Note: Descriptions are shown in the official language in which they were submitted.
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GRADIENT CORRECTOR FOR CYCLOTRON
Field of the invention
[0001] The present invention concerns cyclotrons. In particular, it
concerns
isochronous sector-focused cyclotrons having enhanced focusing of an extracted
beam
of energized charged particles.
Tech nnical background
[0002] A cyclotron is a type of circular particle accelerator in which
negatively or
positively charged particles are accelerated outwards from the centre of the
cyclotron
along a spiral path up to energies of several MeV. Unless otherwise indicated,
the term
"cyclotron" is used in the following to refer to isochronous cyclotrons.
Cyclotrons are
used in various fields, for example in nuclear physics, in medical treatment
such as
proton-therapy, or in radio-pharmacy. In particular, cyclotrons can be used
for
producing short-lived positron-emitting isotopes suitable for PET imaging
(positron
emitting tomography) or for producing gamma-emitting isotopes, for example,
Tc99m,
for SPECT imaging (single photon emission computed tomography).
[0003] A cyclotron comprises several elements including an injection
system, a
radiofrequency (RF) accelerating system for accelerating the charged
particles, a
magnetic system for guiding the accelerated particles along a precise path, an
extraction
system for collecting the thus accelerated particles, and a vacuum system for
creating
and maintaining a vacuum in the cyclotron.
[0004] A particle beam constituted of charged ions is introduced into a
gap at or
near the center of the cyclotron by the injection system with a relatively low
initial
velocity. As illustrated in Fig. 3, this particle beam is sequentially and
repetitively
accelerated by the RF accelerating system and guided outwards along a spiral
path
comprised within the gap by the magnetic field generated by the magnetic
system.
When the particle beam reaches its target energy, it can be extracted from the
cyclotron
by the extraction system provided at a point of extraction, PE. This
extraction system
can comprise, for example, a stripper consisting of a thin sheet of graphite.
For
example, IT ions passing through the stripper lose two electrons and become
positive.
Consequently, the curvature of their path in the magnetic field changes its
sign, and the
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particle beam is thus led out of the cyclotron towards a target. Other
extracting systems
exist which are well known to the persons skilled in the art.
[0005] The magnetic system generates a magnetic field that guides and
focuses the
beam of charged particles along the spiral path until it is accelerated to its
target energy.
In the following, the terms "particles", "charged particles", and "ions" are
used
indifferently as synonyms. The magnetic field is generated in the gap defined
between
two magnet poles by two solenoid coils, 14, wound around these poles. Magnet
poles of
cyclotrons are often divided into alternating hill sectors and valley sectors
distributed
around a central axis. The gap between two magnet poles is smaller at the hill
sectors
and the larger at the valley sectors. A strong magnetic field is thus created
in the hill gap
portions within the hill sectors and a weaker magnetic field is created in the
valley gap
portions within the 'valley sectors. Such azimuthal magnetic field variations
provide
radial and vertical focusing of the particle beam every time the particle beam
reaches a
hill gap portion. For this reason, such cyclotrons are sometimes referred to
as sector-
focusing cyclotrons. In some embodiments, a hill sector has a geometry of a
circular
sector similar to a slice of cake with a first and second lateral surfaces
extending
substantially radially towards the central axis, a generally curved peripheral
surface, a
central surface adjacent to the central axis, and an upper surface defining
one side of a
hill gap portion. The upper surface is delimited by a first and second lateral
edges, a
peripheral edge, and a central edge.
[0006] In practice, a particle beam has a cross sectional area. An
objective of
cyclotrons is to produce charged particle beams having a given energy which
are as
much focused as possible (i.e. having a small cross sectional area) The
variations of the
magnetic field created by the succession of hill sectors and valley sectors
contributes to
the focusing of the =beam in a similar way as a light beam can be focused by
lenses.
Upon extraction of the particle beam out of the gap defined between two magnet
poles,
however, the particle beam crosses boundary regions where the magnetic field
loses its
homogeneity, which is detrimental to the focusing of the particle beam. This
is a
particularly sensitive issue because, on the one hand, the particle beam has
its highest
energy at the point of extraction and, on the other hand, it is more difficult
to control the
magnetic field at the peripheral edges of the magnet poles where the magnetic
field
drops rapidly. To enhance the focusing of an extracted particle beam, it has
been
proposed in the art to modify the geometry of the peripheral edges of hill
sectors by
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forming protrusions to said peripheral edges by addition of gradient
correctors. Gradient
correctors are relatively small blocks of steel with respect to the size of a
hill sector,
which are coupled to the peripheral surfaces of the hill sectors. Such
gradient correctors
allow the modification of the magnetic field near the peripheral edges and
thus locally
modify the magnetic field near the peripheral edge of a hill sector to improve
the
focusing of the outgoing particle beam. The use of protruding gradient
correctors has,
however, several drawbacks. First, the volume of the vacuum chamber hosting
the
magnet poles must be increased accordingly, thus requiring more energy and
time to
pump the gases from the vacuum chamber. Second, the overall weight of the
cyclotron
is increased because of, on the one hand, the weight of the gradient
correctors
themselves and, on the other hand, the increased overall size of the outer
walls of the
vacuum chamber and, consequently, the size of the flux return yoke; both
contributing
to a substantial increase of the cyclotron weight. Third, the position of the
protruding
gradient correctors is essential; small deviations of position may yield large
variations
of the magnetic field. Gradient correctors must be fixed manually by a skilled
artisan at
precisely the same position of the peripheral surface of all the hill sectors.
This is of
course, a critical and expensive operation. Fourth, these protruding gradient
correctors
have the effect of deviating the magnetic field outwards, which pulls outwards
the path
of the particle beam towards the peripheral edge of a hill gap portion between
a pair of
opposed hill sectors where the magnetic field loses its homogeneity. This
shift also
leads to a loss of useful magnetic field and thus requires an increase of the
coil current
in order to compensate this loss. It is therefore more difficult and expensive
to control
the properties of the extracted particle beam.
