A Rotating Coil Apparatus with Sub-Micrometer Magnetic Center Measurement Stability Page: 3 of 5
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2000. Continual improvements in the system have been made
since then which will be described below, so that other labs
faced with measuring magnetic centers to 0.05 pm precision
will be warned about potential problems. The major challenge
is to keep the axis of rotation of the measuring coil fixed
relative to the magnet's geometric center. Fig. 1 is a photo of
an already improved set-up, as it was in late 2002, measuring
an NLC prototype water-cooled electromagnetic quad [4].
w ... o .1, .. e . s n . c. .,-- .-
a9-. . '
~- --
-- -
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Fig.1. Rotating coil set-up in late 2002 measuring an NLC linac electro-quad.
B. Double Rotating Coil Construction
The two measuring coils are wound from Multifilar 38
gauge magnet wire formed into a 10 wires-wide ribbon. Five
turns of the ribbon are wound around a 355.6mm long G-10
form; slots in its ends keep the wires in place. Two identical
forms are inserted into a 431.8mm long 9.525mm diameter G-
10 rod such that the windings are in a plane with one side
each at the center of the rod, the other sides each at a 3.8mm
radius. The individual wires are soldered together to make 50
turns in each coil and the 4 leads come out near one end of the
G-10 rod. The two coils can be connected in 3 different ways
as described below. The rotating coil itself has not been
changed since 2000. The challenge is to find the best way to
connect shafts to the G-10 rod ends and cause the rod to rotate
without bowing, wobbling or vibrating relative to the
geometric centerline of a quadrupole. The quad itself must
also be prevented from changing shape or vibrating, so that
mechanical reasons for movements of the magnetic center can
be distinguished from truly magnetic reasons.
C. Double Coil Connections and Signal Processing
For integrated field strength measurements the two coils are
wired in series summation, so that the two coils act as one
large coil. This arrangement reduces the effect of coil bowing
and of coil offset on the integrated strength measurement. A
stretched wire system is used to calibrate the accuracy of the
strength measurement. The center and multipole harmonics
measurements are done with the two coils wired in series
opposition (bucked), so that the main quadrupole field is
somewhat cancelled; the surviving odd terms have a greater
signal to full scale ratio. The main quadrupole harmonic and
all even harmonics are measured using a single winding. The
ratio of the dipole strength to the quadrupole strength is
proportional to the offset of the coil from the magnetic centerof the magnet.
The measurement coil rotates, at 1Hz, 13 times in one
direction, then 13 in the opposite direction, with only the
voltages from the middle 8 rotations being used; this is
repeated 4 times for one magnetic center measurement. The
voltages induced in the coils as they rotate through the quad's
magnetic field are sent through 6 mercury wetted slip rings to
a multiplexer. This allows real-time switching between the
individual coil signals or the sum of the two coils.
The selected signal goes to a Metrolab digital integrator,
which is the heart of the coil measurement system; it
integrates the coil signal with respect to pulses received from
an encoder attached to the coil drive system. To have the
largest signal to noise ratio the gain of the integrator
preamplifier is set by turning the coil, measuring the
maximum signal and then picking a gain that does not exceed
3/5 of the maximum integrator range. By averaging over 4
measurements the electronic noise and integrator drift are
minimized. The coil is driven by a stepper motor and its
associated driver. A GPIB Ethernet transfer box allows a
computer running C language programs in Lab Windows/CVI
to control and monitor all of the instruments of the double coil
system. See the electronics diagram in Fig. 2.
Double Cad
Compumotor
LN 83-62 Motor Gudoy Encoder -- -- --
---------I
Compu'votorCM 2f0
Motor Driver, Controller Keditley 7D01 PDI-5D25
Mu[p lexer D n roControl Signals .- - -
Measuremei Sinas Is
GPIi NetOrkNI GPIB-Ethemet Computer
convrte-rFig. 2. Rotating coil drive mechanism and signal processing electronics.
III. FINDING THE BEST SUPPORTS
Most of the mechanisms which lead to movements of the
magnetic center are caused by variations in the temperatures
of various parts of the set-up. We have 6 thermocouples
measuring different parts of the set-up, their readings are
stored by the computer. There are 3 separate supports in the
set-up and their structure can lead to unwanted movement. So
as to test the precision of the measurement system we placed
the magnet on a twin rail horizontal slide so we could move it
deliberately, but discovered that a change of 0.80C in the
ambient temperature caused a 3pm shift in X. The "X
position" in Fig. 3 is the horizontal distance of the measured
magnetic center from the center of the rotating coil which is at
X=0, Y=0 by definition. We believe the varying expansion of
the spring in the slide, varying with the ambient temperature,
caused the X variation. The similarity in the shapes of the two
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Spencer, Cherrill M.; Anderson, Scott D.; Jensen, David R. & Wolf, Zachary R. A Rotating Coil Apparatus with Sub-Micrometer Magnetic Center Measurement Stability, article, December 2, 2005; Menlo Park, California. (https://digital.library.unt.edu/ark:/67531/metadc880205/m1/3/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.