U.S. patent number 9,271,385 [Application Number 13/881,315] was granted by the patent office on 2016-02-23 for magnetic structure for circular ion accelerator.
This patent grant is currently assigned to Ion Beam Applications S.A.. The grantee listed for this patent is Alessio Capelluto, Sebastien De Neuter, Roberto Marabotto, Patrick Verbruggen. Invention is credited to Alessio Capelluto, Sebastien De Neuter, Roberto Marabotto, Patrick Verbruggen.
United States Patent |
9,271,385 |
Verbruggen , et al. |
February 23, 2016 |
Magnetic structure for circular ion accelerator
Abstract
A magnet structure for use in a circular ion accelerator, such
as e.g. a synchrocyclotron comprises a cold-mass structure
including superconducting magnetic coils (20, 25), at least one dry
cryocooler unit (10, 11, 12, 13) coupled with the cold-mass
structure for cooling the latter and a magnetic yoke structure (30)
with a return yoke (35) configured radially around said coils (20,
25). The return yoke (35) comprises an opening in which said dry
cryocooler unit (10, 11, 12, 13) is received so as to be in thermal
contact with said cold-mass structure.
Inventors: |
Verbruggen; Patrick (Orbais,
BE), De Neuter; Sebastien (Jandrenouille,
BE), Marabotto; Roberto (Genoa, IT),
Capelluto; Alessio (Genoa, IT) |
Applicant: |
Name |
City |
State |
Country |
Type |
Verbruggen; Patrick
De Neuter; Sebastien
Marabotto; Roberto
Capelluto; Alessio |
Orbais
Jandrenouille
Genoa
Genoa |
N/A
N/A
N/A
N/A |
BE
BE
IT
IT |
|
|
Assignee: |
Ion Beam Applications S.A.
(Louvain-la-Neuve, BE)
|
Family
ID: |
43978034 |
Appl.
No.: |
13/881,315 |
Filed: |
October 25, 2011 |
PCT
Filed: |
October 25, 2011 |
PCT No.: |
PCT/EP2011/068691 |
371(c)(1),(2),(4) Date: |
July 01, 2013 |
PCT
Pub. No.: |
WO2012/055890 |
PCT
Pub. Date: |
May 03, 2012 |
Prior Publication Data
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|
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Document
Identifier |
Publication Date |
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US 20130270451 A1 |
Oct 17, 2013 |
|
Foreign Application Priority Data
|
|
|
|
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Oct 26, 2010 [EP] |
|
|
10188946 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H05H
7/04 (20130101); H05H 13/02 (20130101) |
Current International
Class: |
H05H
15/00 (20060101); H05H 13/02 (20060101); H05H
7/04 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2010089574 |
|
Aug 2010 |
|
WO |
|
2012/055958 |
|
May 2012 |
|
WO |
|
Other References
International Search Report, International Patent Application No.
PCT/EP2011/068691, date of completion Nov. 29, 2011, 3 pages. cited
by applicant .
Joonsun Kang et al., "Design Study of a K22 Prototype
Superconducting Cyclotron Magnet." IEEE Transactions on Applied
Superconductivity, vol. 20, No. 3, Jun. 2010, pp. 192-195. cited by
applicant .
International Search Report, International Patent Application No.
PCT/EP2011/068844, date of completion Nov. 28, 2011, 3 pages. cited
by applicant .
C.B. Bigham, "Magnetic Trim Rods for Superconducting Cyclotrons."
Nuclear Instruments and Methods, vol. 131, 1975, pp. 223-228. cited
by applicant .
A. Garonna, "Synchrocyclotron Preliminary Design for a Dual
Hadrontherapy Center." Proceedings of IPLAC'10, Jun. 2010, Kyoto,
Japan, pp. 552-554. cited by applicant .
S. Holm, "Factors Affecting Beam Intensity and Quality in
Synchrocyclotrons." Proceedings of the Fifth International
Cyclotron Conference, 1971, pp. 736-748. cited by applicant .
