Beam-Position Monitor System for the KEKB
Injector Linac
T. Suwada, N. Kamikubota, K. Furukawa, and H. Kobayashi
KEKHigh Energy Accelerator Research Organization
1-1 Oho, Tsukuba, Ibaraki 305–0801, JapanAbstract. About 90 stripline-type beam position monitors (BPMs) have been newly installedin the KEKB injector linac. These monitors easily reinforce handling beam orbits andmeasuring the charge of single-bunch electrons and positrons which are injected to the KEKBrings. The design value of the beam position resolution is expected to be less than 0.1 mm. A
new data-acquisition (DAQ) system has been developed in order to control these monitors inreal time. The hardware and software of 18 front-end computers were tuned for the linac
commissioning. This report describes the hardware and software system, the monitor
calibration, and preliminary beam test results.

INTRODUCTION
The KEK B Factory (KEKB) project (1) is in progress in order to test CP violationin the decay of B mesons. KEKB is an asymmetric electron-positron collider comprisedof 3.5 GeV positron and 8 GeV electron rings. The PF 2.5 GeV linac (2) is also being
upgraded to the KEKB injector linac in order to inject single-bunch positron and electron
beams directly into the KEKB rings. The injected beam charges are required to be
0.64 nC/bunch and 1.3 nC/bunch, with a repetition rate of 50 Hz, for the positron andelectron beams, respectively. High-charge primary electron beams (~10 nC/bunch) arerequired in order to generate sufficient positrons. Therefore, it is important to easily
control the orbits of the beams; most importantly, the beam positions and charges of theprimary high-current electron beams have to be controlled so as to suppress any beam
blowup generated by large transverse wakefields. A BPM system has been developed toperform this function since 1992. The goal of the beam position measurement is todetect the charge center of gravity with a resolution of 0.1 mm. The amount of the beam
current needs to be precisely controlled in order to maximize the positron production andthe beam injection rate to the KEKB rings. This facilitates a well-controlled operation of
the injector, allowing us to reach an optimum operational condition with as a short atuning time as possible and maintain such a condition through a long-term operation

period. A new DAQ system based on VME/OS-9 computers, which are connected withthe linac control system, has also been developed in order to control these monitors inreal time. The performance of the new system has been tested using electron beams at an
extended linac beam line during its commissioning.

HARDWARE SYSTEM
Design of a Beam Position Monitor
A conventional stripline-type BPM made of stainless steel (SUS304) was designedwith a π/2 rotational symmetry. A drawing of the monitor geometry and a photographare shown in Figures 1 (a) and (b), respectively. The total length (195 mm) was chosen
so as to make the stripline length (132.5 mm) as long as possible, so that it can beinstalled into limited space in the new beam line of the linac. Each electrode is comprisedof a 50 Ω transmission line. The angular width of the electrode is 60° in order to avoid
strong electromagnetic coupling between the neighboring electrodes. A 50 Ω SMA
vacuum-feedthrough is connected to the upstream side of each electrode, while thedownstream ends are short-circuited to a pipe in order to simplify the mechanicalmanufacturing. Quick-release flange couplings (manufacturer's standard KF flange) areused at one end of the monitor for easy installation into the beam line. More detailed
descriptions are given elsewhere (3–6).

(a)

(b)
FIGURE 1. Geometrical drawing (a) and photograph (b) of the stripline-type beam position monitor
installed in the new beam line.

Data-Taking System
All analog signals of the BPMs are connected with monitor stations in conjunction
with those of wall current monitors (WCMs). Eighteen monitor stations, each of whichis comprised of a front-end computer (VME/OS-9 with a MC68060 microprocessor at50 MHz), a signal digitizing system (an oscilloscope), and a signal-combiner box, arelocated on the linac klystron gallery at nearly equal intervals along the beam line. Eachmonitor station can control a maximum of twelve BPMs. A schematic drawing of themonitor station is shown in Figure 2. Four signals of a BPM are sent directly to asignal-combiner box through 35 m-long coaxial cables. The frequency response andcharacteristics of the coaxial cable are given elsewhere in detail (5). Two signal
combiners combine the horizontal and vertical signals from each BPM, respectively. Inthe combiner box, one of two horizontal (vertical) signals is delayed with a time of 7 ns
in order to reject its signal-mixing. The two signals from the signal combiners are fed totwo input channels of a digital sampling oscilloscope (Tektronics TDS680B) with asampling rate of 5 GHz and a bandwidth of 1 GHz. The Unix workstations and thefront-end computers communicate with each other through a network system. As shownin Figure 3, all of the front-end computers are linked to a switching-hub with a star
topology. Fiberoptic cables are used for physical connections in order to avoid
electromagnetic interference from high-power klystron modulators. The hub has a linkto the linac control network, where Unix workstations (WSs) and man-machineinterfaces (PCs and X-terminals) are connected.
Monitor Station
Digital Oscilloscope GPIB
y x I 5GHz

X-terminals

Touch-sensitive
terminals

VME/OS9X

Switching-HubX

Combiner Box
8ch-Combiner*8ch-Combiner

4chCombiner

Linac Control Network(TCP/IP)*

Front-end
computer(VME)

Klystron Gallery
35-m-longcoaxial cable
BPM

fiber-optic cables
to other front-end
computers

Linac TunnelCombiner

Front-end
computer(VME)

Front-end
computer
(VME)

UNIX
workstation UNIX
workstation

WCM*Delay Cable (7ns)

FIGURE 2. Schematic drawing of the datataking system in a monitor station.

