CA2071209A1 - Sectorally focused cyclotrons - Google Patents

Abstract

Magnetic sectors known as ridges for focusing a beam are merged into a single device with the accelerator system, known as a high-frequency resonator, through a suitable choice of design and size. These ridges (1) are designed for high-frequency electromagnetic resonance so that the alternating voltage necessary for particle acceleration can develop.

Description

~ 20~209 WO 91/0'78~4 - 1 - PCT/BE90/00067 ... .
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.
`:
.
, ",;
, .,-. - , . . . I - ; , . . : .

.` : ~ , ,: . .

.'; . W0 gl/078~4 2 ~ 20 7 12 ~ ~C~rlBE90/00067 ~~ 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~
9~)h/Zt)t)~ 110.3S ~ s~r)0~ ZfiZ~ Zb~91b~ Sl:SI Zfi~LZ~L

. .. .
'. ~
.

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 9t~/~ 110~ S~3!:)0~ Z6Z~ Zb~9lb~ 9l :SI Z6/LZ/L(~
, . , .. , ~.. - .
~ . ~ - ; . :, :

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~.
`

Sh(3/b~ L~ .L LO:~S ~ ';213r~0~1 7,6Z~ o~ 91~ SI ~6/LZ/L(I

- .: J-' ~
,, ' : ;. ~ :

~ ~wo ~l/07a64 ~ 0712 09 PCT/BEgOtOo067 `^ 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.

S~ S~ 110~ !)0~ Z~Z~ Zb~9tb~ Ll :SI Z~/LZ/L0 .
.. . ~ . :: :
::
~, ,."~ ~

,: .: ::' ,: , : : , ' .

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.

, .
- : : ~ :~. :
:' ::'" '. ~ ~ '- ' ,, .

.

Claims (6)

1. Sectorally focused cyclotron of the superconduct-ing or non-superconducting type, comprising essentially a magnetic structure consisting of:
- two base plates (2 and 2') called yokes;
- at least two upper magnetic sectors (1 or 9) and a same number of lower sectors (1' or 9') called ridges, situated symmetrically with respect to a median plane (7), the upper and lower magnetic sectors being separated by a small air gap, - a number, equal to the number of ridges, of sector-shaped spacings (11), called valleys, where the air - gap is of larger dimension, - a flux-return path (3) rigidly joining the lower yoke (2) to the upper yoke (2'), and;
a vacuum enclosure (5) surrounding the ridges (1 or 9) and the valleys (11);
characterised in that the frequency used for accelerating the particles, which is equal to the frequency of rota-tion of the said particles in the cyclotron or to a multiple of this frequency, is equal to the electro-magnetic resonant frequency, greater than 10 MHz, of an assembly formed by at least one pair of ridges (1) which is separate from the other pairs of ridges (9) and from the vacuum enclosure (5) and which constitutes a resonator of the coaxial line type with short-circuited ends, this frequency being such that the associated wavelength is greater than four times the depth of the valleys (11) measured from the median plane (7), so that this assembly of pair(s) of ridges (1) at one and the same time secures the focusing of the particles and serves as accelerating system for the said particles.

2. Cyclotron according to Claim 1, characterised in that the wavelength associated with the accelerating frequency lies between five times and ten times the depth of the valleys (11) measured from the median plane (7).

3. Cyclotron according to either one of Claims 1 or 2, characterised in that the two ridges (1) situated sym-metrically wish respect to the median plane (7) and intended to act as a resonating accelerating electrode are electrically connected to one another (10).

4. Cyclotron according to any one of the preceding claims, characterised in that an adjustable capacitance is provided between the pair of resonating ridges and an electrode at ground potential.

5. Cyclotron according to any one of the preceding claims, characterised in that it possesses four pairs of ridges (1 or 9), of which two pairs of ridges (1) are of the resonant type and thereby secure the acceleration of the particles, the other pairs of ridges (9) being connected electrically to the vacuum box (5) and hence not resonating.

6. Cyclotron according to any one of the preceding claims, characterised in that the angular distance between two acceleration gaps which are the gaps situated between two ridges acting as high frequency resonator, can be equal to the angle separating two successive valleys.