SuperKEKB POSITRON SOURCE CONSTRUCTION STATUS
T. Kamitani∗ , M. Akemoto, D. Arakawa, Y. Arakida, A. Enomoto, S. Fukuda, Y. Funakoshi,
K. Furukawa, T. Higo, H. Honma, N. Iida, M. Ikeda, E. Kadokura, H. Kaji, K. Kakihara,
H. Katagiri, M. Kikuchi, H. Koiso, M. Kurashina, S. Matsumoto, T. Matsumoto,
H. Matsushita, S. Michizono, K. Mikawa,T. Mimashi, T. Miura, F. Miyahara, T. Mori,
A. Morita, H. Nakajima, K. Nakao, T. Natsui, Y. Ogawa,Y. Ohnishi, S. Ohsawa,
M. Satoh, T. Shidara, A. Shirakawa, M. Suetake, H. Sugimoto, T. Suwada, T. Takatomi,
T. Takenaka, M. Tanaka, M. Tawada, Y. Yano, K. Yokoyama, M. Yoshida, L. Zang, X. Zhou,
KEK, Tsukuba, Japan, D. Satoh, Tokyo Inst. of Technology, Tokyo, Japan
Abstract
The KEKB electron/positron injector linac is under the
upgrade for SuperKEKB. A new positron source with a target, a beam spoiler, a flux-concentrator and large-aperture
S-band accelerating structures inside DC solenoids and anelectron/positron separator have been installed. Preliminary positron beam commissioning has been started. The
first positron beam has been observed after the upgrade. A
construction status of the new positron source and a recent
positron beam performance are reported.

components and a preliminary positron beam performanceare described in the following sections.

INTRODUCTION
A positron source upgrade is one of the major challenges
for the SuperKEKB injector linac [1]. In the upgrade, thepositron bunch intensity is boosted from 1 nC to 4 nC by
increasing the positron capture efficiency and the positron
beam emittance is shrunk from 2100 µm to 92, 7 µm inthe horizontal and vertical planes by introducing a damping ring. More concretely, we have been introducing or
upgrading the following items, (1) a flux concentrator (FC)as a positron focusing solenoid with a larger energy acceptance, (2) a positron production target which fits in the FC
geometry and a beam spoiler for target protection, (3) bridge
coils to make smooth solenoid field distribution between
the FC and downstream DC solenoids, (4) large aperture
accelerating structures for the positron capture section anda subsequent accelerator module for enlarging transverse
phase space acceptance, (5) an electron/positron separator
chicane and a beam stopper for eliminating secondary electrons, (6) a focusing system of quadrupole magnets whichmatches to the enlarged acceptance of the capture section,(7) beam collimators to eliminate positron beam halo, (8)a damping ring (DR), (9) beam transfer lines to/from DR
including an energy-spread compression system at the injection line and a bunch compression system at the extraction
line, (10) an upgrade of the another energy-spread compression system and the switchyard beam lines at the end of thelinac corresponding to the change of the electron/positron
beam energy asymmetry from 8.0/3.5 GeV to 7.0/4.0 GeV.
The components (1) to (6) have already been installed in the
beam line as shown in Fig. 1 and a preliminary positron
beam commissioning has been started. A status of these∗ E-mail: < takuya.kamitani @ kek.jp >Figure 1: SuperKEKB positron capture section.

PRIMARY ELECTRON, TARGET AND
BEAM SPOILER
Primary electron beams are accelerated up to 1.5 GeV
before the 180 degree J-arc by the eleven accelerator modules of 160 MeV energy gain per module (effectively ninemodules for reserving two modules for a stand-by and foran energy adjustment margin) and further up to 3.3 GeV byanother eleven modules. Because one of the module is notoperated because a klystron modulator has been converted
to a temporary pulse power supply for the flux concentrator,
primary electron beam energy at the stage of June 2014 is
3.1 GeV.
Positrons are produced with a 14 mm thick amorphoustungsten. If a primary electron beam pulse of the designed
intensity (two bunches of 10 nC each) is focused in a spot size
(σ x , σ y ) smaller than 0.4 mm, the peak energy deposition
density (as an index of the target destruction condition) can
exceed the limit 35 J/g obtained from the SLAC operation
experience. To robustly prevent the target from this situation,a beam spoiler is introduced at 3 meters in front of the target
for enlarging the spot size more than 0.7 mm by multiple
scattering in a thin plate. Details of the beam spoiler is
described in Ref [2] and [3].
For a pulse-by-pulse switching of the electron and positron
injections, we have adopted a scheme to switch an electron
orbit by pulsed steering magnets to hit the target to produce
positrons or to pass through small holes beside the target andin the beam spoiler. The orbit of the electron for injection isset on the axis of the beam line to avoid an emittance growth.
Meanwhile, the target is set with a 3.5 mm offset from theaxis. Degradation of a positron yield by the target offset
amounts to 50 %, however, it is shown to be reduced to 20 %by proper optimization of an electron incident position andof flux concentrator orientation with respect to the target as
described in Ref [4].

