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 <[email protected]> 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 < > [email protected] > > 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 > > _______________________________________________ > > 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. > _______________________________________________ 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.
