1. FP420 Alignment With Beam Position Monitors Jo Pater (Manchester) 14-16 July 2008

2. July 2008J.Pater - BPM-based Alignment2 FP420 Alignment Plan LHC button-style BPMs on fixed beampipe + Wire Positioning System (WPS) –Alignment wire is absolute reference Similar but modified (larger-aperture) BPM on each Hamburg pipe –Referenced to detector by knowledge of mechanics Offline track-based alignment using exclusive dileptons (M.Albrow et.al)

3. July 2008J.Pater - BPM-based Alignment3 Hamburg pipe BPM beam LHC beampipe Alignment wire WPS sensors bracket Fixed BPMs + WPS Overall accuracy of ~10  challenging: tolerances of individual components add up quickly: –WPS sensors: known to be accurate to < 1  –Mechanics: <10  tolerances possible but not easy ! –Complicated by the moving bit –BPMs: need micron-scale accuracy and resolution

4. July 2008J.Pater - BPM-based Alignment4 BPM issues for FP420 –Preliminary study (JP) suggests that BPMs are capable of it (see following slides) –Will need carefully designed readout electronics (see following slides) –UCL engineer (A.Lyapin) on board, experienced with BPMs for linear colliders Electrode design could be tailored to give better performance if necessary –Two larger electrodes (instead of four smaller) would give better performance but only in one dimension Workshop April 2007: what BPMs are the best choice for us? –1st choice: LHC BPMs (electrostatic button-type) Already used in large numbers in LHC –minimises integration issues Can be optimised to special diameters –e.g. to mount on Hamburg pipe Micron-level precision/resolution believed possible, although not demonstrated, by LHC team:

5. July 2008J.Pater - BPM-based Alignment5 Manchester, Cockcroft Alignment Test Bench Damped, floating optical table 1.2m wide x 3m long (see next slide) 3 WPS sensors + wire + readout 2 LHC BPMs (i.e. the fixed BPMs) –Horizontal setup: beam wire stretched by hanging weight over pulley ends of wire on micro-positioners –Read out with network analyser (courtesy of Cockcroft Laboratory): 40 MHz CW on beam wire Electrode signals (unamplified) compared internally with reference  calculated offline –NB: as yet no large-aperture BPMs (i.e. the ones that will move with the Hamburg pipes) 2 Schaevitz LVDTs and signal conditioners –Read out via DMM + Labview

6. July 2008J.Pater - BPM-based Alignment6 LHC BPMs in FP420? Preliminary Study (JP, July 2008) Resolution –A quick study: 6 repeated measurements of  under identical conditions, at two different wire positions, yields standard deviations of 0.00025 and 0.00035. Corresponds to spatial displacement of the wire of about 5 microns. Taken as a measure of what the BPM itself is capable of, this can be considered a ‘worst possible’ resolution as specialised readout electronics can only help. Linearity –See next slides

7. July 2008J.Pater - BPM-based Alignment7 LHC BPM Linearity ± 6mm either side of centre

8. July 2008J.Pater - BPM-based Alignment8 LHC BPM Linearity in 50  steps ~2.5mm from centre

9. July 2008J.Pater - BPM-based Alignment9 LHC BPM Linearity in 10  steps around centre

10. July 2008J.Pater - BPM-based Alignment10 LHC BPM Linearity further from the centre

11. July 2008J.Pater - BPM-based Alignment11 LHC BPM Performance Based on preliminary study –Resolution (~ a few microns) looks acceptable –Linearity Very good over short distances near centre of BPM Less good further away from centre –As expected! –Should be repeatable and therefore correctable Further work needed to determine –Repeatability  correctability  accuracy –Other corrections, e.g. temperature dependence

12. July 2008J.Pater - BPM-based Alignment12 Possible Hardware Solutions for BPM Processing Electronics A. Lyapin (UCL) Narrow bandwidth electronics  commercially available (i-tech)  high resolution as noise is rejected (down to a few um)  gain/offset drifts compensation implemented (stable over hours and days!)  averaging over a few hundred consequent bunches Wide bandwidth electronics  single bunch measurement  poor single bunch resolution (LHC electronics: ~100  m)  Averaging turn-by-turn could improve resolution by sqrt(N)  e.g. standard LHC front-end electronics + custom next-level board  need to take care of drifts  LHC frontend electronics + specialised next-level board

13. July 2008J.Pater - BPM-based Alignment13 BPM tests: next steps Have in hand front-end LHC readout boards 1)Commission them need to bricolage connection to power supplies (don’t have the custom backplane) 2)Test them Use e.g. LabView to simulate averaging over individual bunches 3)If that works well, AL to design next-level board.