XiaoYu Wu, "Conceptual Design and Orbit Dynamics in a 250 MeV
Superconducting Synchrocyclotron." A Dissertation submitted to
Michigan State University, 1990, 172 pages. cited by
applicant.
|
Primary Examiner: Le; Tung X
Assistant Examiner: Sathiraju; Srinivas
Attorney, Agent or Firm: Fitch, Even, Tabin & Flannery
LLP
Claims
The invention claimed is:
1. A magnet structure for use in a circular ion accelerator
comprising: a cold-mass structure including superconducting
magnetic coils; at least one dry cryocooler unit coupled with the
cold-mass structure and configured to cool the cold-mass structure;
and a magnetic yoke structure including a return yoke configured
radially around the superconducting magnetic coils; wherein the
return yoke comprises an opening in which the at least one dry
cryocooler unit is received so as to be in thermal contact with the
cold-mass structure.
2. A magnet structure for use in a circular ion accelerator
comprising: a cold-mass structure including superconducting
magnetic coils; at least one dry cryocooler unit coupled with the
cold-mass structure and configured to cool the cold-mass structure;
and a magnetic yoke structure including a return yoke configured
radially around the superconducting magnetic coils, wherein the
return yoke comprises an opening in which the at least one dry
cryocooler unit is received so as to be in thermal contact with the
cold-mass structure, the at least one dry cryocooler unit being
received in the opening in a position essentially perpendicular to
a central axis of the superconducting magnetic coils.
3. A magnet structure for use in a circular ion accelerator
comprising: a cold-mass structure including superconducting
magnetic coils; a plurality of dry cryocooler units coupled with
the cold-mass structure and configured to cool the cold-mass
structure; and a magnetic yoke structure including a return yoke
configured radially around the superconducting magnetic coils,
wherein the return yoke comprises an opening in which the at least
two dry cryocooler units are received so as to be in thermal
contact with the cold-mass structure, the at least two dry
cryocooler units being superimposed at a same radial position.
4. The magnet structure according to claim 1, comprising two
openings spaced by an angle of 180.degree.in the return yoke,
wherein at least one cryocooler unit is received in each of the two
openings.
5. The magnet structure according to claim 4, wherein two
cryocooler units are superimposed in each of the two openings.
6. The magnet structure according to claim 1, wherein the cold-mass
structure includes a bobbin associated with the superconducting
magnetic coils, and the at least one dry cryocooler unit is in
thermal contact with the bobbin.
7. The magnet structure according to claim 6, wherein the
superconducting magnetic coils includes a current lead that is in
thermal contact with the at least one dry cryocooler unit, so that
the at least one dry cryocooler unit simultaneously cools the
bobbin and the current lead.
8. The magnet structure according to claim 6, wherein the at least
one dry cryocooler unit comprises a terminal cooling stage member
that is in thermal contact with an outward wing of the bobbin, and
the outward wing is in contact with a radial outer part of the
magnetic coils.
9. The magnet structure according to claim 1, wherein the magnet
structure has a central axis and a median plane perpendicular to
the central axis, and the opening in which the at least one dry
cryocooler unit is received is symmetric with regard to the median
plane.
10. The magnet structure according claim 1, comprising a cryostat
enclosing the cold-mass structure and forming a vacuum chamber for
keeping the cold-mass structure under vacuum, the vacuum chamber
comprising a radial vacuum chamber extension, and the at least one
dry cryocooler unit including at least one cooling stage housed in
the vacuum chamber extension, and a head part with a connector
protruding out of the radial vacuum chamber extension.
11. The magnet structure according to claim 10, further comprising
tie rods configured to support the cold-mass structure, each of the
tie rods being positioned partly within a hollow tube, which
extends the vacuum chamber for passing through the magnetic yoke
structure.
12. The magnet structure according to claim 11, wherein at least
one of the hollow tubes is coupled to a vacuum pump configured to
create a vacuum in the cryostat.
