PRE-INJECTOR OF THE KEKB LINAC
S. Ohsawa, A. Enomoto, K. Furukawa, M. Ikeda, K. Kakihara, T. Kamitani, H. Kobayashi,
Y. Ogawa, T. Oogoe, A. Shirakawa, T. Suwada, T. Urano, S. Yamaguchi, KEK, Tsukuba, Japan

Abstract
The pre-injector of the KEKB linac has been completed
and is in operation. 10-nC electron bunches, withoutsatellite bunches, are created and used for producing
positrons, while 1-nC electron bunches are directly
injected into the KEKB 8-GeV ring. Observed bunchwidths are typically 10 ps (FWHM) or less. The highintensity beam is stably accelerated to a positron
production target with a transmission efficiency of greaterthan 90%. Major efforts to stabilize the beam and theirresults are described.

1 INTRODUCTION
Since 1994, the KEK electron / positron linac has been
upgraded in order to inject 8-GeV electrons and 3.5-GeV
positrons directly into the KEKB rings. The major goalsof the linac upgrade have been to increase the acceleratingenergy from 2.5 to 8 GeV and to increase the positron
intensity by about 20 times. One of the most important
points at issue has been to stably accelerate a high-density
single-bunch beam of 10 nC to the positron-producing
target without loss. Fundamental characteristics of thebeam are strongly dependent on the performance of thepre-injector, which makes a high-density, short bunch of
10-ps width (FWHM) from a 200 keV low energy beamof 2-ns width.
The linac was extended upstream and 19 accelerator
modules were newly installed in a new building. The first
12 accelerator modules including the pre-injector werecompleted by the end of September 1997. The beamcommissioning was initiated on October 7th, 1997 before
completing the 180°-bending section. After having
combined the old and extended linacs at the end of March
1998, the beam was transported through the entire linac.
The linac rf repetition rate has been raised from 25 pps up
to 50 pps since operation started in September 1998.
We have carried out the commissioning step by step
while gradually increasing the electron beam intensities[1]. By precisely tuning the rf phases and beam orbit,
about 10 nC electron bunches accelerated by October
1998. But, as was expected, it was difficult to switch
beams to deliver to one of four different rings at every
injection. In the linac system some very old componentsthat have been used since the start of operation in 1982 arestill in use. After completion of the upgrade, we
immediately started a project to improve thereproducibility of the beams. In September 1999, the linac

reached the stage wherein a high intensity beam of 10nC
bunches could be accelerated stably to the positron
production target with a transmission rate of greater than90%.
The main elements of the pre-injector had been moved
except for two accelerating sections from the previous
pre-injector of the old linac. Some parts of the preinjector were improved step by step so as to make beams
more stable in the high current region. A cavity of the firstsub-harmonic buncher (SHB1) that had been utilized from1985 was replaced with a newly designed one in May
1998. There was a problem with the cooling performanceof the cavity at high power. In August 1999, a solid-state
amplifier replaced an old planar-triode-type rf amplifierfor SHB1. At the same time a newly designed SHB2cavity was installed, the shunt impedance of which wasimproved two times in order to relax power requirement
for its power amplifier.
As the beam charge increased, it became difficult tomaintain some equipment within tolerance limits forkeeping the beams stable without feedback systems. Some
software feedback loops were added during thecommissioning. They cover the gun accelerating voltage,
the gun-beam trigger timing and the power levels of theSHBs [2].

2 SYSTEM CONFIGURATION
The pre-injector consists of an electron gun, a bunchingsystem and two accelerating sections with focusing,vacuum and monitor systems. The electron gun is a2
triode-type thermionic gun with a 1cm cathode. The
bunching system is composed of two SHBs withththfrequencies set at the 25 and 5 sub-harmonics of 2856
MHz, and a prebuncher (PB) and a buncher.
The SHB frequencies are chosen from sub-harmonics
near 100 MHz and 500 MHz with the condition that eachfrequency should be a multiple of the linac/ring common
frequency of 10.385 MHz, which is a 25x11th
subharmonic of 2856 MHz.
Table 1: Operation parameters
Element
Frequency (Power)Gun50 Hz200kV
SHB1 (standing wave)
114 MHz
11 kW
SHB2 (standing wave)
571 MHz
7 kW
Prebuncher(travelling wave) 2856 MHz1 MW
Buncher (travelling wave)
2856MHz23 MW
Accelerating sections
2856MHz
12 MW(travelling wave)
x2

