Under Construction

Vibration Stabilization for the NLC final focus

(Special Projects Group)

Link to software group vibration feedback page 

The final focus doublets for the NLC need to be stabilized to a relative position of ~1nm. On long timescales (>1second) the beam - beam deflection provides the best measure. For shorter timescales, several options have been suggested:

• Passive isolation: The vibration levels measured at various sites suggest that passive isolation may provide sufficient isolation. This approach has the advantage of simplicity, however it may fail if "cultural" noise due to the accelerator is larger than expected.
• "Optical Anchor" This system uses an interferometer to measure the relative positions of the magnets. Interferometers can provide sub-nanometer resolution at high data rates (>1KHz). The primary disadvantage is that differential ground motion can mimic the effects of magnet motion. In addition the optical anchor requires penetrations through the detector for the light paths
• "Inertial anchor" This system uses accelerometers mounted to the magnets and feedback. This system locks the magnets to the "fixed stars". This system has the disadvantage of not providing information at low frequencies, and of locking the final focus quads to the "fixed stars" while the rest of the accelerator is presumably referenced to the ground.
• "Super fast feedback" This system would use a very fast (10ns) feedback (or feedforward) to correct the trajectory of the later parts of the bunch train based on deflection measurements on the early bunches in the train.

Ground motion: Ground motion varies substantially between sites. For many sites, the measured ground motion would allow the use of passive isolation for the final focus quads. There is concern, however, that the installation of the accelerator and associated systems may produce an unacceptable increase in ground motion, necessitating the use of some form of active isolation. A good discussion of ground motion effects and measurements for the NLC can be found in the NLC ZDR.

Note on overall vibration issues for NLC vibration.pdf

Vibration control systems consist of three components:

Sensor: Detects the motion of the device which is controlled (Electron beam, Laser Interferometer, Accelerometer)

Feedback Loop: An algorithm for controlling the actuators based on the sensor readings (Analog, Digital, Adaptive)

Actuators: These provide the actual mechanical feedback. (Piezo-pushers, Inertial pushers)

Most recent talk MAC2002Stabilization.pdf   

Old material (some out of data) 

Page by Josef Frisch frisch@slac.stanford.edu