FFT-oriented feedback

In the SLC many feedback systems keep the beam under control. Here we concentrate on feedback systems which operate at 120 Hz or lower frequencies, so software can be used to make some decisions. The linac steering feedback uses beam position monitor data and corrects magnet settings to keep the beam orbit to the desired values (mostly flat). Looking at the fast Fourier transformation (FFT) of the data, a reduction in the zero and very low frequency component is observed, while on the other hand noise at higher frequencies is amplified. To improve this situation, the FFT can be used to alter the feedback so a flat spectrum can be achieved, indicating the lowest white noise level. If there is a spike at, for example, 2 or 7 Hz or another feedback is oscillating, this feedback would adjust itself till this spike is reduced to the white noise level (not beyond). Different simulation results for the frequency response are presented