Note: Descriptions are shown in the official language in which they were submitted.
~ 20~209
WO 91/0'78~4 - 1 - PCT/BE90/00067
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CTORAIJLY FOCUSED CS~CLOTRONS
Subject of the invention
The present invention relates ~o cyclotron~ in
which the particle beam is focused sectorally. More
particularly, the present invention relates to iso
chronous cyclotxons comprising a magnetic circuit con-
sisting of at least two sectors called "ridges" in which
the air gap is narrow, separated by sector-shaped spac-
ings called "valleys where the air gap is of greater
dLmension.
The present invention relates to both supercon-
ducting and non-superconducting cyclotron~.
State of the art
Cyclotrons are particle accelerators used in
particular to produce radioactive isotopes.
Cyclotrons are usually made up of three main
; separate assemblies consisting of the electromagnet, the
- hi~h-frequency resonator and the vacuum enclosure with
PUm~?s .
The electromagnet secures the guiding of the ions
over a path representing approxLmately a spiral of
increasing radius, during acceleration. This magnet may
`- use conventional or superconducting windings.
In modern cyclotrons, known as isochronous or
sectorally focused cyclotrons, the poles of the electro-
magnet are divided into sectors having alternately a
narrow air gap and a larger aix gap. The e~fect of the
azimuthal vaxiation in the magnetic ~ield which re~ults
from this is to secure the vertical focusing of the beam
during acceleration.
The accelerating electrodes (often called ~dee~
for historical reasons) are intended to accelerate the
particles in rotation in the cyclotron. ~n alternating
voltage of several tens of kilovolts i3 applied to the
electrodes, at the frequency of rotation o~ the particle~
in the magnet or, alternatively, at a frequenc~ which is
- an exact multiple of the frequency o~ rotakion o~ the
particles in the magnet, thi~ having the e~fect of
accelerating the par~icle beam~ revolving in the machine.
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~~ In order to be able to g~n~.r~te high volt~g~0 ~
very high f~equencle~ on the accel0r~king electrodas, it
~ nerally ~lt tO be necee~A~y to conneCt ~he
- acceleratlng electrode to the gro~nd ~l~ctric ~otential
S through an electrlc circuit elemont having an inductive
cha~acter. The as~embly ~urni~hed by the acceler~ting
el~ct~ode, whlch po~e~ses a c~p~cltiv~ aspect ln rela-
tlon to ground, and to the inductive elem~nt thus
~escribed, form~ a re~onan~ electromagne~lc cl~cult,
10 po~seBs~ng a high magnifica~i~n factor. Th~ ~8e~bly khus
formed i~ called the "hig~ frequency resonator" or
"rc60nant accelerating electrodes".
Finally, the act~on of pump8 enables a high
~acuum to ~e obtaincd ln the he~metic enclosure surround-
lS ing the rid~es Hnd tho hlgh frequency reson~tors, and
:. con~quently called ~h~ vaCuUm enclo5ure.
A partlcularly favourable embodiment of a
~ EP hc~clotron is de6c~lbed in European pAtRnt ~ppllcatlon No.
/\ ~, in which the air gap of the sector~ called
~idges ~ B narrowe~ to a value close to th~ ~iize of the
accele~ated beam, where~s ~he alr ~ap of the ~cter8
cAlled valley~, which ~eparate ths ridge~, i8 very l~rga
80 that the m~gnetic f ield there is appro~cimAtely z2ro .
In thi~ pxeferred embodiment, the hlgh freguenc~
- 25 reson~tor~ are in~talled ln two oppo~ite valleys. The~0
h~gh~frequency re~onatlng s~tem~ are made up o~ the
acceler~ting olectrode~ and of electrlcally con~ucting
vertical plllars connecting the electrode~ to ~round and
~o~m~ng, with the electrodes, ~ elec~romagnetlc circui~
~Bonat~ng at a multiple 0~ ~he f~e~u~ncy selected for
th~ ~Gceler~ting o~ th~ b~am.
~i~B of ~Çhq invention
. Tho pre~ont in~ent~on aim~ to ~u~nish a ~l~pl~
~: and ~ore economic~l conetruction of 8 sec~orAlly focu~d
cyclo~ron.
The pro~ent inventlon also alm~ to enhanc~ the
reliabil~ty of the cyclo~on owing to it~ ~impl~fisd
design.
