[Reece]

Ions are atoms of material off the cathode that ride along the electron beam. Electrons are deflected by magnetism, but ions aren't. Older tubes had their electron guns "bent" so they didn't shoot straight. The ion trap had magnet(s) that bent the beam back straight during operation of the tube. Since ions aren't bent by magnetism, they went off at the preset angle and impacted the side of the tube neck, doing no damage. Newer tubes have the electroluminescent phosphor coated on the back side with a thin coating of aluminum, which deflects the ions. Their electron guns are aimed straight at the screen, so no ion trap is required. Someone else may give a more technical explanation.