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

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Motivation Seek to measure spin-correlation observables in p+ 3 He scattering at energies between 2 and 5 MeV. Need a polarized 3 He target for such measurements. Need a very uniform magnetic field to maintain 3 He 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 Required field uniformity Mu-metal cylinder –Enhances B-field inside cylinder –Shields internal region from external fields. Side apertures are possible Blue: outward Red: inward Max 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 7.5 cm 24 current rods (3 mm diam) mu-metal (1 mm thick) 5 cm

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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 Delrin cut horizontally. Mu-metal shield cut vertically. Assembled coil with rods and current carrying wires. Horizontal slot provided for scattered particles.

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Setup for B-field Measurement 3-axis robot 3-axis Hall probe Wired sine-theta coil in mount 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, B x, B y, B z

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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 B x = 10 Gauss Scanned interior with a 3-axis Hall probe Found < 2 x 10 -3 /cm Transverse-component midplane contour maps (in Gauss) BzBz ByBy x y z

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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 3 He 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

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