Analysis of NSTX TF Joint Voltage Measurements Page: 3 of 10
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Analysis of NSTX TF Joint Voltage Measurements*
Princeton Plasma Physics LaboratoryPOBox 451, Princeton, NJ08543
Abstract-This report presents findings of analyses of recorded
current and voltage data associated with 72 electrical joints
operating at high current and high mechanical stress. The
analysis goal was to characterize the mechanical behavior of each
joint and thus evaluate its mechanical supports. The joints are
part of the toroidal field (TF) magnet system of the National
Spherical Torus Experiment (NSTX) pulsed plasma device
operating at the Princeton Plasma Physics Laboratory (PPPL).
Since there is not sufficient space near the joints for much
traditional mechanical instrumentation, small voltage probes
were installed on each joint and their voltage monitoring
waveforms have been recorded on sampling digitizers during
each NSTX "shot".
Strong mechanical forces arise during pulsed operations, far
stronger than the joint conductors could long survive without the
restraining assistance of a mechanical support system. A joint's
apparent electrical resistance changes dynamically if sufficiently
strong net lateral force on the conductors causes a reduction in
the joint's area of high pressure contact. Since the
electromagnetic forces are well known, this circumstance would
arise if the mechanical supports were not working properly.
Analyzing the nonlinear relations between pulsed magnetic forces
and joint electrical resistances can thus identify and even
diagnose mechanically overstressed joints.
The present design of the joints and their supports was operated
in two successive run periods, February-July 2004 and April-
September 2005. Because of indications from analyzing the first
run period's voltage probe data that the mechanical support
system's fabrication was flawed, the joints and their mechanical
supports were rebuilt before the second run period without
changing the design. Analyses of voltage probe data from the
second run period indicate improved mechanical support
Low aspect ratio tokamaks such as the NSTX are being
researched at PPPL as a plasma confinement scheme with
promising attributes for future fusion reactors. The Figure 1
schematic depicts the toroidal plasma surrounding a "center
stack" inside a roughly spherical vacuum vessel approximately
twelve feet tall. During pulsed operations the plasma conducts
a toroidal electrical current (looping around the center stack) of
about one million amperes. Ten separate multiturn Poloidal
Field (PF) coil winding sets, including the Ohmic Heating
(OH) coil which induces the plasma current, conduct toroidal
currents parallel to the plasma current.
NSTX Cross section
The Figure 2 cross section view of the NSTX depicts
features relevant to this analysis. The 36 series-connected
Toroidal Field (TF) coil turns are mechanically divided into a
cylindrical 8 inch diameter 18 foot tall inner-leg subassembly
located at the middle of the centerstack, and twelve 3-turn
outer-legs returning the TF current between top to bottom
outside the vacuum vessel in symmetrical poloidal planes. The
TF inner-leg, threaded inside the OH solenoid winding, has a
12 turn inner layer and a 24 turn outer layer. At four
elevations, the outer-legs are electrically connected via "TF
Flexible Connectors" to rigid "TF Radial Flag" assemblies
which in turn are bolted in tiers against the two layers of inner-
leg turns, thus forming the 72 TF electrical joints which are the
subject of this analysis.
Figure 3 shows one of the radial flags in isolation without
any of its mechanical support features.
*Work Supported by U.S. DOE Contract No. DE-AC02-76CH03073)
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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/m1/3/: accessed September 18, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.