Proceedings of the 11th Annual Meeting of Particle Accelerator Society of Japan
August 9-11, 2014, Aomori, Japan

PASJ2014-SUOM01

A design study of ILC positron source by electron driven scheme
M. Kuriki∗ , Y. Seimiya, T. Takahashi, HU/AdSM, Higashihiroshima
T. Okugi, M. Satoh, J. Urakawa, Accelerator Lab., KEK, Tsukuba
T. Omori, IPNS, KEK, Tsukuba
S. Kashiwagi, RCEPS, Tohoku U., Sendai
Abstract
ILC (International Linear Collider) is an e+ and e- collider based on linear accelerators. ILC is the most desirable future project of high energy physics for detail studyof Higgs sector and hunting new particles such as SUSY
particles. The current baseline design of ILC positronsource is based on gamma ray conversion from undulator
radiation. This scheme is attractive from physics point ofview because polarized positron is generated. On the other
hand, this approach is totally new and it is very difﬁcult todemonstrate the system prior to the construction because
it requires more than 100 GeV beam as the driver. Froma project management point of view, a technical backup
based on a well established technology is desirable. A conventional positron generation (e- driven) for ILC can be thesolution. In this method, the technology is well established,but the heat load on the production target can be the biggest
issue. We present a result of a simulation study of the edriven ILC positron source. By employing 6 GeV e- beam
as the driver, an enough amount of e+ can be generated in
the acceptance deﬁned by DR (Damping Ring).The target
heat load is kept below the practical damage limit established by SLC.

INTRODUCTION
International Linear Collider (ILC) is a future project ofhigh energy physics. It is an electron and positron linear
collider based on the Super-conducting accelerator with its
CME (Center of Mass Energy) 500 GeV in the ﬁrst phaseand 1 TeV in the second phase. The design luminosity at
500 GeV is 2.0 × 1034 cm−2 s−1 . Technical Design Reportof ILC has been published in 2013[1]. The Japanese candidate site has been selected as Kitakami Mt. area, Iwate prefecture and the technical detail design including site speciﬁc parts (e.g. access tunnel layout, etc. ) is progressed.
In ILC, the positron is generated by undulator method. In
this method, the driver electron beam generates high energy gamma ray by passing through undulator. The gamma
ray is converted to positron by pair-creation process with
Ti-alloy target. For the efﬁcient conversion, the gamma rayenergy is at least more than 10 MeV which requires 130
GeV drive electron beam energy with 10 mm undulator period. An electron linac dedicated to the driver is not realistic and the electron beam for collision is shared with theundulator. This is a totally new approach as positron source
and a system demonstration prior to the real construction is∗ mkuriki@hiroshima-u.ac.jp

desirable, but it is therefore practically difﬁcult. By considering the risk control of a project, it is not an ideal situation.
If we have a technical backup for the ILC positron source, it
reduces unknown technical risks related to this totally new
approach.
Conventional positron generation for linear colliders has
been proposed and it is also considered for ILC[2]. In this
proposal, several GeV electron beam impinges on a heavy
metal target (typically W-Re) and positron is generated byBremsstrahlung. Possible target destruction is the biggest
issue in this case. According to SLC experience, Peak Energy Deposition Density (PEDD) given by incident electron beam has to be less than 35 J/g[3][4][5]. Our goal isestablishing the positron injector design to achieve enoughamount of positron for ILC keeping PEDD less than thelimit.

