The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

New BPM System and Its Calibration for Upgraded PF Ring
Kaiichi HAGA, Tohru HONDA, Mikito TADANO and Toshio KASUGAHigh Energy Accelerator Research Organization (KEK)
Institute of Materials Structure Science, Photon Factory
1-1 Oho, Tsukuba, Ibaraki 305, JAPAN

Abstract
Accompaning the brilliance-upgrading project of thePhoton Factory storage ring, the beam position monitor(BPM) system will be newed. The new system wasdesigned to enable precise and fast measurements to correct
closed orbit distortion (COD), as wsll as to feed back theorbit position during user-runs. There will be 42 BPMs
newly installed, making a total of 65 BPMs. All of theBPMs will be calibrated on the test bench using a
coaxially strung metallic wire. The measured electricaloffsets are typically 200 jlm in both direction, which is
1/2 - 1/3 of those of old-type BPM. In the signalprocessing system, PIN diode switches are to be adapted inorder to improve the reliability. In the fastest mode, thissystem is capable of measuring COD within about 10msec, and this fast acquisition will allow us fast
suppression of the beam movement for frequencies of up
to 50 Hz using a global feedback system.

dispersion function. The new system was designed toenable precise and fast measurements to correct the COD,
as well as to feed back the orbit position during user-runs.
New BPM system comprises electrostatic pickup units
and a signal-processing system. The pickup units includes
42 new-type ones on newly fabricated vacuum chamber and17 old-type ones. Calibration measurement of new BPMs
was made on a test stand in order to know the offset ofelectriical center. In addition, the responce of the new BPM
to the: beam-induced field was calculated using a method ofBoundary Element Method (BEM) to deduce the position
sensitivity.

2 BPM System
2.1 Pickup Unit

The pickup units for the normal cell sections are to bed:mbled in number, according to the addition of thequadmpole magnets in the same sections. Vacuum

1 Introduction

Photon Factory (PF) ring is a 2.5 GeVelectron/positron storage ring dedicated for synchrotron
radiation (SR) experiments. It was planned to provide
much brilliant SR by reducing the ring emittance 130nmrad to 27 nmrad, and reconstruction of the various
components of the PF ring is now in progress.
Accompaning the brilliance-upgrading project, the beam
position monitor (BPM) system will be newed. The
purpose of the BPM system is to accurately measure thebeam position in order to correct for any closed-orbit
distortion (COD), to stabilize the beam position using
feedback, and to correct such optical functions as theFig. 2 Cross-sectional view of a pickup unit for new PFBPM.

Fig. 1 Planned location of BPMs in the upgraded PFstorage ring.

chambers are also replaced to new ones, therefore 42 BPMs
are newly installed, making a total of 65 BPMs. The
newly installed pick up unit has four pickup-electrodes of abutton type. The whole BPM lication in the upgraded PF
ring was shown in Fig.1.
A cross-sectional view of the new BPM unit is shownin Fig. 2. It was designed to fit in the narrow space
between the magnets. · A commercial product (fromKYOCERA corp.) was acbpted for the electrode assembly,
which consists of a button electrode (10.3 mm indiameter), a feedthrough and an SMA-type connector. The
button is set at the center of the assembly within ±50 jlm.
Each BPM unit is fixed to an end of the quadrupole
magnet. All of the BPMs will be tested at a calibration
bench where the electric center of each BPM will beidentiJtied with an overall accuracy of about 100 jlm using
a coaxially strung metallic wire.

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The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

DC P.S.
------------~------[-------:

l levelling UnH---- ---~-:!

!!

:r------:~-~t:~:----------------------------------~!! :·-L.O.:----~!
Fig. 3 Schematic diagram of the signal-processing system.

2.2 Signal-processing System
A schematic of the layout of the signal-processing
system is shown in Fig. 3. These circuit units are to be
distributed in 12 local control racks around the ring. Thebeam signals from the pickup electrodes are to betransmitted to the processing system, and at the front endof the system, one of the signals will be selected by RFswitches. To improve the reliability, the currently-usedmechanical coaxial switches will be abanooned, and PIN
diore switches are to be aropted. The fluctuation of theinsertion loss over many switchings is less than ±0.01 dB,
which correspond; to a position error of -3 Jlm. The
signal retection circuit consists of a super-heterodyne
circuit with synchronous retection. The bandwidh of thesignal retection is retermined by the response time (- 3Jlsec by 10 to 90% rise time) of a low pass filter in the
final retection stage. In the fastest more, this system iscapable of measuring the COD within about 2 msec,
which is determined by the sum of the switching time, theo.5.... . . ......•::•-----------------=-=--=--:..: ...: .. !.!.:...................
.........
......... .
~:•·~·::::::!·1·

0

3 Simulated Responce of the New BPM
In order to measure the beam position precisely, theposition sensitivity and the electriCal center of the BPM
should be known before installing into the storage ring. In
the calibration measurement, an antenna which isconnected to a rf-source is used to simulate the field of thehigh energy beam. However, the electromagnetic field
produced by the antenna is somewhat differe~t from thatinduced by a real beam. Thus, we calculated the responceof the BPM to deduce the position sensitivity.
Using a method of Boundary Element Method (BEM)[2][3], we simulated the responce of the BPM to electricfield induced by the beam. The cross section of the 6eam
wet was approximated with small line segments. The
potential and inwced charge rensity of each element werecaluculated and summed to the inwced charge& of the fourelectrodes A - D. The beam position data are given as theratio (U and V): of these induced charges.

