THE BPM DAQ SYSTEM UPGRADE FOR SUPERKEKB INJECTOR LINAC
M. Satoh#, F. Miyahara, T. Suwada, and K. Furukawa, Accelerator Laboratory, High Energy
Accelerator Research Organization (KEK), 1-1 Oho, Tsukuba, Ibaraki 305-0801, JapanT. Kudou and S. Kusano, Mitsubishi Electric System & Service Co., Ltd, 2-8-8 Umezono, Tsukuba,
Ibaraki 305-0045, JapanThe KEK electron/positron linac is a 600-m-long
injector that provides the beams with different energy tofour independent storage rings. The non-destructive beam
position monitor (BPM) is an indispensable diagnostic
tool for a long-term stable beam operation. In the KEK
linac, about one hundred BPMs with the four strip-line
type electrodes are utilized for the beam orbit and charge
measurement. The measured beam orbit data is used forthe beam orbit and energy feedback loops. The current
data acquisition (DAQ) system for BPM comprises 24
fast digital oscilloscopes. They can work as aWindowsXP-based EPICS IOC.
Toward the SuperKEKB project, the upgrade of injector
linac is going on for increasing the beam intensity andreducing the emittance. For the SuperKEKB injector linac,
the electron beam emittance will be reduced one-fifthsmaller than that of former KEKB project by using a new
RF gun. For this reason, the measurement precision of
BPM is strongly required to be increased. In this paper,
we present the upgrade plan and status of DAQ system
for BPM towards SuperKEKB project in detail.

INTRODUCTION
The KEK linac sequentially provides the electron andpositron beams with different energies and intensities forfour independent storage rings as shown in Table 1. For
increasing the integrated luminosity and stored current
stability, the simultaneous injection between KEKB
electron and positron rings has been strongly required. In
addition, the PF top-up injection has been also strongly
demanded even during the KEKB injection. For thesereasons, the injector upgrade project started in 2004 sothat the simultaneous top-up of KEKB electron/positronand PF rings. This upgrade was completed in April 2009,
and the simultaneous top-up injection among three
independent rings was successfully achieved [1, 2, 3, 4].
Whereas the KEKB project has completed in thesummer of 2010, the Super KEKB project has started foraiming at the peak luminosity of 40 times higher than thatof former KEKB project. For this purpose, the injector
linac upgrade is going on for increasing the beam
intensity and reducing the emittance. In this linac upgrade,main issues are the construction of positron damping ring,the development of the new positron capture system forincreasing the positron charge of four times present, and___________________________________________ _____________#

masanori.satoh@kek.jp

the installation of a low emittance electron gun as shownin Fig. 1. The performance required of beam position
measurement is a higher precision of ten micro meters orless since the emittance of electron and positron beams
are less than 20 mmmrad.
Table 1: Injection beam energy and charge for each ring

Injection
beam energy(GeV)
Beam charge/bunch (nC)

KEKB e/SuperKEKB
e8/7

KEKB
e+/SuperKEKB
e+
3.5/4

PF

PFAR

2.5

3

1/5

1 (10*)/4(10*)

0.1

0.1

*

Charge of primary electron for positron production

Figure 1: Schematics drawing of SuperKEKB injector
linac. The coloured parts will be newly installed forSuperKEKB project.

LOW EMITTANCE ELECTRON BEAM
DELIVERY
For the SuperKEKB project, the emittance of positron
beam will be reduced 10 mmmrad from 2100 mmmradby using the damping ring newly constructed. The
q = 5 nC, Accelerating structure misalignment x : 0.5 mm

Normalized rms
emittance (mm.mrad)

Abstract

nx Minimum

200

nx Maximum

180

nx Average

160

168

140
120
100
80
60

42.66

40
20
0

12.25
0

100
#C

200
#1

300
#2

#3

400

500

#4

#5

s (m)

Figure 2: Simulation result of horizontal emittance
growth in the KEK injector linac from Sector C to
Sector 5.

