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AdV-TCS (TCS studies actuation (simulations + experiments))
izumi, chiummo, allocca - 19:16 Sunday 18 December 2016 (35867) Print this report
common CO2 exploration

This is a quick summary of today's TCS fun, but in short, we are confused. Look at the attached pictures.

The goal of the day was to change the common CO2 setting while checking the interferometer properties such as carrier's recycling gain, mode matching and contrast defect. At the beginning, we did a quick test where we let CITF freely swing. This test showed something unexpected to us -- the recycling gain decreased as we increased the common CO2 power. Later, we further investigated this by fully locking CITF on the carrier. We will perform careful analysis tomorrow, but it seems that CITF consistently drops the recycling gain as we increase the common CO2 power. By the way, it would be extremely helpful if we could get a power sensor for each CO2 beam so that we don't have to go through an offline calibration.

In addition, one thing we immediately noticed was the beam shape. The size of the beam resonating in CITF significantly increased which is opposite of what we have thought. This needs further theoretical investigations. We are leaving the interferometer without the CO2 lasers and both arms locked.

Images attached to this report
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genin - 21:14 Sunday 18 December 2016 (35868) Print this report

This afternoon I have been called by the commisioning team since the Injection system was very unstable. In order to solve this issue I have just disabled the full bandwidth control of the IMC cavity and I enabled it again after the RFC have been relocked.

It seems that this kind of instability we've already seen in the past might be triggered but the PR mirror direct reflection or some spurious light coming from that beam (To be investigated).

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mantovani - 15:18 Monday 19 December 2016 (35875) Print this report

It happened also now when aligning PR for PR-NI configuration. I leave for the PR-ITF lock the IMC AA in drift control.

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Izumi, Allocca, Chiummo - 18:02 Tuesday 20 December 2016 (35886) Print this report

Here are some plots and thoughts. No conclusion though.

35884_20161220173638_tcstestdec18.png

The plot starts at 16:38:00 UTC and the duration is 80 minutes. At the end we lost lock and never tried re-locking CITF. During this period, we changed the CO2 setting, mostly in the common CO2 lens to see how CITF behaves. As shown in the plot, we had five decent lock stretches. The vertical dashed lines with a number represent the check points where we downloaded the B1 and B4 camera images to see what kind of mode was resonating in CIUTF. The camera images are shown below. Note that we kept manually aligning PR and BS trying to maintain a good axisymmetric mode.

 

 

35884_20161220172406_outputldafn5.gif

At the last check point or (7), the camera exposure time was significantly reduced as we noticed saturation. Unfortunately, all the other pictures are highly saturated.

 

Some observations:

  • Overall, having a common CO2 seems to have decreased the build up.
  • The beam shape became Lagguerre-ish between check points (2) and (4).
    • This may be correlated with the engagement of both CO2 lasers.
  • We had a highest build up of about 12 (mW?) at B4_DC at around check point (2).
  • After check point (2) the build up kept decreasing.
  • Meanwhile, B2_DC kept increasing.
    • Roughly speaking, this means less amount of input power is coupled to CITF. Bad.
  • Contrast defect at the beginning without CO2s was bad as expected (see 35843).
  • The contrast defect seemingly improved right after we engaged the CO2 lasers as expected.
  • However, the contrast defect then degraded between check points (4) and (5). This could be a degraded alignment of the beam splitter.
  • There is a sudden drop in B4_DC which seemed to be related to alignment.
    • However a funny thing is that according to the camera images, we seemed to have succeeded in reducing the 1st order modes by aligning CITF even though B4_DC dropped.
izumi - 18:47 Tuesday 20 December 2016 (35888) Print this report

Another plot.

I have filtered the CO2 power time series with a single pole low pass filter with an 1/e time scale of 850 sec. This should simulate the effective CO2 powers for forming the thermal lenses in the compensation plates. It seems that the highest build up was achieved when both lasers were around 20 mW.

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izumi - 15:40 Wednesday 28 December 2016 (35931) Print this report

Re-attaching the plots for the same reason. cryingcryingcrying

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