[0007] There therefore remains a need in the art to provide an
isochronous sector-
focused cyclotron allowing the extraction of a more focused and more
predictable
particle beam in an efficient and cost effective manner.
Summary of the invention
[0008] The present invention is defined in the appended independent
claims.
Preferred embodiments are defined in the dependent claims.
[0009] The present invention concerns a magnet pole for a cyclotron
comprising at
least 3 hill sectors and a same number of valley sectors comprising a bottom
surface,
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said hill sectors and valley sectors being alternatively distributed around a
central axis,
Z, each hill sector comprising:
(a) an upper surface defined by:
= an upper peripheral edge, said upper peripheral edge being bounded by a
first and
a second upper distal ends, and being defined as the edge of the upper surface
located furthest from the central axis;
= an upper central edge, said upper central edge being bounded by a first
and a
second upper proximal ends and being defined as the edge of the upper surface
located closest from the central axis;
= a first upper
lateral edge connecting the first upper distal end and first upper
proximal end;
= a second upper lateral edge connecting the second upper distal end and
second
upper proximal end;
(b) a first and second lateral surfaces each extending transversally from the
first and
second upper lateral edges, to the bottom surfaces of the corresponding valley
sectors located on either sides of a hill sector, thus defining a first and
second
lower lateral edges as the edges intersecting a lateral surface with an
adjacent
bottom surface, said first and second lower lateral edges each having a lower
distal end located furthest from the central axis;
2 0 (c) a
peripheral surface extending from the upper peripheral edge to a lower
peripheral line defined as the segment bounded by the lower distal ends of the
first and second lower lateral edges;
characterised in that, the upper peripheral edge of at least one hill sector
comprises a
concave portion with respect to the central axis defining a recess extending
at least
partially over a portion of the peripheral surface of the corresponding hill
sector.
[0010]
Preferably, the recess is generally wedge shaped with a first and second
converging lines (preferably straight lines) extending away from the upper
peripheral
edge, with a converging angle, 0, preferably comprised between 70 and 1300,
more
preferably between 80 and 1100, most preferably 90 5 .
[0011] The recess has a converging portion, away from the upper peripheral
edge,
said converging portion having one of the following geometry:
= a sharp corner forming a triangular recess;
= a straight edge forming a trapezoidal recess; or
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= a rounded edge forming an arched recess.
[0012] Preferably, the upper peripheral edge has an azimuthal length,
Ah, and
wherein the concave portion extends between 3% and 30% of the azimuthal length
of
the upper peripheral edge, preferably, between 5% and 20%, more preferably,
between
5 8% and 15%.
[0013] Preferably, the recess is separated from the first and second
upper lateral
edges. Alternatively, the recess is adjacent to the first upper lateral edge.
[0014] The recess can extend over a portion of the peripheral surface.
[0015] Preferably, the portion of the peripheral surface correspond to a
fraction,
of the height of the peripheral surface measured parallel to the central axis
between the
upper peripheral edge and the lower peripheral line, wherein the fraction,
is
comprised between 25% and 75%, preferably between 40% and 60%, most preferably
between 45% and 55%.
[0016] In order, to have smooth variations of the magnetic field, the
peripheral
surface forms a chamfer adjacent to the upper peripheral edge.
[0017] Preferably, the upper peripheral edge is an arc of circle which
centre is
offset with respect to the central axis, and which radius is not more than 85%
of a
distance from the central axis to a midpoint of the upper peripheral edge,
which is
equidistant to the first and second upper distal ends.
[0018] The number, N, of hill sectors is preferably 3, 4, 5, 6, 7, or 8,
more
preferably N = 4.
[0019] The invention also relates to a cyclotron for accelerating a
particle beam
over a given path comprised within a gap, said cyclotron comprising first and
second
magnet poles such as described above, wherein the first and second magnet
poles are
positioned symmetrically with respect to a median plane normal to the central
axes of
first and second magnet pole forming said gap in between, with hill gap
portion being
formed between two opposite hill sectors and valley gap portions being formed
between
two opposite valley sectors.
[0020] Preferably, the recess of a cyclotron has a first and a second
recess distal
points, said first and second recess distal points being separated from one
another by a
distance L10, and wherein the hill gap portion between a pair of hill sectors
of the first
and second magnet poles has an average height, Gh, and wherein the ratio Gh /
L10 is
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comprised between 5 and 100 %, preferably between 10 and 50 %, more
preferably, 20
and 33 %.
[0021] The
cyclotron can also comprise a point of extraction, located in a hill gap
portion between two opposite upper surfaces of hill sectors of the first and
second
magnet poles, wherein the given path of the particle beam is an outward spiral
path
cycling about the central axis until said first point of extraction whence the
particle
beam can be driven out of the cyclotron with a given energy along an
extraction path,
and wherein the recess is located downstream from said point of extraction
wherein
downstream is defined with respect to the direction of the particle beam, such
that the
extraction path crosses on line of the recess with an angle comprised between
80 and
1000, preferably between, 85 and 95 .
[0022]
Preferably, the cyclotron further comprises a second point of extraction in a
hill sector defining a second extraction path, and comprising a second recess
located
downstream from the second point of extraction, such that the second
extraction path
crosses one line of he second recess with an angle comprised between 80 and
1000
,
preferably between, 85 and 950..