emittance of electron beam will be reduced 20 mmmradfrom 100 by a low emittance rf gun. The low emittanceelectron beam should be delivered to the ring without
damping ring due to cost reduction. In the high intensity
electron linacs, emittance growth can be caused by themisalignment of accelerating structures and magnets.
Figure 2 shows the simulation result of horizontal
emittance growth in the KEK linac. Here, it is assumed
that the KEK injector linac configuration from Sector C to
Sector 5, bunch charge of 5 nC and initial normalized rms
emittance of 10 mmmrad, and the accelerating structure
misalignment of 0.5 mm with standard deviation. In thissimulation, the emittance calculations were carried out by
using simulation code elegant and 100 different error
seeds. The simulation result shows that the average andmaximum emittances at the end of Sector 5 are 42.66mmmrad and 168 mmmrad, respectively. The final
emittance strongly depends on the seeds of error.
On the other hand, the fine control of beam orbit cancure the emittance deterioration. Figure 3 shows thesimulation result of emittance growth compensation byusing the fine control of initial beam offset and angle. In
this simulation, we used the seed of alignment error
corresponding to the maximum emittance growth of 168
mmmrad. The simulation result shows that the final
emittance can be reduced 11.5 mmmrad from 168
mmmrad by adjusting of initial beam angle and offset.
The control values of initial beam angle and offset are 100
rad and 10 m, respectively. From these simulation
results, the high precision beam position measurement isstrongly required for the fine beam orbit control ofSuperKEKB injector linac.
200

168 mm.mrad

120
80
40
1.0
0
0.7
5

0.5
0

11.5 mm.mrad

0.2
5

00

-0

0.
50

1.
00

0.
75

)
m
(m

x 0

0.
25

0.

0.0
0

-0.
25

-0.
50

-0.
75

-1.
00

' (m
rad
)

5

0
-0.5

.2

-0

0

00
-1.0 .75

x

 nx (mm.mra

d)

160

Figure 3: Simulation result of emittance growthcompensation by using beam orbit control.

BPM DAQ SYSTEM,
Present System
In the KEK linac, many kinds of feedback loops havebeen developed and utilized to stabilize the beam orbit,

energy, and energy spread [5, 6, 7]. These feedback loops
make use of the beam position information acquired by
the non-destructive BPMs [8]. About one hundred
stripline-type BPMs have been installed in the KEK linac.
The twenty four front-end systems have been installedin the linac klystron gallery at a nearly equal interval
along the beam line. The each DAQ system deals with theanalogue signals of 3 to 6 BPMs. A schematic drawing thepresent DAQ system is shown in Fig. 4. It comprises afast digital oscilloscope (Tektronix DPO7104; 10 GSa/s, 4channels, 8 bits, CPU P4/3.4 GHz, Gigabit-Ethernet) anda cable combiner box.

Figure 4: Schematic drawing of the present BPM DAQ
system.
The four signals coming from one BPM are fed to two
signal combiners (vertical and horizontal) together withthe signals from other BPMs. The delay cables
corresponding to a 7 ns time delay are used to avoid
waveform overlaps at the signal combiners. The each
output of combiner box is divided again into two signals.
Since it is impossible to change the vertical scale ofoscilloscope in every 50 Hz, CH1/CH2 and CH3/CH4 areused for the low charge and the high charge modes,
respectively. The waveforms digitized at a sampling rateof 10 GSa/s are analyzed and converted into the beam
parameters (beam charge, horizontal position, vertical
position), taking into account the calibration coefficients.
The DAQ software has been developed by using
Microsoft Visual Studio 2005 C++, TekVisa, and EPICS
R3.14.8.2 libraries. The DAQ software is running on thefast oscilloscope, and each DAQ can work as an EPICS
IOC [9]. The similar DAQ system is also utilized for the
KEKB and PF-AR beam transport lines [10].
The beam position measurement precision of thepresent system is listed in Table 2. These figures show theexperimental results measured by three BPMs method [5].
The result of 1 nC shows worse precision than that of 0.1

nC since the same vertical scale settings are used for themeasurements of 1 nC and 7 nC. There, the maximum
ADC counts of 1 nC are smaller than the case of test with
0.1 nC electron. From these experimental results, it isdifficult for the present DAQ system to achieve the beam
position measurement precision of 10 m or better.
Toward Super KEKB linac, a new BPM DAQ systemshould be implemented since the ADC resolution of 8 bits
limits the position measurement precision in the present
system.
Table 2: BPM precision of the present DAQ system
Beam charge (nC)
0.1
17

Measurement precision (mm)0.06626
0.106450.05052

New System
For the new DAQ system, the measurement precisionof about 10 m or better is one of key requirement. In
addition, all beam position should be measured in every
20 ms interval. The SuperKEKB injector linac should
perform the simultaneous injection to four independentrings since the beam lifetime of SuperKEKB is much
shorter than that of KEKB rings. In the current design, it
is estimated to be around 10 minutes. Figure 5 shows thebeam operation scheme of SuperKEKB injector linac. The
electron/positron beams with different energies andamounts of charges are delivered in every 20 ms. For thisreason, the fast and precise attenuation control is also key
requirement for the new system.
We adopt Libera Brilliance Single Pass unit as a

Figure 5: Schematics drawing of SuperKEKB injector
linac beam operation. The beams with four different
energies and amounts of charges should be managed.

