VUV & SX Beamline Design

Kenta Amemiya
Photon Factory, High Energy Accelerator Research Organization

1Outline
1. Choice of monochromator type1.1. energy region
1.2. resolution & intensity
1.3. some hints for the choice
1.4. examples for soft X-ray monochromator
2. Design procedure
2.1. optimization of parameters2.2. analytical estimation of energy resolution
2.3. ray-tracing simulation
3. Beamline installation
3.1. alignment
3.2. optical adjustments using SR
2.3. experimental estimation of beamline performance 2

1.1. energy region
1. High-energy region (>~150 eV)Grazing incidence (>~85 deg) monochromator is inevitable*except for multilayer grating
2. Low-energy region (<~50 eV)(Near) normal incidence monochromator is also available*Medium incidence monochromator?
Strongly affects the polarization
3. Wide-energy beamline (e.g. 30 eV – 1500 eV)(a) Combination of grazing and normal incidence monochromators(b) Variable included angle monochromator(c) Interchangeable gratings
Included angle

31.2. resolution & intensity
1. Energy resolution depends on…
Dispersion & Focus
Focus size depends on…
Source size, demagnification, aberration, slope error,…
Some of them drastically change according to technical progress

No absolute solution !!
e.g. aberration-free monochromator

Parabolic mirror(collimation)

Parabolic mirror(focusing)

Plane grating(dispersion)
Point source

Perfect monochromator, in principle,
except for the reflectivity loss
Slope errors in parabolic mirrors are large
Use of cylindrical mirrors ⇒ large aberration

Exit slit(wavelengthselection)

Recent progress in SR sources;
Small divergence ⇒ negligible aberration
4

1.2. resolution & intensity
2. Intensity depends on…
Number of optical elements
Incidence angle & acceptance (* more grazing needs larger mirror)
Diffraction efficiency of the grating* High groove density ⇒ large dispersion but low efficiency

e.g. the simplest monochromator
Minimum intensity loss (no mirrors)

Point source

Exit slit(wavelengthselection)

Focal condition depends on wavelength
Aberration might be serious

Concave grating(dispersion & focusing)

We must compromise !!
Intensity, resolution, energy range,…

5

1.3. some hints for the choice
1. Grating shape (plane, spherical, …)
Spherical： dispersion & focus ⇒ small number of optical elements
be careful for aberrations

2. Groove density (uniform or varied)
Varied line spacing： simpler optics (or higher resolution with the same optics）
be careful for precision in the groove parameters

3. Included angle (constant or variable)
Variable： higher degree of freedom ⇒ resolution & intensity in wide energy range
scanning mechanism is more complicated ⇒ be careful for reproducibility

4. Entrance slit
Without slit： Source size of SR itself directly affects the resolution
Higher resolution than the source-size limit is never obtained !
With slit： Higher resolution can be achieved at the sacrifice of intensity
pre-focusing optics is necessary
5. Focusing elements in monochromator (upstream, downstream of G, or nothing)
Effects of the slope errors in the focusing mirror are smaller in the upstream case

The choice depends on properties of light source, precision of mirrors, reliability
6of scanning mechanism, needs from applications, costs, …

1.4. examples for soft X-ray monochromator(1) Plane grating monochromators
Collimated-light illumination
Parabolic mirror(collimation)

Essentially no aberration

Parabolic mirror(focusing)⇒ α and β can be freely chosen⇒ Demagnification can be controlled
Precision of parabolic mirrors is relatively poor

Plane grating(dispersion)
Point source

Exit slit(wavelengthselection)

One can use cylindrical mirrors if divergenceis small enough
Focusing
mirrorGrating
Collimating
mirrorincluded angle
determination

http://sls.web.psi.ch/view.php/beamlines/adress/optics/index.html

7

1.4. examples for soft X-ray monochromator(1) Plane grating monochromators
Diverging light illumination (SX-700)
Plane grating & post-focusing mirror (e.g. elliptical mirror) with variable included angle
Precision of elliptical mirrors was essential due to high demagnification factor
One can use cylindrical mirrors if divergence is small enough
Horizontal focus Grating(not essential)
Focusing mirror
included angle
determination

Number of optical elements is reduced compared to the collimated case
Relation between α and β must be properly chosen to keep focal condition
81.4. examples for soft X-ray monochromator(2) Spherical (or cylindrical) grating monochromators
Rowland mount
Monochromator itself consists of a grating only
But…
Relation among α，β，r, and r’ must be properly chosen

Exit slit(wavelengthselection)

Spherical grating(dispersion & focusing)“Rowland condition”: r = R cos α, r’ = R sin β⇒ Many optical elements and complicated scanning mechanism
DRAGON mount
Monochromator consists of a spherical (cylindrical) grating only
Fixed included angle⇒ Simple scanning mechanism
Kinds of aberration arises, but only the defocus term can be canceledby moving the exit slit
“Active grating” (variable radius) is developed to achieve fixed exit slit 9

