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
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.
Sine-Theta Coil - Concept
•
0
– Current I  sin q
•
Required field uniformity
•
Mu-metal cylinder
– Enhances B-field
inside cylinder
Red:
inward
Current
q
Max
Max
B
– Shields internal region
from external fields.
•
Current Direction
Blue:Current
outward
Variable surface current
Side apertures are possible
Mu-metal Shield
0
Sine-Theta Coil – B-Field Calculation
• Poisson/Superfish
7.5 cm
LANL code used to calculate
magnetic field from currents and
magnetic properties
• Geometry
5 cm
Shielded infinite cylinder
• Results
A 5 cm diam central region has
1 B
103

B x
cm
24 current rods
(3 mm diam)
mu-metal
(1 mm
thick)
Sine-Theta Coil – Design Details
• 24 Copper rods placed
on Delrin cylinder.
1/8" Copper Rods
12" (30 cm)
3 1/8 " (8 cm)
• Six separate currents
are regulated to 10-3.
• Coil is covered with a
mu-metal shield with
windows for emerging
scattered particles.
Windows
Mu Metal Shield
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.
Setup for B-field Measurement
• The robot moved the
3-axis Hall probe
3-axis robot
Wired sine-theta coil in mount
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
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.
Sine-Theta Coil – Measured B-Field
x
• Currents adjusted to provide Bx = 10 Gauss
• Scanned interior with a 3-axis Hall probe
• Found
y
-3
1 B< 210x310 /cm

B x
cm
Transverse-component midplane contour maps (in Gauss)
By
Bz
z
‘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.
Flip-Book Movie of B-field Inside Coil