[^1]

*Abstract*. The Toroidal Field (TF) coil of the National Spherical Torus
Experiment (NSTX) \[1\] suffered a failure on February 14, 2003, after
approximately 3 years of service and 7200 pulses. The failure occurred
at an electrical joint connecting one of the radial flags to the inner
leg bundle. Damage was extensive such that the coil could not be
repaired. Analysis of the failure revealed structural design problems
which have been addressed in the re-design effort described herein.
Construction of a new TF coil is now underway.

# INTRODUCTION

[T]{.smallcaps}he NSTX is a spherical torus (ST) magnetic confinement
fusion device which employs a de-mountable TF coil, meaning that the
inner legs of the coil are bundled together into a single assembly which
is separable from the outer legs. This feature is central to the ST
concept, in which future reactors are envisioned to utilize a single
turn central conductor carrying mega-amps of current to create the
toroidal field. Although NSTX has a multi-turn TF coil, the low aspect
ratio and low toroidal field of the ST configuration still lends itself
to the grouping of the inner leg turns in to a bundle for convenience of
manufacture, assembly, and maintenance. With the demountable
configuration comes the challenge of an electrical joint, however, at
high field. On NSTX, the TF inner leg bundle is threaded through an
ohmic heating (OH) solenoid. Therefore, in order to preserve the ability
to separate the OH and TF coils by sliding the OH coil over the TF
bundle, the TF joint occurs at the radius of the inner leg bundle where
the field is maximum.

# TF Configuration

An isometric view of NSTX shown in Fig. 1, and a zoomed-in view of the
upper TF joint region in Fig. 2. These views are of the original design,
but the general features of the new design are similar. The TF inner leg
bundle and OH coils are enclosed in a vacuum jacket and form the center
stack, which is completely demountable from the rest of the machine. The
TF coil has 36 water cooled copper turns connected in series, and
produces 6kG at the major radius R~0~=0.85m with 71.2kA per turn. The 36
turns are arranged in two layers, 12 on the inner layer and 24 on the
outer. Radial flags are affixed to the inner leg conductors using long
thru-bolts. Further out, L-shaped connectors join the flags to flexible
links which in turn connect to the outer legs. A hub assembly consisting
of several stainless steel disks structurally supports the radial flags.
A torque collar attaches to the bundle below the elevation of the disks
and transmits torque from the bundle to the hub.

In the original TF assembly, shown in Fig. 3, the fit-up and
communication of loads from the radial flags to the hub disks, which
were split into 180^o^ segments, was accomplished by a set of G-10
wedges and manually selected shims.

![](media/image1.jpeg){width="3.4993055555555554in"
height="3.1493055555555554in"}Fig. 1. Isometric View of NSTX showing
de-mountable TF/OH Center Stack.

![](media/image2.jpeg){width="3.495138888888889in"
height="2.140277777777778in"}

Fig. 2. Zoomed View showing Connections from Inner to Outer Leg

![](media/image3.jpeg){width="3.5416666666666665in"
height="2.5194444444444444in"}

Fig. 3. G-10 Wedges and Shims of Original Assembly

These features proved to be the primary defects leading to the failure
of the original design. The shimming was by nature imprecise, resulting
in uncertain communication of EM loads from flags to hub disks. Second,
the shimming allowed the flags and hub disks to slide radially with
respect to one another. This, plus the non-toroidally continuous hub
disks, led to a lack of stiffness against vertical loads and a tendency
toward "dishing". In the original finite element analysis (FEA) of NSTX,
although G-10 properties were assumed in a layer of elements forming the
interface between the flags and hub disks, the modeling did not allow
sliding between the parts. This proved to be a major oversight because,
without sliding, the flags would tend to act as the web of I-beams
formed with the disks, which would be quite stiff. It resulted in a
serious underestimate of the loads on the flag bolts which was likely
the primary cause of the failure.

