The 12th Symposium on Accelerator Science and Technology, Wako, Japan 1999

A CONSTRUC TION AND COMISSIONING OF SYNCHROT RON RADIATIONMONITOR FOR KEK B-FACTORYToshiyuki MITSUHASHI , John W. FLANAGAN and Shigenori HIRAMATSU
High Energy Accelerator Research Organization, Oho, Tsukuba, Ibaraki, 305-0801 Japan

ABSTRACTA synchrotron radiation(SR) monitor for KEK Bfactory has been designed and constructed. The monitor
consists of following systems; 1) visible SR beam
extraction system having a real time surface flatness
measurement system; 2) SR beam relay system; 3)focusing system for the observation of the beamprofile,; 4) SR-interferometer for the measurement ofthe beam size; 5) extra branch beamline for othermeasurement such as streak camera. The two sets ofSR monitor systems are constructed for the HER andthe LER. The results on commissioning of SR monitor
are also reported.

beam source. The optical paths from the source point
to the optical hat for the HER and the LER have almost
same design [4]. The SR beam is extracted by watercooled Beryllium extraction mirror. The outline ofextraction mirror system is shown in Fig. I.
Surrounding structure of the mirror is designed byelectrically smooth to reduce the corrective effects forthe stored beam.

1 INTRODUCTION

Monitor 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. A synchrotron radiation (SR) monitor
for KEK B-factory was designed and constructed. The
visible SR beam for the monitor is produced by a
dedicated week bending magnet and is extracted by amirror system which has a real- time surface flatness
measurement system. The extracted SR beam istransferred to the SR monitor hat on the ground by
about 40m long optical path system. An image of theelectron (positron) bean is observed by the use of a
focusing system based on the adaptive optics[l]. Since
the diffraction limit of the focusing system, we cannot
measure the small beam size precisely. We use the SR
interferometer [2] for the precise measurement of thesmall beam size. The extra few branch beam lines areprepared for the other measurements such as streakcamera.
rn the commissioning of the B-factory,
immediately after the beam storage in the ring, wesucceeded in observing the beam image. After getting afewl OmA beam current, the SR interferometer isapplied to measure both of the horizontal and thevertical beam size. Now the beam images for the HER
and the LER are continuously displayed in control room
and automatic analysis system for the interferogram via
SR interferometer is running on the control
computer[3].

outer chamber

S-H wavefront sensor port

extracted visible SR beamFig.l Outline of SR extraction mirror system.
After extraction, the SR beam is divided into two
beams by a half mirror, one is for focusing system bymeans of adaptive optics and other is for the SRinterferometer and extra branch beamlines. Total
length of the optical path is about 40m. At the end of
the optical path, the clean rooms (monitor hat) are seton the ground floor. The outline of optical path isshown in Fig.2. The clean room (SR monitor hat) is
designed to class 100 dust- free room.

Deformablemirrorrelay lens
13.0 mTotal length of the optical pass: about 40 m

2. OPTICAL PATH AND CLEAN ROOM FORTHE MONITOR

To reduce a strong power of SR beam due to hard
X-rays component, we insert dedicated 5mrad weakbending magnets into the both HER and LER as a SR-99-Fig. 2 Outline of optical path.

The 12th Symposium on Accelerator Science and Technology, Wako, Japan 1999

3. IMAGING SYSTEM FOR PROFILEMEASUREMENT
Because of the extraction mirror for SR beam will beirradiated by a strong power of SR beam. The surface
flatness will be deformed beyond Rayleigh's criterion(/c/8) of diffraction. This deformation of the extraction
mirror introduce a wavefront error and finally makes ablurred image of the beam.
To compensate thiswavefront error, we designed an imaging system bymeans of adaptive optical system[ I]. The outline of thedesign of imaging system is shown in Fig. 3.
lectron b am orbit

extraction mirror

the beam will be observed with magnifier lens andCCD.
The deformable mirror system will be installed in theLER in coming autumn. Right now the beam profile
images of the HER and the LER are observed without

the deformable mirror.
Figure 5 shows typicalexamples of the beam profile of the HER and the LER.

SR(a)HER(b) LER

Fig. 5 Observed beam profiles at the HER and theLER.

4 SR-INTERFEROMETER FOR BEAM SIZE
MEASUREMENT

deformable
ITIIffOf

imaging

deformable mirror controller

f~ 1000 mm

To measure the vertical and horizontal beam sizes, theSR interferometers were installed in both of the HER andLER [2]. The set up of SR interferometer is shown mFig.6.
~ doubleslit

image processor

Fig.3. Outline of the imaging system
The wavefront error is transferred on the deformablemirror using a relay lens system (2: I) [4] andcompensated by deformable mirror.
To measure a wavefront error caused by thedeformation of extraction mirror, we designed andconstructed a Shack-Hartmann wavefront sensor[4].
The outline of the S-H wavefront sensor is shown inFig.4.
h113b>e relay lens systen(20: I)
f=600!nmHe-Ne la<;er collimator--"'33mnA'-"q,25 mmB.S.