Short description of the drawings
[0023] 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
schematically shows (a) a side cut view and (b) a top view of a cyclotron
according to the invention;
Fig. 2 shows an example of hill and valley sectors of a cyclotron according to
the invention;
Fig. 3 shows a partial perspective view of a half cyclotron and the path of
accelerates charged particles (the outlets for the extracted particles in the
flux
return yokes are not shown for enhancing visibility);
Fig. 4 shows an example of a hill sector according to the present invention
comprinsing a gradient corrector;
Fig. 5 shows an example of lines of a magnetic field with and without gradient
corrector;
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Fig. 6 shows examples of geometries of the concave portion of the upper
peripheral line of a hill sector according to the present invention;
Fig. 7 shows examples of geometries of the recess according to the present
invention;
Fig. 8 shows an example of a magnet pole according to the present invention
having two recesses and two points of extraction
Fig. 9 shows another example of a hill sector according to the present
invention
comprising an improved upper peripheral edge design of a hill sector;
Fig. 10 shows a third example of a hill sector according to the present
invention
and having a recess (a) and a pole insert (b).
Detailed description
Geometry of a cyclotron according to the present invention
[0024] The
present invention concerns isochronous sector-focused cyclotrons,
hereafter referred to as cyclotron of the type discussed in the technical
background
section supra. As illustrated in Fig. 3, a cyclotron according to the present
invention
accelerates charged particles outwards from a central area of the cyclotron
along a spiral
path 12 until they are extracted at energies of several MeV. For example, the
charged
particles thus extracted can be protons, H+, or deuteron, D. Preferably, the
energy
reached by the extracted particles is comprised between 5 and 30 MeV, more
preferably
between 15 and 21 MeV, most preferably 18 MeV. Cyclotrons of such energies are
used, for example, for producing short-lived positron-emitting isotopes
suitable for use
in PET imaging (positron emitting tomography) or for producing gamma-emitting
isotopes, for example, Tc99m, for SPECT imaging (single photon emission
computed
tomography).
[0025] As
illustrated in Fig. 1 a cyclotron 1 according to the present invention
comprises two base plates 5 and flux return yokes 6 which, together, form a
yoke. The
flux return yokes form the outer walls of the cyclotron and control the
magnetic field
outside of the coils .14 by containing it within the cyclotron. It further
comprises first
and second magnet poles 2 located in a vacuum chamber, facing each other
symmetrically with respect to a median plane MP normal to a central axis, Z,
and
separated from one another by a gap 7. The yoke and the magnet poles are all
made of a
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magnetic material, preferably a low carbon steel and form a part of the
magnetic system.
The magnetic system is completed by a first and second coils 14 made of
electrically
conductive wires wounded around the first and second magnet poles and fitting
within
an annular space defined between the magnet poles and the flux return yokes.
[0026] As illustrated in Fig. 1(b) and Fig. 2, each of the first and second
magnet
poles 2 comprises at least N = 3 hill sectors 3 distributed radially around
the central
axis, Z (Fig. 1(b) illustrates a preferred embodiment with N = 4). Each hill
sector 3,
represented in Fig. 1(b) as light shaded areas, has an upper surface 3U
extending over a
hill azimuthal angle, ah. Each of the first and second magnet poles 2 further
comprises
the same number, N, of valley sectors 4, represented in Fig. 1(b) as dark
shaded areas,
distributed radially around the central axis Z. Each valley sector 4 is
flanked by two hill
sectors 3 and has a bottom surface 4B extending over a valley azimuthal angle,
'iv, such
that oth + iv = 360 /N.
[0027] The hill sectors 3 and valley sectors 4 of the first magnet pole
2 face the
opposite hill sectors 3 and valley sectors 4, respectively, of the second
magnet pole 2.
The path 12 followed by the particle beam illustrated in Fig. 3 is comprised
within the
gap 7 separating the first and second magnet poles. The gap 7 between the
first and
second magnet poles thus comprises hill gap portions 7h defined between the
upper
surfaces 3U of two opposite hill sectors 3 and valley gap portions 7v defined
between
the bottom surfaces 4B of two opposite valley sectors 4. The hill gap portions
7h have
an average gap height, Gh, defined as the average height of the hill gap
portions over
the areas of two opposite upper surfaces 3U.
[0028] Average hill and valley gap heights are measured as the average
of the gap
heights over the whole upper surface and lower surface of a hill sector and a
valley
sector, respectively. The average of the valley gap height ignores any opening
on the
bottom surfaces.
[0029] The upper surface 3U is defined by (see Fig. 2):
= an upper peripheral edge 3up, said upper peripheral edge being bounded by
a first
and a second upper distal ends 3ude, and being defined as the edge of the
upper
surface located furthest from the central axis Z;
= an upper central edge 3uc, said upper central edge being bounded by a
first and a
second upper proximal ends 3upe and being defined as the edge of the upper
surface located closest from the central axis;
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= strai
= a second upper lateral edge 3u1 connecting the second upper distal end
and second
upper proximal end.
[0030] A hill sector 3 further comprises (see Fig. 2):
= a first and second lateral surfaces 3L each extending transversally from
the first and
second upper lateral edges, to the bottom surfaces of the corresponding valley
sectors located on either sides of a hill sector, thus defining a first and
second lower
lateral edges 311 as the edges intersecting a lateral surface with an adjacent
bottom
surface, said first and second lower lateral edges each having a lower distal
end
3Ide located furthest from the central axis;
= a peripheral surface 3P extending from the upper peripheral edge to a
lower
peripheral line 31p defined as the segment bounded by the lower distal ends
31de of
the first and second lower lateral edges.
[0031] The average height of a hill, Hh, sector is the average distance
measured
parallel to the central axis between lower and upper lateral edges.
[0032] An end of an edge is defined as one of the two extremities
bounding a
segment defining the edge. A proximal end is the end of an edge located
closest from
the central axis, Z. A distal end is the end of an edge located furthest from
the central
axis, Z. An end can be a corner point which is defined as a point where two or
more
lines meet. A corner point can also be defined as a point where the tangent of
a curve
changes sign or presents a discontinuity.
[0033] An edge is a line segment where two surfaces meet. An edge is
bounded by
two ends, as defined supra, and defines one side of each of the two meeting
surfaces.