1.4. examples for soft X-ray monochromator(3) Varied-line-spacing (VLS) plane grating monochromators
Diverging light illumination
Monochromator itself consists of a VLS plane grating only
Relation between α and β must be properly chosen⇒ A precise variable included angle system is inevitable

Horizontal focus(not essential)

included angle
determination

10

1.4. examples for soft X-ray monochromator(3) Varied-line-spacing (VLS) plane grating monochromators
Converging light illumination (Monk-Gillieson mount)

Plane grating
SourceCylindricalmirror

CylindricalmirrorSpherical
mirrorPre-focusing optics

Toroidal mirror

Entranceslit

Monochromator

Exit slit

Planemirror

Post-focusing optics

Pre-focusing mirror upstream of VLSG
Constant included angle ⇒ Simple scanning mechanism
Moderate aberration in spite of constant included angle
Variable included angle system is also adopted recently

111. Choice of monochromator type1.1. energy region
1.2. resolution & intensity
1.3. some hints for the choice
1.4. examples for soft X-ray monochromator
2. Design procedure
2.1. optimization of parameters2.2. analytical estimation of energy resolution
2.3. ray-tracing simulation
3. Beamline installation
3.1. alignment
3.2. optical adjustments using SR
2.3. experimental estimation of beamline performance122.1. Optimization of the parameters
Overview of a typical soft X-ray beamline

Pre-focusing optics

Monochromator

Post-focusing optics

Pre-focusing optics： focuses X rays onto the entrance slit
Monochromator： from the entrance slit to the exit slit
Post-focusing optics： focuses monochromatized X rays onto sample position
Higher order suppression (Mc)：utilizes energy dependence of reflectivity (or transmittance)

132.1. Optimization of the parameters
1. Source-size limit

Dispersion:dz / dλ = r ' nm / cos βSource size &Demagnification
SourceGrating

Beam size at the exit slit
Slits’ (lower limit) = s r’/r
⇒ λ/Δλ ∝ nmr/scosβ(a) If the source size is the same, longer monochromator gives higher resolution.
(b) If the monochromator length (r + r’) is the same, longer entrance arm (r)gives higher resolution. ⇒ Higher demagnification factor is better !
But…
(a’) Long monochromator needs large mirrors to keep enough acceptance⇒ higher cost, or intensity loss by reduced acceptance(b’) High demagnification factor causes large aberration.
⇒ Eventual decrease in energy resolution
Most people choose ~1:1 (r ~ r’) optics, though it might not be the best solution.
Groove density (n) and included angle are chosen, considering thebalance among dispersion, demagnification, diffraction efficiency, etc.

14

2.1. Optimization of the parameters
2. Monochromator parameters (mirror radius, groove parameter, etc.)- highly depends on the type of monochromator
Design example: Variable-included-angle Monk-Gillieson mountvaried-line-spacing (VLS) grating monochromator

pre focusing
2K=included angle determination
15

2.1. Optimization of the parameters
Parameters:
ρ (sagittal radius of M1)
Groove parameters of VLSG
N = N0 (1+a1w+a2w2+a3w3)

2K=

K. Amemiya & T. Ohta, J. Synchrotron Rad. 11 (2004) 171.

1. Choose two energies (E1 and E2) and respective included angles (K1 and K2)
2. Optimize ρ and a1 so that the defocus vanishes at (E1, K1) and (E2, K2)
3. For other energies, included angles are set so that the defocus vanishes
4. Choose an energy (E3) and optimize a2 so that the coma aberration vanishes
5. Choose E4 and optimize a3 so that the spherical aberration vanishes
162.1. Optimization of the parameters

N = N0 (1+a1w+a2w2+a3w3)
17

2.2.analytical estimation of energy resolution
From light path function
Includedangle (deg)defocus

Source size

slope error

182.2.analytical estimation of energy resolution

19

2.3. ray-tracing simulation
Source parameters:
σx = 350 μm, σy = 20 μm, σx‘= 20 μrad, σy‘ = 5 μrad, 4.5 m undulator
Optimization conditions for N0 = 600 l/mm:
E1 = 50 eV, E2 = 500 eV, K1 = 164o, K2 = 174o, E3 = E4 = 100 eVSpot diagram at the exit slit=> E/ΔE~26,000

20

2.3. ray-tracing simulation
Simultaneous scan mode
Included angle is scanned simultaneously with the grating

Source size or slope error limited resolution

21

2.3. ray-tracing simulation
Fixed included angle mode

Relatively high resolution over wide energy range* Analytical estimation is consistent with ray tracing simulation
222.3. ray-tracing simulation

