[time-nuts] Maser info (vacuum levels)

J. Forster jfor at quik.com
Thu Sep 2 18:14:03 UTC 2010

Back to the Palladium plug for a minute. The problem you mentioned about
the "valve" not shutting off completely is analogous to the problem with

In a HeCd LASER, there is an oven with Cd metal in it that is heated to
provide Cd ions to the discharge tube. There is also He in the tube. The
balance between the partial pressures of He and Cd needs to be within
fairly close tolerances to sustain suffcient population inversion for the
thing to lase.

Well, the electric field causes Cd to migrate to the negative end of the
tube and go splat against the cathode, entrapping He atoms, just like a
sputter pump. This depletes the He in the tube and it stops working, so
they add a He reservoir with a heated glass diaphragm as a valve to
replenish the He in the discharge tube.

Problem is, the diaphragm leaks He, even when the thing is shut off at
room temperature. And, it uses up Cd to sputter pump He when you turn it
back on.

Adjusting one of these beasts is non-trivial. You have to get the He/Cd
mix right (by looking at the strength of spectral lines) then adjust the
mirrors at both ends of the resonator. It takes a while.  :))




> This EFOS maser typically runs with the two vacuum pressures below 1.5 X
> 10-6 Torr. (as measured via the ion pump current)
> Maximum should not exceed about 3.6 X 10-6 Torr for either pump.
> The internal vacuum will drop to about 1 X10-7 Torr if the Hydrogen to
> the disassociator is turned off.
> One pump pumps the "internal" which is the maser bulb, the disassociator,
> and the connecting lines, so mainly just pumps Hydrogen. (fairly low
> volume)
> The "external" pump pumps the insulating outer shell and the interior of
> the maser cavity. It pumps any leakage or outgassing that occurs. (much
> larger volume)
> Pumps are 20 LPS varian triodes.
> Corby Dawson
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