For reasons of machining tools limitations, as well as for reduction of stress
concentrations, two surfaces often meet with a given radius of curvature, R,
which
makes it difficult to define precisely the geometrical position of the edge
intersecting
both surfaces. In this case, the edge is defined as the geometric line
intersecting the two
surfaces extrapolated so as to intersect each other with and infinite
curvature (1/R). An
upper edge is an edge intersecting the upper surface 3U of a hill sector, and
a lower
edge is an edge intersecting the bottom surface 4B of a valley sector.
[0034] A peripheral edge is defined as the edge of a surface comprising
the point
located the furthest from the central axis, Z. If the furthest point is a
corner point shared
by two edges, the peripheral edge is also the edge of a surface which average
distance to
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the central axis, Z, is the largest. For example, the upper peripheral edge is
the edge of
the upper surface comprising the point located the furthest to the central
axis. If a hill
sector is compared to a slice of tart, the peripheral edge would be the
peripheral crust of
the tart.
5 [0035]
In an analogous manner, a central edge is defined as the edge of a surface
comprising the point located the closest to the central axis, Z. For example,
the upper
central edge is the edge of the upper surface comprising the point located the
closest to
the central axis, Z.
[0036] A
lateral edge is defined as the edge joining a central edge at a proximal end
10 to a peripheral edge at a distal end. The proximal end of a lateral edge
is therefore the
end of said lateral edge intersecting a central edge, and the distal end of
said lateral edge
is the end of said lateral edge intersecting a peripheral edge.
[0037]
Depending on the design of the cyclotron, the upper / lower central edge
may have different geometries. The most common geometry is a concave line (or
concave curve), often circular, of finite length 0), with respect to the
central axis,
which is bounded by a first and second upper / lower proximal ends, separated
from one
another. This configuration is useful as it clears space for the introduction
into the gap
of the particle beam and other elements. In a first alternative configuration,
the first and
second proximal central ends are merged into a single proximal central point,
forming a
summit of the upper surface 3U, which comprises three edges only, the central
edge
having a zero-length. If a hill sector is again compared to a slice of tart,
the pointed tip
of the slice would correspond to the central edge thus reduced to a single
point. In a
second alternative configuration, the transition from the first to the second
lateral edges
can be a curve convex with respect to the central axis, Z, leading to a smooth
transition
devoid of any corner point. In this configuration, the central edge is also
reduced to a
single point defined as the point wherein the tangent changes sign. Usually,
even in the
first and second alternative configurations, a hill sector does not extend all
the way to
the central axis, the central area directly surrounding the central axis is
cleared to allow
insertion of the particle beam or installation of other elements.
[0038] As shown is Fig. 2, the first and second lateral surfaces 3L are
preferably
chamfered forming a chamfer 3ec at the first and second upper lateral edges,
respectively. A chamfer is defined as an intermediate surface between two
surfaces
obtained by cutting off the edge which would have been formed by the two
surfaces
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absent a chamfer. A chamfer reduces the angle formed at an edge between two
surfaces.
Chamfers are often used in mechanics for reducing stress concentrations. In
cyclotrons,
however, a chamfered lateral surface at the level of the upper surface of a
hill sector
= enhances the focusing of the particle beam as it reaches a hill gap
portion 7h. The
5 peripheral surface 3P of a hill sector can also form a chamfer at the
upper peripheral
edge, which improves the homogeneity of the magnetic field near the peripheral
edge.
[0039] A cyclotron according to the present invention
preferably comprises N = 3
to 8 hill sectors 3. More preferably, as illustrated in the Figures, N = 4.
For even values
of N, the hill sectors 3 and valley sectors 4 must be distributed about the
central axis
10 with any symmetry of 2n, with n = 1 to N/2. Preferably, n = N/2, such
that all the N hill
sectors are identical to one another, and all the N valley sectors are
identical to one
another. For odd values of N, the hill sectors 3 and valley sectors 4 must be
distributed
about the central axis with a symmetry of N. In a preferred embodiment, the N
hill
sectors 3 are uniformly distributed around the central axis for all N = 3-8
(i.e., with a
15 symmetry of N). The first and second magnet poles 2 are positioned with
their
respective upper surfaces 3U facing each other and symmetrically with respect
to the
median plane MP normal to the respective central axes Z of the first and
second magnet
poles 2, which are coaxial.
[0040] The shape of the hill sectors is often wedge shaped
like a slice of tart (often,
20 as discussed supra, with a missing tip) with the first and second
lateral surfaces 3L
converging from the peripheral surface towards the central axis Z (usually
without
reaching it). The hill azimuthal angle, ah, corresponds to the converging
angle,
measured at the level of the intersection point of the (extrapolated) upper
lateral edges
of the lateral surfaces at, or adjacent to, the central axis Z. The hill
azimuthal angle, tzh,
25 is preferably comprised between 360 / 2N 10 , more preferably between
360 / 2N 5 , most preferably between 360 / 2N 2 .
[0041] The valley azimuthal angle av, measured at the
level of the central axis Z is
preferably comprised between 360 / 2N 10 , more preferably between
360 / 2N 5 , most preferably between 360 / 2N 2 . The valley azimuthal
angle av
30 can be equal to the hill azimuthal angle, ah. In case of a degree of
symmetry of N,
av= 360 /N - cth; for example, for N = 4, ay is the complementary angle of ah,
with
av = 90 - ah.
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[0042] The
largest distance, Lh, between the central axis and a peripheral edge is
preferably comprised between 200 and 2000 mm, more preferably between 400 and
1000 mm, most preferably between 500 and 800 mm. For a 18 MeV proton
cyclotron,
the longest distance, Lh, is usually less than 750 mm, and can be of the order
of 500 to
750 mm, typically 520 to 550 mm. The upper peripheral edge has an azimuthal
length,
Ah, measured between the first and second upper peripheral ends, and can be
approximated to, All= Lh x ah [rad].
