# Presentation: 593123

## Magnetic Interference in the SNS Instrument Hall

**Wai-Tung Hal Lee**
***Neutron Polarization Scientist ***
***Instrument Systems Group***

**September 10, 2004**

**HYSPEC I.D.T. Meeting**

## 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?**

## 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

## 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

## 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.

## NCNR: NSE interfered by magnets at SANS and SPINS

**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

## SNS Policy on Magnetic Interference

- “The fringe fields do not exceed  (field limit to-be-decided)  at or beyond the boundaries of that instrument.”
- “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**

## SNS Policy on Magnetic Interference

- 	We propose a limit of 0.5 Gauss at the boundary.

**SNS Policy on Magnetic Interference**

## SNS Policy on Magnetic Interference

**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**

## 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

## Suggestions

- 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**