Today we decreased the power injected to the interferometer.
- we first adjusted the calibration of the INJ IMC TRA signal. It was mistuned by error on the 27th and 29th of January, while working on the EIB for ALS (the attenuation before the photodiode has been moved). We moved this attenuation to maximize the signal and we better clamped it. We took as reference PSTAB PD2 DC which is also monitoring the IMC transmission power: we computed the ratio IMC TRA over PSTAB and we tried to align the beam on the IMC photodiode to recover the previous ratio. We didn't manage to recover it, so we increased the gain of IMC TRA from 7.015 to 7.388 W/V (2nd plot). We measured the power impinging the IMC TRA photodiode after the rectangular density (as shown in the 3rd picture) finding 13.6 mW.
- we moved the waveplate of the IPC 2 on SIB1 (the actuator is located in the CEB minirack, look at picture 4 ) which changes the amount of power toward the interferometer. We took as references IMC_TRA for the power before the IPC and SIB2 B2 for the power injected to the ITF. In order to have a good signal on SIB2 B2 photodiode the PR was aligned until reaching its maximum (around 10 mW). We realized that we were slightly below the maximum transmission from IPC2. We rotated the waveplate until reaching a minimum (09:50:50 UTC) and we waited some min to collect data with all the light dumped on the BD of SIB1 tower (the water cooled one). We then set it at the max of transmission (look at plot 5). We didn't register any IMC unlock during this operation.
- we reduced the IMC transmitted power acting on the waveplate of the IPC1 located on the EIB. We directly measured the power going towards SIB1 with the calorimeter: we lowered the power sent to SIB1 from 48.1 W to 34.8 W , which corresponds to IMC TRA = 27 W (plot 6). We made a measurement of the IMC TF and of the spectrum of the IMC error signal and PSTAB error signal (temporarily plugged on the channel named ML D1 TEMP ERR) before and after lowering the power (plots 7 and 8). We had to modify the attenuation of the IMC error signal from 8 to 6 dB, and we also had to increase the power on the PSTAB phd acting on IPC3 from 1.3 V to 1.6 V (because we noticed some oscillations at 12 kHz, which already were seen when varying the laser power #47534), look at plot 9. We also updated the thresholds in INJ_MAIN.ini for the IMC TRA, RFC TRA locks and also the thresholds in the DSP for the lock of the RFC (from 5 to 4 V).