The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

COMPACT ACCELERATOR DEVELOPMENT AT S-LSR
A 。Noda, H。Fadil, S. Fujimoto, M. Ikegami9Y. Iwashita, T. Shirai9M. Tanabe，H. Tongu
ICR, Kyoto University，Uji-city，Kyoto 611-0011，JapanK。Noda, S., Shibuya, T. Takeuchi，S。Yamada#，NIRS，Inage-ku9Chiba, 263-8555, Japan
M. Grieser$，MPI, D-69029, Heidelberg，Germany
E。Syresin*9 1419805Dubna, Moscow Region，Russia
Abstract
At ICR, Kyoto University, a compact ion
accumulation/cooler ring S-LSR with the circumferenceof 22.557 m is now under construction in collaboration
with NIRS. Possibility of realizing laser produced 12C6+
beam with 2 MeV/u in the energy bin of ±0.1% is to bestudied by combination of phase rotation and electroncooling. Trial for downsizing of the accelerator byadopting compact sizes in electron cooler andconventional equipments such as beam monitors is also to
be evaluated experimentally at S-LSR.

INTRODUCTION
Usage of the accelerators for various applications thanbasic physics has recently become more and more
important. Carbon therapy, for example, has been foundto be very effective to cure lung and liver cancers within arather short treatment time (less than a week) due to its
high radiobiological effectiveness (RBE). So as to realize
widespread use of such facilities, downsizing of thefacility is inevitable. From this point of view, Advanced
Compact Accelerator Research Project has been promoted
by Ministry of Education, Culture, Sports, Science andTechnology since 2001 [1]。One of the major items ofsuch approach is utilization of very high electro-magnetic
field created by a high power laser for beam acceleration.
Phase space manipulation through phase rotation in thelongitudinal phase space and beam cooling have been
main scopes of the collaboration between ICR, Kyoto
University and M RS。Collaboration with APRC at JAERI
Kansai Research Establishment on laser ion production
has also take a very important role in the project. In thepresent paper, however, approach for the downsizing of
the accelerator in connection with the construction of SLSR is presented due to limit of available space.
Electron Beam Cooler

MATCHING IN PHASE SPACE
It is attractive to be able to attain ion beam up to several
MeV/u directly by irradiation of a high power short pulselaser. The beam divergence, however, is very large due tothe focusing condition of the laser to attain the laser
power density higher than 1018W/cm2. The energy
spectrum is also very wide (~100%) and we aim atcollection of ions in the energy bin of ±5% around thecentral energy of 2 MeV/u. Rotations of the laserproduced ion beam in phase spaces both in longitudinal
and transverse directions are needed.

Longitudinal Phase SpaceThe ion production by ahigh-power short-pulse laserhas been reported from USA
and Europe [2],[3]. The energy
spectrum, however, amounts aslarge as 100 %, which causes
serious limitation for realapplication of such a beam. So
as to cope with such situation,
we have proposed such ascheme to select the ion beam in
the energy bin of ±5%，which is
reduced to ±1% by the phaserotation in the longitudinal
phase space with use of a
debuncher synchronized to thepulse laser. In Figure 2, the Figure 2. Phase Rotation
completed phase rotation cavity,
Cavitywhich is two gap quarter waveresonator, is shown. The ion beam is further reduced in
fractional energy spread another one order of magnitude
by the electron cooling in a cooler ring in order to matchwith the rather small energy acceptance of the short pulse
synchrotron with the high peak magnetic field ( 〜 3 T) [4]。
In Table 1，program of the reduction of energy spread ofthe laser-produced ion beam is listed up.
Table 1 :Energy Spread of the Laser-Produced Ions
(Design Specifications)Figure l:Scheme of the Compact Ion Synchrotron forCancer Therapy

At theProduction
TargetAfter PhaseRotation

After Electron
Cooling±5%±1%±0.1%

The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

Transverse Phase Space

cool beam [7] because cooling force given by the relation,
The high-power short pulse laser is focused into a small
size ( 〜10ドm in radius) and thus the laser-produced ion
beam has a small size but a rather high divergence. So as所) =_綠
胸抛を、( i )
to utilize such a beam as an injection beam for the RF
decreases according to the increase of ion-electron
accelerator, it is also needed to rotate such a divergent
relative
velocity: u (Lc is the Coulomb logarithm and ne
beam in the transverse phase space and make a almostandme
are electron density and electron mass,parallel beam. One possible scheme for the transverse
respectively). In order to improve this situation, we havephase rotation is utilization o f a solenoid because o f its
proposed the scheme to sweep the relative velocity by
merit as can equally rotate both in horizontal and verticalchaging the ion energy with use of an induction
directions The scheme， however, needs rather strong
accelerator (Figure 3(a)) or electron energy by changing
solenoid field realized only by superconducting techniquethe high voltage applied for the electron GUN (Figureand it needs some distance from the laser-irradiation3(b)). Feasibility of such schemes has already beentarget [5].
A scheme to install compact and strong
demonstrated by the experiments utilizing TSR for thequadrupole magnets made of permanent magnets justinjection beam from the MP Tandem and heated in thedown stream o f the target inside the vacuum vessel is alsolongitudinal or horizontal direction [8]. The cooling time,investigated as the alternative [6], which needs careful
however, largely depends on the transverse beam size andstudies about magnetic force penetrating outside andexperiments with real laser-produced ion beam isstrength flexibility。It is urgent requirement to establish
inevitable, which we plan at S-LSR. In Figure 4? thethe ion optics to guide the laser-produced ion beam to thecompact electron cooler to be installed in a straightphase rotation cavity and downstream momentum
analysing magnets for evaluation o f energy spread, whichis to be investigated at JAERI, Kansai from now on 。
Based upon the results obtained there, improvement ofsuch ion optics for phase rotation both in longitudinal andtransverse phase spaces will become one o f the major
research scopes at S-LSR 。

