[time-nuts] Cs beam cavity: why is it U shaped?

[Rick Karlquist]

The tube is like an interferometer.  Think of two telescopes
spaced apart x meters.  It has the resolving power of an x
meter diameter telescope.  It doesn't have the light gathering
power of an x meter telescope.

There is sufficient RF power to flip the state of all the Cs
atoms, so additional interaction time would not be helpful.
Also, the accuracy of the standard depends on the phase error
between the two ends.  (The big machines send the beam through
in both directions to cancel this out).  There is no way to
do this (AFAIK) if you had to excite the atoms over their
entire flight.  In the 5071 CBT, there are proprietary manufacturing
techniques that reduce the random phase error to parts in
10^13 and the systematic phase error to parts in 10^14, or
so I have been told.

This cleverness is the kind of thing that gets noticed in Stockholm.

Rick Karlquist, N6RK


Attila Kinali wrote:
> Moin,
>
> For some time now, i'm wondering why the microwave cavity of Cs
> beam standards is U shaped. Ie why does the Cs beam fly first
> trough the first "sub"cavity, leaves it, flies a substantial
> length trough free space, passes the second "sub"cavity and
> then goes to the detector.
>
> If the interaction time with the microwave field would be an
> issue, i would expect the beam to pass trough a longer stretch
> of the cavity, and not two time trough a short stretch that
> are widely spaced.
>
> Unfortunately, none of the papers i've read has shed any light
> on this, and google isn't helpfull either.
>
> Could anyone here enlighten me?
>
> 			Attila Kinali
>
> --
> Why does it take years to find the answers to
> the questions one should have asked long ago?
>
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