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

HIGH INTENSITY PERFORMANCE OF THE KEK 12GEV-PS FOR
THE K2K EXPERIMENTHikaru Sato and the Task Force TeamHigh Energy Accelerator Research Organization (KEK), Tsukuba, 305-0801, Japan
Abstract
The long-baseline neutrino oscillation experimenthas been successfully running since Aprill999. In order torealize this experiment, improvement of the KEK-PS
such as intensity upgrare studies and the hardware
improvements had been perfonned A factor of 50%
increase of the beam intensity was achieved It repends onthe every effort to overcome the beam loss mechanism
such as the micro-wave instability, the space charge effects
at the various periods from injection through extraction
period The neutrino oscillation experiment requests thefast extraction. It needs to construct the fast kicker magnet
system and to re-construct the septum magnets and someother equipment. The switching of the fast extraction andthe slow extraction is possible without the evacuation of
the ecpipment and the multi-users experiments areavailable in short time. The perfonnance of theimprovements of the KEK-PS and the high intensity
operation status will be presented.

1 INTRODUCTION
The KEK-PS complex comprises two 750 keV
Cockcroft-Walton pre-injectors, 40 MeV injector linac,
500 MeV booster synchrotron (BR) and 12 GeV main ring
(MR). It has been operated successfully to serve a proton
beam for 25 years[l]. Beams have been served by the half
integer slow extraction to East and North counter halls.
Beam bunches accelerated in the booster except to the main
ring are utilized as NML (Neutron and Meson Laboratory).
An intensity upgrac:E was coming to the urgent problem
for the long-baseline neutrino oscillation experiment,
K2K (KEK to Kamioka)[2]. This meant the fast extracted
high-mtensity beam creates the high current neutrino beam
to S uper-Kamiokande about 250 km west of KEK. The
hardware upgrac:E, intensity upgrac:E study [3, 4, 5] andconstruction of the fast extraction system had been
perfonned [6]. The beam channel group in the Institute ofParticle and Nuclear Studies constructed the neutrino beamline and the prodlction target for same days [7].
Commissioning was started in January 1999 and the K2K
experiment is continuing since Apri11999.
P~ers on accelerator development devoted to K2K
experiment by the end of 1999 were collected into one
volume [8]2 FAST EXTRACTION
Fast extraction of full circulating beam is requested
for the K2K experiment. The EP 1 extraction system was

modified so that both of slow and fast extractions arepossible. The careful orbit analysis suggested thefeasibility using existing slow extraction devices, such as
bumps and septum magnets system. The septum magnets
C,D,E are excited in series by one power supply for theslow extraction. However, the septum C should be excitedin imkpendently for the fast extraction according to theomit analysis. Then, the septum C magnet power supply
and the switching devices to change the eX.tl:action mode
were constructed The automatic changing system of theextraction kicker and electro-static septum in the same
vacullm chamber was also constructed Fast extracted
beam ooes not need septum A,B field so the moving
region of these magnets were expanded The beam ducts
near these· septum magnets were also replaced.
In ord:r to rouble of the kicker magnetic field andsave the transmission time, the Blumlein system wasdecidld to construct. The field strength is requested higher
than 0.11 T and this should be realized within the space of
3m in one long straight section. C-typed dstributed
magnets with half widh of the nonnal type are connected
face to face each other [6]. A neutrino beam line extendld
from EP 1-A primary beam line of the north counter hall
towards the direction of the Super-Kamiokande [7]. A
fast-extracted proton beam is transported about 400 m andfocused onto a production target.
Main ring beam tuning and the neutrino beam line
tuning started on January 27, 1999. This was performedby the slow extraction since the beam monitoring in theextraction beam line was easier than fast extraction. On themorning of February 3, the extraction system was changed
from the slow extraction mode to the fast extraction mode
after tuning the neutrino beam line except the hom system.
System change was oone in only three hours. The kicker
magnets were excited and the fast extracted beam wasobserved on the monitor screen at the exit of the septum
magnet E. After short beam-off for the check of some
instrumentation, the fast extraction tuning started again
and the beam was confinned on the production target.