13. The magnet structure according to claim 1, wherein: the cold
mass structure includes at least two superconducting magnetic coils
comprising a material being superconducting below a nominal
temperature, the at least two superconducting magnetic coils being
configured for having a common central axis; and a bobbin
configured to support the at least two superconducting magnetic
coils; and the magnet structure further includes: a cryostat
enclosing the cold-mass structure and forming a vacuum chamber for
keeping the cold-mass structure under vacuum, wherein the magnetic
yoke structure surrounds the cryostat.
14. A synchrocyclotron comprising a magnet structure according to
claim 1.
15. The magnet structure according to claim 1, wherein the at least
one dry cryocooler is received in the opening such that the at
least one cryocooler is essentially in a horizontal position.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a national phase application of International
Application No. PCT/EP2011/068691, filed Oct. 25, 2011, designating
the United States and claiming priority to European Patent
Application No. 10188946.7, filed Oct. 26, 2010, both of which are
incorporated by reference as if fully rewritten herein.
TECHNICAL FIELD
The invention generally relates to a circular ion accelerator, more
particularly to a superconducting synchrocyclotron. More
specifically, the invention relates to a magnet structure for a
circular ion accelerator, more particularly to a magnetic structure
for a superconducting synchrocyclotron.
DESCRIPTION OF PRIOR ART
A typical magnetic structure of a superconducting synchrocyclotron
generally comprises a cold-mass structure including at least two
superconducting magnetic coils, i.e. magnetic coils which comprise
a material that is superconducting below a nominal temperature, and
a bobbin associated with the magnetic coils. A cryostat generally
encloses this cold mass structure and forms a vacuum chamber for
keeping the cold mass structure under vacuum. The cold mass
structure is cooled with one or more dry cryocooler units below the
nominal temperature at which the magnetic coils are
superconducting. The magnet structure further comprises a magnetic
yoke structure surrounding the cryostat. Such a yoke structure
generally comprises an upper part, a lower part, a pair of pole
parts and a return yoke arranged radially around the magnetic
coils.
U.S. Pat. No. 7,656,258 describes such a magnetic structure for
generating a magnetic field in e.g. a superconducting
synchrocyclotron. The magnet structure comprises several dry
cryocooler units as shown in FIG. 10 of the referenced patent
(units identified with reference number 26) to cool the cold-mass
structure (21) below a temperature where the coils become
superconducting. A first dry cryocooler unit (26) is positioned
vertically on top of the upper part of the yoke (36) and extends
vertically through a hole in the upper part of the yoke structure
towards the cold mass structure (21). A second cryocooler unit (26)
is positioned vertically below the lower part of the yoke structure
(36) and extends vertically through a hole in the lower part of the
yoke structure. Two additional dry cryocooler units (33) are
installed on top of the upper part of the yoke structure and
configured for cooling the current leads (37, 58) of the coils (12,
14). Such a vertical orientation of the dry cryocooler units is
necessary for reaching the specified nominal refrigeration capacity
(e.g. Gifford-McMahon type of cryocooler units). Other types of
cryocooler units (e.g. pulse type of cryocooler unit) only operate
in a vertical position.
Although the design of the magnetic structure as disclosed in U.S.
Pat. No. 7,656,258 may work in a satisfactory manner, it has
nevertheless some disadvantages.
A first disadvantage of the magnetic structure as disclosed in U.S.