[0043] The two
magnet poles 2 and solenoid coils 14 wound around each magnet
pole, form an (electro-)magnet which generates a magnetic field in the gap 7
between
the magnetic poles that guides and focuses the beam of charged particles (=
particle
beam) along a spiral path 12 illustrated in Fig. 3, starting from the central
area (around
the central axis, Z) of the cyclotron, until it reaches a target energy, for
example of
18 MeV, whence it is extracted. As discussed supra, the magnet poles are
divided into
alternating hill sectors and valley sectors distributed around the central
axis, Z. A strong
magnetic field is thus created in the hill gap portions 7h of average height
Gh within the
hill sectors and a weaker magnetic field is created in the valley gap portions
7v of
average height Gv > Gh, within the valley sectors thus creating vertical
focusing of the
particle beam.
[0044] When a
particle beam is introduced into a cyclotron, it is accelerated by an
electric field created between high voltage electrodes called dees (not
shown), and
ground voltage electrodes attached to the lateral edges of the poles,
positioned in the
valley sectors, where the magnetic field is weaker. Each time an accelerated
particle
penetrates into a hill gap portion 7h it has a higher speed than it had in the
preceding
hill sector. The high magnetic field present in a hill sector deviates the
trajectory of the
accelerated particle to follow an essentially circular path of radius larger
than it
followed in the preceding hill sector. Once a particle beam has been
accelerated to its
target energy, it is extracted from the cyclotron at a point called point of
extraction PE,
as shown in Fig. 3. For example, energetic protons, 1-1 , can be extracted by
driving a
beam of accelerated ions
through a stripper consisting of a thin foil sheet of graphite.
A Fr ion passing through the stripper loses two electrons to become a
positive, H+. By
changing the sign of particle charge, the curvature of its path in the
magnetic field
changes sign, and the particle beam is thus led out of the cyclotron towards a
target (not
shown). Other extracting systems are known by the persons skilled in the art
and the
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type and details of the extraction system used is not essential to the present
invention.
Usually, a point of extraction is located in a hill gap portion 7h. A
cyclotron can
comprise several points of extraction in a same hill portion. Because of the
symmetry
requirements of a cyclotron, more than one hill sector comprises an extraction
point. For
degrees of symmetry of N, all N hill sectors comprise the same number of
points of
extraction. The points of extraction can be used individually (one only at a
time) or
simultaneously (several at a time).
Gradient corrector
[0045] Fig. 4(a) and Fig. 4(b) show an example of a preferred embodiment
of a
magnet pole for a cyclotron comprising N = 4 hill sectors and N = 4 valley
sectors
comprising a bottom surface, said hill sectors and valley sectors
alternatively distributed
around a central axis, Z, with a symmetry of N = 4. Such hill sector of
cyclotron
according to the present invention comprises a first and second lateral
surfaces 3L, a
peripheral surface 3P and an upper surface 3U such as defined above. The upper
peripheral edge 3up of the upper surface of at least one hill sector comprises
a convex
portion adjacent to a concave portion with respect to the central axis
defining a recess
extending partially over a portion of the peripheral surface of the
corresponding hill
sector. Preferably, The upper peripheral edge 3up of the upper surface of at
least one
hill sector comprises 2 convex portions separated by a concave portion.
[0046] Because of the symmetry requirements of 2n for even values of N and
of N
for odd values of N; discussed supra, the same symmetry must apply to the
presence or
not of a concave portion with respect to the central axis on the upper
peripheral edges of
the various hill sectors. Therefore, the upper peripheral edge of each hill
sector,
preferably, comprises a concave portion 3upc with respect to the central axis
defining a
recess 10 extending partially over the peripheral surface of the corresponding
hill sector
between two convex portions.
[0047] The term "concave" means curving in or hollowed inward. The
concave
portion with respect to the central axis of an edge, is a portion of the edge
curving
towards the central axis. This term is opposed to the term "convex" that means
curving
out of or extending outward from the central axis.
=
CA 2965016 2017-04-25
14
[0048] The position of the recess can either be separated
from the first and second
lateral edges, or adjacent to the first or second lateral edge. Preferably, a
hill sector
comprises at least one recess separated from the lateral edges.
[0049] In prior art cyclotrons, protruding gradient
correctors were used. Protruding
5 gradient correctors have several drawbacks:
= increase of the volume of the vacuum chamber,
= increase of the volume of the yoke, and of the whole cyclotron,
= increase of the weight of the cyclotron,
= difficulty of precise positioning of the gradient correctors which must
be done
10 manually,
= outwards deviation of the magnetic field.
[0050] Using recessed gradient correctors instead of
protruding gradient correctors
has several advantages. First, it allows the reduction of the size of the
vacuum chamber
hosting the magnet poles leading to a decrease of energy required for
evacuating the
= 15 gases from the vacuum chamber and reducing the time of
the gas evacuation. Second,
the overall weight of the cyclotron is decreased because, on the one hand, the
weight of
the hill sectors is slightly reduced instead of being increased and, on the
other hand, the
overall diameter of the inner surface of flux return yoke is decreased. Third,
the position
of the recesses can be precisely manufactured and positioned by numerically
controlled
20 machining allowing the optimization of the angle at which the particle
beam crosses the
peripheral edge of the hill sector. Fourth, when protruding gradient
correctors deviate
the magnetic field outwards, the magnetic field is deviated inwards by
recessed gradient
correctors resulting in an inwards shift of the last cycles of the particles
path, further
away from the peripheral edge of the hill sector, where the magnetic field is
more
25 uniform than close to the peripheral edge. Fig. 4 shows an example of
the lines of the
magnetic field deviated by the recessing gradient corrector (Fig. 5(a)) and
without any
gradient corrector (Fig. 5(b)). It is therefore easier and more predictable to
control the
properties of the extracted particle beam, and particularly the focusing
thereof. This
deviation towards the acceleration area also allows the power fed to the coils
to be
30 decreased.
[0051] Preferably, the upper peripheral edge 3up comprises
a first and a second
recess distal points lOrdp, defining the boundaries of a recess, and which are
defined as
the points where the tangent of the upper peripheral edge changes sign or
presents a
CA 2965016 2017-04-25
discontinuity. The first and second recess distal points are separated from
one another
by a distance L10. The recess also comprises a recess proximal point lOrpp
defined as
the point of the recess located closest to the central axis, Z. The first and
second recess
distal points lOrdp join the recess proximal point lOrpp by a first and second
recess
5 converging edges lOrc.
[0052] The recess depth, H10, is defined as the height of
the triangle formed by the
first and second recess distal points lOrdp and the recess proximal point
lOrpp, and
passing by the recess proximal point lOrpp. The depth of the recess, HIO is
comprised
between 3% and 30%, preferably, between 5% and 20%, more preferably, between
8%
10 and 15% of the azimuthal length, Ah, of the upper peripheral edge.
Preferably, the ratio
of the recess depth, H10, to the largest distance, Lh, between the central
axis and a
peripheral edge of a hill sector, H10 / Lh, is comprised between between 2%
and 20%,
preferably, between 4% and 15%, more preferably, between 6% and 10%.
[0053] The upper peripheral edge 3up has an azimuthal
length, Ah, measured
15 between the first and second upper distal ends 3ude. The first recess
converging edge
10r1 joining the first recess distal point to the recess proximal point has a
length L101
and the second recess converging edge 10r2 joining the second recess distal
point to the
recess proximal point has a length L102. The lengths L101 and L102 of the
first and
= second recess converging edges are comprised between 5 and 30% of the
azimuthal
20 length, Ah, of the upper peripheral edge. Preferably, the length L101 is
equal 40% to
the length L102 (L101 = L102 40%).
[0054] Preferably, the distance L10 between first and
second recess distal points
ranges between 5% and 50%, more preferably, between 10% and 30%, most
preferably,
between 15% and 20% of the azimuthal length, Ah, of the upper peripheral edge.
25 [0055] Preferably, the recess also extends over a portion of the
peripheral surface
3P from the upper peripheral edge 3up towards the lower peripheral line 31p.
The recess
thus extends over the peripheral surface over a fraction, of a height of the
peripheral
surface measured parallel to the central axis between the upper peripheral
edge and
lower peripheral line. The fraction, c, is preferably, comprised between 25%
and 100%,
30 preferably between 40% and 75%, most preferably between 45% and 55%.
[0056] As illustrated in Fig. 6, the concave portion of
the upper peripheral edge can
have any of the following geometries open between the first and second recess
distal
points: (a) a rectangle, (b) a trapeze, (c) a triangle having straight edges
or curved
CA 2965016 2017-04-25
16
(inwards or outwards) edges, (d) an arc of circle, (e) two arcs of circle, (f)
a parabola,
(g) a square, (h) a parallelogram, (i) a polygon, (j) a smooth curve.
Basically, any
geometry determined by numerical analysis can be implemented. For example, in
the
case of a trapeze, the small base can comprise the recess proximal point
lOrpp, and the
large base can be defined by the first and second recess distal points lOrdp.
Alternatively, the small base can be defined by the first and second recess
distal points
lOrdp, and the large base can comprise the recess proximal point lOrpp. A
triangle can
be scalene, isosceles or equilateral. It can also be right, with the right
angle formed at
the recess proximal point lOrpp. In the case of two (or more) arc of circle,
they can be
1 0 curved inwards ((e) right) or outwards ((e) left). The concave portion
is preferably
designed such that one edge thereof intersects the extraction path of a
particle beam
with an angle of 80-1000, preferably 85-95 , substantially 90 .
[0057] Preferably, a recess 10 extends over a portion of the peripheral
surface
parallel to the central axis. Alternatively, it can extend downwards from the
upper
surface with an angle with the central axis, Z. The distance L10 and/or the
height H10
can increase or decrease independently of one another or simultaneously along
the
height of the peripheral surface. The area of the cross-section of the recess
normal to the
central axis, Z, can thus decrease or increase with the distance from the
upper surface.
In a more complex embodiment, the geometry and the area of the cross-section
of the
recess can change over the peripheral surface. The height of the recess can
also vary
over the peripheral surface. Fig. 7 illustrates some examples of geometries of
recesses.
For example, the recess can have a shape of: (a) a prism extending from the
upper
surface parallel to the central axis, (b) a prism extending from the
peripheral surface
normal to the central axis, (c) a (portion of) pyramid, or more complex
volumes
extending over the peripheral surface.
[0058] Preferably, the recess is generally wedge shaped with the first
and second
recess converging edges being straight (or slightly curved inwards or
outwards) lines.
The tip of the wedge corresponds to the recess proximal point and points at
the general
direction of the central axis. The converging angle, 0, at the tip of the
wedge is
preferably comprised between 70 and 130 , more preferably between 80 and 110
,
most preferably 90 5 . The expressions "inwards" and "outwards" used herein
are to
be understood as "towards" or "away from" the central axis, respectively.
CA 2965016 2017-04-25
17
[0059] More generally, the converging portion of the wedge-shaped recess
can
have one of the following geometries:
= a sharp corner forming a triangular recess, corresponding to the wedge
shaped
recess discussed supra;
= a straight edge forming a trapezoidal recessed wedge; or
= a rounded edge wedge forming an arched recess.
