BEPCII Performance and Beam Dynamics Studies on Luminosity

Abstract

The upgrade of the Beijing Electron Positron Collider, BEPCII, is now in a good performance for both high energy physics and synchrotron radiation experiments. The luminosity at the design energy of 1.89 GeV reached the design value 1.0*10^33 cm-2 s-1 recently. A lot of work, including accelerator physics study and technical progress , has been done for the luminosity enhancement, not only at the design energy, but all the energy region run for HEP experiments from 1.0 to 2.3 GeV. The performance of BEPCII and the process of luminosity enhancement will be described in detail.

Full text

BEPCII PERFORMANCE AND BEAM DYNAMICS STUDIES ON
LUMINOSITY*
C.H. Yu
†
, Y. Zhang, Q. Qin, J.Q. Wang, G. Xu, C. Zhang, D.H. Ji, Y.Y. Wei, J. Xing, X. H. Wang,
X.M. Wen, Z. Duan, Y. Jiao, N. Wang, Y.M. Peng, Y.Y. Guo, S.K. Tian, Y.S. Sun, J. Wu, T. Yue,
X.Y. Huang, Z.C. Liu, H.F. Ji, S.D. Gu, Key Laboratory of Particle Acceleration Physics and
Technology，Institute of High Energy Physics, Chinese Academy of Sciences, Beijing, China
Abstract
The upgrade of the Beijing Electron Positron Collider,
BEPCII, is now in a good performance for both high
energy physics and synchrotron radiation experiments.
The luminosity at the design energy of 1.89 GeV reached
the design value 1.01033 cm-2s-1 recently. A lot of work,
including accelerator physics study and technical pro-
gress, has been done for the luminosity enhancement, not
only at the design energy, but all the energy region run for
HEP experiments from 1.0 to 2.3 GeV. The performance
of BEPCII and the process of luminosity enhancement
will be described in detail.
INTRODUCTION
The Beijing Electron-Positron Collider (BEPC) has
been well operated not only for high energy physics, but
also for synchrotron radiation application for more than
15 years since 1989. Its upgrade scheme, BEPCII, is a
double-ring collider, in which two beams have same en-
ergies. It aims at the research of -charm physics with a
designed luminosity of 1.01033 cm-2s-1, which is about
two orders higher than BEPC at the beam energy of 1.89
GeV. The two new rings of BEPCII have been built in the
existing BEPC tunnel while keeping the machine as a
synchrotron radiation source. According to the require-
ments of high energy physics, BEPCII has been operated
in a large energy region from 1.0 GeV to 2.3 GeV since
2009 [1]. The performance of the BEPCII as a synchro-
tron radiation source provides a high flux of synchrotron
radiation at the beam energy of 2.5 GeV for 14 beam lines
about 3 months every year.
ROAD TO THE DESIGN LUMINOSITY
Commissioning with Designed Lattice
The layout of BEPCII as a double-ring collider is
shown in Figure 1, with the main parameters shown in
Table 1. The BESIII detector was installed in May 2008.
The commissioning of BEPCII with the designed lattice
at the energy of 1.89 GeV started on June 22nd, 2008.
Both electron ring (BER) and positron ring (BPR) accu-
mulated beams successfully on June 22nd and June 26th,
respectively. Tuning on collision started on July 16th,
2008. The luminosity reached 1.31032cm-2s-1 with the
beam current of 520 mA*520 mA before December 2008.
A long time was taken to study the source which caused a
strong longitudinal instability shown in Figure 2 in both
BPR and BER. Dipole mode oscillation was observed in
BER, while both dipole and quadrupole mode oscillation
were observed in BPR.
Figure 1: Layout of BEPCII as a collider.
Table 1: Main Design Parameters of BEPCII
Parameters Values
Operation energy 1.0~2.1 GeV
Optimized energy 1.89 GeV
Beam current 910 mA
Bunch current 9.8 mA
Circumference 237.5 m
Number of particles 4.51012
 function at IP x/y 1.0 m/1.5 cm
Horizontal emittance 144 nmrad
Working point x/y 6.53/5.58
Harmonic number 396
Bunch number 93
Bunch spacing 2.4 m
RF voltage 1.5 MV
RF frequency 499.8 MHz
RF cavity number per ring 1
Energy loss per turn 121 keV
Synchrotron radiation power 110 kW
Damping time

