FROM KEK-PS TO J-PARC
Yoshishige Yamazaki, J-PARC, KEK & JAEA, Japan
Abstract
The user experiments at J-PARC have just started.
JPARC, which stands for Japan Proton Accelerator
Research Complex, comprises a 400-MeV linac (atpresent: 180 MeV, being upgraded), a 3-GeV rapidcycling synchrotron (RCS), and a 50-GeV main ring(MR) synchrotron, which is now in operation at 30 GeV.
The RCS will provide the muon-production target and thespallation-neutron-production target with a beam powerof 1 MW (at present: 120 kW) at a repetition rate of 25
Hz. The muons and neutrons thus generated will be usedin materials science, life science, and others, including
industrial applications. The beams that are fast extracted
from the MR generate neutrinos to be sent to the Super
Kamiokande detector located 300-km west of the J-PARC
site. The slowly extracted beams generate kaons for
hypernuclei experements, kaon rare decay experiments,and so forth. This unique accelerator scheme and its
usage scheme both originate from those of KEK-PS. It
can be said that the J-PARC is an upgraded version of
KEK-PS in both the beam energy and beam power. It isdetailed how the world-class machine of J-PARC has
been developed from KEK-PS.

target are located in series. Every 3 s or so, depending
upon the usage of the main ring (MR), the beam isextracted from the RCS to be injected to the MR. Here, it
is ramped up to 30 GeV at present and slowly extracted to
Hadron Experimental Hall, where the kaon-production
target is located. The experiments using the kaons are
conducted there. Sometimes, it is fast extracted to
produce the neutrinos, which are sent to the Super
Kamiokande detector, which is located 295-km west of
the J-PARC site. In the future, we are conceiving thepossibility of constructing a test facility for an
accelerator-driven nuclear waste transmutation system,
which was shifted to Phase II. We are trying every effort
to get funding for this facility.

INTRODUCTION
This paper reports on how KEK-PS has developed to
JPARC [1] and then the recent progress in J-PARC,
which has already started user runs. Here, KEK stands for
National Laboratory for High Energy Physics (originally),which right now is High Energy Accelerator Research
Organization, while PS stands for proton synchrotron.
JPARC is the acronym of Japan Proton Accelerator
Research Complex. It will be seen that many aspects,
making J-PARC unique among accelerator projects in theworld, originate from KEK-PS. The linac features andring features are then presented, including the newly
developed ring RF systems. Throughout this report, therelation and the comparison between KEK-PS andJPARC will be shown at many places.

Figure 1: Bird’s eye view image of J-PARC.
Figure 2 shows the J-PARC accelerator scheme. It isnoted that this scheme is essentially the same as that of
KEK-PS, which Yoshitaka Kimura presented in his
lecture [2]. In other words, it can be said that the J-PARC
accelerator is an upgrade version of KEK-PS byapproximately several times in energy and by twentytimes in beam intensity, that is, by two orders ofmagnitude in beam power.

ACCELERATOR SCHEMES OF KEK-PS
AND J-PARC
J-PARC was built in Tokai, Ibaraki, as a joint project
between KEK and Japan Atomic Energy Agency (JAEA).
The 330-m long linac accelerates a negative hydrogen
beam to 181 MeV at present (right now, we are upgrading
it to 400 MeV) to be injected to the rapid-cycling
synchrotron (RCS), where the beam is ramped up to 3
GeV with a repetition rate of 25 Hz. It is fast extracted toMaterials and Life Science Experimental Facility, where
the muon-production target and the neutron-production

Figure 2: J-PARC accelerator scheme.

It is also noted that KEK-PS already placed the booster
RCS in between the injector linac and the high-energy
synchrotron MR. At that time, it was a very new idea,
which was proposed by T. Kitagaki for KEK-PS. Perhaps,
this was also proposed by others independently, because
Fermi National Accelerator Laboratory (FNAL) began to
use this scheme almost at the same time. Now thisscheme is employed everywhere for high-energy
accelerators and is a kind of global standard.
The booster synchrotron is idle after injecting its beams
to the MR until the next injection. It was KEK-PS whichused this period for the first time in order to produce
spallation neutrons and muons. These two secondary
particles thus produced have been very useful tools formaterials and life science. The ISIS followed this RCS
scheme, but increased the beam power much further, andit was the world’s highest power sources for neutrons andmuons, until the spallation neutron source (SNS) [3]achieved higher beam power.

