aLIGO LHO Logbook

H1 IOO (ISC)

kiwamu.izumi@LIGO.ORG - posted 15:35, Tuesday 20 January 2015 - last comment - 08:53, Wednesday 21 January 2015(16166)

IMC ASC maintenance

Rana, JeffK, Kiwamu,

We did some maintenance items on IMC ASC. The major activities are:

We had a difficulty in restoring the IMC alignment back as we ran the offloading script when the HEPIs were down and therefore we screwed up the alignment.
- Interestingly, restoring the suspensions using the witness sensors as references did not work at all. Simply burt-restoring the suspensions and ASCIMC frontends brought it back to a good lock-able alignment.
- After the recovery, we ran the offloading script again and so all the MC suspensions now should be in good alignment.
We set a limiter in each loop with a hope that this is going to reduce a risk of tripping the suspensions (alog 16138, 16128) . Here are the counts we set for the limiters:
- DOF_1_P(Y)_LIMIT = 125
- DOF_2_P(Y)_LIMIT = 125
- DOF_3_P(Y)_LIMIT = 250
- DOF_4_P(Y)_LIMIT = 3000
- These values are derived such that they are roughly the same as their 5 sigma deviations in 100 days trend except for the DOF4 loop which had hit the new limiter once in this evening. So we set the DPF4 to a high value.
We disabled the 2nd whitening stage in MC2_TRANS QPD because the signals were saturating most of the time when IMC was not locking.
- Therefore the MC2_TRANS QPD now has one whitening filter.
We cleaned up the control filters and installed a new low-pass in every DOF. Rana will report some more details about this.

Also, tomorrow or sometime this week, we might insert a lens in the REFL gigE path as the REFL beam now is a bit too big to be able to cover the spatial higher order modes.

Comments related to this report

rana.adhikari@LIGO.ORG - 18:32, Tuesday 20 January 2015 (16170)IOO, ISC

Link

Some issues with the current implementation:

Its not clear that DOF4 servo makes any sense. Using the extinguished field (dark, messy HOM stuff) seems like a not stable way to make a centering loop. Certainly we cannot assume that putting digital offsets into these loops will be stable in the long run. The MC REFL image visibly changes week-to-week. If the carrier contrast defect was symmetric or if the SB power was larger or if we used 2f RF centering, this might be better.
We had two low pass filters in all of these loops. This is now replaced with a single 4th order low pass at 7 Hz. Since we have no noise budget or noise estimate its not clear what these filters were for.
There was a 'not1' filter in these filter banks which seems like a hacky way to inverting the plant, but only would be so if we were driving single pendula. We'll now look at modifying the MC SUS damping filters and then making the pendulum Q's lower and inverting the pendulum response (M1 P -> M3 P).
Not sure when the Input Matrix was measured or diagonalized. This ought to be done to make sure we're not shooting ourselves in the foot.
The FM2 (1:0) integrator was getting clicked off during each lock loss and sending the full control signals as a transient to the suspensions. This FM triggering is now turned OFF and things seem better. No DOF4 transients have happened after turning these off.
The beam sizes on these MC WFS seem too small. According to Kiwamu (log #6560) the beam diameter is ~0.7 mm on both of them. According to Hartmut, they are ~1.5 mm at LLO.

sheila.dwyer@LIGO.ORG - 22:45, Tuesday 20 January 2015 (16173)

Link

Tonight we have twice had MC lock losses where MC1+MC3 got completely misalinged, although they didn't trip. The second one of these was at the same time as a lockloss of the ALS, but we don't know if the IMC caused it or ALS. This was at 6:35 UTC, Jan 21st.

daniel.sigg@LIGO.ORG - 08:53, Wednesday 21 January 2015 (16177)

Link

We made the decision not to increase the spot size. This was done at LLO to decrease the jitter coupling into the ISS. The in-vacuum ISS doesn't require this.

The IMC seem to have worked much more reliably, before we added DOF4. Please turn it off for good.