HIGH-INTENSITY, TWO-BUNCH ACCELERATION FOR DOUBLINGPOSITRON PRODUCTION AT THE KEKB LINAC
Y.Ogawa, A. Enomoto, Y.Funakoshi, K. Furukawa, T. Honda, N. Iida, N. Kamikubota,T.Kamitani, E. Kikutani, M. Kikuchi, H. Kobayashi, H. Koiso, T. Matsumoto, Y. Ohnishi, S.
Ohsawa, K. Oide, T. Suwada, KEK, Tsukuba, Japan•Abstract
The rapid injection of positrons into the KEKB ring isone of the most critical issues affecting the accumulationof the integrated luminosity. The KEKB linac has until
now injected single-bunch positron beams with a chargeof about 0.4 nC into the KEKB ring, requiring several
minutes of injection time. In order to make this positron
injection time as short as possible, we have introduced ahigh-intensity, two-bunch acceleration scheme in the
primary electron beam for positron production, which
could eventually double the positron intensity and halve
the injection time. The bunch separation has been
determined to be 96.29 ns, taking account of the SLED
pulse width at the linac as well as the common frequency(10.385 MHz) of both the linac (2856 MHz) and the ring(508.887 MHz). The first beam test results are reported.

1INTRODUCTION

In high-energy factory machines, like KEKB andPEPII, it is strongly required that the beam-injection
time be as short as possible for the effective
accumulation of the integrated luminosity [1]. In orderto improve the injection time, we must primarily
increase either the beam intensity or the bunch number in
the injector linac. In the case of the KEKB linac, wherea high-intensity single-bunched beam is accelerated to thepositron production target, the single-bunch charge
amounts to about 10 nano Coulomb [2], which is almost
the limit of stable acceleration without introducing any
method of special cures. We therefore doubled the bunch
number to increase the positron beam intensity per pulse
and to halve the injection time (Table 1). The results ofthe first acceleration and injection tests as well as thetwo-bunch acceleration scheme are described.

2TWO-BUNCH ACCELERATION

2 . 1 Scheme
The generation [3] and acceleration of two-bunch
beams is schematically illustrated in Figs. 1. The mainconstraints for doubling the bunch number are:
• The bunch interval time must be 96.29 ns, whichcorresponds to the common period of thefrequencies of the linac and the ring. This is due
to complex frequency relations (Table 2) between
the linac and the ring.

Loading compensation should be carried out
within a SLED gain curve (Fig. 3) in order toequalize the two-bunch energies.
Table 1: Injection time for KEKB (Example)
LER
HER
Physics Run
Injection/day
LER
Single Bunch[Two Bunch]
HER
Beam
Switching/ Others
TotalPresent0.4 ->0.7 A0.43 ->0.58 A
90 min14 times

Full Injection
2.1 ->2.6 A
1.0 ->1.1 A
2 0 min
5 3 times

3.8 min(1.3 mA/s)[2 min](2.5 mA/s)
1 min(2.5 mA/s)
3 - 4 min

6.4 min(1.3 mA/s)[3.3 min](2.5 mA/s)
0.7 min(2.5 mA/s)
3 - 4 min

8 - 9 min
[6 - 8]

1 0 - 12 min[7 - 9]

Table 2: Various frequency relations.
Beam Trigger
Grid Pulser

Delay

Gun
Delay

Grid Pulser
96.29 n sGun

SHB1SHB2114 MHz
11571 MHz 2856 MHz
55275

96.29 n s10.4 MHz1PBFirstBuncher

SecondKEKB Ring
96.29 n s

SLEDLinac
508.9 MHz2856 MHz
Every 49 bucket
275

Figure. 1: Two-bunch generation and acceleration.

Multiple
Common Frequency

1SHB1
SHB2
LinacRing

115*1 1
5*5*1 1
7*7

Frequency[MHz]
10.38545
4114.240
571.200
2856.000
508.887

Period[ns]
96.28
98.754
1.751
0.350
1.965

accelerated along the linac. We have so far achieved
successful acceleration of two-bunched positrons withcharges of 0.4 – 0.5 nano Coulomb at the end of thelinac. Fine-tuning is under way.

Horizontal Orbit
ren 1.00
E

VerticalSecond Bunch

e 0.99v
i 0.98tFirst Bunch
al 0.97
eRe- Charge
Loading Compensation
2.5 %e+ Charge

Positron Production Target

1st bunch

96.29 ns

0.96
0.95
-100-50050

100

Gun Timing [ns]SLED Timing [ns]Figure 2: SLED gain curve.
2nd bunch

2.2 Bunch Profiles
For stable acceleration of the two-bunch beam it isfairly important to have the same bunch profiles for eachbunch at the exit of the bunching section. Figure 3
shows the results of streak-camera measurements byutilizing the optical transition radiation (OTR) generated
from a thin piece of metal inserted into the beam line at
the exit of the bunching section, indicating that twobunches have almost the same bunch profiles.

