Analysis of NSTX TF Joint Voltage Measurements Page: 4 of 10
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locations. For instance, this can cause a brief measured joint
voltage reversal at the end of a unidirectional current pulse, as
appears in Figures 4. Because eddy current patterns change
nonlinearly with TF joint conditions, it is not practical to model
and cancel them out as would be possible for purely inductive
effects. Sophisticated data mining methods  have been used,
but focusing on stationary TF current conditions avoids
confusing these effects with true apparent resistance variations.
TF Radial Flag
A TF radial flag is a 5 inch tall copper bar about 12 inches
long, with its 1 inch width tapering down to 0.78 inches at the
TF joint interface, in the foreground of Figure 3. The outer end
of the radial flag has a "tee" for bolting to the TF flexible
connectors. The inner end forming the TF joint is silvered to
improve electrical conductivity. The four holes provide access
for long bolts, screwed into threaded inserts in the associated
inner-leg TF turn and pretensioned to 5,000 pounds each. The
horizontal grooves visible in Figure 3 accommodate the voltage
monitoring probes and their cables (not shown). The probes
are each insulated coaxial assemblies with a spring-loaded
center conductor to press against the associated TF turn and a
press-fit electrical contact with the flag a short "setback"
distance (about 0.2 inches) back from the joint face. Although
both sides of each flag have voltage probes installed, only the
"B" probes (on the right sides) have always been connected to
digitizers during each shot since February 2004. A small and
variable subset of the "A" probes (on the left sides) have also
been connected to spare digitizer channels.
Joint voltage signals are sensitive to noise and interference.
Probe voltage differences are in the millivolt range while the
common mode voltage is in the kilovolt range. Since the ac
common mode rejection of the instrumentation amplifiers is
not perfect, the result is that differentiated TF circuit voltage
transients couple to the recorded signals in addition to the
intended joint voltage drops. The unwanted signal components
include thyristor power supply noise as well as spikes at
breakpoints in the slope of the TF current waveform. Since
each TF joint voltage signal is sampled at a 2000/second rate,
thyristor ac common mode contribution to the signal is easily
recognized (see Figure 4a), and is well removed by a 15
millisecond FWHM triangular FIR filter without severely
distorting real signal dynamics (see Figure 4b). However, a
voltage spike remains from the power supply turn-on transient,
even though it is not part of the true joint voltage drop.
In addition, changes in the TF current level cause decaying
eddy current patterns to circulate through different parts of a
joint, transiently affecting voltage measurements at probe
Figure 4a: Raw TF Joint Voltage Measurement Data
Figure 4b: Filtered Raw TF Joint Voltage Measurement Data
In order to interpret the voltage signals a multiphysics
ANSYS model of a TF joint was created without any support
structure, as depicted in Figure 5.
Figure 5: ANSYS Multiphysics Model of TF Joint
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Woolley, R. Analysis of NSTX TF Joint Voltage Measurements, report, October 7, 2005; Princeton, New Jersey. (digital.library.unt.edu/ark:/67531/metadc890648/m1/4/: accessed December 17, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.