x/

y/

E 25/25/12.5 ms
Natural energy spread 5.1610-4
Momentum compaction 0.0235
Natural bunch length 1.35 cm
Crossing angle at IP 112 mrad
Beam-beam parameter 0.04
Luminosity 1.01033cm-2s-1
___________________________________________
* Work supported by NSFC U1332108
† yuch@ihep.ac.cn
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Figure 2: Longitudinal multi-bunch instability.
Two profiles, which caused the quadrupole mode oscil-
lation in the BPR, were removed in Jan. 2009. Collision
tuning resumed on February 2nd, 2009. The maximum
luminosity reached 2.31032 cm-2s-1 with a beam current
of 520 mA*520 mA on April 12th, 2009 during the opera-
tion for high energy physics.
During the period of dedicated machine study the hori-
zontal tune was moved from 6.530 to 6.505 on May 5th,
2009. The luminosity reached 3.31032 cm-2s-1 with beam
current of 520 mA*520 mA in 10 days. The maximum
beam-beam parameter reached 0.021. However, the di-
pole mode oscillation in longitudinal direction still existed
in both BER and BPR, which led to obvious luminosity
reduction.
Longitudinal feedback systems were installed into BPR
and BER during the summer shutdown in 2009. The
commissioning of BEPCII with longitudinal feedback
system began on December 18th, 2009. The longitudinal
instability was suppressed effectively so that the lumi-
nosity was improved about 30%. The data taking at the
energy of 1.89 GeV started from the beginning of 2010.
The beam current and luminosity were improved step by
step, together with the control of detector background and
the luminosity optimization systematically. The maxi-
mum beam current and luminosity reached 750 mA and
6.491032 cm-2s-1, respectively until April 28th, 2011, the
time that BESIII began the data taking plans for other
energy points(1.0~2.3 GeV).
Figure 3: Some hardware failures which limited the in-
crease of beam current.
There are two limitations to restrict the luminosity im-
provement. One is the beam current, and the other is
beam-beam parameter. It’s very hard to increase the beam
current, especially above 700 mA due to heating problem,
which were mainly caused by synchrotron radiation pow-
er and high order mode. Several serious hardware failures
were happened during the operation, such as kicker mag-
net, RF coupler, SR monitor, bellows, feedback system,
etc., shown in Figure 3. The beam-beam parameter was
suppressed obviously under 0.033 at any bunch current
shown in Figure 4 even with sufficient collision tuning for
the luminosity optimization. Bunch lengthening effect
was considered to explain the phenomenon. Several
bunch length measurements by streak camera had been
done. However no believable results were obtained due to
worse measurement accuracy.
Lattice Upgrade to Control the Bunch Length
There is only one RF cavity for each ring due to the
limitation of free space in the tunnel. It’s unavailable to
suppress bunch length by increasing the voltage of RF
cavity. A new lattice was designed to control the bunch
length [2] with the main parameters shown in Table 2. The
natural bunch length was reduced from 1.35 cm to 1.15
cm by decreasing the momentum compaction from
0.0235 to 0.0170. More collision bunches are required
since the designed bunch current is reduced from 9.8 mA
to 7.0 mA with lower emittance for the consideration of
bunch lengthening. The bunch spacing is modified from 4
RF buckets to 3, which will lead to a slight luminosity
reduction due to stronger parasitic collision.
Table 2: Main Parameters of Low Momentum Compac-
tion and Low Emittance Lattice
Parameters Values
Optimized energy 1.89 GeV
Beam current 910 mA
Bunch current 7.0 mA
 function at IP x/y 1.0 m/1.5 cm
Horizontal emittance 100 nmrad
Working point x/y 7.505/5.580
Harmonic number 396
Bunch number 130
Bunch spacing 1.8 m
RF voltage 1.5 MV
Momentum compaction 0.0170
Natural bunch length 1.15 cm
Beam-beam parameter 0.04
Luminosity 1.01033cm-2s-1
Since BESIII doesn’t take data at the energy of 1.89
GeV during period of April 28th, 2011 to 2018. A dedicat-
ed machine study to test the new lattice at the energy of
1.89 GeV was performed during February 28th to March
7th, 2013. The restriction to the beam-beam parameter was
broken, as shown in Figure 4. The maximum beam-beam
parameter reached 0.043.
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Figure 4: C
o
schemes of
d
During th
e
verse multi
-
beam stable
timization
w
stable beam
c
ity reached
7
mA*735 m
A
0.0349. The
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r
p
ossible to r
e
1 with the ne
w
b
ack system.
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The bunc
h
sible to kee
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The emittan
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increase the
number can
b
was reduced
essary
b
eam
Table 3: M
a
tion and Lar
g
Paramete
r
Optimized
Beam curr
e
Bunch cur
r
 function
Horizontal
Wor k i ng p
o
Bunch nu
m
Bunch spa
c
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m
Natural bu
n
Beam-
b
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y
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mparison of
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unch insta
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y the feedb
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as done wit
h
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urrent was 7
7
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while th
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alize the des
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low moment
u
c
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from 1.5 cm
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a
in Parameter
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e Emittance
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at IP x/y
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ch length
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o
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120 bunch
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m
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unch nu
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ealize 1.0
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ess. The bet
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o
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o
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attice
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arameter wi
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t
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T
he luminosit
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aximum lu
m
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am current
o
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w
as reduced
a
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ber case. It’
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ity 1.01033
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t upgrade of
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-2
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-1
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led as less a
s
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o
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0
nm to 122
n
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o that the
b
a
function at
t
o
decrease th
e
o
f 1.01033c
m
o
mentum Co
m
Va lu es
1.89 GeV
910 mA
8.3 mA
1.0 m/1.35 c
m
122 nmrad
7.505/5.580
110
1.8 m
0.0181
1.15 cm
0.04
1
.11033cm-2s
-
t
h the
t
rans-
keep
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op-
i
mum
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inos-
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to realize th
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with the
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e
d obviously
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ld be expec
t
design lumi
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2
014, the ma
x
to series ha
r
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eached 8.53

