1. Reflectivity and Photo Yield measurements of technical surfaces
1/06/2017
Reflectivity and Photo Yield measurements of technical surfaces
Eliana La Francesca1,2, G. Gwalt3, F. Schäfers3, F.Siewert3,A. Sokolov3, M. Angelucci1 and R.Cimino1
1 Laboratori Nazionali di Frascati- INFN
2 Università “Sapienza” di Roma
3 Helmholtz Zentrum Berlin
Acknowledgements:
INFN- NCV project MICA;
We thankfully acknowledge HZB for financial support.
FCC Week 2017, 1 June Berlin
FCC Week 2017, Berlin

2. Outline Synchrotron radiation in FCC-hh Reflectivity
1/06/2017
Synchrotron radiation in FCC-hh
Reflectivity
Carbon Reflectivity
Bessy II measurements
Conclusions
I will introduce the problem we are studying which is related to the production of SR in FCC-hh. We are looking at the reflectivity of the material of the beam screen, and in particular our approach consists in using carbon as potential coating to increase reflectivity. We started an experimental campaign at Bessy II and I will show you some preliminary results and some preliminary conclusions. 2
Eliana La Francesca
FCC Week 2017, 1 June Berlin
FCC Week 2017, Berlin

3. Synchrotron Radiation detrimental effects
1/06/2017
Heat load on the accelerator walls
Photon stimulated desorption
Production of secondary electrons
Beam instability
LHC has a non negligible SR production.
In the Highest Energy Proton Circular Collider ever designed, FCC-hh, large production of Synchrotron Radiation is expected
The detrimental effects of SR can be summarized in heat load on accelerator walls, photon stimulated desorption, production of SE and consequently beam instability. SR historically typically occurred in lepton machines, but the high energy of new hadron colliders is such that in LHC SR is non negligible, and in FCC-hh a large production of SR is expected. 3
Eliana La Francesca
FCC Week 2017, 1 June Berlin
FCC Week 2017, Berlin

4. Parameters LHC H-L LHC FCC-hh
FCC Parameters
1/06/2017
Parameters LHC H-L LHC FCC-hh
Version 1.0 ( )
Dipoles at cryogenic temperature of 1.9 K
Here some relevant FCC Parameters are summarized. In particular l want to underline the last three which are: SR power per ring, SR heat load per meter, and critical photon energy. Thus a fundamental step will be to dissipate the SR heat load, also in order to protect the dipoles at 1.9K. 4
Eliana La Francesca
FCC Week 2017, 1 June Berlin
FCC Week 2017, Berlin

5. Heat Load Dissipation VS Temperature
1/06/2017
PTOT ~3 𝐺𝑊
Power to dissipate 1W (W)
PTOT ~80 𝑀𝑊
Credits: R. Kersevan -- Beam Dynamics meets Vacuum, Collimations, and Surfaces
FCC needs a Beam Screen at the highest possible temperature compatible with vacuum stability, impedance…
If we consider the Power needed to dissipate 1W VS Temperature we can see that at cold bore temperature would be necessary 3 GW for the whole machine. This means that FCC needs a BS at highest possible temperature compatible with all other constraints (vacuum, impedance ecc).
It is important to underline that the majority of this heat load is in Dipoles. 5
Eliana La Francesca
FCC Week 2017, 1 June Berlin
FCC Week 2017, Berlin

6. Synchrotron Radiation interaction with Matter
1/06/2017
X-Ray Range
FCC-hh SR incidence angle: deg (0.62 mrad)
~ 21 m from source
Photon fan strip ~ 2mm
R. Cimino, V. Baglin and F. Schäfers, PRL. 115 (2015)
Let’s focus about FCC SR with more detail.
FCC SR is expected to impinge accelerators walls at 21 m from source, with a very grazing angle (0.035 degree) and with 2mm of photon fan strip.
Critical energy is in X-ray range. SR Power is principally carried by high energy photons. 6
Eliana La Francesca
FCC Week 2017, 1 June Berlin
FCC Week 2017, Berlin

7. Synchrotron Radiation interaction with Matter
1/06/2017
Arc SR Heat Load = 28.4 (44.3) W/m/aperture
Reflected + Absorbed + Transmitted
To be increased in high temperature parts (BS)
Specular reflected
Scattered
To be increased in cold parts (Superconductor Magnets)
Arc SR Heat Load = 28.4 (44.3) W/m/aperture
In general, the radiation interacting with matter can be reflected, absorbed and transmitted; neglecting transmission we have just reflection and absorptions. For technical surfaces it is important to consider that total Reflectivity is equal to specular reflectivity and scattered light. Reflectivity is an extremely important parameter to be controlled. Experimental data are important to validate simulations to optimize design. 7
R. Cimino, V. Baglin and F. Schäfers, PRL. 115 (2015)
Eliana La Francesca
FCC Week 2017, 1 June Berlin
FCC Week 2017, Berlin

8. Reflectivity
1/06/2017
X-Ray Reflectivity depends on a limited number of parameters:
Photon energy and light polarization
Angle of incidence
Surface roughness
Material 8
Eliana La Francesca
FCC Week 2017, 1 June Berlin
FCC Week 2017, Berlin

