[time-nuts] Rubidium cell or lamp rejuvenation.

[cdelect at juno.com]

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.