[time-nuts] Cs beam cavity: why is it U shaped?
magnus at rubidium.dyndns.org
Tue May 1 18:48:09 UTC 2012
On 05/01/2012 06:47 PM, Attila Kinali wrote:
> 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?
In combination with Tom's link to the Noble lecture, the U-shaped form
is really a bent transmission line such that the same source provides
the same signal. It's also important that the phase delay from the
source to both branches be very closely matched. Miss-align them and you
get a systematic frequency error as a result. Notice how the RF
interaction has the RF field being oriented orthogonally to the
direction of the beam.
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