together with KEK in 1997 for starting Japan Hadron
Project (JHP) [4] or Japan Hadron Facility (JHF) [5],another effort was still necessary. Finally in 2001, we
could start the construction of J-PARC by joining the JHF
project of KEK and the Neutron Science Project (NSP)[6] of JAERI. Here, JAERI stands for Japan Atomic
Energy Research Institute, which was joined with Japan
Cycle Organization (JCO) to form JAEA in 2005. The
joint project could take advantage of the R&D results, fora period of more than ten years, of both KEK and JAERI.
During this long R&D period, many young accelerator
scientists have been brought up in both the institutes. Thisis one of the reasons why we could start the pretty
challenging project immediately after the funding. During
the course of the construction, SNS in US started thebeam commissioning and the user run. In 2009, Large
Hadron Collider (LHC) [7] started the beamcommissioning. These are the historical background for
KEK-PS and J-PARC.

BRIEF HISTORY OF JAPANESE HIGHENERGY ACCELERATORSPROGRESS FROM KEK-PS LINAC TO JPARC LINAC

Figure 3 shows a brief history of the high-energy
proton and electron accelerators in Japan for discussing
KEK-PS and J-PARC. In the previous papers, the authors
mentioned Ernest O. Lawrence’s visit to Japan, which
had a big impact on starting the accelerator project inJapan. In 1955, the Institute for Nuclear Study (INS),
University of Tokyo, was founded and an electron
synchrotron was built there. Immediately after that,Japanese scientists began to make a plan to have a highenergy proton accelerator because CERN-PS and BNLAGS started their operations. However, the constructionof KEK-PS was delayed for quite a long time until KEK
was founded and its construction started in 1971. Duringthat time, radio-frequency quadrupole (RFQ) linac wasinvented and Los Alamos Meson Physics Facility(LAMPF) began its beam operation as shown in Fig. 3.

As mentioned above, during the time period between
KEK-PS and J-PARC, the RFQ linac was invented.
Therefore, J-PARC could take benefit of using a compact
RFQ linac rather than a gigantic Cockcroft-Walton
electrostatic injector. One may see from Fig. 4 how thesesizes are different, where the J-PARC 3-MeV RFQ linac
is compared with the KEK-PS 750-keV CockcroftWalton electrostatic injector.

Figure 4: Cockcroft-Walton Electrostatic Accelerator for
KEK-PS (left) by courtesy of KEK Archives Office andJ-PARC RFQ linac (right). Compare the sizes of people
there.

Figure 3: Brief history of high-energy accelerators inJapan.
After KEK-PS started its operation at 12 GeV in 1976,
the long period was again needed to start the nextgeneration proton synchrotron. Although INS was joined

In addition, the RFQ linac is doing the adiabatic
bunching before the acceleration, guaranteeing ideal
bunching. For this reason, it does not give rise to any
whisker shaped particle distribution in the longitudinal
phase space ( p/p and the longitudinal distribution),
which is inevitable in the case of the conventional
buncher system. The ideal bunching contributes a lot tothe minimization of beam loss downstream, in particular,
in the ring injection. This is a very important progress forhigh-intensity accelerators, where one has to minimize
the beam loss, which would otherwise give rise to highradioactivity. The radioactivity prevents people fromhands-on maintenance, which is indispensable foraccelerator components. Since an RFQ can transversely
focus the particles frequently, the transverse emittance
growth is suppressed well. For this reason, it is preferable
to accelerate the beams up to the energy as high as
possible by an RFQ linac. This can also ease the design
and manufacturing of the drift tube linac (DTL) following
the RFQ. The low-energy drift tubes (DTs), housing the
focusing/defocusing quadrupole magnets with watercooling, are difficult to design and manufacture becauseof their small sizes. The beam energy of the J-PARC RFQ
was set at 3 MeV by inventing the -mode stabilizing
loop (PISL) [8] which will be detailed in the following
section.
In contrast to the RFQ, the J-PARC 50-MeV DTL is
not so different from the KEK-PS 40-MeV DTL, but theaccelerating frequency was increased by more than 1.5
times (200 MHz to 324 MHz, Figs. 5-6). This makes it
possible to use klystrons, which are much more stable RF
power sources than are vacuum tubes like triodes, whichhave been conventionally used.

heritage or tradition to use or to stick to the axial
symmetry, since we believe that it is very important for
the quality of the beam. In this way, the scheme of the JPARC linac was formed, as shown in Fig. 6.