# Failure of Original TF Assembly

On February 14, 2003, following the morning test shots, the first plasma
attempt of the day resulted in a loud bang heard on the control room
audio monitors followed by a plume of smoke visible on the control room
video monitors. Waveforms of TF current during the last successful test
shot and the faulty shot are shown in Fig. 4[.]{.smallcaps} The target
flat top level was 53.4kA which produces Bt=4.5kG. The fault occurred
just prior to flat top as the current passed 50kA. Several protective
devices tripped and limited energy dissipation in the fault to 1.4MJ.

![](media/image4.jpeg){width="2.828472222222222in"
height="2.1555555555555554in"}

Fig. 4. TF Current Waveforms During Last Test Shot and Faulty Shot

Subsequent inspection of the coil revealed extensive melting and
carbonization at one of the flag joints on the bottom of the machine.
Voltage and current waveforms were analyzed and a fault sequence was
developed as shown in Fig. 5.

![](media/image5.jpeg){width="2.9402777777777778in"
height="1.5909722222222222in"}

Fig. 5. TF Current Waveforms During Last Test Shot and Faulty Shot

An open circuit fault occurred initially at the flag joint of turn 3,
which was located close to the start and finish leads of the coil, where
the full 1kV terminal voltage was present. The resulting plasma then
initiated a short circuit fault across the coil terminals, and the spike
of fault current from the power supply led to the power supply trip and
subsequent L/R decay. Thanks to the current spike the power supply
tripped quickly. Had this not occurred, more energy would have been
dissipated, leading to more extensive and perhaps collateral damage.

Following the event, more detailed FEA was undertaken and it became
apparent that the mechanical support of the flat was inadequate. The
cyclic vertical EM loads on the flags was causing excessive deflection
and moment on the flag bolts, gradual loosening of the fasteners, loss
of contact pressure, and high localized current density, eventually
leading to failure.

# Performance Requirements

## Magnetic Field and Current

The TF is required to supply a long pulse at 3kG with 4.5 second flat
top, and a short pulse at 6kG with a 0.6 second flat top. Although both
pulse waveforms have the same prospective ∫I^2^(t)dt =
6.5x10^9^amp^2^-sec, the 6kG operation is clearly the most challenging
because of the high force and the minimal time available for heat
dissipated at the joint to diffuse to surrounding copper.

## EM and Thermal Loads

## ![](media/image6.jpeg){width="2.4722222222222223in" height="2.701388888888889in"} {#section .unnumbered}

Fig. 6. Electromagnetic Loads on TF Flag Joint

EM loads are shown in Fig. 6. The in-plane loads are vertical and result
from the magnetic pressure of the TF coil itself. The out-of-plane loads
consist of torsion on the inner leg bundle and lateral loads on the
flags. The torsional load results from the radial field at the end of
the OH coil crossing the vertical TF current, when the lateral load
results from the vertical fields from the OH and PF coils crossing the
radial TF current.

Additional forces arise from thermal effects. The 80^o^C temperature
rise in the inner leg conductor causes an axial thermal expansion of
0.35" overall and, importantly, an increase in the length of the inner
layer conductors as they extend beyond the outer layer, which amounts to
an increased separation between the inner and outer layer flags. The
radius of the bundle also increases by around 0.006" during a pulse.

## Contact Resistance

Contact resistance is a key performance driver since it determines the
heating of the joint. To characterize the achievable contact resistance
using the machining and silver plating processes typical during TF coil
manufacturing, tests were performed to quantify the relationship between
contact pressure and contact resistivity as shown in Fig. 7.

![](media/image7.jpeg){width="2.8208333333333333in"
height="2.6743055555555557in"}

Fig. 7. Contact Resistivity vs. Pressure for Silver Plated Joint

To put perspective on Fig. 7, an FEA calculation of peak temperature,
which is always located at the corner of the joint due to current
bunching, shows that that an average contact resistivity of2.5μΩ-in^2^
leads to a peak temperature of 120^o^C. Although contact pressure
typically varies across the joint, this finding suggests that contact
pressures of 1ksi and above are sufficient.