9. Specular Reflectivity VS Incidence angle
1/06/2017 Cu
Here we show a simulation of specular reflectivity VS incidence angle and Photon Energy of Copper sample with 50 nm of roughness. 9
Eliana La Francesca
FCC Week 2017, 1 June Berlin
FCC Week 2017, Berlin

10. Reflectivity
1/06/2017
X-Ray Reflectivity depends on a limited number of parameters:
Photon energy and light polarization
Angle of incidence
Surface roughness
Material 10
Eliana La Francesca
FCC Week 2017, 1 June Berlin
FCC Week 2017, Berlin

11. Specular Reflectivity VS Roughness
1/06/2017
REFLEC simulations
Here we show REFLEC simulations of Specular reflectivity vs roughness, for a Copper sample at FCC incidence angle and for four different values of roughness. We can see that increasing roughness from 0 to 100 nm the specular reflectivity drops down. However we must note that as the roughness increases also the scattered light contribution increases. 11
Eliana La Francesca
FCC Week 2017, 1 June Berlin
FCC Week 2017, Berlin

12. Reflectivity
1/06/2017
X-Ray Reflectivity depends on a limited number of parameters:
Photon energy and light polarization
Angle of incidence
Surface roughness
Material 12
Eliana La Francesca
FCC Week 2017, 1 June Berlin
FCC Week 2017, Berlin

13. Specular Reflectivity VS Material
1/06/2017
Specular Reflectivity VS Material 13
REFLEC simulations
Reflectivity depends on materials. Here you can see calculated specular reflectivity at fixed roughness and incidence angle for three typical technical bulk materials, such as aluminium, copper and stainless steel. We can see all the drops are due to the existence of absorption edges in materials.
The value and position of absorption edge depends on material atomic structure and composition. So ideally if you want to obtain a high reflectivity in x-ray regime you should find a material in which has no absorption energy in the same range. This is the Carbon case.
Eliana La Francesca
FCC Week 2017, 1 June Berlin
FCC Week 2017, Berlin

14. Specular Reflectivity: the case of Carbon
1/06/2017
REFLEC simulations
Attenuation depth (𝜆):
P(x)=exp(-x/𝜆)
For x= 𝜆 the intensity of X-rays
falls to 1/e of its value at
the surface.
R. Cimino, V. Baglin and F. Schäfers, PRL. 115 (2015)
20 nm of C can reflect all photons
𝜆(C) ~ 3.5 nm (X-ray range)
Here you can see reflectivity vs energy for different coating materials. The carbon coating is characterized by highest reflectivity. If you calculate the attenuation depth of the X-ray at the incidence angle of FCC you find out that 20 nm of Carbon are enough to reflect all photons at such grazing angle. So we can think of solutions with accommodating impedance issues. 15
Eliana La Francesca
FCC Week 2017, 1 June Berlin
FCC Week 2017, Berlin

15. BESSY-II Optic Beamline and Reflectometer
1/06/2017
With the aim of cross check all these simulations we went to Bessy II optic beamline, which is designed to measure Reflectivity for optical elements.
A.A.Sokolov,et al,Proc.of SPIE92060J-1-13(2014)
Eliana La Francesca
FCC Week 2017, 1 June Berlin
FCC Week 2017, Berlin

16. BESSY-II Optic Beamline and Reflectometer
1/06/2017
Incidence angle θ: -90° – 90°
Detector in-plane 2θ: -180° – 180°
Detector off-plane χ : -4° – 4°
Sample – detector: 310 mm
Six axes sample positioning
Sample current measurement
GaAsP-Photodiodes
Detector slits, pinholes
We don’t go into details, these are the specifics of setup we used 16
A.A.Sokolov,et al,Proc.of SPIE92060J-1-13(2014)
Eliana La Francesca
FCC Week 2017, 1 June Berlin
FCC Week 2017, Berlin

17. Bessy II Measurements Photo Yield: PY= Ne/N𝜸
1/06/2017
Photon Energy range 35÷1800 eV
Beam height h=0.3 mm
Incident Beam measurement
GaAsP Photodiodes (4x4mm) (1.2*4mm)
Incidence angle 0.25, 0.5 deg
Reflectivity measurement
Photo Yield:
PY= Ne/N𝜸
Specular Reflectivity
Scattered Light
Specular
Total
Detector
𝜽 𝒓 =𝟐 𝜽 𝒊
This is a scheme of the setup we used: photon energy ( eV), we keep the beam height as small as possible to allow a low grazing angle of incidence, we use two photodiodes, same material but different acceptance, and then we performed reflectivity measurements both in specular (placing the detector at the geometrical theta/2theta angle) and scattering position.
In every measurement we registered Photo Yield too.
Mirror
𝜽 𝒊 17
𝜽 𝒊
Eliana La Francesca
FCC Week 2017, 1 June Berlin
FCC Week 2017, Berlin

