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Message ID: 1191     Entry time: Thu Jan 15 18:27:02 2015
 Author: Zach Type: DailyProgress Category: SiFi - ringdown Subject: Sapphire washers added, ringdown setup rebuilt, higher Q measured

[Nic, Zach]

Yesterday, we opened up the small cryostat and installed the sapphire washers (SwissJewel SP-175). This is hypothesized to increase the resonator Q by reducing the strain energy leaking into the lower-Q steel clamp.

We found that the inner diameter of the washers is slightly too small to accomodate the inner lip of the lower part of the clamp. We were able to make do just by having the lower sapphire washer sitting on this lip---rather than on the full wider area of the lower clamp section---but it is not ideal.

Nevertheless, we clamped it, resealed and pumped the chamber down. As it pumped, I rebuilt the HeNe optical lever readout. When I finished, I was quickly able to tap the cryostat and see a mode ringing at almost exactly 250 Hz, which is known to be the frequency of this cantilever at room temperature. At a respectable pressure of several x 10-5 Torr, I made a quick-and-dirty ringdown measurement using a scope and a stopwatch. I estimated $\tau$ at roughly 2.5 seconds, giving Q ~ 2000. This was already a few times higher than Marie was able to measure at room temperature (see below).

I went down today and did an actual measurment, using the Zurich box sampling at 7 kHz as DAQ. Fitting the envelope by eye, I found a time constant closer to $\tau$ = 5.55 s, giving Q ~ 4300 (I don't think my stopwatch method was all that wrong yesterday, but I do think the residual gas might have been contributing at the time---the pressure is now at 10-7 Torr). This is not only much better than the previous result, but also within a factor of less than 3 of the expected result for Si, according to Marie's data. Given how cavalier we were with the clamping, I'm fairly confident that the sapphire washer idea (and therefore also the monolithic thicker-clamp idea) works as intended.

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