PERFORMANCE OF THE KEK 8-GeV ELECTRON LINAC
A.Enomoto, e-/e+ Injector-Linac Group, Linac Commissioning Group,KEK, Tsukuba, 305-0801, JapanAbstract
The KEK 8-GeV electron linac has been operated for
the KEK B-Factory accelerator (KEKB) since December,1998. After being upgraded from 2.5 GeV, it presently
provides 8-GeV electrons and 3.5-GeV positrons for theKEKB colliders. It also injects 2.5-GeV electron beams
for the synchrotron radiation (SR) experiments. To
deliver different kinds of beams, many parameters areautomatically switched; at the same time, many software
programs are used to correctly reproduce these beams.
This paper describes the current performance of theupgraded linac.

1 INTRODUCTION
The KEKB accelerator includes an 8-GeV electron ring(HER) and a 3.5-GeV positron ring (LER); its goal is aluminosity of 1 x 10 34 cm-2s-1 with collisions between
1.1-A electrons (e-) and 2.6-A positrons (e+).
Two major goals are required of the linac: the first is
to deliver full-energy beams for both rings; the second isto provide positrons of more than 0.64 nC/bunch at 50
Hz, which, assuming no beam loss, corresponds to an
accumulation rate of 3.2 mA/s and an injection time of
13.5 minutes to accumulate from 0 to 2.6 A.
As shown in Figure 1, the linac contains two electron
sources at Sectors A and C, a positron source at Sector 2,
and 57 s-band accelerator modules with an average energy
gain of 160 MeV each. During routine operation, itdelivers different beams: 8-GeV e- for the HER, 3.5-GeV
e+ for the LER, and 2.5-GeV e- for the SR research.
Electron beams from the A-Gun are bunched by double
sub-harmonic bunchers (SHBs) at 114.24 MHz and 571.2
MHz, as well as bunchers at 2856 MHz, in order to forma single-bunch beam with a 10-ps FWHM.
An electron bunch with 1 nC is accelerated to 8 GeVby the entire linac. When a positron beam is accelerated,a positron radiator is inserted at an energy point of 3.7
Sector A

Sector B

180-deg.
ArcSector 3

GeV; then, by increasing the gun-grid voltage, an intenseelectron of 10 nC hits the radiator and emerging positrons
are accelerated through the remainder of the linac.
The reason to use the C-Gun is historical. This gun
has been used since the old 2.5-GeV linac started, and also
during the linac upgrade. It was, however, reset out of thebeamline. When it is used, the gun beam is merged tothe beamline by a 90-degree bending magnet. A multibunch short pulse beam with a 1 ns FW is formed bybunchers at 2856 MHz, and accelerated to 2.5 GeV inorder to inject into the storage rings.

2 OPERATION HISTORY
The linac upgrade had been progressing since 1994, andwas completed by March, 1998. On April 27th, 1998,
the first acceleration of a 2-nC electron bunch to 8 GeV
was achieved by employing 54 accelerator modules [1][2];however, the necessary energy margin was later obtainedby installing three more accelerator modules.
Though beam studies have continued since October,
1997, a positron goal of 0.64 nC at the end of the linac
was achieved in April, 1999. Table 1 shows the best
beam performance of the linac obtained so far.
In December, 1998, routine injection for the KEKB
rings was initiated. At the beginning of the KEKB ringcommissioning, the linac continued injection to theHER/LER and the PF storage ring. In June, injection tothe AR storage ring was started.
During 7300 hours of the linac operation in FY 1999,
to reproduce these beams correctly and as quickly as
possible and to maintain the positron intensity for a long
period became a major effort.
The different beam modes have been successfully
switched using a software panel within about twominutes. Regarding the average intensity of positrons,
two remarkable steps were made after the 1999-summer
and the 2000-winter shutdown periods, as shown in

A-Gun
Positron Source
Sector 1

Sector C

Sector 2

C-GunPFECS
Sector 4

Sector 5

Beam Switchyard

Figure 1: The KEKB 8-GeV electron/positron linac.