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a
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3
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ity for singl
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am paramete
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a
m current r
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April 2016.
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am paramet
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5
. The collisi
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so that the
r
t
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i
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osity. Durin
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x
imum stable
r
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
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nd 92 bunc
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2
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ached 910
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ailure was f
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illing
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atter
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0
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osity could
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0
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as too stron
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ack system.
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agnets
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ity.
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ated lattice a
t
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med in Nov
.
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eam para
m
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-
of 8.6m
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i
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n bunch cur
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elatively les
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gh beam cur
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g
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t
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-
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ith the bea
m
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e
2
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i
on and corre
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ed lattice.
m
A with 10
5
e
w during th
e
n
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e
r luminosit
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800mA. Th
e
a
chieved wit
h
119
b
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s
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384 on Apri
l
n
ot be highe
r
0
mA becaus
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to be su
p-
T
he dedicate
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The upgrad
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n proposed.
-
g
d
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.
-
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-
s
-
-
t
-
m
e
-
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y
e
h
s
l
r
e
-
d
e
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A lower vertical beta function of 1.2 cm at the IP was
tested for the luminosity optimization during the dedicat-
ed machine study period. But it was not successful due to
quite low beam-beam parameter. Different coupling coef-
ficients were tested for the luminosity optimization. 1.0%
was the suitable choice for BEPCII to keep collision beam
stable and luminosity higher.
Collision Tuning System of BEPCII
The collision tuning system was developed from 2003.
It consists of BESIII solenoid compensation [3], global
coupling correction, X-Y coupling tuning at the IP, rela-
tive orbit deviation tuning, optics deviation tuning, chro-
maticity (dQx,y/dE, dβx,y/dE, dαx,y/dE) knob, etc.
Figure 6: Collision tuning loop of BEPCII.
About 85 parameters, which could be scanned during
the online routine luminosity optimization. The collision
tuning loop of BEPCII is shown in Figure 6. The luminos-
ity reduction caused by deviations and multi-bunch insta-
bility could be eliminated effectively.
Luminosity Evolution at the Energy of 1.89 GeV
The luminosity evolution at the energy of 1.89GeV is
shown in Figure 7. There were four major periods for the
luminosity tuning at the energy of 1.89 GeV. From Jan.
2010 to April 2011, within which the BESIII detector
took data. March 2013, Nov. 2014 and April 2016 were
the dedicated machine study periods in a few weeks.
Figure 7: Luminosity evolution at the energy of 1.89GeV.
OPERTION WITH ENERGY 1.0~2.3GEV
Lattice Selection for Different Energy Region
The operation energy of BEPCII is decided by the BE-
SIII working plan. The operation energy region of BEP-
CII is from 1.0 to 2.3 GeV. Actually, BEPCII has been
operated in this energy region with a full energy injection
according to the requirements of high energy physics. The
energy region is quite large so that it is very important to
