TWO-BUNCH ORBIT CORRECTION USING THE WAKE FIELD KICKM. Satoh†, T. Suwada, K. Furukawa, Y. Ogawa, M. Kikuchi and H. Koiso
Accelerator Laboratory, KEK, Ibaraki 305-0801, Japan

Abstract
In the KEKB injector linac, the two-bunch
acceleration scheme has been used for doubling thepositron injection rate to the KEKB Low-Energy-Ring(LER).
In this operation mode, the multi-bunchtransverse wake field caused by first bunch affects thebeam orbit of second bunch. In the KEKB linac, the orbit
correction method based on the average minimum of twobunch orbits has been adopted and has worked stably.
However, a new tow-bunch orbit correction method isstrongly required to make the loss of charge less. We
propose the new two-bunch orbit correction method basedon the local bump method. In this scheme, some local
bumps are intentionally constructed in a low-energy area.
Adjusting the local bump height can control the wakefield strength affecting second bunch. In this paper, wereport the results of preliminary beam test to confirm thisnew method being useful.

1 INTRODUCTION
The KEKB project started in 1994 to investigate CPviolation in B-meson decays with the double-ring collider[1]. It consists of 8-GeV electron and 3.5-GeV positronstorage rings. The beam injection efficiency from theinjector linac to the ring should be boosted as high as
possible since the performance of the experiment depends
strongly on the integrated luminosity. In order to achieve
efficient full-energy injection, the original 2.5-GeV
electron linac was upgraded up to 8-GeV, while enforcing
the acceleration gradient by a factor of 2.5 and byextending the length of the linac. The layout of theKEKB linac is shown in Fig. 1. Because of the site limit,
two linacs with 1.7-GeV and 6.3-GeV were combinedusing a 180-degree bending magnet system to form a J-

shape linac. In the J-arc section, the beam opticalparameters are determined so that the achromatic andisochronous conditions are fulfilled [2].
A beam starting from an electron gun passes two
subharmonic bunchers (SHB1: 114.24 MHz and SHB2:571.2 MHz) and an S-band bunching section (2856 MHz)to accomplish a single-bunched beam with a bunch widthof about 10 ps (FWHM). After acceleration to the end ofsector B (1.5-GeV), it enters into the J-arc section. It isthen re-accelerated either to the end of the linac (8.0GeV) or to the positron production target (3.3-GeV),
depending on the operation mode. In order to obtainhigh-intensity positrons, large amount of primaryelectrons should be transported to the positron production
target. The primary electron beam was designed to be 10nC per bunch to produce 3.5-GeV positrons with 0.64 nC.
We therefore doubled the bunch number to increase thepositron beam intensity per pulse and to halve theinjection time [3]. Figure 2 shows the orbit and current
status display for two-bunch operation. The bunchinterval time must be 96.29 ns, which corresponds to thecommon period of the frequencies of the linac and thering.
In the KEKB linac, the orbit correction method basedon the average minimum has been successfully used fordaily operation. The orbit distortion causes the beam loss
especially in the J-arc section. Using this method, theorbit correction is carried out so that the average orbit
distortion of both bunches can be minimized. Figure 2
shows the orbit and charge status panel in the two-bunch
operation. This example shows that a charge loss of
second bunch is lager than that of first bunch because of
the deterioration of second bunch orbit. In order to avoid
such beam loss due to orbit distortion and obtain morehigh-intensity positrons, a new orbit correction method is125 m
180‑deg.
LinacArc

Sector B

Sector A Electron Gun(Gun̲A)

Buncher
B‑8 B‑7 B‑6 B‑5 B‑4 B‑3 B‑2 B‑1 A‑4 A‑3C‑1 C‑2 C‑3 C‑4 C‑5 C‑6 C‑7A‑2A‑1C‑8 1‑1 1‑2 1‑3 1‑4AssemblyHall(Gun̲C)

2‑12‑2 2‑3 2‑42‑62‑7 2‑83‑1 3‑2 3‑3 3‑4 3‑5 3‑6 3‑7 3‑84‑1 4‑2 4‑34‑5 4‑6 4‑7 4‑85‑1 5‑2 5‑3 5‑4 5‑5 5‑6 5‑7 5‑8

KEKB /

Positron Source

Beam Switchyard
5040

Sector C

Sector 1

Sector 2

Sector 3

Sector 4

Sector 5483 mFigure 1: Layout of the KEKB injector linac. J-arc section combines two straight sections.
_______________________________________†masanori.satoh@kek.jp

PFECS
1‑5 1‑6 1‑7

strongly required.
primary electron

charge loss in the J-arc section of the KEKB injector linac.

3 RESULTS OF BEAM TEST

positron

Horizontal Orbit
Vertical Orbit
ChargetargetFigure 2: Orbit and charge status panel for the two-bunch
operation mode. Top and middle show the horizontal andvertical orbit respectively. Bottom shows charges of eachbunch. Blue and light green indicate first and secondbunches.