KEKB

Table 1: Design Beam Goal and Achieved Performance(1) Gun(2) Buncher(3) Arc(4) e+ target(5) e+ Solenoid exit
(6) Linac end

EnergyIntensity
Pulse width
EnergyEnergy spread (σ)
Intensity
Efficiency
Emittance γβε (σ)
Bunch width
EnergyEnergy spread (σ)
Jitters (p-p)
Drift (with feedback)Emittance γβε (σ)
TransmissionEnergy
Intensity
Transmission
Intensity
Specific yield
EnergyEnergy spread (σ)
Intensity
Specific yield
Transmission
Emittance γβε (σ)
Pulse repetition

keV
nC/pulsens
MeV
MeVnC/pulse
mmps
GeV
MeV

1999

1.4
0.06
5
1.5
0.6%

1.9
95%
0.04
6
1.7
0.29%

mm

0.17
100%

GeV
nC/pulse
nC/pulse
e+/e-GeV
GeV
MeV
nC/pulse
e+/e-GeV
mm
pps

Figure 2. It increased from 0.32 nC to 0.45 nC, andthen again to 0.62 nC. The best injection rate of 1.7mA/s was recorded in January, 2000.

0.8

8-GeV electronGoal Achieved
2002001.522
1.816162000

0.48
0.15%
1.28>80.05%
>1.28

0.25
50>80%
0.31
50

3.5-GeV positron
GoalAchieved
20020013142
2.81515
22>10
1190%
0.06
0.08
16
10
1.5
1.7
0.6%
0.38%
0.1%
<0.2%/h1.7>95%100%
3.7
3.7>10
1096%
2.4
6.8%
3.5>3.5
0.125%
0.15%
>0.64
0.82
2.3%
1.5
50

1.450

Table 2: Injection Beam (June 2000)
RingParticle
EnergyPulse
WidthStore
Refill fromTimeInterval

HERLER
electron positron8 GeV
3.5 GeV
1 nC
0.6 nC
single bunch
500 mA 700 mA~350 mA
1 min
10min
1-2 hPFARelectron electron
2.5 GeV 2.5 GeV
0.2 nC
0.2 nC
1 ns
400 mA
40 mA~250 mA
0 mA
3 min
2 min
24 h
2-4 h

Table 3: Switching parameters and devices
04

7

10

14

7

Figure 2: Positron intensity (nC/bunch) vs. Month.
To increase the positron intensity further is stillnecessary for decreasing the injection time. Instead of asingle-bunch beam, a preliminary study for a two-bunch
scheme was recently attempted.

3 BEAM SWITCH AND INJECTION
The linac must inject four kinds of beams (Table 2).
To deliver these beams, some devices must be switchedand the parameters must also be changed, as shown in
Table 3. However, the number of changed items is
suppressed as much as possible, in order to decrease thechanging time and to reduce the error probability.
The data of the A-gun have been changed in order toemit a 1 nC pulse or a 10 nC pulse. In addition tocurrent control, a grid timing adjustment is essential
because the pulse width increases with charge.

Ring
HERGunA
gun data#1e+ target
off
pulse solenoid
offchicaneoff
rf timing#1
rf phase#1
magnet data#1
monitor range
#1

LERA
#2
on
on
on
#2
#2
#2
#2

PFC
#3
off
off
off
#3
#3
#3
#3

ARC
#3
off
off
off
#3
#4
#4
#3

When a positron beam is accelerated, a water-cooledtungsten target of 14 mm thick, as a positron radiator, isinserted into the beamline by a pneumatic actuator, and achicane (positron-electron separator) is also used. Whenan electron beam is accelerated, these devices are turned off
and a 2-T pulsed solenoid just after the positron radiator isalso switched off.
When some accelerator modules are not necessary to
be used, the rf timing is delayed by 100 µs. The rf phasevalues of sub-booster klystrons, each of which drives 8
high-power klystrons, are changed when the positronbeam is accelerated.
To adjust the beam energies
precisely, both at the 180-degree arc and the linac-end
beam switchyard, the rf phase values of two pairs of highpower klystrons are changed.
The parameters of the transport magnets are changed
according a simple standardize sequence. The up/down
speeds of the magnet currents are controlled so as to avoid
any problems due to abrupt switching that would damage
the power supplies.
Regarding a parameter change, the feedback software foreach beam detects the beam mode and runs automatically.

