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, Japan
Abstract
The KEK electron/positron linac is a 600-m-long
injector that provides the beams of different energies withfour independent storage rings. A non-destructive beam
position monitor (BPM) is an indispensable diagnostic
tool for a long-term stable beam operation. In the KEKlinac, around one hundred BPMs with the four strip-line
type electrodes are utilized for the beam position andcharge measurement. The measured beam positions canbe used for the beam orbit and energy feedback loops.
The current data acquisition (DAQ) system of BPM
comprises 23 fast digital oscilloscopes based on
WindowsXP.
Toward the SuperKEKB project, the upgrade of injector
linac is now going on for increasing the beam intensity
and reducing the beam emittance. For the SuperKEKB
injector linac, the electron beam emittance should be
reduced one-fifth smaller than that of former KEKBinjector. For this reason, a high precision of the beam
position measurement is strongly required. In this paper,
we present the performance of a new BPM DAQ system
in detail.

INTRODUCTION
The KEK linac sequentially provides the electron andpositron beams of different energies and intensities withthe four independent storage rings. For increasing theintegrated luminosity and stored current stability, thesimultaneous injection between KEKB electron andpositron rings has been strongly required. In addition, thePF top-up injection has been also strongly demanded evenduring the KEKB injection. In 2004, the injector upgrade
project has started aiming at the simultaneous top-up
among KEKB electron/positron and PF rings. This
upgrade has been completed in April 2009, and thesimultaneous top-up injection for the three independentrings has been successfully achieved [1-4].
Whereas the KEKB project has been completed in thesummer of 2010, the SuperKEKB project has been started
for aiming at the peak luminosity of 40 times higher thanthat of KEKB project. For this purpose, the injector linac
upgrade is now going on for increasing the beam intensity
and reducing the beam emittance. In this upgrade, the key
issues are the construction of a positron damping ring, thedevelopment of a new positron capture system forincreasing the positron charge of four times present, andthe development of a new electron gun for a low___________________________________________ _____________#

masanori.satoh@kek.jp

emittance beam production [5-11]. The desired precisionof beam position measurement is ten micro meters or lesssince the transverse (vertical) normalized emittancerequired for electron (positron) beam is less than 20 (6)mmmrad.

SYSTEM DESCRIPTION OF BPM DAQSYSTEM
Present System
In the KEK linac, many kinds of feedback loops havebeen developed and utilized to stabilize the beam orbit,
energy, and energy spread [12-14]. These feedback loops
make use of the beam position acquired by a nondestructive BPM [15]. Around one hundred BPMs withthe four stripline-type electrodes have been installed inthe KEK linac.
The twenty three DAQ systems have been installed in
the linac klystron gallery at a nearly equal interval along
the beam line. The each DAQ system deals with theanalogue signals from 3 to 6 BPMs. Figure 1 shows aschematic drawing of the present DAQ system. It
comprises a fast digital oscilloscope and a cable combinerbox.
The four analog signals from one BPM are fed to two
signal combiners (vertical and horizontal) together withthe signals from other BPMs. The delay cables
corresponding to a time delay of 7 ns are used to avoid
the waveform overlaps at the signal combiners. The each

Figure 1: Schematic drawing of the present BPM DAQsystem.

output of combiner box is divided again into two signals
with equal amplitude. Since it is impossible to change thevertical scale of oscilloscope in 20 ms interval, CH1/CH2
and CH3/CH4 are used for the measurement of low
charge (0.11 nC) and high charge (10 nC) beams,respectively. The waveforms digitized at a sampling rateof 10 GSa/s are used for the calculation of beam positionand charge. The DAQ software is running on the fast
oscilloscope, and each system can work as an EPICS IOC[16, 17].
The beam position measurement precision of thepresent system has been evaluated by using the 3-BPM
method [15], and its result shows around 50 m for boththe horizontal and vertical directions. Toward
SuperKEKB linac, the new BPM DAQ system should be
developed since the beam position measurement precisionof about 50 m is not enough for the control of a low
emittance beam transport.

New System
For the new DAQ system, the beam position
measurement precision of about 10 m or less is one ofkey requirement. In addition, a fast beam positionmeasurement is required for all beam pulses of 20 ms
interval since the SuperKEKB injector linac should
perform the simultaneous injection among the four
independent storage rings. The electron and positron
beams of different energies and intensities are delivered inevery 20 ms. For this reason, a fast and precise
attenuation control is also one of key requirement for thenew system.
We adopt the Libera Brilliance Single Pass (LBSP) unit
as a candidate of the new system [18]. It is a commercial
product of Instrumentation Technologies. It is a module
dedicated for BPM DAQ and widely used at many recent
accelerator facilities. The LBSP unit comprises four
signal input ports, four analog-to-digital converters(ADCs) of 125 MHz sampling frequency and 16 bit
resolution, two pass band surface acoustic wave (SAW)
filters of a center frequency 511 MHz with a 3 dB
bandwidth of 24 MHz, a single-board computer (SBC), afield-programmable 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. After applying two SAWfilters, the signal is stretched to around 100 ns because of
the limited band pass width. Using 12 of digitized datapoints, the sum of squares of them is utilized for the beam
position and charge calculation. The data processing ofdigitized waveform is carried out on the FPGA part. The
calculated beam positions and other parameters aretransferred to SBC part which is the embedded Linux
operation system with a fast Gigabit Ethernet port. All theparameters can be accessible via EPICS channel accessprotocol from a client software.

PERFORMANCE EVALUATION
Using the 3-BPM method, we evaluated the beam
position measurement precision of LBSP unit with theelectron beam of 0.3 nC. In this experiment, three LBSP
units were connected to three adjacent BPMs as shown inFig. 2. In this figure, we measured the three BPMs
encircled by light blue colour for the beam test. The
eleven different settings were applied for the horizontal
and vertical steering magnets situated at upstream of used
BPMs. For each steering setting, the beam positions weremeasured over 1000 successive beam pulses.
The fine phase adjustment among four signals fromeach electrode was carried out prior to the 3-BPM
measurement. To attain a higher precision of position
measurement, the fine phase adjustment is indispensable
since the ADC clock of LBSP is not synchronized withthe rf frequency used for the beam acceleration. Figure 3
shows the phase differences among the four signals before
and after fine phase adjustment. The maximum phase

Figure 2: Electron beam positions of horizontal (upper),vertical (middle) directions, and beam charge (bottom)along the KEK linac during the 3-BPM measurement.

(a)(b)
Figure 3: Phase differences among four signals fromdifferent electrodes before (a) and after (b) fine phaseadjustment.

(a)(b)
Figure 4: Measurement precision of horizontal direction
evaluated by the 3-BPM method. (a) presents themeasured versus expected beam position at BPM#3, and(b) shows the histogram of the residual between them.

difference of about 1.5 ns was corrected to around 100 ps
by inserting the different number of SMA connectors toeach signal input port of LBSP.
Figure 4 shows the 3-BPM result for horizontaldirection. Using the linear multiple regression analysis,
the expected beam positions at BPM#3 were calculated
from the measured beam positions at BPM#1 and BPM#2
over the 11000 measured data as shown in Fig. 4-(a). Theresiduals between the expected and measured beam
positions at BPM#3 were calculated, and its histogram
was fitted by a Gaussian distribution function as shown inFig. 4-(b). The similar result was also obtained for thevertical direction. From these results, the position
measurement precisions of LBSP in horizontal andvertical directions are 20.24 m and 19.12 m,respectively. These results show that the precision ofLBSP is much improved in comparison with that ofcurrent DAQ system though the target precision is still
not archived.

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 a damping ring, the highprecision beam position measurement and control arestrongly required. For this purpose, we evaluated theperformance of a candidate LBSP unit of Instrumentation
Technologies.
To eliminate the pulse-by-pulse jitter of beam position
and angle, we carried out the 3-BPM method with theelectron beam of 0.3 nC at the KEK linac. The test resultshows that the measurement precision of LBSP archives
approximately 20 m for both the horizontal and verticaldirections. The further improvements of using ADC withhigher sampling rate and tuning the analog frontend are
now going on for aiming at the measurement precision ofbeam position less than 10 m. We will conduct the 3BPM test with the improved Libera units at KEK linac in
this autumn.

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

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