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.
Cheers
John P
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