ILC ELECTRON DRIVEN POSITRONSOURCEIn this section, ILC electron driven Positron source is
described. The layout is shown in Fig. 1. It consists from electron linac, conversion target, AMD (Adiabatic Matching Device) for transverse momentum suppression, positron injector with focusing solenoid for positron
capturing, chicane to remove electrons energy deviatedpositrons, positron booster up to 5 GeV, and ECS (Energy Compressor Section). Our goal is providing an
enough amount of positron to DR whose dynamic aperture is γAx + γAy < 0.07 in the transverse space andz < ±35mm and δ < ±0.0075 in longitudinal space,
where Ax and Ay are action value, δ is relative energy deviation. As a design criteria, 50% margin on the numberof positron in DR is required. Number of positron in eachbunch at IP (Interaction Point) should be 2.0 × 1010 , then
3.0 × 1010 positrons in DR acceptance is required.
PEDD gives a practical limit on the positron intensity
on the production target. It should be less than 35 J/g according to SLC experience[3][4][5]. In the ILC positron
generation, number of required positron per second is 50
times larger than that of SLC and the target might be easily broken. To compensate PEDD, 63 ms out of 199 ms
which is ILC pulse interval, is used for positron generation. In the 63 ms, 20 RF pulses are ﬁred in 300 Hz. For
each RF pulses, 132 bunches are contained in a form of a
triplet where each mini-trains contain 44 bunches with 6.15
ns spacing and the mini-train interval is 100 ns[2]. Duration and average beam current of one triplet is about 1
μs and 0.63 A and it is feasible to employ Normal Con-- 119 -

Proceedings of the 11th Annual Meeting of Particle Accelerator Society of Japan
August 9-11, 2014, Aomori, Japan

PASJ2014-SUOM01

e+ injector
driver linac

targetchicane e+ booster

ECS

DR

AMD
Figure 1: Layout of the ILC electron driven positron source which consists from electron linac, target, AMD, positron
injector, positron booster, and ECS.
ducting (NC) RF system for the acceleration. By considering positron capture performance and cost effectiveness,
L-band and S-band NC accelerators are employed.
The beam energy and bunch intensity of the driver linac
is typically 6 GeV and 2.0 × 1010 , respectively. The target
is 14 mm thick rotating target made from W-Re alloy which
has a good conversion efﬁciency. The rotation could be up
to 5 m/s tangential speed to suppress PEDD below 35 J/g
and spread out the heat load. AMD induces a strong magnetic ﬁeld along the beam axis. The peak ﬁeld is typically
5 Tesla and the ﬁeld is smoothly connected to the solenoid
ﬁeld at the positron injector, 0.5 Tesla. AMD magnetic ﬁeld
is generated by Flux concentrator which should be similar
to that designed for Super-KEKB factory at KEK, Japan[6].
In this scheme, 132 bunches in a triplet impinge on asame spot on the target. PEDD is not exceeded 35 J/g withthis one triplet. Before bunches in the next triplet arrive at
the target, it takes 3.3ms and the target moves by 16.5mm.
The target shift is more than 4 σ by assuming 4 mm rms
spot size on the electron beam. According to this consideration, PEDD is kept at that by one triplet even we continue
injection over 63 ms.
The positron injector linac is composed from L-band NC
accelerators with 0.5 Tesla focusing solenoid ﬁeld. Theenergy is up to 250 MeV. After the injector linac, a chicane
section is inserted to remove electrons and positrons with alarge energy deviation. Figure 2 shows beta and dispersion
functions of the chicane section.
The positron booster is composed from L-band and Sband NC accelerators as a result of optimization which will
be mentioned in the next section. The positron is accelerated by the booster up to 5 GeV. Figure 3 shows the lattic conﬁguration and optical design of the booster. Afterthe booster, ECS (Energy Compressor Section) is placed.
DR acceptance in the longitudinal space is 70 mm in z and1.5% in δ, respectively. The z acceptance is too wide comparing to the δ acceptance, because the energy spread by
RF curvature assuming L-band or S-band acceleration with
70 mm bunch length is much larger than 1.5%. Phase-spacerotation by ECS in the longitudinal space improves the effective area of the DR acceptance. In other words, ECS
optimizes the capture efﬁciency. Figure 4 shows allowed
energy spread before the booster as a function of the bunch
length. In this calculation, the energy spread δ at the endof the positron injection and that by the RF curvature in

Figure 2: Beta function (upper) and dispersion function(lower) in the chicane section. Horizontal axis shows distance from the top of the chicane section. The lower picture
shows magnet lattice.

