The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

DEVELOPMENT AND INSTALLATION OF A SINGLE-PASS BEAM
POSITION MONITOR IN KEKB
H. Ishii, Y. Onishi, M. Tobiyama, K. Mori, M. Tejima and S. Hiramatsu
KEK, Tsukuba, Japan
Abstract
A single-pass beam position monitor (SPBPM) wasdeveloped for measuring o f the injection beam orbit of theKEK B-factory (KEKB), and 21 sets have already been
installed. We adopted log-ratio processing using alogarithmic (log) amplifier for the SPBPM o f KEKB. The
measurement circuit consists o f four log-amplifiers, fourpeak-hold circuits, four analog-digital converters (ADC),a memory circuit and a timing control circuit. Moreover,it coexists with the existing beam position monitor (BPM)system for a closed-orbit distortion (COD) measurement.
In order to choose the BPM signal o f the low energy ring(LER) and the high energy ring (HER) of KEKB, an RF
signal switch circuit was also manufactured. Thesecircuits were controlled by the same VXI standard as theexisting BPM system. This report summarizes theSPBPM system and its performance.

INTRODUCTION
The KEKB accelerator is a collider comprising apositron ring (LER) o f 3.5 GeV and an electron ring(HER) o f 8 GeV. This May, the KEKB accelerator
achieved the design value o f the peak luminosity o f above
1034 cm^sec'1. For high-luminosity operation, a precise
control o f the beam orbit in an accelerator based on theBPM system is required. The existing BPM system [ 1 ] isa high-precision measurement BPM system with anaccuracy o f a few micron-meters or less, and has beenused to measure the COD since the start ofcommissioning in December, 1998. HER and LER areequipped with 443 and 454 BPM pickups, respectively.
The beam signals from the four button electrodes of each
BPM pickup are transmitted through independent coaxial
cables with lengths o f 40〜 240 m. In order to realize ahigh measurement accuracy to the present system, thedetection circuit method o f the so-called narrow band wasadopted. The signal component at the 2nd harmonicfrequency (1018 MHz) o f the accelerating RF frequency(509 MHz) is detected using super-heterodyne circuits
and a spectrum analysis by means o f a fast Fouriertransform (FFT). Although the existing BPM system isadvantageous for processing a signal, like anaccumulation beam at hardly changed in time, there is adisadvantage, which cannot respond to an oscillation of as e v e r a l 10 kHz beam signal, like in the case o f beam
injection. In order to compensate for such a fault, aSPBPM system has been installed in KEKB.

PERFORMANCE SPECIFICATION
There are two purposes for the installing o f the SPBPMin KEKB. One is to search for the optimum parameter toinject a beam to each ring from the linac. The 2nd is tomeasure the betatron phase advance of each ring foroptics diagnosis. Since the electric charge per beam bunchis very small in the case of beam injection, the SPBPM
must detect very weak signals, especially the positron
beam for LER, it is about 0.3 nano-coulomb (nC). Thus,
the required resolution is less than 0.5 mm.
Moreover, in the case o f optics diagnosis o f each ring,
the SPBPM must measure the beam position with a highresolution o f less than 0.1 mm while accumulating asingle-bunch beam charge o f 10 nC. Such requiredconditions are summarized as shown in Table 1.
Table 1 :Specifications for the SPBPM
Beam current

Expected signalpower (250MHz)

Requiredresolution

10nC(6.25*1010PPP)-30 dBm

0.1 mm0.3 nC(1.88*109PPP)-60 dBm

0.5 mm

SIGNAL PROCESING CIRCUIT
To satisfy the above requirements, a signal detector
must cover a wide dynamic range (50 dB). In order torealize wide-range detection, we decided to adopt a logamplifier circuit. Such a signal processing method iscalled the log-ratio BPM [2].
The electronics for the SPBPM consists o f four logamplifiers, four peak-hold circuits, four ADCs, a memory
circuit and a timing control circuit, as shown in Figure 1.
Firstly, each signal voltage o f four electrodes from a BPM
pickup is inputted into a linear amplifier through a band
pass filter (a center frequency of 250 MHz with a band
width o f 12.5 MHz). The output signal o f a linear
amplifier is inputted into a log-amplifier, and the peakvoltage is sampled and held.
Synchronizing with a beam signal, four peak voltages
are converted to a 12-bit digital value to be recorded inmemory, respectively, by the clock of a timing control
circuit. The SPBPM circuit can store digital data of eachelectrode o f turn by turn for a maximum of 16384 times.
The timing control circuit has a function to synchronize
the signal for every BPM and the time of conversion for
the ADC by lag time adjustment.

The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

BEAM POSITION FORMULA

f8 = 25 8 M H zBUJI2.SMHZ
lo % % -3 d B

The beam position o f the log ratio-BPM o f KEKB isable to be calculated using the following formulas:丨161•貴urn lU b it *l 28dB,NF = 3Ch.l O---M em o ry

16k

X ^ C ^ ^ o ^ J V .^ L o g iV jV ^Ch.2 O—

Y = C y x { L o g { V j V c ) + Log { V j \ ^ ) Y

Ch.3
Ch.4

The four-signal voltage o f each electrode is set to VA,V B, Vc, and V[). Cx and Cy are the position conversion
cocfricients.
In order to estimate the position conversion coefficiento f the log-ratio BPM, the mapping data for the log-ratio
BPMs were calculated by log conversion o f the usual
mapping data, as shown in Figure 3.

M em o ry

16k；

Q~|hhp>--- toc^Hpcati~~ 隱S t a r t T r ig g e r In
S t a r t T r ig g e r Out
R e u o lu tio n p u l$ e

Mapping for LERBPM(944>)

R e iio lu tio n p u U e0.4

QI 0.

