1 / 19 M. Gateau CERN – Geneva – CH 14th International Magnetic Measurement Workshop September 2005, Geneva, Switzerland Uncertainty on magnetic measurements of the LHC magnets at CERN M. Gateau, L. Bottura, M.Buzio, S. Sanfilippo

2 / 19 M. Gateau CERN – Geneva – CH 14th International Magnetic Measurement Workshop September 2005, Geneva, Switzerland Overview Introduction Uncertainty on magnetic field Repeatability of measurements Reproducibility of measurements Summary & Conclusion

3 / 19 M. Gateau CERN – Geneva – CH 14th International Magnetic Measurement Workshop September 2005, Geneva, Switzerland Overview

4 / 19 M. Gateau CERN – Geneva – CH 14th International Magnetic Measurement Workshop September 2005, Geneva, Switzerland The 3 magnetic measurement systems used for cold characterization of LHC magnets (1) Superconducting dipole on the cold test bench in SM18 equipped with rotating coil system A pair of shafts for measurements of 15-meter- long dipoles  Field quality needs to be determined with high accuracy  10-20% of the 1706 cryo-assemblies will be measured magnetically during cold series tests  3 systems for magnetic measurements at cold 1) Rotating coils Used for dipoles or Short Straight Sections (SSS) of standard length 12 sectors for dipoles & 6 for SSS’s over total magnet length Voltage integral vs. angular position is recorded Field strength & multipoles for dipoles, quadrupoles and associated correctors

5 / 19 M. Gateau CERN – Geneva – CH 14th International Magnetic Measurement Workshop September 2005, Geneva, Switzerland The 3 magnetic measurement systems used for cold characterization of LHC magnets (2) Automated scanner installed on the special SSS’s test bench in SM18 Installation of SSW for special SSS measurement 3) Single Stretched Wire (SSW) Can be used on any length of magnet 1 wire loop over total magnet length Voltage integral vs. wire displacement in transversal plane Integrated strength of quadrupoles and dipoles, (field direction, magnetic axis) (See talk of G. Deferne, Fiducialisation/ Alignment / Axis, Today) 2) Automated scanner Used for SSS & special SSS’s of variable lengths One 600mm-long rotating coil Longitudinal scanning over magnet length Voltage integral vs. angular position Integrated gradient & local multipoles of quadrupoles, (axis)

6 / 19 M. Gateau CERN – Geneva – CH 14th International Magnetic Measurement Workshop September 2005, Geneva, Switzerland Overview

7 / 19 M. Gateau CERN – Geneva – CH 14th International Magnetic Measurement Workshop September 2005, Geneva, Switzerland Uncertainty on magnetic field To fulfil requirements of the beam dynamics, the main field should be known: better than 8 units of uncertainty for dipoles better than 10 units of uncertainty for quadrupoles From cold measurements, we expect to reach less than 1 unit on main field random error for dipoles & quadrupole 0.1 units or better on higher harmonics random error few units on systematic error Measurement uncertainty = random error + systematic error

8 / 19 M. Gateau CERN – Geneva – CH 14th International Magnetic Measurement Workshop September 2005, Geneva, Switzerland Overview

9 / 19 M. Gateau CERN – Geneva – CH 14th International Magnetic Measurement Workshop September 2005, Geneva, Switzerland Rotating coils - Recent results on dipole field integral b1, (bn & an) normalised multipoles (n = 2 to 15)  (units)  (b1)<1 unit  (bn&an) <0.02 units MB Measurement repeatability on integrated 11850A Shafts are not identical: in this particular case, the rotating coil of aperture 1 gives higher accuracy for multipoles Zero sensitivity for n = 12.5 due to measurement coil geometry WITHIN EXPECTED LIMITS

10 / 19 M. Gateau CERN – Geneva – CH 14th International Magnetic Measurement Workshop September 2005, Geneva, Switzerland Rotating coils - Noise study  per bn&an (units) Magnet current (A) MB Noise analysis on normalised multipoles vs. current Noise signal is decreasing as magnet current is increasing  ELECTRICAL NOISE At high currents, noise signal is constant  NOISE MECHANICAL COMPONENT Current ripple not compensated on b1