FLUX CONCENTRATOR
We have developed a flux concentrator (FC) based on
the SLAC-IHEP model design [5]. A temporary pulse
power supply has been constructed by modifying an existing
klystron modulator to supply pulse current of 6 µs width tothe FC. A snubber circuit is connected to the pulse feeding
line for a pulse wave form shaping. The maximum current
is limited to 6 kA that is half of the design specification
12 kA, since this temporary power supply uses only single
thyratron instead of two in the complete power supply. Thus,
the available FC field strength at this stage is 1.8 T instead of
the specified 3.5 T. The bridge coils outside the FC generate
an additional 0.5 T field at 600 A current. After successful
operation tests of two prototype FCs up to 6 kA, we have
manufactured a FC assembly for operation in the beam line.
It is composed of a FC in a vacuum chamber with pumping
ports and two ion pumps, a positron production target infront of the FC with 2 mm gap, two bridge coils and horizontal/vertical steering coils of DC current operation outside
the chamber, magnetic flux return yoke, a girder to support
the assembly components, a multi-coupling plates for quickconnection/disconnection of the various connectors for the
DC power cables, the interlock signal cables and the coolingwater pipes. The FC is set with 2 mm offset from the beamline axis. Thus, the target and the beam hole are placed
within the aperture diameter of 7 mm keeping the hole on
the axis as shown in the Fig. 2.

Figure 2: Flux Concentrator viewed from downstream.
The pulse current from the power supply is fed throughten parallel coaxial cables of 18 m long to reduce the inductance of the power line. To avoid the radiation damage
in the insulation material at the cable terminals, the cables
are connected to a power line of parallel copper plates at a
position 3 m from the FC. The plates are connected throughshort removable copper bars to the electrodes at the endplate of the FC chamber. After the operation in a test stand,the FC assembly has been installed in the beam line. In ahigh-voltage processing of the FC, the solenoid field by thebridge coil induced gas bursts. Thus, we have performed acareful processing by scanning the FC voltage and the bridge
coil field strength. After the processing, we still have somebursts in the beam operation, however, the frequency is lessthan a few per day.

LARGE APERTURE CAPTURE SECTION
We use the large aperture S-band (LAS) accelerating structures in the positron capture section since their aperture diameter (30 mm) enlarge the transverse phase space acceptancetwice compared to that with the conventional S-band structures (21 mm). Details of the LAS structure is describedin Ref [6]. In the capture section for SuperKEKB, six LAS
structures are installed inside the DC solenoid modules asshown in Fig. 3.

Figure 3: LAS structure insertion into DC solenoid.
While the first two structures are connected to a set of a
klystron and a SLED cavity for higher field gradient as 14
MV/m, the following four structures to an another set for
10 MV/m. The high gradient in the first two structures isessential in reducing the satellite particles that can make radiation problem during the damping ring injection. The LAS
structures are also used in the subsequent accelerator module in a quadrupole focusing region. Four structures were
installed there. Acceleration length in the capture section isincreased from 6 m for the KEKB capture section to 12 m
for SuperKEKB. As a result, the positron beam energy fromthe capture section is boosted to 110 MeV compared withthat of KEKB as 70 MeV even with the initial deceleration.
This boost improves the beam transmission efficiency in thematching section after the capture section. As described in
Ref [6], accelerating structures immersed in a magnetic field
by DC solenoids or quadrupole magnets, can suffer frombreakdown and multipacting problems. Since all the LAS
structures are still in an rf processing stage, the operation
for the preliminary positron beam commissioning has been
performed with the SLED cavity detuned. The achieved
field gradient are 10 MV/m for the first two structures and7 MV/m for the others. Due to the limited power available

at this stage, the DC solenoids are operated at half of thedesign current. Thus the DC solenoid field strength is 0.2 T
instead of the specified 0.4 T.

