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

DEVELOPMENT AND INSTALLATION OF A SINGLE-PASS BEAMPOSITION 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 of 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 of KEKB. The
measurement circuit consists of 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 of 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.

PERFORMANCE SPECIFICATION
There are two purposes for the installing of the SPBPM
in 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 for
optics 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 of optics diagnosis of each ring,
the SPBPM must measure the beam position with a highresolution of less than 0.1 mm while accumulating asingle-bunch beam charge of 10 nC. Such requiredconditions are summarized as shown in Table 1.
Table 1: Specifications for the SPBPMINTRODUCTION
The KEKB accelerator is a collider comprising a
positron ring (LER) of 3.5 GeV and an electron ring(HER) of 8 GeV. This May, the KEKB accelerator
achieved the design value of the peak luminosity of above
1034 cm-2sec-1. For high-luminosity operation, a precisecontrol of 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 of 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 of 40~240 m. In order to realize ahigh measurement accuracy to the present system, thedetection circuit method of the so-called narrow band wasadopted. The signal component at the 2nd harmonicfrequency (1018 MHz) of the accelerating RF frequency(509 MHz) is detected using super-heterodyne circuits
and a spectrum analysis by means of a fast Fouriertransform (FFT). Although the existing BPM system isadvantageous for processing a signal, like anaccumulation beam at hardly changed in time, there is a
disadvantage, which cannot respond to an oscillation of aseveral 10 kHz beam signal, like in the case of beam
injection. In order to compensate for such a fault, aSPBPM system has been installed in KEKB.

Beam current

Expected signal
power (250MHz)

Requiredresolution

10 nC(6.25*1010PPP)-30 dBm

0.1 mm-60 dBm

0.5 mm90.3 nC(1.88*10 PPP)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
log-amplifier circuit. Such a signal processing method iscalled the log-ratio BPM [2].
The electronics for the SPBPM consists of fourlog-amplifiers, four peak-hold circuits, four ADCs, amemory circuit and a timing control circuit, as shown inFigure 1. Firstly, each signal voltage of 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 of 12.5 MHz). The output signal of alinear amplifier is inputted into a log-amplifier, and thepeak voltage 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 controlcircuit. The SPBPM circuit can store digital data of eachelectrode of 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
The beam position of the log ratio-BPM of KEKB isable to be calculated using the following formulas:

X = Cx × {Log(VA VC ) − Log(VB VD )},Y = C y × {Log(VA VC ) + Log(VB VD )}.
The four-signal voltage of each electrode is set to VA,VB, VC, and VD. Cx and Cy are the position conversion
coefficients.
In order to estimate the position conversion coefficientof the log-ratio BPM, the mapping data for the log-ratio
BPMs were calculated by log conversion of the usual
mapping data, as shown in Figure 3.

Figure 1: Block diagram for SPBPM
Since the SPBPM system coexists with the existing
BPM system, we have manufactured a relay module tochange the signal of the BPM button electrode between
the SPBPM and the existing BPM. The function can alsochange the signals of HER and LER, that is, the SPBPM
measures the beam position of 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 of 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 of the output voltage of alog-amplifier is linear to an incoming signal of -70 ~ -10dBm, but the signal level is less than -70 dBm, this has
been shown for the characteristic of a log-amplifier thatlinearity becomes a worse rapidly. The SPBPM has aposition resolution of 0.3 mm at the point of -60 dBm
input, which corresponds to a beam charge of ~ 0.3 nC
injected from the linac, and 0.1 mm in the region of -40 ~-10 dBm.

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 [mm/V]
HER arc section Cx =22.18 [mm/V], Cy=31.78 [mm/V]

Figure 4: Log ratio value to the beam position on an axisof coordinates of LER BPM and HER BPM.

ACTUAL BEAM POSITION
MEASUREMENT
Figure 2: Output voltage and resolution for the SPBPM tothe input signal power

21 SPBPMs were installed in KEKB by this June. Inorder 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 of the injection beam
An orbital measurement of first injection of the positron
beam to the LER is shown in Figure 5. The vibration ofthe beam based on the injection error was observed using
a beam signal of 0.5 nC (-56 dBm). Since the betatrontune 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.

Figure 5: Orbital measurement of first injection of thepositron beam to LER.

Measurement of a single bunch beam in theaccumulation mode
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.

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

Comparison of SPBPM and existing BPM
Last, the consistency of the position sensitivity between
the existing BPM and the SPBPM was measured bysingle kick orbit of 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.

Figure 7: The consistency of the position sensitivity
between 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 of a low electric charge bunch of 0.3 nC could bemeasured, and the single pass BPM could measure anincoming beam signal of -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 by
Sony-Tektronix Ltd and Artek Inc.

REFERENCES[1] M.Tejima, et al., “BEAM POSITION MONITORSYSTEM FOR KEKB”, Proc. EPAC2000[2] Robert E. Shafer, “Log-Ratio Signal-Processing
Technique for Beam Position Monitors”, Proc.
Fourth Accelerator Inseminations Workshop,Barkeley1992.
Figure 6: Beam position of turn by turn. Upper,
Horizontal beam position; Middle, Vertical beam
position; Lower, Four outputs of SPBPM.