ThePSI
PSIHigh
HighIntensity
Intensity Cyclotron
Cyclotron and
and its
The
its Extrapolation
Extrapolation
to
a
10
MW
Driver
to a 10 MW Driver
P.A. Schmelzbach, Th. Stammbach, S. Adam, A. Mezger, and P. Sigg
P.A. Schmelzbach, Th. Stammbach, S. Adam, A. Mezger, and P. Sigg
Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland
Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland
Abstract. The acceleration of a 2 mA, 590 MeV proton beam has been demonstrated at the PSI High Intensity
Abstract.
The
a 2machine,
mA, 590theMeV
protonmode
beam
demonstrated
at theupgrade
PSI High
Intensity
Cyclotron.
Theacceleration
peculiaritiesof
of the
operation
for has
highbeen
currents,
and the ongoing
program
are
discussed.The
Thepeculiarities
conceptualofdesign
of a 10theMW
driver mode
basedfor
onhigh
the currents,
extrapolation
of ongoing
the present
performances
is
Cyclotron.
the machine,
operation
and the
upgrade
program are
presented.The conceptual design of a 10 MW driver based on the extrapolation of the present performances is
discussed.
presented.
INTRODUCTION
INTRODUCTION
The
ThePSI
PSIaccelerator
acceleratorfacility
facilitywas
wasbuilt
builtinin1974
1974with
with
thethe
goal
of
reaching
a
beam
intensity
of
100
goal of reaching a beam intensity of 100|iA
µAatatan
an
energy
ofof590
energy
590MeV.
MeV.The
Themain
mainstage
stageofofthe
theaccelerator
accelerator
chain,
chain,thethePSI
PSIRingcyclotron
Ringcyclotron isis aa separated
separated sector
sector
cyclotron,
cyclotron,specially
speciallydesigned
designedfor
forhigh
highbeam
beamintensities
intensities
[1].
[1].The
Theconcept
conceptproved
provedtoto bebe successful
successful and
and an
an
upgrade
upgradewas
wasundertaken
undertakentotoachieve
achievea abeam
beamcurrent
currentofof
1.51.5mA.
mA.InIn 1985
1985 a a new
new injector
injector cyclotron
cyclotron was
was
commissioned[2]
[2]and
andinin1991-95
1991-95the
theRF
RFsystems
systemsofof
commissioned
Ringcyclotronwere
wererebuilt
rebuiltininorder
ordertotoprovide
providethe
the
thethe
Ringcyclotron
necessaryRFRFpower.
power.InInthe
thecourse
courseofofthis
thisupgrade
upgradethe
the
necessary
peakvoltage
voltageininthe
thefour
four accelerating
accelerating cavities
cavities was
was
peak
raisedfrom
from450
450kV
kVtoto730
730kV.
kV.According
Accordingtoto aa law
law
raised
described
by
W.
Joho
in
reference
[3],
the
beam
described by W. Joho in reference [3], the beam
currentlimit
limitsetsetbybylongitudinal
longitudinalspace
spacecharge
chargeforces
forcesinin
current
Ringcyclotronisisproportional
proportionaltotothe
thethird
thirdpower
powerofof
thethe
Ringcyclotron
accelerationvoltage.
voltage.The
Thepredicted
predictedbeam
beamintensity
intensity
thetheacceleration
wasreached
reachedinin1995,
1995,thus
thusvalidating
validatingthis
thislaw.
law. With
With
was
stronger
bunching,
better
beam
matching
and
other
stronger bunching, better beam matching and other
minorimprovements
improvements the
the beam
beam loss
loss was
was further
further
minor
reduced
and
the
beam
current
for
routine
operation
reduced and the beam current for routine operation
couldbeberaised
raisedtoto1.8mA.
1.8mA.InIna ashort
shortbeam
beamtest
testininJuly
July
could
2000
a
maximum
beam
current
of
2
mA
was
2000 a maximum beam current of 2 mA was
accelerated
and
extracted
from
the
Ringcyclotron.