14. July 2008J.Pater - BPM-based Alignment14 Potential BPM Calibration Scheme On bench: –Attach fiducials to outside of BPM –Survey --> position of fiducials wrt WPS sensor and beam-wire, fold in BPM response –Must be temperature-dependent (e.g. BPM expansion) In-situ: –Mount some BPMs on positioners calibrate them by offsetting a known amount Cross-calibrate the others by fitting the orbit –Inject pulse to compensate for gain/offset drifts (it should last for at least one normal fill) - method studied by T-474 at SLAC ESA (A.Lyapin) FP420: two of our BPMs already move

15. July 2008J.Pater - BPM-based Alignment15 WPS sensors use a capacitive measurement technique along 2 perpendicular axes. On each axis the wire lies between 2 electrodes Proven resolution ~0.1-0.3 microns LEP energy spectrometer study Reproduced on Manchester bench Wire Positioning Sensors

16. July 2008J.Pater - BPM-based Alignment16 The Moving Bit Need to relate detector position precisely to alignment wire, while allowing detector (on Hamburg pipe) to move freely –LVDT is an obvious potential solution, but off-the-shelf examples not accurate enough: best are ~0.25% of full scale i.e. ~100  on 4cm –Schaevitz ® designed special (rad-hard, very accurate) LVDTs for LHC collimator alignment (see next slide) 0.1-0.04% of full scale i.e. 16-40  on 4cm Compact package (20cm) Rad-hard to 50 MGy, very good temperature stability Company confident they can provide shorter version with significantly better accuracy (at least at one end of stroke.) Have 2 examples of LHC device at Manchester…

17. July 2008J.Pater - BPM-based Alignment17

18. July 2008J.Pater - BPM-based Alignment18 LVDT study at Manchester Resolution Accuracy Temperature dependence and compensation

19. July 2008J.Pater - BPM-based Alignment19 LVDT Resolution As expected, resolution is a function of displacement –(plots show resolution in volts; 10V=25mm) At 25mm,  ~115nm At 10mm,  ~60nm At centre,  ~30nm

20. July 2008J.Pater - BPM-based Alignment20 LVDT Accuracy: Calibrate by scanning across length of LVDT, plotting voltage against nominal x position; fit a line to this data.

21. July 2008J.Pater - BPM-based Alignment21 Can get better accuracy near centre by fitting to central points More work needed: e.g. calibrate at constant temperature

22. July 2008J.Pater - BPM-based Alignment22 LVDT Temperature Dependence Data taken at several displacements… –several days per displacement –Tracking room temperature …shows clear temperature dependence –Probably more than one effect, e.g. Difference in CTEs of support components Effects of temperature on electrical characteristics of LVDT (e.g. wire resistance)

23. July 2008J.Pater - BPM-based Alignment23 LVDT Temperature dependence (2) Should be correctable. First try: Needs more work –Different correction factors for e.g. dT/dt –Better temperature control --> better calibration Have programmable ‘oven’ at Manchester

24. July 2008J.Pater - BPM-based Alignment24 Other BPM-based Alignment Jobs Integration –Must put together working group to integrate alignment hardware. Action JP to coordinate, someone from each relevant area. DAQ requirements (inputs/outputs) need to be defined

25. Reserve Slides

26. July 2008J.Pater - BPM-based Alignment26

27. July 2008J.Pater - BPM-based Alignment27 Resolution/Precision/Accuracy A. Lyapin (UCL) Resolution – the smallest change of the measured value an instrument can see –depends on the sensitivity, noise/adc bit resolution Precision – if multiple measurements of the same value are taken, how far they fall from each other –mainly depends on resolution and scale calibration Accuracy – how far the averaged measured value is from the true value –depends on the offset calibration, drifts and non- linearities Precision and accuracy are usually defined over some period as they degrade High accuracy, low precision From Wikipedia: High precision, low accuracy

28. July 2008J.Pater - BPM-based Alignment28 Narrow- vs. Wide-Band Electronics At 420m, will individual bunches be… –…same size, same orbit as overall beam? Then use narrow-band solution, nothing to be gained from bunch-by-bunch analysis –…smaller than beam, each bunch having a stable individual orbit? Could win by using wide-band electronics, averaging over ~hundreds of turns for each bunch At IP, individual bunch orbits vary by ~1  (for an r.m.s. beam size of 16  )

29. July 2008J.Pater - BPM-based Alignment29 Gain/offset drifts compensation A. Lyapin (UCL) T-474 experiment at SLAC ESA  active monitoring system sending CW burst into processing electronics when there is no beam induced signal  clear gain drifts have already been observed  compensation hasn’t yet been demonstrated  method under study