OTR

Analyser Accelerator Sections Q-mag. Beam Scraper Buncher

PB SHB2

SHB1

Gun

Figure 1: Schematic layout of the pre-injector. ORT is the optical-transition-radiation monitor for measuring bunches.
The klystron for the pre-injector is a special one thathas a generating capacity of 60 MW. Since the buncher rf
power was doubled, the bunching performance wasimproved especially in the high intensity region, as wasexpected from computer simulations [3].

3 PERFORMANCE OF MAIN ELEMENTS
2.1 Gun grid pulser
We introduced a new grid pulser in September 1999 todeliver stable and short electron beams from a triode-type
thermionic gun. Its time jitter is remarkably small (σ~4ps) , with a pulse width ( FWHM) as small as 1ns. We
designed the new pulser to be divided into two parts. Thepulse generating part is installed in the conical portion of
the gun body to avoid degradation of the pulsecharacteristics from transmission over long cables. The
other part is composed of power supplies and voltage
control circuits, which are assembled in a NIM crate.
Another feature of the pulser is that it has an electricanalog time-delay unit that continuously varies the trigger
timing over a 3-ns range. This function is useful forcontrolling the beam timing with a feedback system,
which is described in the section 2.3.

resistance depends on its temperature. Under the present
operation conditions, the temperature rise is about 10°C
when it becomes stable, which causes a gun-voltagechange of 1.5%. A software feedback system has been
adopted to compensate the change by optimising themodulator reference voltage depending on the change.
The gun-pulse voltage is kept constant by the feedback
system within a deviation of 0.05%. This satisfies therequirement of less than 0.1%, which corresponds to a
1.1-ps change of the gun-beam timing at a beam monitor
located upstream the SHB1.

2.3 Gun beam timing control
If the pulse timing of the gun beam changes, the beamis bunched, in general, at a different rf phase with adifferent bunch shape. The requirement on the timing
stability is as severe as 100 ps in the case of a 10-nC beamthat is used to produce positrons. When a timing change
exceeds this value, more than 10% of the beam is lost
during acceleration to the target.
The beam intensity delivered from the gun is controlled
by both grid-pulse and grid-cathode bias voltages. We aremonitoring the timing and amplitude of the grid pulses
after averaging 30 pulses to eliminate noises. The pulse
timing is compared with a reference wave of 571MHz at adigital oscilloscope on the ground level. The timedifference is kept at a constant value by a feedback loop.
Triggering signals for the grid pulses are synchronisedwith a reference rf wave of 571MHz by a time delay unit
(TD4R) at the 200KV level. The synchronisation system
is very effective for decreasing the time jitter (σ~2ps).
ThereFigure 2: Pulse shape of the new grid pulser. A typicaltiming jitter (σ) is about 4 ps. The h-axis is 500ps/division.

2.2 Gun voltage control
The electron gun is operated at 200 kV with 50 pulses
per second. When the gun modulator continuouslyoperates without stops, the pulse amplitude remains stable.
It actually stops, however, at an interval of two or fourhours in regular operation, when the Photon Factory rings
require injections from the other electron gun. This causes
temperature change in load resistors of the gun modulator.
This means that the gun voltage changes whenever themodulator starts operation again because the load

Figure 3: Timing history of grid pulses for 5 days: thevertical axis shows time at 50ps/division. The lower bluepoints represent grid-pulse timing, while the upper red
points indicate the time difference between grid pulses
and gun-beam pulses.

is, however, some element sensitive to ambient
temperature somewhere. The feedback system isnecessary to assure of the long-time stability.
Operational history of the grid-pulse timing is shown in
Fig.3. The pulse timing is maintained usually within 20 ps.