Comparison with diverging illumination optics

Monk-GIllieson
converging X rays
illuminate VLSG

non Monk-GIllieson
diverging X rays
illuminate VLSG

23

2.3. ray-tracing simulation

Comparison with diverging illumination optics

Monk-Gillieson(converging illumination)

non Monk-Gillieson(diverging illumination)

Simultaneous scan

Simultaneous scan

Fixed included angle
Fixed included angle

24

1. Choice of monochromator type1.1. energy region
1.2. resolution & intensity
1.3. some hints for the choice
1.4. examples for soft X-ray monochromator
2. Design procedure
2.1. optimization of parameters2.2. analytical estimation of energy resolution
2.3. ray-tracing simulation
3. Beamline installation
3.1. alignment
3.2. optical adjustments using SR
2.3. experimental estimation of beamline performance25

3.1. alignment
Determination of beamline center

Hole on theShield wall

to beamline
Q-magnet

Undulator

Q-magnet
Shield wall

Target on the Q-magnet
26

3.1. alignment

Grating adjustment(roll and yaw)
Adjusted by using
diffraction of Laser light

Mirror adjustment

Adjusted by using a dummy
mirror and Laser light27

3.2. optical adjustments using SR
Example: BL-16A at the Photon FactoryM0:

vertical focusing to entrance slit (S1) [r = 15 m, r’ = 5 m]

M1:

vertical focusing to 90 mm upstream of exit slit (S2)[r = 4 m, r’ = 7.91 m]

28

3.2. optical adjustments using SR(a) Vertical focusing of M0

Light intensity was monitored downstream of S1 during M0 roll-angle scan.
Upper and Lower parts of light were taken by using an aperture.
optimized

designed

M0 pitch = 1.9975 deg (-0.125%)

M0 pitch = 2.0 deg
Upper part
Lower part

Intensity (arb. units)

-5

6x10

a little peak shift

-5

4x10

-5-5

6x10

no peak shift
-5

4x10

-5

2x10

0-0.61

2x10

-0.60

-0.59

0-0.61-0.58

M0 Roll Angle (deg)M0Upper part
Lower part

-5

8x10

Intensity (arb. units)

-5

8x10

aperture

-0.60

-0.59

-0.58

M0 Roll Angle (deg)S1

Focal point is upstream of S1

M0aperture

S1Focal point is just at S1!

Sagittal radius of M0 is ~0.1% smaller than the designed value (262 mm).
29
This coincides with the inspection report!

3.2. optical adjustments using SR(2) Vertical focusing of M1
M1 is designed so that light is focused at 90 mm upstream of S2.

S1

M1GratingS2
designed focal point-6

3.0x10designed Zero-th order light intensity was monitored
M1 pitch = 2.0 deg
downstream of S2 during Grating angle scan.
Upper part
Lower part

-6

Intensity (arb. units)

2.0x10Upper and Lower parts of light were taken byusing an aperture

S2: -45 mm-6

1.0x1045 mm upstream ofdesigned position

0.0-6-1.0x10-6-2.0x10

A peak shift between the upper and lower partsmeans that the focal position is upstream of S2.
However…

S2: 0 mmdesigned position(90 mm downstream
3.97503.9755ofthefocus position)
Grating Angle (deg)The peak shift should be reduced by ~50 % when S2is placed at -45 mm position.

Focal position is far from S2 !?

30

3.2. optical adjustments using SR(2) Vertical focusing of M1optimized
M1 pitch = 1.9535 deg (-2.3%)

M1 pitch

Upper part
Lower part1.0x10Peak Shift (arb. units)

Intensity (arb. units)

S2: -45 mm

5.0x10150-6-7

S2: 0 mm1.9775 deg
1.9612 deg
1.9535 deg

10050

0.0

S2: +45 mm
0focal position-7-5.0x10
2.98122.98142.9816

Grating Angle (deg)2.9818-100 -80 -60 -40 -20

02040

S2 position (mm)The focal point became 90 mm upstream of S2 when the pitch angle of M1
was changed to 1.9535 deg (-2.3% from the designed value).

Sagittal radius of M1 is ~2.3 % smaller than the designed value !?

31

3.2. optical adjustments using SR
Problem of the Mirror Holders
VLSG plane

M1cylindrical

M2
plane
1000

depends on included angle !!
Plane mirror (M2) and/or
Grating (VLSG) are not plane!
Mirror distortion by the holder?

