which causes less frequent or optimally no refilling of the
helium tank during operation of the magnet system. The inventive
system therefore provides continuous measuring operation without
having to organize the supply and refilling of helium. Moreover,
the second helium tank may be smaller than in conventional magnet
systems due to the reduced helium consumption. This reduces the
overall size of the apparatus.
[0016] In a preferred embodiment of the magnet system, the pulse
tube cooler has several, preferably two, stages. The second stage
of the pulse tube cooler projects directly into the first tank,
wherein the temperature of the second stage during operation is
T<3K, whereby the helium in the first tank is further cooled
directly and without removing helium thereby completely avoiding
consumption of liquid helium in the first tank. The helium located
in the second tank is also cooled through residual heat conduction
via the thermal barrier. Minimizing the heat transfer into the
second tank and suitable selection of the insulation properties of
the thermal barrier ensures that no helium is discharged from the
second helium tank, which is at atmospheric pressure or slight
overpressure.
[0017] In a particularly suitable design of the insulation
properties of the barrier, the second helium tank is slightly
under-cooled and can be maintained at atmospheric pressure or
slight overpressure through introduction of a heater therein,
without having helium escape from the second helium tank. In this
design, no helium is removed from the first and second tank during
operation, which avoids the need to refill the cooling agent.
[0018] In a particularly preferred embodiment of the invention, one
stage of the pulse tube cooler upstream of the cold end is
thermally conductingly connected to a radiation shield disposed in
the cryostat. The radiation shield can be cooled by the pulse tube
cooler stage connected thereto.
[0019] In a particularly preferred fashion, the radiation shield
connected to the pulse tube cooler stage surrounds the helium tanks
and the pulse tube cooler replaces a tank of liquid nitrogen in the
cryostat. In this case, supply of liquid nitrogen to the
arrangement can be omitted. Due to omission of the nitrogen tank,
the arrangement may be more compact.
[0020] The pulse tube cooler preferably comprises a regenerator
material substance which has a phase transition at a low
temperature of around 4K or below, in particular a magnetic phase
transition. The phase transition increases the specific heat of the
regenerator material to permit heat exchange from the working gas
to the regenerator material, even at very low temperatures
(T<4K).
[0021] In particular, for regenerator materials having a magnetic
phase transition, the regenerator material is advantageously
magnetically shielded in the cryostat thereby preventing
disturbance of the main field by the magnetic phase transition.
[0022] In a further embodiment, the pulse tube cooler additionally
or exclusively contains helium as the regenerator material. Since
helium has no magnetic phase transition, it has no disturbing
effects in connection with magnetic applications and is relatively
inexpensive compared to other conventional regenerator materials.
DE 199 24 184 A1 has already disclosed the use of high-pressure
helium as a regenerator material.
[0023] In a particularly preferred embodiment of the invention, the
section of the pulse tube cooler comprising the regenerator is
disposed at a location in the cryostat having a minimum magnetic
field during operation, e.g. radially outside of the magnet coil,
approximately in the region of its central plane. Interaction
between the regenerator material and the main magnetic field is
thereby minimized.
[0024] The cryostat and the pulse tube cooler are preferably
designed and dimensioned such that no helium must be refilled into
the cryostat during operation to increase the user friendliness of
the magnet system and permit continuous operation thereof.
[0025] In a particularly advantageous embodiment, the magnet system
comprises a main field magnet coil and an active shielding coil
which is disposed coaxially thereto and radially outside of the
main field magnet coil, wherein the axes of the two coils are
disposed vertically, and the cold end of the pulse tube cooler is
disposed between the main field magnet coil and the shielding coil.
The cold end of the pulse tube cooler is then in a low magnetic
field or in a zero magnetic field, thereby minimizing or preventing
disturbance of the main magnetic field by the pulse tube
cooler.
[0026] In a further advantageous embodiment of the invention, a
heating device is provided in the second helium tank to heat the
helium. This is advantageous in that the pressure of the helium
located in the helium tank can be regulated. The undercooled helium
in the first helium tank can thereby be maintained at atmospheric
pressure to realize a stable operating state.
