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

A DESIGN OF SYNCHROTRON RADIATION MONITOR FOR
KEK B-FACTORYToshiyuki MITSUHASID, Norishige IDRAMATSU ,Naoya TAKEUCID*,Masahide ITOH*and Toyohiko YATAGAI *High Energy Accelerator Research Organization, Oho, Tsukuba, lbaraki, 305 Japan*Institute of Applied Physics, Tsukuba University, Tennodai, Tsukuba ,Ibaraki,305 Japan

Abstract
A synchrotron radiation(SR) monitor for KEK B-factoryhas been designed. The monitor consists of 1), visible
SR beam extraction system having a real time surfaceflatness measurement system, 2),SR beam transfersystem, adaptive optics system for the correction of
deformation of the SR extraction mirror, 3), focusing
system for the observation of the beam profile, 4), SRinterferometer for the measurement of the beam size,and 5),extra branch beamline for other measurement·
such as streak camera.

optical system and other is for the SR-interferometer.
Total length of the optical passes are 41m for each. At
the end of the optical pass, it will be located clean
room for devices of the optical monitor. The clean roomis designed to class 100 dust free room and thedimension of 4.5 x 4.5 m2.

1 INTRODUCTION

relay lensMonitor to measure the beam profile or beam size
which based on a using the synchrotron radiation will
greatly improve the efficiency of the commissioning of
KEKB project. In this paper, it is descried that thedesign of the SR monitor. Not only the ordinal SR
monitor based on an imaging , we introduce newly
developed techniques for this monitor. One is so called
adaptive optical system(l),(2)which is an active
correction system for distortion of the wavefront andother is the SR-interferometer (4),(5) for themeasurement of the beam profile and the size.

14.0m

T otallength of the optical pass = 39. 98 m

2 DESIGN of OPTICAL PASS AND
CLEAN ROOM FOR THE MONITOR
The design of optical pass is shown in Fig.l. A
dedicated weak bending· magnet is insert in the both
HER and LER as a SR beam source. The parameters ofthe source points are listed in Table 1.
Table 1 Preliminary parameters of the weak bend
bending radius angularNwer of SR source size
183m
28.86W/mrad at 2.5A O'x 472J..Lm
LER
O'y 69J..Lm *49.31W/mrad at lA O'x 650)lm
HER 1172m
lo'v l20Jlm **vertical beam Size assumes an emittance couphng of2%

The optical pass is designed to 11m pass from thesource point to SR extraction mirror, and then theoptical pass is divided into two passes by the use of ahalf mirror, one for focusing system having a adaptivee+/emirror

Fig. I Outline of the SR-interferometer.

3 DESIGN OF ADAPTIVE OPTICALSYSTEM FOR THE COMPENSATION
OF WAVEFRONT ERROR FOR THE
FOCUSING SYSTEM
3-1 Design of the adaptive optical system
Because of the extraction mirror for SR beam will beirradiated by a strong power of SR beam as listed inTable 1. The surface flatness will be deformed beyond
Rayleigh's criterion (A/8) of diffraction limited optics.
This deformation of the extraction mirror introduce a
wavefront error and finally makes a blurred image of thebeam. To compensate this wavefront error, we designed
an adaptive optical system by the use of deformable-273-

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

mirror(!). The design of the adaptive optical system isshown in Fig.2.
electron beam orbitSR

extraction mirror X~ysShack-Hartmann

the image spot. We can measure the rms spot
displacement by 1/10 of the pixel, then thecorresponding sensitivity is 'A/20 ('A =633nm).
Performance of the wavefront sensor was tested at theoptical laboratory 2 at Photon Factory. A result of themeasurement of a wavefront error caused by SRextraction mirror at BL-27 is shown in Fig. 4.

deformablemirror

position(mm)deformable mirror controller

position (mm)Fig.2 The design of adaptive optical system for themonitor
The wavefront error is transferred on the deformablemirror using a relay lens system (1:1) and compensatedby deformable m~rror. The deformation of deformablemirror is watched by the Shack-Hartmann sensor. We
use a deformable mirror CILAS BIM31 which is a"bimorph" type mirror having 31 electrodes in thebehind of the mirror surface. After the wavefront
correction, SR beam will focused by a diffraction
limited ED doublet lens. Then the image of the beam
will be observed with magnifier lens and CCD.

Fig.4 A result of the measurement of a wavefront error
caused by SR-extraction mirror at BL-27
3-3 Performance test of the adaptive optical system at
Photon Factory
Performance of the adaptive optical system was testedby the use a pinhole source by the use of Xe-discharge
lamp and SR-extraction mirror of BL27 at Photon
Factory. As a result, we succeeded to compensate thewavefront error less than A/5.6 in rms. A result ofpoint spread functions before and after the wavefront
correction is shown in Fig.5.

3-2 Wavefront sensing
We designed and constructed a Shack-Hartmannwavefront sensor(2) to measure a wavefront error causedby the deformation of extraction mirror. The outline ofthe S-H wavefront sensor is shown in Fig.3.
Image relay lens systern(20: 1)
f~JOOOHe-Ne laser

collimator

A.=633nm

1{:125 mm

image relay lenssystem(!:l)

r~sommi[=50 mrn
digital CCD camera
lOxlO~tm each pixel

mmBe-mirror

I• !200mm •Ia) before wavefront correction

Image relay lens system(20: 1) magnifies
the wavefront gradient 20 times
If we can measure the root mean squarespot displacement by 1110 of the pixel,
then the corresponding sensitivity is)J20 (ic~633 run).

