SPS-BGI Coating discussion

Name: SPS-BGI Coating discussion
Start: 2023-09-26T15:00:00+02:00
End: 2023-09-26T16:00:00+02:00
Location: No location set

Tuesday 26 Sept 2023, 15:00 → 16:00 Europe/Zurich

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Talking points:

Ti or NEG coating? According to minutes both are possible. The baseline, according to BI, was NEG coating, as it was already used for coating the ceramic plates for the LHC-BGI in 2006.
Protection of the laterals, ion trap and top-side?
VSC group can't make a thicker coating of the electrical feedthrough slot. Is there a solution for this item?
In the past, RF group noted that the coating of the ceramic rod should be 10-30 ohms / sq. VSC group proposes to coat it with the same metal as the one used for the top electrode. Is there an available drawing of this rod (or dimensions)?
Is it needed electrical contact between the ceramic rod and the metallic arms?

377 ohms is the vacuum impedance
The higher the vacuum pressure, the more ions are created and bombarded. Consequently, the coating layer can be brushed. In conclsuion, the thicker the layer, the longer lasting it is
The resistivity depends linearly on the coating thickness ( $\rho =R{S \over \ell }$ ). The thinner is the layer, the higher the resistivity
The thinner the coating thickness is, the higher is the probability that the coated surface, once outside a vacuum enviroment, oxidates. Consequently, it will become non conductor and the resistivity will increase
NEG is not as good conductor as Titanium
The roughness of the coated surfaces is an important paramaeter
The coating process for this dimension has a non-uniformity of around 10% on thickness of coating

Requirements

Material: MACOR
High surface resitivity:10-30 kohms/sq (>> 377 ohms, to make the cathode transparent to the RF calculation domain)
Operating at -30 kV continously (worst case scenario)
Thick coating layer

Conclusions

The coating material will be NEG
The target resistivity will be 10 kohms/sq
Resistivity will increase, but should stay below 30 kohms/sq
The laterals can be coated. For the feedthrough slot, a vacuum compatible silver glue will be used. Since the silver glue doesn't need to be cured after the application, we can first NEG coat the cathode and silver paint the slot later on
A study concerning the coating of the ion trap and top electrode will be done by Gunn Khatri, to analyse the possibility of a non coated surface in terms of operation of the ion trap
The bottom side of the electrode needs to be coated to provide the electrical field. Uniformity of the coating surface is not very important, because there is no current flow (10% is acceptable)

Requirements

Conclusions

The coating material will be Titanium, because it has been already validated and it is more robust to temperature increase
The target resistivity will be 10-30 ohms/sq
The coating on the sides of the rod will not be uniform (due to the fixation process). In any case, the surface in contact with the arm will be the cylindrical because the rod is embedded

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