The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

Synchrotrons of Joint Project
S. Machida and Accelerator Group of High Intensity Proton Accelerator Joint Project
KEK, Tsukuba-shi, lbaraki-ken, 305-0801, Japanand JAERI, Tokai-mura, Naka-gun, lbaraki-ken, 319-1195, Japan(e-mail: shinji.machida@kek.jp)

Abstract
The construction of Joint Project between KEK and
JAERI has started in April 2001. Hardware R&Ds still
continues in parallel. Here we focus on mainly the designprinciple and issues of two synchrotrons, namely 3 GeV
Proton Synchrotron (PS) and 50 GeV PS.

1 INTRODU CTION
The Joint Project, formerly JHF at KEK and NSP at
JAERI, has been approved and construction started in
April 2001 [1-12]. That will be· a great outcome ofintensive design study and hardware R&Ds for last several
years. The goal of the accelerators is to deliver 1 MW
pulsed beams from 3 Ge V Proton Synchrotron (PS) andpulsed (fast) and continuous (slow) beams of 0.75 MWfrom 50 GeV PS. It will open variouS new research fields
from atomic energy to biophysics.
Besides the applications with this new facility, theaccelerator complex itself is a tremendous challenge.
Beam intensity of one or two order magnitude more thanthe present facilities requires the same or even lower beamloss. Therefore, this accelerator complex demands a very
careful beam handling, which was not realized in the othertype of accelerators.
In this paper, we will discuss the overview of theaccelerator complex, design considerations, and an
example of commissioning simulation.

2 TIME STRUCTU RE, BEAM INTENSIT Y,
AND EMITTAN CE

2.1 Time Structure
Let us first look at the time structure of the two
synchrotrons. The 3 Ge V PS runs with 25 Hz. Since theexcitation pattern of the magnets is sinusoidal andacceleration occurs on the rising side, the beams stay inthe .ring for 20 ms in one cycle from injection to
extraction. Injection requires only around 0.5 ms. It
corresponds to about 300 turns revolution. Charge
exchange injection is performed during that time. As an
injector of 3 GeV PS, it is sufficient to run linac with thesame 25 Hz. However, the other function of linac, that is adriver of Accelerator Driven System (ADS), makes it runs
with 50 Hz.
Most of the extracted beams from 3 GeV PS aredelivered to the neutron facility. However, four batches,
which consists of two bunches because of harmonic
number of two in 3GeV PS, are transported to 50 GeV PS
every 3.42 s, which corresponds to one cycle of 50 GeV
PS. In fact, depending on operation modes of 50 GeV PS,
3.42 s can be shorter or longer. After the first batch is
injected, only two buckets out of nine is occupied and 50
GeV PSis just waiting for the next batch from 3 GeV PS.
Then second batch fills in two of the empty backets and soon. The transfer and injection of 50 GeV PS with four
batches takes 0.12 s (= (4-1) x 0.04 s). Once injection iscompleted, acceleration starts with linearly excitedmagnets up to the final energy. It takes 1.9 s. When the
beam is extracted with slow extraction scheme, firstly RF
is turned off and debunched beams are continuously
extracted in 0. 7 s. After the beam is extracted, another 0. 7
s is required to reset the magnetic field for the next
injection. When fast extraction mode is taken, one turn
extraction takes negligibly short time compared with one
cycle. In principle, one machine cycle becomes 2. 72 s(=3.42- 0.7).

2.2 Beam Intensity
The beam intensity at each stage becomes following.
The peak beam intensity of linac is supposed to be 50 rnA.
In fact, pulse trains of linac with 324 MHz structure arechopped with the RF frequency of 3 GeV PS (1.22 MHz)and its existing ratio is 56%. The averaged peak intensity
becomes 28 rnA. When 3 GeV PS accepts 293 turns of thelinac beams, the total accumulated particles are 8.333 x
10 13 or 333 ~A in average. With extraction kinetic energy
of3 GeV, that makes 1 MWbeampo wer.
Four batches of 3 GeV PS injection are injected to 50
GeV PS so that the total accumulated particles are 3.33 x
10 14 or 15.6 ~A in average and that makes 0.75 MW (here,
slow extraction scheme and its cycle time are assumed.)
Just for comparison, the maximum number of particles per
pulse in the existing machines is 7 x 10 13 in the AGS at
BNL. The 50 GeV PS will be the first proton synchrotron
with number ofproton per pulse ofthe order of10 14 •- 15-The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

