Experimental Facilities Division Oak Ridge
April 14-16, 2004
Magnetic Interference in the SNS Instrument Hall
Wai-Tung Hal LeeNeutron Polarization Scientist
Instrument Systems Group
September 10, 2004
HYSPEC I.D.T. Meeting
Experimental Facilities Division Oak Ridge2
April 14-16, 2004
Potential Magnetic Interference Problems at HYSPEC
• HYSPEC’s neighboring instruments are field-sensitive:
BL 13 - Fundamental Physics.
BL 15 - Spin Echo Spectrometer (NSE)
• Magnetic materials such as steel may over-stress a magnet in its vicinity.
• Field-sensitive components at HYSPEC?
Experimental Facilities Division Oak Ridge3
April 14-16, 2004
Problem identified after Asterix at LANSCE was built and a 11 T magent was purchased
• Asterix and Fundamental Physics are separated by ~ 5 m
• 11 T magnet on Asterix
Asterix
Fundamental Physics
Lujan Center
Experimental Facilities Division Oak Ridge4
April 14-16, 2004
Asterix – Fundamental Physics interference
• Steel shielding (Asterix, Fundamental Physics Hub)• Active field compensation with “bucking coils”• 11 T magnet will not operate for half a year on Asterix
Additional arc-shaped steel shielding
“Bucking” coil
Access door
Experimental Facilities Division Oak Ridge5
April 14-16, 2004
NCNR: NSE interfered by magnets at SANS and SPINS
• NSE – SPINS: ~10 m• NSE – SANS: ~10m• NSE – PHYSICS: 15 m• PHYSICS – SANS: <10 m
NSE
SANS (behind) SPIN (see magnet)
At present, conflict avoidance by scheduling is the only method used to deal with interference to the NSE. Iron shielding box on SANS is being designed.
Experimental Facilities Division Oak Ridge6
April 14-16, 2004
NCNR: NSE interfered by magnets at SANS and SPINS
• Fringe field recorded at NSE sample position: Lots of NSE data must be thrown out due to interference
Experimental Facilities Division Oak Ridge7
April 14-16, 2004
1. “The fringe fields do not exceed (field limit to-be-decided) at or beyond the boundaries of that instrument.”
2. “After careful review the fringe fields at any other instrument can be shown to be less than the acceptable threshold at that instrument (acceptable thresholds will depend on specific instruments and specific locations within those instruments, and are defined as field levels that result in a degradation of performance of that instrument)”
Acceptable for normal and abnormal (quench or other failure) operation
Exception
“In rare cases there may be sufficient scientific justification to exceed the magnetic fringe field limits defined above.” Facility Review
SNS Policy on Magnetic Interference
Experimental Facilities Division Oak Ridge8
April 14-16, 2004
1. “The fringe fields do not exceed (field limit to-be-decided) at or beyond the boundaries of that instrument.”We propose a limit of 0.5 Gauss at the boundary.
Dipole field profiles with 0.5 G at the wall
distance from source (m)
0 1 2 3 4 5 6 7 8 9 10
Fie
ld (
ga
uss
)
0.001
0.01
0.1
1
10
100
50 mG
5 G
0.5 Gat wall
15 T Oxford at 15T
15 T Oxford at 1T
10 T shielded at 10 T (best case scenario)
5-CNCS7-VULCAN
6-QSA
NS
3-SN
AP
4A-M
ag R
efl4B
-Liq Refl
Mag Reflto SNAP
Mag ReflLiq Refl
Liq Refl toQSANS
QSANS toLiq Refl
Liq ReflMag Refl
SNAP toMag Refl
2-Backscattering
Dipole field profiles with 0.5 G at the wall
distance from source (m)
0 1 2 3 4 5 6 7 8 9 10 11
Fie
ld (
gaus
s)
0.001
0.01
0.1
1
10
100
50 mG
5 G
15 T Oxford at 15T
10 T shielded at 10 T (best case scenario)
15 T Oxford at 1T
0.5 Gat wall
20 mG
18-ARCS
12-SC
D
11A-POWGEN3
17-SEQUO
A
15-NS
E
14B-H
YS
PE
C
SCD toFu Phys
SEQUOAto ARCS
ARCS toSEQUOA
NSE toFu Phys
Fu Physto SCD
13 Fu Phys
Fu Physto NSE
SEQUOAto NSE
ARCS toNSE
SNS Policy on Magnetic Interference
Experimental Facilities Division Oak Ridge9
April 14-16, 2004
2. “After careful review the fringe fields at any other instrument can be shown to be less than the acceptable threshold at that instrument (acceptable thresholds will depend on specific instruments and specific locations within those instruments, and are defined as field levels that result in a degradation of performance of that instrument)”
Criteria for Acceptable Thresholds:
1. Each instrument is responsible for defining acceptable threshold levels for externally-produced fields on any sensitive component or sample on that instrument. These definitions must be approved by an appropriate review process.
2. Each instrument having a sensitivity to externally-produced magnetic fringe fields should make reasonable efforts to mitigate that sensitivity. The definition of acceptable threshold levels for that instrument must take into account the mitigation in place, and may not be approved if the mitigation is assessed to be inadequate.
SNS Policy on Magnetic Interference
Experimental Facilities Division Oak Ridge10
April 14-16, 2004
Self-interference
• Magnetic materials such as steel in shielding and structural steel near a magnet stress the magnet coils:
– Magnet quench due to movement of the coil– Magnet damage = months to year-long down-time
• Sample stage made of steel and/or motor location too close to a magnet– “Frozen” sample stage– Damaged motor and encoders
• Spin-transport
Experimental Facilities Division Oak Ridge11
April 14-16, 2004
Evaluate the use of magnetic shielding to lower the fringe field.
Passive shielding: steel plate, -metal or its variants. Use to shield both field source and field sensitive devices.
Steel plate: Moderately effective (shielding factor 2-5); Does not saturated up to Tesla field; Thick – a fraction of a cm to several cm, Relatively cheap.
-metal: Very effective for small field; Saturates at 10 Gauss; Thin; Expensive
Active shielding: compensation coils. Shielding the magnets and specific instrument that requires extremely uniform and stable field
Use non-magnetic materials for shielding and structure within 2-3 m of the sample position
Place motors of the sample stage away from the sample position
Suggestions