Pat. No. 7,656,258 resides in the fact that for each cryocooler
unit installed in the upper, respectively lower part of the yoke
structure, a corresponding hole must be made in a symmetrical way
in the opposite lower part, respectively the opposite upper part of
the yoke structure. This symmetry of the holes in the magnetic yoke
structure is indeed necessary for warranting the required magnetic
field properties. It will be appreciated that these supplementary
holes result in an increased machining time when manufacturing the
yoke structure. A great number of holes in the yoke structure also
results in a second disadvantage, namely a reduction of the
efficiency of the yoke structure and an increase of the magnetic
stray field. A third disadvantage is due to the fact that
vertically positioned dry cryocooler units increase the height of
the accelerator and hence require a larger building with
sufficiently high ceilings to house the cyclotron. Moreover, for
maintenance purposes, such cyclotrons are opened by removing the
upper part of the yoke structure. Hence, before opening the
cyclotron, it is necessary to first disconnect the vertically
arranged cryocooler units from the cold mass structure, which is a
major fourth disadvantage. This fourth disadvantage further results
in longer down time periods of cyclotron operation, when the
cyclotron must be opened for e.g. maintenance purposes.
The publication of JOONSUN et al: "Design Study of a K22 Prototype
Superconducting Cyclotron Magnet", IEEE Transactions on Applied
Superconductivity, IEEE Service Center Los Alamitos, Calif., US,
vol. 20, no. 3, 1 Jun. 2010, pages 192-195, discloses a cryogenic
system comprising three 1.5 W GM cryocoolers arranged in separate
re-condensing vessels located laterally of the cyclotron. Conduits
for evaporated and re-condensed helium pass through a radial
opening in the upper half of a return yoke.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a magnetic structure
for use in an ion accelerator (e.g. synchrocyclotron) which
overcomes or alleviates at least some of the aforementioned
problems of prior art magnetic structures.
This object is achieved by magnet structure in accordance with
claim 1, respectively by a synchrocyclotron comprising such a
magnet structure.
A magnet structure for use in a circular ion accelerator comprises
a cold-mass structure including superconducting magnetic coils, at
least one dry cryocooler unit coupled with the cold-mass structure
for cooling the cold-mass structure and a magnetic yoke structure
comprising a return yoke configured radially around the coils. In
accordance with one aspect of the present invention, the return
yoke comprises an opening in which the dry cryocooler unit is
received so as to be in thermal contact with the cold-mass
structure.
In a preferred embodiment, the dry cryocooler unit is received in
the opening in a position essentially perpendicular to a central
axis of the magnetic coils.
Preferably two dry cryocooler units are received in the same
opening in the return yoke, wherein they are preferably
superimposed at a same radial position. Indeed, by superimposing
two cryocooler units at the same radial position with respect to
the return yoke, the return flux of the magnetic field remains the
same when compared to the use of a single cryocooler unit at the
same radial position and hence there is no need to increase the
diameter of the cyclotron to compensate for the loss of magnetic
flux capacity when installing a second cryocooler unit for
increasing the refrigeration capacity.
In a preferred embodiment, the return yoke comprises two openings
spaced by an angle of 180.degree., wherein at least one cryocooler
unit is received in each of these openings. Thus, symmetry of the
yoke structure is warranted with a minimum of openings therein.
Preferably, two cryocooler units are superimposed in each of these
openings.
The cold-mass structure typically includes a bobbin associated with
the superconducting magnetic coils, wherein the at least one
cryocooler unit is advantageously in thermal contact with the
bobbin.
The superconducting magnetic coils advantageously include a current
lead that is in thermal contact with the cryocooler unit, so that
the latter simultaneously cools the bobbin and the current lead.
Hence, no dedicated or additional dry cryocooler units must be
installed for cooling the current leads and, consequently, no
additional openings must be made in the yoke structure
The cryocooler unit advantageously has a terminal cooling stage
member that is in thermal contact with an outward wing of the
bobbin, and the outward wing is in contact with a radial outer part
of the magnetic coils.
In a preferred embodiment, the magnet structure has a central axis
and a median plane perpendicular to the central axis, and the
opening in which the dry cryocooler unit is received is symmetric
with regard to the median plane.
The magnet structure typically comprises a cryostat enclosing the
cold-mass structure and forming a vacuum chamber for keeping the
cold-mass structure under vacuum. This vacuum chamber
advantageously comprises a radial vacuum chamber extension in which
at least one cooling stage of the dry cryocooler unit is housed.