[0027] Moreover, a valve for filling-in helium is advantageously
provided on the cryostat, which is connected to at least one helium
tank and through which helium can be refilled if e.g. helium has
escaped via an overpressure valve.
[0028] The magnet system is preferably part of a magnetic resonance
apparatus, such as an NMR spectrometer, a nuclear magnetic
resonance tomograph or an ICR mass spectrometer. These apparatuses
particularly depend on a homogeneous, stable and undisturbed
magnetic field in a volume under investigation such that they
considerably profit from the advantages of the inventive magnet
system.
[0029] Further advantages of the invention can be extracted from
the description and the drawing. The features mentioned above and
below may be used individually or collectively in arbitrary
combination. The embodiments shown and described are not to be
considered as exhaustive enumeration but have exemplary character
for describing the invention.
BRIEF DESCRIPTION OF THE DRAWING
[0030] FIG. 1 is a schematic illustration of a magnet system with
installed pulse tube cooler; and
[0031] FIG. 2 is a schematic illustration of a preferred embodiment
of an inventive magnet system with installed pulse tube cooler.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0032] FIG. 1 shows a magnet system with a first helium tank 4
which is disposed in a cryostat 1 and which contains a main field
magnet coil 3 for generating a highly homogeneous magnetic field. A
second helium tank 2 is disposed above the first helium tank 4 and
is separated from the first helium tank 4 via a thermal barrier 5.
The second helium tank 2 contains liquid helium at atmospheric
pressure having a temperature of more than 3K, preferably 4.2K. The
two helium tanks communicate with each other such that helium can
flow from the upper into the lower chamber, where the helium is
cooled (further cooled) to a temperature of considerably less than
3K, preferably 1.8K, using a pulse tube cooler 11. In the
embodiment of FIG. 1, the pulse tube cooler 11 passes through the
second helium tank 2 such that the warm end 10 of the pulse tube
cooler 11 is disposed outside of the cryostat 1 and the cold end 19
of the pulse tube cooler 11 projects into the first helium tank 4
thereby cooling the helium located in the first helium tank 4 to
the desired temperature. The arrangement of the pulse tube cooler
11 permits cooling of the helium thereby preventing helium from
escaping from the helium tanks 2, 4 such that refilling of the
helium tanks 2, 4 is not necessary during normal operation, thereby
avoiding the demanding liquefaction of helium gas. For safety
reasons, the magnet system may be provided with an overpressure
valve through which helium can escape into the atmosphere in case
of heating of the helium e.g. as a result of a quench of the main
field magnet coil 3. In this case, refilling of helium into the
second helium tank 2 may be required. Towards this end, the
inventive magnet system comprises a fill-in valve 12.
[0033] The pulse tube cooler 11 which is integrated in the
inventive magnet system has two stages to cool the helium below its
boiling temperature at atmospheric pressure. The cold end 19 of the
second stage 14 of the pulse tube cooler 11 projects into the first
helium tank 4 to cool the helium in the first helium tank 4. The
second helium tank 2 comprises a heating device to control the
pressure in the helium tank e.g. to maintain the undercooled helium
in the first helium tank 4 at atmospheric pressure. The first stage
13 of the pulse tube cooler 11 may be thermally conductingly
connected to a radiation shield 15 located in the cryostat 1. The
radiation shield 15 reduces incoming radiation energy. The
radiation shield 15 may be cooled via the pulse tube cooler 11
through thermal connection to the radiation shield 15 such that a
separate nitrogen tank 16 can be omitted. For this reason and
through reduction of the size of the second helium tank 2, the
inventive magnet system can be realized with compact size compared
to conventional magnet systems.
[0034] The pulse tube cooler 11 is preferably disposed within a
vacuum safety device which projects through the radiation shield 15
and the second helium tank 2 and is mounted to the vacuum safety
device in a pressure-tight manner. The vacuum safety device
comprises side walls 8 of a material with poor conducting
properties, e.g. stainless steel, and an end piece 9 which contacts
the cold end 19 of the pulse tube cooler 11 and is made from a
material having good conducting properties, e.g. copper, such that
heat exchange between the liquid helium in the first helium tank 4
and the pulse tube cooler 11 is effected mainly via the cold end 9
of the pulse tube cooler 11.
