A Novel Demountable TF Joint Design for Low Aspect Ratio Spherical Torus Tokamaks Page: 3 of 7
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A Novel Demountable TF Joint Design for Low
Aspect Ratio Spherical Torus Tokamaks
R. D. Woolley
Princeton University Plasma Physics Laboratory
Princeton, New Jersey, USA
woolley@pppl.govAbstract-A novel shaped design for the radial conductors and
demountable electrical joints connecting inner and outer legs of
copper TF system conductors in low aspect ratio tokamaks is
described and analysis results are presented. Specially shaped
designs can optimize profiles of electrical current density,
magnetic force, heating, and mechanical stress.
I. INTRODUCTION
Toroidal plasmas in low aspect ratio tokamaks (i.e.
spherical torii) have a much smaller central hole than tokamaks
of higher, more conventional aspect ratio, so their coil systems
use demountable TF system conductors instead of the discrete
wound TF coils common in the higher aspect ratio tokamaks.
The demountable components include a TF Central Bundle of
wedge-shaped turn conductors carrying the TF threading
current from the machine's bottom to its top, and TF Outer Leg
assemblies which return the current in each turn from top to
bottom. At the top and bottom, TF Radial Assemblies carry
TF current between inner joints with TF Central Bundle turns
and outer joints with TF Outer Leg conductors.
TF Outer Leg conductors and their joints don't pose
difficult design problems since they are immersed in a
relatively low intensity toroidal magnetic field and they inhabit
a relatively uncrowded region in which there is plenty of space
available for any needed mechanical support structures. The
TF Central Bundle conductors also don't pose difficult design
problems although they inhabit a relatively strong toroidal
magnetic field region. Due to symmetry the TF Central Bundle
assembly is self-supporting since its large centering forces are
internally reacted by compressive wedging.
On the other hand, design of the TF Radial Assemblies can
be challenging since they are subjected to large uncompensated
magnetic forces and there is little room at their jointed inner
ends for mechanical support structure. The design of these
assemblies and their demountable joints with Central Bundle
conductors is the present subject. Optimized part shaping is
the proposed technique which addresses design issues. The
occasion for this consideration is a proposed upgrade of the
NSTX, so that is used to illustrate the concept.
A key shaping feature configures the demountable
connections between radials and centerstack conductors as lap
joints, so that TF conductor current on both sides of each joint
This work is supported by the U.S. Department of Energy Contract
No. DE-AC02-CH0911466flows in the same vertical direction. The primarily consequence
is that forces resulting from TF current flow increase the
pressure clamping each joint closed. A second consequence is
that the resistive voltage gradient distributes the current
uniformly through each joint, thus reducing local hotspots.
This contrasts with conventional TF radial designs such as the
initial TF Flag design of the National Spherical Torus
eXperiment (NSTX) in which TF current makes a sharp, right-
angle turn as it passes through its demountable joints.
A second key feature is that the mechanical force holding
each demountable joint closed at low TF current is provided by
reacting against hoop tension in an external structural ring. This
contrasts with the initial NSTX TF Flag design in which
threaded fasteners cutting through joints produce local
concentrations of pressure, current density and heating while
also reducing net current-carrying joint areas.
A third key design feature configures the (R, Z) poloidal
plane current path beyond each lap joint to follow thin flexible
straps specially shaped as moment-free constant-tension
curves. This contrasts with the initial TF Flag design of NSTX
whose straight horizontal current path shape generated large
moments requiring extremely precise structural support to
avoid severe nonuniform joint pressure profiles or even joint
liftoff. The use of flexible conductive straps also
accommodates small misalignments resulting from assembly
tolerances or thermal growth.Figure 1: TF Radial Assembly
.
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Woolley, R.D. A Novel Demountable TF Joint Design for Low Aspect Ratio Spherical Torus Tokamaks, article, May 29, 2009; Princeton, New Jersey. (https://digital.library.unt.edu/ark:/67531/metadc929432/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.