The latter advantageously includes a head part protruding out of
the radial vacuum chamber extension.
A preferred embodiment of the magnet structure with a vacuum
chamber for keeping the cold-mass structure under vacuum further
comprises tie rods for supporting the cold-mass structure. Each of
the tie rods is advantageously positioned partly within a hollow
tube, which extends the vacuum chamber for passing through the yoke
structure. At least one of these hollow tubes is advantageously
coupled to a vacuum pump for creating a vacuum in the cryostat.
SHORT DESCRIPTION OF THE DRAWINGS
These and further aspects of the invention will be explained in
greater detail by way of example and with reference to the
accompanying drawings in which:
FIG. 1 is a three-dimensional (3D) view of a synchrocyclotron
comprising a magnetic structure according to the invention;
FIG. 2 is a schematic sectional view of the magnetic structure
according to the invention, the sectional plane being a vertical
plane containing the central axis of the synchrocyclotron;
FIG. 3 is an enlarged detail of FIG. 2 showing a configuration of
dry cryocooler units in a magnetic structure according to the
invention;
FIG. 4 is a schematic sectional view of a synchrocyclotron having a
magnetic structure according to the invention, the sectional plane
being the median plane of the synchrocyclotron, which is
perpendicular to the central axis of the synchrocyclotron;
FIG. 5 is a three-dimensional (3D) view of a cryostat for a
synchrocyclotron according to the invention; and
FIG. 6 is a three-dimensional (3D) view of the cryostat of FIG. 5
within a magnetic yoke structure.
The figures are not drawn to scale. Generally, identical components
are denoted by the same reference numerals in the figures.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
FIG. 1 shows, as an illustration of the invention, a three
dimensional view of a preferred embodiment of a synchrocyclotron 1
comprising a magnetic structure according to the invention. It will
be noted that, for the sake of clarity, the representation of the
synchrocyclotron 1 is only schematic, and that not all its parts
and details are shown. The major part of the magnetic structure
that is visible from the outside of the synchrocyclotron is a
magnetic yoke structure 30, which is usually made of ferromagnetic
iron. The synchrocyclotron with its magnetic structure is supported
on the floor by several feet 5.
FIG. 2 is a schematic sectional view illustrating a preferred
embodiment of magnetic structure according to the invention. The
magnetic structure comprises two circular superconducting magnetic
coils 20, 25. These coils have an annular shape and are
superimposed symmetrically with regard to a median plane of the
synchrocyclotron 1. To fix the ideas, it will be noted that the
coils of the magnetic structure shown in FIG. 2 have e.g. an outer
diameter of 1.370 m and an inner diameter of 1.108 m. These coils
are generally named upper coil 20 and lower coil 25, respectively.
The two coils 20, 50 have a common central axis 50, as indicated in
FIG. 2, going axially through the centres of the coils. This
central axis 50 is also forming a central axial axis for the entire
magnetic structure.
The superconducting coils 20, 50 are generating a coil magnetic
field in an axial direction, i.e. in a direction parallel with the
central axis 50. They comprise e.g. NbTi as superconducting
material and are typically operated at 4.5 K, with current
densities of about 55.6 A/mm2 for providing a coil magnetic field
of about 3.33 Tesla. Alternatively, other superconducting conductor
materials can be used such as Nb-3Sn conductors.
As mentioned above, the magnet structure comprises a magnetic yoke
structure 30, which consists of several parts. Following main parts
of the yoke structure can be distinguished on FIG. 2: an upper yoke
part 31, a lower yoke part 32, a pair of pole parts 33, 34 and a
so-called return yoke 35. The return yoke 35 is radially arranged
around the coils 20, 25. To fix the ideas, it will be noted that
the return yoke 35 of FIG. 2 has e.g. an inner diameter of about
1.590 m and a radial thickness of about 0.455 m.
