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sheila.dwyer@LIGO.ORG - posted 17:25, Friday 07 June 2024 - last comment - 09:00, Thursday 13 June 2024(78308)

pressure spikes and HAM6 powers

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.

Comments related to this report

keita.kawabe@LIGO.ORG - 18:23, Friday 07 June 2024 (78310)

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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).

Pirani for HAM6 (top middle) registered the largest pressure spike, so it seems that the beam was definitely hitting something in HAM6.
DCPDs (top left) seem to have been protected by the fast shutter, neither of them show excessive spike before the shutter closed.
SR2 was moving by a huge amount in YAW (bottom left) due to SRC2 loop (right, second from bottom), which is ASC-AS_C. At the same time, ASC-AS_C_NSUM was getting smaller and smaller (mid bottom) even though ASC-AS_A_DC_NSUM was almost constant.
This means that the beam was clipped in ASC-AS_C path (OM1 transmission) but not in OM1 reflection. It was probably 1" lens holder but it could also be BOSEM cables for OM1 sticking out from the back of the OM1 cage. See https://dcc.ligo.org/LIGO-D1000342.
1. Another bad effect of this is that the trigger voltage for the fast shutter is smaller than it should be. Not an order of magnitude smaller, but smaller nevertheless. We cannot use this alignment at all.

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.

Images attached to this comment

keita.kawabe@LIGO.ORG - 23:03, Friday 07 June 2024 (78313)

Link

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.

keita.kawabe@LIGO.ORG - 09:00, Thursday 13 June 2024 (78410)

Link

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:

FS wire (hit by the beam coming from HAM6 into OM1).
If the beam hit the corner or the side of the FS and deflected/scattered, it could have burnt anything (FS wire, black material for FS, various cables anywhere).
OM1 BOSEM cables (hit by the OM1 transmission, which is still powerful enough)
PEEK ring in the lens in front of AS_C (hit by the OM1 transmission, which is still powerful enough)
AS_C itself (hit by the OM1 transmission, which is still powerful enough)

Images attached to this comment