[0060] The present invention also concerns a cyclotron comprising magnet
poles as
defined supra. As described supra, a cyclotron accelerates a particle beam
over a given
path until a first point of extraction whence the particle beam is driven out
of the
cyclotron with a given energy. The hill gap portion between a pair of hill of
the first and
second magnet poles of a cyclotron has an average height, Gh. Preferably, the
ratio of
the distance L10 between first and second recess distal points lOrdp to the
height of hill
gap portion Gh, is comprised between 1 and 20, preferably between 2 and 10,
more
preferably, 3 and 5. For example, for a hill gap of height Gh = 20-40 mm, the
distance
L10, can be of the order of 10-100 mm, yielding a ratio L10/Gh which can be
comprised between 1-5, preferably between 3 and 3.5, i.e. Gh / LlO < 1.
[0061] Preferably, a point of extraction is located within a hill gap
portion adjacent
to the peripheral edges of a pair of opposed hill sectors. A recess is located
downstream
from said first point of extraction wherein downstream is defined with respect
to the
direction of the particle beam. The recess 10 is precisely machined with
respect to the
point of extraction and to the extraction path such that the particle beam
intersects the
first converging recess edge 10r1 with an angle of 90 15 . The particle
beam thus
leaves the hill gap portion substantially normal to the magnetic field, which
improves
the focusing of the extracted particle beam. The position and the geometry of
the recess
are determined by numerical computation and/or testing.
[0062] As shown in Fig. 8, the cyclotron may further comprise a second
point of
extraction, PE2, located downstream from the first point of extraction, PE1,
and within
the same hill gap portion of the same pair of opposed hill sectors. The
particle beam can
be driven out of the cyclotron at said second point of extraction with the
same energy as
at said first point of extraction. In this case, the hill sector comprising
the two extraction
points, also comprises two recesses, each located downstream from a
corresponding
point of extraction.
CA 2965016 2017-04-25
18
[0063] Fig. 9 shows an example of a preferred embodiment
of a magnet pole for a
cyclotron according to the present invention. In this embodiment, the upper
peripheral
edge 3up is bounded by a first and a second upper distal ends, and the upper
peripheral
edge of a hill sector comprises an arc of circle 3ac which centre is offset
with respect to
5 the central axis, and which radius, Rh, is not more than 85 % of a
distance, Lh, from the
central axis to a midpoint of the upper peripheral edge, which is equidistant
to the first
and second upper distal ends (Rh / Lh < 85%).
[0064] Preferably, the ratio Rh / Lh of the radius, Rh, to
the distance Lh, is not
more than 75% (Rh / Lh <75%), more preferably not more than 65% (Rh / Lh <
65%).
10 [0065] The aim of having the upper peripheral edge comprising an arc
of circle
= which centre is offset with respect to the central axis is to
homothetically approximate at
least a portion of the upper peripheral edge to the highest energy (= last)
orbit of the
spiral path 12 in a hill gap portion 7h of the cyclotron. By "homothetically
approximate
the orbit" is meant that the arc of circle portion of the upper peripheral
edge and the last
15 orbit of particle adjacent to the point of extraction are both arcs of
circle sharing the
same centre with different radii. The arc of circle is thus approximately
parallel to the
= portion of said last orbit directly adjacent to and upstream from the
extraction point. The
length of the path of the extracted orbit and the angle between the orbit and
the upper
peripheral edge becomes independent of the azimuthal position of the
extracting system
20 (for example a stripper). In consequence, the characteristics of the
extracted beam are
(nearly) independent of the position of the point of extraction.
[0066] Preferably, the arc of circle extends from the
first upper distal end to the
second upper distal end of the upper peripheral edge, thus defining the whole
peripheral
edge of a hill sector and the centre of the arc of circle lies on the bisector
of the upper
25 surface, said bisector being defined as the straight line, joining the
central axis to the
midpoint of the upper peripheral edge.
[0067] Preferably, the peripheral surface forms a chamfer
adjacent to the upper
peripheral edge.
[0068] As described supra, a cyclotron accelerates the
particle beam over a given
30 path until a first point of extraction whence the particle beam can be
driven out of the
cyclotron with a given energy. Advantageously, a hill sector may comprise more
than
one point of extraction, for example, two. The arc of circle portion of the
upper
peripheral edges of two opposite hill sectors with respect to the median plan
MP, of two
CA 2965016 2017-04-25
19
magnet poles are parallel to and reproduce homothetically a portion of the
given path
directly upstream of the first point of extraction. The arc of circle shares
the same centre
as, and is parallel to a portion of the given path over the whole peripheral
edge. The
terms "upstream" and "downstream" are defined with respect to the direction of
the
particle beam.
[0069] When
the particle beam has reached its target energy, it is extracted at a
point of extraction and, it then follows an extraction path downstream of the
point of
extraction. A part of this extraction path lies between the first and second
magnet poles
and is thus still comprised within the hill gap portion and subjected to the
magnetic
field. If the pair of opposite hill sectors comprises a first and a second
points of
extraction, the particle beam can be extracted either at the first or at the
second point of
extraction or at both. The particle beam then follows either a first or a
second extraction
path downstream of he first or second point of extraction. With the circular
geometry of
at least a portion of the upper peripheral edge according to the present
embodiment, the
length of the extraction path comprised within the gap downstream of the first
point of
extraction, Li, and the length of the extraction path comprised within the gap
downstream of the second point of extraction, L2, are substantially equal.
[0070] The
main advantage of having the same length of extraction paths
downstream of the first and second points of extraction is to ensure that the
particle
beam extracted from one point of extraction has similar optical properties as
the one
extracted from the second point of extraction.
[0071] Fig. 10
shows an example of a preferred embodiment of a magnet pole for a
cyclotron wherein the upper surface of at least one hill sector further
comprises:
- a recess 8 extending over a length L8 between a recess proximal end 8rpe
and a
recess distal end 8rde along a longitudinal axis 8r1 intersecting the upper
peripheral edge and the upper central edge; said recess is separate from the
first
and second upper lateral edges over at least 80% of its length, L8, and
- a pole insert 9 having a geometry fitting said recess and being
positioned in, and
reversibly coupled to said recess.
