Proceedings of the 2003 Particle Accelerator Conference

A SIMULATION STUDY OF THE JLC POSITRON SOURCE
T. Kamitani∗ , Y. Ohnishi, KEK, Tsukuba, Ibaraki, JapanS. Kashiwagi, Waseda University, Tokyo, JapanAbstract
This paper describes a parameter optimization of the JLC
positron source design by detailed particle tracking simulations. The design parameters are modified from those of
the previous JLC design study [1], considering the energydensity limit of the converter-target damage and the recentchange in the specifications, especially the twice increaseof a charge intensity of a beam pulse [2]. It is shown thata multiple target system to share the deposition energy isinevitable and at least three targets are necessary to yield
sufficient positrons within the energy-density limit.

INTRODUCTION
The design of the JLC positron source [1][2] is based ona conventional scheme of positron production using high-Zmaterial target irradiated by a high-energy electron beam asshown in Fig.1. A combination of a pulsed solenoid (fluxENERGY-DENSITY ESTIMATIONDC Solenoid

Flux concentrator
Rotating
Converter
TargetL-band Acceleratingstructure

PositronsPrimaryElectrons

Figure 1: JLC positron sourceconcentrator) and DC solenoids is used for the transverse
phase space matching. The solenoid field is varied slowly
from 7.0 T to 0.5 T to achieve an adiabatic matching so as
to have a wide energy acceptance. A long flat solenoidal
field of 0.5 T extends in the several meters to cover theaccelerating structures in the capture section. Only thepositrons which can pass through the aperture of the accelerating structure are captured and accelerated. L-band
accelerating structures are used here, because it has largeaperture (20 mm in radius) and long RF wave length (210mm), thus having large acceptance both in transverse andlongitudinal phase space. The positrons are accelerated up
to 180 MeV with the L-band structures in the solenoid focusing region and are transferred to the quadrupole focusing system. In the downstream, the positrons are accelerated by a S-band linac up to 1.98 GeV for injection to thepre-damping ring (PDR).
∗ takuya.kamitani@kek.jp

0-7803-7739-9 ©2003 IEEE

An intensity of 14.4 × 10 11 positrons/pulse, which isequivalent to that of electrons, is require in the present
JLC design. It was increased more than twice, comparedwith the previous design (6.0 × 10 11 particles/pulse) [1]. Abeam pulse has a structure like a train of 192 bunches in 1.4
nsec interval, each having 0.75 × 10 10 positrons. A repetition rate of the beam pulse is 150 Hz. A primary electron
beam, whose intensity is 19.2 × 10 11 electrons/pulse, is accelerated up to 10 GeV at the converter target to produce
positrons. A performance of the positron source is evaluated by the positron yield which is defined by a ratio of thepositron intensity to that of the primary electrons. Assuming a loss of the positron beam from the PDR to the interaction point is negligible, the positron yield is estimated by
calculating a conversion ratio from electrons to positrons
in the converter target and a fraction of positrons whichsurvive during a transmission from the capture section tothe PDR injection. A design goal of the positron yield is
N e+ /N e− = 0.75.

3288

Tungsten-Rhenium alloy has been chosen as a convertertarget, because of the high melting point and the high tensile strength. The thickness of the target was determined tobe 21 mm (6.0 radiation lengths) for the optimum positronyield with the 10-GeV primary electron beam. A rotating
structure is used so that each beam pulse hits a different
part of the target to prevent concentrated heat deposition.
Nevertheless, the energy deposition by a single beam pulse
may exceed the threshold for target damage due to fatigue.
From analysis of the cracked SLC positron source target,
the threshold for damage is estimated to occur when the local density of the energy deposition is 35 J/g, as reported in
the NLC positron source design [3]. This limit is used as areference in the design of the JLC positron target. Increasing the transverse size of the primary beam helps reduce
the energy density in the target. Examples of the energydensity in the target are shown in Fig.2, for two cases of
the incident beam sizes of 2.0 and 3.0 mm. The densities
grow as the beam go deeper in the target and the peaks areat the exit. By increasing the beam size from 2.0 to 3.0mm, the peak energy density becomes almost half. It is because the energy deposition is smeared out in wider region
as shown in the contour plots in Fig.2. The energy densities
in the plots are for the case of the beam intensity which is
one-third of the JLC specification.
Though increasing the beam size reduces the energy density, it also reduces the positron yield, because larger fraction of the produced positrons go out of the acceptanceaperture. As shown later, if the incident beam size is suf-

