I thought this was interesting. It's from a 1952 book on TV repair, and the section I copied below explains some fundamentals on Ion traps. I am posting this as I see from time to time people mentioning a glow in a CRT neck, and mostly attribute this to gas. I wonder if in some cases these are the "Tube Three" variety. See below:

In positioning the ion trap, bear in mind that there are three types of picture tubes on the market.

Tube one has no internal markings to show where the ion trap should be placed. In such cases, the ion trap should be moved back and forth along the neck of the tube while rotating it around the neck in all directions until the light appears on the face of the tube. Continue to position the trap until the four corners of the raster appear. Then carefully move it back and forth until all segments of the picture are clear and well focused.

Tube two has two small metal flanges welded on the electron gun in the neck of the tube and visible through the glass. The ion trap should be placed over these flanges to start with and then carefully moved into the best position.

Tube three is a development of the Rauland Corporation which makes the adjustment of the ion trap quite a simple matter. A portion of the internal structure has a glow when the ion trap is in an incorrect position. This operates in the following manner: the aperture of the first anode is coated with a fluorescent material. When the ion trap is misadjusted, the electron beam is incorrectly aligned and some of the electrons strike the fluorescent substance, causing a glow. If the trap is correctly adjusted, the beam does not strike the anode and there is no glow. Thus, in this type of tube, simply adjust the ion trap until the glow completely disappears or is at an absolute minimum. The advantage of this system is that with the other two tube types it is necessary to watch the results of moving the ion trap by observing the picture as it is reflected in a mirror while one is working at the rear.

Anyone know if these CRT's were prevalent and ever caught on? I can't say I've ever seen one.

Look like nobody else has heard of this either!

I've seen an ad for them once, but never one in person.....

When I read your post , I thought "what a super idea !!!" .... Knowing what a pain it the butt it could sometimes be to find an Ion Trap's perfect position , I would have loved it if all the CRTs of the olden days had that ....

Now , that being said , , I'd never heard of it before your post , , and have no direct experience to share of it .... If you DO find one of these CRTs , and get good pictures of the purposfully glowin neck , I'd love to see em