POSITRON YIELD OPTIMIZATION BY ADJUSTING THE
COMPONENTS OFFSET AND ORIENTATION∗
L. Zang† , M. Akemoto, K. Furukawa,
S. Fukuda, T. Higo, N. Iida, K. Kakihara, T. Kamitani, T. Miura,
F. Miyahara, Y. Ogawa, H. Someya, T. Takatomi, K. Yokoyama
High Energy Accelerator Research Organization (KEK)
1-1 Oho, Tsukuba, Ibaraki, 305-0801, Japan
S. Ushimoto
Mitsubishi Electric System & Service Co., Ltd, Tsukuba
Abstract
The SuperKEKB’s design luminosity is 40 times greaterthan that of KEKB. In order to achieve this goal, we need tokeep the injection electron beam emittance small and let theprimary electron beam generates high positron yield. The
design is challenge but fulfilled after introducing a pulse-topulse switching system for electron and positron injection.
During the operation, the injection electron beam is on axis
to preserve emittance and the primary electron beam will
be steered to strike on the offset target to generate positrons.
The flux concentrator as a part of the matching device in
capture section plays an important role in yield improvement. It is offset due to the constrain of small aperture. As
a result of the straight cut-in slit section the FC has an unsymmetric field distribution, so that the orientation could
lead to a positron yield variation. In this paper, we will
discuss this issue and find out the ideal trajectory for thepositron beam.

INTRODUCTION
From KEKB to SuperKEKB, the luminosity is required
to be 40 times higher, which demands the injection linac toprovide beams of higher charges. [1]. The positron source
principle configuration has been shown in Figṙeflayout. The
primary electron beam would be guided off the central axis
by the pulsed steering coil before striking on the target.
To avoid damaging the target by the extremely small size
electron beam, a beam spoiler will be placed between thequadruple and the target. The beam spoiler is made of aluminum attaching with a screen monitor, so that it could not
only scatter the beam to protect the target but also measure
the beam size. More detailed spoiler discussion could befound in paper [2]. When the primary electron beam strikes
on the target, the beam spot size can be as large as 0.7 mm.
The positron conversion target is made of tungsten whose
radius is 2 mm and length is 14 mm. In the other hand, tooptimize the injection electron beam, it needs to be placedon a common central axis of the DC solenoid.
A hole of 1 mm radius for electron beam passage is bored
0.5 mm away from the edge of the target. Considering thetarget radius is 2 mm, then the target center is 3.5 mm off∗ Work supported by KEK† zang@post.kek.jp

Figure 1: Schematic view of SuperKEKB positron source
configuration [4].

from the central axis. The generated positron beam hasa very large divergent angle but small lateral dimension.
Hence, a flux concentrator (FC) would be installed to improve the capture efficiency [5]. The FC has an aperture of
7 mm, in which should contain the target and the passage
hole. As a consequence, the FC center has to have a 2 mm
offset from the beam line and the target center has a 1.5 mm
offset from the FC center. The positron target and the FC
layout are shown in Fig.2. After passing through the FC, thepositron beam enters into the solenoid field with a few millimeters offset from the central axis moving as a cyclotron
orbit. The captured positrons would be accelerated by thelarge aperture S-band accelerating structures before being
delivered to the downstream of the linac [3].
In the following section, we will present the FC field distribution and discuss the positron yield reduction due to theoffset layout of SuperKEKB positron source. The results
are based on the simulation from the target to the end of thecapture section. For the ideal scenario we assumed all thecomponents are well aligned on the central axis. The maximum positron yield equals to 0.89. Whereas SuperKEKB
layout could see 50% yield degradation because of the target and FC offset. We propose that by adjusting the FC slit
orientation, the positrons can go through a path with relatively low transverse components. Combining with a newoffset strike point on the target, the positron yield reduction
because of the components offset will be recovered.