# New Design

The new TF joint design is shown in Fig. 8.

![](media/image8.jpeg){width="3.467361111111111in"
height="2.071527777777778in"} Fig. 8. New TF Flag Joint Design

The flags are potted with injected epoxy in boxes which are bolted to
the hub disks. In this configuration, the flag boxes act like the webs
of an I-beam, resulting in a very stiff structure. In addition,
communication of the EM loads from the flags to the boxes via the
potting is extremely efficient. A286 flag bolts are replaced by larger
Inconel 718 studs at twice the preload. Spring loaded coaxial voltage
probes are installed on all joints.

Key improvements over the old design are listed in Table I.

TABLE I

Improvements of New Design

  ----------------- ---------------------------- --------------------------
                    **Old Design**               **New Design**

  **Hub Stiffness** Not adequate; lacking stiff  Very stiff. Boxes form
                    linkages between disks       webs with disks like
                    because flags could slide    I-beams.
                    w.r.t. disks                 

  **Bolts/Studs**   Shoulder engagement too      Shear Shoe using two 3/8"
                    small (4 bolts x 0.1 depth = dia bolts
                    .4 @ 1/2\" dia, Area = 0.18  
                    in\^2)                       

  ** **             Bolt 5/16" thread was necked Studs necked down to root
                    down too far to 0.232\"      dia of 3/8\" bolt =
                                                 0.314\"

  ** **             0.438" shank diameter was    Neck down to root dia of
                    too large, bolt not          3/8\" bolt = 0.314\",
                    compliant for thermal        Belleville washer
                    cycling                      

  ** **             Torsion in long bolts during Studs with nuts used in
                    tightening, inaccurate       place of long bolts, stud
                    tensioning                   tensioner

  ** **             Dual purpose bolts, combined Loose fitting clearance
                    tension and shear functions, holes for studs, separate
                    tolerance issues,            shear shoes
                    torque(tension) uncertainty  

  ** **             Four 5/16\" bolts @ 2500#,   Four 3/8\" studs @ 5000#,
                    marginal friction to carry   doubling of preload
                    shear (4\*2500=10000lbf      (4\*5000=20000lbf)
                    applied vs. 9230lbs,         
                    requires mu \~1.0)           

  ** **             Thin washers under bolt      1/4\" thick washer plate
                    heads                        over Belleville washers at
                                                 tee-ends

  **Inserts**       Keensert type, marginal      Taplok type, thread
                    thread engagement            engagement \> 0.5\"

  **Shimming**      Manually selected and        Hysol/glass tape potting
                    inserted G10 shim stock      in boxes, mold released to
                                                 permit thermal growth

  **Out-of-Plane    Wedged G10 blocks with       Flags potted in boxes,
  Load Path**       pusher bolts                 boxes bolted to hub disks

  **Torque Collar** Two piece collar bolted      Three piece collar with
                    directly to hub. Wet lay-up  sliding contact with hub
                    0.25" thick Hysol RE2039 &   for torsion-only
                    HD3561. Holes in collar for  connection. Wet lay-up
                    epoxy outflow to enhance     0.180" Hysol E-120HP.
                    adhesion.                    Serrations in collar to
                                                 enhance adhesion.

  **Joint           10A Biddle measurement via   200A precision measurement
  Resistance        connection to two half flags using voltage probes in
  Measurement**     on disassembled joint,       situ, \~ 20x enhanced
                    resolution \~ 1μΩ            resolution
  ----------------- ---------------------------- --------------------------

A simplified diagram of the in-plane force application and the elements
available to react the load is given in Fig. 9. The four primary load
paths are 1) the bolted joint friction, 2) the shear shoe, 3) the torque
collar, and 4) the hub/spline/VV. At the joint, the radial pressure
exerted by the fasteners results in a frictional response to vertical
loads on the flag, depending on the coefficient of friction. In
addition, the shear shoe, which is bolted on to the ends of the inner
leg conductor, provides a vertical stop.

