Winkout in HID discharges

Summary form only given. Metal halide HID lamps are so named because of the metal salts (usually iodides) that are included in the lamps to provide a high vapor pressure source of efficient radiators, viz. metal atoms. In the high temperature plasma, the iodides disassociate and some of the metal atoms react with the silica arc tube vessel to form involatile oxides. The iodine thus liberated can affect the subsequent operation of the metal halide lamp. Normally, the liberated iodine would combine with excess metal vapor (mercury) that is present in commercial metal halide lamps to form metal iodides. However, in reduced mercury discharges the iodine may remain nascent. In extremely mercury reduced metal halide discharges, we have observed the abrupt cessation of current flow during starting when the voltage available from the ballast is limited. This cessation of current flow or "winkout" is attributed to the rapid and uncontrolled vaporization of iodine from the cold electrode surface. The immediate vaporization of the condensed iodine crystals from the electrode surface produces a high density, electronegative plasma which cannot be sustained by the available ballast voltage. The presence of liberated iodine in the discharge vessel was verified by two techniques: microscopic observation of iodine crystals condensed on the cold electrodes, and by absorption spectroscopy of the iodine molecules vaporized when the vessel is heated.