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Superconducting Magnet Division
Magnetic Measurements
Animesh JainSuperconducting Magnet Division
Brookhaven National Laboratory, Upton, NY 11973
Superconducting Magnet Division – S&T Committee Program Review
June 22-23, 2006
Magnetic Measurements: Slide 1Superconducting Magnet Division –S&T Committee Program Review
Magnetic Measurement Goals
• Measurement of field quality in magnets:
– Ensure that field quality meets specified requirements
– Detect any gross construction errors
– Monitor trends and provide timely feedback to ensureuniformity in a large production run (e.g. RHIC).
– Provide feedback during construction (using interim measurements) to improve field quality (e.g., shims for RHIC insertion quads and SNS magnets; pattern modulation in direct-wound magnets)
– Provide data needed for use of the magnets in accelerator(e.g., excitation curves, superconductor magnetization effects at injection, ramp rate effects, iron yoke saturation effects)
• Measurement of field center and field direction:– Provide data needed for magnet installation and alignment
• Superconducting Magnet Division (SMD) has been active in developing new techniques, as needed, to meet these goals.
Magnetic Measurements: Slide 2Superconducting Magnet Division –S&T Committee Program Review
Field Quality Measurement Basics
• Field quality in accelerator magnets is expressed in terms of harmonics (coefficients Bn and An in a series expansion of 2-dimensional field):
Rref is an arbitrary “Reference radius”, typically ~50-70% of coil inner radius.
• These coefficients are most conveniently and accurately measuredusing rotating coils.
• The angular dependence of field is picked up by a rotating coil and a Fourier analysis of the signal gives all the harmonics.
• A rotating coil system typically has several different loops to “buck out” certain harmonics. This is done to minimize errors caused by imperfections in the rotational motion of the coil.
• SMD uses a tangential coil design with a total of 5 windings.
[ ]1
1
)cos()sin(),(
−∞
=∑
+=
n
n ref
nnrR
rnAnBrB θθθ [ ]
1
1
)sin()cos(),(
−∞
=∑
−=
n
n ref
nnR
rnAnBrB θθθθ
Radial component: Azimuthal component:
Magnetic Measurements: Slide 3Superconducting Magnet Division –S&T Committee Program Review
Cross Section of a Typical Measuring Coil in SMD
All SMD coils are wound in grooves machined into cylinders. Most coils have a similar cross section, with 5 active windings.
2 Dipole (3 turns each)2 Quad (3 turns each)1 Tangential (30 turns, 150)
Many coils of different radii and lengths havebeen built to suit different magnet sizes.
Although primarily designed for dipoles and quadrupoles, the same coil design can be used to measure practically all types of magnets (dipole through 12-pole, except octupole) by adjusting the weight factors in a digital bucking scheme.
1 50
Magnetic Measurements: Slide 4Superconducting Magnet Division –S&T Committee Program Review
Examples of Rotating Coils
247 mm Diameter,3 m Long,External Drive Coilin a SNS Quadrupole
Smallest coil availableis 15 mm in diameter.
Shortest coil availableis 51 mm long.
“Moles” for measuring long magnets in several short sections.
55 mm dia., 1 m long
148 mm dia., 0.92 m long
68 mm dia., 0.05 m long
–Developed for SSC
–R&D100 Award 1988
Magnetic Measurements: Slide 5Superconducting Magnet Division –S&T Committee Program Review
Application: Electrical Short in HERA Quadrupole
-6.5
0
6.5
-6.5 0 6.5X (cm)
Y (
cm)
Zero Current
QH0103: Q2
Large changes in harmonics were seen in the Q2 layer after it was buried under three more layers.
Most likely cause was a short between the lead and a pole turn.
This model explained all changes.
Computed Vs Measured Changes
-60
-50
-40
-30
-20
-10
0
10
20
30
40
50
60
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16Harmonic Number
Ch
an
ge i
n H
arm
on
ic (
un
its
at
50 m
m)
Normal: MeasuredNormal: CalculatedSkew: MeasuredSkew: Calculated
Note: Reference radius increased from 31 mm to 50 mm to clearly show the higher harmonics
2 turns of Q2exposed for repair
A repair was performed without sacrificing any layer
End region of a pole
Magnetic Measurements: Slide 6Superconducting Magnet Division –S&T Committee Program Review
Measurement of Magnetic Field Center
• Field center needs to be measured relative to external fiducialsin order to install and align the magnets.
• Rotating coils can measure the magnetic axis relative to the rotation axis (typical noise ~ 2-5 µm in quadrupoles).
• It is often difficult to locate the rotation axis relative to thefiducials, particularly for short coils inside long magnets.