Focus Position (mm)Focal point of zero-th order500
0-500-1000-1500-2000
167 168 169 170 171 172 173 174 175 176

2K (deg)32

3.2. optical adjustments using SR
Effect of Holder Improvement33

3.2. optical adjustments using SRFocal position for diffracted light

2K = 174 deg
20000

N = 500 l/mm Ar 2p (244.4 eV)

2K = 172 deg
30000

Upper part
Lower part25000

S2: -45 mm

Upper part
Lower part60

S2: -45 mm
4010000

20000

15000

10000

S2: +45 mm
5000

S2: 0 mm

S2: +45 mm

Energy Shift (meV)S2: 0 mmIntensity (arb. units)

Intensity (arb. units)

15000

2K = 174
2K = 172
2K = 170

200-205000-40250.8 250.9 251.0 251.1 251.2 251.3

Photon Energy (eV)0251.1 251.2 251.3 251.4 251.5 251.6

Photon Energy (eV)-60-40-20020

S2 position (mm)4060High resolution is not necessary for adjustment!34

3.2. optical adjustments using SR
Focal position

S2 rotation (tilt angle)

N = 1000 l/mm
2K = 173.0 deg
6000

N = 500 l/mm
Upper part
Lower part

Tilt

Right part
Left part

1000

4000

500
410.8 410.9 411.0 411.1 411.2 411.3 411.4
Photon Energy (eV)6000

2K = 172.8 deg

Upper part
Lower part

4000
2000
410.8 410.9 411.0 411.1 411.2 411.3 411.4
Photon Energy (eV)

10000 2K = 172.7 deg

Right part
Left part

50000415.0

Intensity (arb. units)

Intensity (arb. units)2000

415.2 415.4 415.6
Photon Energy (eV)415.8

Right part
Left part

1000
5000415.0

415.2

415.4 415.6 415.8
Photon Energy (eV)416.0

Right part
Left part

1000
500

410.9 411.0 411.1 411.2 411.3 411.4
Photon Energy (eV)0415.0415.2 415.4 415.6
Photon Energy (eV)

415.8

35

3.3. experimental estimation of beamline performance
Absorption spectrum for N2 gas
N = 500 l/mm, S1 = 25 μm, S2 = 25 μm

N = 1000 l/mm, 2K = 172.7 deg
60000

50000

S1 = 25 μm
S2 = 20 μm

Intensity (arb. units)

Intensity (arb. units)

50000
40000

30000

20000

40000

30000

20000
10000

0S1 = 50 μm
S2 = 40 μm

10000

400.5

401.0

401.5

402.0

Photon Energy (eV)

402.5

0

400.5

401.0

401.5

402.0

402.5

Photon Energy (eV)36

3.3. experimental estimation of beamline performance
Absorption spectrum for Ar gas
Ar 3s → np
0.9 meV20251530

λ / Δλ > 30,000
37

3.3. experimental estimation of beamline performance
Photon Flux: photodiode is available101386

Photon Flux (photons/sec)

42101286421011864210

10050

100

150200

Photon Enregy (eV)25030038

3.3. experimental estimation of beamline performance
Beam size: knife-edge scan
Intensity (arb. units)

4

Light intensity is monitored atdownstream of the knife edge

Intensity
derivative

3
32
2
1
10-1
52.6

52.7

52.8

52.9

53.0

Horizontal position (mm)Horizontal Size

Vertical Size
4

Intensity (arb. units)

Vertical position (mm)

22.64

12 μm
22.62

22.60

22.58

0

102030Intensity (arb. units)

4050

3280 μm

1052.6

52.7

52.8

52.9

53.0

Horizontal position (mm)39(3.2.) optical adjustments using SR
Adjustment for the focus on the sample
Knife edge

Aperture
Horizontal Focus
M3 pitch = 2.0 deg

Right part
Left part2M3 pitch = 1.994 deg

6 M3 yaw = -0.25 deg

Upper part
Lower part

1

20
54.9

0
22.2

022.4

3

M3 pitch = 1.96 deg
255.4

Right part
Left part1058.1
358.2
58.3
58.4
58.5
Horizontal position (mm)M3 pitch = 1.99 deg

258.6

Right partLeft part22.7

Upper part
Lower part

2103

22.3

00

22.4
22.5
22.6
Vertical position (mm)M3 pitch = 2.1 deg

2155.9
56.0
56.1
56.2
Horizontal position (mm)

22.4
22.5
22.6
Vertical position (mm)3 M3 pitch = 2.06 deg

155.8

22.3

22.3

22.7

Upper part
Lower part

22.4
22.5
22.6
Vertical position (mm)22.7

Intensity (arb. units)

55.0
55.1
55.2
55.3
Horizontal position (mm)Right part
Left part

4

1

Intensity (arb. units)

Intensity (arb. units)

2Tilt angle

Vertical Focus

2022.5Vertical position (mm)22.6
Right part
Left partM3 yaw = -0.30 deg

151050
22.4
25
2015
1050

22.5
Vertical position (mm)M3 yaw = -0.289 deg

22.5

22.6

Right part
Left part

22.6
Vertical position (mm)4022.7

Thank you for your attention !

41