Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to improvements in the extrac-
tive electrode positioned in the vicinity of the particle
source substantially located in the centre of accelerators of
the cyclotron type, this improved extractive electrode making
: it possible to reduce the vertical focusing and defocusing
effects attributable to the hyperfrequency electrical accelera-
tor field created between the "Dees" and the mass of a cyclotron
(or a synchrocyclotron).
;~; In a cyclotron, the beam of particles issuing from
- 10 the particle source situated in the vicinity of its centre is
subjected to a hyperfrequency electrical accelerator field and
to a magnetic field which enable the path followed by the beam
to develop in spiral form in the "Dees".
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, Acceleration is obtained after each rotation through
a predetermined angle 0 by means of a hyperfrequency accelera-
tor field created in the accelerating spaces between the "Dees".
The value of the magnetic field is normally higher
in the central zone of the cyclotron so as to ensure suitable
focusing of the beam during its initial revolutions~ However,
according to the phase of the hyperfrequency electrical fie~d,
- the particles passing through the accelerator space are
defocused or focused by the hyperfrequency electrical field
and may become lost in the walls of the accelerator because
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an excessive focusing or defocusing, resulting in a reduction
in the "effective phase" zone.
- The extractive electrode according to the present
invention makes it possible to reduce the vertical focusing
or defocusing effect which the hyperfrequency accelerator
; electrical field has upon the beam of particles.
, . . .
According to the invention, there is provided an
; extractive electrode intended to be positioned in the vicinity
of the source of a charged particle beam of an accelerator
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;- of the cyclotron type, said particle source being substantially
located in the center of said cyclotron, said accelerator
` comprising at least two electrodes or "dees" located between -
the pole pieces of an electromagnet and means enabling an
accelerating hyperfrequency electrical field to be created
; in accelerating spaces situated between said "dees" for periodi-
cally accelerating said particles, said extractive electrode
J.' being constituted with a metallic block substantially prismatic
in shape and removably fixed to the extremity of one of said
"dees", said block having an entry face facing said particle
source and an exit face, said entry face being provided with
at least a window, for the passage of said particles, said
window opening onto a deep notch formed in said block, sald
notch being only delimited by two curved lateral walls forming
a channel for the passage of said particles, said channel
opening at said exit face onto one of said accelerating spaces,
said hyperfrequency electrical field having in said channel
a component of low value along a line perpendicular to the
plane of the mean path of said particles in such a manner
to eliminate vertical focusing and defocusing effects of a
high value electrical field component.
` For a better understanding of the invention and to
, show how the same may be carried into effect, a description
follows having reference to the appended drawings illustrating,
; by way of example, an embodiment of the invention. In the
drawings:
Fig. 1 illustrates an ion source associated with an
extractive electrode according to the invention.
: Figs. 2 and 3 show two examples of embodiment of an
` 30 extractive electrode according to the invention, figure 3
being on the first sheet of drawings.
Figs. ~ to 10 illustrate further examples of embodi-
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ment of an extractive electrode according to the invention.
An accelerator of the cyclotron type comprises at its
centre a particle source S (for example an ion source), as
illustrated in Fig. 1, this ion source being generally formed
by a metallic tube 1 of narrow cross-section, at the two ends
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, of which a cathode 2 and an anticathode 3, respectively, are
arranged for ionisation of the gas introduced into the tube l.
A magnetic field created along the axis XX of the tube 1 or
"source bo~y" ensures suitable confinement of the plasma
obtained in the tube l. ~n extractive electrode E is arranged
outside the "source body" 1 in front of a slot 4 formed in
-~ the centre thereof. If the particles are ions, they are
extracted from the source l during the negative alternation
of the hi~h frequency electrical field.
Fig. 2 shows one example of embodiment of an extrac-
tive electrode according to the invention. This extractive
electrode i9 formed by a metallic block 10 (for example of
copper) substantially prismatic in shape and comprising an
entry face 12 and an exit face 13 for the beam. The entry
; 20 face 12 comprises a window 14 positioned in front of the
;; particle source S. This window 14 opens onto a deep notch ll
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; which becomes progressively wider and opens at the exit face 13.
This notch ll forms a channel C constituting a passage for
the beam F of particles, this passage C being defined by lateral
walls 15 and 16, the form of which is determined by the
; characteristics of the beam and the magnetic field. The
metallic block lO forming the extractive electrode according
to the invention may be mechanically fixed to the edge of one
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of the "Dees" 17, for example by means of screws 18 (Fig. 2).
