Thank you John. That was most informative. dana
On Tue, Nov 21, 2017 at 2:26 PM, John Ponsonby <[email protected]> wrote: > 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 > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/ > mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
