DESIGN, MANUFACTURE AND OPERATION OF THE BEAM SPOILERFOR POSITRON TARGET PROTECTION∗
L. Zang† , K. Kakihara, T. Kamitani, K. Mikawa, F. Miyahara, T. Suwada
High Energy Accelerator Research Organization (KEK)
1-1 Oho, Tsukuba, Ibaraki, 305-0801, Japan
Abstract
In order to produce positrons, intensive pulsed electron
beam is used to strike on a tungsten target. The energy deposition is distributed non-uniformly over the target, leading to a mechanical stress. As a result of large thermal gradient, the target could be potentially damaged. To avoid
the target destruction, the peak energy deposition density(PEDD) in the target should be well below the critical limit(35 J/g) based on the SLAC operational experience. Withan expected primary electron spot size on the target of SuperKEKB positron source, the PEDD will exceeds the limit.
We will introduce a beam spoiler to enlarge the spot size bymultiple scattering in thin beam screen and aluminum plate.
It reduces the PEDD down to half of the limit. This paper describes the design of the spoiler and the beam screen systemused in the positron beam commissioning of SuperKEKB
positron source started in 2014.

INTRODUCTION
SuperKEKB requires an intensive beam with a large number of positrons, which is generated by a 3.5 GeV primaryelectron beam with a charge of 20 nC/pulse and the frequency repetition rate is 50 Hz [1]. The pulsed electron
beam distributes the energy non-uniformly over a tungsten
target. A mechanical stress appears due to the large thermal gradient during each pulse, which could potentially destroy the target. Such kind of target damage has been experienced at SLAC [2]. After a series of experiments, thepeak energy deposition density (PEDD) has been describedas the parameter to evaluate the safety margin and to determine the incident beam size to avoid the target damage.
The threshold value of the PEDD is about 35 J/g. The detailed study of correlation amount electron beam spot size,
target PEDD and positron yield has been discussed in thepaper [3]. One of the conclusions is that a spot size of
0.7 mm is an optimum value considering the safety marginof PEDD and positron yield reduction. For SuperKEKB, at
the upstream of the positron target, as we can see from theFig.1, there are pulsed quadruples for the beam focus purpose. The pulse quadruples could mis-focus the primaryelectron beam leading to extremely small spot size. In order to make sure the primary electron beam strike on thepositron target with a PEDD lower than 35 J/g, a protection
spoiler needs to be placed between the focus magnet andthe target. The spoiler should be placed close to the pulsed
quadruple to prevent to be destroyed by the extremely small∗ Work supported by KEK† zang@post.kek.jp

Figure 1: Schematic view of SuperKEKB positron source
configuration and spoiler principle design.
size electron beam. The beam spoiler is made of aluminumattaching with a screen monitor who can help us observe
the beam through view port. In the following section of thispaper, the spoiler design and assembly will be introduced.
Furthermore, the spoiler and target alignment would also bediscussed. In the final section, some early stage spoiler testwith electron beam will be presented.

SPOILER DESIGN AND ASSEMBLY
The spoiler has a double layers structure as we can see
from the Fig.1, because it can be used not only as a scatter
plate but also a screen monitor. In fact, the first layer is ascreen and the second layer is a scatter plate. It is 45 degree tilted for the purpose of reflecting the beam image by
screen through the view port. The screen and scatter plate
is made of AF995R (Al2 O3 +CrO3 ) and pure aluminum respectively. The properties of both of the materials have been
listed in table1. Although we only expect a small amountof energy deposition in the spoiler and the screen, for thesafety reasons, water cooling system is applied while a thermocouple is installed to monitor the temperature. The assembled spoiler has been implemented to the linac beamline as shown in Pic.2
Table 1: positron yield
Properties
Density
Melting Point
Thermal Cond.
Specific heat
Radiation Length

Unit
g/cm3
K
W/m/K
J/g/C
mm

Aluminum
2.7
993.5
237
0.9
89AF995R
3.98
2000
250
1.09
70.4

Due to the restriction of the beam line layout, there arefew places where the spoiler could be installed. The cho-Figure 2: Beam spoiler has been installed in the linac beamline. The hole aperture is 3 mm.

Figure 3: Telescope image of the spoiler alignment.

sen position locates 3 m upstream of the positron target. In
fact, 3 m is a good drift distance requiring relatively thinner
spoiler, which could see less positron yield reduction. And
also the 3 m position is close to the pulsed quadruple, which
is secure from the mis-focusd electron beam. As we havediscussed in previous section that a spot size of 0.7 mm isdesirable. Therefore, a combination of 0.1 mm screen and
0.2 mm scatter plate should sufficiently scatter the electronbeam. After drifting 3 m to downstream target, the aiming
spot size could be achieved.

