UPGRADE OF BPM DAQ SYSTEM 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, Japan
Abstract
The 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 utilized
for the beam orbit and energy feedback loops. The present
data acquisition (DAQ) system for BPM comprises thetwenty four 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. The electron beam emittance will
be reduced one-fifth smaller than that of former KEKB
project by using a new rf gun. The measurement precisionof beam position is strongly required for the lowemittance beam transport. Two new DAQ systems areunder development as a candidate aiming at themeasurement precision of 10 m. In this paper, thesystem description and the result of performance
evaluation are presented 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. Forthe improvement of integrated luminosity and stored
current stability, the simultaneous injection between
KEKB electron and positron rings has been stronglyrequired. In addition, the PF top-up injection has been
also strongly demanded even during the KEKB injection.
For these reasons, the injector upgrade project started in
2004 so that the simultaneous top-up of KEKB
electron/positron and PF rings. This upgrade wascompleted in April 2009, and the simultaneous top-up
injection among three independent rings was successfullyachieved [1].
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 ongoing for increasing the beam intensity
and reducing the emittance. In this linac upgrade, main_______________________________________________________#masanori.satoh@kek.jp

issues are the construction of positron damping ring, thedevelopment of a new positron capture system forincreasing the positron charge of four times present, andthe 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 forthe stable low emittance beam transport.
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

PFPFAR

2.5

3

1/5

1 (10*)/4(10*)

0.3

0.3

*

Primary electron for positron production

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

LOW EMITTANCE PRESERVATION
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. Theemittance of electron beam will be reduced 20 mmmradfrom 100 by using a low emittance rf gun underdevelopment. The low emittance electron beam should bedelivered to the ring without a damping ring due to cost
reduction.
In the high intensity linacs, the accelerating structures
should be precisely aligned since its misalignment causes
the large emittance growth due to the short range
transverse wake field. The simulation result based on theSuperKEKB linac parameters shows that the acceptablemisalignment of accelerating structure is less than thestandard deviation of 0.1 mm. However, it seems to bedifficult to achieve such fine component alignment in apractical manner. In our plan, we will apply BNS
damping scheme at Sector A and Sector B, and the bunch

compression at the arc section to mitigate the emittance
growth. In addition, at the beginning of Sector C, the finebeam position and angle controls will be carried out forthe emittance preservation at the end of Sector 5. In prior
to these tactics, the ordinary beam base alignment will
also be conducted. For these reasons, the beam position
should be precisely measured and controlled.

SYSTEM DESCRIPTION
Present System
In the KEK linac, many kinds of feedback loops havebeen developed and utilized to stabilize the beam orbit,energy, and energy spread [2]. These feedback loops makeuse of the beam position information acquired by the nondestructive BPMs [3]. About one hundred stripline-type
BPMs have been installed in the KEK linac. The twenty
four front-end systems have been installed in the linac
klystron gallery at a nearly equal interval along the beamline. The each DAQ system deals with the analog signalsof 3 to 6 BPMs. A schematic drawing the present DAQ
system is shown in Fig. 2. It comprises a fast digital
oscilloscope (Tektronix DPO7104; 10 GSa/s, 4 channels,8 bit, CPU P4/3.4 GHz, Gigabit-Ethernet) and a cablecombiner box.
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 the time delay of about 7 ns are installed
to avoid the waveform overlapping inside the signal
combiners. The each output of combiner box is equallydivided again into two signals. Since it is impossible tochange the vertical scale of oscilloscope in every 50 Hz,
CH1/CH2 and CH3/CH4 are used for the low charge andthe high charge modes, respectively. The waveforms
digitized at a sampling rate of 10 GSa/s are processed andcalculated into the beam parameters (beam charge,horizontal and vertical positions), taking into account thecalibration coefficients measured in advance. The DAQ

software has been developed by using Microsoft Visual
Studio 2005 C++, TekVisa, and EPICS base R3.14.8.2.
The DAQ software is running on the fast oscilloscope,and each oscilloscope works as an EPICS IOC [4]. The
similar DAQ system is also utilized for the KEKB andPF-AR beam transport lines [5].

