Eleventh International Magnet Measurement Workshop September 21 - 24, 1999

1. A Mole for warm magnetic and optical measurements of LHC dipoles1 L Bottura1, M Buzio1, G Deferne1, H Jansen2, C Glöckner2, A Köster2, P Legrand1, A Rijllart1, P Sievers1 , F Villar1 1 CERN, European Organization for Nuclear Research - 1211 Geneva 23, Switzerland 2Frauenhofer Institut IPT, Aachen, Germany Eleventh International Magnet Measurement Workshop September 21 - 24, 1999 Brookhaven National Laboratory, Upton, New York, USA Thursday, 23 Sept.1999Berkner B10:50-11:20 1Also published in the proceedings of the 16th International Conference on Magnet Technology, Ponte Vedra Beach, IL, 26 Sept- 2 Oct 1999

2. Introduction Mechanical layout VR Demo1 Magnetic performance Optical performance Conclusions A Mole for Warm Magnetic and Optical Measurements of LHC Dipoles 1 http://home.cern.ch/m/mtauser/www/applications/mole/virtualmole.wrl

3. Introduction • Why use a mole ?¾Purpose: measure magnetic axis ¾Cold measurements done for max. efficiency with 15 m long coil shaft, obstructing line-of-sight to the coil center Þtravelling probe is needed¾Additional duties: warm field quality of all dipoles, cold field of samples • Requirements for series measurements¾ Coil center position: precision better than 0.1 mm¾ Warm measurements:Æ 50 mm, B  25mT¾ Rugged, quick and simple to setup and operate ¾ As many off-the-shelf components as possible

4. Mole test bench overview magnet front reference bench(telescope) rear reference bench(transport motor, cable connections)

5. Front reference bench Telescope CCD camera mole support shell (closed) focusing system Sliding support

6. Rear reference bench Axis transfer jig Mole Cable receptacle Transport motor

7. Front reference bench optical alignment targets (4) Laser for alignment measurement axis transfer carbon fiber jig Telescope/CCD/DSP unit granite table transport belt

8. Status • Warm Mole ¾First unit accepted and under commissioning ¾Two more units to come, will perform warm test of all LHC dipoles¾Can be readily adapted to warm quadrupole tests • Cold Mole¾ Order started, last design details now being finalized¾ Will perform cold tests for a sample populations of LHC dipoles to get warm-cold axis correlation ¾ Further units may be slightly modified for cold quadrupole tests

9. Mechanical layout • Main features¾three 750 mmlong radial coils ¾ coil drive: Shinsei travelling-wave ultrasonic piezo motor ¾ 4096 CPR Heidenhein angular encoder ¾On-board feedback auto-leveling system ¾Spring-loaded roller system¾Motorized traction belt drive¾Two granite benches to establish reference positions¾LED+lens system to create virtual light-spot in the coil centre¾Telescope + CCD camera + DSP to measure light-spot position¾All functions remotely controlled via RS232¾Auxiliary optical system to transfer axis to magnet fiducials

10. Main components 3:1 reductiongearbox Coil Piezo motor Wyler encoder Potentiometer Ball-bearing support flange Elastic coupling(with helical groove for the cable)

11. Longitudinal transport • closed-loop pre-tensioned Inconel transport belt • feedback-controlled DC motor drive • referencing given by optical barriers at both ends of the magnet (self-calibration at the end of each trip) • max. velocity about 1 m/s • precision of positioning better than 0.5 mm • mole must be switched manually between apertures

12. Longitudinal reference position spring-loadedroller LED viewingaperture Optical barrier(receptor) lower half-shellsupport front transport belt attachment

13. Safety • interlock system: mole will stop if covers are removed, transport cable gets tangled, transport motor drains overcurrent • reduced velocity close to the edges • longitudinal position cross-checked with two independent measurements (angular encoder + linear potentiometer)

