Guenther Rehm, Head of Diagnostics Group FOFB Optimisation Guenther Rehm, Head of Diagnostics Group 03/04/2008 FOFB Optimisation

Topics Problems Improvements FOFB trips due to BPM position jumps Slow build up of excessive corrector values Improvements Goal to reduce absolute corrector strengths Modal and BPM weights Smooth stopping 03/04/2008 FOFB Optimisation

BPM position jumps trip FOFB 03/04/2008 FOFB Optimisation

TbT-PM triggered by FA glitch 03/04/2008 FOFB Optimisation

Sudden changes on SA channels Manual Switches (DSC off) Automatic Switching (DSC on) 03/04/2008 FOFB Optimisation

Slow build up of correctors 03/04/2008 FOFB Optimisation

Reducing unobservable modes Put “pressure” into the feedback loop to reduce each corrector Implemented by including a slow decay for each corrector current Where orbit correction needs it, it will keep the corrector almost at the right value (deviation to small to observe on orbit) Unobservable modes will decay away with a decay constant of many minutes 03/04/2008 FOFB Optimisation

Modal and BPM weights In principle, Diamond has 168 EBPMs and 168 correctors each plane, so decoupled response matrices are 168x168 Square matrices could be directly inverted However, system should also work with fewer EBPMs or correctors Use SVD for pseudo inversion Orbit motion can then be transformed into “mode space” Weights can be applied to modes and BPMs, reducing measurement noise contribution. 03/04/2008 FOFB Optimisation

Orbit Motion in Mode Space dB 03/04/2008 FOFB Optimisation

Response Matrix Eigen Values Condition number 4400 03/04/2008 FOFB Optimisation

Inversion and Conditioning Tikhonov regularization Inversion 03/04/2008 FOFB Optimisation

Gains of individual modes 03/04/2008 FOFB Optimisation

Closed Loop Response per Mode 03/04/2008 FOFB Optimisation

BPM weights in each cell 1 0.05 0.2 03/04/2008 FOFB Optimisation

Result of BPM weights 03/04/2008 FOFB Optimisation

Smooth Stopping of FOFB Calculations run distributed on 24 CPUs Synchronous stopping is essential as otherwise correction is applied on part of correctors for some time All feedback CPUs communicate status (stopped, ready, running) through fast network When any is stopped, all stop on next cycle Stopping can be caused by: command out of limits for total corrector current (4.9 A of 5 A) out of limits for corrector current step (0.5 A around 2 s average) out of limits for beam position (100 um) When stopped, trailing average is written to correctors to remove dynamic component from correction. 03/04/2008 FOFB Optimisation