select lattice parameters for the optimized luminosity. The
energy region was separated into three parts: from 1.0
GeV to 1.6 GeV, 1.6 GeV to 1.9 GeV and 1.9 GeV to 2.3
GeV. The horizontal Emittance is the key parameter for
the low energy region for the consideration of collision
bunch current and bunch number. Bunch length is the key
parameter for the high energy region due to voltage limi-
tation of RF cavity. The main lattice parameters for low
and high energy regions are shown in Table 4 and Table 5,
respectively.
Table 4: Main Lattice Parameters for Low Energy Region
Parameters Values
Beam energy 1.0 GeV
 function at IP x/y 1.0 m/1.2 cm
Horizontal emittance 54 nmrad
Working point x/y 6.505/5.580
Momentum compaction 0.0286
Natural bunch length 0.6 cm
Table 5: Main Lattice Parameters for High Energy Region
Parameters Values
Beam energy 2.3 GeV
 function at IP x/y 1.0 m/1.5 cm
Horizontal emittance 144 nmrad
Working point x/y 7.505/5.580
Momentum compaction 0.017
Natural bunch length 1.5 cm
The Operation Status from 1.0 GeV to 2.3 GeV
During the past 5 years, data taking at 21 low energy
points and 106 high energy points have been finished as
scheduled. The peak luminosity estimation and realization
at different beam energy are shown in Figure 8.
Figure 8: The peak luminosity from 1.0 GeV to 2.3 GeV.
BEPCII is optimized at 1.89 GeV and the RF system
can provide a maximum 110 kW beam power with the
beam current of 910 mA. For the operation of high energy
region the beam current had to be decreased due to the
limitation of RF power. Moreover, the bunch length and
emittance could not be well controlled so that the beam-
beam parameter was lower than expected. For the opera-
0
1
2
3
4
5
6
7
8
9
10
0
100
200
300
400
500
600
700
800
900
1000
Jan.23, 2010
Jan.28
Feb.1
Feb.12
Feb.15
Mar.1
Mar.10
Mar.11
Mar.21
Mar.26
May.3
Nov.27
Dec.9
Dec.14
Dec.15
Dec.17
Dec.22
Dec.23
Dec.23
Dec.24
Dec.25
Jan.12, 2011
Feb.25
Apr.8
Mar.8, 2013
Nov.18, 2014
Nov.19
Nov.20
Apr.3, 2016
Apr.4
Apr.5
Luminosity (×10
32
cm
-2
s
-1
)
Beam Current (mA)
e- beam current e+ beam current Luminosity
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tion of low energy region there was no restriction of beam
power and bunch length. However, the multi-bunch insta-
bility was very serious due to longer damping time. The
injection efficiency was also affected by the longer damp-
ing time. The beam current was limited by the ability of
feedback system. The statistic of beam-beam parameter is
shown in Figure 9.
Figure 9: The maximum beam-beam parameter achieved
at different beam energies.
SYNCHROTRON RADIATION RUNNING
The outer rings of BER and BPR are connected to be
the third ring BSR, which is designed as a synchrotron
radiation facility, as shown in Figure 10 with the main
parameters shown in Table 6. There are 14 beam lines
including 8 extracted from 5 wigglers in the BSR. Every
year, 3 months dedicated experiment time is spent to the
users.
Figure 10: Layout of BSR as a synchrotron radiation
facility.
The operation for synchrotron radiation facility was de-
signed as a decay scheme. Beam from linac was injected
into storage ring every 6 hours. The dedicated machine
study for top-up operation started in April 2014. With the
well control for both injecting beam and circulating beam
and well control of radiation dose for both detector and
beam stations, top-up operation was realized on October
27th, 2015. The operation from decay mode to top-up
mode is shown in Figure 11.
Parasitic operation with 2 wigglers on was realized in
2014 after the fine tuning for the luminosity. During the
data taking of high energy physics, 9 beam lines of syn-
chrotron radiation facility, which are distributed in the
outer ring of BER, could provide synchrotron light for the
users without affecting the luminosity.
Figure 11: The beam current and lifetime in decay mode
and top-up mode during one month SR operation.
Table 6: Main Parameters of the BSR Lattice
Parameters Values
Energy 2.5 GeV
Beam current 250 mA
Circumference 241.13 m
Horizontal emittance 160 nmrad
Harmonic number 402
RF voltage 3.0 MV
RF frequency 499.8 MHz
RF cavity number 2
Energy loss per turn 336 keV
Synchrotron radiation power 84 kW
Damping time

x/

y/

E 12/12/6 ms
Working point x/y 7.28/5.20
Natural energy spread 6.6610-4
Momentum compaction 0.0165
Natural bunch length 1.2 cm
CONCLUSION
BEPCII is now in a good performance for both high
energy physics and synchrotron radiation users. The de-
sign luminosity of 1.01033 cm-2s-1 has been achieved with
continuous efforts of luminosity optimization and hard-
ware improvements. Top-up injection has been realized
for synchrotron radiation facility. The top-up operation of
the collider is being studied for much higher integral
luminosity.
REFERENCES
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moniumlike Structure in e+e−→π+π−J/ψ at s =
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Proceedings of IPAC2016, Busan, Korea Pre-Release Snapshot 13-May-2016 09:00 TUYA01
01 Circular and Linear Colliders
A02 Lepton Colliders
ISBN 978-3-95450-147-2
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