• The technique of “Harmonic Antenna” was developed at SMD for RHIC magnets
– Uses stationary pick up coils and AC excitation of magnet at~10-50 Hz
– Special coil geometries measure only the relevant harmonics
– Fiducials on the antenna to measure its location in external frame
Magnetic Measurements: Slide 7Superconducting Magnet Division –S&T Committee Program Review
Non-rotating Harmonic Coil (Antenna)
6 Windings (2 Dipole; 2 Quadrupole; 2 Octupole):Can measure Quadrupoles through 12-pole magnets
Cross-wires on each end to locate antenna position
Sinusoidal AC excitation of the magnet. FFT of pick up signal.
A similar concept was adopted later at CERN for the LHC
Magnetic Measurements: Slide 8Superconducting Magnet Division –S&T Committee Program Review
Quadrupole Vibration Measurements at 4.5 K
Actively Stabilized Isolator
(< 1 nm RMS integrated above 10 Hz)
Laser source and fiber head on isolation table
HorizontalGeophone
VerticalGeophone
Viewport
RHIC Spare CQS Magnet
Effect of cryogen flow on the cold mass motion was measured with ~ a few nm resolution using this set up.
Magnetic Measurements: Slide 9Superconducting Magnet Division –S&T Committee Program Review
Quad Center Motion at nm Level in Linear Collider(Work is in early stage)
2500-turn coil from CERN (32 µm wire)
coil mounted in asolid G-10 block
Platformfor
geophone to monitor coil motion
End Support
FiberGlassTube
81.3 mmOD
2.53 m2.53 m
125 mm x 13.5 mm125 mm x 13.5 mm
Magnetic Measurements: Slide 10Superconducting Magnet Division –S&T Committee Program Review
Harmonic Measurements in Time Varying Fields
• Rotating coils are ideally suited only for DC fields.
• In many instances, it is necessary to measure the field quality under dynamic conditions.– Time decay and snapback due to persistent current effects– Eddy current induced harmonics while ramping
• Typical rotating coil speed is ~3.5 sec per revolution.– 128 angular positions; 1 power line cycle integration at each reading.
• A time resolution of 1.3 sec has been achieved with 64 angular positions (useful for time decay and snapback studies).
• A new iterative analysis procedure was developed to allow rotating coil measurements during ramping at relatively low rates (below ~0.1T/s).
• An entirely new system consisting of many coils was developed for very high ramp rates (> 1 T/s) for two recent projects.
Magnetic Measurements: Slide 11Superconducting Magnet Division –S&T Committee Program Review
Sextupole Time Decay & Snapback in a LHC Dipole
-21.0
-20.5
-20.0
-19.5
-19.0
-18.5
-18.0
-17.534
5
350
355
360
365
370
375
380
385
390
395
400
405
410
415
420
Current (A)
No
rma
l S
extu
po
le (
un
it a
t 2
5 m
m)
40 A/s pre-cycle; 10 A/s ramp
Dec
ay o
ver
~ 3
0 m
in.
Snapback when ramping is resumed
Dependence on pre-cyclehistory & ramp rate has been studied
1.3 sec between data points
10 A/s= 7 mT/s in this magnet
Rotating coil data with an iterative analysis for slowly varying fields
Magnetic Measurements: Slide 12Superconducting Magnet Division –S&T Committee Program Review
Harmonic Measurements at Very High Ramp Rates
16 Printed Circuit coils
6 turns/layer; 10 layers
300 mm long
0.1 mm lines with0.1 mm gaps
Radius = 26.8 mm (GSI)35.7 mm (BioMed)
System of Coils Used in Stationary Mode
Magnetic Measurements: Slide 13Superconducting Magnet Division –S&T Committee Program Review
Harmonic Measurements at Very High Ramp Rates
-1.0E-04
0.0E+00
1.0E-04
2.0E-04
3.0E-04
4.0E-04
5.0E-04
6.0E-04
7.0E-04
8.0E-04
9.0E-04
0 1 2 3 4 5 6 7
Current (kA)
B3
[D
n–
Up
] (T
esla
, 2
5m
m)
DC 1.5 T/s
2 T/s 3 T/s
4 T/s
Sextupole Hysteresis at Ramp Rates of 0 to 4 T/s
DC
4 T/s
3 T/s
GSI Dipole1 m Long Prototype
Magnetic Measurements: Slide 14Superconducting Magnet Division –S&T Committee Program Review
Summary
• The Superconducting Magnet Division is well equipped to carry out field quality measurements in most accelerator magnets.
• Rotating coils are the primary tools employed for magnetic measurements, although other techniques, such as Hall probes and NMR, are also used where needed (not covered in this talk).
• New measurement and analysis techniques are developed to meet measurement challenges– e.g., Harmonic Antenna; Fast Rotating Coil; Stationary Coil
• SMD has successfully measured large production runs, such as for the RHIC and SNS, meeting all the measurement goals.
• We look forward to new measurement challenges– Electron Cooling, NSLS-II, Rapid Cycling Synchrotron, ILC, ....