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