ADC counts (Arb. units)

30000

candidate of the new system [11]. It is a commercial
product of Instrumentation Technologies. It is a module
dedicated for BPM DAQ and widely used at many recent
accelerator facilities. Libera Brilliance Single Pass unit
comprises four 16-bits ADCs of 125 MHz sampling
frequency, two pass band saw filters in each analogue
channel, a single-board computer (SBC), a fieldprogrammable gate array (FPGA), and so on. It has also
the variable attenuator with maximum attenuation of 31
dB. The attenuation level can be adjusted in 1 dB step.
The typical analogue signal shape from BPM electrodeis a bipolar of 3 ns in width. Applying double saw filters,
the signal is stretched to around 100 ns as shown in Fig. 6.
After about 10 data points are sampled, the sum ofsquares of them is used as signal amplitude from eachelectrode. The data processing of digitized waveform iscarried out on the FPGA side. The calculated beam
position and other data are transferred to SBC side whichis the embedded Linux operation system with fast Gigabit
Ethernet. A client software like beam orbit display can
retrieve the information of beam position and amount ofcharge via EPICS CA protocol.

EVALUATION OF DAQ PERFORMANCE
We evaluate the basic performance of Libera Brilliance
Single Pass unit. Figure 7 shows the result of dataacquisition defect by using test pulse of 50 Hz while fourhours of continuous measurement. In this graph, thetimestamp intervals between previous and current
acquisitions are plotted as a function of test pulse number.
From this result, all beam pulse can be measured up to 50
Hz since all timestamp differences are less than 40 ms.
The more long-term experiment will be also carried out
soon.
Figure 8 shows the precision of beam position
measurement by using 1000 successive pulses of 0.1 nC
electron beam. In this experiment, we used the elevendifferent steering settings and three LIBERAs connected
to different BPMs. These results include the beam
position fluctuation. The result of vertical direction at
steering setting #5 of BPM#1 shows the standard
deviation of 15 mm. The measurement precision of

Ch A
Ch B
Ch C
Ch D

20000
10000
0
-10000-20000-30000

0

20

40

60

80 100 120 140 160 180 200

t (ns)
Figure 6: Raw waveform data measured by LIBERA
Brilliance Single Pass unit.

Figure 7: Result of data acquisition defect by using test
pulse of 50 Hz while four hours of continuous test.

LIBERA Single Pass unit could be less than 15 mm whenthe effect of beam fluctuation is subtracted from thisresult. From these results, it is convinced that LIBERA
Single Pass unit is feasible for our purpose.
Horizontal (BPM#1)
Horizontal (BPM#2)
Horizontal (BPM#3)
Vertical (BPM#1)
Vertical (BPM#2)
Vertical (BPM#3)

Standard deviation of
measured beam position (mm)

0.35
0.30
0.25
0.20
0.15
0.10
0.05
0

1

2

3 4 5 6 7 8 9Steering setting number

10 11 12Figure 8: Precision of position measurement by using
0.1 nC beam and eleven different steering settings.

SUMMARY AND FUTURE PLAN
Toward the SuperKEKB project, the injector linac
upgrade is going on for increasing the beam intensity andreducing the beam emittance. For the low emittanceelectron beam transport without damping ring, highprecision beam position measurement and control are
strongly required. For this purpose, the current BPM
DAQ system should be replaced by new one with higher
measurement precision. Recently, we preliminary
evaluate the performance of a candidate Libera Brilliance
Single Pass unit of Instrumentation Technologies.
In this performance test, we measured the speed
performance of BPM signal detection and the precision ofbeam position measurement by using the KEK linac beamof 0.1 nC. The test result shows the speed performance isenough high for our application of 50 Hz beammeasurement. The possibility of position measurement
precision less than about 15 m is also confirmed by thebeam test.
The functionality of fast and precise attenuation
control will be implemented soon. The complete test of
position measurement precision by three BPMs method
will also be carried out by using the KEK linac beam in
the near future. At the same time, other schemes forgetting the higher precision are also under consideration.

ACKNOWLEDGMENTS
The authors would like to thank Dr. Rok Hrovatin, Mr.
Matjaz Znidarcic, Mr. Peter Leban of Instrumentation
Technologies for their fruitful discussions.

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