4 STABILITY OF THE LINAC
4.1 Tolerance of the linac parameters
Among four kinds of linac beams, it is most difficult
to maintain a high-current primary electron beam toproduce positrons [3].
Though many factors wereconsidered possibly to be responsible for degradation in
the positron intensity, when the positron beam intensity
decreased, we found that the primary electron orbit had
usually changed just below the buncher section, and thatthe primary electron intensity had particularly decreased
downstream of the 180-degree arc.
Some studies were carried out to understand the beamphysics concerning high-current single-bunch accelerationand to suppress beam blowup, both theoretically andexperimentally [4][5]. The conclusion was that it ispossible to transport a high-current beam if the intensity
is 10 nC per bunch and the initial beam orbit andaccelerator alignment distortion could be controlled to 0.1
mm. To control the beam orbit, the tolerances weremeasured by changing the parameters around the beam
condition that gave a nearly maximum current (9.4nC/bunch at the positron target). Table 4 gives theresults needed to keep the current at more than 90% of themaximum.
Table 4: Tolerance for High-Current Beam
Beam timing
Gun high voltage
SHB1 phase
SHB2 phase
Buncher phase±45 ps± 0.38%±1.1 deg±1.3 deg±1.7 deg

These parameters, except for the gun high voltage, isnot so easy to maintain for a long time, and had notactually been stabilized before the monitors and thefeedback system were installed [6].

4-2 Hardware improvements
The first initiative was to improve the monitors for therf and beam timing. A 114-MHz, 10-kW main amplifier
and a 2856-MHz 1-kW drive amplifier were replaced. The
cooling performance of the SHB2 was improved. These
were undertaken during the summer shutdown of 1999.

During the new-year shutdown, the gun-beam timing
became monitored and stabilized. A gun modulator and ahigh-power klystron modulator for the fundamental
bunchers were finely tuned and stabilized.
According to the rf and timing monitors and a beam
study of the linac parameter tolerances, it seems that thelinac beam instability was due to multiple factors
regarding the pre-injector.

4-3 Feedback system
At fast, using a host computer, we had developed
feedback software to stabilize the linac beam andequipment: 1) the beam energy at the linac arc and at theend of the linac; 2) the beam orbit along the linac; 3) thegun high-voltage; 4) the 114-MHz SHB power; and 5) the571-MHz SHB power.
Subsequently we have added feedback loops to: 6) the
gun grid pulse timing; 7) the 114-MHz SHB rf phase; 8)
the 571-MHz SHB rf phase.
Although some of these feedback tasks should be run
on local hardware or computers, this has not been done sofar. One reason for not using hardware is noise problems
in the klystron gallery. A fast host computer (CPUs:Alpha 21264, 500 MHz x 2) was added for this purpose
before the summer shutdown.

6. SUMMARY(1) The KEKB injector linac had been commissioned
and went into routine operation in December, 1998.
Two major goals of the energy (8 GeV) and the positron
intensity (0.64 nC/bunch) were achieved.
(2) During 1999, the average intensity of the positron
beam was remarkably increased, and the reproducibility ofthe different kinds of beam switchings has also been
improved by the linac stabilizing project.
(3) To further increase the positron intensity, apreliminary study of two-bunch acceleration has just been
initiated.

REFERENCES[1] A. Enomoto, et al., “Commissioning of the KEKB 8GeV e- / 3.5-GeV e+ Injector Linac”, EPAC’98,
Stockholm, June 1998, pp.713-715.
[2] Y. Ogawa, et al., “Commissioning of the KEKB Linac”,
LINAC98, Chicago, August 1998.
[3] S. Ohsawa, et al., "Pre-injector of the KEKB Linac", thisconference.
[4] Y. Ogawa, et al., "Quadrupole Wake-Field Effects in the
KEKB Linac", this conference.
[5] T. Kamitani, J. Q. Wang, and S. H. Wang, "Simulationsof Wake Effects on a High-Current Electron Beam at the
KEKB Injector Linac", KEK Report 2000-4.
[6] K. Furukawa, et al., "Energy Feedback Systems at the
KEKB Injector Linac", ICAREPCS99, Trieste, October,
1999, pp.248-251.