HIGH-INTENSITY AND LOW-EMITTANCE UPGRADE OF 7-GeV
INJECTOR LINAC TOWARDS SuperKEKB
K. Furukawa∗ , M. Akemoto, D. Arakawa, Y. Arakida, A. Enomoto, S. Fukuda, H. Honma,R. Ichimiya, N. Iida, M. Ikeda, E. Kadokura, K. Kakihara, T. Kamitani, H. Katagiri, M. Kurashina,
S. Matsumoto, T. Matsumoto, H. Matsushita, S. Michizono, K. Mikawa, T. Miura, F. Miyahara,T. Mori, H. Nakajima, K. Nakao, T. Natsui, Y. Ogawa, S. Ohsawa, F. Qiu, M. Satoh, T. Shidara,A. Shirakawa, H. Sugimoto, T. Suwada, T. Takatomi, T. Takenaka, M. Tanaka, Y. Yano,
K. Yokoyama, M. Yoshida, L. Zang, X. Zhou, KEK, Tsukuba, Japan, D. Satoh, TIT, Tokyo, JapanRF GUNAbstract
After a decade of successful operation at KEKB a new
asymmetric electron/positron collider, SuperKEKB, is being constructed upgrading KEKB. It aims at a luminosity of
8 × 1035 cm−2 s−1 , 40-times higher than that of KEKB, inorder to study the flavor physics of elementary particles further, by mainly squeezing the beams at the collision point.
The injector linac should provide high-intensity and lowemittance beams of 7-GeV electron and 4-GeV positron by
newly installing a RF-gun, a flux concentrator, and a damping ring with careful management of emittance and energy
spread to be 20 mm mrad and 0.1%. It should also perform simultaneous top-up injections into four storage rings
by pulse-to-pulse beam modulations not to interfere between three facilities of SuperKEKB, Photon Factory andPF-AR. This paper describes the injector design decisions
and present status of the construction. SuperKEKB will becommissioned within FY2014

INTRODUCTION
During a decade of successful operation, KEKB asymmetric electron/positron collider offered important insights
into the flavor structure of elementary particles [1]. KEKB
is being upgraded towards SuperKEKB with a design toachieve 40 times higher luminosity in order to elucidate
new physics beyond the standard model of elementary particle physics. The injector linac should be upgraded to enable a beam size of 50 nm at the collision point and twicelarger stored beam currents with short expected lifetimes of10 minutes. The requirements to the injector are the emittance of 20 mm mrad and the energy spread of 0.1% forboth of electron and positron at the end of linac, and thebunch charge of 5 nC for electron and 4 nC for positron.
Two bunches in a pulse are expected at a rate of 50 Hz.
Low-emittance and high-current electron would be delivered employing a photo-cathode RF gun. High-current
positron would be generated using a flux concentrator andlarge aperture accelerating structures, and then it is damped
to low emittance through a damping ring (DR). All the storage rings of SuperKEKB HER, LER, PF and PF-AR should
be filled in top-up injection mode by using pulse-to-pulse
modulation (PPM) at 50Hz (Fig. 2). Recent progress is described in following sections.
∗ < kazuro.furukawa @ kek.jp >Low emittance electron beam source and its transport
are indispensable to realize nano-beam scheme for higher
collision rate at SuperKEKB. While DR is employed for
positron emittance reduction, the same solution is not possible for electron because of the cost and space. Thus, aphoto-cathode RF gun has been developed. Generation ofa high-charge electron bunch up to 5 nC is already challenging, and it is also preferable to generate a bunch morethan 10 nC as a primary beam for positron generation.
The primary target of the gun is the bunch charge of 5
nC and the emittance of 10 mm mrad to give a room forslight emittance blow-up along the linac. Each componentof RF gun, such as a laser, a photo cathode or a cavity,
was examined carefully for the stable long-term operation.
As a baseline, the combination was chosen with Nd:YAGlaser for higher power, LaB6 cathode for longer lifetimeand reasonable quantum efficiency, and side-coupled disk
and washer (DAW) cavity for larger aperture and focusing(Fig. 1). The combination was installed at a test station 3-2
at the middle of linac.
Laser and microwave systems were tuned carefully.
However, the performance of the cathode was not favorable, and then it was replaced with Ir5 Ce cathode [2]. After
further tuning at the laser optical system and beam transport, 4.8-nC beam was achieved, which is the designed
beam current, and 4.4-nC beam was transferred to the endof linac. This baseline electron source can be used at thebeginning of the commissioning.
However, in order to deliver an electron beam with required characteristics, space charge effect mitigation by alonger bunch length (30ps) and especially energy-spread

Figure 1: DAW-type cavity for the baseline RF gun.

