Since yesterday we worked on the recovery of the system after the rearrangement of the filter cavity end bench.
1. Recovery of the SC alignment
We first tried to recover the SC alignment into the filter cavity. The SC was quite easy to find and with IR AA we could reach 5V in transmission. To investigate what is the reason for this low power, we did the following attempts:
1. Check the possible clipping of the IR PD. We manually scanned the mirror angle in front of the PD, and we could see in both x and y direction, that there is a period the power stop changing which means the beam is well inside the sensor.
2. Slow scan of the SC frequency to check the higher-order mode, it seems the TEM01 and TEM02 modes are both quite small.
3. We moved the IR PD along the beam, in this way we could reach 8V. It seems the problem would be the beam size on the sensor, but this is not consistent with what we saw in step 1.
I did a rough lock-unlock measurement to check the RTL, it seems the RTL is around 70ppm which is quite similar to what we have in the past. We will try to repeat this measurement in the following shift, if the RTL is fine, the low power on the IR PD maybe only caused by some local reasons on the FCEB bench.
One strange thing we noticed tonight is that the IR seems to oscillate with 0.5Hz frequency, which may be from the suspension. The possible reason could be the temperature in the clean room change due to our presence there. But during the work on Sunday, we unplugged and replugged the cable of the sensors on the end bench, it seems we could not get any signal after that(pic 1), we will inform the responsible person tomorrow about this.
2. Recover the green pointing loop
Then we tried to recover the green beam pointing loop. To be noted, the power on the PSD was reduced to about 2V instead of 13V after the bench was rearranged, and an ND= 2.0 filter was installed in front of it. The new sensing matrix was made in the same way as usual. The new driving matrix inserted in the Acl is shown below:
ACL_MATRIX_BEGIN GR_BPC2_DRIVING 1 PSD_X_NORM PSD_Y_NORM
ACL_MATRIX_CH GR_M7_BPC2_X "" 66.94 -11.38
ACL_MATRIX_CH GR_M7_BPC2_Y "" -9.11 -66.48
The new set points of the loop are
ACL_CONST_CH GR_M7_BPC2_X_SET "" 1 LOOP_FREQ 1.65
ACL_CONST_CH GR_M7_BPC2_Y_SET "" 1 LOOP_FREQ 1.95
The loop was closed with the new settings, we will check the performance of the loop tomorrow.
In pic 2, we show the picture of the end bench now.