The vibration damper needs a sensor capable of sensing sub-nanometer motions. The Electron beam is the only option for low frequency feedback. For all other sensors, the magnetic center will drift relative to the sensor position by more than a nanometer. Does not require additional equipment. Readout frequency lower at lower beam rate Signal disappears when the beam is lost.
Interferometers provide high accuracy measurements with sub-nanometer resolution.
An interferometer can be arranged to measure the difference in position between the two final focus quadrapoles. An interferometer can have sufficient resolution from DC to Kilohertz. Accelerometers and geophones. Standard accelerometers do not have sufficient noise, and standard geophones cannot operate in high magnetic fields. A capacitive inertial sensor could be constructed with sufficient resolution.