The bus bars for the NSTX-U magnetic coils underwent multiple design iterations to accommodate structural and electrical requirements. The final design used co-axial, water-cooled and air-cooled copper conductors to distribute current from the power supplies to the OH, TF, PF-1abc, and bakeout coils. Fabrication involved forming, machining, brazing, and insulating the conductors. Installation of the complex bus bar assemblies required mechanical lifts and trial fittings. Testing confirmed the bus bars could safely meet the increased current and voltage loads required for the NSTX-U upgrades.

1. BUS BARS DESIGN AND STRESS ANALYSIS
The bus bars and supports design underwent multiple design iterations due to the complex nature of the
surroundings, results from stress analyses and manufacturability. The bus bars provide electric current to the
magnetic coils including the OH, TF, PF-1abc and Bakeout. Mechanical and structural supports were provided as
required, by results from the FEM, to lower stresses to within acceptable limits. The final design used co-axial,
water-cooled and air-cooled conductors which were made from oxygen-free copper materials.
NSTX-U COILS BUS DESIGN AND CONSTRUCTION*
NEWAY D. ATNAFU, L. DUDEK, S. GERHARDT, A, KHODAK, S. RAMAKRISHNAN, M. SMITH, P. TITUS
Princeton University Plasma Physics Laboratory, Princeton, New Jersey
Abstract
The construction of the NSTX upgrade project was completed in the fall of
2015. The multi-year capital project was budgeted at $94 Million. NSTX-U
successfully ignited its first plasma. The reactor will be used to run
experiments under increased Toroidal Field (TF), Plasma Current (Ip), Beam
Injection Power, and pulse length. The Bus Bars connect the magnetic coils
to the power supply lines. The bus bars design consists of co-axial, water-
cooled and air-cooled bus bar systems. The bus bars design was analyzed
and satisfied the NSTX structural design criteria. FEM analysis was
performed using ANSYS software to verify the performance of the bus bars
under the increased current loads. The processes used for fabricating the bus
bars include forming, machining, brazing, welding and water-jet cutting.
Individual conductors were insulated using Kapton Tapes for electrical
insulation and Fiber Glass Wetted with Epoxy Hysol to provide further
electrical insulation and a protective mechanical coating. The joint surfaces
were silver plated and the bolts torqued appropriately to maintain joint
resistances within an acceptable range. Structural supports were provided as
necessary to counter forces against the bus bars due to the magnetic fields,
short circuit conditions and thermal boundary conditions. The insulated bus
bars and assemblies were hi-pot tested to verify insulation; the joints were
resistance checked; and the water-cooled buses were leak checked using
hydrostatic pressure testing. The installation of the coils bus bars was a
tedious process because it required the use of mechanical lifts for carrying
the conductors, wood mocks for trial fitting, and design changes due to
interferences in the field. At completion of the bus bars installation, pre-
operational testing was performed to verify that the coils bus systems are
capable of meeting the required current and voltage ratings.
References
1. Neway D. Atnafu et al., “NSTX-U Vacuum Vessel Design
Modification,” SOFE, IEEE/NPSS, San Francisco, CA, 2013
2. J. E. Menard et al., “Overview of the physics and engineering
design of NSTX upgrade,” SOFE, IEEE/NPSS 24th Symposium,
Chicago, IL, 2011.
3. H. Zhang, P. Titus, P. Rogoff, A. Zolfaghari, D. Mangra, and M.
Smith, “Analysis Efforts Supporting NSTX Upgrades,” PPPL,
2010, http://www.osti.gov/bridge/servlets/purl/1001666-
tpFMBS/1001666.pdf.
4. H.M. Fan, M. Ono, G. Sheffield, J. Bialek, and J. Robinson,
“Conceptual Analysis and Design of NSTX Vacuum Vessel and
Support Structures,” SOFE, 16th IEEE/NPSS Symposium ,
Champaign, IL, 1995.
5. Stevenson et al., “NSTX Second Neutral Beam: General
Requirements Document,” PPPL, April 2009.