COMPARISON BETWEEN KEK-PS AND
J-PARC RINGS IN TERMS OF SPACE–
CHARGE FORCEIn the above section, the differences between the KEKPS linac and the J-PARC linac are presented. For those inrings, some preliminary introduction is necessary
regarding the space–charge effect. In a ring, particles
exert betatron oscillations, both horizontally andvertically, around the equilibrium closed orbit during thecourse of the circulation. The space–charge force
depressed the numbers of the oscillations per cycle,
referred to as the tunes in the accelerator field, since it is arepulsive Coulomb force against the focusing function of
the quadrupole magnets. In general, the tunes are set far
from any integer numbers and any dangerous resonances,which give rise to emittance growth and/or the beam loss.
The space–charge force shifts and/or spreads the tunes
thus set, ultimately pushing them close to theseresonances. It is understood that this is the mechanism tolimit the beam intensity in a ring. This tune shift, which
has a negative sign for its defocusing character, issometimes represented by “Lasslette Tune Shift” defined
as follows: y = Figure 5: KEK-PS DTL (left) by courtesy of KEK
Archives Office and J-PARC DTL (right).

Figure 6: Scheme of J-PARC linac.
The J-PARC DTL was followed by a 180-MeV
separated DTL (SDTL), while the KEK-PS linac energy
was 40 MeV. Recently, upgrade to 400 MeV was funded
for construction. Annular-ring coupled structure (ACS) [9,10] is now under mass production for upgrade.
Geometrically speaking, the ACS is an axially symmetricversion of the side-coupled structure, which is commonlyused everywhere. As Y. Kimura mentioned [2], it is ourN rpF  y (1 +  x )  2 3 B f
ywhere N is the number of protons; y, the verticalemittance; x, the horizontal emittance; rp, the classicalproton radius; F, the form factor; Bf, the bunching factor;
, v/c, and , the relativistic mass divided by the rest mass.
It is not insisted here that the Lasslette tune shift can beused for the detailed discussion on the space–charge force.
Instead, we are quoting these values as a useful measure
or a scaling law regarding the space–charge force. In
particular, it is noted that the tune shift is inversely
proportional to 2 and 3 for the following reason. In
general, the beam generates a magnetic field, whichcancels the repulsive force between the charges. In
addition, if the beam is accelerated, the mass is increased,making the space–charge force less effective. This
inverse proportionality to 23 is the reason why one
increases the injection energy for upgrading the beam
intensity. The large emittances and the large bunching
factor respectively imply less-dense charge distribution
transversely and longitudinally. The form factor and thebunching factor are respectively introduced to represent
the deviation from the uniform distribution transversely
and longitudinally.
Table I and Table II show the comparisons between theRCSs and MRs, respectively, of KEK-PS and J-PARC.
Figures 7 and 8 show the photographs of these machines.
First of all, the energy of the injector linac is increased bya factor of 10 from KEK-PS to J-PARC. In addition, thecircumference of J-PARC RCS is about ten times as longas that of KEK-PS booster. As a result, the extractionenergy of the RCS is increased by a factor of 6 from
KEK-PS to J-PARC. The repetition rates are about thesame. Another big difference exists at their apertures insuch a way that the aperture of J-PARC RCS is about fourtimes as wide as that of KEK-PS Booster. In the case of
the MRs, however, the difference is not so much (1.8
times). Here is some difficulty in J-PARC MR, if one
tries to maximize the beam power. The circumference of
J-PARC MR is five times that of the KEK-PS MR toaccelerate the beam to 30–50 GeV, which is several times
as high as that of KEK-PS MR.
Table I Comparison between KEK-PS booster RCS and
J-PARC RCS.
Parameters
Ring Circumference,m
Repetition, Hz
Beam Stored Energy
per pulse, kJ
Number of protons per
pulse, 1013
Beam Energy, GeV
Beam power, MW
Beam current, μA
Injection energy, GeV
23 at injection
Beam emittance after
painting,  mm mrad
Beam aperture,  mm
mrad
Lasslette tune shift

KEK-PS(Achieved)
38

J-PARC(Designed)
348

J-PARC(Achieved)
-

20
0.2

25
40

25
12

0.25

8.3

2.5

0.5
0.004
8
0.04
0.0908

3
1
333
0.4
1.475
216

3
0.3
100
0.18
0.505
150

79a)