The superconducting coils 20, 25, together with the magnetic yoke
structure generate a combined magnetic field between the two poles
of the magnetic structure. The prototype referred herein is e.g. a
250 MeV proton synchrocyclotron having a magnetic structure
designed for providing a total magnetic field of about 5.6 Tesla
for bending protons during a circular acceleration process. To fix
the ideas, it will be noted that the entire magnetic structure of
such a synchrocyclotron has e.g. a diameter of about 2.5 m and a
height of 1.56 m and has a total weight of about 45.000 kg.
FIG. 3 is an enlarged view of part of the sectional illustration of
FIG. 2. The superconducting coils 20, 25 are supported by a coil
supporting structure which comprises a mechanical containment
structure 27, referred to as bobbin 27, and coil supporting plates
28, 29. The bobbin is usually made of aluminium. When in operation,
the upper and lower coils 20, 25 exercise large axial attractive
forces on each other and also generate radial forces outward. The
bobbin 27 is designed and has a shape for withstanding these
forces: it has basically an outward wing that is contacting the
radial outer part of the two coils and an inner wing in between the
coils for withstanding axial attractive forces between the coils.
Both the outer wing and inner wing of the bobbin have multiple
holes for providing access to various parts of the
synchrocyclotron. The bobbin 27 supporting the two coils 20, 25 is
also thermally coupled with the two coils 20, 25. The coil
supporting structure also comprises an upper and a lower 29 coil
supporting plate having an annular shape and which are fixed to the
bobbin 27. These coil supporting plates 28, 29 are preferably made
of stainless steel. These coil supporting plates 28, 29 and the
bobbin 27 cooperate for encapsulating and holding the coils in
place. The coils 20, 25 are further surrounded by heat shields 60.
Those heat shields are preferably made of an aluminium alloy. The
upper and lower superconducting coils 20, 25 with the supporting
structure 27, 28, 29 are called the cold-mass structure of the
magnet structure, as these parts are kept below a temperature where
the conductors of the coils 20, 25 are becoming superconducting.
The whole cold-mass structure is preferably encapsulated in a
cryostat 70 that is forming a vacuum chamber for keeping the
cold-mass structure under vacuum (see e.g. FIGS. 4, 5 and 6).
The cold-mass structure is cooled by using a dry cryocooler unit.
With the wording "dry" it is understood that the coils are
maintained in a dry condition, i.e. they are not immersed in a
cooling liquid (e.g. liquid He). Instead, the cold-mass structure
is thermally coupled with one or more dry cryocooler units. These
dry cryocooler units are commercially available.
As shown in FIG. 2 and FIG. 3, a through opening in a radial
direction is made in the return yoke 35 for receiving a dry
cryocooler unit 10. In this example, the dry cryocooler unit 10 is
in a position in which its longitudinal axis is essentially
perpendicular to the central axis 50 of the synchrocyclotron 1. In
other words, if the synchrocyclotron 1 is positioned on the floor
on its feet 5, as shown in FIG. 1, the dry cryocooler 10 unit is
essentially in a horizontal position. When positioning a dry
cryocooler unit 10 perpendicular to the central axis 50, there is a
certain tolerance with respect to this orientation. In the example
presented, the cryocooler unit 10 is preferably at an angle of
90.degree.+/-5.degree. with respect to the central axis 50 and more
preferably at an angle of 90.degree.+/-2.degree..
When the dry cryocooler unit (e.g. a dry cryocooler unit of the
Gifford-McMahon type) is in such a horizontal position with respect
to the floor, the refrigeration power will be lower than its
nominal refrigeration power, i.e. the refrigeration power is
typically reduced by 15%. For example, a dry cryocooler having a
nominal refrigeration power of 1.5 W in a vertical position will
only have a refrigeration power of 1.3 W in a horizontal position.
With a refrigeration power of 1.3 W per cryocooler unit and with a
synchrocyclotron in operation (i.e. producing beam), four dry
cryocoolers units are needed to cool the cold-mass structure of the
present example to a temperature of 4.5 K. In FIG. 2, the
horizontal arrangement of the four cryocoolers 10,11,12,13 is
shown.