New System
The measurement precision of 10 m or better is one of
key requirements for the new DAQ system. In addition,
all beam positions up to 50 Hz should be measured. Since
the SuperKEKB injector linac will be operated base on asimultaneous top-up of the four independent rings, theelectron/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.
Currently, we have two candidates as a new system.
One of them is a VME-based module which has beenreported in elsewhere [6]. Another is Libera Single Pass
Electron (LSPE) unit from Instrumentation Technologies
tested in this study [7]. It is a module dedicated for BPM
DAQ and widely used at many recent accelerator
facilities. LSPE unit comprises a 16-bit ADC with thesampling frequency of 160 MHz, two surface acoustic
wave (SAW) filters with the center frequency of 522
MHz and the bandwidth of 24 MHz in each channel. It
has also the variable attenuator with maximum
attenuation of 31 dB. The attenuation level can beadjusted in 1 dB step. A single-board computer (SBC)
and a field-programmable gate array (FPGA) are utilizedfor processing the digitized waveform.
The typical analog signal shape from the BPM
electrode is a bipolar with the duration of 3 ns. Applying
double SAW filters inside LSPE, the signal is stretched toaround 100 ns as shown in Fig. 3. Using about 15
sampled data points, the square root of square summationof them is used as signal amplitude corresponding to theeach electrode. The data processing of digitized waveform
is carried out on the FPGA. The calculated beam positionand other data are transferred to SBC which is based on
the embedded Linux operating system with fast Gigabit
Ethernet. The client software like beam orbit display canretrieve the information of beam position and bunch
1500
1200A/DC counts

9006003000-300-600-900-1200-1500

Figure 2: Schematic drawing of the present BPM DAQsystem.

050100150200250300t (ns)Figure 3: Raw signal waveform measured by LSPE.

RESULTS OF 3-BPM MEASUREMEN
Present System
The beam position measurement precision of thepresent system was evaluated by the 3-BPM method [3].
We conducted the beam test by using the electron beamwith the charge of 0.3 nC at Sector 3 in Fig. 1. Applying
the 5 different settings to a steering magnet situatedupstream of the used BPMs, the beam positions weresynchronously measured with three BPMs. For eachsteering settings, around 400 data were measured andanalyzed. Figure 4 shows the residual distribution
between the measured and estimated horizontal beam
positions at the third BPM. Here, the estimated beam
positions at the third BPM can be calculated with thelinear multiple regression analysis by using the measured
position data at first and second BPMs. The results of thehorizontal and vertical directions are 26.3 m and 25.1
m, respectively. In the present system, the position
measurement precision is limited by the ADC resolutionof 8 bit.

measurement precision of 1.6 m though the precision
deteriorates larger than 10 m with the large phasemisalignment. In Fig. 5, the maximum setting of 4000
corresponds to the timing delay of around 1.5 ns.
After the optimization of phase alignment, we carriedout the 3-BPM experiment with the same BPMs andsteering magnets used for the present system evaluation.
The result is shown in Fig. 6. The measurement precisionsof horizontal and vertical directions are 7.05 m and 7.06
m, respectively. We confirmed that the LSPE unit cansatisfy our target performance of 10 m.
15.0
12.5

x (m)charge through the EPICS CA protocol.

10.0
7.5
5.0
2.5
0.00500 1000 1500 2000 2500 3000 3500 4000

Delay setting of channel A

Figure 5: Measurement precision for changing thedelay settings of channel A.

Figure 4: Residual distribution between the measured
and estimated horizontal beam positions at the third
BPM by the present system.

New System
In prior to the 3-BPM experiment by using LSPE, weshould precisely adjust the four signals from differentelectrodes since a small fraction of signal leaks beyond
the predetermined sampling window of 96 ns duration. InLSPE, the channel A and channel C are used for
calculating the horizontal beam position. In order toestimate the sensitivity of the signal phase misalignment
to the measurement precision, we evaluated themeasurement precision by changing the delay setting ofchannel A. Here, the input signals are generated by apulse signal generator instead of the real electron beam.
The optimum adjustment of delay setting shows the highFigure 6: Residual distribution between the measured
and estimated horizontal beam positions at the third
BPM by the LSPE unit.