Proceedings of the 2003 Particle Accelerator Conference
JLC design (beam sig-x = 3.0 mm)

Energydensity
50454035302520151050

Energydensity
50454035302520151050

Beam

Peak density = 49 (J/g)

864Xpos)(mm20-2-4-6 0-8

15 63 4(Xo)
Depth

2

Peak density = 25 (J/g)

864Xpos)(mm20-2-4-6 0-8

15 63 4(Xo)
Depth

2

beam sig-x = 3.0 mm

886644X-pos (mm)X-pos (mm)beam sig-x = 2.0 mm

Beam

100

20-2-6345660504030201 target2 targets
3 targets
4 targets
SLC Limit
0.5

1

1.5

2

2.5

33.54

4.5

5Incident e- beam size (sig-x=sig-y) [mm]Figure 4: Energy density as a function of beam size-82700-6

18000-2-40E(e-) = 10.0 GeV
N(e-) = 192. x 10^10 /pulse
Target Thickness = 6.0 Xo

90102-4-8

Energy deposit density [Joule/gram]

JLC design (beam sig-x = 2.0 mm)

012

3Depth (Xo)

45POSITRON YIELD ESTIMATION

6Depth (Xo)

Figure 2: Energy density distribution

ficiently large concerning the energy density, the positronyield is far below the requirement. To keep the density well
below the threshold with sufficient positron yield, multiple
target system is used for the energy deposition to be sharedas proposed for the NLC [3]. A candidate multiple tar-Figure 3: Multiple target system

get system for the JLC is shown in Fig.3. To divide thebunches in a beam pulse into three, an RF deflector cavity
whose oscillation is synchronized to every third bunch isused. The three groups of the sparse bunches go towards
the different target. Each target is followed by a positron
capture section and a collimator to define energy and emittance. The three groups of the produced positrons are thenmerged with another RF deflector cavity. To see the effectof using the multiple targets, the peak energy density andits dependence upon the incident beam size is evaluated forthe case of one to four targets. The result is shown in Fig.4.
The EGS4 shower simulation code [4] was used to calculate the energy depositions in small volumes in the target.
The acceptable minimum beam sizes to clear the density
limit are 4.8, 3.2, 2.5, 2.1 mm for single, 2, 3, 4 target systems, respectively. By using multiple target, the acceptablebeam size become smaller.

3289

In order to estimate the positron yield for each case of
the multiple target systems, detailed particle tracking simulations were performed.
As a first step, positron production in the target wassimulated using the EGS4 code. Approximately, twentythousand positrons were generated in the 21-mm thicktungsten-rhenium target from a thousand initial electrons of
10-GeV beam energy. Samples for different incident beam
sizes were generated, assuming the transverse incident particle distribution is a two-dimensional gaussian and wereused as input data for later simulations.
Capture efficiency of the solenoid focusing system wasevaluated by tracking simulations with a step-by-step integration of the equation of motion, including a focusing
magnetic field, an accelerating field and a detailed aperture
constraints in the capture section [5]. The step size was
0.25 - 1.0 mm. Typical number of the particles after thecapture section was three thousand.
To estimate a beam transmission efficiency of the 1.98GeV positron injector linac, particle tracking was performed with the SAD code [6] in a six-dimensional symplectic manner. The positron focusing system in the linac
is a combination of a FODO of singlets and periodic triplet
quadrupoles. In Fig.5, a preliminary design of the beam
optics and the layout in the linac is shown, as well as an example of the beam transmission efficiency along the linac.
Here, the beam optics from the defining collimator till thejunction of the three beams and that for the transfer line
to the PDR were omitted. In this simulation, an aperture
radius in the linac was assumed to be 10 mm and an acceptance cut by the energy and emittance defining collimator,
located just after the capture section, were also taken intoaccount.
Finally, PDR acceptance cuts were applied to the sample
particles after the tracking simulation in order to estimate
the positron yield. The acceptances for PDR injection wereassumed to be 0.027 rad.m in the transverse emittance, ±1
percent in the particle energies and 24 mm in the longitudi-

Proceedings of the 2003 Particle Accelerator Conference
numbers of the targets is shown in Fig.7 at each acceptable minimum beam size. The single or two target systems cannot yield sufficient number of positrons. In thecase of three target system, the incident beam size canbe 2.5 mm in radius and the estimated positron yield at
PDR is N e+ /N e− = 0.99 and the requirement of theN e+ /N e− = 0.75 is satisfied.

