NEW DATA-ACQUISITION SYSTEM OF BEAM-POSITION AND
WALL-CURRENT MONITORS FOR THE KEKB INJECTOR LINAC
T. Suwada, N. Kamikubota, K. Furukawa and H. Kobayashi
KEK, 1-1 Oho, Tsukuba-shi, Ibaraki-ken, 305-0801, JapanAbstract2
About 90 stripline-type beam-position monitors(BPMs) and 70 wall-current monitors (WCMs) have been
newly installed in the KEKB injector linac. Thesemonitors reinforce easily handling beam orbits andmeasuring the amount of beam charges for single-bunchelectrons and positrons which are injected to the KEKBrings. A new data-acquisition (DAQ) system has beendeveloped in order to control these monitors in real time.
Hardware and software of 18 front-end computers weretuned for the linac commissioning. This report describes
the DAQ system, the calibrations of the hardware system,and preliminary beam-test results.
1

INTRODUCTION

The KEK B-Factory (KEKB) project [1] is inprogress in order to test CP violation in the decay of B
mesons. The KEKB is an asymmetric electron-positron
collider comprising 3.5-GeV positron and 8-GeV electronrings. The KEKB injector linac [2] is also being upgradedin order to inject single-bunch positron and electron beams
directly into the KEKB rings. The beam currents are
required to be 0.64nC/bunch and 1.3nC/bunch for thepositron and electron beams, respectively. High-current
primary electron beams (~10nC/bunch) are required inorder to generate sufficient positrons. Therefore, it isimportant to easily handle the orbits of the beams, andespecially, the beam positions and current of the primary
high-current electron beams have to be controlled so as tosuppress any beam blowup generated by large transverse
wakefields. A BPM system has been developed toperform this function since 1992. The goal of the beamposition measurement is to detect the charge center ofgravity within a resolution of 0.1mm. The WCMs havebeen also newly developed in order to reinforce themonitoring system of the beam currents. The amount of
the beam current needs to be precisely measured in order tokeep the positron production and the beam-injection rate
to the KEKB rings higher, because a well-controlled
operation of its injector is required so that we can reach an
optimum operational condition with as a short tuning
time as possible and also keep it during a long-term
operation period.
A new DAQ system based on
VME/OS-9 computers, which are connected with the linac
control system, has been also developed in order to control
these monitors in real time. The performance of the newsystem has been tested using the electron beams at an
extended beam line of the linac during its commissioning.

HARDWARE SYSTEM

2.1 Beam-Position and Wall-Current Monitors
A conventional stripline-type BPM was designedwith a π/2 rotational symmetry.
The total length(195mm) was chosen to make the stripline length(132.5mm) as long as possible so that it can be installed
into limited spaces in the new beam line of the linac. The
angular width of the electrode is 60 degrees in order toavoid a strong electromagnetic coupling between theneighboring electrodes.
A 50Ω SMA-vacuumfeedthrough is connected to the upstream side of eachelectrode, while the downstream ends are short-circuited toa pipe. The WCM comprises a disk-shaped ceramic solid
resistor, a Mn-Zn toroidal ferrite core inside an aluminum
case, and four pickup BNC-type receptacles with π/2
rotational symmetry. The resistance was selected to be
2.5Ω so as to have a good frequency response for shortpulse beams. More detail descriptions are given elsewhere[3-7].
2.2

Monitor Station

All the BPMs and WCMs are connected with themonitor stations. Eighteen monitor stations, each ofwhich comprises a front-end computer (VME/OS-9 with a
68060 microprocessor at 50MHz), a signal digitizing
system (an oscilloscope), and a signal-combiner box, arelocated on the linac klystron gallery at a nearly equalinterval along the beam line. Each monitor station cancontrol twelve(six) BPMs(WCMs) at maximum. The
schematic drawing of a monitor station is shown in Fig.
1. The 4 signals of one BPM are directly sent to asignal-combiner box. Two signal combiners combine thehorizontal and vertical signals of the BPM with thesignals from the other BPMs, and the WCM signal is also
sent to the same combiner box. The two signals from thesignal combiners are fed to two channels of a digital
sampling oscilloscope (Tektronics TDS680B) with asampling rate of 5GHz, and for the WCM the combined
signal is also fed to another channel of the same digital
oscilloscope.
The Unix workstations and the front-end
computers communicate with each other through theextended network system. As shown in Fig. 2, all thefront-end computers are linked to a switching-hub with astar-topology. Fiber-optic cables are used for physicalconnections in order to avoid electromagnetic interference
from high-power klystron modulators. The hub has a link

to the linac control network, where Unix workstations andman-machine interfaces are connected.

