UPGRADE STATUS OF INJECTOR LINAC FOR SuperKEKB
T. Miura#, M. Akemoto, D. Arakawa, Y. Arakida, A. Enomoto, S. Fukuda, Y. Funakoshi,
K. Furukawa, T. Higo, H. Honma, R. Ichimiya, N. Iida, M. Ikeda, E. Kadokura, H. Kaji,K. Kakihara, T. Kamitani, H. Katagiri, M. Kurashina, S. Matsumoto, T. Matsumoto,H. Matsushita, S. Michizono, K. Mikawa, F. Miyahara, H. Nakajima, K. Nakao, T. Natsui,Y. Ogawa, Y. Ohnishi, S. Ohsawa, F. Qiu, M. Satoh, T. Shidara, A. Shirakawa, H. Sugimoto,T. Suwada, T. Takenaka, M. Tanaka, Y. Yano, K. Yokoyama, M. Yoshida, L. Zang, X. Zhou,KEK, Ibaraki, Japan, Daisuke Satoh, TIT, Tokyo, Japan
Abstract
KEK injector linac is being upgraded towards theSuperKEKB which is an asymmetric collider of 7-GeVelectron and 4-GeV positron. Full-energy, high-charge andlow-emittance beams are necessary in order to achieve adesign luminosity of 8×1035 cm-2s-1 for the study of theflavour physics of elementary particles. New components
such as a photocathode RF gun and a positron capturesystem with a flux concentrator were already integrated.
Electron and positron beams are commissioned to confirm
the beam properties.

INTRODUCTION
The KEK injector linac is currently being upgraded inorder to deliver the high-charge and low-emittance beams
to SuperKEKB. The SuperKEKB is an electron-positron
collider, and aims 8×1035 cm-2s−1, which is 40 times larger
than KEKB based on the nano-beam scheme. The
requirements of the injection beam are 7-GeV electrons of
5 nC and 4-GeV positrons of positrons of 4 nC. Lowemittance (less than 20 mm-mrad), and two-bunch
injection of pulses with 50 Hz repetition rate as shown in
Table 1. A photocathode RF gun has been installed in orderto produce low-emittance and high-charge electron beams.
Commissioning has begun, and in parallel, thedevelopment of high-intensity laser system has beencontinued. A new positron-capture section with a fluxconcentrator (FC), large aperture S-band accelerating
structures (LAS) and solenoid focusing coils has beenconstructed. The construction of the positron damping ring(DR) for achieving low emittance is in progress. The
alignment of the components for emittance preservation
has been performed within a tolerance of 0.1 mm in sigma
locally and 0.3 mm in sigma globally [1,2]. It should also
perform simultaneous top-up injections into four storagerings of High Energy Ring (HER), Low Energy Ring(LER) of SuperKEKB, Photon Factory (PF), and Photon
Factory Advanced Ring (PF-AR) by pulse-to-pulse beam
modulations at 50 Hz. The RF system was also developed
to correspond such a pulse modulation. Commissioning ofelectron and positron beams has begun to confirm the beamproperties [3]. This paper describes the recent upgrade
status of injector linac.

________________________________________# takako.miura@kek.jp

Table 1: KEKB / SuperKEKB linac parameters
Parameters

ElectronPositron

Beam Energy [GeV]

8.0 / 7.0

3.5 / 4.0

Bunch charge [nC]

1/5

1* / 4*

Emittance [mm mrad]

100 / 202100 / 10

Energy spread [%]0.05 / 0.1

0.125 / 0.1

Number of bunch

2/2

2/2

(*) bunch charge of primary electron =10 nC

PHYSICAL CONFIGURATION OF THE
INJECTOR LINAC
The injector linac should deliver beams with variable
quantities of charge and energy to four different rings. The
beam mode is switched with a 50-Hz repetition rate of thelinac. That is termed simultaneous injection. The layout of
the injector linac is shown in Fig.1.

Figure 1: Layout and beam energy patterns of KEK
injector linac.
Upstream of the positron target, electrons are accelerated
with a common energy for each beam mode. The electron
beam from the RF gun reaches an energy of 3.3 GeV at thepositron target.
The section of the beam line between the target and theDR is designed to include compatible optics for low-energypositron and high-energy electron. The optics are mainly
designed for the transport of large-emittance positrons.
Hence, the focusing force for high-energy electrons is very
weak in this section.
Downstream of the DR, the acceleration energy depends
on the beam mode. Therefore, pulsed-quads and pulsedsteering magnets will be installed in order to optimize theoptics for different beam energies and to simplify the beamhandling.

vertical emittance were obtained 32.7 ± 3.1 and 10.7 ± 1.4mm mrad, respectively[5]. Currently, commissioning of25-Hz operation is proceeding.

