SUPERKEKB POSITRON SOURCE TARGET PROTECTION SCHEME∗
L. Zang† , T. KamitaniHigh Energy Accelerator Research Organization (KEK)
1-1 Oho, Tsukuba, Ibaraki, 305-0801, Japan
Abstract
The SuperKEKB requires an intense beam with a large
number of positrons, which is generated by a high energy
electron beam strike on a solid tungsten target. The pulsed
electron beam distributed the energy non-uniformly overthe target. In that case, a mechanical stress appears due tothe large thermal gradient during each pulse, which could
potentially destroy the target. Based on the analysis of
the SLAC damaged target, peak energy deposition density(PEDD) should not exceed 35 J/g to ensure a long term ofsafe operation. One way of reducing PEDD is increasing
the beam spot size. Hence we proposed a target protection scheme, in which a beam spoiler is placed upstreamof a generation target. The aim is to maintain the generation targets PEDD below 35 J/g even with a very small size
primary electron beam. In this paper, we will introduce
graphite, aluminum and copper as the protection target material candidates. And also present the PEDD and positronyield evaluation as a function of various parameters such asprotection target thickness and drift space.

INTRODUCTION
The SuperKEKB primary electron is 3.5 GeV with intensity of 20 nC/pulse, and the frequency repetition rate is
50 Hz. The electron beam will strike on a tungsten target
after passing through the pulsed steering coil and quadruple. The generated positrons are focused by the opticalmatching device before injecting into a 2 m long large aperture s-band accelerating structure for acceleration. The
principle layout is shown in Fig.1. In order to achieve requested luminosity at the interaction point (IP), a high intensity positron beam is necessary. In this scheme, the thermal gradient difference due to the pulsed irradiation may
lead to the target destruction. Such kind of target damage
has been experienced at SLAC [1]. After a series of experiments, the peak energy deposition density (PEDD) has
been described as the parameter to evaluate the safety margin and to determine the incident beam size to avoid thetarget damage. The threshold value of the PEDD is about
35J̇/g. The primary electron beam spot size is one of thekey parameters that determine the PEDD. For SuperKEKB,
at upstream of the positron target, there are pulsed steering
coil and quadruple. That does bring the possibility of misfocused primary electron beam strikes on the target with anextremely small spot size which could destroy the target.
∗ Work supported by KEK† zang@post.kek.jp) . /
primary e-bridge coils

DC QM

spoiler
targetsolenoid

Accel.
structuree+beamholeinjection e-
FluxConcentratorpulse ST
pulse QMFigure 1: Schematic view of the SuperKEKB positronsource configuration.
Hence in this paper we would like to introduce a spoiler asa protection system, whose function is to enlarge the spot
size to make sure the PEDD on the target is well below thedestruction limitation value of 35 J/g.

SPOT SIZE, PEDD AND POSITRON YIELDThe basic design of the SuperKEKB positron target is a
14 mm long cylinder made of tungsten with 2 mm radius,
which is surrounded by copper conductor including cooling system. The rate of PEDD in tungsten target according to a Monte Carlo calculation is shown in Fig.2 in thesame frame with the theoretical calculation and simulation
results of the positron yield.
In Fig.2, black line is the PEDD value as a function of incident primary electron spot size on target, and the horizontal red dash line represent the destruction limitation valueof 35 J/g. The blue stars show tracking simulation results ofpositron yield until Damping Ring. There are three examples has been shown which are pinpoint electron beam, spot
size 0.5 mm and 0.7 mm. The blue dash line is obtainedby applying theoretical cut to the positrons generated fromtarget then normalized to fit the tracking simulation results(blue stars).
From the Fig.2 we could see that when the spot size is
reduced from 1 mm to 0.2 mm, the PEDD increase exponentially. Comparing the PEDD of the 0.5 mm and 0.7 mm
spot size cases, for 0.5 mm beam spot the PEDD is about
25 J/g which is below the threshold of 35 J/g. However,the safety margin is rather thin. In this case, a little bit

Table 1: Material Candidates for the Positron Source Spoiler
Copper Aluminum Graphite
Atomic No.
29
1363Density (g · cm )
8.96
2.7
2.09
Melting Point(K)
1358
933
3500
Thermal Cond.
44.7
11.3
32.2
(W · m−1 · K −2 )Radiation length (mm)
14.4
89
188

Figure 2: PEDD (black line) and positron yield (blue color)
as a function of electron beam spot size on the target.
mis-focus from upstream pulsed quadruple could lead to asmaller spot size then seriously damage the target. Hence,
enlarge the electron spot size is necessary, and half of thethreshold value or lower would be a reasonable choice.
When we increase the beam size, we need to bear in mind
that the positron yield will be reduced as the blue stars andline shows. Hence, a spot size of 0.7 mm is a better option.
With this parameter the PEDD is about 15 J/g. This value isless than half of the limitation. As a tradeoff, the positronyield would suffer a degradation about 10%.

PROTECTION SPOILER
In order to make sure the primary electron beam strikeon the positron target with a spot size larger than 0.7 mm,
a protection spoiler needs to be placed between the focus
magnet and target. We have investigated three kinds of materials including Copper, Aluminum and Graphite as listed
in table 1. Each of them has the unique physical properties
that could lead to some advantages to be used as a spoiler.
For example, the copper is a well-known conductor that
could deal with heat much easier. The aluminum is a popular choice for beam size enlargement. And graphite is lightmaterial with much longer radiation length, which could
avoid too thin target when drift space is long.
In the previous section we have discussed the relationship between spot size on target and PEDD. A spot size of
0.7 mm is preferred. To achieve that, we have evaluatedthe required spoiler thickness as a function of drift spacefrom spoiler to target for different materials as shown in
Fig.3. In the plot, it also shows the positron yield corresponding to the thickness and material. These three kindsof materials show an identical contribution to the positronyield and required similar thickness in unit of radiation
length. When the drift distance reduces, which means thespoiler get closer to the generation target, a thicker spoiler
is needed to enlarge the scattering angle. In this process,the positron yield could also be affected that reduce from
1.2 positrons/electron to 0.7 positrons/electron.