[0072] The term "fitting" means that the pole insert has a general shape
able to be
precisely inserted into and nested in the recess.
[0073] In
prior art cyclotrons comprising pole inserts, the pole inserts were
positioned in a recess machined off a lateral edge of the upper surface of the
hill sectors.
CA 2965016 2017-04-25
Access to such pole inserts is, however, rendered difficult by part of the RF
accelerating
system overlapping the upper lateral edge area. Access to such pole inserts
requires
removing the overlapping part of the RF system first. By positioning a pole
insert on the
upper surface, it can be accessed easily and directly for removal, machining
and re-
5 insertion into the recess. With the present embodiment, it is thus much
easier and
efficient to reach the optimal insert topography yielding the predicted
magnetic field
and particle path.
[0074] Preferably, all pole inserts have the same shape
and are made of the same
material. Preferably, the pole insert is made of the same material as the
corresponding
10 hill sector.
[0075] Preferably, the recess extends along a longitudinal
axis intersecting the
central axis, and it is open ended at both ends and extends from the upper
central edge
all the way to the upper peripheral edge. In yet a preferred embodiment, the
longitudinal
axis intersects the upper peripheral edge at a point located at equal distance
from the
15 first and second upper distal ends, and wherein the first and second
upper distal ends are
preferably symmetrical with respect to the longitudinal axis. For example,
except for the
proximal portion 9p adjacent to the central edge, the pole insert has a
general
parallelepiped shape, as illustrated in Fig. 6(b).
[0076] In the embodiment of Fig. 6(a), the recess extends
to and is open ended at
20 the upper peripheral edge, the distal end of the pole insert 9dc forms a
portion of the
upper peripheral edge. The portion of the upper peripheral edge formed by the
pole
insert is preferably not more than 10%, more preferably not more than 50/s of
the length,
Ah, of the upper peripheral edge. Preferably this distal end forms a chamfer
at the
peripheral surface.
25 [0077] The pole insert is nested in the recess and is reversibly
fastened to the
corresponding hill sector. For example, it can be coupled to the hill sector
with screws.
[0078] As discussed supra, the pole insert preferably has
a prismatic geometry
along the longitudinal axis over at least 80% of its length, L9, excluding the
converging
proximal portion 9p, of length L9p. The ridges between the hill upper surface
3U and
30 the hill lateral surfaces are chamfered, then the corresponding ridges
of the proximal
= portion of the recess can be chamfered too.
[0079] The topography, illustrated in Fig. 6, of the pole
insert upper surface 9U
and/or first and second lateral surfaces 9L can be machined to form grooves
9gu, 9g1
CA 2965016 2017-04-25
21
either transverse, or .parallel to the longitudinal axis, of the upper surface
or of a lateral
surface. The grooves may extend along a straight, curved or broken line.
Alternatively,
holes 9hu, 9h1 can be drilled through the surfaces. The holes can be blind
holes (i.e., of
finite depth) or can be through holes. As explained supra, each hill sector
comprises a
pole insert for symmetry reasons, the pole inserts are thus machined
individually or
aligned side by side and all machined together. The resulting aspect of the
machined
pole insert may differ considerably from its aspect before machining.
[0080] In conclusion, the present invention offers the advantages that
it allows the
reduction of the size of the vacuum chamber and a decrease of the overall
weight of the
cyclotron. Third, the position of the recesses can be precisely manufactured
and
positioned. Fourth, the magnetic field is deviated inwards by recessed
gradient
correctors resulting in an inwards shift of the last cycles of the particles
path where the
magnetic field is more uniform than close to the peripheral edge. It is
therefore easier
and more predictable to control the properties of the extracted particle beam,
and
particularly the focusing thereof.
CA 2965016 2017-04-25
22
Ref # Feature
1 Cyclotron
2 Magnet pole
3 Hill sector
4 Valley sector
Yokes
6 Flux return yoke
7 Gap
8 Recess
9 Pole insert
Recess
12 Spiral path
14 Coils
3ac Arc of circle
3ec Chamfered edge
3L Lateral surface
31de Lower distal end of lower lateral edge
311 Lower lateral edge
31p Lower peripheral line
3P Peripheral surface
3U Upper surface
3uc Upper central edge
3ude Upper distal end of upper lateral edge
3u1 Upper lateral edge
3up Upper peripheral edge
3upc Upper peripheral edge concave portion
3upe Upper proximal end of upper lateral edge
4B Bottom surface
7h Hill gap portion
7v Valley gap portion
8Ir Recess longitudinal axis
8rde Recess distal end
8rpe Recess proximal end
9dc Pole insert distal end chanfered
9g1 Pole insert groove lateral
9gu Pole insert groove upper
9h1 Pole insert hole lateral
9hu Pole insert hole upper
9L Pole insert lateral surface
91p Pole insert proximal portion length
9p Pole insert proximal portion
CA 2965016 2017-04-25
23
9pe Pole insert proximal edge
9U Pole insert upper surface
9pe Pole insert proximal edge
9s Screw =
9U Pole insert upper surface
10r1 Recess converging edge (1st)
10r2 Recess converging edge (2d)
lOrdp Recess distal point
lOrpp Recess proximal point
Ah Azimuthal length of the upper peripheral edge
dh Distance upper peripheral edge - highest orbit
Gh Gap height at hill
Gv Gap height at valley
H10 Recess height
Hh Hill height
Li, L2 Length of the extraction path comprised within the gap downstream of
a
point of extraction
L 1 0 Length between first and second recess distal points
L101, Length of the recess converging edge
L102
L8 Recess length
L9 Pole insert length
L9p Pole insert length of proximal portion
Lh Distance between the central axis and a peripheral edge
MP Median plane
PE Point of extraction
Rh Radius of radial pole contour
Central axis
och Hill azimuthal angle
cv Valley azimuthal angle