The
accelerated and extracted from the Ringcyclotron. The
layout
acceleratorcomplex
complexisisshown
shownininfigure
figure1.1.
layout
ofof
thethe
accelerator
To achieve 1 MW of beam power on the PSI
To achieve 1 MW of beam power on the PSI
spallation source SINQ, 2.7 mA have to be extracted
spallation source SINQ, 2.7 mA have to be extracted
from the Ringcyclotron and transported to the last Pion
from the Ringcyclotron and transported to the last Pion
production target, after which about 60-70% of the
production target, after which about 60-70% of the
scattered beam is accepted for further transportation.
scattered beam is accepted for further transportation.
An upgrade of the accelerators to a current of 3 mA is
Anunder
upgrade
accelerators
to a current
of 3with
mA the
is
way.ofItthe
bases
on the experience
made
under
way.
It
bases
on
the
experience
made
with
the
special operation mode of the Injector 2 and on the
special
operation mode
Injector development
2 and on theof
facts established
duringof
thethe
continuous
facts
established
during
the
continuous
development
the Ringcyclotron RF system. The demand of
for
thestronger
Ringcyclotron
system.andTheapplications
demand forin
neutron RF
sources
stronger
neutron
sources calland
applications
in
transmutation
technologies
for ever
more powerful
transmutation
technologies
call
for
ever
more
powerful
drivers. The feasibility of cyclotrons to generate a
drivers.
The feasibility
generate
a
beam power
in the orderofof cyclotrons
10 MW hasto
therefore
to be
beam power in the order of 10 MW has therefore to be
discussed,
layout of
of aa cyclotron
cyclotron
discussed, also.
also. A
A tentative
tentative layout
facility
with
a
beam
current
of
10
mA
at
1
GeV has
has
facility with a beam current of 10 mA at 1 GeV
been
extrapolation of
of the
the beam
beam
been presented
presented [4].
[4]. The
The extrapolation
performance
10 mA
mA is
is aa realistic
realistic
performance from
from 2 mA up to 10
issue
the present
present report.
report.
issue and
and will
will be
be also discussed in the
The
due to
to space
space
The understanding
understanding of the limitations due
charge
of high
high
charge effects
effects is
is the
the key to the development of
intensity
will be
be summarized
summarized
intensity cyclotrons.
cyclotrons. Some aspects will
here,
in [5].
[5].
here, aa more
more detailed
detailed discussion is presented in
72 MeV Injector 2
Variable energy
Injector 1
FIGURE 1. Layout of the PSI accelerators. The Injector 1 is
FIGURE
1. Layout
theenergy
PSI accelerators.
presently only
used foroflow
applications.The Injector 1 is
presently only used for low energy applications.
THE INJECTOR CYCLOTRON
THE INJECTOR CYCLOTRON
For the high intensity beam production the soFor Injector
the high2 intensity
productiondeliver
the socalled
is used. Itbeam
can presently
a
called
Injector
2 ismA..
used.
can presently
beam of
up to 2.2
TheItpreinjector
is an deliver
870 keVa
beam
of up to 2.2 mA..
The preinjector
is an
870 keV
Cockcroft-Walton
generator
equipped with
a cusp
ion
Cockcroft-Walton generator equipped with a cusp ion
CP642, High Intensity and High Brightness Hadron Beams: 20th ICFA Advanced Beam Dynamics Workshop on
High Intensity and High Brightness Hadron Beams, edited by W. Chou, Y. Mori, D. Neuffer, and J.-F. Ostiguy
© 2002 American Institute of Physics 0-7354-0097-0/02/$ 19.00
197
energy gain per turn can be provided in order to assure
separated orbits. A code to perform full six
dimensional simulations is being developed [9, 11] in
order to optimise the injection process and to further
improve the capture rate.
source operated at DC currents of 8 to 12 mA. For a 10
MW facility 50 to 60 mA would be necessary. Sources
delivering such currents are already available. The
corresponding beam power and the importance of a
high beam stability is an important issue for the choice
of a future preinjector. The possible use of a RFQ as
preaccelerator has to be considered, also.
