Proceedings of IBIC2012, Tsukuba, Japan

MOPA24

PHOTON BEAM POSITION MONITOR AT SIAM PHOTON SOURCE
P. Sudmuang, S. Chaichual, N. Suradet, S. Boonsuya, N. Sumano, S. Krainara, H. Nakajima,
S. Rugmai, P. Klysubun
Synchrotron Light Research Institute, 111 University Avenue, Muang District,
Nakhon Ratchasima 30000, Thailand
Photon beam position monitors (PBPM) have been
designed and installed in the beamline front-ends at Siam
Photon Source (SPS). Up till now, these blade-type
PBPMs have been successfully installed at three bending
magnet and an insertion device (planar undulator)
beamlines. Their performance has been tested andcompared with that of the electron beam position monitor.
The achieved resolution is found to be better than 3 μm.
The obtained PBPM data proved to be extremely
invaluable in the investigation of the sources of theobserved beam positional fluctuation, and forcompensation of the orbit perturbation caused byundulator gap change. In this paper, the details of thecalibration procedure will be presented. Various factors
affecting reading of the signal such as undulator gap
change effect, choice of bias voltage, and temperature
variation have been investigated and the results will be
discussed herewith.

PBPM DESIGNPBPM Structure
Figure 1(a) shows the design of the 4-blade type PBPM
structure. The blades are triangular and were made of 0.2
mm-thick tungsten. The upper pair was affixed to a watercooled copper block at the front facing the beam, while
the lower pair was placed at the rear end. The blades wereslotted in sapphire plates for electrical insulation andthermal conduction. A biased photoelectron collector wasinstalled on each side of the copper block to remove thescattered photoelectrons. The PBPM block is attached toan XY translation stage for alignment and calibration
processes. The schematic layout of the whole PBPM setup
is shown in Figure 1(b). Generated photocurrent ismeasured by a picoammeter (Keithley 6485). To
investigate thermal effect on the measured photocurrent, athermocouple was installed on the copper block.

INTRODUCTION
The Siam Photon Source (SPS) is a dedicated 1.2 GeV
synchrotron light source in Thailand [1]. Currently, there
are seven photon beamlines in operation, with three more
under commissioning. In addition, there are three newphoton beamlines under construction. As the number ofexperiments increases, coupled with the fact that more
complicate experiments are being carried out, there ismore demand from the users for higher quality beam,
especially with regards to the beam positional stability
aspect. In recent years the machine group has focused its
effort to achieve this goal [2]. To investigate the sourcesof the beam position fluctuation and observe thesubsequent improvement quantitatively, Photon Beam
Position Monitors (PBPMs) have been developed andinstalled. Several types of PBPMs have been developed atsynchrotron facilities around the world [3-4]. For SPS,
the 4-blade type PBPM is chosen.
In this paper, the PBPM structure together with thecriteria for blade spacing determination are presented,
followed by the details of the calibration procedure, andvarious factors affecting the sensitivity and linearity
threshold of the PBPM. In addition, the experiments fortesting PBPM performance are described.

(a)(b)
Figure 1: 3D drawings of (a) the 4-blade type PBPM
block, and (b) the whole PBPM setup.

Determination of the Blade Spacing
The spacing between the blades ( Gx , G y ), as shown in
Figure 2, are determined by the photon beam power
densities at the location of the PBPM. In order to optimize
the sensitivity and linearity range, the spacing should be
two times the Gaussian width, i.e. G 2V , except for
Gx of the bending magnet PBPM, which is specified by
the horizontal opening angle of a particular photon
beamline. To avoid cross-talk among the blades, the upperand lower blades were shifted by 2 mm (D) apart.

ISBN 978-3-95450-119-9
Beam Position Monitor System

1c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Abstract

MOPA24

Proceedings of IBIC2012, Tsukuba, JapanThe sensitivity was obtained from the slope of thelinear fit of the measured beam position signal withrespect to the PBPM block translation, displayed inFigure 3. By comparing the photon beam signals P' / 6 andBM Source

PLog , it was found that the linear region of PLog extends

DUP_Right

UP_LeftUpper

further than that of P ' / 6 . Thus, we chose PLog over P' / 6in our implementation.

GxGy

2V

LO_Left

LO_Right

Lower

2V
Side View

Undulator
SourceFront View

Figure 2: Blade spacing geometry.

PBPM CALIBRATION

Figure 3: Measured PBPM linearity.

