The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

MEASUREMENT OF BEAM-BEAM KICKUSING BUNCH-BY-BUNCH BEAM-POSITION MONITOR AT KEKB
T. Ieiri*, Y. Funakoshi, T. Kawamoto and M. Masuzawa,KEK, 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan
Abstract
This note describes an orbit measurement at theinteraction point (IP) at KEKB, an asymmetric collider,where the beams collide with a horizontal crossing angle.
A position offset due to the beam-beam kick was detectedusing a bunch-by-bunch beam-position monitor (BPM)
that measured the beam position of collision and noncollision bunches independently. Varying the distance ofthe two beams at the IP, we found that the beam-beamkick was not symmetrical, and that the optimum collision
was performed with a horizontal offset. The asymmetry
was caused by changes in the beam sizes.

1. INTRODUCTIONKEKB [1,2] is a multi-bunch, high-current,electron/positron collider for B meson physics. The
collider consists of two storage rings: the Low Energy
Ring (LER) for a 3.5-GeV positron beam and the HighEnergy Ring (HER) for 8-GeV electrons. Both rings store
more than 1,000 bunches, where the harmonic number is
5120 with an RF frequency of 509 MHz. The two beams
collide at one interaction point (IP) with a horizontal
crossing angle of 22 mrad.
Unlike conventional single-ring colliders, the beam
parameters are different between the two rings, whichmakes collisions complicate. Since KEKB is operated at
the horizontal betatron tune just above a half integer, thebeam-beam collision changes the emittance and the beta
function due to the dynamic effect. Under these situations,various parameters, such as the beam orbits, the x-ycoupling, the beam sizes and the betatron tunes of bothrings are optimised to perform the best collisions.
Beam-position monitors for the collision tuning havealready been installed at both sides of the IP [3]. The
monitors obtain an orbit change at the IP from detecting
the average orbit for all bunches. The orbit at the IP iscontrolled with the cooperation of dipole magnets near
the IP, called “iBump”; however, the orbit control is very
severe and depends on the experience and skills of theoperators. [4]. We have experienced that the luminosity isvery sensitive for a horizontal orbit.
Therefore, it is required to investigate the beam-beameffects. A turn-by-turn beam-position monitor [5] with agate function turns into a bunch-by-bunch monitor. The
beam-beam kick can be detected from a positiondifference between collision and non-collision bunches.
We do not need to install a position monitor near the IP
region, and the monitor would not be affected by a global
orbit correction.

2. MEASUREMENT METHOD
When two beams collide with a position offset, ∆x * ,they are kicked by the space charge of the opposite beamand the orbit is distorted around the ring. A position
monitor located at a phase advance of ∆ϕ from the IPdetects a position offset due to collision. The position
offset at a detector is given byβ det .β *θ cos(πν − ∆ϕ ) .
2sin( πν ) b−b
Here, β det . and β * are the beta functions at a detector andthe IP, respectively and ν is the betatron tune and θ b−b isthe beam-beam kick angle. Assuming a rigid Gaussianbunch, the beam-beam kick for a bunch with the linear
approximation is given by2 πξθ b−b ≈ − * ∆x * ,βwhere ξ is the beam-beam parameter. Since a position
offset at a detector is proportional to the beam-beam kick,we can estimate an offset at the IP, assuming a beambeam parameter.
KEKB is usually operated with a single train ofbunches followed by an empty gap. Bunches are spacedby 6 ns or 8 ns in a train. Additional bunches, called pilot
bunches, are placed just after the train, at different
location in each ring so that they do not collide with eachother as illustrated in Fig. 1. We can evaluate the beambeam effects, assuming that effects of the wakes are equal
to bunches to be measured, by comparing the beam
parameters of a bunch in a train with those of the pilot
bunch. A beam-beam kick induces a difference in thebeam position. The measurement has the following
features: we do not install a detector near the IP, gain
errors of a detector would cancel out due to a subtraction
and the measurement would not be affected by the global
orbit correction. However, an error may occur when there
is a large imbalance in the intensity between bunches tobe measured.
∆Xdet . =e+
Pilot Bunches

eBucket Address

4831

4835

4839

4843

Figure 1: Configuration of bunches at the tail of a train.
* Email: takao.ieiri@kek.jp

