HYSPEC I.D.T. Meeting. Magnetic Interference in the SNS Instrument Hall. Wai-Tung Hal Lee Neutron Polarization Scientist Instrument Systems Group. September 10, 2004. Potential Magnetic Interference Problems at HYSPEC. HYSPEC’s neighboring instruments are field-sensitive:
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Wai-Tung Hal Lee
Neutron Polarization Scientist
Instrument Systems Group
September 10, 2004
BL 13 - Fundamental Physics.
BL 15 - Spin Echo Spectrometer (NSE)
Additional arc-shaped steel shielding
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.
SPIN (see magnet)
Acceptable for normal and abnormal (quench or other failure) operation
“In rare cases there may be sufficient scientific justification to exceed the magnetic fringe field limits defined above.” Facility Review
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.
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.
Evaluate the use of magnetic shielding to lower the fringe field.
Passive shielding: steel plate, m-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.
m-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