How difficult is threading at the LHC ?

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