ELECTRON/POSITRON SEPARATOR
A particle tracking simulation study has shown that a capture efficiency of the secondary electrons from the target
is comparable to that of the positrons. The electron bunchcomes 175 ps (a half of an rf wave length) apart from thepositron bunch as a result of phase slipping in the capture
section. The BPMs used in the SuperKEKB linac cannot separate signals by the electrons and the positrons. Precise beam
position and intensity cannot be measured due to the cancellation of these signals. In order to obtain pure positrons after
the capture section, we have installed an electron/positron
separator chicane as shown in the Fig.4. In the middle of thechicane, positrons pass 30 mm aside from the beam line axis
and electrons pass approximately the same distance on theother side. These beam spots are well separately considering
the spot sizes and the energy spreads. The secondary electrons are absorbed by a 60 mm thick tungsten-copper alloy(W 70% + Cu 30%) block. While, the high energy electrons
from the rf-gun propagate on the axis slightly aside from thestopper block. We have a positron stopper block as well, on
the other side of the separator. These blocks can be moved
separately by pulse motors with a position resolution of 50µm. By measuring beam transmission with changing thebeam stopper block positions, energy distributions of thepositrons and of the secondary electron can be measured.
180

125

180

325

180

125

180

e+
90

e60

Figure 4: Electron/positron separator chicane.

PRELIMINARY POSITRON BEAMPERFORMANCE
An operation of the upgraded positron source has been
started in May 2014. At this stage of the beam commissioning, some components were operated at parameters lower
than the design specifications. Due to a limitation of thetemporary power supply, the FC was operated at a half current of the specification. The DC solenoids in the capture
section were also operated at a half current due to a power
station limit. The LAS accelerating structure were operated
at 70 % of the design field gradient. The electron bunch
intensity from the photo-cathode rf gun was still low as 1
nC compared with the design intensity of 10 nC. Due tothe beam loss in the beam bunching section and the limited
energy acceptance of the J-arc, the primary electron beam

intensity on the positron production target was 0.5 nC. With
this low intensity primary beam, we started to search for apositron beam signal. In the beginning of the beam tuning,
two independent rf phases of accelerating structures in thecapture section have been adjusted to change synchronously
by measuring crest phases with beam passing through thetarget hole reaching to the energy analyzing station a the endof the linac. By proper adjustment of the common rf phase,primary electron incident position on the target and otherparameters, we observed a faint large spot on the screen
monitor after the capture section. To identify it to be thepositrons, we observed the BPM signal wave form with inserting the electron stopper block in the electron/positron
separator chicane. Finally, we observed a signal induced bypositively charged particles. The positron bunch intensity
was 0.02 nC at the entrance of the beam transfer line to thedamping ring. The observed conversion efficiency was 4%. It will be improved to the design capture efficiency (40%) by boosting the FC pulse current, the DC solenoid currents, the LAS structure field gradient and the positron beamtransmission efficiency in the quadrupole focusing region.

SUMMARY
The positron source of KEKB injector linac has been upgraded for SuperKEKB. The components of the new positronsource have been installed in the beam line. Though the operation parameters of some components are significantly
lower than the design specification, a preliminary positron
beam commissioning has been started. We have observed
the first positron beam in the upgraded system. With theprimary electron bunch intensity 0.5 nC at the positron production target, the positron bunch intensity was 0.02 nC at
the entrance of the beam transfer line to the damping ring.
The conversion efficiency was low at this stage, however,it will be improved by boosting the component operation
conditions to the design values.

REFERENCES[1] T. Kamitani et al., “Positron Injector Linac Upgrade for SuperKEKB”, Linac’12, Tel Aviv, Israel, August 2012, MOPLB02[2] L. Zang et al., “SuperKEKB Positron Source Target Protection
Scheme”, IPAC’13, Shanghai, China, May 2013, MOPFI017[3] L. Zang et al., “Design, Manufacture and Operation of theBeam Spoiler for Positron Target Protection”, IPAC’14, Dresden, Germany, June 2014, MOPRI002[4] L. Zang et al., “Positron Yield Optimization by Adjusting
the Components Offset and Orientation”, IPAC’14, Dresden,
Germany, June 2014, MOPRI003[5] L. Zang et al., “Design Optimization of Flux Concentrator
for SuperKEKB”, IPAC’12, New Orleans, USA, May 2012,
TUPPD032[6] S. Matsumoto et al., “Large-aperture Travelling-wave Accelerator Structure for Positron Capture of SuperKEKB Injector
Linac”, IPAC’14, Dresden, Germany, June 2014, THPRI047