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