POSITRON INJECTOR LINAC UPGRADE FOR SUPERKEKB
T. Kamitani∗ , M. Akemoto, Y. Arakida, D. Arakawa, A. Enomoto, S. Fukuda, K. Furukawa,Y. Higashi, T. Higo, H. Honma, N. Iida, M. Ikeda, E. Kadokura, K. Kakihara, H. Katagiri,
M. Kurashina, H. Matsushita, S. Matsumoto, T. Matsumoto, S. Michizono, K. Mikawa,T. Miura, F. Miyahara, T. Mori, H. Nakajima, K. Nakao, T. Natsui, S. Ohsawa,Y. Ogawa, M. Satoh, T. Shidara, A. Shirakawa, T. Suwada, H. Sugimoto, T. Takatomi,
T. Takenaka, Y. Yano, K. Yokoyama, M. Yoshida, L. Zang, X. Zhou, KEK, Tsukuba, Japan
D. Satoh, Graduate School of Science and Engineering, Tokyo Inst. of Technology, Tokyo, Japan
Abstract
This paper reports on a present status of a positron injector linac upgrade for SuperKEKB. A development statusof a ﬂux concentrator for positron focusing is shown. An
inﬂuence of offset layout of the ﬂux concentrator and thetarget on a positron yield is described. Positron capture by
L-band and large aperture S-band accelerating structures
are compared in a viewpoint of satellite bunch elimination.
Beam optical design compatible to electrons and positronsof different beam energies is discussed.

INTRODUCTION
The KEKB-factory is now under an upgrade to SuperKEKB for a forty times higher luminosity [1]. The injector linac is required to supply beams of higher charge
and smaller emittance as shown in Table 1.
Table 1: Linac Beam Speciﬁcations
KEKB
SuperKEKB(e− /e+ )(e− /e+ )Injection Energy (GeV)
Bunch Intensity (nC)
Number of Bunch /pulse
Emittance (µm)

8.0 / 3.5
1.0 / 1.0
2/2
300 / 2100

7.0 / 4.0
5.0 / 4.0
2/2
20 / 92[H] :7[V ]To generate low emittance electron beams of 5 nC bunch
intensity for injection, a photo-cathode RF gun is introduced [2] to replace the existing thermionic gun and the
RF bunching section. The electron beams are accelerated
up to 7.0 GeV over the entire linac and injected into the
HER. Electron beams of 10 nC for positron production will
also be generated with the RF gun. In a case high intensity operation of the RF gun is not stable, the thermionic
gun is also used and the beams from these two guns are
switched. To generate low emittance positron beams, a
damping ring (DR) of 136 m circumference is introduced
at a side of the linac and connected at the beam switch-yard
No.2 (SY2). Beams generated in a positron capture section(PCS) are accelerated up to 1.1 GeV and injected into the
DR. Positrons are extracted after 40 ms from the DR, injected back to the linac and accelerated up to 4.0 GeV for∗ E-mail: < takuya.kamitani @ kek.jp >injection to the LER. Energy gain of the positrons from existing six accelerator modules (typically 160 MeV per each
module) from the PCS to the SY2 is not sufﬁcient for this
1.1 GeV injection to the DR. The PCS is relocated 40 m
upstream to have a sufﬁcient energy margin for the DR. To
increase positron beam intensity four times as much, capture efﬁciency is enhanced in two aspects. At ﬁrst, an existing 2-T pulsed coil of short ﬁeld length (45 mm) rightbehind a positron production target, is replaced with a ﬂux
concentrator (FC) type of 5-T pulsed solenoid of long ﬁeld
length (200 mm). An adiabatic matching characteristics ofa solenoidal ﬁeld distribution with a FC, gives wider energy acceptance for positrons. As a second aspect, existing
S-band accelerating structures with conventional aperture(21 mm in diameter) used in a solenoidal ﬁeld of the PCS
are replaced with large aperture S-band (LAS) structures(30 mm) or L-band structures (35 mm) to enlarge transverse phase space acceptance. Total length of the PCS isextended from 8 m to 16 m to boost positron beam energy
from the PCS from 80 to 120 MeV. This increase of thebeam energy at a transition from solenoidal to quadrupole
focusing region is effective in reducing beam loss aroundan optical matching section.
In December of 2013, we will start a preliminary
positron beam commissioning of the injector linac within
limited operation parameters. A beam commissioning with
the DR will be started in February of 2015. Development,
fabrication and installation of the components are ongoing
to be in time for the schedule.
In the following sections, as topics of signiﬁcance in thepositron injector linac upgrade, ﬂux concentrator development, target protection and offset positron production,
large aperture accelerating structures development, satellite
bunch elimination and electron/positron compatible optics
design are described .

