Creating and Measuring Very Uniform Magnetic Fields for a Polarized 3He Target

1
Creating and Measuring Very Uniform Magnetic
Fields for a Polarized 3He Target
University of North Carolina at Chapel Hill

• Mark Fassler, Tatsuya Katabuchi, and Thomas B.
  Clegg,
• University of North Carolina at Chapel Hill and
  Triangle Universities Nuclear Laboratory (TUNL),
  Durham, NC, USA
• and
• John Nouls, Amersham Health, Research Triangle
  Park, NC, USA

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Motivation

• Seek to measure spin-correlation observables in
  p 3He scattering at energies between 2 and 5
  MeV.
• Need a polarized 3He target for such
  measurements.
• Need a very uniform magnetic field to maintain
  3He polarization in the target.
• Experimental constraints dictated that a Sine
  Theta coil be used to create the magnetic field,
  and that this B-field should be uniform to better
  than 10-3 per centimeter.

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Sine-Theta Coil - Concept

• Variable surface current
• Current I ? sin q
• Required field uniformity
• Mu-metal cylinder
• Enhances B-field inside cylinder
• Shields internal region from external fields.
• Side apertures are possible

Current Direction
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Sine-Theta Coil B-Field Calculation

• Poisson/Superfish
• LANL code used to calculate magnetic field from
  currents and magnetic properties
• Geometry Shielded infinite cylinder
• Results
• A 5 cm diam central region has

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Sine-Theta Coil Design Details

• 24 Copper rods placed on Delrin cylinder.
• Six separate currents are regulated to 10-3.
• Coil is covered with a mu-metal shield with
  windows for emerging scattered particles.

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Sine-Theta Coil Realization
Assembled coil with rods and current carrying
wires. Horizontal slot provided for scattered
particles.
Delrin cut horizontally. Mu-metal shield cut
vertically.
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Setup for B-field Measurement

• The robot moved the Hall probe around inside the
  sine-theta coil. At regular spacings on a 3D
  grid, a computer with a 3D gaussmeter took
  measurements of the 3D B-field.
• A typical scan produces thousands of data
  points, each with 6 dimensions of data x,y,z,
  Bx, By, Bz

3-axis Hall probe
3-axis robot
Wired sine-theta coil in mount
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Visualizing the Data

• The red arrows inside the sine-theta coil are
  actual data taken during a scan with the robot.
  Each arrow is a vector indicating the magnitude
  and direction of the B-field at a point in space.
  The B-field varies by less than 1 throughout
  the volume of interest, so differences are not
  visible when plotted in this way. To analyze
  variations which are important for the physics,
  we plot instead variations in the B-field.

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Sine-Theta Coil Measured B-Field

• Currents adjusted to provide Bx 10 Gauss
• Scanned interior with a 3-axis Hall probe
• Found

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Flip Book of Cross Sections

• These 18 pages (included on the next page as a
  short mpeg movie) show a sequence of
  cross-sections of the B-field in the sine-theta
  coil down the z-axis from about center 3.8 cm
  to center 3.8 cm.
• The greatest irregularities are on the top and
  bottom. These are caused by joints between the
  two halves of the mu-metal. In particular, in the
  center along the top there is great irregularity
  along the z-axis, caused by holes in the mu-metal
  which provide clearance for the tubes for the 3He
  to enter and exit the cell.
• Along the left side on can see slight B-field
  irregularities cause by holes in the mu-metal
  which allow the scattered particles to escape.

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Flip-Book Movie of B-field Inside Coil