On reproducibility

1
On reproducibility

• From several inputs of N. Sammut, S. Sanfilippo,
  W. Venturini
• Presented by L. Bottura
• LHCCWG - 4.10.2006

2
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

3
GeometryEffect of repeated cycles
Data courtesy of N. Sammut
Six loadline measurements separated by 100 cycles
Static nominal current
Standard deviation for both cycles is below 0.01
units for b3 which is lower than the measurement
repeatability
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4
GeometryChanges over the magnet life
Data courtesy of N. Sammut
MB1017 - magnetic measurement in April 2003 -
magnetic measurement in September 2005
Static nominal current
Effect is small within measurement uncertainty
but still larger than measurement repeatability
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5
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 _at_ 17 mm

6
Persistent currentsEffect of precycle - MB - 1
Data courtesy of N. Sammut, S. Sanfilippo
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7
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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8
Persistent currentsEffect of precycle - MQY
Data courtesy of W. Venturini
Differences in TF in the range of 10 units
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9
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 _at_ 17 mm
• MQY (Imin 50 vs. 200 A)
• u(b2) 10 units _at_ 17 mm

These values are relevant only if the pre-cycle
is changed from run to run
10
Decay Effect of powering cycle
Data courtesy of N. Sammut
Large effects observed on harmonics
Main field dependency has larger random Also
because it is more difficult to measure (range
12 units)
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11
DecayModel of powering cycle
Courtesy of N. Sammut
Median of the model error
b1 b3 b5
IFT 0.835 0.03 0.016
tFT - 0.02 -
tpreparation - 0.07 -
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12
DecayAperture difference - 1
Standard cycle (30 flat-top), 1000 s injection
Negligible systematic difference
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13
DecayAperture difference - 2
Influence of flat-top current, 1000 s injection
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14
DecayAperture difference - 3
Influence of flat-top time, 1000 s injection
Influence of wiaiting time, 1000 s injection
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15
DecayEffect of repeated cycles
Error is small and comparable to median of max
scaling error for powering history
b3 ? 0.05 units
b5 ? 0.004 units
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16
DecayChanges over the magnet life
MB1017 - magnetic measurement in April 2003 -
magnetic measurement in September 2005
Dynamic decay amplitude
Change is comparable to the static and dynamic
model error
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17
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

18
Uncertainty after correction
Values estimated for MBs 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
19
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
20
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

21
ExamplesEffect of precycle - MQT
Data courtesy of W. Venturini
The effect low current cycling can be massive
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