FLUX CONCENTRATOR DEVELOPMENT
Flux concentrator 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 a inner surface and generates a strong ﬁeld of several Tesla. Achievable ﬁeld strength is mainly determined by a hole diameterand a primary pulsed current. They are constrained from arequired aperture size, a power supply capacity and break-down limit. We have been collaborating with BINP, SLAC,
Frascati and IHEP on the FC development for KEKB linac
upgrade. We have performed a high power operation testof a BINP-type FC at KEK and experienced a breakdown
problem. The prototype is disassembled and under careful inspection at BINP. For a coming preliminary positron
beam commissioning in December of 2013, we will use amore conventional FC, two type are considered as candidates. One is a spiral slit FC based on the SLAC design.
Frascati and IHEP also have been using this type of FC.
The other type is a simple straight slit FC. A peak longitudinal ﬁeld strength at a same current is lower with the
SLAC type, but a contribution of a transverse ﬁeld component which is not axial-symmetric is also smaller. Considering an effect of target offset from a ﬁeld center described later, evaluation of positron capture efﬁciency by aparticle tracking using detailed 3D ﬁeld distribution is ongoing. Since the FC is installed right behind the target andits radio-activation level will be very high and a replacement work will not be easy. Thus, an engineering design
to avoid mechanical failure or water and vacuum leakage
problems is critically important. Decision of installation
model among the candidates will be made soon with a comparison in the capture efﬁciency performance and engineering design consideration.

POSITRON PRODUCTION TARGET
We have used 14 mm thick (4.0 radiation lengths) amorphous tungsten and later 10 mm thick crystal tungsten forthe positron production target. In the beam commissioning for SuperKEKB, an amorphous tungsten will be used
to avoid precise axis alignment mechanism. For a pulseby-pulse switching of electron and positron injections, asmall hole for electron beam passage is bored a few mmapart from a center of the target. To optimized a positronyield, the target was placed on a common central axis of thepulsed coil and the DC solenoids in the KEKB linac. Thus,
electron beam entered into solenoid ﬁeld at a few mm offset from the axis and traveled through an excursion orbit bya fringe ﬁeld. It becomes a problem at SuperKEKB, since
an electron beam for injection have a small emittance as 20µm. Emittance growth in this excursion orbit is not acceptable and a hole for electron passage should be centered on
the axis of the DC solenoids. An aperture diameter of the
FC is only 7 mm in the present design and the tungsten target of 4 mm diameter and the hole of 2 mm diameter should
be well contained in the aperture. As a consequence, center axis of the FC has 2 mm offset from the beam line andtarget center has 1.5 mm offset from the FC center. We expected the offsets degrade the positron yield 20 to 30 % by
a preliminary simulation study. More detailed study on thisoffset effect is to be done by the tracking simulation.

LARGE APERTURE CAPTURE SECTION
To enlarge a transverse phase space acceptance of the
PCS, accelerating structures with larger aperture will be

used. As a ﬁrst candidate, we started R and D for L-bandstructure which had an appropriate ﬁeld strength and an
aperture diameter as described in Table 2. A ﬁrst prototype L-band structure has been fabricated and under a highpower RF processing in a test stand. Since a large outerdimension of an RF coupler of the structure gives strong
impact on a DC solenoid design, an L-band structure ofslimmer body is in consideration by installing a dummyload inside the structure in a collinear geometry. In a selection of the L-band frequency, we cannot use simple halffrequency (1428 MHz) of the S-band because it is not compatible with 2 bunch operation with 96 ns interval which
was deﬁned from phase synchronization constraint of thelinac and the storage rings. Instead, we adopted 1298 MHz
which is 5/11 of the S-band frequency. This co-prime frequency relationship was found to be useful in eliminationof satellite bunch particles as described later.
The other candidate is a large aperture S-band (LAS)
structure as described in Table 2. It has a larger aperture diameter (30 mm) compared with conventional S-band structures (21 mm). The LAS has a very large group velocity and a ﬁeld strength is relatively low, however operation with a pulse compression by the SLED gives comparable ﬁeld strength. The LAS structures are used not onlyin the PCS, but in subsequent two accelerator modules ofquadrupole focusing region to improve transverse acceptance as described in the next section.
Table 2: Speciﬁcations of large aperture structures
L-bandLAS
RF frequency (MHz)1298.182
2856.000
number of cells
24 + 257 + 2
accel. length (mm)
2001
2064
disk aperture 2a (mm) 39.4 → 35.0
31.9 → 30.0
group veloc. vg /c (%) 0.61 → 0.39
4.2 → 3.5
shunt imp. r0 (MΩ/m) 45.7 → 47.6
46.1 → 48.4
ﬁlling time Tf (µs)
1.360
0.180
accel. ﬁeld (MV/m)
12.2@15MW 6.93@10MW
same with SLED–
16.42
Positrons emerged from the target have wide range ofenergy due to a cascade shower process and long bunch taildue to path differences in spiral orbits. Simulation studies
show that a positron capture in decelerating phase and a formation of a main bunch by RF phase slip is useful to reduce
particles at tails of an energy distribution [3]. However,particles which are not captured in the main bunch, experience further phase slips. They can be captured in subsequent phase positions and form satellite bunches. They canhave comparable beam energy as those in the main bunch
and will not be eliminated by an energy collimation. A
fraction of satellite particles go out of RF buckets are lostat the DR injection. Due to a civil engineering constraint,
a beam loss at DR is strictly limited to be below 0.2 % of adesign beam intensity (4 nC x 2 bunch). Thus, eliminationof satellite particles in the linac is a critical issue for a DR