Figure 1 : Block diagram for SPBPM

JSince the SPBPM system coexists with the existing
BPM system, we have manufactured a relay module tochange the signal o f the BPM button electrode between
the SPBPM and the existing BPM. The function can alsochange the signals o f HER and LER, that is, the SPBPM
measures the beam position o f both rings, HER and LER
in a common circuit. The SPBPM system is controlled by
a VXI-bus (VME-bus Extension for Instrumentation)
standard, like the existing system.

BENCH TESTING
A sinusoidal wave o f 250 MHz from signal-generating
equipment was inputted into a single-turn BPM circuit,and the performance was examined by a bench test. As
shown in Figure 2, the line o f the output voltage o f a logamplifier is linear to an incoming signal o f -70 ~ -10 dBm,
but the signal level is less than -70 dBm, this has beenshown for the characteristic o f a log-amplifier thatlinearity becomes a worse rapidly. The SPBPM has aposition resolution o f 0.3 mm at the point o f -60 dBm
input, which corresponds to a beam charge o f 〜 0.3 nC
injected from the linac, and 0.1 mm in the region o f -10 dBm.

CD)
O_l_i-0.6
LogA-LogB-LogC+LogD

Figure 3: Mapping for the SPBPM
Figure 4 shows the Log ratio to the beam position on anaxis of coordinates of the LER BPM and HER BPM. Bothfigures show that not only a circular LER beam chamber,but a racetrack HER chamber, has very good linearity.
The inclination of this straight line is equivalent to a beam
position conversion coefficient. Their typical coefficientsfor HER and LER are estimated from these lines asfollows:
LER arc section Cx and Cy = 38.0 [mni/V]
HER arc section Cx =22.18 [nun/V], Cy=31.78 [mni/V]
HERarc(race-tracK)g _ - ■Log(D/t»il+Log(A/C)-Log<B/D) 1
j-*-Loq(A/C)+Loa(B/D)_
<Cl

1.5〇

10.5v

メ 00 ..
•身0 .
0 .

'，’ '

パ
レ ，，. - --ゲ パ

\

0-90

-70(0.1)-50(1.0)-30(10.0)-10ln p U t [ d B m ] (Bunch charge[nC])

Figure 2: Output voltage and resolution for theSPBPM to the input signal powerーコ2\/lK u l0>

i+KM

1.

Eeuol losay8642.…
一'T ........

Resolution

2.5

1\ I_—\23ノパノPosltion[mm]ダ•丨••

Posilio_叫Figure 4: Log ratio value to the beam position on anaxis of coordinates of LER BPM and HER BPM.

ACTUAL BEAM POSITION
MEASUREMENT
21 SPBPMs were installed in KEKB by this June. In
order to check the performance of these SPBPMs, wemeasured the beam position using an actual beam.

The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

Measurement o f the injection beam
An orbital measurement o f first injection of the positron
beam to the LER is shown in Figure 5. The vibration o f
the beam based on the injection error was observed using
a beam signal o f 0.5 nC (-56 dBm). Since the betatrontunc is close to a half integer, the beam position ischanging to the opposite direction during every turn. At
the first turn after beam injection, an electric charge ofabout 0.5 nC was injected in LER, which decreased toabout 0.2 nC after ten or more turns.
LER arc section 94phi

卜X -e-Y — Power[dBm]-4 丨-5 丨i

85i l

26Existing BPM

OSPBPM丨
5!01lobo1

Single kick orbit

i-66

1zcIm1iaee

20Last, the consistency o f the position sensitivity between
the existing BPM and the SPBPM was measured bysingle kick orbit o f a beam. Since only 21 sets are still
installed, an SPBPM measurement has few measuring
points compared with the existing BPM measurement
system, as shown in Figure 7. The measured values ofexisting BPM and SPBPM mostly corresponded.

Q .

-700Comparison o f SPBPM and existing BPMjxE【E.

_E m p ¥M O

45m

The system resolution lor a beam position measurement
was estimated from the standard deviation o f the beam
position o f 4096 turns. It was confirmed that theresolution o f the LER arc section and the HER arc section
was about 0.1 mm for a beam of 1mA (10 nC) as arequirement.

4060
Turn numberr

Next, a single bunch beam (1 mA ) was examined in theaccumulation mode by SPBPM. Since the beam waskicked by a horizontal kicker magnet, the horizontal
damped oscillation was observed as shown in Figure 6.

*iい】

J3000

F igure 5: O rbital m easurem ent o f first injection o f thepositron beam to LER.

Measurement o f a single bunch beam in theaccumulation mode

1esiin1

Figure 7: The consistency o f the position sensitivity
betw een the existing BPM and the SPBPM .

CONCLUSION
For beam injection tuning and optics diagnosis, asingle-pass BPM system based on log-ratio BPM wasdeveloped. We investigated the performance of thissingle-pass BPM using an actual beam. That is, the beam
position o f a low electric charge bunch o f 0.3 nC could bemeasured, and the single pass BPM could measure anincoming beam signal o f -30 dBm with about 0.1 mm
resolution. At KEKB, it is scheduled to extend eight more
sets next year and to extend in the future to 120 sets.

ACKNOWLEDGMENTS
The author appreciates the support of this work bySony-Tektronix Ltd and Artek Inc.

REFERENCES

4000
Turn number

Figure 6: Beam position o f turn by turn. Upper,
H orizontal beam position; M iddle, Vertical beam
position; Low er, Four outputs of SPBPM .

[ 1 ] M.Tejima, et al., “BEAM POSITION MONITORSYSTEM FOR KEKB", Proc. EPAC2000
P ] Robert E. Shafer, “Log-Ratio Signal-Processing
Technique for Beam Position Monitors”，Proc. FourthAccelerator Inseminations Workshop, Barkeley 1992.