LONG-TERM STABILITY
In the KEK linac BPM system, a strip-line electrode ofBPM is connected by 35-m-long coaxial cable with BPM
DAQ system installed at klystron gallery. For the longterm operation, the measured beam position could bevaried even for the constant real beam position since thetemperature variation may affect the ADC gain and theelectrical conductivity of long coaxial cable. For thisreason, the system stability during a long-term period wasHorizontalVertical2

Temperature (tunnel)
40
Temperature (gallery)
353012502015-1

10-2-3130716 0900

5130717 0900

Temperature (deg.)

Measured position (m)

30130718 0900

Date and time (yyMMdd hhmm)

Figure 7: Long-term measurement accuracy during twodays by a pulse signal generator.

SECOND BUNCH MEASURMENTACCURACY
The measurement accuracy of second bunch beam
position is also an important figure of merit. We measured
the impact of first bunch signal variation on the secondbunch measurement. By using the experimental setup as
shown in Fig. 8, the simulated second bunch position wasmeasured when the simulated first bunch signal intensity
was intensively changed. As consequence of thesimulated first bunch intentional damping, the simulated
second bunch measurement accuracy is estimated within
10 m as shown in Fig. 9. Another experimental resultshows the first bunch position variation from -2.25 mm to2.25 mm affect the measured beam position of second
bunch with 3.44% as reported in elsewhere [8]. Its resultmeans that the 100 m position offset of first bunchcauses the second bunch measurement error of 3.44 m.
These results are acceptable performance for the real
beam operation.

Fig. 8: Experimental setup for evaluating the simulated
second bunch beam position accuracy.

108
x2 (m)also evaluated. In this measurement, the signal
generated by a pulse signal generator was equally divided
into four signals, and each of them was connected to theSAM connector at the linac tunnel. The other side ofcable was fed to each channel of LSPE situated at linacklystron gallery through the 35-m-long coaxial cable.
Figure 7 shows the measured position variations in twodays measurement period. In this experiment, thetemperature variations at the linac tunnel and the klystrongallery were 1.5C and 4.5C, respectively because the air
conditioner operated only from 8 a.m. to 8 p.m. due to thesummer maintenance period. From Fig. 7, the maximum
position drift is around 2 m. Its stability is enough forour purpose because the temperature can be controlled
within 1C during the real beam operation.

6

Attenuation settings of LSPE (dB)20212225283142005101520
Attenuation values of Att#1 (dB)Figure 9: Variation of second bunch measurement
accuracy by changing the attenuator setting of firstbunch simulated signal.

SUMMARY AND FUTURE PLAN
Toward the SuperKEKB project, the high precision
beam position measurement and control are strongly
required for the low emittance electron beam transport
without a damping ring. For this reason, the present BPM
DAQ system should be replaced by the new one with thehigher measurement precision. We conducted theperformance evaluation of LSPE unit as a new system
candidate. The result of 3-BPM measurement confirmsthat the both of horizontal and vertical measurement
precisions are around 7 m. These results are surpassing
the design goal of 10 m. After comparing theperformances between LSPE and another candidate ofVME-based one under development, one of them will bestarted in mass production and installed in the next fiscal
year.

ACKNOWLEDGMENTS
The authors would like to thank Mr. Matjaz Znidarcic
from Instrumentation Technologies for helping 3-BPM
experiment.

REFERENCES[1] M. Satoh et al., in Proceedings of Linear Accelerator
Conference LINAC2010, Tsukuba, Japan, pp.703707 (2010).
[2] T. Suwada et al., Phys. Rev. ST Accel. Beams 8,112802 (2005).
[3] T. Suwada et al., Nuclear Instruments and Methods
in Physics Research A 440, pp.307-319 (2000).
[4] M. Satoh et al., in Proceedings of ICALEPCS2009,
Kobe, Japan, pp.567-569 (2009).
[5] T. Aoyama et al., in Proceedings of ICALEPCS2009,
Kobe, Japan, pp.495-497 (2009).
[6] R. Ichimiya et al., in Proceedings of IBIC2013,
Oxford, UK, Sep. 16-19, 2013.
[7] http://www.i-tech.si/.
[8] M. Znidarcic et al., in Proceedings of FEL2013,
Manhattan, USA, Aug. 26-30, 2013.