Today it has been decided to perform the HWS-DET measurement to check the alignment CH vs DAS.

Before starting, Fabio and Nicola restored the same positions for BS/SR/NI/SDB1/SDB2 during the HWS-DET measurement performed 'in gentle unlock configuration' Saturday Feb 11th (fig. 1).
The HWS-DET measurement started at 16.07 UTC, the reference has been acquired with the inner ring switched OFF and outer ring and CH ON at the steady state.
Thus, at around 16.15 UTC, the outer ring has been switched OFF to monitor the cooling phase.
The OPL induced by turning OFF the DAS appeared in the map (see fig. 2).
Then, between the WF 173 and WF 174 acquired at 17.41.12 UTC and 17.41.45 UTC, respectively,  the beam disappeared from the map (see fig.3). Looking at the signals, something happened on SDB1 LC_TX/TY (see fig.4).

The measurement was aborted at 17.51 UTC.

Yesterday morning we continued the work on CO2 benches.

We started with the WI bench, and we checked the intensity distribution of the two DAS rings, we adjusted a little bit both the Inner and Outer rings, since the outer doesn’t enter in the camera in the image plane, we took 4 different photos (see figure 1 and 2).

We verified the percentages of transmission and reflection of the pickoff power meter and they are 14% and 86% respectively. We checked again the alignment in the far field and near field (see figure 3 and 4), it was still good.

At this point, we moved on the NI bench and we checked the intensity distribution and the alignment after that we rotated the BSrec in order that the three actuators enter completely in the Pickoff as done for the WI bench (see entry 56383).

Also in this bench we did a small movement to adjust the intensity distribution and we had to realign well the DASes on the CH. The final results are shown in figure 5, 6 and 7.

We verified the percentages of transmission and reflection of the pickoff power meter and they are 14% and 86% respectively, as for the WI bench.

At the end, we left the TCS configuration:

Today at 8.00 UTC again a thermal state change of the CO2 laser on WI bench happened. In attachment the VIM detail on WI power and temperatures (fig1., fig.2). After a fast check, we can say that the jump down of the WI CO2 power can be linked to an increase of the laser temperature, but it is not related to any change in room temperature or chiller temperature. Furthermore the NI does not show analog behaviour (see fig.3). The ITF state seems to be not affected by this change. We will monitor powers and temperatures and regular visual checks to chillers will be asked to operator. 

At 14.55 UTC we decreased the power applied to the ETM RHs:

P NE: 6.7 W--> 6 W

P WE: 7 W--> 6.3 W 

In order to rule out the possibility that the 150kHz is due to a mechanical resonance in the long arm cavity we have decreased emitted RH power both on WE and NE. At 9:02 UTC we have set the NE RH voltage to 15.79 V and WE RH voltage to 16.14 V. 

The power applied to the WE RH has been increased by 100 mW at 14.41 UTC.

Today we continued the tuning of TCS parameters.

We tried to increase power on NI CP to increase CMRF adjusting both NI DAS_OUT and NI DAS_IN. We found at the end a no good status of the entire ITF, both sideband power and CMRF were bad. We are not sure this was up to our work, but in any case we decided to restore the situation obtained on 17th at 14.00 UTC.
New DAS parameters are:
NI DAS_IN :     400 mW
NI DAS_OUT : 2.38 W
WI DAS_IN :    159 mw
WI DAS_OUT: 191 mW
Trend will be shown.

Yesterday morning we have started the tuning of the DAS actuators following this procedure:

starting point: CMRF = -0.4e-9; 112MHz mag = 0.081; CD = 19;

- 1st step: increase of the sideband power using the WI outer ring (from 170 mW to 200 mW)

CMRF = -0.5e-9; 112MHz mag = 0.087; CD = 18.4;

- 2nd step: decrease the CMRF using the NI outer ring (from 2.36 W to 2.41 W). This tuining is not differential thus the common mode (the one that maximise the sidebands)  is spoiled  and has to be compensated

CMRF = -0.12e-9; 112MHz mag = 0.083; CD = 18.6;

- 3rd step: increase back the sidebands using the WI inner ring (from 154mW to 124 mW)

CMRF = it was @ LN1.  112MHz mag = 0.085; CD = 18.6;

the trend is visible in Figures 1 and 2  to highilight the pourpouse of the actuator tuning).
Figure 3 shows the tuning to reduce the CMRF.

This morning we have shined both CPs with DAS in order to observe their effect on the ITF.
We shined both NI and WI DAS with three different power levels:

As expected from simulations (VIR-0855A-17), we observed a decrease of the BNS horizon and of the gain of 56 MHz sidebands (see attached plot). In particular, the measured gain of the 56 MHz sideband with 0.3 W inner DAS power and 0.6 W outer power is in fair agreement with that computed with SIS (VIR-0889A-17).

The wavefront images acquired with the RH on (2017-03-15) allowed to identify the ITM center in the HWS-INJ reference systems (36791, VIR-0097A-17):

• X=[-1.85+/-0.05]mm
• Y=[1.25+/-0.04]mm

The wavefront images acquired with the HWS-INJ with CH on (figure 1 shows an example) have been analyzed to check the alignment on the CP center. 

