The Negative-Ion Blemish in a Cathode-Ray Tube and Its Elimination

This paper is a critical review of the widely scattered and somewhat conflicting data regarding negative ions in cathode-ray tubes and blemish formations. The form of the ion blemish is a function of the form of focusing and scanning used, being a small central spot with electric focus and magnetic scan, and being a circle about an inch in diameter with magnetic focus and scan. The blemish is due to chemical poisoning of fluorescence by negative ions formed on or near the cathode. The paths of such ions are substantially unaffected by the magnetic fields normally used with cathode-ray tubes. The path followed by an ion through an electric field is, however, the same as that of an electron starting from the same point of rest. Thus the ion blemish has the same shape as would the electron spot were the magnetic fields only removed from the crt. The use of a backing layer such as aluminum reduces the blemish but does not eliminate it, apparently owing to porosity of the backing layer. Such a layer has, however, other beneficial properties not related to the blemish problem. The ion blemish can be eliminated by the use of an ion trap, which is a device usually located in or near the gun.