FIGURE 3. Block diagram of the linac
control network and the new network (right side)system.

Calibration of the Monitors
All of the monitors were installed into the beam line after bench calibration. Thebench calibration system for the BPMs is described in detail elsewhere (7). Here, only
the bench calibration and the beam position calculation are briefly described. All of theBPMs have been calibrated by “mapping,” which is performed on the test bench with athin current-carrying wire (500 µmφ), stretched through the center of the monitor, tosimulate the beam. The calibration coefficients of the map function, which relates to thepulse-height information obtained from four pickups for various wire positions, aremeasured by the bench calibration. The horizontal (x) and vertical (y) beam positions arerepresented by map functions with a third-order polynomial, as follows:
3x = ∑ aij (∆ x / Σ x )i (∆ y / Σ y ) j ,i, j = 0y=3j∑ bij (∆ x / Σ x )i (∆ y / Σ y ) ,i, j = 0

(1)

(2)

4

Q = G ∑ gk Vk .

(3)

∆ x = g1V1 − g3V3 , Σ x = g1V1 + g3V3 ,(4)∆ y = g2V2 − g4V4 , Σ y = g2V2 + g4V4 ,(5)k =1Here,

where aij and bij are the coefficients of the map functions (derived by fitting the map data
to the map functions by using a least-squares fitting procedure); V 1 and V 3 (V 2 and V 4)
are the horizontal (vertical) pickup voltages; and gk (k=1–4) are the gain correction
factors. Q is the beam charge, which is calculated by summing four-pickup voltages,
and G is a conversion factor used to calculate the beam charge, which can be measured
by wall-current monitors. The gain correction factors (gk), which correct any signal-gain
imbalance caused by attenuation losses of the cables and the difference of the couplingstrength of the combiners, are measured by using fast test-pulses with a width of 500
ps. These parameters (aij, b ij, gk, and G) for each BPM are stored in the Unix
workstations as a calibration table, and are loaded into each front-end computer at everystartup.

SOFTWARE SYSTEM
Control Software
Several DAQ processes are running concurrently on the front-end computers and on
Unix workstations. The processes, with data and control flow, are shown in Figure 4.
DAQ processes for the WCM are almost the same as those for the BPM, except for thedata format, as shown in the figure. The read-out process resides on each front-end
computer. It reads waveforms of BPM signals from the digital oscilloscope, and thencalculates the beam positions and currents while taking into account the calibration
coefficients described above. Trigger pulses synchronized with the linac beam areprovided to all of the oscilloscopes at the 0.67 Hz cycle. This rate is limited by thecommunication throughput between a front-end computer and a oscilloscope through aGPIB line. The calculated beam positions and currents are transferred to two Unixworkstations over the linac control network with the UDP-protocol (8). In order toreduce the network traffic, data transfer is done only when the beam current data at thefront-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 × 18 VMEs × 2 WSs).
Unix workstations receive beam currents from eighteen front-end computers and store
ten recent data for each front-end on shared-memory regions. The data servers for BPM
use the data on the shared memory. It is worth noting that the data requests fromapplications do not increase the network traffic to the front-end computers.
Unix, PC, or
Touch-terminal

TCP
protocolUnix
workstation

BPM
data server

for
Frontend #1

...

TCP
protocolWCM
data servershared memory
for BPM Read

Front-end
VME/OS9

WCM
application
WCM
application

BPM
BPM
application
application

shared memory
for WCM Readfor
Frontend #18

for
Frontend #1

...

Write
recv

Write
recv

UDP
protocol
VME #1
send
send

UDP
protocolRead
BPM
dataRead
WCM
data

Write

Write

VME #2for
Frontend #18

VME #18

BPM/WCM
waveform read-out
from oscilloscope

FIGURE 4. Block diagram of the control software and data flow for the data-taking system.