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Photon Beam Position Calculation
The PBPM calibration was performed by scanning theBPM block across the photon beam with the translation
stage. Measured photocurrent on the blades are used todetermined the photon beam position (x,y) in the linearregion asx, yPS(1)Bias Voltage Determination
Bias voltage is another factor influencing sensitivity
and linearity of the beam position signal. Increasing thebias voltage widens the linear region while lowers thesensitivity, as depicted in Figure 4. In our case we decided
that the linear region should extend at least to ± 2 mm,with acceptable sensitivity, resulting in the bias voltage of
100 V.

where P is the beam position signal and S is thesensitivity.
The beam position signal was commonly calculated via
the difference-over-sum formulae, i.e.

Px '/66( I R )  6( I L )6( I R )  6( I L )(2)

Py '/66( IUP )  6( I DN )6( IUP )  6( I DN )(3)

The photon beam signal can also be calculated with thelog-ratio method as

Px L og§ 6( I R ) ·log ¨¸© 6( I L ) ¹(4)

Py Log§ 6( IUP ) ·log ¨¸© 6( I DN ) ¹(5)where Px and Py is the photon beam signal in horizontal
and vertical directions, respectively. IUP and I DN are thecurrent from the upper and lower blades, and I R and I L
are the current from the right and left blades, in that order.

Figure 4: Measured PBPM linearity and sensitivity atdifferent bias voltages.

Undulator Gap Change EffectCalibration of insertion device PBPM was meticulously
carried out in order to avoid or circumvent various
detrimental factors such as background radiation from thebending magnet and variation of the current signal oneach blade at different undulator gaps. Influence of thegap change on each individual blade is shown in Figure 5.
To compute the beam position at different gaps, thesensitivity was fitted with an exponential function asS D  E exp(J G)(6)

where G is the insertion device gap, D , E , and J are
fitting parameters.

ISBN 978-3-95450-119-92Beam Position Monitor System

Proceedings of IBIC2012, Tsukuba, Japan

MOPA24

air temperature fluctuation, vacuum pressure problem,etc.

Figure 5: Variation of the (top) individual blade signals,and (bottom) sensitivity as a function of the undulatorgap.

Figure 7: Measured photon beam position
temperature variation during user beam operation.

andAfter the PBPM was installed, we first looked at thereading while varying the setting of a vertical corrector.
The measurement results are shown in Figure 6. It was
found that the orbit change monitored by the PBPM is ingood agreement with the electron BPM, with the PBPM
giving more sensitive reading, reflecting more closely tothe change of the corrector.

Figure 8: Compensation of the orbit perturbation caused
by undulator gap change with (left) and without (right)
incorporating the PBPM reading.

CONCLUSION

Figure 6: Measured photon and electron beam position as
a function of the corrector magnet current.
Figure7 shows the vertical beam position obtained frombending magnet and insertion device PBPM together withthe temperature variation measured on the copper block
inside the vacuum chamber. Temperature variation of thecopper block was restricted to within r 0.25 OC by thecooling water, and thus does not affect the photon beam
position reading. By measuring the short-term beam
position, the resolution was observed to be better than 3μm.
The undulator look-up table was created byincorporating the photon beam position reading obtained
from the PBPM. In doing so, significant improvement
over the normal case, where only electron BPM is taken
into account, has been achieved (Figure 8). Since theinstallation the more accurate PBPM reading enable themachine group to pinpoint more easily the various
sources perturbing the beam such as power supply ripple,

The PBPM were successfully installed in the beamline
front-ends at SPS. The measured photon beam position
obtained by PBPM of each beamline is in good agreementwith the BPM data. The PBPM has linear region thatextends over ± 2 mm and has better than 3 μm resolution.

ACKNOWLEDGMENT
The authors would like to thank Changbum Kim ofPLS and June Rong Chen of NSRRC for their support andadvices related to PBPM design and improvement.

REFERENCES[1] P. Klysubun et al., “Operation and improvement of theSiam Photon Source”, Nuclear Instrument and Methods inPhysics Research, 582 (2007), 18-21.
[2] S. Klinkhieo et al., “Commissioning results of slow orbit
feedback using PID controller method for Siam photonsource”, IPAC’12, Louisiana, USA.
[3] H. Aoyagi et al., “Blade-type X-ray beam position monitors
for SPring-8 undulator beamlines”, Nuclear Instrument andMethods in Physics Research, 467 (2001), 252-255.
[4] S.F. Lin et al., “The Study of New Signal Processing
Technique in Photon Beam Position Monitors”, IPAC’05,
Knoxville, Tennessee.

ISBN 978-3-95450-119-9
Beam Position Monitor System

3c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○PBPM PERFORMANCE