![](media/image9.pct){width="3.301388888888889in"
height="2.4381944444444446in"}Fig. 9. Temperature Distribution at EOFT

The torque collar is attached to the inner leg bundle via a wet lay-up
of room temperature cure epoxy. This collar serves to transmit the
torque generated on the inner leg bundle to the hub assembly. The collar
is connected to the hub at anchor points which are designed to transmit
torque, but not vertical or radial load. The hub assembly takes the
vertical moment generated on the flags. In addition, it communicates the
torsional load from the collar and the lateral load on the flags out to
the vacuum vessel by way of the spline. The spline is designed to
transmit torque, but not vertical or radial load. It is intended to
allow axial thermal displacement of the inner leg assembly with respect
to the umbrella and vacuum vessel.

# Analysis of New Design

Detailed FEA was performed using NASTRAN which includes non-linear
effects, e.g. friction and sliding. Analysis was performed for Start of
Flat Top (SOFT), End of Flat Top (EOFT) and End of Pulse (EOP)
conditions. The FEA provided information concerning the peak stresses in
the various components of the joint and, importantly, the contact
pressure distribution over the joint. Using this contact pressure
information with linear extrapolation between the time points, another
FEA using ANSYS was used to simulate the temperature distribution during
the pulse, including thermal diffusion and resistivity variation with
temperature. Prior analyses showed that magnetic effects on current
distribution were not significant. These results showed that peak
temperatures occur at EOFT, and are located at the corner of the joint
where the current bunches up. A typical result is shown in Fig. 10.

Cases were run at various off-normal conditions including friction
coefficient and preload pressure. In all cases the temperature was less
than 120^o^C, which was established early on as a goal limit. It was
found that reduction in contact pressure near the corner of the joint
led simply to a redistribution of current to areas of higher pressure
and lower resistivity.

Since large regions of the joint are at low current density under
nominal conditions, a relatively high tolerance for pressure reduction
exists. With this in mind, cases were run without the torque collar. It
was found that peak temperatures were approximately 10^o^C higher but
still below 120^o^C. Based on this result it has been decided to defer
the implementation of the new torque collar, at least initially. During
assembly and re-commissioning, using new voltage probes measuring the
joint voltage drop along with fiber optic temperature and strain gauges,
joint performance will be monitored and compared to the predictions of
the analysis. If necessary to gain margin, the installation of the
torque collar remains an option.

# ![](media/image10.jpeg){width="3.3673611111111112in" height="2.298611111111111in"} {#section-1 .Reference-Head}

Fig. 10. Temperature Distribution at EOFT

.

# Conclusions

The demountable TF coil is a central feature of the ST configuration. On
NSTX, the demountable joints are based on a bolted configuration. The
joint is located at the radius of the TF inner leg bundle, because of
the desire to retain the ability to slide the OH coil over the bundle.
As such it is located at peak magnetic field, and EM forces are maximal.

The original design utilized G-10 wedging and shimming which proved
inadequate in terms of its ability to communicate the loads to the hub
structure. The new design uses stainless steel boxes into which the
flags are potted, for direct communication of the loads. The boxes are
bolted to the hub assembly, forming a stiff I-beam like structure.

Detailed FEA shows that the structural support is adequate not only in
terms of its ability to react the loads but also in maintaining high
contact pressure on the joint, which is necessary for high electrical
conductivity.

# References {#references .Reference-Head}

1.  C. Neumeyer, et al, "Engineering Design of the National Spherical
    Torus Experiment," *Fusion Engineering and Design.* vol. 54, pp.
    275-319, 2001.

2.  M. Kalish, et al, "NSTX Flag Joint Design, Testing, and Fatigue
    Analysis," Proceedings of 20^th^ SOFE

[^1]: ^^