3 INTENSITY UPGRADE
Every effort has been devoted to realize the intensity
upgrare of KEK-PS. The BR accelerates more than 2 x
1012 ppp (proton per pulse) for NML. However, the MR
cannot accept the beam of this intensity. Machine studies
for the intensity upgrac:E have continued to make clear thereason and cure of the dfficulty. Several tools for themachine study were developed ancVor upgraded, such as aninjection error monitor [9], a fast beam loss-monitor [10],-49-The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

a tune measurement system with an RF kicker [11], a fast
wire scanner [12] and NMR field monitor for the main
ring bending magnet [13].
According to the concentrated studies results in thespring of 1995 [3], some parameter changing and hardware
improvements were done. The vertical tune was changed
from 7.21 to 5.21 to avoid the effect of the space charge
incbced fourth-order resonance since there were no
installation space in the ring to install the correction
magnets. At this present, vertical tune is set to 5.24 toavoid the effect of space charge tune shift at the highintensity beam operation. In order to increase the verticalaperture, re-alignment of the main ring magnet in thevertical plane was done during summer shut down 1996.
The injection efficiency has increased up to 95-97% fromformer 90-92%.
During the magnet re-alignment work, the fast floor
movement was observed and it seemed depend on therainfall. In order to confirm this phenomenon, themeasurement of the floor tilt in the ring and the moisturein the soil above the tunnel have been going on [14].
The loss observed just after the beginning ofacceleration resulted from the horizontal head-tailinstability, which due to a large change in the chromaticity
produced by the sextupole field induced in the beam-pipe ofthe dipole magnet [4]. In order to suppress thisphenomenon, octupole magnets were installed in the mainring.
According to an understanding of a lot of highimpedance materials in the ring, the two-third of resonant
impedance devices, vacuum ducts between the bending
magnet and the quaciupole magnet and the beam position
monitor were replaced by low impedance ones in 1996 [5].
To cross the transition energy in stable condition, y-jumpmagnet power supplies were upgraded to make the voltage
twice [15].
These improvements expected to the intensity upgrade,but a high intensity operation should be waited until fast
extraction start on March 3 after the completion of thehom system.
Serious and final problems are the beam loss wring
injection pon:h and the transition crossing. Measurementof the beam profiles using flying wires and a multiparticle
tracking simulation mare clear the space charge effects,
nonlinear effects of the sextupole and octupole magnets
and closed orbit distortion during injection porch-[16].
A unique technique to increase the bunching factor andto reduce the space charge effect is the bunch shaping by
modulating acceleration RF voltage with a band-limited
white signal [17]. This is not necessary to remake the highvoltage hardware but the band-limited white signal acrossthe twice of the synchrotron frequency is fed into thevoltage control loop in the low-level system [18]. Centerof the frequency of the band should track the synchrotron
frequency. This was realized by frequency modulating theclock of the arbitrary waveform generator. This method

was applied to the BR, which contribute to reduce thespace charge effect during the injection porch of the MR,
and also to the MR before transition energy. These
applications contributed to achieve more than 6 x 1012
ppp.
Remaining problems were to reduce the beam loss at
the fast extraction and the closed orbit distortion. In spiteof the retailed extraction fine-tuning, the extraction
efficiency did not oven:ome to 93-95%. Results of theextraction study and some other resean:h, we understandthat the beam loss was caused as follows. 1. Effect of thetitanium foil window and the air between the septum
C,D,E magnets : 1. 7% (calculation). 2. Cancellation of
the secondary particle into the beam current transcbcer :about 1% (calculation). 3. Minimized loss near the septum
C magnet: about 2% (study). 4. Totally, 4.7 + x %beam
loss is existing. x% is still unknown factor, but it depends
on the difficulty of the bump orbit optimization due to therestriction of the extraction hardware system layout. Thereis conclusive cin:umstantial evidence about the air effect,that is, about 40m extraction beam line became to 1
atmosphere pressure due to the failure of the vacuum pumpsystem [19] and the extraction efficiency decreased to 9293%.

Q)

.!!l

::JCl.

lii

6x10 12Cl.
U>

-ec:

05x10 12.9;·ic: 4x1012~~

3x10 12CDIllc:~

2x10 12a..

1x10 12Ox10 12FISCAL YEAR

Figure 1: History of the intensity upgrade of theaccelerated beam in the MR.

4 OPERATION STATUS
Following the first stage commissioning during
January and February 1999, commissioning started again
on March 3 after the completion of the hom system.
Extraction beam intensity of more than 6 x 1012 ppp was-50-

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

achieved on May, 1999 and then 8 x 1012 ppp wasaccelerated owing careful and fine-tuning following theban~limited white signal manipulation. Figure 1 shows
the history of the intensity upgrade of the accelerated beamin the MR. Until now, stable operation has been kept with
the intensity of 6.2 - 6.5 x 1012 ppp for the K2K
experiment as shown in Fig.2. At this present, more
intensity than this value is restricted by the radiation safety
limit in the extraction beam line.