A cornplementaxy a.im i~ to enhance th~ e~iciency
.i~
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2~712~9
0 91/07a64 - 3 - P~T/~E9o/ooo67
of the ~cceleratiny of the beam th~ough ~n appropri~t~
choice of variou~ parAmeter~.
Other aims and advantage~ wll~ em~rge ln the
descrlption which follow~,
S Main ch~r~cteri~ element~
The present invention con~i~t~ in mergin~ into a
single d~lce, magnetic 3ectors, c~lled ridges, securing
the focuR~n~ of th~ beam, with th9 ~cel~tor ~ys~em,
known ~s tho hiyh-fxequency reson~tor, through a suit~bl~
cholce of their configuration ~nd d~enslons.
Th~e rldge~ are dLmensioned 50 a~ to exhihit a
high-frequency el~ctrom~gnetic resonance, thus per~itting
the devQlopmen~ of alternating volt~geq ~e~uired for
acceleratlng th~ particles.
Hi~h f~quen~y i8 undex~tood as values great0r
X than 10 ~ega ~ .
In particular, ~h~ effect deslred according to
~h~ inventlon can ~e ~chieved throu~h the following
arrangementss
` 20 - the two ri~ge6 6epara~ed symm~trically with re~pect to
the median plane ~nd intended to act as re~on~tins
accelera~ing electrode should be sub~tantlally
separate from the rid~es neighbour1ng the vacuu~
chamber. They are connected to th0 ground potontial
2~ only at their base, on the ~ide furth~t from the
med1an plane. ~heeo ~wo ridge~ may be electrically
connectsd to o~e ano~her (but in ~uch a way A8 not to
inter~ere wlth the acceler~ting of the particle~).
Thi~ la~t connection, ~ecurirlg parfec~ ~ynunetry of th~
r~diofrequency volt~gefi on elther slde of the m~di~n
I plane, i~ recommended but L~ not howeYer ind1~pen~able
to achieve the desired effect of the invention.
- The ~elecked ~c~elexa~ng requency, which i~ ~n exact
`` mul~lple o~ the frequency of rotatio~ of the ion~ ~n
~5 the cyclotron, muat be ~uch that the a~ociated
wavel~ngth is g~eat~r than four timo~ the depth o~ th~
Y~lley~. A partlcul~rly ~vourable o~or~tion i~
obtained when th~ wavel~gth a~oclated wlth the
~cceleratlng frequency lie~ between ~ive t~me~ ~nd ten
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W0 91/07~64 ~ 4 ~ 7 ~ % 09 PCT/BE90/OU~67
: t~me~ the depth of th~ vall~y~.
- An ad~u~tahle c~pacitance is provided for between the
pair of re80n~ting ridges and an eLect~ode ~t ~round
- potential (~or example th~ vacuum box) so ~ to ~ecure
- 5 the f ine ad~u~tment o~ the r0~0nant frequency o~ the
: re~on~or.
Brlef description of the f iqures
- Fi~ure 1 showa a ~ection~l view through the medl~n
pl~ne of the cyclo~ron ~caorcling to the ~nven'cion.
- Fi~ure 2 ~how~ ~ radial sectional ~i~w along line A-A
: of F~gure 1.
Flgure 3 show~ ~n ~imuthal ~ectional view ~long line
B-B of Flgure 1.
~e3cription of a ~refçrred e~bodiment of the cyclotron
- 15 ac~or~ina ~o the i~vention
; The cyclotron lllustrated diagrammatic811y in
Figures 1, 2 ~nd 3 ie a cyclo~ron i~tended for ~ccele~at-
` lng proton~ up to an ~ne~gy of 250 ~eV.
The ~agnetic ~tructure of the cy~lotron is made
up of a certain n~nber of elemen~s made of a ferromag-
netic material ~1, 2, 3 and 9~ and of coil~ 4 made o~ a
preferably conductlng ~r ~uperconducting~ matQrial.
The fer~emagnetic struc~ure con~l3t3 of:
- - two base plat~ 2 and 2'~ zalled yoke~;
2S - at lea~t two uppar ~ectors, called ridges 1 And~or 9,
end Sho ~ame number of lower ~ectors 1 ' a~d/or 9'
X situated symmetrlc~lly with the upper 5ector~ ~and/or
9 with re~pect to the ~mmetry plan~ 7 known a~ the
median plane, ~nd ~ep~r2ted by a ~all air gap;
betwe0n e~ch ridge 1~ si'cuate~ A gap where the alr gap
i~ o~ g~eater ~ize, called a "vall~y~ 11 and 11 ~;
- . at lea~t two f lux-return p~th~ 3 rigidly ~ oining the
lower yeka 2 to the uppe:r yok~3 2 ' .
he coil~ 4 are of essentially circular ~hape and
ar~ located in the ann~llar gap left between the se tor~
i 1 and 9 ~nd the flu~-return p~th~.
The~e c:o~l~ m~ be mat~ from a ~uperconductlng
~; material, howev~r in thi~ ca~e provi~ion w111 h~ve to be
, -, m~de ~or the nece~y cxyc~genlc ~ppliance~.
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`^ The c~ntral duct 8 ia intended to r~eive, at
lea~t in part, th~ sourco o~ partlcles to be ~ccelerated
whlch are in~ected ~t th~ centr~ of the apparA~u8 ~ by
m0~no knewn per ~e.
5In ~n e~pe~i~lly pr~erred ~mbodlm~nt of ~he
inv~ntLon, tha cyclotron ad~nt~geoualy poBs~se~ 4 pa~s
of rldge , of which 2 palrs 1 and 1' ~re ef the r~sonant
typ~ and thereby secu~Q the ~cc~lar~ion.
The other two p~irq of ridge~ 9 and 9' are
10electrically c~nnected to the vacuum box S and hence do
no~ resonate.
The mean magnetic fleld at the extractlon radlu~
i~ 2.4 Te~l~. The corresponding ~requency of rotation of
the p~oton~ ~ ~ about 30 ,106 rotationR per ~econd. The
15~elected acceleratlng frequency is twice a~ la~e, namely
X 60 m~gac-~ , the coxresponding w~velen~th
baing 4 . 62 m.
The dopth o~ the v611eys 11 and 11', me~ured
from the med~ an plane, 1~ 55 ~m.
20In this instance, obtalning re~vnance at 60
X ~ ~ ~4~d requlre~ tha addltion of an ext
c~pacitance of 120 picof~rad~ per re~onator a~embly.
Pa~t of thi~ c~pacitance re~ults from the a~oining of
electrodes aldlng the accelerating of the bo~m at the
25centre o~ the machlne and the rem~lnd~ provided by
variable capaci~or ~ in8~alled in ths median plane 7
~etween the reaonator 1 and ~h~ ~cuum ~ox 5.
The electrical connections 10 between the
resonating ridges pe~mlt the ~ecuring ~f perfec~ ~ymmetry
I ~0of tho r~dlo f~equency volt~ges either ~id~ of th~ median
¦ pl~ne 7.
It is po~ le to re~ard the rid~s thu~
de~cr~bed, s~paxate from the other ~id~ nd ~xom the
cuum pump~, as constltuting the conductor of a coaxi~l
35tXAn~mi~iOn line whoee out~ conductor conqist3 of the
v~cuu~ box and tha other ridges. It iB po~ible ~o rogard
the coax~al line ~egmen~ thus formed a~ being ~hort-
ci~cuit~d a~ it~ two ~nd~, wher~ th~ ridges A~e ~tt~ch~d
to ~he h~e p1~te~ o~ the cyclotron.
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WO 91/07864 2 ~ 7~ 2 0 9 pcT/BEgo/ooo67
It is well known in electromagnetism that such a
coaxial line thus short-circuited at its two ends ex-
hibits electromagnetic resonance at a req~ency such that
the associated wavelength is equal to twice the length of
the segment of the short-circuited coaxial line.
For this electromagnetic resonance, the voltage
maxLmum is observed in the middle of the line, namely in
the cyclotron, at the level of the median plane where the
particles are accelerated.
Finally, it is well known in electromagnetism
that the abovementioned resonance can be displaced to a
lower frequency, by positioning an extra capacitance
be~ween the inner and outer conductors of th0 coaxial
line, preferably at the middle thereof.
Mareover, the angular distance between two
acceleration gaps (gaps situated between two ridges
acting as high-frequency resonator) may advantageously be
the angle separating two successive valleys, whereas in
conventional configurations the maximum angle between two
accelerating gaps has to be less than the angle of a
valley.
This permits the use of larger angles between the
acceleration gaps, this permitting in certain instances
much more efficient acceleration of the beam.
In the case of the a~ovementioned geomet~y, to
obtain an alternating voltage of lOO kW re~uires a radio
frequency power of just 30 kW per resonator.
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