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

A BEAM PROFILE MONITOR BASED ON AN X-RAY ZONE PLATE OF
THE SPRING-8 STORAGE RING
S. Takano*, M. Masaki, K. Tamura and H. Ohkuma
Japan Synchrotron Radiation Research Institute, SPring-8, Hyogo 679-5198, JapanAbstract
A beam profile monitor based on an X-ray zone plate
has been installed in the accelerator diagnostics beamline
I (BL38B2) of the SPring-8 storage ring. A single phasezone plate and an X-ray zooming tube are used to image
the synchrotron radiation from a dipole magnet source.
Monochromatic X-ray is selected by a double crystal
monochromator. The zone plate is focused by changing
the output photon energy of the double crystal
monochromator. Focused images of the electron beam
have been successfully obtained. Based on preliminary
calibrations of the X-ray zooming tube, the horizontal
and vertical beam sizes (1σ) are roughly estimated to be
120 µm and 15 µm, respectively.

INTRODUCTION
Measurement of small vertical size of electron beam isamong the most important emittance diagnostics of lowemittance synchrotron radiation sources. In the case of
the SPring-8 storage ring, because of the high energy ofelectrons, 8 GeV, and the large bending radius of 39.3m,visible synchrotron radiation from a bending magnet isnarrowly collimated in the vertical direction, anddiffraction effect severely limits the resolution of
conventional electron beam imaging with visible light.
Interferometry is an alternative method to overcome
the diffraction-limited resolution. We have developed atwo-dimensional visible synchrotron light interferometer,
which enables us to obtain two-dimensional information
on transverse profile of an electron beam with twicebetter resolution than imaging systems[1]. However, thenarrow aperture of the vacuum duct transporting visible
synchrotron light limits the spatial resolution of theSPring-8 interferometer to about 50 µm in the verticaldirection, which is apparently larger than the verticalbeam size evaluated by other methods[1].
Improvement of the diffraction-limited resolution byorders of magnitude can be achieved by utilizing
synchrotron radiation in shorter wavelength regions suchas X-rays[2]. To measure the size of the electron beam of
the SPring-8 storage ring by observing X-ray synchrotron
radiation, we have constructed the accelerator
diagnostics beamline I (BL38B2), and have installed abeam profile monitor based on an X-ray zone plate.

*

Email:takano@spring8.or.jp

ACCELERATOR DIAGNOSTICS
BEAMLINE I (BL38B2)
The accelerator diagnostics beamline I (BL38B2) ofthe SPring-8 storage ring has a bending magnet lightsource, and wide band spectral availability including
visible/UV light, and soft and hard X-rays has beenachieved. The beamline consists of a front end in theaccelerator tunnel, an optics hutch on the experimenthall, a visible light transport line, and an X-ray transportline in the optics hutch. The visible and UV light isextracted by a mirror in a vacuum chamber installed
between the front end and the X-ray transport line
(Fig.1). The visible and UV light extracted is transported
in the bent shielded pipe out of the optics hutch to a darkroom on the experiment hall. In the dark room, single
bunch impurity is measured by a gated photon counting
method, which utilizes fast Pockels cells for switchinglight pulses, and bunch length is measured by a streakcamera.
The X-ray transport line (Fig. 1) has a double crystal
monochromator. It covers the energy range of 4 to 14
keV by Silicon (111) reflection. The monochromator
crystals and their mechanisms can be moved off thephoton beam axis in the monochromator vacuum
chamber when use is made of white X-rays. The X-ray
transport line as well as the front end has no windows,
which obstructs soft X-ray and visible/UV light andpotentially could distort wavefront of X-ray synchrotron
radiation. Therefore, all the components of the X-ray
transport line are in vacuum chambers under UHV
pressure. The X-ray transport line has been designed sothat it can be used not only for beam diagnostics but also
for synchrotron radiation experiments on accelerator
components such as photon absorbers. There are twodummy vacuum pipes, which can be replaced by specificchambers for such experiments.

ZONE PLATE BASED X-RAY BEAM
PROFILE MONITOR
In order to measure the size of the electron beam of theSPring-8 storage ring, we have installed a beam profile
monitor based on an X-ray zone plate in the X-raytransport line of the diagnostics beamline I(BL38B2).
The resolution target of the beam size measurement is 1
micron (1σ).

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

Figure 1: X-ray transport line of the SPring-8 diagnostics beamline I (BL38B2). All the components are in the optics
hutch.