tom'3 "
A major aspect in the 6-dimensional matching of
the phase space of the beam to the injector cyclotron is
a strong bunching. A double gap buncher is installed at
a distance of 4.9 m from the injection point on the first
turn. It is operated at a peak voltage of 7.2 kV. At first
glance one would expect that this high buncher voltage
introduces an undesirable energy spread at the time
focus at the injection point. However, with a proper
combination of the buncher voltage and the DC beam
current, the space charge forces can be employed to
reduce this energy spread [6].
team'
The special features of a beam bunch of equal
dimension in radial and longitudinal directions have
already been noted by Chabert [7] in 1981. The PSI
Injector 2 is, however, the first machine taking
advantage of the properties of such a beam. In order to
operate a cyclotron in this regime the beam has to be
longitudinally matched. The ideal phase width at
injection is about 20Q in our case. The centre region of
the Injector 2 with phase-defining and cleaning
collimators is descibed in ref. [8].
FIGURE 2. Beam size at the extraction from Injector 2. The
width of the beam profile (4o) in function of the extracted
beam current averaged over the last seven turns is shown.
The width is proportional to the cubic root of the intensity
THE RINGCYCLOTRON
The beam bunches in the Ringcyclotron are not
circular as in the case of the Injector 2, but very
elongated. Hence, space charge forces and especially
longitudinal effects [3, 9, 6 and references therein]
have to be accounted for. Due to the strong coupling
between longitudinal and radial motion the
longitudinal components generate a tilt of the
elongated bunch in the radial direction. This tilt can be
compensated by using a flattop system and properly
adjusting its phase against the accelerating cavities.
Under space charge dominated conditions in a
cyclotron, such a circular bunch has a stable
configuration [7, 9, 6 and references therein],
especially at high beam intensities. The bunch remains
circular and of equal size during acceleration, and has
therefore an extremely narrow phase width at
extraction. In the case of Injector 2 it is about 2Q.
Consequently, a flattop system is not needed. The
originally installed two flattopping cavities are now
used for additional acceleration. In the upgrade
programme to reach 3 mA it is planned to replace at
least one of them by a true accelerating system [10].
Based on simple models W. Joho [3] predicted that
the maximum beam current should be proportional to
the third power of the average energy gain per turn.
The limit reached when each year one cavity was
upgraded from 450kV to 730kV followed exactly the
prediction as shown in figure 3. With the installation
of new, 1 MV cavities now in development [10] the
space charge limit of the Ringcyclotron will be
increased to 4 mA.
The Injector 2 cyclotron has well separated turns up to
the extraction radius, where the separation amounts to
23 mm. This corresponds to about 7a of the beam
width at 1.9 mA. The beam loss at extraction is
correspondingly low and the extraction rate is about
99.98%. The beam width depends on the beam
intensity as shown in figure 2.
The Ringcyclotron also has well separated turns at
extraction. The separation due to the acceleration only
amounts to 6 mm. It is doubled by a coherent betatron
oscillation induced by off-centre injection on the first
orbit in the cyclotron. The resulting separation of 12
mm corresponds to about 7a of the observed beam
profile width at 1.8 mA. As for the Injector 2 the beam
loss at extraction is low and the extraction rate is
99.98%. The predicted performance of the 1 GeV
The extrapolation to the case of a 10 MW facility is
straightforward. With an extraction energy of 120
MeV the injector cyclotron would be larger, but it
could be operated in the same regime. The beam size
is expected to increase with beam current, but using
four instead of two accelerating cavities enough
198
TABLE 1. Comparizon of some parameters of the PSI
cyclotrons with design values of a 10 MW driver.
Cyclotron beam bases on the experience from the
upgrade of the PSI Ringcyclotron as shown in figure 3.
It is assumed that longitudinal space charge effects set
Energy
Beam Intensity
Cavities
dR/dn
AR(4o)
t LJmit [mA]
Magnete
Cavities
Flat top
Rav
Number of turns
Egain at extr.
