The optimum mercury vapor pressure for production of 2537 .ANG. radiation to excite a phosphor coating in a fluorescent lamp is approximately six millitorr, corresponding to a mercury reservoir temperature of approximately 40.degree. C. Conventional tubular fluorescent lamps operate at a power density (i.e., typically measured as power input per phosphor area) and in a fixture configuration to ensure operation of the lamp at or about a mercury vapor pressure of six millitorr (typically in a range from approximately four to seven millitorr); that is, the lamp and fixture are designed such that the coolest location (i.e., cold spot) of the fluorescent lamp is approximately 40.degree. C. Compact fluorescent lamps, however, including electrodeless solenoidal electric field (SEF) fluorescent discharge lamps, operate at higher power densities with a cold spot temperature typically exceeding 50.degree. C. As a result, the mercury vapor pressure is higher than the optimum four to seven millitorr range, and the luminous output of the lamp is decreased.
One approach to controlling the mercury vapor pressure in an SEF lamp is to use an alloy capable of absorbing mercury from its gaseous phase in varying amounts, depending upon temperature. Alloys capable of forming amalgams with mercury have been found to be particularly useful. The mercury vapor pressure of such an amalgam at a given temperature is lower than the mercury vapor pressure of pure liquid mercury.
Unfortunately, accurate placement and retention of an amalgam to achieve a mercury vapor pressure in the optimum range in an SEF lamp are difficult. For stable long-term operation, the amalgam should be placed and retained in a relatively cool location with minimal temperature variation.
Commonly assigned U.S. Pat. No. 4,262,231 of Anderson et al., issued Apr. 14, 1981, which is incorporated by reference herein, describes situating a lead-tin-bismuth amalgam in an electrodeless SEF fluorescent lamp by wetting the amalgam to a metal wire structure, such as a helical structure or a cylindrical screen, which is fixed within the tip-off region of a lamp envelope. Alternatively, Anderson et al. describe melting the amalgam onto an indium-coated, phosphor-free portion of the interior surface of the lamp envelope.
Smeelen U.S. Pat. No. 4,622,495 describes another scheme for locating an amalgam within an electrodeless SEF fluorescent lamp by attaching an amalgam holder to a tubular indentation (hereinafter referred to as a re-entrant cavity) within the lamp envelope. Disadvantageously, this requires a glass-to-metal seal; and a reliable glass-to-metal seal is difficult to achieve in manufacturing.
Accordingly, it is desirable to provide a relatively simple method for locating an amalgam in an electrodeless SEF fluorescent discharge lamp which provides an optimal operating location for the amalgam, while not requiring a glass-to-metal seal or an internal amalgam holder. Moreover, the amalgam should be held in place during lamp manufacturing without significantly interfering with other lamp processing steps.