A Polarity Checker for LHC Magnets

1. Introduction Measurement method Hardware Software Characterization Test results Conclusions and outlook A Polarity Checker for LHC Magnets L. Bottura, G. Brun, M. Buzio, G. Fievez, P. Galbraith, J. Garcia Perez, R. Lopez, A. Masi, S. Russenschuck, N. Smirnov, F. Thierry, A. Tikhov Contents Ref: M Buzio et al, “Checking the Polarity of Superconducting Multipole LHC Magnets”, paper presented at MT-19

2. 1.1 – Introduction: Purpose of the system • the LHC will include about 1750 cryomagnet assemblies, up to almost 16 m long, housing a total of about 10000 superconducting magnets connected in 1612 electrical circuits • magnet connection errors are always detrimental and may be unacceptable in some cases, including esp. main dipoles and quadrupoles, insertion region magnets, skew and tuning quadrupole correctors • need to check systematically multipole order, type and polarity of all LHC magnetic elements • automated, self-contained probe based on a rotating Hall sensor designed and built • similar system used with success at BNL for RHIC and presented at IMMW XI, Brookhaven (A. Jain et al.) 1232 cryodipoles, including 3696 corrector spool pieces 360 arc Short Straight Sections, divided in 61 sub-types including 260 main quadrupolesand 1080 corrector magnets 106 Short Straight Sections for the insertion regions ~8000 total superconducting corrector magnets

3. 1.2 – Introduction: Specifications Specifications: - general-purpose system for any multipole order and type (normal or skew) - automatic, self-contained, fast - room temperature measurements, fit inside beam pipe (Ø 50 mm) - minimum field 60 mT (~earth field !)

4. 2.1 – Measurement method: Harmonic analysis of radial field • Radial component (normal to Hall sensor) • Vector of N values sampled at regular intervals • DFT of the radial field vector • Inverse DFT of the radial field vector expand, equate term by term Harmonic field coefficients as a function of the DFT of sampled values * denotes complex conjugation

5. Cn I1 I I2 Cn I I1 I2 2.2 – Measurement method: Transfer function • For greater accuracy, polarity is determined on the basis of (approximate) transfer function rather than raw harmonic measurement • An arbitrary number of current points can be specified; minimum is two, of opposite sign if possible • Linear best fit to {Cn,I} pairs Magnets without diode installed Transfer function [T/A @ 17 mm] Magnets with diode installed Remanent field [T @ 17 mm](+ Hall voltage offset)

6. Functional block diagram 3.1 – Hardware: Probe (“mouse”) • 4+1 units in operation at CERN • 24x256-stage stepping motor with 22:1 reduction gearbox • electrolytic tilt sensor • longitudinal transport motor not in use (manual positioning)

7. Voltmeter (tilt sensor) Power supplies for rack, DC motor, tilt sensor, stepping motor 50 mA Hall supply 3.2 – Hardware: DAQ system Windows PC(+ DAC for Hall output acquisition) Keithley 2001 multiplexer DAQ electronics rack Custom data switching unit Bipolar Kepco magnet power supply Mobile rack

8. 3.3 – Hardware: Connections to main cryoassemblies • standardized connections to cryodipoles and arc Short Straight Sections allow fully automatic sequential powering of the magnets • connections to magnets for the Insertion Regions are done manually (106 cryoassemblies, 16 types) cryodipole short straight section

9. 4.1 – Software: LabView user interface Manual input of assembly/magnets to be tested On-line assessment of results by cross-checkingwith expected values Automatic generation of pdf test report User panel, wizard-style interface

10. 4.2 – Software: Configuration files (examples) Magnet definition file

11. 4.3 – Software: Configuration files (examples) Assembly configuration file (…) 4 types of cryodipoles, 61 + 16 types of short straight section in the arcs and insertions Expressed in the magnetic measurement reference frame Magnet configuration file

12. 5.1 – Characterization: Calibration The calibration of the probe concerns mainly two parameters: • Voltage-to-field transfer function of the Hall plate + preamplifier combination (~8.5 mT/V):determined by measuring the loadline of a reference dipole and cross-checking with a Metrolab NMR teslameter • Angular offset between Hall plate and tilt sensor (~10 mrad):determined as the average of the field direction obtained from two harmonic measurements in a reference dipole, inserting the probe from both ends. Other systematic and random factors affecting the measurement that were neglected include:- roll/pitch angle error of the Hall plate ( pick-up of tangential/longitudinal field component) - error R in the radial position R of the Hall plate ( error (n-1)R/R in the field coefficients) - planar effect - temperature drift linearity error of the Hall sensor:<3% for all cases of interest

13. + - magneticfield B force S current N 5.2 – Characterization: Validation (1) The polarity of Hall sensor output was verified with two methods:1) deformation of current-carrying wire from right-hand rule: F = I × B 2) commercial 3D Hall probe teslameter (Metrolab THM7025) results were consistent in both cases

14. 5.3 – Characterization: Validation (2) A systematic verification procedure was carried out on magnets of order n=1 to 5 (total = 5x4x2 measurements): 1) Install magnet so that field is normal positive 2) Check polarity with commercial Hall teslameter 3) Verify multipole order, magnet type and polarity with the Polarity Checker in four cases: { current, insertion from connection or non-connection end} polarity must reverse with the current (always) and with insertion side (only n=2,4) 4) Turn magnet by -/n to make it skew, repeat step 3) expected Br() measured Br() e.g. Normal negative quadrupole results were conforming to expectations in all cases

15. 5.4 – Characterization: Repeatability tests The repeatability of the system was checked by running 60 consecutive measurements in the reference magnets. affected by errors in the dynamic readout of the electrolytic tilt sensor + feed-forward control of the stepper motor repeatability of the main field: better than ~1% in all tested casesrepeatability of field direction: between 2 and 8 mrad

16. 5.5 – Characterization: Overall performance • main field accuracy: depends on repeatability + linearity error • field direction accuracy: depends on repeatability + main field error • time required for - single acquisition: 0.75 s (motor must be switched off)- harmonic measurement: 90 s- full standard cryomagnet: ~1 hour 10% rejection threshold on the difference between measured and expected T.F. inadequate for field direction measurements main field accuracy << amplitude of other harmonic components field direction accuracy << threshold to discriminate phase of main component (worst case=dodecapole=15°) no errors reasonably expected for the target measurement results

17. 6 – Test results: Summary of results of first 505 cryoassemblies 1% of faults in cryodipoles, 20% in Short Straight Sectionsnon-critical errors, all easily rectified at CERN

18. 7 – Conclusions and outlook • 4 units built and in use at CERN, proved reliable and easy to use • 505 cryoassemblies tested, 1200 to go before end 2006 • Automated test procedures for cryodipoles and short straight sections fully established: inner-region insertion quadrupoles/correctors being finalized now • Possible further developments (not really necessary for series tests) include:- improving the mechanics of the longitudinal transport system - characterization of neglected error sources