The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

CountsCounts4030
20100260 280 300 320 340 360µHorizontal Position (µ m)µVertical Position ( µm)

(c)

50

Figure 2: Reproducibilityof the measured average
positions, (a) and (b), andthe phase, (c).
(a): Arg=318.6, Std=15.5(b): Arg=56.9, Std=9.1(c): Arg=36.00, Std=0.05

40Counts60
504030201000 20 40 60 80 100

302010035.8 35.9

3636.1 36.2

Phase (degrees)

4. MEASUREMENT
4.1 Measurement during Physics Run
A position offset is obtained from subtracting theposition of the pilot bunch from that of a collision bunch

Position Offset (mm)A bunch-by-bunch beam-position monitor system isinstalled at two locations in each ring. A common
detector is used for the two beams. A beam signal picked
up by button electrodes is processed in parallel. A gate [6]following a low-pass filter (LPF) selects a specific bunchwith a pulse-width of 6 ns. The on/off isolation of the gate
is more than 60 dB at 1 GHz. A beam pulse is filtered bya band-pass filter with a center frequency of 509 MHz.
An I/Q (in-phase and quadrature phase) demodulator
working at an RF frequency of 509 MHz gives two
orthogonal detected signals to sampling ADCs with amemory. The amplitude of the beam pulse is given by twosignals in orthogonal phase, Vsin and Vcos . The beam
position can be obtained from a difference-over-sum
algorithm using four amplitudes corresponding to eachelectrode. The relative bunch intensity is obtained bysumming up the four amplitudes. On the other hand, thephase difference between the beam signal and thereference RF can be given from the ratio Vsin / Vcos .
A bunch signal selected by the gate is sampled every
revolution of 100 kHz and the beam position and thephase are calculated turn by turn. The reproducibility ofaveraged values over 32,000 data was obtained with astored beam. The histograms are shown in Fig. 2. The
standard deviations of the positions are about 10 to 15 µm
over 200 data; note that the measurement includes astability of the beam itself. On the other hand, thestandard deviation of the phase is 0.05 degrees,
corresponding to a time resolution of 270 fs.
(b)
70(a)
50

located next to the pilot bunch. The LER-2 monitor wasused for detecting a horizontal offset of the positron beamand the LER-1 was used for the vertical detection, since
they were placed in a suitable phase advance from the IP.
Before measuring the collision bunches, the positions
without collisions were measured. There was a positiondifference between two bunches of about 5 µm
horizontally and of 20 µm vertically. These values werecaused by the wake and/or an error of the detectors andsubtracted from data measured with the collision. A
position offset due to the beam-beam kick was measured
during usual physics runs. Figure 3-(a) and (b) show theposition offsets as a function of the LER beam current; (a)measured in a relatively high-luminosity run and (b)measured in a relatively low-luminosity run. Though bothcases run with the same number of bunches and withalmost the same beam current, there was a difference of
35 % in the specific luminosity. We notice in Fig. 3-(a)that the horizontal position offset changes from –100 to –150 µm, depending on the LER beam current. An offset
of –150 µm is equivalent to an offset of -61 µm at the IP,
assuming ξx = 0.07 . In Fig. 3-(b), the horizontal position
offset scatters at high beam current and is larger than thatin Fig. 3-(a) as a whole, which may be related to a lowerluminosity. Some unknown parameters might havechanged between the two runs. These results indicate thatthe optimum luminosity is obtained with a negative
horizontal offset. We may notice a small dip in thehorizontal position offset at the beam current of 1100 to
1200 mA, which corresponds to scanning a tuning knob at
the IP. The vertical position offset is almost zero andconstant in both cases.
(a)
0.1
0.05
0-0.05-0.1-0.15-0.28001000 1200
1400 1600
LER Beam Current (mA)
(b)
0.1
0.05
0
-0.05
-0.1
-0.15
-0.2
800
1000 1200
1400 1600
LER Beam Current (mA)
Position Offset (mm)3. BUNCH-BY-BUNCH BPM

Figure 3: Horizontal (dots) and vertical (squares) offsets
as a function of the LER beam current, (a) measured in a
relatively high luminosity run, where a luminosity of
33
210.5 × 10 /cm /sec was obtained, and (b) measured in a
relatively low luminosity run, where the maximum

The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

24.2 Experiment with iBump Scan

Horizontal Offset (mm)Figure 4: Horizontal position offset (dots) and theluminosity (crosses) as a function of the iBump height,
measured with a bunch current of 0.78 mA (LER) and
0.66 mA (HER). The offset data were corrected using
values without any collision. The arrows indicate thedirection in the iBump height.