Figure 5: Positron Injector Linac Beam Optics and BeamTransmission
nal position in a bunch. Due to large energy spread of thepositrons, approximately 50 % of them were eliminated by
the PDR energy acceptance. To improve this efficiency, thesingle-bunch energy-spread compression system (SBECS)
was considered, which has already been used in the KEK
B-factory injector linac [7]. This system is composed of thechicane to produce path difference for the particles withdifferent energies and an accelerator section which gives
acceleration (or deceleration) to compensate the energy difference in a bunch. As shown in Fig.6, the energy-spreadcan be compressed with this system and the injection efficiency to PDR was expected to be more than 70 %.
Not only the energy-spread of the positrons in a bunch,but an energy deviations between the bunches due to thetransient beam loading along the bunch train is also large.
In the positron linac, ∆T multi-bunch energy compensation system (∆T ECS) is used as in the JLC electron injector linac [2]. Here, the amplitude modulation technique isapplied to an input rf pulse into the SLED cavities to obtain
the desirable slope of unloaded voltage in the accelerating
structure. A combination of two klystrons are needed tomodulate the amplitude of rf pulse for the SLED cavities
with a constant phase. By using the ∆T ECS, the multibunch energy deviation can be negligible compared to thePDR energy acceptance and therefore, it was not includedin the tracking simulation.

Particle Energy [MeV]2000

Before SBECS2020(PDR acceptance)Longitudinal acceptance = 24 mm
Energy acceptance = +/- 1.0 %2000

Particle Energy [MeV]

2020

1980

1960

1940

1920

1900

1880-40

After SBECS(PDR acceptance)Longitudinal acceptance = 24 mm
Energy acceptance = +/- 1.0 %-20-10010Particle position in a bunch [mm]201980

1.25

10.75

0.5

0.25

00

1 target (sig-x=4.8)
2 targets (sig-x=3.2)
3 targets (sig-x=2.5)
4 targets (sig-x=2.1)
Design Yield
0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

Incident e- beam size (sig-x=sigy) [mm]Figure 7: Positron yield as a function of beam size

CONCLUSION
The tracking simulations of the JLC positron sourceshows that, the multiple target system is inevitable to makethe energy density below the threshold of the target damage
and to yield sufficient intensity of the positrons. The simulation shows the minimum number of the targets are three.
The single bunch energy compression system is effective in
improving the injection efficiency to the pre-damping ring.

REFERENCES[1] ”JLC design study”, By N. Akasaka, et al., KEK Report 971, 1997 April[2] ”JLC Roadmap Report”,
http://lcdev.kek.jp/RMdraft/February

12,

2003

see

[3] ”2001 Report on the Next Linear Collider (A report submittedto Snowmass ’01)”, By the NLC collaboration, SLAC-R-571,2001 June[5] ”Positron Production at CLIC”, By T. Kamitani, L. Rinolfi,
CLIC Note 465, March 2001

1960

1940

1920

1880
-50

E(e-) = 10.0 GeV
N(e-) = 192. x 10^10 /pulse
Target Thickness = 6.0 Xo[4] ”The EGS4 Code System”, By W. R. Nelson, H. Hirayama,
D. W. O. Rogers, SLAC-Report-265, December 1985

1900

-30

Positron Yield at PDR (Ne+/Ne-)

1.5-40-30-20-10010Particle position in a bunch [mm]Figure 6: Single-Bunch Energy-spread Compression System
After all the simulations, the positron yield for different

3290[6] ”SAD (Strategic Accelerator Design) Home Page”, By K.
Oide, et al.,
http://www-acc-theory.kek.jp/Accelerator/index.html[7] ”The KEKB injector linac”, By I. Abe, et al., Nuclear Instruments and Methods in Physics Research Section A, Volume499, Issue 1, 21 February 2003, Pages 167-190