How to clean RF coils?

Dear all,

we have not measured many samples, only some organic samples. Could someone please explain, what cause the RF coils change the color like this? Does it affect the plasma ignition? How to clean it properly? 

Is it correct, I take the coils off firstly, then scrub with alcohol. Maybe need aluminum oxide powder to polish it?

Does the cone need to be cleaned as well?

Thank you very much!

Best regards,


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