Just how bad can an Ion burn be?

[benman94]

Ion bombardment definitely does do damage to the screen. Whether you notice it is a different story. Since the ions are deflected as well, the damage that would normally be a big nasty dot in the center of the screen is spread out over the whole screen. To get adequate brightness out of a damaged phosphor back you crank up the controls. Of course the cathode is wearing simultaneously as well, so eventually, the pairing of a damaged phosphor and a weak cathode gives the tell-tale shimmering, and eventually negative, picture. If you could isolate the effect of the ion bombardment though, let's say with some super long lasting cathode material, you would see detriment to the screen.

You're also correct in saying that on a 7JP4 you would expect to see the ion damage in the region of the previously scanned raster.

On the 10BP4 or 12LP4, it was assuredly damage from the electrons themselves; the ions were shot off into the wall of the neck. Remember that an electron in the region of ~2 keV in a lowly 7 inch Transvision to ~75 keV in some projection sets can and will do incremental damage to the phosphor. Ever seen a burned 5TP4? That was done by a beam of electrons at 25-27 keV over some extended period of time.