BEAM TESTS
Beam and DAQ System Tuning
DAQ system tuning and the beam tests have been performed (2) using single-bunchelectron beams at the extended linac section (sectors A and B). Single-bunch electron
beams can be generated by the new pre-injector (9), which is comprised of two
subharmonic bunchers, a prebuncher and a buncher. The electron gun can generate abeam charge of about 18 nC/pulse with a repetition rate of 50 Hz. Single-bunch electron

beams greater than 10 nC/bunch were stably accelerated from the outlet of the buncher
until the end of sector B without any observed beam loss. The beam energies were about
500 MeV and 1.5 GeV, at the end of sectors A and B, respectively. The longitudinal
width of the beams was measured by an optical transition radiation monitor to be about
16 ps with a full width at half maximum after tuning of the preinjector. The beam tests
during the commissioning were performed at a 5 Hz repetition rate. Figure 5 shows anexample of the beam tests at a beam current of 10 nC/bunch, in which the DAQ
processes of four monitor stations (22 BPMs) concurrently worked well. It shows thehorizontal displacements of the beam from the center (the top drawing), the verticaldisplacements (middle), and beam charge (bottom), simultaneously with a refresh rate ataround 1 Hz after elaborate software tuning. The DAQ system operated stably during thebeam tests. Figure 6 shows pickup signals obtained by a beam of 6.2 nC/bunch at thethird monitor station.

FIGURE 5. Variation of the beam orbits and charge intensity along the linac. The beam current of
the single-bunch electrons is 10 nC/bunch.

FIGURE 6. Pickup-pulse signals measured by the digital oscilloscope at a beam charge of 6.2 nC.

Position-Resolution Measurement
The position resolution of the BPM has been measured using single-bunch electron
beams with a beam charge of 7 nC on the basis of the “three-BPM method”. The
principle of this method is simple. The beam orbit of a charged particle through a systemwith both magnetic lenses and acceleration can be generally represented by amultiplication process of transfer matrices (R) (10). If the beam orbit is represented by acoordinate vector (X), passage of the charged particle through the system can berepresented by an equation neglecting the second-order transfer matices:
iXi (1) = ∑ Rij X j (0),

(6)

j=1

where X i(0) and X j(1) are the components of the initial and final coordinate vectors
through the system action, and R ij are the components of the transfer matrix. Here, if
any three beam positions (x 1 , x 2 and x 3 ), which are the first components of thecoordinate vector X, are measured in the system, the third beam position (x3) is linearlycorrelated by the expected beam position (xc3) using the beam positions (x 1 and x 2) asfollows:x 3c = ax1 + bx 2 + c,(7)x3 = x3c .

(8)Here a, b, and c are constants obtained by a least-squares fitting of the correlation
plot in accordance with Equations (7) and (8). Thus, the position resolution can beobtained by calculating the standard deviation of the correlation plot and assuming thatthe resolutions of the three BPMs are the same. Beam tests were performed using
single-bunch electron beams of 1 and 7 nC. The beam positions were changed within
the range of ±2 mm by two correction dipole magnets placed just after the buncher.
Figure 7 gives an example.
x =0.12 (mm)a, b, c=-0.591, -0.01, 0.3271
Q=1 nC/bunch=0.098 (mm)a, b, c=0.372, 0.282, -0.308
Q=7 nC/bunchax1 +bx2+c (mm)ax1 +bx2+c (mm)xx3 (mm)(a)x3 (mm)(b)

FIGURE 7. Correlation plots between the measured and calculated beam positions for the beam
charge of (a) 7 nC/bunch and (b) 1 nC/bunch by using the three-BPM method.

CONCLUSIONSThe new beam position monitor system has been installed in the linac in order toreinforce the beam monitoring of the beam positions and currents. The present system
was inspected with electron beams in an extended linac beam line. The data-taking rate
became 0.67 Hz after elaborate system tuning of the new DAQ. The rate is mainly
limited by the data-transfer rate from the oscilloscope through a GPIB line. The present
DAQ system has been found to be sufficiently fast and stable. The position resolution
has been measured by single-bunch electron beams for beam charges of 1 and7 nC/bunch to be around 0.1 mm.

ACKNOWLEDGMENTS
The authors gratefully acknowledge Mr. T. Obata for his help in developing thesoftware of the front-end computer. They also thank Drs. H. Fukuma and H. Koiso for
their help in developing beam-orbit analysis programs, and the KEKB linac
commissioning group for their help in the monitor calibrations and beam tests.

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[4] Suwada, T., Proceedings of the 10th Symposium on Accelerator Science andTechnology, (Hitachinaka, Japan) 1995, p. 269.
[5] Suwada, T., et al., AIP Conference Proceedings 390, 1996, p. 324.
[6] Suwada, T., et al., presented at the First Asian Particle Accelerator Conference,KEK, Tsukuba, Japan, 1998.
[7] Suwada, T., in preparation.
[8] Kamikubota, N., et al., Proceedings of the 1994 International Linac Conference(KEK, Tsukuba, Japan), 1994, p. 822.
[9] Ohsawa, S., et al., Proceedings of the XVIII International Linac Conference(CERN, Geneva, Switzerland), 1996, p. 815.
[10] Brown, K. L., et al., “TRANSPORT: a computer program for designing charged
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