Number of turns
dR/dn = (Ravy/
(y+l)vr2)(Efiain/E)
2
3
4
5
Etiei^gjf £kiix per Turn [MeV]
6
x 2 with betatron
oscillation
AR(4o)
Turn separation
Space charge lim
Beam power
7 8
FIGURE 3. Maximum beam current extracted from the
Ringcyclotron as a function of the average energy gain per
turn as established during the upgrade of the cavity voltage.
The solid line is the dependence due to the longitudinal
space charge. Also shown is the extrapolation to a tentative
facility with a 1 GeV, 10 mA cyclotron (dashed line).
PSI Injector 2
72MeV
2.2mA
120 MeV
120 MeV
10mA
2 + 2 FT(see text)
4 (=2-2.5 MeV/t)
23 mm
13 mm at 1.9mA
28-35 mm
23mm
PSI Ring
8
4(730kV)
l(460kV)
IGeV
4462mm
215
2.44MeV/turn
215
6 mm
12
8(1000kV)
2(650kV)
5677mm
140
6.3 MeV/turn
140
11 mm
12mm
7 mm at 1.8 mA
7o
2mA
1.18MW
7o
10mA
10 MW
REFERENCES
H.A.Willax, "Status Report on SIN", Proc. 7th Int Conf.
on Cycl and their AppL, Zurich 1975, p.33.
U.Schryber et al., "Status Report on the New Injector at
SIN", Proc. 9th Int Conf. on Cycl. and their AppL, Caen
1981, p. 43.
W.Joho, "High Intensity Problems in Cyclotrons", Proc.
9th Int. Conf. on Cycl. and their AppL, Caen 1981, p.337.
Th.Stammbach et al., "The 0.9 MW Proton Beam at PSI
and Studies on a 10 MW Cyclotron", Proc. 2nd Int. Conf.
on Accel.-Driven Transm. Tech., Kalmar 1996, p. 1013.
Th.Stammbach et al., "The PSI 2 mA Beam and Future
Applications", Proc. 16th Int. Conf. on Cycl. and their
AppL, East Lansing 2001, p. 423.
J.Stetson et al., "The commissioning of PSI Inj.2 for high
intensity, high quality beams", Proc. 13th Int. Conf. on
Cycl. and their AppL, Vancouver 1992, p. 36.
A.Chabert et al., Proc. 7th Int. Conf. on Cycl. and their
AppL, Zurich, 1975, p245 and IEEE Trans. NS 22-3,
1975, p. 1930.
U.Schryber et al., "High Power Operation of the PSI
Accelerators", Proc. 14th Int Conf. on Cycl. and their
AppL, Cape Town 1995, p. 32.
9 S.Adam, "Step to enhance the knowledge on space
charge effects", 16th Int. Conf. on Cycl. and their AppL,
East Lansing 2001, p. 428.
10 P.K.Sigg et al., "Upgrade Concepts of the PSI
Accelerator Systems for a Projected 3 mA Operation",
Proc. 16th Int. Conf. on Cycl. and their AppL, East
Lansing 2001, p. 300.
11 A. Adelmann et al., Contribution to this workshop.
the intensity limit. With eight accelerating cavities
operated at a peak RF voltage of 1MV [10], the
proposed IGeV cyclotron has an averaged energy gain
of 6.3 MeV/turn, compared to 2.4 MeV/turn in the PSI
Ring with four accelerating cavities at 730 kV. Taking
into account the different size and the different final
energy a maximum beam current of 10 mA is possible,
as shown by the dashed line in figure 3. A turn
separation of 7a similar to the situation today in the
Ringcyclotron is expected under the following
assumptions: acceleration of the beam into the fringing
field where vr drops to 1.5, increased beam emittance
from the injector cyclotron as extrapolated from figure
2 and no limitation from other sources. A rebuncher in
the injection transport line might be necessary if the
energy spread of the beam from the injector cyclotron
becomes too large. A choice of parameters of the PSI
and of the 10 MW facility are summarised in table 1.
CONCLUSION
Available technologies and experience at PSI are a
realistic basis for the design of a cyclotron as a 10 MW
proton driver. While scaling laws are seen to apply in a
wide range, further progress in the simulation of space
charge dominated beams is needed to estimate the
ultimate performances of this type of accelerators.
199