Figure 3: Beta function (upper) and dispersion function(lower) in the chicane and the booster. Horizontal axis
shows distance from the top of the chicane section. The
lower picture shows magnet lattice.

the booster linac are linearly added. Both axes are in full
width. Areas below the solid and dashed lines corresponds
to the allowed regions by S-band and L-band booster, respectively. For example, if the captured positron bunch in
the injector has 200 MeV in energy and 10 mm in z, it canbe accepted by DR with the ECS phase space rotation. It is
not possible without ECS, because 200 MeV energy spreadis already more than 1.5% of 5 GeV. R56 of ECS is 1 m orless. From this point of view, both S-band and L-band are- 120 -

Proceedings of the 11th Annual Meeting of Particle Accelerator Society of Japan
August 9-11, 2014, Aomori, JapanMeV

PASJ2014-SUOM01

Booster Acceptance
BeforeAfter

20

L-band2

10S-band

y(mm)

10

10

-10

1

10-1
0.002

0

-20
0.004

0.006

0.008

0.01

0.012

0.014

0.016

0.018

0.02
z (m)

-20

Figure 4: Allowed region of energy spread and bunch
length at the injector with ECS is shown. Below each lines(solid for S-band and dashed for L-band) are allowed.
Table 1: A typical parameter set. Aperture is given in radius.
Parameter
Value
Unit
Drive Beam energy
6.0
GeV
Beam size
4.0
mm (RMS)AMD peak ﬁeld
5.0
Tesla
RF Gradient25
MV/m
Injector L-band RF aperture
20
mm
Booster L-band RF aperture
17mm
Booster S-band RF aperture
10mm
Solenoid
0.5
Tesla

acceptable for the booster, however, the ﬁrst half should be
L-band and the last half can be S-band by considering thetransverse aperture discussed in the next section.

POSITRON CAPTURE SIMULATIONIn this section, results of the tracking simulations are presented. Positron generated by the electron injection with
W-Re target is simulated by GEANT4 and the data usedin this simulation are almost identical to those in Ref.[2].
The data are imported to GPT[7] to perform the tracking
simulation in the positron injector. After the chicane, including the booster up to 5 GeV and ECS, the simulation is
performed by SAD[8]. As a reference, the simulations are
performed with parameters as shown in table1.
Positron generated in the target is captured by AMD followed by injector linac with solenoid focusing. At the endof the injector, the beam energy is 250 MeV, but the energy spread is large. After the injector, there is a chicane
section. There are two roles for the chicane. One is removing electrons from the beam. From the EM shower,
not only positrons, but also electrons are obtained. The
electron has opposite electrical charge and captured by the
injector on the other RF phase than that for positron. Dueto this reversed phase, the electron gives the same beam

-10

0
x(mm)

10

20

Figure 5: Particle distribution in transverse space at the endof the injector where the beam energy is 250 MeV. The
diagonal and right cross symbols show particles before andafter chicane.
loading as that by the positrons. Then, electrons have tobe removed as early as possible to compensate the beamloading effect. Another role of the chicane is removing
positron with a large energy deviation. Such positron will
be lost anyway before DR injection, but the beam loss after acceleration causes high radiation and activation. Then,such positron should be removed as early as possible. By
considering these reasons, the chicane at the lower energy
is better. On the other hand, the capture efﬁciency for the
positron will be affected if the chicane is placed at the lower
energy region. The place of the chicane is determined bycompromising between these two facts. In this simulation,
the chicane is placed after the injector where the beam energy is 250 MeV. It is not fully optimized, but an enoughcapture efﬁciency is obtained with this conﬁguration as we
will see. Quantifying the beam loading effect and the beamloss will be studied as next issues. Figure 5 and 6 show
the particle distributions in the transverse space (xy) andlongitudinal space (z − δ), respectively. In both ﬁgures,
particles before and after the chicane sections are plottedwith diagonal and right cross symbols. In Fig. 5, the accelerator aperture is shown by the red circle for reference.
The chicane increases the horizontal beam size, but it canbe focused again after the chicane. Efﬁciency loss by inserting the chicane is not signiﬁcant. In Fig. 6, z position
shift for lower energy particles is recognized. This shift
is due to the dispersion. By comparing capture efﬁciency
with and without chicane, it is slightly improved with thechicane. For each cases, other parameters like RF phaseare optimized and the exact reason is not well understood.
One possibility is that the x − δ correlation made with thischicane compensates energy spread after the booster. Lessenergy spread might improved the capture efﬁciency, but
the detail should be studied.
The longitudinal phase space distribution after ECS isshown in Fig. 7. The particle distribution is rotated by ECS
as recognized. The particle distribution after ECS is exam-- 121 -

