### 5.1.7 Magnetic Measurements and Testing [^1]

In this section, we focus discussion on testing of the combined function
magnets during production since these magnets are the dominant element
of the Stage 1 machine. No doubt measurement and testing of other
magnetic elements in the ring are necessary, but most of these are of
more conventional design which can be accommodated by existing equipment
and approaches. Also because of their length and small bore, the
combined function magnets are the most difficult to measure.

Unlike 'conventional' superconducting magnets, the conductor of the
combined function magnets (transmission line) is not in a high field
region and quench 'training' is not a significant issue for magnet
performance. Thus magnetic field measurements dominate the magnet
testing process, both as a quality control measure and as
characterization necessary for machine operation.

Field measurements will be performed on all magnets. This must be done
before the beam tube and vacuum pumping chamber is installed, because
the test probe must be placed in the area occupied by the beam tube.
Roughly 10% of magnets will be measured with the transmission line
cooled to cryogenic temperatures allowing characterization of the field
to full current. The remainder will be measured with the transmission
line at room temperature. During initial stages of production when the
fabrication rate is lower, we will cold test all magnets. As rates
approach one magnet per day, the fraction of total magnets cold tested
will be determined by test facility capacity and test needs as indicated
by initial results.

We first discuss a conceptual design for a measurement system to be used
for the combined function magnets. We then follow with a brief overview
of the approach to cold testing and conclude with a few remarks on other
test and measurement considerations.

#### 5.1.7.1 Measurement System

As described above, magnetic measurements are the primary concern of the
test program. While one would also expect to perform an extensive series
of measurements, possibly using several measurement techniques, during
the R&D phase of magnet development, here we have chosen to present one
concept for measurements of production magnets. This concept utilizes a
rotating coil. We try to define the most important issues related to
this concept and examine the consequences. A brief discussion of other
approaches follows.

The 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.[^2] 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 fractional accuracy of the measured field goes as ∆*R/R*
where *R* is the radius of the rotating coil \[1\]. This requires that
the radius of the probe be known with high accuracy. 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. At least an order of magnitude
greater precision (3e-4 strength error) is 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 need know it in the subject
magnets.[^3] 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 difficult, this can be achieved using mechanical
fixtures referencing to the magnet laminations. Vertical placement of
the probe is not as critical.

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 fixture
    ('strong-back') on an edge which describes the magnet sagitta.

-   The strong-back slides horizontally in a series of guide brackets
    ('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 magnet laminations at the edge of the magnet gap,
    referencing the horizontal probe position with respect to the
    laminations.

-   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 could
    also be mounted outside the magnet bore (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.

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. This
requires knowing the probe radius to 10e-3. 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 \[1\]. This should be
possible for room temperature measurements. We would no doubt increase
the gain and run at the highest tolerable current for increased
accuracy.

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 \[1\]. This certainly allows for
the required production throughput. Automation and optimization of
procedures will 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 \[2\] and
LHC magnets \[3\] which are probes with drive and angular alignment
systems built in. The obvious advantage of this approach is that the
probe can be larger by as much as 25% as insertion is from the magnet
end rather than the side. A larger probe produces larger signals.
However, development of drive apparatus operating in a strong magnetic
field is neither trivial nor cheap. We have no doubt that this could be
done; however, we have chosen a simpler approach that removes elements
of the measurement system other than slip rings and pre-amplifiers from
the bore accepting as a consequence somewhat smaller signals.

#### 5.1.7.2 Cold Testing

As a quality control measure and to characterize the magnetic field of a
sample of the magnets to full current, we plan to cold test at least 10%
of produced magnets. The peak rate of 2 magnets built per day requires 1
magnet cold test per week. As discussed in the introduction, quench
training is not a significant issue for the superferric design employed.
However, it is important to have a full set of field measurements of
magnets produced early in the program to check the overall design, iron
saturation, yoke fabrication, and related effects on the field quality
and transfer function. This increased testing also produces a
statistically significant warm-cold correlation for the field harmonics.
As the production rate rises, measurements check mechanical properties
of the magnets as well as properties of the materials used to build
them; and the set of measurements can be reduced to those necessary for
quality assurance.

The cold parts of the magnet are the transmission line and return bus
and their relatively small cryostat. Refrigeration requirements are
modest; cool down and warm up times, short. The associated test
equipment -- cryogenic end boxes, transfer lines, pumps, and
refrigerators -- is simple and inexpensive when compared to facilities
developed for testing 'conventional' superconducting magnets. Cold
testing can be done in a small area in the production facility in
conjunction with warm measurements rather than in a large, separate cold
test laboratory. The actual time spent in hooking up, cooling down,
testing, warming up, and disconnecting the magnet is small when compared
with conventional superconducting magnets. A peak production cold test
cycle occupying the test stand for one five day week would allocate a
two shift day to each of these tasks: installation and connection of the
magnet on the stand, cool down, testing, warm up, removal.

#### 5.1.7.3 Other Testing Considerations

One of the difficult issues in design and construction of a magnet
system of the scale proposed is the understanding of its overall
reliability. Failure rates for the magnets must be very small if
collider availability is to be kept at the level necessary to carry out
the physics program. Determining failure rate is itself a difficult task
as it is unrealistic to plan for the number of tests necessary to
'prove' that the magnets as constructed will meet the lifetime
reliability requirements for the collider (even assuming that we could
agree on a 'proof'.) However, since the iron yoke design and peak fields
are similar to conventional magnet designs in use for decades, we do not
envision reliability problems here. Key components will be subjected to
'lifetime' testing -- equivalent magnetic and thermal cycles (if
appropriate) --to verify that they meet requirements. While it is
impossible to argue the statistical validity of a small sample of test
magnets, accelerated life testing of a few early magnets will be used to
validate design and provide feedback early in production should any
degradation in performance be detected.

References

\[1\] P. Schlabach, \"Magnetic Measurements during Production of VLHC
Low Field Combined Function Magnets\" TD-01-007, 2001.

\[2\] G. Ganetis, et al., \"Field Measuring Probe for SSC Magnets\",
Particle Accelerator Conference, 1987.

\[3\] L. Walkiers, \"Measurements for the Acceptance Tests of the LHC
Superconducting Magnets\', Eleventh International Magnet Measurement
Workshop, 1999.

![](media/image1.jpeg){width="3.201388888888889in"
height="2.2319444444444443in"}

Figure 1: Measurement Fixture. Note that the magnet sagitta is highly
exaggerated.

[^1]: current file:
    /media/pszemraj/PeterTrove/project-files/DATA/ms-word-files/big-DOC/349910.doc

[^2]: One unit is 10^4^ • the harmonic normalized to the main field.

[^3]: Calibration in both a dipole and quadrupole magnet are required
    due to the large gradient which we want to measure with high
    accuracy.