To evaluate the coordinates of the minimum Delta OPL in the CH case we have considered 1.30 h of data:

• X:[-1.84+/-1.00]mm
• Y:[ 1.80+/-1.20]mm

The CH is aligned on the CP center within 0.6 cm.

Yesterday morning at the beginning of our shift we noticed that the WI CO2 laser was shining a lower total power (about  25% less). Looking at the data it seems that this strange behaviour has begun on 7/03/2017 afternoon (around 17.00 UTC) after 4 hours of large instability (see fig1). During the shift, the WI laser was switched off to allow actions on NI bench (moving the RF TTL generator closer to the laser RF driver). When we  tried to switch it on again, it did not shine anymore. Since the two diagnostic LEDs on the RF driver where red we supposed a failure of the driver itself. Today we replaced the damaged driver with a new spare and the laser is now working properly.  During the replacement we noticed that the mammut  and the cables that connect the RF driver with the power supply were a little bit burnt (see fig 2 and 3).  We replaced the connector and we separated the power supply on two different cables feeding the two RF modules (before the power supply was connected to the mammut by a single cable). 

Yesterday 14 March we worked on NI bench trying to understand why the beam size was not as expected by our simulations (see 36836). After some trials, we decided to look at the beam without the spatial filtering, removing the pinhole, after and before the CH lens. The spot is not as expected and it seems to be different in shape and dimension from previous images in same conditions (31/1/2017 and 21/02/2017). Further investigations needed.

At the end of the shift we realized that the WI CO2 laser was not shining anymore. A detailed entry will follow.
 

The iris has been checked, the dimension of the hole was 2.65 mm in diameter instead of 2.65 mm in radius. We check both FFT and Zemax simulations, placing the iris at the object plane of the CP,  the right dimension is 2.65 mm of radius. The loss of power, according to simulations, should be about 10% and the shape on the CP circular. See fig.1 for the cross section of the central heating on the CP simulated by FFt and Zemax. The iris has been machined again and will be placed on WI during the maintenance of  the 21 Feb.

As planned (Federico's suggestion) this morning the settings of the two chillers were carefully compared.  Indeed Elisabetta and Lorenzo found that the (silent) NI chiller had "energy save mode" set on, while the (noisy) WI chiller had not. At 8:35 LT the WI chiller was set in "energy save mode". No more 1Hz noise has been observed since then, see Figure 1. The temperature stability performance of the two chillers look similar (Figure 2).

In the last week a few tests have been performed with TCS chillers to investigate better and find possible mitigations. Mitigation attempts are up to now not effective. Although some strategies are still to be attempted at the chiller side, we will try to investigate also the the possibility to mitigate the coupling. Here a summary of the latest attempts and possibile further strategies:

1. on Feb 9 the heater of the WI chiller has been disconnected. Of course the noise disappeared, but in spite of many trials modifying control parameters, the temperature stability could not be recovered. One limitation is that the chiller firmware does not allow to completely disable the heating loop. The WI chiller heater was re-connected on Feb 13 but the WI chiller was kept off till Feb 16.
2. on Feb 14 the NI chiller has been switched on alone (i.e. while the WI chiller was off). All along this time the noise never appeared and as it was confirmed by the chiller on-board display, the heater never turned on (note that the chiller does not provide remote information on the heating status, the information is provided only at the on-board display). It was also tried to increase the T set point from 19°C  to 22°C: the heater turned just for a few minutes during the transition, than it kept silent again. See Figures 1 (UPS spectrogram 24h Feb 14), 2 (UPS spectrogram 1hour during T set change) and 3 (chiller temperature and T set). We have to conclude that "something is different with the NI chiller" which makes it never turn on the heater. (for conclusions see point 2. below)
3. on Feb 16 the WI heater was plugged on IPS (ENEL) instead than on UPS as usual, and switched on. We observed that the 1Hz noise at the level of INJ signals somehow reduced (a factor 2 or 3) but it was still present. As expected, the 1Hz comb noise disappeared from UPS and appeared on IPS, the noise is roughly the same at the magnetometer location: Figure 4 (shows noise with WI chiller on IPS compared to chiller OFF) and Figure 5 (compares noise with WI chiller on IPS and on UPS). (for conclusions see point 3 below)

Conclusions and further steps:

1. operating the chiller with the heater off seems not feasible.
2. we will further investigate for possible differences between chiller NI (not noisy) and WI (noisy). One possiblity is that there is a difference at the level of the firmware or the settings (indeed the Chiller technician we spoke with mentioned that up to his knowledge the heater PWM cycle is 3Hz, while in order to explain our observations it should be 1Hz cycle) . Another possibility is that the "magic" condition of the NI chiller is due to the difference in thermal load for example because of the less heat dissipation in the shorter water pipes (also to be checked if there is a different regulation of the water flow).
3. The fact that the noise does not disappear when the chiller is powered on ENEL, makes us suspect  that even plugging it on a dedicated UPS would not solve the problem.
4. One possible mitigation strategy is to replace the chiller with that of Virgo (which was only cooling) and its custom control (however we know from data that the old chiller performance was at the level of 0.1°C at the most, while the new chillers performs at 0.01°C, and seems that the new CO2 lasers need this level of T stab.)
5. Another strategy that we will start pursuing is to investigate further the coupling path and act on it.