SPOILER AND TARGET ALIGNMENT
For a pulse-to-pulse switching system, the pulse for generating positrons will be steered off axis and scattered byspoiler. And the injection pulse needs to pass through thespoiler and target. Therefore, the spoiler has a passage holeof 3 mm in diameter in the center, and the target hole is
2 mm in diameter. Alignment of those two passage holes
need to be defined. The spoiler hole and the target hole need
to be well aligned because the injection electron beam has topass through without any interference. The desirable levelof alignment is about ±0.1 mm in both horizontal and vertical direction. After installing the spoiler and the target into
the linac beam line, an alignment measurement has beendone. A telescope was placed at the upstream of the pulsed
quadruple. It was perfectly aligned with the beam line. TheCCD camera attaching to the telescope could help us to get
the image. The measurement results showed spoiler was
about 1.5 mm and 0.1 mm offset to the west and up respectively. The vertical misalignment was fine. But in the horizontal direction, certain action for correction was necessary.
Therefore, a well-defined 0.2 mm shim was inserted to correct the spoiler to the east direction. The Pic.3 shows thespoiler had a misalignment of 0.05 mm after the correction.
Other than the spoiler alignment, we have also done thealignment measurement for the target. As we can see fromthe Pic.4, the positron target system had an offset to east
about 0.3 mm, and vertically down about 0.05 mm. We

Figure 4: Telescope image of the target system alignment.

cannot correct the offset because the reference alignment
track is on the west side, which means only the west offset could be corrected. To sum it up, both of spoiler andthe target system had a precise alignment in the verticaldirection. Whereas in the horizontal direction, the spoiler
and target system had a 0.15 mm misalignment to west and0.3 mm misalignment to east respectively. The spoiler offset has been reduced to 0.05 mm to east by inserting a shim.
Hence, the target system is 0.25 mm offset with respect tothe spoiler center. And the electron injection needs to becarefully adjusted.

SUPERKEKB POSITRON BEAM
COMMISSIONING
SuperKEKB positron beam has started to commission
since 2014. The detailed positron source construction situation could be found in paper [5]. The beam spoiler as
an important protection component has been installed 3 m
upstream of the target. The electron beam charge in thisstage is about 0.8 nC. After passing the J_arc, the charge
for positron generation is about 0.4 nC. The experimentcharge is reduced from 0.08 nC to 0.045 nC. The reduction
is mainly because of introducing the beam spoiler. The
spoiler’s design value is to enlarge a pin point electron beam
to a RMS radius of 0.7 mm. At this stage of operation, theelectron beam is not well optimized, which means the spot
size is fairly large. After scattering, it could strike out of thetarget area. Then combine with the effect of the steering angle and other contribution, the positron charge degradationis reasonable. Further beam test has been planned, and willtake place later this year.

CONCLUSIONS
Figure 5: BMP(SP_16_5) measurement of positron charge
as a function of the steering (BX_13_5) current. Primaryelectron beam passed through the spoiler center hole.

Figure 6: BMP(SP_16_5) measurement of positron charge
as a function of the steering (BX_13_5) current. Primaryelectron beam was scattered by the beam spoiler.
has been carried out to test the influences of the spoiler topositron yield.
At first, we would like to let the primary electron beampass the spoiler hole before being steered (BX_13_5) tostrike on the target. The generated secondary particles will
be captured and accelerated. In the chicane section, secondary electrons would be separated and dumped. Thenthe positron beam charge would be measured by the downstream BPM (SP_16_5). The result is shown in Fig.5.
As we can see from Fig.5, the positron charge is position
dependent. With a 0.4 nC primary electron beam, without
being scattered by spoiler, the maximum positron charge
was about 0.08 nC. The detailed positron yield and position
dependent could be found in paper [6]. In order to investigate the beam spoiler impact, we then used the spoiler toscatter the primary electron beam, and followed the sameprocedure as the previous test to evaluate the positron beam
charge. The BPM measurement result is shown in Fig.6.
The Fig.6 shows the same position dependent as the previous measurement. However, the peak value of the positron

In this paper we have reported the beam spoiler assembly and beam test. It has been successfully. The spoiler
is installed in the linac, locating 3 m upstream of the target. The water cooling system is applied to make sure thespoiler will not be damaged by the high intensity electron
beam. At the same time, the thermocouple would help us tomonitor the temperature changes. Before firing the electron
beam, the spoiler and the target misalignment has also beenmeasured. After correction, the spoiler would only have a0.05 mm offset from beam line center. Whereas the target
system is 0.3 mm offset with respect to the beam line center.
Therefore, the electron injection needs to be carefully adjusted. After all the preparations, we had a 0.4 nC primaryelectron beam for spoiler investigation. For the purpose ofcomparison, we firstly let the electron beam pass the hole tostrike the target. And then repeat the same measurement but
make the electron beam go though the spoiler before hitting
the target. The results indicate that the spoiler could enlarge
the beam spot size, which has caused a positron charge reduction from 0.08 nC to 0.045 nC. Considering this is theearly stage of the SuperKEKB electron and positron commissioning, such result is expected. Further spoiler investigation and positron beam study will be carried out later thisyear.

REFERENCES[1] SuperKEKB Task Force, “Letter of Intent for KEK Super BFactory”, KEK Report 2004-4, June 2004[2] V. Bharadwaj, Y. Batygin and J. Sheppard et al., “Analysis ofBeam-Induced Damage to the SLC Positron Production Target”, SLAC-PUB-9438, August 2002.
[3] L.Zang. Other, “SuperKEKB Positron Source Target Protection Scheme”, IPAC’13, ShangHai, May 2014, MOPFI017(2013),[4] T.Miura. Other, “Upgrade Status of Injector LINAC for
SuperKEKB”, IPAC’14, Dresden, June 2014, MOPRO001(2014),[5] T.Kamitani. Other, “SuperKEKB Positron Source Construction Status”, IPAC’14, Dresden, June 2014, MOPRI002(2014),[6] L.Zang. Other, “Positron Yield Optimization By Adjusting
The Components Offset And Orientation”, IPAC’14, Dresden,June 2014, MOPRI003 (2014).