Today, first of all, we made a test moving the lock from ITMs marionettes to ETMs marionettes. It worked as expected (a better balancing would be required), but it unlocked with the usual 1.8 Hz oscillation when the old filters for MIR/MAR splitting were engaged. Since this fact exclude that the oscillation depends on some strange coupling with DRMI dofs, we checked again other possibilities ad we found an oscillation of CARM SLOW (fig 1). We had excluded this possibility because the first time the problem was observed, CARM SLOW appeared stable. Probably we were wrong since the beginning and the right thing to do was to work on CARM SLOW loop. Anyway, it is not clear at all why a change of actuators - supposed to be transparent in terms of loop gain - should make CARM SLOW unstable. We decided to postpone an activity on that loop, because we need more time to understand better this issue and because we should also have a look on its noise projection.

For the moment, we improved the current reallocation strategy implementing a new High-Pass filter, which is a bit more efficient everywhere and shows no issue at 1.8 Hz (fig 2). We also validated the direct transition to LN2, gaining a factor of 2 in distance from saturation during the lock acquisition. This transition is currently used by default.