On reproducibility

From several inputs of N. Sammut, S. Sanfilippo, W. Venturini

Presented by L. Bottura

LHCCWG - 4.10.2006

Components & reproducibility Geometric

Proportional to conductor and iron positions and shapes

May change from cycle to cycle (powering and thermal) due to conductor displacement because of the effect of Lorentz and thermal stresses

Persistent currents Depends on the integral of the

magnetic moments of each strand in the coil (including iron contribution)

May change from cycle to cycle (powering) due to the hysteretic nature of the magnetic moments

Saturation Depends on the shape and

characteristics of the iron yoke There is no physical

mechanism that could produce a relevant change during the magnet lifetime

Decay & Snapback Depends on the powering

history and on the cable characteristics

Different magnet to magnet Changes from cycle to cycle

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Static – nominal current

GeometryEffect of repeated cycles

Data courtesy of N. Sammut

Six loadline measurements separated by 100 cycles

3.05

3.055

3.06

3.065

3.07

3.075

3.08

3.085

1 cycle 100 cycles 200 cycles 300 cycles 400 cycles 500 cycles

b3 geometric (units)

aperture 1

3.97

3.975

3.98

3.985

3.99

3.995

4

1 cycle 100 cycles 200 cycles 300 cycles 400 cycles 500 cycles

b3 geometric (units)

aperture 2

Standard deviation for both cycles is below 0.01 units for b3 which is lower than the measurement repeatability

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0.00001

0.0001

0.001

0.01

0.1

1

10

b1 b2 a2 b3 a3 b4 a4 b5 a5

0.00001

0.0001

0.001

0.01

0.1

1

10

b1 b2 a2 b3 a3 b4 a4 b5 a5

harmonic (-)

aperture 1

aperture 2

Effect is small within measurement uncertainty but still larger than measurement repeatability

Static – nominal current

MB1017 - magnetic measurement in April 2003- magnetic measurement in September 2005

GeometryChanges over the magnet life

Data courtesy of N. Sammut

GeometrySummary of uncertainty

uncertainty estimated as 3 of multipoles repeatedly measured on the same magnet (few magnets tested)

after powering after training

u(b1)=2.8 units u(b3)=0.3 units @ 17 mm

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Persistent currentsEffect of precycle - MB - 1

Data courtesy of N. Sammut, S. Sanfilippo

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Persistent currentsEffect of precycle - MB - 2

Data courtesy of N. Sammut, S. Sanfilippo

Differences in TF up to ≈ 1.5 units, on b3 up to ≈ 1 unit

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Persistent currentsEffect of precycle - MQY

Data courtesy of W. Venturini

Differences in TF in the range of 10 units

Persistent currentsSummary of uncertainty

The effects are large (of the order of 10 units)

The variability associated with powering cycles is very large

MB (IFT 2 kA vs. nominal) u(b1) ≈ 1.5 units u(b3) ≈ 1 units @ 17 mm

MQY (Imin 50 vs. 200 A) u(b2) ≈ 10 units @ 17 mm

These values are relevant only if the pre-cycle is changed from run to run

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Decay Effect of powering cycle

Data courtesy of N. Sammut

Large effects observed on harmonics

Main field dependency has larger randomAlso because it is more difficult to measure (range 1…2 units)

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DecayModel of powering cycle

Courtesy of N. Sammut

b1 b3 b5

IFT 0.835 0.03 0.016

tFT - 0.02 -

tpreparation - 0.07 -

Median of the model error

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DecayAperture difference - 1

Standard cycle (30’ flat-top), 1000 s injection

Negligible systematic difference

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DecayAperture difference - 2

Influence of flat-top current, 1000 s injection

57

DecayAperture difference - 3

Influence of flat-top time, 1000 s injection

Influence of wiaiting time, 1000 s injection

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t (s)

nominal current

injection current

I (A)

quench measurement 1 measurement 2 measurement 3 measurement 4

1.06

1.08

1.1

1.12

1.14

1.16

1.18

1.2

1.22

1.24

0 1 2 3 4 5Number of preceding LHC cycles

-0.137

-0.136

-0.135

-0.134

-0.133

-0.132

-0.131

-0.13

-0.129

-0.128

-0.127

0 1 2 3 4 5Number of preceding LHC cycles

b3 0.05 units

b5 0.004 units

Error is small and comparable to median of max scaling error for powering history

DecayEffect of repeated cycles

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Change is comparable to the static and dynamic model error

0.001

0.01

0.1

1

b1 b3 b5harmonic (-)

decay amplitude difference (units)

aperture 1

aperture 2

MB1017 - magnetic measurement in April 2003- magnetic measurement in September 2005

Dynamic – decay amplitude

DecayChanges over the magnet life

DecaySummary of uncertainty

Although we have seen (much) better, we maintain that the empirical model (data fits) has a typical error that can amount to up to 20 % of the effect

Main source of uncertainty is from the modelling of powering history, all other effects (aperture differences, cycle details, ageing) are small and have negligible systematic

Why so cautious ? The sample of magnets used for the

data-fitting is limited (10 magnets) This adds an uncertainty in the

projection of the average

Uncertainty after correction

Values estimated for MB’s in July 2004, RMS rview

NOTE: variations of pre-cycle from the nominal one (e.g. due to limitations during commissioning or changes in optics) will cause an additional uncertainty that can be much larger than the above values

Open issues

We know what we know

and we know how well we know what we know

but

we do not know what we do not know

nor do we know how badly we do not know what we do not know

think we

think we

think we

think we

think we

think we

I think we

I think

Examplesa2 anomaly in Ansaldo-2 (2002)

The shape of the a2(I) has a strong anomaly in one aperture of on Ansaldo-2 (2002) reassembled

This data is real ! not a cable hysteresis measurements are OK as far

as we can tell a magnetic piece (protection

layer, shim,…) in the collared coils?

Observed in few other magnets

Depends linearly on maximum current reached

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ExamplesEffect of precycle - MQT

Data courtesy of W. Venturini

The effect low current cycling can be massive