SOFT September 5th – 9th, 2016 • Prague, Czech Republic
Introduction
•National Spherical Torus Experiment (NSTX) is the world’s highest
performance Spherical Torus (ST) experiment and the centerpiece of the U.S.
fusion program.
•Construction of a $94 million NSTX Upgrade project was completed
recently.
•The objective of the project was to expand the NSTX operational space and
thereby the physics basis for the next-step ST facilities.
•The new name for the upgraded machine is NSTX-U, which U stands for
Upgrade.
•The magnetic coils are used to create electro-magnetic fields which is vital
for the creation of a plasma inside the vacuum vessel.
•These coils are connected to their power supply lines using bus bar
assemblies.
*Work supported by U.S. DOE Contract No. DE-AC02-09CH11466
Fig. 1. NSTX-U Coils and Bus Runs
CHI BUS DESIGN
The CHI/Bakeout Bus bars were comprised of 2
inch-square cross-section conductors vertically, 1”
X 3-½” conductor rings and 1-1/2 inch-square
water-cooled conductors horizontally. The bus bars
were designed to take 4 KV of the CHI Operation
voltage. These bus bars are also used for bakeout
system and were designed to carry a continuous
bakeout current of 8 KA.
TF BUS DESIGN
The TF Coils Bus Bars design included 1” X 6” cross-section bus conductors and 2”
X 2-¾” water-cooled conductors. The TF Coils bus bars were designed to carry 130
KA current at 1 KV circuit voltage.
UPPER AND LOWER PF-1ABC BUS DESIGN
The PF-1abc Coils Bus Bars were designed using 1-1/2 inch-square conductors. These
bus bars were designed to carry up to 19 KA current at 2 KV circuit voltage.
Fabrication and Construction
The bus bars fabrication processes included forming, water-jet cutting, CNC
machining, brazing and surface finishing. The water-cooled buses were hydrostatic
pressure tested to ensure that there was no leaking. The fabricated conductors were
then insulated using Kapton tape for electrical insulation and fiberglass wetted with
Epoxy Hysol for additional insulation and protection of the Kapton from physical
damage. The insulated conductors were hi-pot tested to insure no electrical leakage.
The installation of the NSTX-U Bus Bars was a tedious process. Since the bus bars
were complex in shape and too heavy to carry without the use of mechanical
supports, mocks were made from wood material for trial fitting. The design and
fabrication processes were revised using changes generated in the field. When the
bus bars were ready for final installation, their joint surfaces were silver platted to
enhance conductivity. At completion of installation, the joints were resistance
checked and the bus bar assemblies were hi-pot tested. Pre-operational tests were
then performed to confirm that the coils bus systems are capable of meeting the
required current and voltage ratings.
OH BUS DESIGN
The OH Coils Bus Bars design included co-axial conductor, 1-1/2 inch-square cross-section
bus conductors and 1 inch-square cross-section water-cooled conductors. The OH Coils bus
bars were designed to carry 24 KA current at 6 KV circuit voltage. The OH Co-axial conductor
design included a copper rod at the center for power supply line, a hollow copper conductor for
the return line and isolated with G-10 insulation.
Conclusion
The NSTX-U Coils Bus Bars were designed to meet the upgrade criteria and
undergone the necessary stress iterations. The design mainly used Pro/E for
creating models and generating drawings. Analysis was performed using ANSYS
software. The design included water-cooled, air-cooled and co-axial bus designs.
The bus bars were fabricated while maintaining the design features. Field fitting
was done using wood mocks. The final products were trial fitted to ensure that
the tight tolerances at the joints would be met. The fabricated bus bars were
insulated and electrical and hydrostatic pressure tests were performed. The
installation of the bus bars was performed following the appropriate safety
procedures. Pre-operational tests were performed to ensure that the finished
products were acceptable and ready for use.
Image of the CHI Bus Bars,
taken from a Pro/E Model
The TF Bus Bars Assembly
The PF-1abc Bus Bars Assembly
The OH Bus Bars Assembly
PPPL Technicians [J. Bartzak, T. Guttadora
and C. Ennis from left to right] Working on
the Bus Bars Fabrication