∆ y = g2V2 − g4V4 , Σ y = g2V2 + g4V4 ,where x and y show the horizontal and vertical beam
position, aij and bij are derived by fitting the map data to
the map function by using the least-squared procedure,
and V1 and V3 (V2 and V4) are the horizontal (vertical)pickup voltages, and gk (k=1-4) the gain correction
factors, and G the conversion factor for the summation of
the four-pickup voltages in order to calculate the beam
charge Q. The gain correction factors gk, which correct
signal-gain unbalance caused by attenuation losses of thecables and difference of the coupling strength of thecombiner, are measured by using fast test pulse with awidth of 500 ps. These parameters (a ij, b ij and gk) arestored in the Unix workstations as the calibration table
and are loaded into each front-end computer at every
startup.

4chCombiner

8ch-Combiner**

Klystron Gallery
Linac Tunnel
Combiner

35-m-longcoaxial cable
BPM

WCM*Delay Cable (7ns)

Figure: 1 Schematic drawing of a monitor station.
Touch-sensitive
terminals

X-terminals
XSwitching-Hub(for front-end computers)

X3

fiber-optic cables
to other front-end computers
Linac Control Network(TCP/IP)
Front-end
computer(VME)

Front-end
computer(VME)

Front-end
computer
(VME)

UNIX
workstation UNIX
workstation

Figure: 2 Block diagram of the linac control network andthe extended network (right side) system.
2.3 Calibration of the monitors
All the monitors have been installed into the beamline after the bench calibration. The bench calibration
system for the BPMs and WCMs are described in detailelsewhere [5,8]. Here, only the bench calibration for theBPMs and how to calculate the beam positions are briefly
given. All the BPMs have been calibrated by “mapping”
by use of the test bench with a thin current-carrying wire
(500 µmφ) stretched through the center of the monitor tosimulate the beam. Calibration coefficients of the mapfunction, which relates the pulse-height information
obtained from the four pickups with the wire positions, ismeasured by the bench calibration. The map function
using third-order polynomials can be given as thefollowing:
3x=3∑ a (∆ / Σ ) (∆ / Σ ) , y = ∑ b (∆ / Σ ) (∆ / Σ ) ,iiji, j = 0xx

jyk kk= 1∆ x = g1V1 − g3V3 , Σ x = g1V1 + g3V3 ,VME/OS9Combiner Box
8ch-Combiner∑gV,and here,Monitor Station
Digital Oscilloscope GPIBy x I 5GHz

4Q= Giyiji, j = 0xx

jyy

SOFTWARE SYSTEM

3.1 Control Software
Several DAQ processes are running concurrently atfront-end computers and at Unix workstations. The
processes with data flow are shown in Fig. 3. The readout process resides on each front-end computer. It reads
waveforms of BPM/WCM signals from a digital
oscilloscope, then calculates beam positions and currents
taking into account the calibration coefficients (seeSection 2.3). The trigger-pluses synchronized with thelinac beam are provided to all of the oscilloscopes at the0.67 Hz cycle.
This rate is limited from thecommunication throughput between a front-end computerand a oscilloscope through a GPIB line. Simultaneous
measurements of both BPMs and WCMs are not possible
at present, since there is only one oscilloscope for eachmonitor station. In addition, it takes five seconds tochange the DAQ mode from the BPM/WCM mode toanother. Calculated beam positions and currents aretransferred to two Unix workstations over the linac control
network with the UDP-protocol. In order to reduce thenetwork traffic, data transfer is done only when the beamcurrent data at front-end is renewed. As a result, the total
traffic between front-end computers and Unix workstations
is always constant (24 packets per second; 0.67 Hz x 18
VMEs x 2 WSs x 1). Unix workstations receive beam
currents from eighteen front-end computers and store
recent ten data for each front-end on the shared-memory
regions. The data servers for BPM/WCM use the data onthe shared-memory. It is worth noting that the datarequests from applications do not increase network traffic
to front-end computers.
Unix, PC, or
Touch-terminal

Unix
workstation

TCP
protocolBPM
data servershared memory
for BPM Readfor
Frontend #1

Front-end
VME/OS9

WCM
WCM
application
application

BPM
BPM
application
application

...

TCP
protocolWCM
data servershared memory
for WCM Readfor
Frontend #18

for
Frontend #1...

Writerecv

Writerecv

UDP
protocol
VME #1sendsend

UDP
protocolRead

VME #2

forFrontend #18

VME #18

ReadBPM
dataWrite

shows the horizontal displacements of the beam from thecenter (the top drawing), the vertical displacements(middle), and beam charge (bottom), simultaneously withthe refresh rate at around 1 Hz after the elaborated software
tuning. The DAQ system has operated stably during thebeam tests.

WCM
data
Write

BPM/WCM
waveform read-out
from oscilloscope

Figure: 4 Variation of the beam orbits and charge
intensity along the linac. The single-bunch electrons of
the beam current is 10nC/bunch.

Figure: 3 Block diagram of the control software and dataflow.
3.2 Man-Machine Interface
Applications for the BPMs/WCMs send requests to adata server at Unix, then receive the latest beam-current
data. The communications are made with the TCPprotocol using the standard format of the KEK-linac
control system [9]. Typical response times are 1-10 ms,
depending on the CPU power of the client computer. So
far some applications for the BPMs/WCMs have beendeveloped. A graphical presentation for the BPMs, which
was developed with the X-window environment, is shownin Fig. 4.
4BEAM TESTSThe DAQ system tuning and the beam tests havebeen performed by using single-bunch electron beams at
the extended linac section (sectors A and B) [2]. The
single-bunch electron beams can be generated by the new
pre-injector [10] which comprises two subharmonicbunchers, a prebuncher and a buncher. The electron gun
can generate a beam charge of about 18nC/pulse with abeam repetition rate of 50Hz. The single-bunch electron
beams greater than 10nC/bunch were stably accelerated
from the outlet of the buncher until to the end of thesector B without any observational beam loss. The beamenergy was about 1.5 GeV at this end. The beam tests
have been performed under the condition of 5Hz repetition
rate. Figure 4 shows an example of the beam tests. It

5CONCLUSIONSThe new DAQ system has been developed in order toreinforce the beam monitoring for the beam positions andbeam currents. The present system was inspected withelectron beams at the extended beam line of the injector
linac. The data-taking speed is 0.67 Hz, mainly limitedby the data transfer rate of the oscilloscope through aGPIB line. The present DAQ system is found fast andstable enough.
6ACKNOWLEDGMENTS

The authors gratefully acknowledge Mr. T. Obata forhis help in developing the software of the front-endcomputer. They also thank Dr. H. Koiso and the KEKB
linac commissioning group for their help in the monitor
calibrations and beam tests.
7[1]
[2]
[3]
[4]
[5]
[6]
[7]

REFERENCES

S. Kurokawa, et al., KEK Report 90-24 (1991).
I. Sato, et al., KEK Report 95-18 (1996).
T. Suwada, et al., in AIP Conf. Proc. 319, (1993) p.334.
T. Suwada, et al., in AIP Conf. Proc. 390, (1996) p.324.
T. Suwada, et al., to be presented in this conference.
T. Suwada, et al., Nucl. Instr. and Meth. A396 (1997) 1.
T. Suwada, Proc. the 10th Symp. on Accel. Sci. andTech., Hitachinaka, 1995, p.269.
[8] T. Suwada, to be in preparation.
[9] N. Kamikubota, et al., Proc. the 1994 Intern. Linac
Conf., KEK, Tsukuba, 1994, p.822.
[10] S. Ohsawa, et al., Proc. the XVIII Intern. LINAC Conf.,
CERN, Geneva, Switzerland, 1996, p.815.