It’s the combination of the DC plus high RF circulating voltages in the cavity. It’s especially bad in the 35kW units. Keep the plate voltage below 12kV and it’s less likely to happen.
From: John Stortz [mailto:email@example.com]
I worked on an FM-30T before retiring. The bellows used to get what used to appear to be random holes burned thru the thin bellows copper. BE was aware of the problem but said
they didn't know why it happened.