2.3 Emittance
The emittance evolution is assumed in the following
way. First we show transverse one. Although the 100%
emittance from linac (after beam collimation) is 4 pi mmmrad, because of the injection painting process in 3 GeV
PS, it becomes 216 pi mm-mrad (4rms.) We assume
further increase of emittance by a factor of 1.5 due tospace charge effects and other instabilities but not beyond.
That is limited with a beam collimator inside the ring. As aresult, most of the particles (99%) should exists in theemittance of81 pi mm-mrad (adiabatic damping of216 pi
mm-mrad.) at extraction of3 GeV PS. Then, it is delivered
to the neutron facility.
For the batch to the 50 GeV PS, we change painting
pattern at injection so that the resulting emittance isbetween 144 and 216 pi mm-mrad because 81 pi mm-mrad
at extraction is almost equal to the physical acceptance of
50 GeV PS and no room is left for further increase. Whenit starts from 144 pi mm-mrad, adiabatic damping makes
down to 54 pi mm-mrad right before injection of 50 GeV
PS. Then, we expect again the space charge effects, other· instabilities, and optical mismatch at the injection of 50
Ge V PS, that makes the emittance growth by a factor of
1.2 to 1.5 but not beyond again by a beam collimatorinside. Therefore, it becomes 10 pi mm-mrad at the kinetic
energy of30 GeV, and 6.1 pi mm-mrad at 50 GeV.

3 BEAMLOSS
Practically speaking, operational beam intensity islimited, especially in the early phase, by the level of radio
activation or the components. Once the beam loss becomes
more than expected, even at a single pinpoint, we must
decrease beam intensity so that further activation isprevented. At the same time, a careful study to identity
possible reasons that make beam loss should be initiated.
It is important to distinguish controlled anduncontrolled loss. Controlled loss refers to the loss that is
expected and localized to specific places. Uncontrolled
loss is unpredictable. Although the beam loss is inevitablein any case, it is desirable to have only controlled beamloss to some moderate amount.

3.1 Linac to 3 GeV PS Beam Transport and 3
GeVPS
In each stage, the maximum allowable beam loss is
assumed. That is also the basis of shielding plan.
Generally speaking, 1 W/m is a rough criterion to keep themachine accessible with hands-on-maintenance. It istherefore the limit of uncontrolled loss. Controlled loss
may be more than that. After linac, at the beam transport
to 3GeV PS, two different limits are set. One is before arc
and the other is in the arc. In the straight section justdownstream of Iinac, not much beam loss is expected
because of an enough aperture. The allowable beam loss is

0.1 W/m. In the arc, the nominal limit of 1 W/m isenforced. Both are uncontrolled loss.
When a H. beam is injected into 3GeV PS throughcharge exchange foil, some of particles remain as H" or H0
instead of becoming protons. We expect that the fractionof those particles is 2% of the total. In fact, it is possible tocapture most of them to the dump line since the orbit ofthose particles is predictable. A dump target that canhandle 4 kW will capture them. Nevertheless, somefraction of those, especially protons as a result of decaying
H0 at some excited level, behaves as a circulating beamwith large amplitudes and may hit some of components,
resulting in uncontrolled loss.
In order to clean up those particles as well as large
amplitude particles due to space charge effects and otherinstabilities, there is a beam collimator. The loss at thecollimator is controlled loss. It allows up to 4 kW, whichcorresponds to 3% of the total at injection energy.
Other than that, loss during acceleration is expected andit should be less than 1 W/m. Beam loss at extraction isanother source due to large amplitude particle but notcollimated during acceleration or some errors in extraction
kicks. Although it is not controlled loss, beam loss of 1
kW around the extraction region is allowed. In fact,
aperture of the extraction region has the same amount asthat of a collimator, so that there should be no larger
amplitude particle than the aperture.