Figure 2: The optical system of the X-ray beam profile monitor based on a zone plate.
The optical system of the X-ray beam profile monitor
is shown in Fig. 2. The synchrotron radiation emitted by
the electron beam moving in a dipole magnet is imaged
by a single phase zone plate. Monochromatic X-ray at anenergy of 8.2 keV is selected by the double crystal
monochromator. The magnification factor of the zone
plate is about 0.27, and an X-ray zooming tube is used asa detector to compensate for demagnification. The
distance between the source point and the zone plate is
32.2m, while that between the zone plate and the input
photocathode of the X-ray zooming tube is 8.8m. Crossed
wires made of tungsten mounted on a wire scanner areplaced in front of the zone plate as a center stop, and a 4jaw slit is placed in front of the X-ray zooming tube as anorder selecting aperture.
The phase zone plate was fabricated by NTT
Advanced Technology Co. The characteristics of thezone plate are summarized in table 1. The total numberof zones is 468, and the width of the outermost zone
is 0.75 µm. The thickness of the zone material, tantalum,
was optimized to obtain maximum diffraction efficiency.
The X-ray zooming tube (Hamamatsu Photonics K. K.,

C5333) has resolution better than 0.5 µm (FWHM) atthe input photocathode, which is sensitive to X-raysbelow 10keV.
Table 1: Parameters of the phase zone plate
Diameter
1.4 mmNumber of zones
468
Outermost zone width
0.75 µm
Zone material
Tantalum
Zone thickness
2.0 µm
6.92 m
Focal length*
32 %Diffraction Efficiency *1.5 µmSpatial Resolution* (1σ)*
Calculated Value at E = 8.2 keV (λ= 0.15 nm).

EXPERIMENTS
The focal length of a zone plate is proportional to thephoton energy. We focus the zone plate by changing theoutput photon energy of the double crystal
monochromator. In Fig. 3 we show the results fromthe initial experiments. The horizontal size σx of a beam

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

σx400

40500

301020σy506.5

77.5 8 8.5
Energy (keV)

σ (a.u.)15xσ y (a.u.)

2050

09.5

9600

50

204030x2010σ88.5
Energy (keV)

0700 800 900 1000
X (pixel)

50
100
150
Intensity (a.u.)

Figure 5: An example of images of electron beam of theSPring-8 storage ring observed by the zone plate X-ray
beam profile monitor.
vertical sizes of a beam image are minimum at an energyof about 8.15 keV, which is consistent with the design ofthe zone plate. The bend of crystals which initiallyaffected the vertical beam profile has been cured, and afocused image of the electron beam has been successfully
obtained. In Fig. 5 we show an example of images ofelectron beam of the SPring-8 storage ring observed by
the zone plate X-ray beam profile monitor. The fillingmode was the multi-bunch mode, and the total beam
current was 100 mA. The gaps of all the insertion
devices were opened. The shutter time of the X-rayzooming tube was 1ms. Based on preliminary
calibrations of the X-ray zooming tube, the horizontal
and vertical beam sizes (1σ) were roughly estimated to be
120 µm and 15 µm, respectively. More accurate
evaluation of the beam size awaits for carefulcharacterization and calibration of the X-ray zoomingtube.

xσ

σ (a.u.)σ y (a.u.)

2507.5700
800

was minimum at an energy of 8.15 keV, while thevertical size σy was minimum at a significantly lower
energy of 6.9 keV. The photon energy of 8.15 keV giving
minimum horizontal beam size was consistent with thedesign of the zone plate. However, the zone plate was out
of focus at 6.9 keV and additional lenses were necessary
to obtain a focused image of the electron beam in thevertical direction. After the initial imaging experiments,
flatness of the monochromator crystals was examined. It
was confirmed that the crystals were bent by clamping to
the holders, and the shift of energy giving minimum
vertical size was attributed to small bend of the crystals.
The crystal holders of the monochromator have been
improved to cure the bend of the crystals. The results ofimaging experiments after improvement of the crystal
holders are shown in Fig.4. Both the horizontal and the5600

10Figure 3: Horizontal and vertical sizes of a beam image,as a function of photon energy, measured before
improvement of the monochromator crystal holders.

15Y (pixel)

2510y

90Figure 4: Horizontal and vertical sizes of a beam image,as a function of photon energy, measured after
improvement of the monochromator crystal holders.

REFERENCES[1] M. Masaki and S. Takano, “Two-dimensional visible
synchrotron light interferometry for transverse
beam-profile measurement at the SPring-8 storagering”, J. Synchrotron Rad. (2003) 10, pp295-302.
[2] S. Takano et al, “Planned X-ray Imaging of theElectron Beam at the SPring-8 Diagnostics
Beamline BL38B2”, Proc. of
5th European
Workshop on Diagnostics and Beam Instrumentation,
Grenoble(2001),
http://www.esrf.fr/conferences/DIPAC/Proceedings/s
tampedpdfs/PS-18lowerstamp.pdf