Synchrotron, KEK Internal 95-12 (in Japanese)
T. Toyama eta/., Proc. of the 1999 Part. Accel. Conf., p.ll41
K. Takayama et al., Proc. of the 1997 Part. Accel. Conf., p.l548
T. Kawakubo et al., Proceedings of the 7th European Particle
Accelerator Conference, p.2443 and p.2446
M. Ieiri etal., Proc. of the 1st Asian Part. Accel. Conf., Tsukuba,Japan, 1998, p.579
H. Noumi et al., Nucl. Instrum. & Methods A 398 (1997) 399
Edited by I.Yamane and H.Sato, Accelerator Development forK2K Long-Baseline Neutrino Oscillation Experiment, Jan. 2000

[4]
[5]
[6]
[7]

[8]

M. Shirakata et al., AlP Conf. Proc. 315(1994)145[9]

5 CONCLUSION AND CURRENT
SUBJECTS
From April 1999 to July 2001, 5.41 x 10 19 pot(protons on target) are accumulated and 4.79 x 1019 pot areused for experimental analysis [20]. K2K team presented
that a total of 44 neutrinos from KEK have been identified
in the S uper-Kamiokande detector. The number of events
expected in the absence of neutrino oscillations would be64. This result is statistically inconsistent with the nooscillations hypothesis at about the 97% confidence level[21].
Re-alignment of the main ring magnet in the verticalplane was oone for several times, but the horizontal realignment has been never oone. Measurement of thehorizontal alignment, which was oone in 2000 summer
shut oown, indicates the significant distortion [22].
Present steering magnets system has not enough power tocorrect the closed orbit up to 12GeV. In order to realize
more stable operation and further intensity upgrade, themain ring magnet re-alignment of both direction and/or
upgrade of the steering magnets system should be desired.
Although, a life of 12GeV-PS will be nearly overuntil 50GeV-PS (JAERI-KEK joint project) completion,
every effort for further upgrade is under consideration in thegrappling with the deterioration of almost ecpipment. A
second harmonic RF cavity, loaded by magnetic alloy, isunder studying in the MR [23]. A plan of the intensity
ooubler, which is a combination of beam stacking in
500MeV accumulation ring and acceleration of a superbunch in the main ring, has been proposed and unckr
consideration [24].

[10] T. Kawakubo et al., ASN-436, Feb. 14, 2001[11) J. Kishiro et al., Proc. of thel997 Int. Nat.. Conf.on Ace. And
Large Exp. Phys. Control Systems, Beijing, China, 1997, p.208[12] S. Igarashi et al., To be published in Nucl. Instrum. & Methods A[ 13] H. Sato et al., Proc. of the 1999 Part. Accel. Conf., p.3360[ 14] H. Sa to et al., Proc. of the lst Asian Part. Accel. Conf., Tsukuba,Japan, 1998, p.263
H. Sato et al., To be presented at the 2nd Asian Part. Accel. Conf.,
Beijing, China, 2001
[ 15] K. Furuya et al., Proceedings of the 5th Symp. on Power Supply
Technology for Accelerators, KEK Proceedings 99-20, January
2000, p. 97 (in Japanese)[ 16] S. Igarashi et al., Proceedings of the 5th European Particle
Accelerator Conference, p. 1300
[ 17] T. Toyama, Nucl. Instrum. & Methods A 447 (2000) 317[18] T. Toyama et al., Proceedings of the 7th European Particle
Accelerator Conference, p. 1578
[ 19] K. Tanaka, Failure Report FR-322 (in Japanese), July 9, 2001
M. Shirakata, to be reported in ASN-445, Sep. 3, 2001[20] M. Ieiri and T. Maruyama, Private communication.
[21] S.H.Ahn et al., (K2K collaboration), Phys.Lett.B511 (200 1) 178
htto://neutrino.kek.jp/news/200 1.07 .1 O.News/index-e.html[22] M. Shirakata et al., To be presented at the 2nd Asian Part. Accel.
Conf., Beijing, China, 2001[23] M. Yoshii et al., Proc. of the 2001 Part. Accel. Conf., (PAC2001),
MPAH054[24] K.Takayama, Presented at the Int. Workshop on Neutrino
Oscillatrions and their Origin, Frontiers Sciences 35(2000) 115,
Universal Academy Press, Tokyo

EPI BEAM INTENSITY(xEI2ppp)'r------------------------------------,....,-.... ... . ..

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5 ACKNOWLEDGMENTS
The authors would like to express their sincere
thanks to M. Kihara, former director general of accelerator
laboratory and I. Yamane, former director of PS division,for their encouragement. They are much indebted to manycolleagues of PS and beam channel staff for theirdiscussions and collaborations.

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REFERENCES

K. Nakai and T. Ohshima, Summary of Experimental Programs at
the KEK-PS in 1980's and 1990-1994.
K. Nishikawa et al., KEK Preprint 93-55/INS Report 297-93-9
Report on the 1995 Spring Machine Study at KEK 12 GeV Proton-51-

DATE

Figure 2: Fast extracted beam intensity for the K2K
experiment from Apri11999 to July 2001.