1. Paul Edwards Supervisor: Pedro Costa Pinto
Production of carbon film coatings for the SPS to mitigate the electron cloud effect
Paul Edwards
Supervisor: Pedro Costa Pinto

2. But the beam pipes of the SPS are non bakeable,
Electron clouds, formed via secondary electron multipacting emission, in the SPS are a major limitation for delivering beams to the LHC that will enable the machine to achieve its ultimate luminosity.
Thin film coatings have been shown to be very effective at mitigating electron clouds in particle accelerators:
i.e. NEG coatings in the LHC beam pipes provide a max secondary electron yield (δmax) of 1.1 after activation at 180°C
But the beam pipes of the SPS are non bakeable,
>>>> Carbon films a viable alternative?
Possible deposition techniques:
DC Circular Magnetron sputtering (DCCM)
Plasma Enhanced Chemical Vapour deposition (PECVD)
DC Planar Magnetron sputtering (DCPM)
DC Hollow Cathode sputtering (DCHC)
Paul Edwards TE/VSC/SCC 08/04/2011

3. Paul Edwards TE/VSC/SCC 08/04/2011
So far, using DC cylindrical magnetron (DCCM) sputtering, trial carbon coatings have shown positive results both in laboratory and accelerator environment tests.
δmax of ~1 (threshold limit for SPS = 1.3)
Minimal change after prolonged exposure to air
No degradation of the coating after 2 years of operation in the SPS (including 3 months air exposure during shutdowns)
Paul Edwards TE/VSC/SCC 08/04/2011

4. Paul Edwards TE/VSC/SCC 08/04/2011
DCCM sputter coating system
In DCCM,
a uniform electric field is applied between the graphite cathode and the positively earthed beam pipe to be coated.
At the same time, perpendicular to the electric field, a radial magnetic field is applied in the z-axis.
An inert gas (neon) is pumped into the system and ionized in the E-field
Ne+ ions then impact upon the graphite cathode removing neutral charged carbon atoms and secondarily emitted electrons.
The see are instantly trapped by the B & E field combination and orbit the graphite target.
This enhances the gas ionization efficiency resulting in a dense stable plasma.
Carbon atoms condense on the beam pipe surface and film growth results
Paul Edwards TE/VSC/SCC 08/04/2011

5. Difficulties with DCCM
DCCM sputter coating
Difficulties with DCCM
High outgassing from beam pipe, vacuum chamber and graphite target itself results in significantly raising the δmax of the carbon film!
Solution:
Use a cathode assembly with getter assisted deposition.
Paul Edwards TE/VSC/SCC 08/04/2011

6. Paul Edwards TE/VSC/SCC 08/04/2011
DCCM sputter coating
RGA analysis of plasma during film growth
with NEG simultaneously
The deposition temperature for these coatings was 300C except for MBA 9 (the triangle) which was 200C
Similar curve plotting δmax against CO signal was obtained
Thickness of the coatings was around 900 ± 50 nm
Paul Edwards TE/VSC/SCC 08/04/2011

7. Paul Edwards TE/VSC/SCC 08/04/2011
DCCM sputter coating
Effect of varying substrate temperature
300 C
Films grown at less power, and hence lower substrate temperature, have an elevated δmax, but a smoother surface
200 C
Paul Edwards TE/VSC/SCC 08/04/2011

8. Paul Edwards TE/VSC/SCC 08/04/2011
Using NEG assisted DCCM the technology now exists to coat the SPS beam pipes, but a major problem with DCCM is in order to coat the beam pipes, they have to be removed from the magnets and then re-inserted after coating, greatly increasing the time and cost of the whole process.
Is there a better solution?
Paul Edwards TE/VSC/SCC 08/04/2011

9. Plasma Enhanced Chemical Vapour Deposition (PECVD)
Carbon coat chamber via the dissociation of acetylene in DC glow discharge
In PECVD carbon film deposition is achieved by dissociating a hydrocarbon source gas, in this instance acetylene (C2H2), in a glow discharge.
This chemical activation is accomplished by supplying a direct current bias to the gas. Resulting in the breakdown of the gas and formation of a glow discharge plasma, consisting of ions, electrons and electronically excited species.
The energetic electrons in the plasma ionise the gas. A large proportion of the gas is chemically activated by the electrons which results in the dissociation of the molecules into radicals.
Paul Edwards TE/VSC/SCC 08/04/2011

10. Plasma Enhanced Chemical Vapour Deposition (PECVD)
PECVD film growth mechanism
C2H2 + 2e C2H+ + H+
Dissociation of acetylene
Abstraction of hydrogen, formation of H2
Chemisorption of ethynyl
Carbon film growth
In the DC plasma, the acetylene is dissociated and ionized into ethynyl and hydrogen radicals which impinge on the cathode chamber wall.
As the film grows, atomic hydrogen abstracts surface hydrogen from the film forming H2 and creating dangling bonds that enable chemisorption of ethynyl resulting in film growth and effectively adding carbon to the lattice.
Ion subplantation also occurs due to the smaller H ions penetrating deeply into the film. These can again abstract H from C-H bonds creating subsurface dangling bonds. Some of these dangling bonds will be re-saturated by H.
Paul Edwards TE/VSC/SCC 08/04/2011

