The following references describe the main failure mode of Rubidium lamps which is Rubidium depletion. If the thermal design is correct and the oven and supporting circuitry are working properly any condensed Rubidium will be redistributed (back into the tip-off region) during operation. On the 5065A I have seen two completely dead bulbs that were caused by Rubidium depletion. Dana, as far as why the liquid Rubidium does not appear to be moved by gravity I can only assume that surface tension holds it in place. On the two lamps I have seen with Rubidium on the surface of the bulb the Rubidium definitely melts when using the heat gun. You can see it wiggle if shaken but it will not run down into the tip by gravity. You have to heat the envelope hot enough to vaporize the Rubidium and "chase" it back down into the tip. Also I have operated several 5065A for years with the optical unit sitting vertically on the bench. This position puts the tip-off at the top and did not cause the Rubidium to migrate down into the envelope! I suspect any observed Rubidium on the bulb envelope to be caused by excessive storage temperature or oven problems. The older blue optical units are particularly prone to oven problems.
Cheers, Corby https://apps.dtic.mil/dtic/tr/fulltext/u2/a494259.pdf page616 The lamp network presents a range of loads as a function of lamp mode, and the circuit has been characterized under these various conditions to ensure proper operation. Starting may require the exciter to redistribute condensed rubidium inside the lamp by rf induction heating. https://apps.dtic.mil/dtic/tr/fulltext/u2/a150787.pdf page 11 The excess metal charge is constrained by thermal gradients in the tip-off region. The tip-off is formed when the lamp is "pulled" off the vacuum manifold. The lamp envelope is mounted in a metal base with a thermal potting material. The thermal potting material facilitates the transfer of heat from the base, which is the point of temperature control, to the glass envelope. The metal atoms in the vapor are due to the saturated vapor pressure above the metal charge, which is determined by the temperature of the metal, and thus good temperature control is essential for stable lamp operation. http://www.wriley.com/A%20History%20of%20the%20Rubidium%20Frequency%20Sta ndard.pdf page 5 The classic problem with Rb lamps has been life and reliability due to rubidium depletion as the excess Rb diffuses into the glass envelope. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
