### 5.1.7 Magnetic Measurements[^1]

For this study we focus discussion on magnetic field measurements of the
combined function magnets during production. No doubt measurements of
other magnetic elements of the ring are necessary. One would also expect
to perform an extensive series of measurements, possibly using several
measurement techniques, during R&D. Here, however, we have chosen to
present one concept for measurements of production magnets which
utilizes a rotating probe of specific design[^2], try to define the most
important issues related to this concept, and examine the consequences.
Thereafter, a brief discussion of other approaches.

Main parameters of the arc dipole magnets are \~2 T field, ≤12%
horizontal gradient, 65 m length, and 20 mm gap. Measurement of the
integrated dipole field is needed with an accuracy of a few parts in
10^4^. Desired measurement accuracy for the higher order harmonics is
0.1 units.[^3] This implies the horizontal gradient is known relative to
the main field to a part in 10^5^; other harmonics (of order 1 unit), to
10%. We begin by examining the implications of requirements for field
strength measurements. Based on this discussion we present a concept for
a measurement system. Given this system we then address the issue of
measurement of field harmonics.

With respect to measurement of the integrated field strength, there are
two issues. The accuracy of the measured field strength is given by
$\frac{{DB}_{1}}{B_{1}} = \frac{DR}{R}$ where R is the radius of the
probe \[1\]. This requires that the probe radius (actually the radius of
the coil winding) be known with high accuracy. Since manufacturing and
mechanical measurement accuracy are fixed, ∆*R* is fixed. Typical
manufacturing and measurement uncertainty for magnetic measurement
apparatus is \~0.001 inch (25 μm) which translates into a field error of
3e-3 for a probe of 10 mm maximum radius. Calibration of the probe with
at least an order of magnitude greater precision (3e-4 strength error)
is thus required. This level of precision can only be achieved using a
calibration magnet in which the strength is known at least as well as
one desires to know it in the subject magnets. The second issue is
unique to measurement of combined function magnets. Due to the
horizontal field gradient, an error in the horizontal position of the
probe translates into an error in measured field strength. In a magnet
with 10% gradient, measurement of the strength to 1 part in 10^4^
requires positioning the probe with 0.1 mm accuracy. Although it may be
difficult, this can be achieved using mechanical means. Vertical
placement of the probe is not as critical as long as the good field
region is significantly larger than the probe.

The main conclusions from the preceding discussion are that (1) a
rotating coil of the size we are considering can be calibrated with
sufficient precision, and (2) that the horizontal position of the probe
can be set with mechanical fixtures. For reasons we list below, we
choose to insert the rotating probe into the aperture from the side
rather than from the end. The probe is supported on a frame attached to
the magnet as shown in Fig. 1. Principle elements of the design are the
following:

-   The probe body is clamped to a precisely machined strong-back on an
    edge which describes the magnet sagitta.

-   The strong-back slides horizontally in a series of jaws attached to
    the vertical member of the frame. The strong-back is spring loaded
    so that it and the probe are forced against the referenced surface
    at the edge of the magnet gap.

-   The vertical position of the probe is defined by the precision with
    which the frame is attached to the magnet and the precision of the
    jaws relative to the vertical member of the frame and the horizontal
    surfaces of the strong-back. A tilt sensor is mounted on the
    strong-back to monitor its alignment.

-   The motor for rotation and encoder for probe angle readout are
    mounted on the frame and connected to the probe by a mechanism such
    as a toothed belt or right angle drive. Other drive components would
    be similarly mounted (e.g. gearboxes).

-   Development of slip rings small enough to fit the probe bore is
    assumed although one might choose to digitize and transmit the
    signals from inside the bore. A pre-amplifier circuit sized to fit
    the bore is also assumed allowing a boost in signal size.

![](media/image1.wmf){width="2.1902777777777778in"
height="2.997916666666667in"}

Figure 1: Measurement Fixture

A consequence of insertion from the side is that one loses a few extra
mm of probe radius due to the reduction in gap at the magnet edge of the
focusing magnet. One also needs to allow space for the clamps holding
the probe to the strong-back and for clearance during insertion.

Given a probe radius, we need to examine the accuracy of the measurement
of field harmonics. From \[1\] we see that probe calibration matters
little in measuring harmonics. Accuracy of 10% or so is adequate. (i.e.,
If the harmonic is 1 unit; you don't care if it is 0.9 or 1.1.) This
requires knowing the probe radius to 10e-3. *This is probably true even
of the gradient although it needs to be considered.* What is of greater
interest is the evaluation of the interplay of signal size with harmonic
measurement. This has implications for the required level of excitation
current during test. For a reasonable set of probe parameters,
pre-amplifier gain of 100, measurement accuracy of 0.1 unit through n=9
requires at least 200 A excitation current \[3\]. This should be
possible for room temperature measurements.

A probe length of 6.5 m was assumed above. Ten measurements are then
required to construct the integral field. The time required for
measurement is estimated as 2.5 hours with 2 crews of 3 technicians
using 4 sets of measurement apparatus \[2\]. This certainly allows for
the required production throughput. Automation and optimization of
procedures could reduce the time required. We also note that the
apparatus described could be used for either warm or cold measurements.
The only special consideration for cold measurements would be to
construct the strong-back of a nonmetallic material to prevent the
formation of eddy currents during ramping.

We have chosen a probe inserted in the magnet bore from the side and
supported by an external frame. An alternative approach would be a
\"mole\", such as have been developed for measurements of SSC \[3\] and
LHC magnets \[4,5\] which are probes with drive and angular alignment
systems built in. The LHC apparatus includes both warm and cold moles
for a magnet bore of 70 mm. There are advantages to this approach. There
is no external frame. The probe can be larger by as much as 25% as it is
inserted in the magnet from the end instead of from the side. As was
obvious from the discussion above, a larger radius is better although an
increase of 25% doesn\'t help all that much. However, development of
apparatus of the size necessary for a mole that can operate in a strong
magnetic field is not trivial. The LHC apparatus would have to be scaled
down by a factor of 3.5 or more. While we have no doubt that this could
be done, we have chosen an approach that removes elements of the
measurement system other than slip rings and pre-amplifiers from the
bore. This seems like a less daunting task.

#### References {#references .Reference-heading}

\[1\] P. Schlabach, \"Effect of Probe Manufacturing Errors on Field
Measurements\", TD-98-060, 1998.

\[2\] P. Schlabach, \"Magnetic Measurements during Production of VLHC
Low Field Combined Function Magnets\" TD-01-007, 2001.

\[3\] G. Ganetis, et al., \"Field Measuring Probe for SSC Magnets\",
Particle Accelerator Conference, 1987.

\[4\] L. Bottura, et. al., \"A Mole for Warm Magnetic and Optical
Measurements of LHC Dipoles\", Eleventh International Magnet Measurement
Workshop, 1999.

\[5\] L. Walkiers, \"Measurements for the Acceptance Tests of the LHC
Superconducting Magnets\', Eleventh International Magnet Measurement
Workshop, 1999.

[^1]: current file:
    /media/pszemraj/PeterTrove/project-files/DATA/ms-word-files/big-DOC/513135.doc

[^2]: For this exercise, we specify a probe has windings of dipole and
    quadrupole symmetry sensitive to n=1, 3, 5, ... and n=2, 6, 10,...
    respectively as well as a tangential winding with sensitivity to all
    harmonics. This allows for bucking the harmonic signal.

[^3]: One unit is 10^4^ • the harmonic normalized to the main field.