5. DISCUSSION
The horizontal offset and the luminosity are presented
as a function of the iBump height as shown in Fig.4. We
find that they are not symmetrical for the iBump scan.
The beam size might be changed during a scan. Actually,
we observed variations of the beam sizes not only in thehorizontal direction, but also in the vertical direction.
When the iBump height was positive, the size of the+ 2− 2size, defined by Σy = (σ y ) + (σ y ) , is also asymmetric,
as shown in Fig. 5, where the minimum size corresponds
to the maximum luminosity. Moreover, the horizontal sizeof the electron beam rapidly decreased at the negative
height region, which may enhance the horizontal beambeam kick and the horizontal offset of the positron beam.
These phenomena in the sizes reflect the asymmetry in
the luminosity and in the horizontal beam-beam kick.
However, a simulation did not indicate any asymmetry in
the luminosity for a horizontal scan [7]. The effects of thecrossing angle and the electron cloud are unclear. A shift
in the waist is suspected [8]. The studies should becontinued.
The authors would like to thank Prof. K. Oide for hissupport. They also thank the members of the KEKB
commissioning group and the crew of Mitsubishi Electric
System Co. Ltd. for their help.
400
8350

7300

6250

5200
4-200 -150 -100 -50 0 50 100 150
iBump Height (µm)

Effective Vertical Size (µ m)

7000
6000
5000
4000
3000
2000-200 -150 -100 -50 0 50 100 150
iBump Height ( µm)

20.1
0.05
0-0.05-0.1-0.15

Luminosity (/cm /sec)

The iBump makes an orbit bump of the electron beam
to control the distance between the two beams. The
iBump height was scanned to investigate the beam-beameffects, while the vertical orbit and the betatron tune were
kept constant by feedbacks. The global orbit correction
was off, to avoid any interaction with the iBump control.
Figure 4 indicates variations in the horizontal position
offset at the detector and in the luminosity, as a functionof the horizontal iBump height. The measurement started
from a large positive height, where the electron beam
orbited outside and the positron beam was fixed inside.
The position offset of the positron beam indicated anegative value of about –120 µm, which corresponded toa position offset of –115 µm at the IP, assuming ξx =0.03.
Reducing the distance by the iBump height, theluminosity increased and the position offset approachedzero. However, the position offset was not zero, evenwhen the maximum luminosity was obtained. Upon
reducing the iBump height further, the position offset
moved to the positive side and the luminosity reduced
rapidly. We understand that the two beams exchange theirpositions. When the iBump height moved again in thereverse direction, we found differences in the luminosity
and in the position offset at the same iBump height. We
confirm that the optimum collision exists with a positionoffset of -50 to -70 µm.
30
X 100.15
8000

electron beam was larger than that of the positron, whichsuggested that the positron beam was stronger than theelectron beam at the positive height region. When theiBump height was negative, the relation in the sizes wasreverse. We found that the positron vertical size increased
rapidly at a negative height. The effective vertical beam

Horizontal Size (µm)

33luminosity was 7.0 × 10 /cm /sec . The offset data in thisfigure are corrected using values without the collision.

Figure 5: Horizontal size of the electron beam (dots) andthe effective vertical beam size (crosses) as a function ofthe horizontal iBump height. The sizes were notcalibrated.

REFERENCES[1] KEKB Design Report, KEK Report 95-7 (1995).
[2] K. Akai et al., Nucl. Instrum. Methods A499 191(2003).
[3] M. Tejima et al., Proceedings of the 8th EPAC, Paris,France, 1980 (2002).
[4] T. Aoyama et al., Proceedings of the 13th
Symposium on Accelerator Science and Technology,
Osaka, Japan, 307 (2001).
[5] T. Ieiri and T. Kawamoto, Nucl. Instrum. Methods
A440, 670 (2000).
[6] T. Ieiri et al., Physical Review Special Topics –Accelerators and Beams, 5, 094402 (2002)[7] M. Tawada, private communication.
[8] K. Oide, private communication.