Threading / LTC/ JW1 How difficult is threading at the LHC ? When MADX meets the control system … J. Wenninger AB-OP &

Threading / LTC/ JW2 Threading issues Threading can be delicate : Installation ‘surprises’ : polarities, alignment, obstacles… Beam position monitor quality. And at the LHC we also have ‘large’ multipole field errors, and they do contribute (see also A. Verdier, LHC note 308).  Compare : Dipole : 10 units of b3  kick : 2  rad / r = 10 mm Quad : 0.4 mm misalignment  kick : 11  rad  Effect of b3 ~ r 2  Expected |b3| after decay ~ 2-4 units : rather small effect for r < 10 mm !

Threading / LTC/ JW3 Threading strategy Successful LEP strategy : Select a reasonably short range where the amplitude starts to grow : shorter : minimize # errors within region. longer : less sensitive to isolated BPM errors. Use few correctors (1-3) : polarity & bad BPMs. MICADO algorithm.  ‘LEP threader’ Example for a first LHC b1 Horizontal Vertical Detection of poor BPMs : Look at orbit in normalized coordinates Compare predicted and achieved correction. Use your experience (if you have any…). Detection of COD errors : Compare predicted and achieved correction  use few CODs / step.

Threading / LTC/ JW4 Threading strategy / 2 Example for a correction, H plane Difference After correction Target region for correction Before correction Compare this with achieved results – in case of doubt  Good online tools needed : LEP standard with improvements !

Threading / LTC/ JW5 Threader simulation The simulation of threading is tricky because : Involves a lot of pattern recognition. Range selection (start of oscillation), suspicious BPM detection… Normalized coordinate inspection, trajectory fits, response analysis of correction… Unexpected situations. A realistic simulation is very complex and time consuming to set up. A human being is >>> powerful than algorithms for such a task, time investment into automatic algorithm is immense … and difficult to check ! I favor the approach of a CR steering application with well adapted and powerful (manual) tools for the first beams.  idea : couple the steering application which already exists to MADX and make a realistic threader test & training tool ! Verify that online tools are up to the job !

Threading / LTC/ JW6 Threader simulation MADX Corrector changes Trajectory file Aperture cuts, noise…. ‘Conditioned’ trajectory file ‘Real-life’ first turn threading simulation : LHC beam1 treated as a transfer line. Trajectories simulated in MADX with errors… MADX output is conditioned to take into account aperture, add noise… Conditioned data is imported into the control system steering program. Correction evaluated and re-exported to MADX. Iterate, iterate…

Threading / LTC/ JW7 Aperture assumptions Start from arc aperture : 22 / 17 mm. Remove 2 mm for alignment. Remove 2 mm for beam size (2-3 sigma)  50% loss. Remove 2 mm for ‘unexpected/other’ effects.  Effective aperture : 16 mm horizontal / 11 mm vertical. This aperture is applied EVERYWHERE along the ring.  Consider the beam as lost if trajectory exceeds those values. Other choices are of course possible…

Threading / LTC/ JW8 Threading exercise Conditions : Dipoles : -b3 = -20 units (systematic) -other components : standard error table Multipole correctors : OFF BPMs : perfect ! Quads : -misalignment : 0.4 mm rms (gauss). -b2 = ± 50 units random, flat distribution  ~ 25%  -beat (closed orbit) 12 iterations of LEP threader

Threading / LTC/ JW9 And one more.. Conditions : Dipoles : -b3 = -20 units (systematic) -other components : standard error table Multipole correctors : OFF BPMs : ± 3 mm errors, flat distribution. Quads : -misalignment : 0.4 mm rms (gauss). -b2 = ± 200 units random, flat distribution  > 100%  -beat (closed orbit) 13 iterations of LEP threader

Threading / LTC/ JW10 Debriefing / 1 Surprised by the insensitivity to quad errors ? Consider a misaligned quadrupole producing a 10 mm amplitude oscillation. A 2% error on the strength changes the amplitude to 10.2 mm..  very small !  in the noise of the 3 mm BPM errors ! A transfer line is less sensitive to errors (at least for steering), in particular over small sub- ranges. Using  -beat as gauge is not ideal ! BPM effects : Random BPM errors of 3 mm have little effect. The same applies to ISOLATED very bad BPMs (10 mm offsets, signs…). Things get tough if more than ~ 1/4 BPMs are in the ‘very bad’ category, or if you depend on one BPM that falls into the same category (loss over very short range) :

Threading / LTC/ JW11 Debriefing / 2 Threading seems not more complicated than at LEP… Need to add a few more errors : momentum offsets, polarity inversions,… Repeat the exercise for a LEP lattice and check if the threading difficulty corresponds to what was observed at the time. Interesting observation from LEP : we managed (more than once !) to thread the beam through the first turn and establish a closed orbit with the integer tune off by 0.5 ! Errors : The fact that threading is not too sensitive too errors is no reason to relax on measurement and installation quality !

Threading / LTC/ JW12 Outlook  With MADX and the present version of the steering program for the CR, one can already practice threading!  At first sight threading is not so terribly difficult, at least without dramatic problems (like polarity reversals of quads…).  The full monty should be available before next Chamonix :  Injection errors, energy offsets, quadrupole polarity, …  More subtle BPM errors : signs, calibrations, mega-offsets…  Corrector polarity errors.  …  Next step :  From the first turn to the closed orbit !  A sector test is invaluable as preparation for the full first turn to test all the concepts on the ground floor level !