[0035] The pulse tube cooler 11 comprises a regenerator material
having a phase transition in order to generate the required low
temperatures. The phase transition increases the volumetric
specific heat of the regenerator material and permits cooling of
the helium to less than 3K. Pb and rare earth compounds such as
e.g. HoCo, Er.sub.3Ni, ErNi, GdAlO.sub.3 and ErNi.sub.0.9Co.sub.0.1
are suitable regenerator materials. These materials, however, have
a magnetic phase transition which may be disturbing in connection
with magnetic applications. The inventive magnet system therefore
provides magnetic shielding of the regenerator material in the
cryostat 1 via e.g. a .mu.-metal foil which surrounds the pulse
tube cooler 11 or using a highly-conducting housing to shield the
fluctuating magnetization. A superconducting housing can also
surround the pulse tube cooler 11 to minimize the influence of the
above-mentioned disturbing effects resulting from the magnetic
phase transition of the regenerator material.
[0036] FIG. 2 shows a particularly advantageous embodiment of the
inventive magnet system, wherein the warm end 10 of the pulse tube
cooler 11 is disposed below the first helium tank 4. This
arrangement permits use of a shorter pulse tube cooler 11, since
the pulse tube cooler need not pass through the second helium tank
2. In addition to the main field magnet coil 3, the inventive
magnet system may comprise an active shielding coil which is
disposed outside of and coaxial to the main field magnet coil 3 and
which shields the main magnetic field towards the outside. The
pulse tube cooler 11 may be disposed such that the cold end 19 of
the pulse tube cooler 11 is disposed between the main field magnet
coil 3 and the shielding coil. Due to the shielding function of the
shielding coil, the magnetic field between the main field magnet
coil 3 and the shielding coil is zero or nearly zero. This
arrangement of the pulse tube cooler 11 minimizes interaction
between the regenerator material and the main magnetic field even
if regenerator materials having magnetic phase transitions are
used. The inventive magnet system therefore permits use of
conventional pulse tube coolers without having to accept the
usually associated disadvantages.
[0037] The inventive magnet system improves the measuring operation
since the number of helium refilling operations can be considerably
reduced. The second helium tank 2 which, in conventional magnet
systems, contains a relatively large supply of helium in order to
be able to supply helium to the first helium tank 4 over a longer
time period, may be considerably smaller in the inventive magnet
system. The second helium tank 2 thereby mainly serves a
hydrostatic function, i.e. maintains the atmospheric pressure in
the helium tanks 2, 4. In contrast to the conventional magnet
system, the inventive magnet system uses no helium from the helium
tanks 2, 4 for the cooling process. The inventive design is
therefore suited for dry systems and thereby offers a broader
application spectrum.
[0038] In total, a compact magnet system is obtained which is easy
to handle and which avoids heating of the helium in the helium tank
and consequently also re-liquefaction, to permit continuous
measuring operation and largely spares the staff the inconvenience
of having to provide and refill helium.
[0039] The magnet systems of FIGS. 1 and 2 are part of a
high-resolution NMR apparatus at a high magnetic field of around or
over 20 Tesla.
LIST OF REFERENCE NUMERALS
[0040] 1 cryostat
[0041] 2 second helium tank
[0042] 3 main field magnet coil
[0043] 4 first helium tank
[0044] 5 thermal barrier
[0045] 8 side walls of the vacuum safety device
[0046] 9 end piece of the vacuum safety device
[0047] 10 warm end of the pulse tube cooler
[0048] 11 pulse tube cooler
[0049] 12 fill-in valve
[0050] 13 first stage of the pulse tube cooler
[0051] 14 second stage of the pulse tube cooler
[0052] 15 radiation shield
[0053] 19 cold end of the pulse tube cooler
* * * * *