Figure 3: Spoiler thickness (black color) and positron yield(red color) as a function of drift space between spoiler andtarget. The electron spot size on target is 0.7 mm. Threekinds of material are Copper (square), Aluminum (circle)
and Graphite (triangle).

Due to the restriction of the beam line layout, there arefew places where the spoiler could be installed. Based on
the proposed structure, the spoiler could be installed either at positions of 0.7 m or 3 m upstream of the generation
target. The required spoiler thickness for 0.7 m and 3 mschemes is about 0.09 and 0.006 radiation length respectively. In order to place the spoiler near the pulsed quadruple to avoid the small beam size, we prefer to place thespoiler to the position of 3 m. In this case, if we use copperfor spoiler, the spoiler thickness is about 0.08 mm, whichis too thin to handle. Whereas the graphite could have sufficient thickness, but its hardness is a concern. Studies ofchoosing a suitable graphite alloy is undergoing. Therefore, aluminum maybe a better choice because of a goodbalance between strength and radiation length. PEDD isthe other consideration when we make a choice of 3 m
scheme. At the position of 0.7 m, the electron beam spot
size can be in the region of few tens micron, so that thePEDD on spoiler is too high to handle. Whereas at theposition of 3 m, because it is close to the quadruple, theelectron beam could have a large spot size of few hundreds
micron. The detailed investigations will be discussed in thelater section.

Figure 4: ANSYS thermal calculation of temperature increase after applying air cooling on the outer surface of thechamber.

Figure 5: PEDD as a function of electron beam spot size on
the spoiler when it is placed at 3 m upstream of the target.

THERMAL CONDITION
Assuming a 0.5 mm thick aluminum spoiler locating 3 m
upstream of target is irradiated by a 20 nC 3.5 GeV primaryelectron beam. The thermal condition is a concern, which
ought to be looked after by designing a sufficient coolingplan. Our ANSYS analysis indicates that with the simplest
case, an aluminum disk with radiative cooling could reacha peak temperature of more than 300 degree. That’s below the aluminum melting point. However, it will still beconsidered as a threat to the supporting structure and operational environment. Therefore, we apply air flow on thechamber’s outer surface as showing in Fig.4. In this simulation, the spoiler thickness is 0.5 mm and radius is 10 mm.
The spoiler is connected to the chamber which is 10 mmthick made of aluminum. We assume a forced air convection with a film coefficient of 50 W/m 2 ·K has been applied
to the outer surface. The result shows the peak temperature
is well controlled below 50 degree. In fact, due to the relatively thin spoiler, the energy deposition is fairly small, sothat thermal condition is rather trivial.

SPOILER CHALLENGE
As we have mentioned in the previous section, the generation target needs to maintain a PEDD below 35 J/g toavoid destruction. And the PEDD increases hugely witha small beam size. The purpose of the spoiler is to enlarge
the beam spot size to protect the generation target, so an extremely small beam size could hit the spoiler in some cases.
For example, if there is a 0.005 mm spot size electron strikeon the spoiler made of 0.5 mm Aluminum, the PEDD value
is about 1900 J/g, which is far more than the safe threshold.
Even when we assume a spot size of 0.05 mm. The PEDD
is still as high as 177 J/g. The scan of PEDD as a functionof incident beam size has been shown in Fig.5. As the figure show, a spot size increase from 0.005 mm to 0.05 mm
could reduce the PEDD but not enough. Therefore, the 3 mscheme is preferred, because the location is fairly close topulsed quadruple, from where the beam spot size could beas large as few hundreds microns because the distance toquadruple is much shorter than the focus length. Then welook at the Fig.5 again in the range from 0.1 mm to 0.3 mm.
Now the PEDD is reduced from 48 J/g to 6 J/g. To sum itup, we propose to place the spoiler 3 m upstream of target
to make it close to the quadruple maintaining a large spot
size at least 0.15 mm, which will have a PEDD value of
22 J/g. That’s help us overcome the challenge of spoiler
survive under intensive primary electron beam.

CONCLUSIONSIn this paper we have proposed a spoiler protection system for SuperKEKB positron target. The principle of thisscheme is to place a spoiler at upstream of the target. The
spoiler will be used to scatter the electron beam to avoid extremely small beam spot. And the motivation of the scheme
is that the peak energy deposition density could be wellcontrolled under the limitation so that the target destruction
will not happen to SuperKEKB positron target. After investigating a few kinds of material, all the materials shows
an identical behavior for the scattering effect. Aluminumis a preferred choice due to his good balance of strengthand radiation length. And our thermal analysis shows thatan air cooling outside chamber surface would be enough tomaintain the temperature under 50 degree. Regarding thelocation, 3 m upstream of target is chosen, since we needa spot size larger than 0.15 mm. The location close to thepulsed quadruple could help us achieve that. The spoiler
scheme should be able to protect target from damage andmaintain a long term stable operation.

REFERENCES[1] V. Bharadwaj, Y. Batygin and J. Sheppard et al., “Analysisof Beam-Induced Damage to the SLC Positron Production
Target”, SLAC-PUB-9438, August 2002.
[2] A. Bejan and A. Kraus, “Heat Transfer Handbook”, ISBN 0471-39015-1.