324

- 0.312 b)- 0.16

power is considered as a figure of merit is as follows. In
most of these machines, the secondary particles likeneutrons, muons, kaons, neutrinos and so forth, generated
by bombarding the proton beams into targets made ofheavy elements, are made use of for a variety of sciences.
The numbers of the secondary particles thus produced areproportional to the beam energies, that is, the beam
powers.
Table II Comparison between KEK-PS MR and J-PARC
MR.
Parameters
Ring Circumference,m
Repetition, sBeam Stored Energy
per pulse, kJ
Number of protons per
pulse, 1013
Beam Energy, GeV
Beam power, kW
Beam current, μA
Injection energy, GeV
23 at injection
Beam emittance after
painting,  mm mrad
Beam aperture,  mm
mrad
Lasslette tune shift

KEK-PS(Achieved)
339

J-PARC(Designed)
1568

J-PARC(Achieved)
-

2
1.5

2.9
2,175

3.25
228

0.8

48 to 29

5

12
0.7
0.6
0.5
2.07

30 to 50750
25 to 15
3
69.8
216

30
70
2.3
3
69.8
150

30a)

54

54

- 0.30 b)- 0.30 to 0.18a) Effective values estimated by taking into account the difference
between the horizontal apertures of 82 and vertical ones of 20.
b) By assuming that the beam emittance is two thirds as high as theaperture.

- 0.20a) Effective values estimated by taking into account the difference
between the horizontal apertures of 248 and vertical ones of 49.
b) By assuming that the beam emittance is two thirds as high as theaperture.

In order to understand how J-PARC is a world-classmachine, it is useful to classify the world-class machines
into two categories. The first one is easy to understand,that is, the beam-energy front, exemplified by theInternational Linear Collider (ILC) [11], which will be
detailed by the following paper by Marc Ross, and theLarge Hadron Collider (LHC) [7], which would be andhas been built, respectively, to find Higgs particles andexplore beyond the standard model. The second one isreferred to as the beam power front, which J-PARC
belongs to. The beam energies (V; horizontal axis) andthe beam currents (A; vertical axis) are shown in Fig. 9
for major high-energy proton accelerators in the world.
The product of the beam current and the beam energy is abeam power (W), the front of which can be represented
by the tilted lines in the figure. The reason why the beam

Figure 7: KEK-PS booster RCS (left) by courtesy of KEK
Archives Office and J-PARC RCS (right).

Figure 8: KEK-PS MR (left) by courtesy of KEK
Archives Office and J-PARC MR (right).
Another important factor is the time structures of thebeams, for example, the pulse length (say, microsecondlong or millisecond-long) and repetition rate. The

scientific accomplishments are significantly dependent
upon how these requirements are fulfilled. On the other
hand, the difficulty to build and to operate the machines
are dependent upon these parameters.
It is very interesting to see in Fig. 9 that one can makepower-front lines throughout the whole range of the beamenergy, except for PSI and TRIUMF, which are different(CW) from the other machines (pulsed ones) like SNS
and J-PARC. This is not accidental, since theradioactivity is proportional to the power of the beam loss.
In order to keep the radioactivity to allow the hands-on
maintenance, one should control the power of the beamloss below some level. The two lines indicate where theworld levels were in the past and are at present,
respectively. The line was approximately located along
the beam-power of 100 kW, before SNS and J-PARC
were beam-commissioned. The SNS project has alreadypushed and the J-PARC project is trying to push the line
to that of 1 MW, respectively. In order to push the beam
power front from 100 kW to 1 MW, we have to solve thebeam loss problem; in other words, we have to reduce thebeam loss rate by factor of 10. That was and is a big
challenge in SNS and J-PARC, respectively.

Figure 9: Beam currents and beam energies of the major
proton accelerators in the world.
It should be also mentioned that the accelerator scheme
used for J-PARC and ISIS is quite different from that for
SNS and LANSCE in US. The former is RCS-based,
while the latter is based upon a combination of the fullenergy linac and an accumulator ring (AR), that is, noacceleration in a ring. It is very interesting to see [12, 13]

which is more promising for the beam power between the
RCS scheme and the AR scheme.
It can be also seen from Fig. 9 that J-PARC is a worldclass machine being located at the beam-power front inboth the beam-energy ranges of several gigaelectronvoltsand several tens of gigaelectronvolts. J-PARC would bethus an international center for many fields of science.
One field is of course materials and life science, whichuses the pulsed spallation neutrons and muons. For this, JPARC will be one of the three centers together with ISIS
and SNS. The European Spallation Source (ESS) is about
to be funded for construction. For hadron physics, a longtime ago, TRIUMF was trying to build the Kaon Factory.
About the same time, the Institute for Nuclear Research(INR) in Russia, having a 600-MeV proton linac(Moscow Meson Factory, MMF), tried to get funding, but
was not successful. Therefore, now, as a kaon factory, JPARC is at a unique situation. For neutrino physics, again
one of three competitive world-leading centers competingwith each other; FNAL is conducting the project NOvA,
while CERN is conducting the OPERA experiment.

