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I have heard that phosphors burn (age) like that on projection tubes because they're driven so hard. The burn in this case is from electrons not ions, and it's more pronounced in projection tubes because they're driven much harder than regular CRTs. It's aging of the phoshor, like when you see a burned-in image on a computer monitor or how the phosphor on a regular CRT will discolor or even be destroyed if you leave the brightness up with no vertical deflection.
I'm not an engineer and I don't play one on TV, so if anyone feels the need to correct me on this, please do!
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