Upgrade the RIXS Beamline to Ultrahigh Resolution at NSRRC:

Mechanical Stability Issues D.J. Wang, C.C. Chiu, T.C. Tseng, L.J. Huang, H.S. Fung, H.W. Fu, S.W. Lin, C.S. Lin, C.H. Lai, W.B. Wu, S.C. Chung, D.J. Huang, and C.T. Chen

Reporter: C.S. Lin

National Synchrotron Radiation Research Center Hsinchu, Taiwan

outline

• 1. Introduction (optical layout, stability issues)

• 2. Original design and setup

• 3. Measurement instruments

• 4. Upgraded design and engineering

• 5. Performance of the upgrade

• 6. Summary

C. H. Lai, H. S. Fung, W. B.Wu, H. Y. Huang, H.W. Fu, S.W. Lin,C. C. Chiu, D. J. Wang, L. J. Huang, T. C. Tseng, S. C. Chung, C.T. Chen and D. J. Huang Through this new system, the commissioning results successfully showed spin-flip, dd, and charge-transfer excitations of NiO. These results are consistent with previous published results, while maintaining a better spectral resolution and a higher measurement efficiency. The best energy resolution of our setup in terms of full width at half maximum is 77 meV at the incident photon energy tuned about the Ni L3-edge.

Journal of Synchrotron Radiation 2014, Mar 21, p325-332

High-Resolution, High-Efficiency Beamline and Spectrometer for Reasonace Inelastic Soft X-ray Scattering (RIXS)

Layout of RIXS beamline

Base on the energy compensation principle, we build the AGM –AGS beamline for RIXS.

The first optical element is a horizontal focusing mirror (HFM) located at distance 12.05m from the center of an elliptically polarized undulator (EPU), which is the source of X ray.

A vertical focusing mirror (VFM) is located at distance 1.4 m from the HFM to focus the X-ray beam onto an entrance slit.

The light is further focused and dispersed onto the sample by the active-grating monochromator (AGM).

An aperture is placed just before the sample to define the energy spread of the incident photons.

After the sample, a horizontal refocusing mirror (HRFM) enhances the efficiency of collection of scattered X-rays in the horizontal direction.

An active-grating spectrometer (AGS) is used also to disperse and to focus the scattered light onto a two-dimensional charge-coupled device (CCD).

Bendable grating

Entrance slit opening

don’t scarify resolution

Anticipation of Beamline User : High resolution and High Stability

In this innovative optical design,

theoretical resolution is very high but limited by the

realistic optics quality. (so far 0.5urad rms slope

error, A)

High stability is related from source through each

optical component and CCD. (below 0.1urad p-p, B)

Grating

2 PZT actuator

Real resolution〜 〔(A2 +B2+…)〕1/2

NiO RIXS (energy loss) Spectra

3 minutes/spectrum

Spectrometer @ 90 degrees

545 counts/pixel

For the energy of point A, CCD takes the energy loss spectrum 3 min to get FWHM, and further to determine the resolution.

Due to Low stability of AGM, AGS and CCD will then deteriorate the resolution

(weak photon signal expt)

Why is the stability sensitive for this experiment

Monochromator

AGS

CCD

Entrance slit

Sample chamber

Typical RIXS original Setup

To collect RIXS signal at different scattering angle, AGS could be moved to different location. Blue steel frame moves along the circle trace and carries the AGS and CCD to desired location.

How to stably support the movable AGS and CCD?

Before 2013 Feb

0 2 4 6 8 10 12 14 16

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Time(sec)

m

vertical

background RMS:0.010m

steel framework RMS:0.122m

Vertical and horizontal vibration of original

steel frame

Instrument: Lion precision capacitive sensor relative to ground

Vertical vibration is 10 times higher than background

0 2 4 6 8 10 12 14 16

-0.08

-0.04

0.00

0.04

0.08

0.12

0.16

0.20

m

Time(sec)

Horizontal

background RMS:0.004m

steel framework RMS:0.018m

Measurement instruments

• High-resolution autocollimator and stable support for measuring grating angle (20Hz, 0.1urad) on grating surface

• Capacitive sensor for measuring displacement of support (1Kz, 0.001um)

• Linear encoder of grating actuator (20Hz, 2nm)

• PCB accelerometer for measuring chamber (4-100HZ)

Autocollimator

Mirror Granite support

Upgraded engineering

• AGS, CCD granite engineering and air floating.

• AGM and AGS chamber will be rebuilt.

• AGS & CCD are located on granite base with flatness = 5um. The granite base will be lifted height 30um @3Kg/cm^2 when operating 4 air purge holes.