Bunch
Separation
96.29 n s

Figure 4: Beam orbit and charge.

2.4

Twiss Parameters for Positron Beam

The twiss parameters of two-bunched positron beams
were measured at the end of the linac using wire scannermonitors, which were installed at adjacent positions withappropriate betatron phase advances. The results confirm
that each bunch has the same twiss parameters, assuring
proper matching to the beam-transport (BT) line to thering. Figure 5 shows an example of the wire-scanner
measurements.
1st bunch

2nd bunch
Beam-size
Measurements

Bunch Width
15 ps (FWHM)

Bunch Width
13 ps (FWHM)
OpticsFirst

2.3

Second Bunch
Figure 3: Two-bunch profiles.

Orbit and Charge in Acceleration

A two-bunched beam with almost the same bunch
profiles was successfully accelerated without any
conspicuous beam losses to the positron production
target, yielding almost the same number of positrons foreach bunch, which was accelerated to the end of the linac.
Figure 4 shows the orbit and charge of the two bunches

γ εx = 1.8 mm
γ εy = 1.6 mm

γ εx = 1.9 mm
γ εy = 1.6 mm

Figure 5: Measurements of the twiss parameters of
positron beams at the end of the linac by wire
scanners with three wires: horizontal, 45 degree andvertical.

3INJECTION TEST

After passing an energy compression system (ECS)
located at the end of the linac, a two-bunched beam wasguided in the BT line to the KEKB positron ring (LER).
Since the system of beam-position monitors in the BTline are being reconfigured for two-bunch beammeasurements, the beam orbit along the whole BT line isstill unchecked, except for the charge intensity of eachbunch at the end of the BT line. Regardless of thesesituations, we dared to try injection into LER to find out
if there are any essential problems concerning the twobunch injection scheme. To begin with, we injected justtwo bunches into LER, confirming a bunch interval of
49 buckets in the ring. We then tested a special fillpattern of 64/4/4 (234*2), which means 64 trains with 4bunches in each train and 4 rf-bucket spacing, amounting
to a total bunch number of 234*2=468 in the ring.
Figure 6 shows the bunch current distribution measured
along the ring (total current of 100 mA), which indicates
two groups in each train corresponding to two-bunchinjection. The intensity difference between two groups
may be partly attributed to that of the injected beam at
the end of the BT line, which is expected to be improved
after correcting the BT orbit.
We will continue the injection tests with more
realistic fill patterns for high-luminosity experiments.

4Abort Gap

Figure 6: Bunch current distributions along LER,
when two bunches are injected with a special fillpattern of 64/4/4(234*2). Two groups in a train
correspond to two-bunch injection.
Horizontal Orbit
Second Bunch

First Bunch

Vertical Orbit

ISSUES

The remaining main issues for two-bunch operation
are the following:• Fine adjusting of the two-bunch beam orbits at thebunching section, which could assure a wellbehaved orbit downstream.
• The development of a new orbit-correction
algorithm for two bunches; one example (averageminimum algorithm) is shown in Fig.7.
• Simultaneous beam diagnosis for two bunches inboth wire scanners and beam-orbit measurements,
which is being prepared.
• A more sophisticated feedback system forequalising the energy and orbit of two bunches,
which is definitely necessary for stable operation.
• Complicated bucket selection system for twobunch injection, which is due to the non-simple
frequency relations between the linac and the ring
(Table 2).
With all of these items resolved, we expect the stable
operation of two-bunch injection into LER in FY2001.

5One Train

CONCLUSIONS

Two-bunch acceleration at the KEKB linac has beensuccessfully carried out as well as the first injection tests.
Each bunch profile of two-bunched primary electron
beams for positron production was investigated at the endof the bunching section, while an intense two-bunched

Positron Production Target

Figure 7: New orbit correction method based on theaverage minimum of two-bunch orbits applied tothe primary electron beam for positron production.
beam with a charge of 7 and 8 nano Coulomb each wasaccelerated to the positron production target. A twobunched positron beam with a charge of 0.4 – 0.5 nano
Coulomb each was then accelerated to the end of the linac.
The first injection test was performed with a special fillpattern, confirming no conspicuous problems for thetwo-bunch injection scheme. Further fine-tuning withmore sophisticated beam diagnostic tools is in progress
as well as the development of a new bucket selection
system for injection.

REFERENCES[1] Y. Funakoshi et al., “KEKB Performance”,
EPAC’00, Vienna, June 2000, p.28.
[2] K. Furukawa, “Towards Reliable Acceleration ofHigh-Energy and High-Intensity Electron Beams”,
LINAC’00, Monterey, USA, August 2000, p. 630.
[3] S. Ohsawa et al., “Two-Bunch Generation for
KEKB”, in these Proceedings.