Proceedings of the 11th Annual Meeting of Particle Accelerator Society of Japan
August 9-11, 2014, Aomori, Japan

PASJ2014-SUOM01
0.2

6GeV-Sigma4mm-Target14mm-DZ5mm ECS

Before
After

1.5

0.1

1.4
1.3
Yield(e+/e-)

δ

0

-0.1

-0.2

1.1

-0.3

1

-0.4
-50

0.9

-40

-30

-20

-10
z(mm)

0

10

20

30

Figure 6: Particle distribution in longitudinal space at theend of the injector where the beam energy is 250 MeV. The
diagonal and right cross symbols show particles before andafter chicane.
2

6

AMD5T
AMD7T
AMD9T
7
8
9
AMD Aperture(mm)10Figure 8: Yield as a function of AMD aperture for 5, 7, and9 Tesla peak ﬁeld. 5 Tesla peak ﬁeld gives the best yield.
6GeV-Sigma4mm-Target14mm-AMDAp6mm-DZ5mm-7T ECS

1.2
1.1

Dropped Particles
Transmitted Particles

1.5

1
Yield(e+/e-)

1
0.5
δ(%)

1.2

0.9
0.8

0

0.7
-0.5

0.6

-1

0.5

-1.5
-2
-40

-35

-30

-25

-20 -15
z(mm)

-10

-5

0

5

Figure 7: Particle distribution in longitudinal space after
ECS.
ined with DR acceptance and number of accepted positronis counted as yield which is deﬁned as ratio of the accepted
positron with number of electron.
Fig. 8 shows the yield as a function of AMD aperturefor 5 Tesla (solid line), 7 Tesla (dashed line), and 9 Tesla(dotted line) peak ﬁeld. The target end is located at 5 mm
upstream from where AMD ﬁeld is peaked. Larger aperture
gives better yield, but aperture more than 8 mm does notgive any big gain. For the peak ﬁeld, 5 Tesla shows thebest among them. According to this results, 5 Tesla peakﬁeld with 8 mm aperture is an optimum.
Fig. 9 shows the yield as a function of aperture of accelerating structure in radius at the beggining of the booster
linac. Larger aperture gives better yield, but the yield is already saturated at 16 mm. The beam size is reduced withacceleration by adiabatic damping effect and this aperture
corresponds to L-band structure which is used in the ﬁrst
half of the booster.
By considering cost effectiveness, S-band accelerator isbetter than L-band. Up to now, the simulation is performed
with L-band structure. Here, we examined the yield by re-10

L + Transverse Cut
12
14
16
18
Booster Linac and ECS Aperture(mm)20Figure 9: Yield as a function of aperture of accelerating
structure.
placing the L-band with the S-band. Aperture of the S-bandstructure is assumed to be 10mm in radius. The result isshown in Fig. 10. There are totally 40 cells of the latticein the booster linac. In this ﬁgure, the yield is estimatedwhen the L-band structures after the cell are replaced with
the S-band. From this plot, the yield deﬁned as number of
captured positrons and injected electrons is decreased when
we replace large number of cells with S-band. However, as
we mention later, the yield 1.28 gives an enough amountof positron in the DR acceptance. 26 and later cells can be
replaced with the S-band.
Finally, the drive beam and target conﬁguration is optimized according to the yield calculated by the tracking
simulation. By changing the drive beam energy, target
thickness, and the spot size, PEDD and energy deposition per bunch are varied. To compare performances withdifferent conﬁgurations, the bunch intensity is varied giving the same number of positron in the DR acceptance,
3.0 × 1010 /bunch, i.e. the condition is normalized by the
number of captured positron. In Fig. 11, various target andbeam conﬁgurations are plotted in PEDD (horizontal axis)
and Energy deposition per bunch (vertical axis). The numbers associated to each points show the drive beam energy,- 122 -

Proceedings of the 11th Annual Meeting of Particle Accelerator Society of Japan
August 9-11, 2014, Aomori, Japan

PASJ2014-SUOM01

to remove electrons and positrons with a large energy deviation has a limited impact on the capture efﬁciency.
ILC is now in a stage of the technical detail design whichshould be completed in three years. Based on the positronsource design described in this report, we have to establish
a technical design to synchronize to the global ILC schedule. Among various issues which should be studied before
the technical design, the effect of beam loading, especiallyin the positron injector should be carefully studied, because
the beam loading in the positron injector can be very heavy
by electrons. The electrons give the same beam loading
since they are captured in the opposite phase of RF. After
conﬁrming various issues, we can start the technical designof the electron driven positron source for ILC.

6GeV-Sigma4mm-Target14mm-DZ5mm-RAMD8mm-BAMD5T-Sband-ECS

1.4
1.35

Yield(e+/e-)

1.3
1.25
1.2
1.15
1.1
1.05
1

20

25

L + Transverse Cut
30
35
Serial number of cell

40

Figure 10: Yield as a function of cell number where L-bandends.
target thickness, and the beam spot size in rms. As a practical limit, PEDD should be less than 35 J/g to prevent any
target destruction and some conditions are excluded. For
the energy deposition per bunch, there is no clear threshold, however, the lower is better from technical point ofview. Among these conﬁgurations, 6 GeV driver beam energy, 14 mm target thickness, and 4 mm rms spot size is thebest.

ACKNOLEDGEMENT
This work is partly supported by Grant-in-Aid for Scientiﬁc Research (C 26400293).

REFERENCES[1] ILC TDR, ISBN 978-3-935702-74-4 (2013).
[2] T. Omori et al., NIMA672(2012)52-56.
[3] T. Kamitani and L.Rinolﬁ,CLIC-NOTE-465CERN-OPEN2001-025.

Energy Deposition (J/bunc)[4] S.Ecklund,SLAC-CN128(1981).
[5] NLC Collaboration,SLAC-R-571, 2001.

9[6] L. Zang et al., Proc. of IPAC(2012)TUPPD032.

6GeV, 20mm, 6mm

87[7] Pulsar
Physics
Home“http://www.pulsar.nl/gpt/index.html”.

3GeV, 14mm, 6mm

656GeV, 14mm, 6mm

4[8] SAD Home page, “http://acc-physics.kek.jp/SAD/”.

6GeV, 20mm, 4mm

3GeV, 14mm, 4mm

6GeV, 20mm, 3mm
6GeV, 20mm, 2mm

36GeV, 14mm, 4mm

26GeV, 14mm, 3mm
6GeV, 14mm, 2mm

1
0
0

10

20

30

40

50

60
70
PEDD (J/g)

Figure 11: PEDD (J/g) and Energy deposition per bunchwith various conﬁgurations. 6 GeV drive beam energy, 14
mm target thickness, and 4 mm rms spot size is the best.

SUMMARY AND CONCLUSION
We perform a start-to-end simulation for the electron
driven ILC positron source. According to the simulation,
3.0 × 1010 positron per bunch is obtained with PEDD 27
J/g which is below the practical limit by SLC, 35 J/g. The
spot size on the target is 4 mm (RMS) and the bunch intensity of the driver linac is 2.3 × 1010 electrons per bunch.
AMD peak ﬁeld is 5 Tesla with 8 mm aperture. The injector linac is L-band with 0.5 Tesla solenoid-focusing. The
booster linac is a hybrid of L-band and S-band structures.
ECS is important for better acceptance. The beam chicane- 123 -

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