11 / 19 M. Gateau CERN – Geneva – CH 14th International Magnetic Measurement Workshop September 2005, Geneva, Switzerland Rotating coils - Noise on integrator input Magnet current (A)  per flux n (V.s) MB Noise analysis on flux |∆  n|=N.L.2sin(n  /2).r n /r ref n-1. .T.|∆C n |/(n-1).G with integration period T = 7 ms Noise on flux is of the order of noise limit of VFC (Voltage to Frequency Converter) integrators we use For higher frequency integration, R&D with A/D converters has started (see talk of A. Masi, Fast devices, Tuesday)

12 / 19 M. Gateau CERN – Geneva – CH 14th International Magnetic Measurement Workshop September 2005, Geneva, Switzerland Rotating coils - No degradation with time On b1 (units) On higher orders Current (A) BenchOther info MB  10e F2With PGAs MBP > 10e-35000A2Without PGAs MB NA5000A2Without PGAs MB NA5000A2With PGAs MB  10e F2With PGAs In year 2000 Standard deviation averaged on 12 sectors of different magnets Factor affecting repeatability:  Noise on current  Mechanical noise (rotation)  Electrical noise (cabling of bench)  Integrators offset adjustment  Measurement environment (temperature, humidity) STILL WITHIN EXPECTED TOLERANCES

13 / 19 M. Gateau CERN – Geneva – CH 14th International Magnetic Measurement Workshop September 2005, Geneva, Switzerland Overview

14 / 19 M. Gateau CERN – Geneva – CH 14th International Magnetic Measurement Workshop September 2005, Geneva, Switzerland Rotating coils - Same magnet measured with 2 different coils (1) Position of the 12 measurement coil center on magnet length (mm) B1 (T) MB Main 11850A measured with 2 different coils Confirmation of reliable and stable coil calibration (see talk of O. Dunkel, Coils, Tuesday) Average systematic error on B1 between the 2 measurements = 2.34 units

15 / 19 M. Gateau CERN – Geneva – CH 14th International Magnetic Measurement Workshop September 2005, Geneva, Switzerland Rotating coils - Same magnet measured with 2 different coils (2) b1, (bn & an) normalised multipoles per sector (n = 2 to 15) Difference between the 2 positions (units) b1≈2.34 unitsbn&an<0.2 units Cross-check on rotating coils system gives very conclusive results WITHIN TOLERANCES MB Field 11850A measured with 2 different coils

16 / 19 M. Gateau CERN – Geneva – CH 14th International Magnetic Measurement Workshop September 2005, Geneva, Switzerland SSW & rotating coils - result comparison Difference between the 2 systems on b1 (units) Magnet & magnet aperture Field difference on 11850A measured with 2 different systems Comparison between SSW & rotating coils gives a difference of 5.5 units at maximum (1.98 units in average) The difference between the 2 systems is within expectations WITHIN TOLERANCES

17 / 19 M. Gateau CERN – Geneva – CH 14th International Magnetic Measurement Workshop September 2005, Geneva, Switzerland SSW & automated scanner - result comparison Warm data: Courtesy of P.Hagen & E.Todesco, CERN Warm mole TF (T/kA) Cold TF (T/kA) 17 units W/C correlation of the field gradient transfer function using 2 systems of measurements at cold The 17 unit offset correlate with the calibration uncertainty of 15  m on rotation radius (coil radial position should be known better than 8  m)

18 / 19 M. Gateau CERN – Geneva – CH 14th International Magnetic Measurement Workshop September 2005, Geneva, Switzerland Overview

19 / 19 M. Gateau CERN – Geneva – CH 14th International Magnetic Measurement Workshop September 2005, Geneva, Switzerland Summary & Conclusion Courtesy of L. Bottura, CERN Uncertainty 17mm) Uncertainty on the 3 systems 8 units on B1 10 units on B2 Uncertainty on the dipole main field is of the order of 3 to 5 units for all systems used and sufficient for LHC requirements Uncertainty on the quadrupole main gradient of 5 (SSW) to 35 units (coils) has a large variability from system to system: SSW is at present our reference system