Stainless steel retains heat very efficiently, which makes welding it a bit more difficult especially for the novice welder. When faced with excessive welding heat, stainless steel can warp from the high temperatures and even distort during the cooling process.You should have all the tools with you ...

To me you are an expert. all i can say is that your starting point seems lilttle bulky. But Overall it is just wow.

All your ground contact should have good contacts. Ground connected surfaces should not be covered with any insulating coatings. Especially close the target and shielding rings.
There should be no ultra thin metal deposits on insulators used for mounting cathodes. They all should be clean.