2 TWO-BUNCH ORBIT CORRECTIONMETHOD
In a new two-bunch orbit correction method we
proposed, the beam position of first bunch is corrected byusing the wake field kick of first bunch. When the firstbunch beam traverses the off-center of an accelerating
structure, the long-range transverse wake field caused byfirst bunch kicks second bunch. Figure 3 shows theschematic drawing of this correction method. If the bumpheight is adjusted to a suitable value, the orbit of second
bunch can be controlled arbitrarily. If the both beampositions and angles of each bunch can be corrected tozero in a low energy section, the beam orbit keeps
constant in the following beam line. To accomplish suchhigh-quality orbit correction will result in reducing thesecond bunch
first bunch

In this section, the results of beam test are presented.
Only horizontal beam orbit correction was carried out inthis experiment. The procedure of this test is as follows:(a)Construct a first local bump at a beam position
monitor (BPM) where the orbit difference between
first and second bunch are getting larger. Adjust its
bump height so that the orbit difference is minimized
at the downstream side of the bump.
(b) The step (a) will deteriorate the beam orbits of eachbunch at the further downstream side. With keeping
the first bump, construct the second local bump at adownstream of the previous bump. After then, iterate
the above procedures in order to squeeze out the orbit
difference toward the downstream side of the linac,
and minimize the beam position and angle of eachsimultaneously.
If the orbits of each bunch are corrected in the Sector in
A or B without the optics mismatch, it can be expectedthat the charge loss in the J-arc section will bedecreased.
Figure 4 shows the software panel for construction alocal bump. This software can construct a local bump atarbitrary BPM position in the KEKB linac. In this beam
test, three local bumps were successively constructed forthe orbit correction. In Fig. 4, the local bumps (1), (2)and (3) were constructed by changing bump heights of theSPA11, SPA32 and SPB14 BPMs, respectively. First ofall, a bump height of SPA11 was set to 1 mm so that theorbit difference is reduced at SPA32. However, the orbit
difference in the downstream area of SPA32 got worse
after first bump was constructed. In the next step, inaddition to the bump (a), a bump height of SPA32 wasvaried in order to reduce the orbit difference of eachbunch at the downstream of the second bump (b). Afterthe height of the bump (b) was set to 2.5 mm, the bump(c) was constructed, while the bumps (a) and (b) were
SPA11 SPA32 SPB14(1)

(2)

(3)

J-arc sectionSPR01

steering magnets
Figure 3: Schematic drawing of a new two-bunch
orbit correction method using the transverse wakefield kick caused by first bunch.

Figure 4: Software panel to construct a local bump.
Dotted plot shows a range of local bump. Three local
bumps are successively contracted.

0.5

Bump height at SPA12 [mm](a) First Local bump (1)

1.0

1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0.0

rms orbit difference [mm]rms orbit difference [mm]rms orbit difference [mm]1.00.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0.0

0.5

1.0

1.5

2.02.5

Bump height at SPA32 [mm]

1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0.0

0.5

1.0

1.5

Bump height at SPB14 [mm](b) Second Local bump (2)(c) Third Local bump (3)Figure 5: Rms orbit difference between first and second bunches versus bump height. The rms orbit difference calculated fromthe all BPMs between SPA12 (first bump position) to SPR01 (entrance of J-arc section). Three local bumps were contractedsuccessively. In the case of (c), therefore, three local bumps were simultaneously constructed.
kept.
Figure 5 shows the results of the rms orbit differences
between first and second bunches. The rms orbit
difference calculated from the all BPMs between SPA12(first bump position) to SPR01 (entrance of J-arc section).
These three local bumps were contracted successively. In
the case of (c), therefore, three local bumps weresimultaneously constructed. The result of varying thebump height of first bump (1) is plotted in Fig. 5-(1). Therms orbit difference is increased after the bump isconstructed because the orbit at the further downstream of
SPA32 is deteriorated. Increasing the bump height ofsecond bunch (2) can be reduced the orbit difference asshown in Fig. 5-(b). Figure 5-(c) shows that varying thebump height of bump (3) cannot adjust the rms orbit at
the downstream side. The beam energy in the Sector B ishigher than that in the Sector A. For that reason, thelong-range wake field of first bunch cannot fully kick thesecond bunch. In addition to that, the bump (3) wascontracted in the relatively long area where three different
types of accelerating structures are used for compensating
the transverse wake field.
With regard to the beam loss after orbit correction, thetransmission rate decreased to a great degree. To recover
the transmission rate, we tried an optics matching.
However, the emittance could not be measured since thelarge emittance growth prevented the wire scanner
measurement. If the bump positions are selected by thosebetatron phases, some suitable bump combinations will befound without any emittance growth.

4 SUMMARY AND FUTURE PLAN
A new orbit correction method was proposed fortwo-bunch operation in the KEKB injector linac. Thismethod uses the long-range transverse wake field kickcaused by the first bunch. To confirm this correction

method, the preliminary beam test was carried out in theKEKB injector linac. The results of beam test shows that
this correction method can be useful for two-bunch orbit
correction, when the bump positions are suitably selected.
On the other hand, the emittance growth due to local
bump caused also the optics mismatch between thestraight and J-arc section. It became an origin of charge
losses.
In order to avoid such emittance growth,constructing additional bump at the 180-degree phasedifference is very effective. With all of these conditions
fulfilled, the two-bunch beam orbit can be correctedwithout the emittance growth. Therefore, this new orbit
correction method can be used for the daily operation.
More detailed beam test and numerical simulation will be
carried out in the near future, and we will present its
result elsewhere.

ACKNOWLEDGMENTS
The authors would like to thank the member of LCG(Linac Commissioning Group) for their useful discussionsand advices. We would like to thank the linac operators
for their great help in our experiments.

REFERENCES[1] H. Koiso et al., “Present Status of the KEKB BFactory”, EPAC2002, Paris, France, June 3-7,pp.2772-2773.
[2] T. Kamitani et al., “Beam Optics Matching in theKEKB Injector Linac”, APAC'98, Tsukuba, Japan,March 23-27, 1998. pp.429-431.
[3] Y. Ogawa et al., “Two-Bunch Operation of theKEKB Linac for Doubling the Positron Injection Rateto the KEKB Ring”, APAC2001, Beijing, China,
September 17-21, 2001, pp.112-114.
[4] K. Furukawa et al., “Beam Feedback Systems And
BPM Read-Out System for the Two-Bunch
Acceleration at the KEKB Linac”, ICALEPCS2001,
San Jose, U.S., November 27-30, 2001, pp.266-268.