[time-nuts] Hydrogen maser spin exchange

Ole Petter Ronningen opronningen at gmail.com
Fri Jun 17 18:18:06 EDT 2016

There is indeed a quad- or hexa-pole state selection magnet between the
dissociator and the cavity, required to select the atoms of the correct
state to allow a population inversion resulting in oscillation.

My understanding is that the spin-exchange of interest takes place between
atoms already bouncing around inside the cavity - having passed that
particular obstacle.

And while on that subject, if anyone could elaborate on the difference
between the two (quad or hexa-pole), that would be interesting. I have not
found a clear explanation for why one might choose one over the other.


On Fri, Jun 17, 2016 at 11:35 PM, paul swed <paulswedb at gmail.com> wrote:

> Ole,
> Quite looking forward to the replies. Though no intention to own a maser
> currently.
> I thought there was a method of rejecting or reducing types of spin.
> Therefore reducing the impact you mention. Essentially a state selector.
> Pretty sure that thought will get corrected pretty quickly.
> Regards
> Paul
> On Fri, Jun 17, 2016 at 3:33 PM, Ole Petter Ronningen <
> opronningen at gmail.com
> > wrote:
> > Hi. Apologies for a long post.
> >
> > I'm trying to read up on the "care and feeding of hydrogen masers". While
> > they are conceptually simple from a distance, there's quite a bit going
> on
> > in the quantum mechanics department when looked at up close. Somewhat
> > frustratingly, I am not mentally equipped to really grasp the finer (or
> > even coarser) points of that particular department. The topic of this
> post
> > is the concept of spin exchange, and it's relation to cavity (auto)
> tuning.
> > I've read papers on the subject, but I am having difficulties building a
> > "workable intuition", so I turn to the group.
> >
> > Here's what I think I understand, and I respectfully ask for corrections
> if
> > I am way off base here..
> >
> > Spin-exchange in a hydrogen maser happens when two atoms collide, and
> > exchange spin, as it were.. (Hazy on the details here..) The number of
> > spin-exchange collisions is directly proportional to the density of atoms
> > in the cavity. These collisions *will* happen, but is a problem in
> hydrogen
> > masers for two reasons: 1) it takes away energy from the cavity,
> resulting
> > in lower signal output power, which degrades stability, and, 2) more
> > significantly, it results in a frequency shift.
> >
> > The frequency shift, as far as I can gather, is directly related to the
> > cavity resonant frequency - there is no way to *stop* spin exchange
> taking
> > place (apart from reducing the hydrogen density to a level where
> collisions
> > are rare, in which case the density will be too low for oscillation to
> take
> > place), but it is possible to reduce the impact the spin exchange has on
> > the output frequency.
> >
> > While the resonant frequency obviously influences the output power of the
> > maser cavity, the "mistuning" of the cavity also increases the effect
> spin
> > exchange has. In other words, in a perfectly tuned cavity, spin exchange
> > does not result in a frequency shift. In a badly tuned cavity, increasing
> > or decreasing the hydrogen flux (thereby increasing or decreasing the
> > number of collisions taking place) results in a corresponding
> > increase/decrease of the output frequency. Since the cavity ages, and the
> > cavity resonant frequency follows that aging, the long term stability of
> > the maser is degraded unless the aging can be compensated for. Which is
> > what cavity auto-tuning is all about.
> >
> > From my understanding, there are a few ways to implement cavity
> > auto-tuning:
> > 1. From the above, it follows that a modulation of the hydrogen flux
> into a
> > mis-tuned cavity will result in a frequency shift following the
> modulation
> > frequency. Using a stable reference, this shift can be measured, and
> > corrections can be made to the cavity varactor voltage. Once the output
> > frequency no longer shifts in response to the changes in hydrogen flux,
> the
> > cavity is correctly tuned.
> >
> > 2. It is also possible to modulate the cavity varactor voltage. By
> > measuring the output power of the cavity, an error signal can be obtained
> > and used to correct the average varactor voltage. A square wave of i.e.
> > 100hz, centered on the approximate correct varactor voltate is put in the
> > varactor, and cavity output power is measured. If the output power
> measured
> > on the "low" of the square wave is lower than the signal measured when
> the
> > "high", lower the offset by some mV, and vice versa. Suitable filtering
> > would of course be required.
> >
> > The idea is that this method should not result in appreciable degradation
> > of the short/medium term stability of the maser, because the frequency of
> > the atoms interacting with the electromagnetic field in the maser cavity
> > takes time to respond to the changes in the resonant frequency, but the
> > output power responds "instantly". (Hazy on those details as well..) By
> > modulating the cavity varactor voltage (much) faster that the time
> constant
> > of the maser cavity, the modulation is effectively filtered out.
> >
> > I am very interested in this method, as it seems to me that it would be
> > easy (feasible) to retrofit this to older masers never equipped with
> cavity
> > auto tuning.
> >
> > There is at least one more way, which involves injecting a signal into
> the
> > maser cavity through a second coupling loop. At least one vendor I know
> of
> > does this in their newest design. I do not understand even the basics of
> > this method.
> >
> > Any insights and/or corrections of my understanding is most welcome.
> >
> > Thanks,
> > Ole
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