[time-nuts] Hydrogen maser spin exchange

paul swed paulswedb at gmail.com
Fri Jun 17 20:43:35 EDT 2016


Ole I can only point you toward cesium references I remember reading about
the magnet configuration and it did matter. Sorry it was a while ago.
Others may know the reference. It was the HP 5060  as I barely recall.

On Fri, Jun 17, 2016 at 6:18 PM, Ole Petter Ronningen <opronningen at gmail.com
> wrote:

> 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.
>
> Ole
>
> 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
> > WB8TSL
> >
> > 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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