The slow control of CARM length (Tidal Control, acting on NE/WE IP position) has been changed in order to keep the RFC around the resonance. In the previous strategy, the loop was used to correct the error of fMod initial setting. Now this error is corrected once for all just before LN3, directly adjusting fMod.
A long lock is needed, in order to evaluate the stability of the new strategy.
At Automation level this has implied the introduction of the fModErr tuning strategy inside the ACQUIRE_LOW_NOISE_3 state of the ITF_LOCK node.
The strategy is as follow:
- The average over 20Sec of the Sc_BS_fModErr error signal is made
- If this value is over fmod_thr (0.3 at the moment) a correction is calculated as delta of the 8Mhz frequency via the fmod_freq_calib calibration factor. The correction is then applied in steps (of max_freq_delta, currently 0.02 Hz), with one comand sent to the LNFS each second.
- Then a new average of Sc_BS_fModErr is made and the loop continue until we are under threshold
The commands are sent to the LNFS using the already existing set_LNFS_freq.py script. The eightMHz_reference and max_freq_corr parameters are tacken from INJ_MAIN.ini