RECENT J-PARC PROGRESSThe J-PARC accelerator started its construction in
April 2001. On January 24th, 2007, we succeeded inaccelerating the beam up to the designed value of 181
MeV. On October 31st of the same year, we accelerated
the beam up to 3 GeV by the RCS. In both the cases, it
took only one month or so (exceptionally short periods
compared with other similar-size machines) for the
successful accelerations after the start of the beamcommissioning. These are partly because of the excellentalignment of the linac components and the new ring RF
systems, respectively, which will be detailed later. Thesealso promised the future beam-power performance.
On May 30th, 2008, the beam thus accelerated wastransported to the neutron target, producing the neutron
beam. Here, it is worthwhile to note that the neutron
beams from the J-PARC target have shown extremely
sharp energy distribution (by nearly a factor of threebetter than the KENS target of the KEK-PS, eliminating
the tails by nearly two orders of magnitudes), thereby
promising fruitful scientific outcomes. This is one
example of the synergy effects between KEK neutrontarget technology and JAEA nuclear-reactor one. The
neutron beam produced by the high-energy protons
should be cooled down by the moderator in order togenerate thermal neutrons or cold neutrons. For thispurpose, we use the liquid hydrogen entirely covered by
the AIC Decouplers, where AIC stands for silver-indiumcadmium alloy. The excellent energy distribution is theresult of the AIC technology, which is common to the
nuclear reactor technology, combined together with the
KEK neutron target one.
Following the neutron production, the muons wereguided to the experimental area in the MLF on September
26th and was immediately used for the muon spin rotation

(μSR) experiment. Finally, towards the end of 2009, westarted the MR beam commissioning. The MR beam wasaccelerated to 30 GeV on December 23rd, 2008, and,
immediately after that, the beam was slowly extracted to
the Hadron Experimental Facility on January 27th, 2009.
At the end, the MR beam was fast extracted to produce
the mu-neutrinos μ on April 23rd, and finally the firstneutrino event was observed by the Super Kamiokande(SK) Detector on February 24th, 2010. The purpose of theexperiment is to observe the electron-neutrino e to beconverted from μ during the course of its flight from the
J-PARC site to SK.
As such, the J-PARC project is now at the stage foruser runs. In parallel, we are making every effort forbringing the beam power up to the design value of eachcomponent accelerator. Also, we have been encountering
the problems associated with its high-intensity character,which we had never foreseen until the long-term beam
operation started. For example, we had a big trouble with
the RFQ linac, whose remedy took one year to find (infall, 2009). The discharge rate of the RFQ linac had been
gradually increased during the course of beam operation,
until it could not be powered any more after a typicallythree-day beam operation. We have made every effort toimprove the vacuum pressure in the RFQ, including the
installation of the orifice to the low-energy beamtransport between the ion source and the RFQ, theincrease in the number of the high-speed vacuum pumps,
the painstaking baking to a temperature as high as
possible (the RFQ was not designed for the proper
baking), and so forth.

J-PARC LINAC FEATURESIn the preceding sections, it was noted that all the three
J-PARC accelerators have successfully accomplishedtheir first goals of the beam acceleration in surprisingly
short periods. None of these is an accidental incident.
Rather, they have the following reasons and promise
excellent performance in the near future.
It is emphasized here that the importance of the injector
linac has been overlooked in many cases by overconcentrating the attention to the highest-energy ring. In
reality, however, all that the rings can do is to just wait
for the high-quality beams from the injector linac,
preparing their apertures as wide as possible. Here, thehigh quality beam implies the low-emittance, stable beam
both longitudinally and transversely with the minimum
amount of halos and tails. The low-emittances, stability,
and reliability are three key factors for both efficientbeam commissioning and fruitful scientific outcomes.
Regarding these purposes, it is preferable to choose thehigher accelerating frequency (300–400 MHz at the linac
energy front) than conventional (nearly 200 MHz). Firstof all, the higher accelerating frequency implies shortfocusing periods both longitudinal and transverse. Here,longitudinal focusing is provided by acceleration. The