3.2 3 GeV PS to 50 GeV PS Beam Transport
and 50 GeV PS
There are several beam collimators in the beamtransport line between 3 GeV PS and 50 GeV PS. Since
the average beam power going through the beam transport
is about 47 kW (= 4/(25 x 3.42) x 1 MW), 1% beam loss
at the collimator corresponds to 470 W. The rest of thebeam transport is limited to 1 W/m.
The injection of 50 GeV PS is relatively simpler thanthat of 3 GeV PS. It is the inverse process of 3 GeV PS
extraction with fast kicker and septum. The amount ofbeam loss is expected similar. However, the aperture in
injection region is much smaller than the 3 Ge V extraction,and more beam loss is expected, that is 0.3% andcorresponding to 140 W. Most of the controlled loss in 50
GeV PS occurs at the collimator and it is expected to 1%
or 470 W. The loss during acceleration is limited to 0.5
W/m.
The hottest spot in 50 GeV PS is the extraction
insertion region, especially for slow extraction. Since thebeam energy is 50 Ge V, only 1% of the total current isequivalent to 7.5 kW. To make matters worse, the beamloss at septum wires is inevitable for slow extraction
scheme and 1% is the best value we have simulated. Thatis one of the big challenges in the accelerator complex.
The loss in the fast extraction region can be less and it isexpected to 0.15% or 1.1 kW, still not negligible.

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The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

4 DESIGN CONSIDERATIONSIn order to overcome the problems associated with highintensity beams, we emphasize the following design
considerations.

4.1 Basic Optics

septum for slow extraction and minimize beam loss
associated with the extraction.
The overall structure of both synchrotrons obeys FODO
principle and, therefore, flexibility of focusing property is
assured. The number of quadrupole family in the arc isfour and that in the insertion is seven in both synchrotrons.

Both 3 GeV PS and 50 GeV PS have same lattice
structure called 3 cell OOFO high transition gamma latticein the arc. Similar to Chasman-Green or DBA for asynchrotron radiation ring, that structure makes thedispersion function in the bending magnet small and has asmall or even negative momentum compaction factor. In 3
GeV PS, although a normal FOOO design gives transition
gamma high enough so that a beam never cross it, higher
tr~nsition energy than that of a normal FODO lattice
makes more accurate feedback control of RF near the
extraction energy and eases the longitudinal matching to
50 GeV PS without too much voltage reduction (60 kV
instead of 20 kV compared with the peak voltage of 420kV.) In 50 GeV PS, it is essential to make the momentum
compaction factor negative to avoid transition crossing
which has a potential source of beam loss.
Figure 2: Lattice functions of 3 GeV PS. Solid line ishorizontal and dashed line is vertical.

4.2 Emittance Control

Figure I: Lattice functions of 50 GeV PS. Solid line ishorizontal and dashed line is vertical.
One drawback of those lattice structures is chromatic
behaviour, which is relatively inferior to a normal FOOO
because of localized chromaticity correction sextupoles.
However, the tracking study shows dynamic aperture as
much as twice of physical one with only two families of
sextupoles. In order to make the lattice functions
insensitive to momentum deviation, the correction schemewith 4 or more family of sextupoles are underinvestigation.
The insertion is designed independently of the arc tosatisfy basic functions like injection, extraction, RF
acceleration, and collimation. Each function is assigned to
one of three insertions whereas a basic focusing structure
has three-fold synunetry. In 50 GeV PS, there is a section
where a beta function is relatively high and alpha is almost
zero. We believe it gives optimum place for an electricTransverse as well as longitudinal emittance at the
beginning of 3 Ge V PS is enlarged by phase painting to
reduce space charge effects. In addition to a bump orbitthat makes a H" beam from linac and circulating beams
merge at a charge exchange foil, there is another painting
bump orbit in horizontal direction excited during an
injection process. Gradual change of bump excitation
makes an injection point in transverse phase space shift,resulting in the production of almost uniform distribution.
In vertical, a steering magnet located at 180 degree
upstream in betatron phase from the injection point change
the gradient at injection. As a nominal operation, we plan
to fill the beam from the center in horizontal and from theouter edge in vertical, which is called anti-correlated
painting. Once emittance exchange due to coupling (mostprobably due to space charge potential) becomessignificant during injection, we fill the beam from thecenter in both directions, which is called correlated
painting.
It is not clear at the moment what kind of combinationof collimator aperture and painting emittance minimizes
the overall beam loss. Given the physical aperture of 486
pi mm-mrad, which is determined by quadrupole bore andbending gap, there should be optimised setting ofcollimator and painting. Obviously, small painting
emittance increases space charge tune shift and is subject
to deteriorate. On the other hand, larger painting emittanceremoves the margin between emittance and collimator
aperture. Similar argument is applied to the ratio of- 17-

