Proceedings of IBIC2012, Tsukuba, Japan

MOPB88BEAM SIZE MONITOR FOR TPS
Chien-Kuang Kuan, Tse-Chuan Tseng, I Ching. Sheng, Shen-Yaw Perng, June-Rong Chen
National Synchrotron Radiation Research Center, Hsinchu, Taiwan
Third-generation light source TPS under construction inNSRRC has two diagnostic beamlines in the storage ring.
Visible SR interferometers and X-ray pinhole cameras arewidely used to measure the transverse beam profile insynchrotron light sources. In phase I we shall adopt bothmethods for application as beam-size monitor. The visible
SR interferometer uses a double slit to obtain a onedimensional interference pattern along the horizontal orvertical axis. A simple X-ray pinhole camera is designed
to measure the size, emittance and energy spread of theelectron beam. In this paper we present the design andcalculation of these two beam-size monitors for TPS.

INSTRODUCTIONS
Taiwan Photon Source (TPS) is under construction at
National Synchrotron Radiation Research Center(NSRRC). The electron beam stored in the storage ring ofcircumference 518 m has energy 3 GeV and current 500mA. In phase I we shall have two diagnostic beamlines,
using radiation from a bending magnet: one is a dedicateddiagnostic beamline; the other is constructed together
with the bending beamline. The rms beam sizes at theseplaces are theoretically 40 μm in the horizontal plane and16.5 μm in the vertical plane for 1 % coupling. We adopt
two methods to measure the beam size -- a visible SRinterferometer and an X-ray pinhole camera.
Both methods of measuring the beam size are simple,
cheap and reliable, and can measure the vertical beam
size of 16.5 μm. The visible SR interferometer [1,2] has
better resolution than the X-ray pinhole camera [3], but isconstrained by the maximum opening angle between theslits. The X-ray pinhole camera is an imaging system.
There are many effects on the point spread function (PSF)
in the imaging system, including diffraction and pinholedimension. The deconvolution of the PSF determines thesmallest image size measurable with the imaging system.
We use the X-ray pinhole camera to measure the beamsize, emittance and energy spread of the electron beam.
If the coupling is down to 0.1%, the rms beam size istheoretically 5.2 μm in the vertical plane. It is difficult tomeasure accurately a beam size in this range with thesetwo methods; it is possible, but must be done carefully:
there are too many constraints and errors induced by the
optical components. In this paper we present the design,calculation and smallest resolution of the beam size forthese two beam size monitors for TPS.

VISIBLE SR INTERFEROMETER
Figure 1 shows the layout of the visible SR
interferometer. This interferometer is based on an
investigation of the spatial coherence of SR. Aninterferogram in the CCD is shown in fig. 2; fitting theinterferogram yields visibility γ from Eq. (1):(1)
The beam size σbeam comes from the relation [1],(2)
in which λ is the wavelength of the observation, R is the
distance from the light source to the double slit, and S is
the slit separation.

Figure 1: Layout of the visible SR interferometer.

Figure 2: Interferogram in the CCD.
The parameters used are wavelength λ = 500 nm,
distance R = 20 m, and maximum possible visibility γ =0.97. With these values inserted into Eq. (2), the possible
resolution of the beam size depends on the slit separation
S as shown in fig. 3. Using S = 70 mm and visibility γ =0.97, the beam size is about 5 μm. The opening angle
between the slits, 70/20 = 3.5 mrad, is constrained by thebending chamber; the minimum gap in the bending
chamber of the TPS case is 3.8 mrad. The main error ofthe visible SR interferometer arises from the distortion of
the mirror by the radiation power. We use a cooling mirror
made of Be to diminish the mirror distortion. Withvisibility γ=0.97, the background of noise in the CCD

ISBN 978-3-95450-119-9
Transverse & Longitudinal Emittance Measurements

1c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○AbstractMOPB88

Proceedings of IBIC2012, Tsukuba, Japanaffects the accuracy of measurement of the beam size.
The CCD will be tested and calibrated before use.

and YAG:Ce, at length D = 15 m; the magnification M isthen equal to 3.
The photon energy is filtered with Al material. The
transmission ratio is shown in fig. 6. We filter the photonenergy with this Al material of varied thickness to obtain
the photon flux in fig. 7. To use Al with thickness < 3 mm,photon flux > 10-6 photon/s [4] and a CdWO4 screen gives
a compromise between the CCD and the energy range; theultimate value depends on the final choice of CCD type.

Figure 3: Calculation of the beam size with varied slit
separation S and visibility γ.

X-RAY PINHOLE CAMERA

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Figure 2 shows the layout of the X-ray pinhole camera;
the distance from the light source radiated by the bending
magnet to the zero point of the front end (FE) is 4.6 m,from the light source to the shielding wall is 19.2 m.