Preferably, the opening in the return yoke 35 is configured such
that it can receive two superimposed dry cryocooler units as shown
in greater detail in FIG. 3. Both cryocooler units 10, 11 are
preferably positioned to have their longitudinal axis perpendicular
to the central axis 50 and more preferably the two dry cryocooler
units are located at the same radial position with respect to the
return yoke 35. In this way, the return flux of the magnetic field
remains the same and there is no need to increase the diameter of
the cyclotron to compensate for the loss of magnetic flux capacity
due to the installation of a second dry cryocooler unit. To fix the
ideas, it will be noted that the opening made through the return
yoke 35 for receiving two superimposed cryocooler units is
rectangular and has a height of about 50 cm and a width of about 29
cm.
As illustrated in FIG. 4, a second pair of cryocooler units 12, 13
is advantageously separated from a first pair of cryocooler units
10, 11 by a radial angle of 180.degree.. Identical to the first
pair of dry cryocooler units, also the second pair is received
through an opening in the return yoke (see e.g. FIG. 2), preferably
configured for receiving the two cryocooler units superposed at the
same radial position.
Typically and as shown in FIG. 3, a dry cryocooler unit 10, 11
comprises a head part 17, a first stage member 16 and a second
stage member 15. The head part comprises connection means for
making connection with a cooling fluid compressor, e.g. a helium
compressor (not shown). The first stage member 16 is at an
intermediate temperature (for example 50 K) and a lowest
temperature of for example 4.2 K is reached at the second stage
member 15. The second stage member 15 is making a thermal contact
with the cold mass structure such that the cold mass structure is
cooled to a temperature where the conductors of the coils become
superconducting (e.g. 4.5 K). More specifically, the second stage
member 15 is making a thermal contact with the outward wing of the
bobbin 27 (see e.g. FIG. 4). As in this preferred magnetic
structure, two pair of superimposed dry cryocooler units are used,
each second stage member 15 of each dry cryocooler unit is making a
thermal contact with the outward wing of the bobbin 27 of the two
coils 20, 25 as shown in FIG. 3 and FIG. 4.
The dry cryocooler units that are used for cooling the cold mass
structure are at the same time also configured for gradually
cooling the current leads of the two coils 20, 25 by making
appropriate thermal contacts with the first stage and second stage
members. In this way, no dedicated or additional dry cryocooler
units need to be installed for cooling the current leads and hence
no additional openings need to be made in the yoke structure
30.
As discussed above, the cold-mass structure is surrounded by a
cryostat 70 and a vacuum is created in the cryostat to thermally
insulate the cold-mass structure.
FIG. 5 shows a three dimensional view of the cryostat 70, whereas
FIG. 6 shows its integration into the magnetic yoke structure (for
clarity, only the lower part of the yoke 32 and only part of the
return yoke 35 are shown in FIG. 6). This cryostat 70 having a
shape of a hollow cylinder is made of stainless steel and has a
wall thickness of e.g. 5 mm. The pair of horizontally mounted dry
cryocooler units 10, 11 on one side of the cryostat and the pair of
horizontally mounted dry cryocooler units 12, 13 on the other side
of the cryostat are both coupled to the cryostat 70 by means of a
radial cryostat vacuum chamber extension 75. This radial cryostat
vacuum chamber extension 75 houses the first stage member 15 and
the second stage member 16 of a pair of dry cryocooler units. In
FIG. 5, solely the head part of the dry cryocooler 10, 11, 12, 13,
which extends outside or partly outside the return yoke 35, is
visible.
The heavy cold-mass structure, having a weight of about 4.300 kg,
must be supported inside the cryostat 70. For this purpose, tension
links 80, 90 are used, preferably both in the radial direction and
the axial direction. Different types of tension links can be used.