Fig.3 Outline of the Shack-Hartmann wavefront sensor.
The sensor consists a multi-lens array , two image relay
lens systems and a CCD camera. The wavefront error
caused by a deformation of the extraction mirror istransferred to the multi-lens array by a image relay lenssystem (20:1)and it is divided by each lenslet. In thefocal plane of the multi-lens array, the image spot of
each lenslet is shifted by a quantity proportional to thelocal slope of the wavefront. Then the image plane of
the multi-lens array is again transferred to CCD by aimage relay system (1:1), and measure the position of-274-posilion (nun)b) after wavefront correction
Fig. 5 Results of point spread functions.

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

Figure ·5-a) shows a measured point spread function
before the wave front correction. It is measured by theuse of pinhole source which is 200~m in diameter
which is located at 8m from the focusing objective lens.
Figure 5-b) shows a measured point spread function
after the wavefront correction. A strong astigmatismas shown in Fig.4 caused by a cylindrical shape of thewavefront error as shown in Fig.4 is well corrected.

where 9 denotes observation angle of the interferogram,D 1 denotes distance of the first double slits, and D2
denotes distance of second double slits (distance betweenlays). The degree of spatial coherence y is representedby;

4 DESIGN OF
SR-INTERFERO METER FOR
VERTICAL BEAM PROFILE AND
SIZE MEASUREMENT

Jc(D2 ) = /(0)cos(kDzCi:l)d0

The principle of object-profile measurement by meansof the spatial coherency of the light is known as the vanCitterut-Zemike's theorem(3). It is well known that A.
A. Michelson was measured the angular diameter ofstars by his famous stellerinterferometer. Recently, thisprinciple was applied for the measurement of verticalelectron beam profile in the storage ring by one of theauthors by the use of SR-interferometer(interferometerfor synchrotron radiation)(4) at the Photon Factory.
Since this method is based on spatial coherence of theSR beam, it is suitable to measure a small electron
beam size having a good spatial coherence. Recently,
very small vertical beam size (16.5~m) of AURORA at
SR center of Ritsumeikan University was measured bythis method(5).
4-1 Design of the SR-Interferometer
The SR-interferometer is basically a wavefront
division type two beam interferometer by the use ofpolarized quasi-monochromatic rays(4). In this time,
for the purpose with ease of data analysis, we modified
the design of double slit interferometer into
quadruplicate slit design of the interferometer. A
schematic drawing of the new design of the SRinterferometer is shown in Fig.5.
SR beam position shifterII/ :r]-, Ij

double sht D2I.

, ·+ +

i:j '\_ ~ ..,:0 ,_,5 ACKNOWLEDGMENTS
Authors wish to thank to Professors Dr. S. Kurokawa
and Dr.K. Satoh for their encourage of this work.

REFERENCES

Interti:rogram

mm (max)"\"""- Lens 0=30 mm, f=600 mm
1.

where f(e) denotes beam profile as a function of angular
diameter e. In this configuration of the interferometer,
the spatial frequency of interference fringe is a functionof D1 and the modulation of the y is a function of 02.
This means spatial frequency of the interference fringe isnever change according to change of 02. Only themodulation of the y is changing as a function of 02.
This performances of the newly designed SRinterferomater have a great advantage to measure thevisibility (modulation of the y) and phase of theinterference fringe.
Usable opening angle of the visible SR beam isestimated by 3mrad for LER and 1.8mrad for HER.
Under this conditions, it is estimated we can measure
the beam profile having rms beam sizes 49~m for
LER and 85~m (these beam sizes are corresponding tothe case of 1% coupling) for HER with this SRinterferometer. To assuming a Gaussian beam profile,smaller limitation of the beam size measurement isestimated by lO~m for LER and 17~m fot HER.

double slit Dl

9030m-1 s(Dz)q>=tan - c(D2)d1ctuic polanzer

t·H ,_,it~-~:::: SJ~~{ {'~~-:::Objecl

Js(D2 ) = /(0)sin(kD20)d0

Band-pass filter 300 nm +/. 5 nm

Fig.5 A design of SR-interferometer forB-factory.
For the purpose to stabilize the SR beam position infront of the interferometer, a beam position stabilizer isapplied at the entrance pupil of the interferometer.
In the quadruplicate slit design of the interferometeras shown in Fig.5, the first double slit serves as a
movable double slit, and second one (having nophysical slits) serves a fixed double slit. By the use ofthis design, the interferogram is given by;(1) N. Takeuchi, T. Mitsuhashi, M. Itoh and Y.
Yatagai, Technical digest of nonastronomical adaptiveoptics(NAA0'97)lp.26, Munchen,(l997).
(2)N. Takeuchi, T. Mitsuhashi, M. Itoh andY. Yatagai,
to be published in proceedings of Particle Accelerator
Conference (1997) ,Vancouver.
(3) M.Bom and E. Wolf,"Principles of Optics, P459
Pergamon press. (1980).
(4) T. Mitsuhashi, to be published in proceedings ofParticle Accerelator Conference (1997) ,Vancouver.
(5)T.Mitsuhashi, H. Iwasaki,Y. Yamamoto, T.
Nakayama and D. Amano, somewhere in thisproceedings.

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