POSITRON CAPTURE SECTIONPHOTOCATHODE RF GUN
A photocathode S-band quasi traveling-wave sidecouple RF gun [4, 5] was installed as a low-emittanceelectron-beam source in September 2013 (see Fig.3). ThisRF gun consists of seven acceleration cavities and coupling
cavities as shown in Fig.2. It has two side-coupled standing
wave field. Thus, the quasi traveling-wave side-couple RFgun has a strong focusing force and a high accelerating
gradient. The material of the photocathode is Ir 5Ce [6],which has a long life time and a quantum efficiency larger
than 10-4.

Figure 4: Schematic drawing around the positron target.
For SuperKEKB, four times greater bunch charge
compared with the KEKB positron beams is necessary, but
the bunch charge of the primary electron beam for positron
production is the same in KEKB and SuperKEKB.
Therefore, in order to increase positron yield, a newpositron-capture system is developed using FC [8], and RF
capture system using six LAS [9]. Figure 4 shows thepositron target. In the case of electrons, beam passes
through a hole (2mm) in the target located at the center ofthe beam line in order to preserve low emittance. On theother hand, the positron target made of tungsten ( 4 mm,14 mm in length) is placed with a horizontal offset of 3.5mm from the beam line axis. Therefore injection orbits ofthe electron beam and of the primary electron beam for
positron production are switched using two-pulse steering
upstream of the target.

Figure 2: Quasi travelling-wave side-couple RF gun.
LAS in Solenoid
Waveguide

FC/TargetHV line

Figure 5: Positron production system.
Figure 3: Photo of installed photocathode RF gun.
Beam commissioning with the RF gun is in progress. Amaximum beam charge of 5.1 nC has been confirmed withYb:YAG thin-disk laser system [7] at the low repetition
rate of 5 Hz. As a result of a quad-scan for the beam of 1nC with 2 Hz repetition rate, the normalized horizontal andThe positron capture section is one of the most important
upgrades of the injector linac. The positrons are collectedby a FC of 3.5 T followed by the RF accelerator capturesystem with a 5kG solenoid magnetic field. The LAS,
which have a large physical aperture of 30 mm incomparison with typical 20 mm are employed as thepositron-capture RF system. The construction of thepositron production system was already installed as shownin Fig.5. In addition, a spoiler [10] was also installed
upstream of the target to protect damage to the target
caused by the large intensity and focussed beam.
As shown in Fig.6, many quadrupole magnets wereinstalled in a dense arrangement in order to accept andtransport large-emittance positrons downstream of thesolenoid section. Since the quadrupole magnets areoverlapping with the accelerating structures, DC-type isemployed. Both of low-energy positron and high-energy
electron beams must be transported in the same magnetic
condition.

digitized IQ waveform data are sent through a Gb-Ethernet
to the EPICS-IOC server at the 50-Hz repetition rate of theinjector linac.

Figure 7: Commonly designed digital RF drive unit andRF monitor.

SUMMARY
The upgrade of injector linac towards SuperKEKB isongoing. A photocathode RF gun as a high bunch charge
and a low-emittance electron-beam source was installed. A
positron capture section using FC and LAS with solenoid
magnets were installed in order to increase positroncapture efficiency. The RF system have also beendeveloped for four-ring top-up simultaneous injection.
Currently, beam commissioning has been started duringongoing construction.
Figure 6: Densely placed quadrupole magnets fortransportation of large emittance positrons. They areoverlapping with the accelerating structures.

RF DRIVE UNIT AND MONITORThe RF system has also been upgraded in order tosupport RF modulation for four-ring simultaneous top-up
injection. The RF monitor in operation during KEKB could
not distinguish the beam modes. Therefore, in three-ring
simultaneous injection at the time of KEKB operation, wecould not monitor the stabilities of the RF outputs of theklystron and SLED which is an RF pulse compressor. A
new RF drive unit and RF monitor using IQ digitalconversion have been developed. The IQ digital techniquemakes possible the simple amplitude and phasemodulations. Phase-inversion timing of the SLED input RF
is important in order to reduce the beam accelerationenergy jitter. Thus, the RF system should synchronize withthe clock of an “event timing system” of 114.24 MHz,
which is synchronized with the RF frequency (2856 MHz).
A common design was adopted for the RF driver andmonitor. A combination of 14-bit DAC/ADC and IQ
modulator/demodulator is employed for the RF drive unit
and RF monitor as shown in Fig.7. The event mode andphase information from “event generator” is received by aSFP and FPGA directly through an optical fiber. TheREFERENCES[1] T. Higo et al., “Linac Alignment for SuperKEKB Injector”,
TUPRI090, these proceedings.
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[6] D. Satoh et al., “Development of Iridium Cerium CathodeMaterial for High Charge and High Current”, MOPRI037,
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