[time-nuts] Hydrogen maser spin exchange
kb8tq at n1k.org
Sat Jun 18 15:02:02 EDT 2016
Far more than you *ever* wanted to know about state selectors:
> On 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.
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.
More information about the time-nuts