18. Cu 1A Copper samples AFM (20x20µm2) Cu 1A Cu 2A Polished 78.4 nm
1/06/2017
AFM (20x20µm2)
Cu 1A
78.4 nm
Cu 1A
Cu 2A
Sample
RMS Roughness (Ra)
Cu 1A
10 nm
Cu 2A
30 nm
We measured different copper samples with different roughness, with and without carbon coating. These samples have been characterized by means of AFM to measure surface properties, such as roughness.
I will show you first the data of 10 nm roughness sample measured at different angles.
Polished 18
Eliana La Francesca
FCC Week 2017, 1 June Berlin
FCC Week 2017, Berlin

19. Copper sample Cu1A and REFLEC simulations
1/06/2017
Tech. Surf
C K edge
REFLEC
At grazing incidence angle
contaminants are 
influencing Cu Reflectivity
O K edge
BESSY II
Cu L2,3 edge
qi=0.25 deg
REFLEC
Here we show reflectivity measurements for a copper sample (Cu1A, blue line). Data are compared with reflec simulations for pure copper and pure carbon. We can see that our sample consists in bulk copper with some coating of carbon and oxygen, due to air exposure. These are evident from absorptions edges occurring near 280 and 540 eV respectively. Also in metal without any additional coating we can see carbon and oxygen typical reflectivity contributions. This behaviour is significantly different with respect to bulk material. This is observed at 0.25 degree, for more grazing angles this aspect will increase. 19
Eliana La Francesca
FCC Week 2017, 1 June Berlin
FCC Week 2017, Berlin

20. Specular Reflectivity VS Photon energy (Preliminary Results)
1/06/2017
Tech. Surf
Cu 1A
Cu L2,3 edge Cu
C K edge
Carbon coating reduces
reflectivity at low energy
O K edge
Cu + CC
Cu 1A + CC
At high energy reflectivity
is significantly enhanced
Here there is the comparison between copper data (on the top) and copper + Carbon coating data (bottom). We can still see the presence of Oxygen edge, while Copper absorption disappears.
But this is only a part of the story, in fact this is only the specular reflectivity. (a voce direi dove la SR induce più’ HL) 20
Eliana La Francesca
FCC Week 2017, 1 June Berlin
FCC Week 2017, Berlin

21. Total Reflectivity VS Specular Reflectivity
1/06/2017
Ra= 10 nm
Spec. Reflectivity
specular reflection 21
Reflectivity
twotheta (deg)
100
10-1
10-2
10-3
10-4
10-5
10-6
𝜽 𝒊 = 5 deg
small angle
scatter
wide angle
Ra=0.5 nm
hn=1500 eV
Sample
Specular Reflectivity
Total Reflectivity
Cu 1A
0.61
0.73
Cu 1A + CC
0.78
0.90
If we consider even the scattered light and if we compare this results with a Silicon mirror we can see that in this case almost all photons are concentrated within a very small region. Even in a good sample with 10 nm of roughness, we can see a significant difference between specular and total reflectivity, specially in sample with Carbon coating. This can be explained with the possibility that carbon coating increase roughness, but we have to verify it.
Eliana La Francesca
FCC Week 2017, 1 June Berlin
FCC Week 2017, Berlin

22. Photo Yield VS Incidence angle
1/06/2017
𝜽 𝒓 =𝟐𝜽
hn= 1800 eV
Tech. Surf 𝜽
PY= Ne/N𝜸
Sample
Cu 1A
(Ra=10 nm)
Max value
Cu 2A
(Ra=30 nm) Cu
0.47
0.46
Cu + CC
0.23
0.15
Preliminary Results:
little dependence on roughness
Carbon coating seems to reduce PY
Here we can see Photo yield VS Incidence angle for two different samples with different roughness and the comparison between copper data (full line) and copper + Carbon coating data (dashed line)
Eliana La Francesca
FCC Week 2017, 1 June Berlin
FCC Week 2017, Berlin

23. Conclusions
1/06/2017
At FCC-hh SR incidence angle contaminants will influence Reflectivity.
Carbon coating seems to increase Total Reflectivity and reduce absorption and related Heat Load.
For technical surfaces scattered light cannot be neglected.
Photo Yield does not seem to significantly depend on roughness and decrease with CC.
Our preliminary results suggest that further work is needed in order to qualify the use of Carbon Coating to increase Reflectivity.
Experimental data are important to characterize SR behaviour and HL for all materials to be used in FCC-hh dipoles and Interaction points.
In FCC-hh SR will impinge on the accelerator wall at a very grazing angle. In this geometry surface contaminants (Carbon, Oxygen, physisorbed gas) will influence reflectivity
Carbon can increase the Total Reflectivity as predicted and reduce absorption and related Heat Load
All technical surfaces have enough roughness so that light scattering cannot be neglected. This means that in simulations it is important to consider Total Reflectivity and not just the calculated Specular Reflectivity.
Our preliminary Results suggest that further work is needed
- if one wants to pursue the possibility of reducing HL on cold surfaces by using a highly reflecting carbon coating.
- if one just want to characterize SR behaviour and absorption in FCC-hh dipoles and Interaction points. 23
Eliana La Francesca
FCC Week 2017, 1 June Berlin
FCC Week 2017, Berlin

24. Thank you for your attention.
Eliana La Francesca
FCC Week 2017, 1 June Berlin