2.4 Sub-harmonic Bunchers
In order to relax the requirements on the electron gun,two SHBs were introduced into the bunching system. The
cavities are newly designed and fabricated to overcome
difficulties of the old cavities [4]. Copper-plated stainlesssteel (SUS304) was used for the old SHB1 cavity. This
time oxygen free copper was selected as the structural
material for both cavities to improve cooling performancewith a new cooling-pipe configuration. From computer
simulations we had learned before fabrication that, owing
to rf power dissipations, the resulting cavity thermal
deformations changed the resonant frequencies, but they
were estimated to be tolerable. Shunt impedances wereimproved as shown in Table 2 in order to relax powerrequirements for power amplifiers.
For stable acceleration, the phase stability of eachcavity is important, especially for the 10-nC singlebunched beam. The phase tolerance on the beam, comingfrom the goal of keeping the beam transmission rate above
90% at the target, is only 1.5° for each cavity. We aremonitoring the rf phases of these cavities relative to the2856-MHz accelerating rf wave by means of a sampling
oscilloscope. It is useful for measuring relative phaseshifts between different frequencies. It shows all
deviations from the master oscillator wherever they occur.
Since the phases are usually stable at the present time on
the timescale of a week, the SHBs operate without phasefeedback systems.

Table 2: Results of low-power tests
Parameters
SHB1
SHB2
1 .14
3.04
Shunt impedance (MΩ)
Q0 (Measurement)
6989
12580
R0/Q0 (Measurement)163
241
Acceleration Gap (mm)
42.5
30.05
Shuntimpedance
1.7
2.03(new/old)

2.5 Buncher and Accelerating Sections

The prebuncher and buncher were manufactured in
1992 for the purpose of studying the potential of the KEK
PF linac for high current acceleration. After attaining thataim, they were moved and reconstructed in the new preinjector of the KEKB Linac.
In March 2000, RF reflections owing to electric
discharge began occurring often in the buncher. We
opened a beam line downstream of the buncher and founddiscoloration in parts of the output coupler and someadjoining disks.

Since an rf source klystron fed power into the buncher
and two accelerating sections, we decided immediately tochange the rf feed connections of the two accelerating
sections from serial to parallel in order to reduce the total
rf-filling time. By this rearrangement it became possible
to shorten the rf pulse length from 1.5 to 0.5 µs, which
succeeded in avoiding electric discharges.

4 BEAM CHARACTERISTICS
The beam characteristics of the single bunches aremeasured at the buncher and the pre-injector exits. All ofthem satisfy the goals. As a result the beams are stably
accelerated to the target or the end of the linac withtransmission efficiencies of greater than 90%.

Figure 4: The 10-nC electron bunch of 10ps width.
Table 3: Typical beam characteristics
Parametersee- for e+
Beam charge#1 nC
11 nC
Emittance γε#
35 µm
80 µm
Beam energy#
16.1 MeV 14.8 MeV
Bunch Width (FWHM)
8 ps
10 ps
Satellite bunches0%< 1%
Beam charge
1 nC
10.5 nC
Beam energy
64 MeV
60 MeV# at the buncher exit. Others are at the pre-injector exit.

ACKNOWLEDGEMENTS
The authors wish to thank to all of the staff members ofthe linac and the commissioning group for their support.

REFERENCES[1] Y. Ogawa, et al. “Commissioning of the KEKB
Linac”, LINAC98, Chicago, U.S.A., August 1998.
[2] K. Furukawa, et al. “Energy Feedback Systems at the
KEKB Injector Linac”, ICAREPCS 99, Trieste, Italy,October 1999. pp.248-251.
[3] S. Ohsawa, et al. “High-Intensity Single-Bunch Beamof the PF 2.5-GeV Linac”, LINAC94, Tsukuba, Japan,
August 1994. pp.193-195.
[4] S. Yamagichi, et al. “Development of a 114.24MHz
LINAC98, Chicago, U.S.A., August 1998. 558-561