[time-nuts] COTS cesium standard physics package life

[John Miles]

> 1.  Cesium depletion, which only occurs when the tube is operating with
> cesium oven on, and high voltage at the other end; and

The Cs atoms need to be electrically neutral, so their depletion rate shouldn't depend on the presence of HV, just the oven heater.  State selection wouldn't work on ionized atoms, and you also wouldn't want them to be accelerated towards the electron multiplier until after they've reached the ionizer filament at the end of the cavity.  (The longer they spend hanging out with Schroedinger's cat in the Ramsey cavity, the narrower the line width.)

> 2.  Tube vacuum and other physical aspects that may deteriorate over
> time, whether or not the tube is operating.  The routine ion pumping
> helps keep the vacuum up in storage, but is not as effective as the
> continuous pumping that occurs during operation.

Some of the HP standards (5060, 5061, 5071) have a CS OFF setting that enables the ion pump by itself.  Those tubes should last indefinitely in that mode of operation.  Unfortunately the 5062C didn't have a way to disable the Cs oven while leaving the ion pump active, which is probably why there are so few operational 5062C tubes left.  By now, most are either out of cesium or too gassy for the ion pump to recover.  

I added a switch to  my 5062C to allow the vacuum to be maintained without running  the Cs oven.  It gets turned on every so often, maybe a couple of times per year, when I want a noisy signal source with known ADEV characteristics.  Even though the ion pump runs 24/7/365, there are always a few overcurrent cycles on initial powerup for some reason, where the beam current exceeds the trip point and shuts down the power supplies.  Something inside the tube is apparently outgassing during warmup -- whether it's the Cs oven or the hot-wire ionizer ribbon, I don't know.  But the condition always clears itself within a few  seconds.  The same thing happens when I turn my 5061s on after a long period in CS OFF.

- john, KE5FX
Miles Design LLC