frequent focusing, that is, with a short focusing period, isvital for the immunity against the space–charge effect.
Even further important is to enable the use of klystrons,
which are very stable high-power RF sources, with areasonable size by increasing the frequency.
On the other hand, this means that everything has asmaller size, making it difficult to house the quadrupole
magnets within the DTs. In many cases, the permanent
quadrupole magnets have been developed for coping withthe small DTs. Then, one loses the flexible knobs, which
could empirically optimize the beam optics parameters
and/or keep the parameters from any unpredictable
resonances. For this reason, we have concentrated oureffort on developing the DTs with water-cooledelectromagnetic quadrupoles therein. Making full use ofelectroforming and wire-cutting technologies, we havefinally succeeded in manufacturing a very compact
electromagnetic coil with a water cooling channel. By
that, we could produce the 324-MHz DTL. This is basedon the heritage of the collaboration between the Japaneseindustries and KEK, in particular, for the electroplating,electroforming and electropolishing technology, which
was mentioned by Kimura in a preceding paper [2]. We
took a big advantage of this kind of tradition.
It is preferable to accelerate the beam up to the highestpossible energy by using an RFQ linac, since the RFQ
can frequently and strongly focus the beam in bothtransverse and longitudinal directions. However, it had
been said that the RFQ linac longer than four wavelengthsof free space (that is, with a frequency of 300 to 400 MHz
and an acceleration energy higher than around 2.5 MeV)is almost impossible to tune. During the course of thedevelopment of a 3-MeV, 432-MHz RFQ linac for JHP,
we found the reason for this saying. By elongating the
RFQ to this region, the degenerate resonant frequency of
the two second-lowest dipole modes (deflecting modes) islowered down, getting close to the operational quadrupolemode. As a result, these modes are mixed with each other,giving rise to complicated field distributions, whichcannot be used for operation. We have invented the mode stabilizing loop (PISL) [8] shown in Fig. 10 toincrease the dipole-mode frequencies far beyond theoperating frequency, eliminating the mode mixings. As aresult, long RFQ linacs, that is, high-energy RFQ linacs
beyond around 3 MeV, have become possible tomanufacture. By that technology, we have marked theworld record (3 MeV) of the beam energy of a proton
RFQ linac at that time [14].
For the stability and reliability of the linac performance,
the high-power RF amplifiers and high-voltage converter
modulator (HVCM) are two most important components.
For the former, we developed klystrons in collaboration
with a vendor. We have a long history for thecollaboration, starting from KEK Photon Factory. The 3MW, 324-MHz klystrons are most powerful for arepetition of 25 Hz or 50 Hz and a pulse length of 600 μs.
Perhaps because the klystrons were in reliable, stableoperation at the J-PARC linac, the frequency itself now
becomes a kind of global standard (FNAL Project X, ISIS
future project, China Spallation Neutron Source, GSIFAIR project, and so forth).

provides several klystrons with power, while the latterprovides just one klystron. As far as the HVCM
concerned, the former is much advantageous regarding
both cost and reliability. However, the former scheme
requires klystrons with modulation anodes, which perhaps
limit the klystron power to nearly 3 MW for the J-PARC
duty specification. On the other hand, the latter scheme
requires no modulation anode, simplifying the structureof the klystron. As a result, the highest-possible power
would be more than 3 MW. In the J-PARC case, and in
the other cases such as SNS and the proposed future
project, one RF system to be driven by one klystron does
not require the RF power beyond 3 MW. Also, the vendor
has successfully developed the anode-modulated klystron;with an RF power of 3 MW, an RF pulse length of 1 ms,
and a repetition of 50 Hz. The choice of the anodemodulation is one of the most important reasons for the
reliability and stability of the J-PARC linac.

Figure 10: Inside view of the J-PARC linac. Bar-likeobjects are PISLs.
This fact may be for the following additional reasons.
First, the frequency of 325 MHz is a quarter of the Lband frequency of 1300 MHz, which will be used for
future ILC. If one chooses 325 MHz for the front end, one
can choose the L band for the high-energy part, for which
the Superconducting Cavities (SCC) would be a choice.
In this case, one may take a full benefit from a worldwide ILC effort. Second, the 324-MHz system is actuallyworking in the J-PARC linac as an example with adesigned emittance and so forth. This is a very importantfactor since one might have found some serious difficulty
during the course of the detailed design, construction, andoperation, if one chooses a new parameter. In fact, themedium-energy beam transport (MEBT) between the
RFQ and DTL was very difficult to design, fabricate, andinstall since the MEBT is through a very busy area
including many components like bunchers (that is,longitudinal matcher), choppers, and transverse matchers.
Now, here is a working example at the J-PARC.
Another important decision to make is a choice of thepulse-modulation scheme of the HVCM. In general, there
are two major schemes: anode modulation and cathodemodulation. One system of the former scheme comprises
one high-power, big DC power supply with several lowcurrent, pulsed-anode modulators, each of which isinstalled to a klystron. The latter is just a high-power,
pulsed-cathode modulator, in which, it is difficult tomanage the noise issue and reliability. The former

Figure 11: A half cell of the J-PARC ACS. The view isfrom the accelerating cavity side.

Figure 12: Lowest energy ACS module that was alreadypowered.
It is noted that we are going to use the axiallysymmetric accelerating structure, ACS, for the highenergy part of the J-PARC linac as previously mentioned.
This is the result of the 15-year collaboration with the
Moscow Meson Factory (MMF), Institute for Nuclear
Research (INR), Russian Academy of Science (RAS).

The ACS had been proposed by a Russian physicist [9],who invented the famous disk-and-washer (DAW)
structure. Afterwards, the ACS had not been enabled topractical use for its parasitic mode problem inherent to its
coaxial structure of coupling cavities. We could solve thisproblem [10] by increasing the number of slots to connect
the coupling cavity with the accelerating one from two tofour (see Fig. 11), when we were developing the linac for
JHP. Since the frequency of the J-PARC linac is threefourths of that of the JHP one, the size of the J-PARC
ACS would have been four-thirds of that of the JHP one
just by scaling. The collaboration with the MMF managed
to reduce the size of the J-PARC ACS to that of the JHP
one and made the detailed RF design. The five-cell,buncher cavities and the eighteen-cell, lowest-energy
cavity (Fig. 12) was powered well above their design
values. At present, the ACS cavities are under mass
production for the energy upgrade, as previously
mentioned. The collaboration with the MMF iscontinuing, since the MMF people, who once planned theKaon Factory, are very enthusiastic about J-PARC, which
includes the Kaon Factory.

transition. In contrast, the J-PARC MR result showed nobeam loss observed during acceleration. This is one
distinguished example of the progress in the accelerator
technology, during the period of about more than 30 years
from 1977 to 2010.

J-PARC RING FEATURESThe J-PARC RCS has the transition energy, but it isbeyond the acceleration energy, eliminating the beam loss
again. The lattice for this case is referred to as a hightransition energy lattice. Figure 14 shows the result of
300-kW beam delivery of the RCS. At the injection, we
have some beam loss in some cases. But, if we make very
good adjustment of the parameters, the beam loss at the
injection is almost eliminated. During the course ofacceleration, no beam loss was observed at all.

The linac beam that cannot be accepted by the ring RF
is eliminated at the linac MEBT. The RF chopper devised
by T. Kato [15], deflects the beam during the period of
the operation with the same frequency as that of theacceleration. No beam was observed during the chopped
period (world’s best performance) in contrast to the
Meandor-type chopper being used everywhere.
Separated-function scheme of bending magnets andfocusing magnets were invented by T. Kitagaki (Ref. [16]published in 1953) for strong focusing lattice, and they
were used in KEK-PS MR. Here, the focusing anddefocusing functions of the quadrupole magnets are
separated from the bending function of the bending
magnets, being in contrast to the combined-function
lattice, which had been commonly used before thisinvention. Nowadays, the separated-function scheme is socommon that people do not recognize who invented this.
In addition, the transition energy of J-PARC MR is
imaginary for eliminating the beam loss inherent at thetransition. In the case of the normal FODO lattice that iscommonly used for the proton synchrotrons, the proton
beams pass the transition energy during the course of
their acceleration. Here, the transition energy means thatthe frequency of the synchrotron oscillation vanishes, thatis, the beams lose the restoring force for both the energy
and phase deviation from their equilibrium values. The
beam loss is inevitable here. The imaginary transitionenergy means no transition energy in the real world.
Figure 13 shows one of the very early stage results of
the KEK-PS MR acceleration for comparison with that of
the J-PARC MR. After nine-times injection from the
KEK-PS booster to the MR, the bending magnets were
ramped up for acceleration, but the beam was lost at the

Figure 13: Beam current variation patterns during theinjection and the energy ramping. The left is that of the
KEK-PS MR (courtesy of KEK Archives Office), while
the right is that of J-PARC MR. The original data pointsof the right figure were too thin to see. So, the data points
were traced in a thick curve.