Figure 6: Transmission ratio of material Al of varied
thickness.

Figure 4: Layout of the X-ray pinhole camera.

Figure 7: Photon flux after the Al filter with varied
thickness at fixed aperture 13 μm.

Figure 5: Optimum pinhole dimension with varied
magnification D/d and varied energy at fixed length d.
To increase the resolution of the beam size, a smallerlength d from the light source to the pinhole is better,
whereas a greater length D from the rectangular pinhole
to the scintillator is better [3]. The smallest length for a
possible place to install the pinhole is d = 5 m from thelight source. The greater is length D, the weaker is theintensity of the image into the CCD. We must hencecompromise between length D and the intensity of theimage. From fig. 5, the magnification M = D/d increases
little for M larger than 3. We put the scintillator, CdWO4

Figure 8: Image beam size divided by M with varied M
and varied energy at fixed length d.
Figure 8 shows the size of the source beam, obtained on
dividing the size of the image beam by M; we can havethe measuring resolution of the beam size. The relation offilter, energy and pinhole with measurement resolution ofthe source beam size is summarized in Table 1.

ISBN 978-3-95450-119-92Transverse & Longitudinal Emittance Measurements

Proceedings of IBIC2012, Tsukuba, Japan
Table 1: Resolution of the source beam size
Al filter /mm
0.1 0.5

11.5

22.53

MOPB88use the pinhole array with vertical separation 0.5 mm andhorizontal separation 1 mm to prevent overlap of the lightfrom adjacent pinhole. The first choice of pinholedimension is 15 μm, varied pinhole dimension between
10 and 25 μm.

maximum energy
13.0 18.3 21.5 23.0 25.0 26.5 27.1/keV
optimum pinhole
18.5 15.6 14.5 13.9 13.3 12.9 12.8/μm
image beam
size/M /μm

10.1 8.5 7.8 7.6 7.3 7.0 7.0

With varied photon energy, length d and magnification
M, the pinhole has varied resolution of the beam size.
With fixed length d = 5 m, M = 3 and thickness of the Al
filter less than 3 mm, the minimum source beam size islocated between 10 and 20 μm of the pinhole dimension,
as shown in fig. 9.

Figure 11: Resolution of beam size with program SRW
and multiple electrons.

Figure 9: Calculated resolution of the beam size [3].

ACKNOWLEDGEMENTS
We thank colleagues, particularly the Precision
Mechanical, Beam Dynamics, Instrumentation & Control
and Vacuum Groups, for helpful discussion.

REFERENCES

Figure 10: Resolution of the beam size with program
SRW and a single electron.
We use the program SRW [5] to simulate the resolutionof the beam size, with energy 21.5 keV and the profile
fitted with a Gaussian shape. Figure 10 using the singleelectron mode, the data is near the calculation at thepinhole 15μm. Figure 11 using the multi-electron mode,the data is larger than calculated. The minimum source
beam size is between 10 and 25 μm of the pinholedimension.
To align the pinhole, there are two methods: one is anadjustable pinhole; the other is a pinhole array. We shall[1] T. Mitsuhashi and M. Tadano, “Construction of a SR
Monitor for the Photon Factory Advanced Ring”,
Proceedings of the 2003 Particle Accelerator
Conference, 2003, pp. 2506-2507.
[2] T.C. Tseng et al., “The SRI beam size monitor
developed at NSRRC”, PAC 05, Knoxville,Tennessee, USA, 2005.
[3] P. Elleaume et al., “Measuring Beam Sizes and Ultra
Electron Emittances Using an X-ray Pinhole
Camera”, J. Synchrotron Rad., 1995, pp. 209-241.
[4] C. A. Thomas and G. Rehm, “An X-Ray Pinhole
Camera System for Diamond”, in Proceedings ofDIPAC ‘05, Lyon, France, 2005.
[5] O. Chubar and P. Elleaume, “Accurate and Efficient
Computation of Synchrotron Radiation in the Near
Field Region”, EPAC98, THP01G, 1998, pp.11771179.

ISBN 978-3-95450-119-9
Transverse & Longitudinal Emittance Measurements

3c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○DISCUSSION
The main design parameters of the beam size monitor
for TPS are set. Both a visible SR interferometer and an
X-ray pinhole camera are used to measure the beam sizeof the TPS project. From the analysis a beam size larger
than 10 μm is easily measured with the two methods atTPS. For the purpose of machine study, the beam size canbe adjusted below 10 μm. In the range of beam size 10 to5 μm, the relative design parameters should be carefully
adjusted. A beam size less than 5 μm is difficult tomeasure.