Re: EFOS Maser turns 34!
In a hydrogen maser the hydrogen atoms have to be confined in a
storage-bulb within which the oscillating RF magnetic field is all in-phase.
The storage-bulb has to be made from fused quartz for mechanical stability and
for very low dielectric loss. The bulb has to be very thin walled, say 1mm to
keep the dielectric loading and the dielectric loss acceptably small. For an
active maser the product of the loaded Q-factor of the cavity and the "filling
factor" of the storage bulb has to exceed a certain minimum value. The filling
factor is a function of the shape of the storage bulb. Now fused quart is very
difficult stuff and it can only be worked white hot "in the flame" and very few
people can do it. It is much more difficult to work than ordinary glass
blowing. You can't expect a storage-bulb to be made very exactly to an
engineering drawing. As a result the cavity has to be machined to suit the
given storage-bulb. The guys at Oscilloquartz told me that after making a lot
of EFOS m
asers they eventually got their quartz bulb maker to make the bulbs
interchangeable between cavities but you can't expect such accuracy if you want
to make a one-off maser. The first time I had bulbs made the wall thickness was
far too great. The quartz people said thay had no way of measuring the wall
thickness. I said I can weigh the bulb and thus deduce the average thickness,
indeed it was obvious just holding it the hand that it was too heavy.
It turns out that the resonant frequency of the cavity is much more
critically dependent on its diameter than on its length. So it would be best to
be able to mount the bulb in the cavity and to measure the resonant frequency
with the cavity still in the lathe to avoid having to recentre the cavity each
time one needs to take off a few thou (mils to you in the US). The loaded Q
should be about 35,000 and the resonance is so narrow that one has to off-set
tune the cavity so that it is on-tune when it is in vacuum and when it is at
the chosen working temperature (40°C is a common choice). For an aluminium
cavity the resonant frequency shifts about one bandwidth per °C of temperature
change.
These are matters that need to be understood if one contemplates making
one's own H-maser.
John P
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