Figure 14: The beam current variation patterns of the JPARC RCS, which delivered a beam power of 300 kW tothe neutron-production target. Experimental data points
and theoretical curves are shown for various injection
conditions. The upper figure is as measured by DCcurrent transformer (DCCT), while the lower one byslow-current transformer (SCT). The horizontal axis is
the time duration in seconds. Thus, the left-mostcorresponds to the start of the injection, while the rightmost corresponds to the extraction. The beam loss isconcentrated on the injection period.

In addition to the transition-free lattice, we have many
innovations of the accelerator technologies like ceramics
vacuum chamber, MA-loaded accelerating cavities, andso forth, where MA stands for magnetic alloy. The JPARC RCS uses many ceramics vacuum chambers, as
shown in Fig. 15. Ceramics technology was vital for the
RCS, which should keep the system from any effect of
the eddy current to be otherwise induced by rapid-cycling
magnetic field.

it as promise, since they were and are very anxious tohave the high beam power for their science outputs. Thiskind of pressure has been very important and useful forthe accelerator progress. Fortunately, the powers realizedso far for one pulse, for 1 min, and 1 h, exceeded the
expected values. However, for the user runs, it is a little
lower. This is mainly because the neutron-production
target needs development and testing in order to accept
the 300-kW beam power. The J-PARC performance is achallenge not only for accelerator technology, but also fortarget technology.

Figure 15: Ceramics chambers used for the J-PARC RCS.
The left is for the bending magnet, while the right for thequadrupole magnets.
The MA-loaded cavity could generate the very-highfield gradient, which is 2.5 times as high as that of theconventional one. By that, we could realize the world’s
most rapid acceleration, that is, 2.82 GeV with arepetition of 25 Hz. Note that the rapid acceleration
requires the high field gradient. The MA-loaded cavity
has another advantage regarding the cure of the beam
instability. The quality value of the cavity is extremely
low, enabling the powering with both the fundamental
accelerating frequency and the second-harmonics one.
The latter can be used to elongate the beam bunch foreasing the space–charge effect. Conventionally,
additional cavity system to the accelerating one would
have been necessary for that purpose. Finally, we need nocomplicated tuning system for the cavities, since the Q
value is very low. The cavity tuning system isconventionally complicated by applying the magnetic
field to the ferrites to vary the resonant frequency. The
use of the MA-loaded cavities significantly simplifies the
RF control system, easing the beam commissioning a lot.

BEAM POWER DEVELOPMENT FROM
KEK-PS TO J-PARC
Main parameters of both KEK-PS and J-PARC arecompared in Tables I and II. The designed Lasslette tune
shift, which is a measure of the space–charge force, is 0.16 for the J-PARC RCS. However, it is noted that thetune shift of the KEK-PS booster achieved -0.31, and thatof the KEK-PS MR -0.30. This shows how skilful the
KEK-PS people were achieved in beam operation.
Figure 16 shows an expectation of how the beam powerof the J-PARC RCS would be developed. This was just an
expectation or prediction, maybe, optimistic one.
Immediately after this was shown, the J-PARC users took

Figure 16: An expectation of the beam-power
improvement as compared with the realized.

SUMMARY
The J-PARC accelerator technology largely originatedfrom that developed for KEK-PS. It is also based on thedevelopments starting in 1986 for JHP and JHF in KEK
and for Omega Project and NSP in JAERI. It took 22
years. During that time, there has been the progress in thetechnology and young scientists have grown up. This is
the reason for the on-schedule, successful beamcommissioning of the J-PARC accelerator. However, weneed further effort to overcome some technological issues.
In this way, the developments and the operational
experiences in J-PARC will contribute a lot to theworldwide technological advance in the accelerator field
for several-megawatt neutron sources in the future,neutrino factories, and so forth.
Needless to say, J-PARC has been built for its
scientific outputs. As mentioned at the beginning, JPARC will be useful not only for materials and life
sciences, but also for high-energy physics and nuclear
physics. In the KEK roadmap for the high-energy physics,
three important projects are listed: J-PARC, ILC, andSuper B Factory. J-PARC is required for the lepton
number or lepton physics, including the already-started
neutrino experiment, and the future muon g-2 and ultracold neutron experiments. Therefore, many of you areinvited to join us for enjoying the J-PARC world atpresent and in the future.

ACKNOWLEDGEMENT
The author is grateful to Toshio Suzuki, Yoshiro Irie,
and Ainosuke Ando for providing him with KEK-PS data.
He expresses his thanks to Michiko Sekimoto, KEK
Archives Office, for giving him some historicalphotographs and for allowing him to use them in thispaper. He especially acknowledges J-PARC staff
members for the photographs and data used in this paper.

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