• Each granite platform segment was polished at a factory in advance. The installation flatness is about 50um/10m, and pitch direction is below 20urad.

• Slow curing epoxy was used to fill with the gap between granite and ground.

• Repeatability of sliding granite in pitch direction is about 1 urad by lifting it up and down.

Granite engineering

Granite platform

Sliding granite base

Wire pulling mechanism of the sliding granite

AGS & CCD sliding granite base

moving mechanism

Using 8 steel wires with spring force to pull the granite base, and then to prevent it from distortion by steel frame

TLS RIXS BL upgrade

2013 Feb

New AGM chamber system

New system

MEDSI2012 C.C. Chiu D. J. Wang

Using Stable granite base to support chamber

Introducing Vertical big chamber Decoupling Grating adjusting

mechanism with Vertical big chamber

AGM design

Grating rotation center

Flexible pivot

Motor Micos Connection rod, 30cm vertical

Weight balance

The grating support is mounted to granite and decouple with chamber via bellows, and therefore prevents the vibration from both sides of the chamber

Bellows

Grating support

Granite

Resolution of AGM by Micos motor

MEDSI 2014 Resolution is good enough for the grating

Refine chamber mounting

Because the weight and vacuum force are different on both sides of the chamber, we will introduce bellows support to reduce extra moment on the chamber.

beam

Bellows support

bellows

Chamber mounting from view of beam direction

Ion pump

bellows

Due to Weight of IP and GV is more than AGM chamber, we use bellows to decouple with the chamber to further prevent the indeterminate supporting points of this system.

0 50 100 150 200

0.00008

0.00010

RMS= 2.30 nm

Po

sitio

n(m

m)

Time (sec)

Best before Aug. shutdown

20140930 after upstream frame

RMS= 2.33 nm

AGM chamber with bellows support

Data obtained from Micos motor encoder

User response: Bellows support help shorten the pendulum-phenomenon duration of grating holder in the chamber of AGM subjected to external excitation

Bellows support

Without bellows support

367mm

10nm p-p

10nm p-p/367mm

≈ 27.2nrad

AGS design Insert Bellows support locked by screw rod

• AGS is mounted on the bottom flange due to height limitation

• The adjusting mechanism for angle driving of AGS is the same as the one of AGM

Add wt on bottom flange

2000 2500 3000

1.5

2.0

2.5

3.0

Time(sec)

ra

d

AK background on 05A granite

RMS 0.13 rad

60cm

Stability test of Autocollimator

Autocollimator

Mirror

Wind shield

No bellows support No wt

No bellows support Wt 80Kg

1 bellows support Wt 80Kg

2 bellows support Wt 80Kg

Grating encoder (nm rms)

3.6 2.3 2.2 2.18

Autocollimator (urad)

0.38 0.178 0.187 0.167

AGS chamber mounting stability

The bellows support are tentative, and just shows the trend

24

[100] 𝜎-pol

[001]

857

856

855

854

853

852

Incid

en

t P

ho

ton

En

erg

y (

eV

)

-8 -6 -4 -2 0

Energy Loss (eV)

700

600

500

400

300

200

100

0

Spin d-d CT

1.0

0.8

0.6

0.4

XA

S (

arb

. u

nits)

865860855850

Photon Energy (eV)

NiO XAS

300

200

100

0Inte

nsity (

arb

. u

nits)

-0.4 -0.2 0.0 0.2Energy Loss (eV)

853.0 855.0 855.5 857.0

FWHM = 108 meV for several hours

Commissioning results: RIXS spectra on NiO

Excitation of local spin-flip

The surface normal of the NiO sample was along the [001] direction; the incident angle was 30 from the sample surface.

RIXS spectra excited with incident photons of energies 853, 855 and 857 eV.

RIXS spectra in an energy range near the elastic scattering excited with of selected incident energies labeled in units of eV.

Color mapping of RIXS intensity across the L3 absorption edge, obtained by combining RIXS spectra excited every 500 meV.

We chose NiO as a test sample for high resolution RIXS measurements using a side branch of the EPU beamline at the TLS.

NiO is a prototypical system of strongly correlated electrons that exhibit many-body characteristic properties.

Summary

• After upgrading the stability of RIXS beamline, users are

satisfied with the resolution and stability of RIXS system.

• From grating encoder reading, it seems that we can

attain below 30nrad p-p mechanical stability

(corresponding to 10nm obtained from Micos motor encoder) p-p mechanical stability for RIXS system, and

this mechanical stability can last for several hours.

• Beamline signals obtained from CCD are more sensitive

than most commercial measurement tools.