After warming the ETMY up to 48 K, the finesse recovered to 1230(33).
Still, it have not recovered as the privious value as ~1450. Also the transmitted power is 65 % comparing to the value measured in last week. I did not check the consistency of this value yet, and will check afterwards.

Between July 25-26 (klog #9651) and July 30 when X arm finesse dropped, there was also a temperature change in ITMX (see atttached).
According to Saito-san and Uchiyama-san, this was due to recovery of a cryocooler for the duct shield.

For a record:

[X arm finesse]
1230(33) with ITMX 23.3 K, ETMX 48.0 K (July 31) https://klog.icrr.u-tokyo.ac.jp/osl/?r=9730
 (July 31: ETMX warm up started)
990(23) with ITMX 23.3 K, ETMX 22.7 K (July 30) https://klog.icrr.u-tokyo.ac.jp/osl/?r=9714
1428 with ITMX 29.0 K, ETMX 38.6 K (June 12) https://klog.icrr.u-tokyo.ac.jp/osl/?r=9215
1430 with ITMX 35.1 K, ETMX 25.1 K (June 8) https://klog.icrr.u-tokyo.ac.jp/osl/?r=9156
1440 with ITMX 37.0 K, ETMX 35.0 K (June 5) https://klog.icrr.u-tokyo.ac.jp/osl/?r=9097
1447 with ITMX 31.8 K, ETMX 80.7 K (June 3) https://klog.icrr.u-tokyo.ac.jp/osl/?r=9052
1450 with ITMX 30.9 K, ETMX 100.8 K (May 31) https://klog.icrr.u-tokyo.ac.jp/osl/?r=9033
1455 with ITMX 31.5 K, ETMX 106.5 K (May 30) https://klog.icrr.u-tokyo.ac.jp/osl/?r=9014
1447 with ITMX 43.5 K, ETMX 110.1 K (May 28) https://klog.icrr.u-tokyo.ac.jp/osl/?r=8999
Summary of measurements May 16-25, 2019 https://klog.icrr.u-tokyo.ac.jp/osl/?r=8977

[Y arm finesse]
1340 with ITMY RT, ETMY 295.5 K (July 11) https://klog.icrr.u-tokyo.ac.jp/osl/?r=9496
1309 with ITMY 27.2 K, ETMY 68.7 K (June 12) https://klog.icrr.u-tokyo.ac.jp/osl/?r=9211
1241 with ITMY 27.1 K, ETMY 79.5 K (June 10) https://klog.icrr.u-tokyo.ac.jp/osl/?r=9169
 (June 4: ETMY cooling restarted)
1315 with ITMY 26.2 K, ETMY 85.9 K (June 3) https://klog.icrr.u-tokyo.ac.jp/osl/?r=9052
1313 with ITMY 26.2 K, ETMY 85.6 K (June 2) https://klog.icrr.u-tokyo.ac.jp/osl/?r=9047
1313 with ITMY 26.2 K, ETMY 84.9 K (June 1) https://klog.icrr.u-tokyo.ac.jp/osl/?r=9040
1290 with ITMY 26.2 K, ETMY 83.3 K (May 31) https://klog.icrr.u-tokyo.ac.jp/osl/?r=9023
1317 with ITMY 26.6 K, ETMY 82.1 K (May 30) https://klog.icrr.u-tokyo.ac.jp/osl/?r=9014
1315 with ITMY 41.9 K, ETMY 41.9 K (May 28) https://klog.icrr.u-tokyo.ac.jp/osl/?r=8995
 (after May 25, ITMY cooling restarted)
Summary of measurements May 16-25, 2019 https://klog.icrr.u-tokyo.ac.jp/osl/?r=8977
 * Finesse was 184 with ITMY 24.2 K, ETMY 22.3 K (May 16)
 * Increasing ETMY temperature started on May 17
 * Finesse recovered only upto 400-440 by only increasing ETMY temperature to ETMY 33-60 K.
 * So, ITMY temperature increase started (around May 22)
 * Finesse recovered after increasing the ITMY temperature also (1312 with ITMY 65 K and ETMY 75 K (May 25))
See, also http://gwclio.icrr.u-tokyo.ac.jp/lcgtchief/20190617.html

-> These suggest that increasing and reducing ITMY temperature will recover the finesse (note that ETMY tempareture also changed from 60 K to 75 K during this process)
-> We haven't tried if finesse is OK with ETMY re-cooled down

Miyoki-san,

Thanks for your comments.

ETMY -> ETMX ?
Right. Just typo.

>According to this report about finesse recovery within the temp range less than 50 K, it might be also better to warm up ITMX to about 50 K.
I agree with you, but also we should check step by step.

I again measured finesse with the temperature of 51 K (ETMX). The result is 1240(45) almost same as privious measurement.

Finesse: 1184(34)
Temperature: ITMX: 23.3 K, ETMY 57.3 K

Finesse: 1128(17)
Temperature: ITMX: 23.3 K, ETMY 67.4 K

Slightly worse?

Finesse: 920(20)
Temperature: ITMX: 23.3 K, ETMY 67.4 K

Got worse and we are wondering the ITMX Tcam image seems strange. We are not sure if it's camera problem or real issue.

Finesse: 1397(20)
Temperature; 81 K, 45 K

- Finesse was recovered, and it might be due to warming the ITMX up.
- After ETMX warmed up, somehow the ITMX was contaminted and lower the finesse.
- We will cool ETMX down again and see what happens.

I plotted the history of finesse measurements upto klog #9825 today.





Note:

1. It seems like there was a trend of slight finesse decrease in X arm in May to June.

2. There was a ITMX temperature drop from 24.8 K to 23.3 K on July 27. According to Uchiyama-san and Saito-san, this was due to restart of cryocoolers for duct shields. According to Miyoki-san's klog #9821, Ar or O2 frosting temperature (23.8 K) is in between.

3. So, what caused the sudden finesse drop on Aug 5? When did the ITMX frosting happened? Are there Tcam images from the past?

One measurement in this morning.

Finesse: 1477
Temperature: 55.3 K (ITMX), 82.8 (ETMX)

- The ETMX drifted by 3 urad in yaw and 8 urad in pitch in last night
- For some reason, the GR TRANS degraded by factor of ~2.

- Although I forgot to mentioned about it in the 1PM commissioning meeting, but I was sure that the one drifted last night was the ITMY because the beam position on the ETMX also drifted.
- Also I checked the control signal of the DC control, and it indicated the drifting optics was the ITMX.
- I checked the OPLEV signal and it had a huge jump when the cryo cooler was turned on. I would say that it might be worthful to replace the oplev.

Finesse: 1400(46)
Temperature: 71.2 K (ITMX), 80.9 (ETMX)

Still the ITMX image is awful.

[Nakano, Michimura]

X arm finesse was measured to be 1402 (ITMX 120 K, ETMX 118 K).
Only measured once due to laziness.

I modified the finesse fitting code to fit the data with

fitfunc = @(p,x) p(1).*exp(-(x-p(2))/p(3))+p(4)

instead of

fitfunc = @(p,x) p(1).*exp(-(x-p(2))/p(3))+trans(end)

to fit the offset also.
The code lives in /kagra/Dropbox/Personal/Commissioning/201905/ArmRingDownFit.m or  https://www.dropbox.com/s/56thgkorjrac32f/ArmRingDownFit.m?dl=0

I tried fitting past data and found that this will reduce the standard deviation by half.
The effect to the finesse itself is ~10 when the finesse is ~1400.

History of past finesse vs temperature measurements are also attached.