Figure 2: Layout of injector linac.
mitigation by a rectangular bunch shape should be manipulated carefully (temporal manipulation). Another test station A-1 with a Yb-doped fiber oscillator, a fiber amplifier, a Yb:YAG thin-disk regenerative amplifier, a thin-disk
multi-pass amplifier, and two-stage frequency doublers was
installed to examine such a high-power and shaped laser
pulse [3, 4]. A quasi traveling-wave side-couple cavity
will replace the DAW cavity soon for further focusing andhigher gradient [5]. The beam test goes well at lower beam
charge of 0.3 nC without shape manipulation at first for theinitial beam commissioning in autumn 2013.

POSITRON ENHANCEMENT
The positron beam generated in the KEKB injector wasapproximately 0.8nC × 2 bunches at 50 Hz. It shouldbe enhanced up to 4 nC in stages, because of the designof SuperKEKB with twice larger beam current and muchsmaller beam lifetime down to 10 minutes. A high charge
positron bunch will be generated with a conventional tungsten target and will be captured employing a flux concentrator and large aperture accelerating structure with velocity bunching. As the generated beam emittance is large, itwill be damped employing DR at the energy of 1.1 GeV.

Flux Concentrator to capture more positron
One of the keys to increase the positron yield is the development of focusing pulsed coil just after the positron
generation target. An air core pulsed coil was employedin the previous KEKB project. For SuperKEKB, it will
be replaced by a pulsed flux concentrator (FC) with a highfield of 4 Tesla. FC is a pulsed solenoid composed of a
primary coil and a copper cylinder with a conical hole inside. Induced eddy current flows through a thin slit to an
inner surface and generates a strong field of several Tesla.
Achievable field strength is mainly determined by a holediameter and a primary pulsed current.
The design of the FC was examined based on the collaboration with BINP, SLAC, Frascati and IHEP. While a
BINP-type was considered as a future upgrade with higher
field, a conventional SLAC/Frascati/IHEP-type FC was
fabricated in KEK with a spiral slit for the preliminary
positron beam commissioning in December 2013 (Fig. 3).
After a magnetic field measurement, it was tested at a peakcurrent up to 6 kilo ampere, a half of the nominal current,
without any issues such as discharges.
Several modified types of FCs would be fabricated andcompared for different shapes even with a new entrance

Figure 3: Flux concentrator under field measurement.
cone design [6]. Simulation studies from primary electron to DR including FC and large aperture structures were
performed for higher field, better field symmetry, higher
positron capturing ratio and lower beam loss. The target
protection scheme is also under development not to makethe energy deposit exceed a known limit [7].

L-band system as backup
The initial plan of the positron enhancement employed
L-band (1298 MHz) accelerator structure with a larger
aperture and longer wavelength compared with other Sband structure. The frequency was chosen considering
the integer relation between linac and ring frequencies,
filtering feature to reject satellite bunches, and development synergy with the linear collider project. Besides thelarge positron acceptance, elimination of S-band satellite
bunches might be effective to reduce the beam loss at DR.
It was also planned to utilize L-band for the beam bunch
compression system after the extraction from DR.
The design and the fabrication of the system started inFY2010 with a 40-MW klystron and an accelerating structure (12 MV/m at 15 MW). In parallel the detailed design
was examined with iterative beam simulations of positron
capturing and acceleration, and it was found that a comparable performance could be achieved with a large aperture
S-band structure and the velocity-bunching scheme. It isalso known that S-band system has advantage for the costand maintainability. Thus, it was decided to employ S-band
system at least at the first stage of the project.
Nevertheless, since the margin for the requirement isnarrow and L-band system should be ready as a backup
scheme, high-power tests of newly-developed components
were performed in FY2012. A klystron was confirmed
to operate at sufficient parameters (30 MW, 1.5 µs). A
new 2m-long constant gradient travelling wave structure
was also tested up to 6 MW within a limited conditioning

time. A low-weight aluminum vacuum wave guide and a
SiC dummy load gave satisfactory performance.
In order to eliminate the vacant space between the accelerating structure and the positron focusing solenoid magnets and to reduce the size of the solenoids the development of a collinear RF load was conducted. Several ofcells at the end of the structure were covered with thermalsprayed RF absorber (Kanthal alloy). Various tests of thermal spray of the absorber were performed to optimize thesuitable method for the load. It was technically evaluatedby debonding test of the sprayed surface and the measurement of the gas emission in vacuum. A test load was constructed for the high power test. So far no essential issue
has been found [8].