IOOOmmII\lens [F8() ITllT\ f= 1000 mm

dichromic sheet polarizers

band-pass filter 6/c ~ 10 nm

Fig.6 A set up of SR-interferometer forB-factory.

Be-mirror

Right now, one SR interferometer is shared to verticaland horizontal beam sizes measurements. For theindependent measurement for the vertical and thehorizontal beam sizes, we are preparing one more SR
interferometer in this summer. Observed typicalinterferograms of the vertical direction at the HER andthe LER are shown in Fig.7"=100•- 111111

lc

80nnn (max)•I

Fig.4 Outline of the Shack-Hartmann wavefront sensor.
The sensor consists a multi-lens array , two image relay
lens systems and a CCD camera. In the focal plane of
the multi-lens array, the image spot of each lenslet isshifted by a quantity proportional to the local slope ofthe wavefront.
After the wavefront correction, SR beam will focusedby a diffraction limited doublet lens. Then the image of-100-(a) HER(b) LER

Fig.7 Observed interferograms for the verticaldirection at the HER and the LER.

The 12th Symposium on Accelerator Science and Technology, Wako, Japan 1999

where y denotes the visibility, which is measured at a
double slit separation of 0(2]. By using this method, wecan measure the beam size by analysing an interferogramtaken at fixed D automatically. The measured results ofbeam size are relayed back to the control room for acontinuous real-time beam size measurement (3].

The absolute value of the complex degree of spatial
coherence is evaluated from the visibility of theobserved interferograms. The results are shown in
Fig.8.
~~0.

0.

8 0.

~ 0.

~0.4-;;

0.

J3 0.

~-,.

5 MEASUREMENT OF BUNCH LENGTH BY
STREAK CAMER

0.

~-,-

The bunch lengths in the HER and the LER aremeasured by the use of the streak camera. T results ofthe bunch lengths as a functions of ring current are
shown in Fig. 9.

o~~~~~~~~

010

Slit separation (n.~)

Slit separation (mm)(b) horizontal HER(a) vertical HER

~~~

15..
.
.

0.

u

12r--~-~~---,]

0.

10

c

0.

~

0

u

0.

~

,g

0.

0.
000

10

15

20

8~

6·-

0.

o.
0

010~-:;;

~

0.

!

25

10015

25

20

0 0~-"07_5 -~,~-~1.5

o~~~~~~~~~

0Single-b.mch current (mA)

Slit separatioo (nm)

0.5

11.5

ring current (mA)(b) LER(a) HEL(b) horizontal LER(a) vertical LER

Fig.9 Results of bunch length measurements.
Fig. 8 Absolute value of the complex degree of spatial
coherence plots for HER and LER.
Under a Gaussian beam profile approximation, we canevaluate a RMS width of beam size by using the leastsquares analysis of the spatial coherence[2]. The RMS
beam size abeam is given by the RMS width of the spatial
coherence curve cry as follows:
abeam=2

A-·R·n:·ar(1)where R denotes the distance between the beam and thedouble slit. Least-squares fitting of spatial coherence
calculated by the Gaussian beam profile approximation
for the measured spatial coherence are also shown in Fig.
8. We can evaluate the RMS beam size by ar obtained
from this fitting. The results those measured in the earlystage of the ring commissioning are listed in Table 1.
Table 1 Results of beam size measurement
RMS
widthy (mm)LERhorizontal

LER

verticalHERhorizontal

HER

vertical

11.64
10.32
5.86
8.31

magnification
todue
deformation of SR
extraction mirror

0.481
0.8850.800
0.918= A-. Rearnn:·D)..!__In(_!.)
2y

6 CONCLUSIONS

A synchrotron radiation(SR) monitor for KEK Bfactory has been designed and constructed. The beamprofile and size measurement systems are performed
well to deliver continuous real-time data. The bunch
length measurement system via streak camera is alsoavailable at the interferometer beam line. Further
refinements to the measurement systems such as
independent mesurements for the vertical and thehorizontal beam size are under preparation.
7 ACKNOWLEDGMENTS

The authors wish to thank to Professors Dr. S.
Kurokawa and Dr. K. Satoh for their encourage of thiswork. The authors also thank to KEK B commissioning
team for their support and operation.

beamsize
(!lm)

538
361
689
437

We can also measure estimate the RMS. beam size
from one data of visibility, which is measured at a fixed
separation of double slit. The RMS beam size crhcam isgiven by,cr b

We can estimate natural bunch length by
extrapolating the data to zero current, and results are
5.5mm for the HER and 6.0mm for the LER.

(2)
-101-

REFERENCES[1] T. Mitsuhashi, N. Takeuchi, S. Hiramatsu, M. Itohand T. Yatagai, Proc. APAC98, p698,(1998).
[2] Ed. by S-i Kurokawa et.al. "Beam Measurement",
p399-427, World Scientific,(1999).
[3] J.W. Flanagan, TMitsuhashi and S. Hiramatsu,
somewhere in this proceedings.
[4] J.W. Flanagan, S. Hiramatsu and T. Mitsuhsahi,Proc. PAC99, p.2120,(1999).