A Double Crystal Monochromator of Sagittal Focusing at SSRF

J.H. He, S.J. Xia, Z.C. Hou, J.L. Gong, X.M. Jiang and Y. Zhao

SSRF Project TeamShanghai Institute of Nuclear Research,

Chinese Academy of Sciences,

I. Introduction

II. Design and Construction

III. Preliminary Tests

I. Introduction

Characteristics of sagittal focusing monochromator(SFM):– monochromatizing and focusing the beam simultaneously, useful for simplifyi

ng/optimizing the beamline optics

– large horizontal acceptance

– good focusing

Aim and task

Why to make a SFM:

– two beamlins at SSRF proposing to use SFM, which is commercially available, but expensive

– developing the relevant techniques, a first try in China

Major requirements:– a good focusing performance

– bearable to the rather high heat load(~0.5W/mm2), can be used at SSRF bending magnet beamline and BSRF wiggler beamline

II. Design and Construction

• Operating principles and modes:

• Design features

• Products

Operating principles and modes:

when X = H/2sin ， Y = H/2cos, fixed H; when H(max)=30mm ， =5.5-25deg.,

required X,Y translation range:

chosen X,Y range: X = 150mm, Y = 50mm

Incident beam

X

YH

θ

Exit beam

1st crystal

2nd crystal

Schematic operating principles of the monochromator

mmX 121)sinθ

1

θsin

1(

2

H

maxmin mmY 48.1)

cosθ

1

θcos

1(

2

H

maxmin

By controlling X and Y, the monochromator can work at different modes:

a) fixed beam exit height; b) variable beam exit height; c) direct beam;

Design features :

Main structure of the monochromator

Main features of the design:

• independent adjustment of crystal positions to facilitate the different operating modes

• direct water cooling to stand rather high heat-load

• accurate bending of the crystal, driven by the complex flexure hinge mechanism

• independent adjustment of crystal orientations by using the flexure hinge mechanism

• movable support of the main structure to facilitate the installation and maintenance

1) 1st crystal cooling system

Similar to PF-monochromator (H. Oyanagi et al.), modified to be

a) compatible to the silicon manufacturing technique available in China

b) able to stand the required heat-load

Crystal shape and parameters

Photos of 1st crystal

Crystal cooling system:

Cooling water is fed into the crystal through the rotation axis

2) 2nd -crystal and crystal bender

Tilt-table flexure hinge mechanism:

driven symmetrically by two actuator with better than 0.1m resolution

2nd -crystal

2nd -crystal ： ribbed to resist the anticlastic distortion when bent

2nd -crystal parameters ：

w=0.6mm, e=1.4mm, h=10.0mm, t=0.8mm

2

3

111 t

h

e

w

R

R

s

a

Rs :1 ~ 10 m, Ra ≥2200m, corresponding to 12μrad average slope error

Photo of crystal bender

3) Crystal orientation adjustment

Micro-actuator

Balancing spring

Right circular flexure hinge mechanism

3.2/4

3 tR

E

maximal rotation angle ：

beryllium bronze ： E=115GPa ， =1.15GPa

Required rotation angle ： >0.5

Three adjustments for :

1st crystal roll

2nd crystal pitch

2nd crystal yaw

4) Movable support

Photo of main structure

III. Preliminary tests

1) Test of crystal orientation adjustment

Resolution obtained ：

1st crystal roll 0.16,

2nd crystal pitch 0.18,

2nd crystal yaw 0.48

optical auto-collimator

DCM

Ideal focusing condition ：

R=2F1F2sin/( F1+F2),

F1:object distance ; F2 :are and image distance

The focusing image diffuses when • bending curvature deviates from the Ideal R by R

• crystal surface is not ideally cylindric with a spread of R at the average radius of R

image diffuseness: W F1/F2 F2=(F1+F2)(R/R),

2) Test of focusing performance

Source width(H) : 1.2mm ， F1=17.5m ， F2=7m, =11 , spot width ： 0.6m

m ， W=0.12mm, W/W=25% ， R/R 0.25%, R 2m

F1=25m ， F2=14m, =13 , spot width ： 0.9mm,

W=0.13mm, W/W=19% ， R/R 0.2%, R 4m

Laser simulation test

3) Test of cooling effect

A particular testing apparatus:• an electrical gun with maximum power of 800W is used to used to simulate the synchrotron radiation power distribution

•a specially designed interferometer is used to measure the surface profile of the crystal.

0 5 10 15 20 25 30 35 40 45 50 55 60

-0.3

-0.2

-0.1

0.0

0.1

3mmÁ¦±Û£¬10L/minÁ÷Á¿µÄ¾§Ìå ÐαäÇúÏß

76W,0.19W/mm2

152W,0.38W/mm2

228W,0.57W/mm2

304W,0.76W/mm2

380W,0.96W/mm2

456W,1.14W/mm2

532W,1.34W/mm2Ð

αäÁ

¿( m

)

X (mm)

Dis

tort

ion

Crystal surface profile at different heat-load

Conclusion:

By prebending the crystal in an opposite direction, surface distortion due to the heat-load up to ~ 400W and 1w/mm2 can be reduced to a tolerable a level (R>1000m).

4) Other Specifications

Rotation(Bragg) angle

Range: -2°- 30°

Reproducibility: 1.8 〞 Resolution 0.18 〞

Acceptance

Horizontal: >2.0 mrad

Vertical: >0.25 mrad

Vacuum < 5×10-6 Torr

Summary

• a good performance of the adjusting mechanisms, the bending mechanism and the mechanical structure

• on-line test necessary

ACKNOWLEDGMENT

We thank Mr. Sizhong Zhou and his group, Mr. Renkui Zhou, Fanghua Han and Shicuang Liu for their collaboration at the mechanical design and construction of the monochromator. We thank Dr. Freund and his group at ESRF for their kind help in making the focusing crystal and for their helpful discussions. We also thank Dr. Oyanagi for his helpful discussion on the crystal cooling techniques.