The preferred tension link is formed by a tied rod. As shown on
FIGS. 1 and 5, three radial tension rods 80 and six axial tension
rods 90 are attached to the cold-mass structure as supporting
means. These tie rods are preferably made of Inconel. Radial tie
rods have e.g. a diameter of 14 mm, while the axial tie rods have
e.g. a diameter of 8 mm. From the six axial tie rods 90, three pass
through the upper yoke part 31 and three pass through the lower
yoke part 32. The three radial tie rods 80 pass through the return
yoke 35. For passing through the various parts of the yoke
structure 30, each of the axial 90 and radial 80 tie rods is
mounted partially within a hollow tube 85 that is fixed to the
exterior of the cryostat 70 as shown in FIG. 4 and FIG. 5. These
hollow tubes 85 are part of the cryostat vacuum chamber and are
hence vacuum-tight, just as the cryostat body.
As mentioned above, a vacuum is created within the cryostat 70. To
create this vacuum, a tube connection piece 86 is advantageously
connected to one of the hollow tubes 85, as illustrated in FIG. 1.
A vacuum pump can then be connected to this connection piece 86 for
creating a vacuum inside the cryostat 70. The advantage of this
configuration, where a connection piece 86 is connected to a hollow
tube 85 enclosing a tie rod 80, is that no additional specific
opening must be made in the yoke structure 30 for installing a
pumping tube coupled on one end to the cryostat 70 and on the other
end to a vacuum pump installed outside the magnetic structure. With
this configuration, a hollow tube 86 plays the role of being at the
same time a housing of a tie rod 80 for supporting the cold
mass-structure and a pumping channel for pumping vacuum inside the
cryostat 70.
The present invention has been described with regard to a preferred
embodiment of a magnet structure for use in a synchrocyclotron. The
embodiment described is e.g. capable of providing a magnet field of
about 5.6 T and designed for use in a 250 MeV proton
synchrocyclotron. The dry cryocooler units that are installed
through openings in the return yoke of the magnet structure are
positioned in an essentially perpendicular position with respect to
the central axis 50 of the coils. As discussed above, the dry
cryocooler units are preferably installed at an angle of
90.degree.+/-5.degree. with respect to the central axis 50 and more
preferably at an angle of 90.degree.+/-2.degree.. However, the
detailed description of this embodiment just illustrates the
invention and may not be construed as limiting.
More specifically, in alternative embodiments, the dry cryocooler
units installed in openings of through the return yoke may not have
an orientation perpendicular with respect to the central axis of
the synchrocyclotron 1. Thus, the longitudinal axis of the dry
cryocooler unit may define an angle smaller than 90.degree. with
the central axis of the synchrocyclotron 1, for example an angle of
80.degree.. The invention is of course also applicable to other
kinds of circular accelerators (such as e.g. a cyclotron) and to
other magnet field strengths.
More generally, it will be appreciated by persons skilled in the
art that the present invention is not limited by what has been
particularly shown and/or described hereinabove. The invention
resides in each and every novel characteristic feature and each and
every combination of characteristic features.
Reference numerals in the claims do not limit their protective
scope.
Use of the verbs "to comprise", "to include", "to be composed of",
or any other variant, as well as their respective conjugations,
does not exclude the presence of parts other than those stated.
Use of the article "a", "an" or "the" preceding an element does not
exclude the presence of a plurality of such elements.
REFERENCE SIGNS LIST
01 synchrocyclotron 05 foot 10,11 first pair of cryocooler units
12,13 second pair of cryocooler units 15 second stage member 16
first stage member 17 head part 20 superconducting magnetic coil
(upper coil) 25 superconducting magnetic coil (lower coil) 27
bobbin 28,29 coil supporting plates 30 magnetic yoke structure 31
upper yoke part 32 lower yoke part 33,34 pair of pole parts 35
return yoke 50 common central axis 60 heat shield 70 cryostat 75
cryostat vacuum chamber extension 80 radial tension rod 85 hollow
tube 86 tube connection piece 90 axial tension rod
* * * * *