Microphonics and detuning control
To allow operation at highest loaded Q and such at lower power levels for a given field amplitude microphonics detuning control is mandatory. A multi-cell cavity is a complex mechanical system and it was observed that mostly mechanical eigenmodes are excited by the externally induced vibrations.
Figure 5 shows a Wavelet analysis of microphonics detuning, showing the variation with time of the dominating spectral content.
This is confirmed by transfer functions measured between excitation of the piezo tuner and the detuning amplitude of the cavity at the given modulating frequency. The obtained spectrum shows a collection of mechanical eigenmodes of the cavity-tuner system (see Figures 6).
The presence of these eigenmodes prohibits the usage of any classical feedback system. Such for detuning control several advanced adaptive feedforward or state space techniques have been or are currently under study. A detuning reduction by one order of magnitude at 1.8 K was demonstrated using the adaptive feedforward technique (Figure 7), even though the given microphonics background at HoBiCaT was already low.