Target offset 3.5mm
0.90.8

FC offset 2mm

Slit x+FC Aperture
7mm

Slit x-Positron Yield [Ne+/Ne-]

0.70.60.50.40.3x- slit

0.2x+ slit
y slit0.1

0.00.0Tungsten Target
Radius 2mm

Slit yInjection Electron
Passage Hole Radius 1mm

Figure 2: SuperKEKB positron target geometry. The redpart represent the FC, yellow is the cooling copper andbrown is the tungsten target. The injection electron passage
hole is shown in white. The green dash lines are the threepossible slit orientation directions.

Figure 3: CST 2D FC transverse field distribution.

FC FIELD DISTRIBUTION AND SLITORIENTATIONIn KEK, we have manufactured and studied two kinds of
FC: the spiral slit one and the straight slit one. The detailed
introduction of these two models, prototyping and experiment could be found in paper [5]. One of the advantages forthe spiral slit FC is low transverse component. The transverse component of the magnetic field in FC is crucial toachieve high capture efficiency. It could flick positrons off
the axis causing positron yield reduction. The Fig.3 shows
the 2D transverse field distribution on the plane that parallel to the slit direction. The main source of the transverse
component is due to the straight cut-in slit section. The field
could be divided into three sections:
1. Between FC and target: the field is symmetric.
2. Cut-in straight slit region inside FC: the field is offset.
3. Further deeper of FC: field is symmetric again.
This indicates that the transverse field strength is lower
on the slit side. A detailed simulation has been done to in-

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Target Offset [mm]Figure 4: Positron yield (number of positrons per electron)
as a function of target offset. The FC center is 2 mm awayfrom the center axis. The black, red and green marks represent the slit orientation of x-, x+ and y respectively, asmarked in Fig.2

vestigate the influence of transverse field by scanning thepositron’s injection trajectory. The simulation started fromthe target and ended at the last accelerating structure in thecapture section. The results have been presented in Fig.4.
In the figure, we show the positron yield at the end of capture section as a function of the target position. Target offset equals to 0 means it’s on central axis. When it equals
3.5 mm, it’s the SuperKEKB setup. And the different colors represent the various FC slit orientation, which has beenmarked on the Fig.2 with the green dash line.
As we can see from the figure, when the target offset=0 mm (on axis), the target is close to x+ slit (because FC
is 2 mm offset from central axis), the x+ slit positron yieldis 15% higher than the x- slit one. Then as the target offset
increases, which means the target is moved toward to the xdirection, the yield peak at the position of 1 mm∼1.5 mm.
The location in between central axis and FC center returns
the maximum value. In this region, the positron yields are
more or less the same with difference slit orientation. And
then the x- slit start to have a better capture efficiency alongwith the target offset toward to the same direction as slit. For
instant, when the target offset equals to 3.5 mm, and an x+
slit direction only has a yield of 0.43, but an x- slit can improve the yield to 0.53. This simulation has shown us that ahigher yield could be achieved by adjusting the target offset
and slit orientation to the same side.

OFFSET STRIKE ON THE TARGET
Although the FC orientation could recover the positronyield a little bit, it’s still a dramatic reduction. The layout isrestricted due to the small aperture of the FC. Therefore, weneed to carry out a solution based on SuperKEKB positronsource setup which has a 3.5 mm offset target and 2 mm off-apply an offset strike of 1 mm. It’s about 91% of the ideal
layout (everything is on axis)

0.8

Positron Yield [Ne+/Ne-]