11. Plasma Enhanced Chemical Vapour Deposition (PECVD)
δmax typically ~ 1.5 independent of the parameters used during deposition
No matter how the parameters were changed in the coating process, films always had an either an sey of ~1.5 or were non-conducting with an sey of ~1.4.
The high sey is believed to be due to the presence of large amounts of hydrogen inherent in the coating
PECVD
Max Power, W
Bleed pressure, mbar
Max Bias, V
Thickness, µm
Max SEY 49
450
2.0e-1 ± 0.2
900
1.2 ± 0.3
1.54
11/15 30
600
1.0 ± 0.3
1.43 50
1.4e-1± 0.2
1.5 ± 0.3
1.40 51
2.0e-1± 0.2
0.3 ± 0.3
1.50 52
100
1300
0.6 ± 0.3
1.52 53
162
1.0e-1 ± 0.2
1100
Paul Edwards TE/VSC/SCC 08/04/2011

12. DC Planar Magnetron Sputter coating. (DCPM)
A magnetron sputtering system that fits inside the beam pipe?
Use an array of small, circular, strong permanent Samarium Cobalt magnets housed in a slender vacuum sealed container with graphite targets mounted on the top and bottom of the assembly.
B field along the strip traps the electrons results in concentrated plasma rings
Use a small motorized oscillating movement to ensure an evenly distributed plasma (uniform plasma erosion of the graphite target)
Paul Edwards TE/VSC/SCC 08/04/2011

13. DC Planar Magnetron Sputter coating. (DCPM)
Trial coating with a 300mm long prototype shows encouraging results, with the carbon film yielding a δmax of 0.98 with no change after 3 months ageing in air
Paul Edwards TE/VSC/SCC 08/04/2011

14. DC Hollow Cathode Sputter Coating. (DCHC)
Generate a high density plasma without the need of a magnetic field
Use as the cathode an array of rectangular graphite cells
Electrons are trapped between the walls of each cavity and oscillate in a pendulum-like motion
Results in a highly ionized plasma
Paul Edwards TE/VSC/SCC 08/04/2011

15. DC Hollow Cathode Sputter Coating. (DCHC)
Trial runs with a 400mm long prototype have given films that yield a δmax 0.97
Major obstacle is finding a way to uniformly distribute the discharge within the cells
Paul Edwards TE/VSC/SCC 08/04/2011

16. Paul Edwards TE/VSC/SCC 08/04/2011
Conclusions
Carbon coatings are in the last stages of development
DCCM method is ready to be implemented, but will be expensive
DCPM and DCHC show promise, but full scale models need to be assessed.
Paul Edwards TE/VSC/SCC 08/04/2011

17. Paul Edwards TE/VSC/SCC 08/04/2011
Conclusions
The influence of plasma contaminants and the effect of ion bombardment (as well as deposition temperature) during coating needs to be investigated so as to optimize the coating process
DCPM and DCHC coatings have low δmax yet high O content
Is there a link between δ and purity?
Ion bombardment from the bleed gas is enhanced in DCPM and DCHC depositions because of the short distance between the cathodes and the substrate. This ion bombardment could possibly cause hydrogen abstraction from the coating (c.f. PECVD). We believe there is a correlation between δ and hydrogen. With dehydrogenation, we can still reach a low δ even if the O% is high.
More study on the films ageing process is also of paramount importance
Paul Edwards TE/VSC/SCC 08/04/2011

18. Paul Edwards TE/VSC/SCC 08/04/2011
Kudos to
Pedro Costa Pinto
References/Additional reading
M. Davister, R. Locht The Dissociative ionization of C2H2. The H-C2H binding energy. Advances in mass spectrometry 13 (13th IMSC Budapest 94), (1994), pp.172
J. W. A. M. Gielen, W. M. M. Kessels, M. C. M. van de Sanden,a) and D. C. Schram Effect of substrate conditions on the plasma beam deposition of amorphous hydrogenated carbon J. Appl. Phys. 82 (5), 1 (1997), pp
Robertson Diamond like amorphous carbon materials science and engineering r37 (2002) pp
May Diamond thin films a 21st century material Phil trans r soc London a 358 (2000) pp
K. Nishimura, K. Ohya Secondary electron emission from hydrogen-implanted graphite with real depth profile of hydrogen Jpn. J. Appl. Phys. 32 (1993) pp
Y. Kishimoto, T. Ohshima, M. Hashimoto, T. Hayashi A consideration of secondary electron emission from organic solids J. Appl. Poly. Sci. 39 (1990) pp
Questions?
Paul Edwards TE/VSC/SCC 08/04/2011