··················r···················j···-------··-------·t··-----·-··········• f~~ •·:~·•:..

>

response time of the retection circuit and the conversion
time of an analog-to-digital (AID) converter. This very fastacquisition will allow us fast suppression of the beam
movement for frequencies of up to 50 Hz using a global
feedback system [1].

! ..! ..
:.
·!·:::::::·~·:.:• ;: ••• ~ •• ·~l.

-·-· _ _ _ _ _ :i 1u =<(QA+QD) - (QB+Qc) ) 1 (QA+QB+Qc+Qn)
andv = <<QA+QB) - (Qc+QD) ) 1 (QA+QB+Qc+QD)~·r=of these inwced charges. The position sensitivities (Sx andSy) are given by the rerivatives ofU and V with respect tothe beam position at the center of the BPM.
Sx = aU(x,O) I ax ,Sy = aV(O,y) I ay

- 1-1-0.5

00.5uFig. 4

Simulated responce of the new BPM.

The simulated results are shown in Fig. 4. Derived valuesof horizontal and vertical BPM sensitivity are Sx 0.0575=mm·l andSy =0.0283 mm·l, respectively.

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The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

The various distortion of the BPM assembly during thefabrication, especially the fluctuation in the thickness of
the BPM duct, will generate the offset of the electricalcenter. The typical value of this fluctuation is less than
200 Jlm, and will result the simulated offset -200 Jlm.

2.........

E0E.........

r0ol0:a---------·-··--·T--:-------------r~a-·-----------r----------------.....
Cl)-4 Calibration Measurement

VI0Before installing new-type BPMs, every BPM
assembly was calibrated on a test stand in orrer todetermine the electrical center of the BPM relative to its
mechanical center. A precise determination of the offset ofeach individual BPM is important for commissioning andoperating the storage ring.

:0u.E -1>
Cl)

Figure 6 shows the distribution of measured electricalcenter offsets for the new-type BPMs. The horiwntal andvertical offsets of each BPM are shown as closed circles.
All of the measured offsets are less than 500Jlm, andtypically 200Jlm, in both directions. This is in goodagreement with a simulated result of -200Jlm. The offsetsof electrical center for old-type BPMs are also shown inFig. 6 for a comparison. The offsets for the new-type
BPMs were about 112 - 113 of those of the old-type.

l0~· 0l•j0:~-----·---···-----1-----------;;-------[--·---------------+-·---------·------24.2 Distribution of the Measured Offsets~o8l:-2Fig. 6o----------------+-----·E>-~~--~------+----------------0cu

4.1 Test Stand for Calibration

Shown in Figure 5 is the test stand used for thecalibration. The BPM assembly was mounted vertically on
the stand A steel wire of 300Jlm in diameter was strungcoaxially, and simulated the beam. Both ends of the wire
were connected to N-type connectors. The wire wasprecisely aligned using a spring tensioning device locatedat the lower end The calibration was performed at afrequency of 500 MHz, the signal-detection frequency ofthe BPM electronics. RF signals emerging from thebutton electrodes were multiplexed in a SP4T switch andtransmitted to a network analyzer for measuring theirintensities. The electrical offsets were then obtained-using
the BPM sensitivity, which was derived from a simulation
based on the boundary element method.

l0!co:o:!

!

l[-10eNev.IBPM
OOid! BPM!
12(mm)
Horizontal offset
Distribution of the measured offsets of theelectrical center relative to the mechanical center.

4.3 Horizontal and Vertical Offsets

Both the horiwntal and vertical offsets of the new-type
BPMs are nearly zero on the average, which indicates thatno systematic errors were introduced during the course of
fabricating the Q-ducts and BPM assemblies. These
measured offsets for all BPMs will be stored on acomputer and used to calculate the beam position.

5 Conclusion
The BPM system will be newed for the upgraded PF ring.
The 42 BPMs will be newly installed to match the newvacuum duct. In the pickup unit, the button electrode is setat the center of the assembly within 50 Jlm. All of themeasured electrical offsets are typically 200 Jlm in both
direction, which is 1/2 - 1/3 of those of the old-type BPM.
The offsets are nearly zero on the average, which indicatesthat no systematic errors were introduced during thefabrication. In the signal-processing system, PIN diodeswitches are to be acbpted in orrer to improve thereliability. The fluctuation of the insertion loss over many
switchings is less than 0.01 dB, which corresponds to aposition error of 3 Jlm. In the fastest mode, this system iscapable of measuring the COD within about 2 msec. This
very fast acq.1isition will allow us fast suppression of thebeam movement for fre<pencies of up to 50 Hz using aglobal feedback system.
In orrer to accurately determine the BPM offsets
relative to the magnet axis, a beam-based calibration
method will be very useful. This method has the advantagethat it can eliminate various error sources, including
mechanical alignment errors of the BPM assemblies orresidual offsets of the signal-processing electronics.

References[1] Photon Factory Activity Report #12, 1994, p.A-6.

Fig. 5The test stand for calibrating the new BPM.

[2] The Boundary Element Method for Engineers (PentechPress, London,1978).
[3] T. Shintake et al., N.I.M., A254 (1987) 146.

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