operation.
In a case we use six 2-m long L-band structures in the
PCS, satellite bunches are formed in L-band wave-lengthintervals. Due to the co-prime relationship to the S-bandwave-length, satellite bunches are accelerated or decelerated in abnormal phases in the downstream S-band linac
modules and are eliminated in the linac or in an energy
collimation. It is shown that a beam loss at DR injection
will be suppressed to be an allowable level 0.05% [3]. Ina case of a hybrid of two L-band and four LAS structures,the loss becomes 0.2%, but still acceptable. To suppress
an initial construction cost by avoiding L-band component
construction, it is recommended to start a beam commissioning with a conﬁguration of all LAS structures in the
PCS. In this case, the loss becomes 0.4% and further improvement is necessary. A preliminary simulation study
indicates that raising S-band deceleration ﬁeld around 20
MV/m can reduce satellites. More detailed study will bedone soon.

DC steerings. Orbit correction performance and emittance
growth with this limited number of steerings should bechecked by simulations and underway.
In the region after the DR till the end of the linac, bothof the electrons and the positrons have low emittance andnot precisely same but high energy. Quadrupole triplets
and steerings are placed in 20 m interval. Total numberof magnets are not so large for this region and there are
no quadrupoles outside the structure, hence we decide all
the quadrupoles and steerings here to be pulsed magnets.
It will be useful for a precise independent matching to thestorage rings and for an independent orbit control to suppress emittance growths for positrons and electrons.

POSITRON/ELECTRON COMPATIBLE
OPTICS
By using accelerating structures with larger aperture, atransverse acceptance of the PCS which corresponds to an
RMS emittance is enlarged from 2000 µm to 4000 µm. As
described in the previous section, the PCS is relocated to a
40 m upstream position and a beam energy from this section is raised from 80 MeV to 120 MeV. As a consequenceof these changes, a quadrupole focusing system after the
PCS till the SY2 needs an upgrade and an addition and rearrangement of quadrupole and steering magnets will bemade. In the two accelerator modules after the PCS, LAS
structures of 30 mm aperture diameter are used to enlarge
the transverse acceptance and quadrupoles are installed in
70 cm interval outside of the structures. In the subsequent
three modules, ordinary S-band structures of 20 mm aperture are used and quadrupoles are placed in the same interval. In the subsequent four modules, quadrupole triplets
are placed between the S-band structure and the intervalchanges from 3m to 5m according to the beam energy.
In this region between the positron production target andthe SY2 before the DR, both of low energy, huge emittance positrons and high energy, low emittance electrons
pass through the same quadrupole focusing system. Almost all quadrupoles in this region are DC magnets. To
avoid a beam loss of the positrons, focusing strengths are
optimized for them. Thus, electrons are focused in a readymade quadrupole ﬁelds which are weak for the energy ofelectrons. Slight optimization to moderate electron beam
size is performed within a range in which the positron optics is not signiﬁcantly modiﬁed. Fig. 1 shows beam sizeof the positrons and the electrons.
Even with this compromised optics for the electrons, thebeam sizes are kept sufﬁciently small. For independent orbit control of the positrons and the electrons, 12 pulsed
steering magnet pairs are installed in addition to existing

Figure 1: positron/electron beam optics design

SUMMARY
The KEKB positron injector linac is under an upgradefor SuperKEKB. Two types of ﬂux concentrator in different
slit geometry are in comparison by simulations and a prototype fabrication. An inﬂuence of an offset layout of the
FC and the target is discussed. As accelerating structures
to be used in the capture section, L-band and large aperture
S-band structures are compared in a viewpoint of satellite
bunch elimination. Performance of a compatible optics forelectrons and positrons of different beam energies with DC
quadrupoles is studied. A preliminary positron beam commissioning of the injector linac will be started in Decemberof 2013.

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TUPPD035[3] N. Iida, H. Ikeda, T. Kamitani, M. Kikuchi, K. Oide, D.
Zhou “Beam Dynamics in Positron Injector Systems for
the Next Generation B-factories”, IPAC’11, San Sebastian,Spain, September 2011, THYA01