`~ 30 The channel C is open at its upper and lower ends so as to
eliminate the vertical focusing and defocusing effects to which
the particle beam is normally subjected, these focusing and
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defocusing effects being due to that component of the hyper-
, frequency electrical field which is perpendicular to the plane
, of the mean trajectories of said beam. From this point of
view, the channel C will be more effective, the smaller its
width in relation to its height.
- To enable the extractive electrode to be placed as
close as possible to the source S, the entry surface 120 of
the block 100 may be convex in shape, as illustrated in Fig. 3. `
Fig. 4 shows another example of embodiment of an
extractive electrode according to the invention. This elec-
trode formed by a block 101 comprising in its entry face
30 two windows 31 and 32 opening onto two channels Cl and C2
separated from one another by a median element 33, these
two channels Cl and C2 becoming progressively wider and
opening at the exit face 34. These channels Cl and C2 allow
the passage of two particle beams corresponding to two possible
operations of the cyclotron at frequencies of different
harmonic orders. The form of the median element 33 is defined
- by the characteristics of the beam and the accelerator.
- 20 In another example of embodiment shown in Fig. 5,
, the block 110 forming the extractive electrode is formed with
two channels C3 and C4 separated by a displaceable median
element 35 enabling the width of the channels C3 and C4 to be
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varied. In this embodiment, the median element 35 has two
rods 36 and 37 integral with the block 110 extending through
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i~ it. These two rods 36 and 37, which are parallel to the plane
`- of the mean trajectories of the beam, enable the median element
35 to be guided during its positioning relative to the lateral
- walls 115 and 116. One of these rods, for example the rod 36,
acts as a guide whilst the rod 37 is screw threaded and enables
the median element 35 to be gradually displaced. The Figures 6
and 7 show in detail two modified embodiments of the displace-
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1~573~8
able median element 35. The end of this median element 35
s:ituated at the entrance of the extractive electrode is formed
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, with two shoulders 38 and 39 which enable the median element
35 to engage on one hand in an aperture 40 divided in two
parts by the median element 35 to form the windows 31, 32,
and to slide on the other hand, along the upper and lower -
edges 41 and 42, respectively, of this aperture 40 (Fig. 5).
The median element 35 may be kept in the selected position
by means of a plate P fixed to the end of the median element
; 10 35 comprising the shoulders 38 and 39, the edges 46, 47
(Fig. 6) of the plate P resting on the entry face 103 of the
extractive electrode (Fig. 5). This plate P (Fig. 6) has a
flat outer surface 43 corresponding to the surface directed
towards the particle source S. This plate P may have a convex
outer surface 44, as illustrated in Fig. 7, or may have a
projecting outer surface 45 trapezoidal in shape, as illustrated
in Fig. 8. The choice of the shape and dimensions of the outer
surface 43 to 45 of the plate P makes it possible to obtain
a vertical focusing component of the required value, this
focusing component being attributable to the hyperfrequency
electrical field prevailing between the source 1 and the "Dee"
; in the vicinity of the source S.
In another embodiment, the median element 35 may
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have lateral surfaces 50 and 51 as shown in Figs 9 and 10,
these lateral surfaces 50 and 51 being able to apply with
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precision either to the lateral wall 52 or to the lateral wall
53 of the notch 11 in the block 111. The channels C4 as
- shown in Fig. 9, or C5 (Fig. 10) thus obtained would be
; well-defined.
The channels C4 and C5 allow the passage of two
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; beams Fl and F2 coming from one and the same source S, but
~ corresponding to the operation of the cyclotron at two harmonic
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frequencies for example.
The extractive electrode according to the invention
may be used with advantage.
- in cyclotrons comprising "Dees" of low height (in
this case, the vertical focusing and defocusing effects -
attributable to the hyperfrequency electrical field are signi-
ficant in the two first passages of the particles between
the accelerating spaces and the extractive electrode enables
these effects, which are harmful to the smooth operation of
the cyclotron, to be corrected,
; - when the cyclotron is intended to operate at a
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, high harmonic order (heavy ion cyclotron for example). In -
this case, the magnetic field "hump" contributing to the
" vertical focusing of the beam in the central zone of the
cyclotron should not be too large so as to limit the phase
drift which is proportional to the amplitude of this "hump" -
, and to the harmonic order. The use of the extractive electrode
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ii according to the invention by which the focusing and defocusing
effects attributable to the hyperfrequency electrical field
can be made very weak, eliminates the need for a significant
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increase in the value of the magnetic field in the accelerating
space.
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