0.7

CONCLUSION0.60.5

0.4

0.3

x- slitx+ slit

0.2

y slit
0.1

0.0
0.0

0.5

1.0

1.5

2.0

Strike offset [mm]Figure 5: Positron yield as a function of electron bombardment position respect to the target center. Strike offset
equals 0 is the target center and the primary electron beam’s
moving direction is toward to the central axis. The black,red and green marks represent the slit orientation of x-, x+
and y respectively, as marked in Fig.2
set FC. There is one clue we can find out from previous section is that the capture efficiency is better when the positron
beam trajectory is in between central axis and FC center.
This would encourage us to let the electron beam strikes
the target’s off center position to enhance the positron yield.
The Fig.5 shows the simulation results of the positron yield
as a function of the electron bombardment position. Whenx equals to 0, electron beam strikes on the target center. As
we know that the target radius is 2 mm, so the x equals 2 mmmeans it’s the edge of the target. The Fig.5 has also includes
the influences of the FC slit. The x- slit, x+ slit and y slit
represented by the black, red and green marks respectively.
From the Fig.5 we can see that the positron yield would
increase along with the movement of the striking point. The
peak appears when the primary electron beam moves away
1 mm from the target center, which corresponds to 2.5 mm
offset from central axis and 0.5 mm from FC center. As weexpected, the x- slit orientation returns a maximum yield of
0.79, and the y slit is slightly lower (0.76), then the lowest
one is the x+ which equals to 0.73. If the striking point
keeps moving further, although the trajectory gets closer
to the FC center, the primary electron cannot develop sufficient cascade shower to generate positrons. Therefore, thepositron yield is reduced to 0.45 when the offset equals to2 mm, where is the edge of the target. One more thing has
been reflected from this figure is that the slit impact has been
confirmed again. The offset position equals to 2 mm represents the FC center. When x is less than 2 mm, the positron
trajectory is always on the x- side, so that x- slit as expected
would return a higher positron yield all the time. All in all,from this evaluation we can get one important results: Thepositron yield could be recovered from 0.53 to 0.79, if we

The FC plays an important role as a part of matching device. We have found out that the cut-in slit would cause afield offset in the FC. In order to let the injection electron
beam passes the center of the DC solenoid to avoid emittance dilution, the target is away from central axis. Andthe FC could only contain both target and passage hole if it
is 2 mm offset duo to the restriction of the small aperture.
This is an issue that will cause huge yield decrease. As we
can see from the Table.1, if we assume an ideal layout ofthe positron source, which means there’s no offset for any
components and primary electron beam strike at the centerof the target, the positron yield would be 0.87. For the proposed SKB positron source structure, FC offset is 2 mm andtarget offset is 3.5 mm. If the slit is the opposite directionof the offset direction, the positron yield could be reduced
from 0.87 to 0.43. If the slit is the same direction as the offset direction, the positron yield will increase from 0.43 to0.53. Therefore, we suggest letting the target offset and theFC slit on the same side to improve yield. Furthermore, thepositron trajectory should close to the FC center by striking
the primary electron beam off the target center. For example, if the strike position is 1 mm offset, which means 1 mm
closer to the FC center, then the positron yield can be improved from 0.53 to 0.79, which is about 91% of positrons
from the ideal layout. The positron yield is well recovered
without changing the default layout.
Table 1: Positron yield for various calculation scenarios.
On-axisx+ Slitx- Slit

FC/Target
Ideal
OffsetOffsetCenter-Strike
0.87
0.43
0.53

Offset-Strike
N/A
0.73
0.79

REFERENCES[1] SuperKEKB Task Force, “Letter of Intent for KEK Super B
Factory”, KEK Report 2004-4, June 2004
[2] L.Zang. Other, “SuperKEKB Positron Source Target Protection Scheme”, IPAC’13, ShangHai, May 2014, MOPFI017(2013),[3] T.Miura. Other, “Upgrade Status of Injector LINAC for
SuperKEKB”, IPAC’14, Dresden, June 2014, MOPRO001(2014),[4] T.Kamitani. Other, “SuperKEKB Positron Source Construction Status”, IPAC’14, Dresden, June 2014, MOPRI002(2014),[5] L.Zang. Other, “Design Optimization of Flux Concentrator for SuperKEKB”, IPAC’12, New Orleans, May 2012,
TUPPD032 (2012).