During the maintenance  at 8.00 LT we opened the enclosure of the WI CO2 laser bench and set the laser in a pulsed way to allow the positioning of  the pin hole. We profiled again the beam to recover the same beam size. We also put in place  the power meter for the central heating pick off. The last part of the cabling to the ADC is missing.  At 12.50 we tested the laser at full power and everything was ok. At 13.05 we switched off the WI CO2 laser and its chiller.

At 10.20 we switched on the NI CO2 laser and we positioned the power meter for the central heating pick off. The last part of the cabling to ADC is missing. At the end of the maintenance we left the NI CO2 laser switched on, with enclosure closed, to test the 1 Hz magnetic noise difference between two chillers.

The WI CO2 laser and its chiller have been switched off at 16.40 LT to connect again the cable of the chiller heater, that was disconnected to remove the 1Hz magnetic noise (36441). The laser has been put back in operation at maximum power at 17.45 LT. 

In order to implement the slow power control on CO2 lasers we tested the communication between DAQ and Pzt driver and we tried to make a scan between 0 and 50 V. This procedure is essentisal to find the control working point  in a region of linearity.

First of all, the TCS_CEB has been modified in order to be able to drive the TCS DAC (ch0 and ch1 of the first TCS DAC). The noise features have been added (line, ramp, white noise, colored noise) with output TCS_NOISE (which shows the output voltage of the Pzt driver). The noise is driven to the two DAC channels thanks to a driving matrix obtaining the two DAC signals (TCS_N_NOISE and TCS_W_NOISE for the North and West CO2 laser benches respectively showing the voltage sent by the DAC).

After that, we changed the gain of our Pzt driver from 7.5 to 10 and we set a ramp on the DAC of 5 V. Due to differential mode of the DAQ and  single ended mode of the Pzt driver we have a problem of floating ground and half of the voltage at the output. So, to recover this problem we set 10 V ramp on the DAC that gives a maximum output of 5V and a maximum voltage at the Pzt driver output of 50 V. We show in figure the TCS_NOISE channel that gives the Pzt driver output, TCS_N_NOISE and   TCS_W_NOISE, that gives the DAC outputs, the photodiodes ISSIN and ISSOUT for both benches, that monitor the laser power.

We can observe a wide region of linearity suitable to find a good working point. Now the control with UFG of 0.1 Hz can be implemented.

In this afternoon we spent a shift working on the TCS tuning with CITF.

We haven't decided on the working point for the central heating yet as the interpretation of the CITF data is non-trivial.

At this point I might suggest, as a different strategy, relying on the Hartman cameras and providing the dead-reckoned thermal lens values.

[The tests]

We tried two particular CO2 settings as follows.

• Configuration 1:
  • This is to provide a 20 uD single-path lens on each arm to force the power recycling cavity stable.
  • [CO2 NI, CO2 WI] = [140 mW, 140 mW] . These numbers were derived theoretically.
  • rotation stage settings [theta NI,  theta WI] = [35 deg, -22 deg]
  • Initially locked on the carrier, then switched to the sideband in the middle of the measurement as the carrier lock wasn't able to maintain a stable lock.
• Configuration 2:
  • This to set the CO2s at the point where we had a highest build up in the past (35867).
  • [CO2 NI, CO2 WI] = [20 mW, 20 mW]
  • rotation stage [theta NI, theta WI] = [-3 deg, -40 deg]
  • Locked on the carrier throughout the measurement.

[Results]

• Conf 1:
  • Significant degradation in the build up. This was true for both carrier- and sideband-locked CITF. Consistent with the past measurement (35867).
  • B4 image showed Laguerre-type modes both for carrier- and sideband-locked CITF. This is also consistent with the past measurement.
• Conf 2:
  • No significant change in the DC signals.

In addition to these measurement, the highest build up of 15 mW at B4_DC was observed before these measurements when neither of the CO2s was activated. The B2_DC signal also dropped to 4 mW (from 13.8 mW) which is the lowest value I have seen. This is inconsistent with all the past measurements we did with CITF before. This could mean that there are uncontrolled variables somewhere affecting the build up of CITF. Note that since the arm cavities weren't available today, we set the NI alignment using the B2 and B4 cameras/DC signals as referecnes.

[Current interferometer state]

Both NI and WI central heating are switched off. The CITF locking loops are off. Also the below are the demod phases that we found (kind of) optimum for this afternoon.

• B1p_119MHz_phi0 = -0.72 rad
• B2_6MHz_phi0 = -0.67 rad