We cannot make a reasonable assessment of energy deposited in HAM6 when we had the pressure spikes (the spikes themselves are reported in alogs 78346, 78310 and 78323, Sheila's analysis is in alog 78432), or even during regular lock losses.
This is because all of the relevant sensors saturate badly, and the ASC-AS_C is the worst in this respect because of heavy whitening. This happens each and every time the lock is lost. This is our limitation in configuration. I made a temporary change to partly mitigate this in a hope that we might obtain useful knowledge for regular lock losses (but I'm not entirely hopeful), which will be explained later.
Anyway, look at the 1st attachment, which is the trend at around the pressure spike incident at 10W (other spikes were at 60W, so this is the mildest of all). You cannot see the pressure spike because it takes some time for the puffs of gass molecules to reach the pirani.
Important points to take:
- Marker shows when the Fast Shutter was actually fired, which is witnessed by GS13 (bottom left).
- ASC-AS_C cannot be relied on. It started saturating badly even before the FS fired (look at INMONs, 3rd from the top, left), and it kept saturating for a long time.
- OMC QPDs cannot be relied on. They started saturating at about the same time as AS_C though they seem to have stopped saturating earlier (2nd from the bottom, left).
- AS_B and AS_C cannot be relied on. They're much better than AS_C and OMC QPDs, they started railing some milliseconds earlier than the FS, but they railed (right middle for SUM, one below that for INMONs).
- But the beam was NOT clipped by the Fast Shutter after it closed as the power dropped to zero, we can see that after WFS stopped saturating.
This is understandable. Look at the second attachment for a very rough power budget and electronics description of all of these sensors. QPDs (AS_C and OMC QPDs) have 1kOhm raw transimpedance, 0.4:40 whitening that is not switchable on top of two stages of 1:10 that are switchable. WFSs (AS_A and AS_B) have 0.5k transimpedance with a factor of 10 gain that is switchable, and they don't have whitening.
This happens with regular lock losses, and even with 2W RF lock losses (third attachment), so it's hard to make a good assessment of the power deposited for anything. At the moment, we have to accept that we don't know.
We can use AS_B or AS_A data even though they're railed and make the lower bound of the power, thus energy. That's what I'll do later.
(Added later)
After TJ locked the IFO, we saw strange noise bump ffrom ~20 to ~80 or so Hz. Since nobody had any idea, and since my ASC SUM connection to the PEM rack is an analog connection from the ISC rack that also has the DCPD interface chassis, I ran to the LVEA and disconnected that.
Seems like that wasn't it (it didn't get any better right after the disconnection), but I'm leaving it disconnected for now. I'll connect it back when I can.
In a hope to make a better assesment of the regular lock losses, I made the following changes.
- With Richard's help, I T-ed the ASC-AS_C analog SUM output on the back of the QPD interface chassis in ISC R5 rack (1st picture) and connected it to H1:PEM-CS_ADC_5_19_2k_OUT_DQ.
- The SUM output doesn't have any whitening nor any DC amplification, it is just the analog average (SEG1+2+3+4)/4 where each SEG has 1kOhm transimpedance gain, and AS_C only receives ~400ppm of the power coming into HAM6. This will be the signal that rails/saturates later than other sensors.
- The other end of the T goes to fast shutter logic chassis input in the same rack. The "out" signal of that chassis is T-ed and goes to the shutter driver as well as shutter interface in the same rack.
- Physical connection goes from the QPD interface in the ISC rack on the floor to the channel B03 of the PEM DQ patch panel on the floor, then to CH20 of the PEM patch panel in the CER.
- I flipped the x10 gain switch for AS_B to "low", which means there's no DC amplification for AS_B. So we have that much headroom.
- I set the dark offset for all quadrants.
- There was no "+20dB" in the AS_B DC filters, so I made that and loaded the filter (2nd attachment).
- TJ took care of SDF for me.
My gut feeling is that these things still rail, but we'll see. I'll probably revert these on Tuesday next week.
SDF screenshot of accepted values.
Low voltage operation of the fast shutter: It still bounces.
Before we started locking IFO, I used available light coming from IMC and closed/opened the fast shutter using the "Close" and "Open" button on the MEDM screen. Since this doesn't involve the trigger voltage crossing the threshold, this only seems to drive the low voltage output of the shutter driver which is used to hold the shutter in closed position for a prolonged time.
In the attached, the first marker shows the time the shutter started moving, witnessed by GS-13.
About 19ms after the shutter started moving, the shutter is fully shut. About 25 ms after the shutter was closed, it started opening, got open or half-open for about 10ms and then closed for good.
Nothing was even close to railing. I repeated the same thing three times and it was like this every time.
Apparently the mirror is bouncing down or maybe moving sideways. During the last vent we haven't taken the picture of the beam on the fast shutter mirror, but it's hard to imagine that it's close the the end of the mirror's travel.
I thought that it's not supposed to do that. See the second movie in G1902365, even though the movie is capturing the HV action, not the LV, it's supposed to stay in the closed position.
ASC-AS_C analog sum signal at the back of the QPD interface chassis was put back on at around 18:30 UTC on Jul/11.
Unfortunately, I forgot that the input range of some of these PEM ADCs are +-2V, and so the signal still railed when the analog output of ASC-AS_SUM didn't (2V happens to be the trigger threshold of the fast shutter), so this was still not good enough.
I installed 1/11 resistive divider (nominally 909Ohm - 9.1k) on the output of the ASC-AS_C analog SUM output on the chassis (not on the input of the PEM patch panel) at around 18:30 UTC on Jul/12 2024 while IFO was out of lock.