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How difficult is threading at the LHC ?

Explore the difficulties of threading at the LHC, including installation surprises, beam position monitor quality, and the impact of multipole field errors. Learn about the threading strategy used at LEP and the simulation process for threading at the LHC.

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How difficult is threading at the LHC ?

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

  2. 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 mrad / dipole @ r = 10 mm • Quad : 0.4 mm misalignment  kick : 11 mrad • Effect of b3 ~ r2 • Expected |b3| after decay ~ 2-4 units : rather small effect for r < 10 mm ! Threading / LTC/ JW

  3. 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’ • 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. Example for a first turn @ LHC b1 Horizontal Vertical Threading / LTC/ JW

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

  5. 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/ JW

  6. Threader simulation • ‘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… Corrector changes ‘Conditioned’ trajectory file MADX Aperture cuts, noise…. Trajectory file Threading / LTC/ JW

  7. 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/ JW

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

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

  10. 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 b-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/ JW

  11. 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/ JW

  12. 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 ! Threading / LTC/ JW

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