The goal of today’s activity was to verify whether the ITF can survive the switch-on of the NI ring heater.

The test procedure is described in the Git issue created by Michal:
https://git.ligo.org/virgo/commissioning/commissioning-tasks/-/issues/179

At 05:52 UTC, the ITF was manually unlocked.
A relock in CARM NULL 1F was requested and reached at 06:04 UTC.

At 06:40 UTC, the NI outer ring heater power was decreased by 10% (STEP 1).

The NI etalon loop was switched off at 08:43 UTC.

After that, the NI RH was switched on at 08:47 UTC (V = 6.1 V, P = 1 W).

ITF unlocked at 10.28 UTC.

A relock was achieved on the first attempt at 10:39 UTC, but ITF  immediately unlocked due to a loop oscillation.

In agreement with ISC, we decided to park the ITF in CITF and wait for the transient to subside.

At 13:18 UTC, the ITF was successfully relocked in CARM NULL 1F, with no changes applied on the ISC side.

The ITF remained locked for 1 hour before unlocking at 13:58 UTC.

The ITF then unlocked multiple times, all exhibiting the same behavior.

It was therefore decided to decrease the DAS outer ring power by an additional 10% (STEP 2) in order to recover the 12 MHz sideband, with the aim of improving the stability of the ITF.

This step was performed while the ITF was down at 14:49 UTC. An increase in the 12 MHz sideband was observed, however, this did not have any impact on the CARM NULL 1F behavior, where the same type of unlock was observed.

In order to further increase the 12 MHz sideband, an additional 10% reduction was applied at 16:15 UTC (STEP 3).

None of these steps allowed the ITF to remain locked in CARM NULL 1F for more than two minutes. All unlocks showed the same behavior.

The behavior of the main signals is shown in the attached figure.

After that, the RH was switched off at 18:23 UTC, and the original DAS settings were restored at 19:35 UTC.

The activity concluded at 19:36 UTC.