The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

collimator and physical aperture. Larger collimator place in the straight section where dispersion is zero. A
aperture accommodates more beams without increasing
charge exchange foil close to beam tail at each location
space charge tune shift. However, once a particle hits awill strip electron if the incoming H" particle has moreprimary collimator, not enough margin between collimator
amplitude than the limit, which is 0.1% in momentum andand physical aperture makes the particle hit at an
4 pi mm-mrad in transverse emittance. The proton with
unpredictable place before reaching a secondary collimator.· large amplitude will be bended in the opposite direction
Simulation results show the ratio of 1.5 for both cases iswhen it goes in the arc or hit aperture limit by secondary
optimum and that determines the nominal values, namely collimators (or collectors) in the straight. Since we know
collimator aperture of 324 pi mm-mrad and painting
the source point of protons, that is the foil, and can predict
emittance of 216 pi mm-mrad. However, those are one of orbit afterward, it is a controlled loss.
major parameters to be optimised during theStill there are some drawbacks in those systems. In thecommissioning and its necessary knobs are prepared.
momentum collimator, physical size does not reflect
In the longitudinal phase space, painting is not somagnitude of momentum precisely. Although it is locatedobvious as the transverse. The linac pulse train is chopped
at the large dispersion region, the size is partiallysuch that +-1 00 degrees in a 3 GeV PS RF bucket isdetermined by betatron amplitude. A particle with smaller
occupied with the particles. Shift of the injection point in betatron amplitude can survive after collimator with larger
the phase space is introduced in the momentum direction
momentum. In the transverse collimator, we can only
because of the ramping of bending fields whereas the
scrape large amplitude particles with some certain phases.
incoming beam has the constant energy. That is about
For example, four collimators with 45 degree phase
0.1% during the injection period of 0.5 ms. The
advance apart can shape a beam as an octagon, not an
momentum spread of incoming beams are expected to +ellipse. That is one of fundamental limits of collimator in
0.2%. In addition, we plan to have an offset of theone pass beam line.
injection momentum relative to the synchronous
In 3 GeV PS, there are also momentum and transverse
momentum determined by bending fields by 0.2%, that collimators. Now circulating particles are already charge
forms larger longitudinal emittance than that without theexchanged and foil cannot be used to scrape large
offset and therefore gives larger bunching factor.
amplitude particle. Instead, an obstacle called "jaw" is put
Another increase of a bunching factor comes from thenear the beams. In the transverse collimator, the particle
introduction of the second harmonics. A low Q cavity which hits jaw will gain (or lose) transverse momentum.
realized with magnetic alloy makes it possible to drive one
Depending on the sign of additional momentum,
RF cavity with fundamental and second harmonics at once
secondary collimators downstream with a right · phasewithout any extra cost. We certainly plan to use the second relation, or even after one or several turns later, will catch
harmonic from the very beginning of the commissioning.
the particle. In the momentum collimator, much thinner
Painting emittance of 216 pi mm-mrad together with jaw will slightly change the transverse momentum and letbunching factor of 0.42 with second harmonic RF reduces
the particle hit the transverse collimator downstream. The
the incoherent tune shift due to space charge force down
50 GeV lPS has transverse collimator from the beginning,to -0.15, which is moderate.
but no momentum one. It will be added later if necessary.
In the downstream of 3 GeV PS, emittance control isIdeally we can expect all the beam loss occurs in those
simply done with collimator, one in 3GeV PS to 50 GeV
collimators and no other place. However, still somePS beam transport and the other inside 50 GeV PS. The
uncontroHed beam loss occurs, for example, at thetune shift in 50 GeV PS is about -0.15, just happens to bequadrupole magnets between primary and secondary
the same as 3 GeV PS.
collimators. There are also some particles which go
through the collimator region but cannot circulate in the
4.3 Beam Loss Control
rest of the ring. Another concern is whether the beamcollimated at the injection energy will be adiabatically
In any accelerator, beam loss is inevitable and it must be
dampedaccording to acceleration. In fact, as long as all
pretty small ratio in high intensity synchrotron. If it cannot
themachineaperture is larger than the collimator one in 3
be avoided, the best possible way is to localize the loss
GeVPS,
the aperture limit always exists in the collimator
where we make an adequate radiation shielding andwhether beam is damped or not. However, if it is notassume special cares are necessary once machine failure.damped, large amplitude particles will be lost in the beamoccurs.
transport between 3 GeV PS and 50 GeV PS or in 50 GeV
A device to control beam loss is a beam collimator. In
PSbecause we do make the machine aperture as large as 3
the beam transport between linac and 3 Ge V PS, there are
GeV
PS. We need more study especially on the behaviour
two kinds of collimators; one for momentum collimation
oflarge
amplitude particles during acceleration process.
and the other for transverse amplitude collimation. The
former is set at the place in the 90 degree arc section
where beam size due to momentum spread dominates
because of large dispersion function. The latter is set at the- 18-

The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

5 SIMULATION OF COMMISSIONING

REFERENCES

Although careful choice of lattice and beam parameters
is made as much as possible during the design stage,
eventually a lot of machine tuning and beam study using
the real machine are needed to reach the design goal. Fromthe previous experiences of existing synchrotrons, we haveideas how we tune a machine and what kinds of beam
study are necessary to improve beam quality.
Having said that, it should be, however, noted that all
the procedures based on the previous experiences may not
be performed because of the limitation of radio-activation.
For example, with full intensity operation, machineparameters cannot be chosen so that more than 10% of theparticles are lost, simply because such as beam loss is
prohibited even as controlled loss. In that respect, a dry
run of commissioning and machine improvement becomes
more serious compared with the existing facilities.
In the following, we show a simulation of a tune survey.
That is a typical example which can be done in theexisting machines easily, but limited in the high intensity
machines. Figure 3 shows the rms emittance as a functionof vertical bare tune. The horizontal bare tune is fixed at
vy=7.35 (this simulation is based on the older verion of the
3GeV PS. Now the nominal tune is Vx=6.72 and vy=6.35.)
Taking the tune shift into account, some peaks indicating
emittance growth can be attributed to structure resonances.
A peak at Vy=6.20 is due to 4vy=24, one at vy=6.95 is due
to 4vy=27, one at Vy=6.60 is due to 2vx+2vy=27, and one
at Vy=5.50 is due to 4vy=21. A small peak at vy=5.70 may
be due to 2Vx-2Vy=3.

[1] "The Joint Project for High-Intensity Proton
Accelerators", KEK Report 99-4, JHF-99-3 and
JAERI-Tech 99-056 (1999).
[2] Y. Yamazaki et al., "Accelerator Complex for the
Joint Project of KEK/JHF and JAERIINSP", Proc.
1999 Part. Accler. Conf., THOLl (1999).
[3] Y. Yamazaki et al., "High Intensity Proton
Accelerators for the JAERIIKEK Joint Project", Proc.
2000 European Acceler. Conf., THOAF201(2000).
[4] Y. Yamazaki et al., "The Construction of the LowEnergy Front 60-MeV Linac for the JAERI/KEK
Joint Project", Proc. 2000 Linac Conf., TUD07(2000).
[5] Y. Yamazaki et al., "The Present Status of the
JAERI!KEK Joint Project for High-Intensity Proton
Accelerators", Proc. 2001 Part. Accler. Conf. (2001).
[6] M. Mizumoto et al., Proc. 1st Asian Part. Accel.
Conf., 5A001-309 (1998).
[7] M. Mizumoto et al., "A High Intensity Proton Linac
Development for the JAERI Neutron Science
Project", Proc. 1998 Linac Conf., TU1004 (1998).
[8] JHF Project Office, "Proposal for Japan Hadron
Facility," KEK Report 97-3 (JHF-97-1).
[9] JHF Project Office, "JHF Accelerator Design Study
Report" KEK Report 97-16 (JHF-97-10).
[10] Y. Mori, Proc. 1997 Part. Accel.Conf., 920 (1997).
[11] Y. Yamazaki, Proc. 1st Asian Part. Accel. Conf.,(1998).
[12] Y. Yamazaki and M. Kihara, "Development of theHigh-Intensity Proton Linac for the Japanese Hadron
Project," Proc. 1990 Linac Conf., 543 (1990).

vertical tune
Figure 3: RMS emittance at 1,000 turns after injection
when bare tune is surveyed in vertical direction.

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