14. Autoleveling • Why autoleveling ?¾imperfections of beam pipe/rollers  rotation of container during travel¾observed rotation up to 2~3°/m¾ transport belt & cable may be twisted¾coil must be level prior to measurement • How does it work ?¾DC motor engaged in container to rotate coil+encoder+piezo motor ¾feedback system using wide-range level meter ¾ precision achieved better than 4 mrad¾ time needed less than 5 s¾autoleveling done at the end of each longitudinal movement

15. Auto-leveling mechanism Inner gear (linked with container) Motor shaft gear Wide-rangelevel meter Roller (fixed tangentially by friction)

16. Auto-leveling mechanism travel autolevel End of travelling Final configuration Start

17. Optical measurement • Telescope¾accurately leveled and aligned wrt magnet ¾ mounted on rails to serve both apertures • CCD camera¾752 582 pixel, 6.00 4.96 mm size ¾ self-aligned to telescope via reference marks • DSP image processor¾threshold filtering + center of gravity calculation¾ measurements internally averaged over multiple takes¾ software calibration map

18. Virtual lightspot the virtual image remains in the coil center even when the mole rotates LED Magnifying lens Virtual lightspot

19. Optical measurement • Performance¾4 mm 14 mm  20 m range ¾ total error  60 m¾ measurement time  4 s at closest range

20. Optical system calibration

21. Optical system calibration

22. Example: optical measurement

23. Focusing system • How does it work¾no autofocus needed¾ longitudinal position sent to the camera at the end of each movement ¾ focusing motor follows a software calibration table ¾ total error if out-of-focus  1 m/mm

24. Magnetic performance • Harmonic Coils¾41750 mm fiberglass/epoxy coil support ¾ 3 radial coils with 400 turns, 3.4 m2 area¾ 1 coil for main dipole, 1 for dipole compensation + 1 spare • System precision¾Main dipole: better than 10-4¾ Dipole angle: better than 0.2 mrad¾ Multipoles: generally better than 2·10-6 relative to B1¾bucking factor  2000

25. Example: multipoles

26. Example: calibrated dipole angle

27. Axis Transfer • Why transfer the magnetic axis ? ¾ harmonics, and hence magnetic axis, are measured in the coil reference frame¾ the center of the coil is measured in the telescope reference frame¾ the magnetic axis is needed in the reference frame of the magnet fiducials • How the transfer will be done¾ the granite tables establish two reference positions per aperture, vertically and laterally reproducible within 20 m¾ reference positions are measured by the telescope ¾relative position of magnet fiducials and granite tables is measured by means of an auxiliary laser system ¾relative position of coil center wrt magnet fiducials can thus be computed

28. Axis Transfer Granite jig reference line Granite table reference line  Mole coil positionalong magnet   Auxiliary alignment laser beam  Magnet fiducial line   Granite table jig Telescope frame

29. Axis Transfer Z  Magnet fiducial line  Magnetic center (z)  Marble table jigs line   Mole coil position Y Marble table reference line Telescope reference frame

30. Problems encountered • Unsteady velocity of piezo-motor, getting worse with motor heating • Micro cables and connectors delicate and difficult to handle • Fine alignment of the lens • Image distortion due to thermal gradients in the magnet

31. Piezo motor rotation speed • Rotation speed quality ¾ encoder detects low-frequency rotation speed oscillations up to 8%¾ oscillation amplitude gets larger as motor heats up¾ oscillations interfere with measured harmonics (errors in b2 up to 0.5 units observed) • Improvements under way¾ optimization of stiffness and damping of the coil/motor link to minimize response ¾ fine tuning of motor speed map (t) to minimize excitation¾ enhanced motor cooling ¾automatic off-line filtering of spurious harmonics

32. Piezo motor rotation quality Forward run Backward run average

33. Piezo motor rotation quality

34. Piezo motor rotation quality

35. Conclusions • Magnetic and optical performance generally exceeding specifications • Well suited for industrial-type test environment • Test bed for further technological improvements (piezo-motor, remote encoders, etc.) • Software integration under way • Minor improvements under way: smoother velocity control, better optics alignment