EMITTANCE PRESERVATION
If a beam bunch accelerated in the structure has an offset from the center, generated wakefield induces transverse
force to the tail, and projected emittance can be very large.
In order to suppress the effect, mechanical alignment of
quadrupole magnets and structures up to 100 µm and precise beam handling are required. The beam orbit should bemanipulated to find a low-emittance condition empirically,
then the orbit should be stabilized. The beam bunch shapeshould be also manipulated employing laser pulse shaping,a magnetic chicane, and an arc section.
Several tests were performed to manipulate the beam
bunch shape. In order to compress the bunch, one of thebeam optics parameters, R56, was designed and was confirmed (Fig. 4). As the direct longitudinal profile measurement was not possible yet, it was evaluated by measuring
dispersion functions.

BEAM INSTRUMENTATION, CONTROLS
Four storage rings should be filled in top-up injection
mode simultaneously as described before. To this end,
the linac should be operated with precise beam controls.
Dual-layer controls with EPICS and event-based systems
were being enhanced to support beam operation with precise pulse-to-pulse modulation (PPM) at 50Hz. A virtualaccelerator (VA) concept is being introduced to enable asingle linac behaving as four VAs switched by PPM, where
each VA corresponds to one of four top-up injections into
SuperKEKB HER, LER, PF and PF-AR. And each VA
should be accompanied with independent beam feedback
stabilization loops to preserve the orbit with low-emittancecondition.
It is also important to install adequate safety systems
since the beam intensity is very different between beammodes [11].
Several developments have been underway for beam instrumentation as well. An X-band deflector was designed
to measure the vertical beam emittance at the end of thelinac for SuperKEKB. Since the 1-m-long deflector developed for LCLS (SLAC) can fit to the design well, the same
design was adopted. The construction work was conductedunder the Japan-US cooperation framework. The cells weremade in Japan and assembled at SLAC. It was finally vacuum baked and fiducialized at SLAC (Fig. 5).
A single-shot sliced emittance measurement would bepossible, and it is hoped to operate on an unused bunch (astealth bunch measurement) using VA and PPM controls.

Figure 5: A 1-m-long X-band deflector for single-shot andsliced vertical emittance measurement.

CONCLUSION AND FUTURE
The injector linac is being upgraded towards
SuperKEKB. Low-current linac commissioning will
be performed in autumn, 2013, and the first injection will
start at the beginning of 2015.
Figure 4: Designed optics parameter (right) was reproduced in the beam test (upper left).

Alignment
It is desirable to have alignment down to 100 µm locallyand 300 µm globally along 120-m and 480-m straight lines.
Local measurement was performed by a conventional laser
tracker. However, as the linac tunnel was narrow, global
measurement was challenging. Thus, a new long baseline laser beam was developed, which was stabilized by aprecise angle adjuster, guided through vacuum pipes under
the accelerator girders and measured by position sensitive
photo detectors. The measurement precision was 60 µm
over 480-m straight line [9].

REFERENCES[1] M. Akemoto et al., Prog. Theor. Exp. Phys. (2013) 03A002.
[2] D. Satoh et al., these proceedings (IPAC13, MOPFI023).
[3] X. Zhou et al., these proceedings (IPAC13, WEPME018).
[4] M. Yoshida et al., these proceedings (IPAC13, TUPFI004).
[5] T. Natsui et al., these proceedings (IPAC13, TUOCB103).
[6] L. Zang et al., these proceedings (IPAC13, MOPFI018).
[7] L. Zang et al., these proceedings (IPAC13, MOPFI017).
[8] F. Miyahara et al., these proceedings (IPAC13, WEPFI019).
[9] M. Satoh et al., these proceedings (IPAC13, WEPME016).
[10] A. Akiyama et al., Prog. Theor. Exp. Phys. (2013) 03A008.
[11] A. Shirakawa et al., these proceedings (IPAC13, THPEA007).