Keita, Sheila,Robert, TJ, Jennie, Corey, Tony
Dave and Gerardo alerted us that there have been vacuum spikes after locklosses in HAM6 yesterday. We are looking at what circulating powers were and what the lockloss transient was at the time of locklosses that caused and didn't cause a vacuum spike in HAM6. Here is a comparison table:
time | AS_C peak | POP A LF | which alignment | spike? |
6/7 1:12:35 | 0.938*10 = 9.378 | 5505 | bad | 3.6e-8 |
6/7 3:04:08 | 1.35*60 = 81 | 26739 | bad | 4.28e-8 |
6/7 03:51:29 | 1.05*60 = 63 | 27032 | bad | 1.5e-7 |
6/7 23:15:54 UTC | 0.342*2 =0.684 | 1109 | good | no |
6/7 20:54 UTC | 1.58*2= 3.16 | 651 | good, PRG was tanking | no |
In the bad alignment, the beam was clipped on it's way to AS_C, so that diode wasn't catching all the power it would have in the good alignment. AS_A and AS_B saturate at the time of locklosses.
The spike in vacuum pressure was higher with each successive lockloss, although the power on AS_C was largest in the second one.
One of the three pressure spikes from yesterday was the lock loss from 10W, others were from 60W.
1st plot shows the worst one in terms of the pressure spike (the last one at 6/7 1:12:35 UTC, 60W).
This doesn't tell us the cause of the pressure spike, that requires different investigations.
Anyway, the 2nd plot shows the lockloss from today after Sheila, Jennie and TJ moved to a better alignment.
SR2 doesn't move much over the power range from 2W to 25W, and ASC-AS_C_NSUM doesn't show the sign of clipping. Everything looks saner and thus better.
The only concern is if this "better" alignment will still burn things in HAM6. We haven't observed pressure spikes from 2W lock losses with this alignment today but that only happened twice.
To clarify, "bad" alignment that produced pressure spikes was only for Thursday, and the "better" alignment today is supposed to be the same as or at least quite similar to pre-Thursday. Since we haven't had any pressure spikes with pre-Thur. alignment, I doubt that we'll see any pressure spikes with this alignment either.
However, picos had to be moved by large amount to go from pre-Thur. to Thur. and then to Fri. alignment. Even though Fri. alignment should be clouse to pre-Thur., we expect some not-so-large difference in the WFS sled centering, thus in the HAM6 alignment.
Out of caution, we'd like to see that losing lock at 10W won't cause pressure spikes before proceeding further. We chose 10W, because a pressure spike was observed at that power with a loss of lock with a bad (Thu.) alignment. If we don't see any spike for many lock losses with current (Fri.) alignment it would be safe to proceed.
Since automated DC lock won't work for this "good" alignment yet (that problem existed before Thursday and it seems to persist today), Sheila and I asked Tony to lock the IFO, bring it to 10W with RF, kill lock and observe pressure. If there's a spike he will stop, otherwise he will repeat.
We'll reconvene tomorrow.
Attached is an annotated HAM6 picture.
Also, plots with more piranis showing all three pressure spikes was posted in alog 78327.
Since FS seems to have worked, potential point that could have made pressure spikes are: