Download Measuring Magnetic Field Transients

Measuring Magnetic
Field Transients
Philip Keller
Magnetic Field Transients
§  Examples:
•  Eddy currents:
-  Gradient coils in MRI
-  Switching magnets in
particle accelerators
•  “Decay & snapback”
§  Hard to measure:
•  Small effect on top of
strong field
•  Typically short-lived
§  Answer: fluxmeters!
Fluxmeter : history
Flip Coil (ca. 1927)
Ballistic galvanometer (1938)
Photos: University of Nebraska, Dept. of Physics
Fluxmeter: principles
§  Faraday’s law of induction:
t
end
dΦ
V =−
⇒ ΔΦ = − ∫ V dt
dt
tstart
B=0
§  Flux density:

Φ = ∫ B ⋅ dâ ⇒
S
ΔΦ
1
ΔB =
=−
A cosϑ
A cosϑ
V
§  Coil:
tend
∫ V dt
tstart
A = Na
Φ
x
§  Units (SI):
•  Flux (Φ): Wb = V×s
•  Flux density (B): T=V×s/m2
Pros and cons
§  Pros:
•  Very flexible
•  Precise
•  Mapping “built in”
•  Can measure integrated field or gradient directly
•  Can measure flux density inside iron
§  Cons:
•  Requires thought
•  Requires a coil – possibly very high-tech
•  Requires good technique:
drift compensation, noise, coil motion, thermal expansion,
thermocouple effects…
Clever industrial applications (1)
§  Magnetic moment
measurement:
§  Remove
 or flip:
ΔΦ ~ µ ⋅ ẑ
§  Quick check of
permanent magnets
Photo: Magnet-Physik Dr. Steingroever GmbH
Clever industrial applications (2)
§  Magnetic potential
measurement:

  ∂D

∇× H = Jf +
→ 0 ⇒ H = −∇ψ
∂t
∞ 
∞
∫ B ⋅ dlˆ = −µ0 ∫ (∇ψ )⋅ dlˆ = µ0 (ψ (A) − ψ (∞))
A
A
§  E.g. find H in gap:
•  Measure potential
difference ψa-ψb
•  Divide by s
Photo: Laboratorio Elettrofisico Engineering Srl
Diagram: E. Steingroever, G. Ross, Magnetic Measuring Techniques
Industrial applications: integrator
§  Several manufacturers
§  Typical specifications:
• 
• 
• 
• 
• 
• 
Resolution: 10-7 Vs
Accuracy: 0.1%
Drift: 10-6 FS/min
Max rate: 25 meas/s
Vin(max): 60 V
Rin: 100 kΩ
Diagrams: E. Steingroever, G. Ross, Magnetic Measuring Techniques
Research applications
§  Characterization of
accelerator magnets
§  Multipole expansion with
~10-5 resolution
§  Partial integrals (e.g.
triggered by encoder)
§  Measure time as well as
voltage
§  “Bucking” coils to improve
SNR => multiple channels
Research applications: integrator
§  PDI5025 (1986-2009)
§  Specifications:
• 
• 
• 
• 
• 
• 
• 
• 
Channels: 1 or 2
Resolution: 10-8 Vs
Accuracy: ±100 ppm
Drift: ~10-4 FS/min
Max rate: 1 kPI/s
Internal buffer: 2 kPI
Vin(max): ± 5V
Rin:1 MΩ / 1000 MΩ
Vin
VFC
Counter
Fast Digital Integrator FDI2056
§  Introduced 2009,
major upgrade 2012
§  Major improvements over
PDI5025:
•  Channels: 3 (or more)
•  Resolution: 10-14 Vs
(V: < 1µV, t: 12.5 ns)
•  Max rate: 500 kPI/s
•  Internal buffer: 1 MPI
•  Vin(max): ±100 V
•  Interface: Ethernet /
VXI-11 / SCPI
•  Built-in industrial Windows
computer
Vin
ADC
Σ
Measuring magnetic field
transients
Conclusion
Recent upgrade:
§  Embedded PC =>
SCPI, PDI5025 interfaces
§  Triggers: timer, encoder,
external, software
§  Channel synchronization
< 1 ns
§  Sample rate to 500 kPI/s
§  On-board memory 1 MPI
§  Field-upgradable
§  Reduced price of
crate + host interface
§  Free for existing customers!
The end result:
§  New generation of high
resolution, high speed
fluxmeters
§  Designed for
measurements
impossible with other
magnetometers
§  “Heritage” in physics
research, now
industrialized