1 / 19 M. Gateau CERN – Geneva – CH [email protected] 14th International Magnetic...

M. GateauCERN – Geneva – CH

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14th International Magnetic Measurement Workshop26-29 September 2005, Geneva, Switzerland 1 / 19

Uncertainty on magnetic measurements of the

LHC magnets at CERN

M. Gateau, L. Bottura, M.Buzio, S. Sanfilippo


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Overview

Introduction

Uncertainty on magnetic field

Repeatability of measurements

Reproducibility of measurements

Summary & Conclusion


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Overview

Introduction

Uncertainty on magnetic fieldUncertainty on magnetic field

Repeatability of measurementsRepeatability of measurements

Reproducibility of measurementsReproducibility of measurements

Summary & ConclusionSummary & Conclusion


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The 3 magnetic measurement systemsused 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 accuracy10-20% of the 1706 cryo-assemblies will be measured magnetically during cold series tests3 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


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The 3 magnetic measurement systemsused 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)


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Overview

IntroductionIntroduction






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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


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Overview



Repeatability of measurements




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Rotating coils - Recent results on dipole field integral

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

shaft in ap1shaft in ap2

b1, (bn & an) normalised multipoles (n = 2 to 15)

(u

nits

)(b1)<1 unit

(bn&an)<0.02 units

MB3348 - Measurement repeatability on integrated field @ 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


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Rotating coils - Noise study

per

bn

&an

(u

nits

)

0.E+00

1.E-01

2.E-01

3.E-01

4.E-01

0 2500 5000 7500 10000 12500

0.E+00

1.E-02

2.E-02

3.E-02

4.E-02

5.E-02

0 2500 5000 7500 10000 12500

Magnet current (A)MB1222 - 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


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Rotating coils - Noise on integrator input

1.E-08

1.E-07

1.E-06

1.E-05

1.E-04

0 2500 5000 7500 10000 12500

Magnet current (A)

p

er fl

ux n

(V

.s)

MB1222 - Noise analysis on flux

|∆n|=N.L.2sin(n/2).rn/rrefn-1..T.|∆Cn|/(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)


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Rotating coils - No degradation with time

On b1(units)

On higherorders

Current(A)

Bench Other info

MB3348 0.57 10e-3 11850 F2 With PGAs

MBP201 0.14 > 10e-3 5000 A2 Without PGAs

MB1017 0.13 NA 5000 A2 Without PGAs

MB1017 0.13 NA 5000 A2 With PGAs

MB1022 0.27 10e-3 5000 F2 With 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


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Overview




Reproducibility of measurements



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Rotating coils - Same magnet measured with 2 different coils (1)

Position of the 12 measurement coil center on magnet length (mm)

B1

(T

)

MB1222 - Main field @ 11850A measured with 2 different coils

8.310

8.315

8.320

8.325

63

0

18

90

31

50

44

10

56

70

69

30

81

90

94

50

10

71

0

11

97

0

13

23

0

13

86

0

reference position1 sector shifted position

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


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Rotating coils - Same magnet measured with 2 different coils (2)

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

1.E+01b1, (bn & an) normalised multipoles per sector (n = 2 to 15)

Diff

ere

nce

be

twe

en

the

2 p

osi

tion

s (u

nits

)

b1≈2.34 units bn&an<0.2 units

Cross-check on rotating coils system gives very conclusive results

WITHIN TOLERANCES

MB1222 - Field difference @ 11850A measured with 2 different coils


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SSW & rotating coils - result comparison

0

1

2

3

4

5

6

MB

10

09

Ap

1

Ap

2

MB

10

14

Ap

1

Ap

2

MB

10

15

Ap

1

Ap

2

MB

10

19

Ap

1

Ap

2

MB

30

05

Ap

1

Ap

2

MB

30

11

Ap

1

Ap

2

MB

33

33

Ap

1

Ap

2

Diff

ere

nce

b

etw

ee

n th

e 2

sy

stem

s o

n b

1 (u

nits

)

Magnet & magnet apertureField difference on B1 @ 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


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SSW & automated scanner - result comparison

Warm data: Courtesy of P.Hagen & E.Todesco, CERN

Warm mole TF (T/kA)

Co

ld T

F (

T/k

A)

58.2

58.3

58.4

58.5

58.6

57.9 58.0 58.1 58.2 58.3

SSWAutomated scannerScanner ideal fitSSW ideal fit

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)


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Overview







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Courtesy of L. Bottura, CERN

Un

cert

ain

ty(u

nits

@ 1

7m

m)

Uncertainty on the 3 systems

0.01

0.1

1

Long shaft Automatedscanner

SSW

bn&an

8 units on B1

1

10

100

Long shaft Automatedscanner

SSW

B1B2

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

1 / 19 M. Gateau CERN – Geneva – CH [email protected] 14th International Magnetic...

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