In order to damp high frequency noise near 60 Hz, the LI29 fast feedback has been upgraded to include a timeslot control algorithm. Linac pump vibrations cause 59 Hz oscillations, and the timeslot algorithm is able to damp at that frequency. This feedback algorithm has been used in FB69 since 1996. The goal of the timeslot design is to damp at low frequencies and at frequencies near 60 Hz, without causing excess amplification of beam jitter at other frequencies. In preliminary measurements, the 59 Hz oscillation is damped by a factor of about 0.29.
Additional study is needed to ensure the feedback design is optimal for all frequencies.
The feedback now acquires and buffers data at 120 Hz, but feedback control is only performed at 60 Hz. Pulsed Amplitude Units are used to implement separate hardware controls for the alternating pulses. In order to damp the 59 Hz oscillations, the calculated timeslot difference pseudoactuator produces a 1 Hz wave as it beats against the 60 Hz calculation. Yes To combat power supply problems with the fast correctors in LI29, a new functionality has been added to Fast Feedback to enable users to limit actuator movement per feedback iteration. A limiting value has been set for the LI29 feedback loop and it should be changed by knowledgeable users only.