Lucia, Yutaro (remotely), Yoshinori (remotely), Ryutaro (remotely), Kiwamu,

We discovered that the H2 coil on the test mass stage of ETMX did not respond to the DAC request. We will have a look at the electronics tomorrow.

[Background story]

We have been concerning about the pitch motion that were at 0.625 Hz and 0.78 Hz seen by the optical lever (6709) because these modes take a long time to decay and therefore we have been suffering from them. So we wanted to damp these modes but for some reason we kept failing in closing the damping loop which uses the test mass optical lever and feeding signals back to the test mass coils (5906).

[Measurements]

We started from measuring the transfer function of the TM pitch to optical lever pitch. We quickly learned that the mechanical plant now has an extra negative zero at around 0.57Hz which is just before the first peak at 0.625Hz. See the attachment below.

The blue trace in the figure is the same transfer function taken by Miyamoto kun in this past August for coparison. Also his measurement was done when the payload was kept at a cryogenic temperature. Our new measurement clearly shows an extra notch at 0.57 Hz and also the DC level is smaller than Miyamoto-kun's.

We then drove each of four coils at a time and looked at the optical lever signals. We then discovered that the H2 coil didn't respond to the DAC request while the others did. So it is likely that the H2 coil is somehow disconnected or the relevant circuit is busted.