BEAM COOLING
Electron Cooling
For the application of laser-produced ion beam to rather
precise experiment or injection to the following
synchrotron, further reduction of energy spread isinevitable, which is expected to be accomplished by anelectron cooling in our scheme. The electron cooling,
however, has been believed to be effective only for rather

Figure 4 Layout of the Electron Cooler for S-LSR 。
\ of the vacuumin the dipole andquadrupole magnets

W
urr/jcmlBlNlctirici^Xfu^nci

rigure 5 The position relation of
the electron cooler inthe straight section 。

section, 1.86 m in length,
is shown. In Figure 5 ，very tight situation of
the installation of theelectroncooler
isillustrated. Further thedrift tube for electron
beam in the central
solenoid
sectionisdesigned to be an
elliptical shape so as tomatch with the troid
radius as short as 0.25 m。

Laser Cooling(b)
Figure 3 Schemes of electron cooling of hot ion beam,(a)ion energy sweep with use of induction accelerator,(b)electron energy sweep by ramping the highvoltage to accelerate the electron.

Utilizing S-LSR the 3-dimensional laser coolingproposed by H. Okamoto et al.[9] is also to be applied for
4Mg+ ion with kinetic energy o f 35 keV. At S-LSR, thefollowing new approach towards the crystalline beam isto be applied.

The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

Control of Dispersion Function in the Dipole
If the crystalline beam becomes 3 dimensional and thecooling force created by the CW laser drives all the ionsto a certain energy, the shearing force will be applied tothe beam because of the arc length difference. So as toavoid this difficulty, superposition of the electric anddipole magnetic fields is proposed [10]. Detailed designof the electrodes to be installed inside of a rather limited
gap of 70 mm of the dipole magnet has been going on[11]. The vacuum chamber in the dipole is now underconstruction including the movable electrodes to provide
such an electric field in case of beam crystal mode [12] asshown in Figure 5 [13]。 It is expected the dispersion
function in the dipole section can be controlled byadjusting the strength of the electric field relative to themagnetic field. Such scheme is also expected to makeenergy difference almost linear to the horizontal
displacement of the beam from the central orbit by theacceleration and deceleration due to electric potential
difference at the entrance and exit of the dipole section,
which seems preferable for attainment of 3-Dcrystalline
beam [14].

going on [16]. The
beamposition
monitors to control
thebeamorbit
distortion
have
beeninstalledinsidethequadrupole magnet
as shown in Figure
6 in order to reducethe circumference Figure 6。Beam Monitor Installedin the Q-Magnet[17]。

CONCLUSIONSApproach to compact accelerator at S-LSR can belargely divided into two. One is the challenge to use thevery high electro-magnetic field produced by a highpower laser. Basic structure of such a scheme will be
fixed by the test experiments just started using 100 TW,
20fsec laser at APRC, JAERI Kansai ResearchEstablishment. At S-LSR, further improvement o f thephase rotation system is planned together with electron
cooling o f laser-produced “hot” io beam。 The other ispursuit o f the limit of compactness using conventional
accelerator technology. Such approach has become
possible owing to recent rapid advance in computerscienceas3-dimensional
electromagnetic
fieldscalculation. Circumference economy as installation of theposition monitors inside the quadrupole magnets has also
been investigated, which will be directly applicable to theconventional accelerator.

IREFERENCES■Figure 5. Vacuum chamber in the dipole and quadrupolemagnets.

APPROACH TO COMPACTNESSAt S-LSR, ultimate limit of compactness attainable inthe conventional accelerator has also been pursued. As isabove mentioned, the size of the electron cooler isreduced as small as possible keeping the length of theeffective cooling region of 0.5 m.
In addition, thedistance between the dipole and the quadrupole magnets
is determined to be 200 mm9 where interference ofmagnetic fields is anticipated。 In the design of S-LSR,
the presence of the adjacent magnet has been taken into
account with use of 3-dimensional computer code,
TOSCA [15]. The performance of such design procedure
is to be evaluated by field measurements, which are now[ 1 ] S. Yamada, Proc. of the invited talk at this meeting.
[2] P. Stephen et al” Physics of Plasma 7 (2000) pp20762082..
[3] E. L. Clark et al” Phys. Rev。Lett. 85 (2000) pp16541657.
[4] K. Endo et al.，Proc. of EPAC2000 (2000) pp2515=2517.
[5] A. Noda et al., Beam Science and Technology, 8(2003) pp22-24.
[6] S. Nakamura, private communication.
[7] D Mohl, Proc. o f ECOOL84 (1984) pp293-301.
[8] H. Fadil et al., Accepted for the publication in Nucl。
Instr. & Meth。In Phys。Res。
[9] H. Okamoto, A. M. Sessler and D. Mohl, Phys. Rev。
Lett. 72 (1994) pp3977-3980 。
[10] M. Ikegami et al., to be submitted to Phys.
Rev. STAB。
[11]M. Tanabe et al., Proc. o f this meeting (in Japanese)。
[12] T. Shirai et al., Proc. o f this meeting (in Japanese).
[13] H. Tongu et al., Proc. o f this meeting (in Japanese).
[14] H. Okamoto, private communication.
[15] T. Takeuchi et al.，Proc. of COOL03, in print.
[16] T。Takeuchi et al” Proc。o f this meeting (in Japanese).
[17] S. Fujimoto et al” Proc. of this meeting (in Japanese)。