[time-nuts] H-maser drift
k8yumdoober at gmail.com
Tue Nov 21 17:47:56 EST 2017
Thank you John. That was most informative.
On Tue, Nov 21, 2017 at 2:26 PM, John Ponsonby <jebponsonby at gmail.com>
> There seem to be a lot of misunderstandings about H-masers. To set the
> record straight note:
> 1. The flow of hydrogen is generally controlled using a palladium
> membrane, though a palladium-silver alloy is to be preferred because it is
> less likely to crack. Only hydrogen will diffuse through the
> palladium-silver membrane, so as well as being a temperature controlled
> regulator it is also a filter. Indeed it is an isotopic filter through
> which even deuterium doesn’t pass. The protons are thought to migrate
> through the membrane and recombine on the output surface first into atoms
> and then into H2 molecules. I used thin walled palladium-silver tubes which
> had roughly the dimensions of a match stick. Hydrogen on the inside was at
> about twice atmospheric pressure with output into “vacuum” on the outside.
> Control is by heating with a large current flowing along the rather low
> resistance tube. Russian H-masers use nickel tubes rather than the more
> expensive palladium-silver. Such a “palladium leak” requires only a few
> seconds on Turn-On to settle to a steady flow.
> 2. Hydrogen from the "palladium leak” passes to a “dissociator" which is a
> small bulb made of heavily boronated glass, e.g. Pyrex, in which the H2
> molecules are dissociated into H atoms by a non-contacting RF discharge.
> Atomic hydrogen recombines very readily on any metal surface so the
> discharge is either by magnetic or electric field acting through the glass
> wall. Metals are charactersised by having conduction bands full of free
> electrons. Boron is an electron acceptor, so Pyrex is very unlike a metal
> and it has a low surface recombination rate. Not as low as FEP120 (See 5.
> below) but one can’t line a discharge bulb with it.
> 3. The very high Q RF cavity (loaded Q ≈ 36000), which is tuned very
> exactly to the hydrogen frequency of 1,420,405,751Hz, operates in the TE011
> mode in which the oscillating RF magnetic field is toroidal, going up the
> middle and down the outer part of the cavity. The resonant frequency is
> much more sensitively dependent on the cavity diameter than on its length.
> 4. Inside the cavity is the "storage bulb" which is made not of glass but
> of fused quartz. It is typically about 1mm thick. Fused quartz is chosen
> for its exceptionally low RF loss tangent. But of course it has a
> dielectric constant which results in its loading the cavity which is thus a
> little smaller than one first thinks. Since it is very difficult to
> manufacture quartz bulbs to normal engineering tolerances it is not
> possible to calculate how much the cavity will be loaded. So it is not
> unusual to manufacture the cavity to match the given storage bulb.
> 5. The inside of the storage bulb is coated typically with a layer of
> FEP120, a Dupont product akin to Teflon. An H atom can make of the order of
> 10,000 bounces off its surface without change of quantum state. Also H
> atoms won’t stick to the coating. (Non-stick frying pans are coated with
> FEP120 and what is true for an egg is true for an atom.)
> 6. The shape of the storage bulb should be chosen to maximize the “filling
> factor”. This is defined as: η’=Vb<Hz>^2b/Vc<Ha^2>c Here the numerator is
> the product of the storage bulb volume Vb times the square of the mean of
> the z component of the RF magnetic field Hz averaged over the internal
> volume of the bulb b, and the denominator is the product of the cavity
> volume Vc times the mean of the square of the magnitude of the RF magnetic
> field Ha averaged over the entire volume of the cavity c. A spherical bulb
> is non-optimal though may early masers had spherical storage bulbs.
> 7. The RF discharge generates UV. This shines up the beam path and
> illuminates the bulb coating in the region where the incoming atoms first
> make contact with the bulb coating. This UV undoubtledly damages the FEP120
> coating. The deterioration of the coating may be one of the causes of long
> term drift.
> John P
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