![](media/image1.jpeg){width="5.992361111111111in"
height="1.0048611111111112in"}

####### PRELIMINARY 

INFORMATION\
PACKAGE

NSTX TF FLAG JOINT

#### DESIGN REVIEW

E. Baker, A. Brooks, J. Chrzanowski,

P. Heitzenroeder, M. Kalish, R. Marsala, C. Neumeyer,

R. Parsells, B. Paul, H. Schneider, M. Williams, I. Zatz

7/31/3

![](media/image2.png){width="1.9583333333333333in"
height="0.7805555555555556in"}

[Table of Contents]{.underline}

INTRODUCTION [3](#introduction)

PERFORMANCE REQUIREMENTS [7](#performance-requirements)

OPERATIONAL HISTORY [8](#operational-history)

FAILURE OF ORIGINAL TF ASSEMBLY [9](#failure-of-original-tf-assembly)

KEY DESIGN DRIVERS [13](#key-design-drivers)

SUMMARY OF FORCES [15](#summary-of-forces)

LOAD PATHS [16](#load-paths)

DESCRIPTION OF NEW DESIGN [17](#description-of-new-design)

COMPARISON OF OLD AND NEW DESIGNS
[21](#comparison-of-old-and-new-designs)

ANALYSIS [22](#analysis)

TESTING [23](#testing)

SUMMARY OF DESIGN MARGINS [27](#summary-of-design-margins)

SUMMARY OF PERFORMANCE [28](#summary-of-performance)

INSTRUMENTATION [29](#instrumentation)

#   {#section .unnumbered}

# 

# 

# 

# INTRODUCTION

The TF Inner Leg Assembly failed on 2/14/03 at the Inner Leg-to-Flag
joint due to structural weaknesses.

On 4/10/3 a new design was presented for review. Although analysis,
testing, and detailing was incomplete at that time, sufficient
confidence was gained so as to allow release of the inner leg conductors
for machining.

Following the April review the analysis, component testing, and design
detailing has been completed. In addition, the chits generated during
the April review have been resolved as reported on under separate cover.

The purpose of the 8/7/3 review is to examine the final design and
identify any shortcomings which may need correction prior to proceeding
with manufacturing, assembly, commissioning, and operation.

An additional review in September will assess cost and schedule issues
related to the TF recovery effort. It is highly desirable that the
August review establish that the technical basis is sound and not in
question in September.

As stated at the beginning of the recovery effort, the Project seeks a
more robust design which will not suffer from the shortcomings of the
original design. Toward this end, the re-design activity must:

-   *factor in lessons learned from failure*

-   *ensure that all engineering aspects analyzed at appropriate level
    of detail*

-   *perform testing as necessary for engineering input and design
    verification*

-   *reduce dependence on precision manufacturing/assembly*

-   *facilitate ease of maintenance*

Figures 1, 2, and 3 depict the main elements of the joint and serve to
identify the nomenclature. The scope of this review is limited to the
following elements:

-   *Inner Leg / Flag Interface*

-   *Flag*

-   *Flag Fasteners (threaded inserts, studs, washers, nuts, etc.)*

-   *Flag Structural Supports (Boxes, Shear Shoes)*

-   *Hub Assembly/Torque Collar*

-   *Instrumentation Features Built-in to Above*

Other aspects may be discussed but are out of scope.

![](media/image3.pct){width="5.179166666666666in"
height="3.332638888888889in"}

Figure 1 -- TF Coil System

![](media/image4.pct){width="5.182638888888889in"
height="3.1104166666666666in"}

Figure 2 -- Elements Related to TF Inner Leg --to- Flag Joint

![](media/image5.pct){width="4.823611111111111in"
height="4.866666666666666in"}

Figure 3 -- Flag Fasteners and Structural Support

As shown in Figure 4, recovery activities have consisted of three main
thrusts:

-   *Design (drawing) development*

-   *Finite Element Analysis*

-   *Testing*

![](media/image6.pct){width="6.309722222222222in"
height="3.4430555555555555in"}***Figure 4 -- Joint Recovery
Activities***

The testing is subdivided into "component" tests and "prototype" tests.
The component tests are designed to provide information necessary for
the engineering design. The prototype tests are designed to confirm the
design. Ideally, all testing would be complete prior to design review.
However, due to schedule constraints and resource limitations, prototype
testing will not take place until after the review.

Component testing consists of:

-   *Pull-out tests on threaded inserts (one time and 50,000 cycle at
    100^o^C)*

-   *Pull-out tests on bolts threaded directly in copper (one time and
    50,000 cycle)*

-   *Friction coefficient and electrical resistance tests for silver
    plated copper*

-   *Shear tests on simulated torque collar attachment*

Prototype testing consists of:

-   *Mechanical mock-up tested for 50,000 cycles*

-   *Electrical mock-up tested at full current and I^2^T*

Since prototype testing will follow after the design review, and since
chit resolution will also follow, another checkpoint is identified in
the plan which will ensure that all design issues have been closed out.
Most likely this will consist of a review by the PPPL Activity
Certification Committee.

# PERFORMANCE REQUIREMENTS

The base GRD requirement is for B~t~=[3kG]{.underline} at R~0~=0.854m
with 4.5 sec flat top, corresponding to 35580 Amp with the 36 turn coil.
There is also a "high field" GRD requirement for [6kG]{.underline} at
R~0~=0.854m with 0.6 sec flat top, corresponding to 71160 Amp with the
36 turn coil. Waveforms are shown in Figure 5.

![](media/image7.pct){width="5.9944444444444445in" height="5.26875in"}

Figure 5 -- Current Waveforms

(Shot 108712 was an actual 6kG Test Shot)

Allowing for the possibility of a power supply fault at the end of flat
top from full current, the *I^2^T* requirements imposed by the waveforms
are of order 6.0 x 10^9^ A^2^-sec. However the coil design anticipates a
maximum of 6.5 x 10^9^ A^2^-sec, corresponding to an 80^o^C rise in the
main conductor.

The 6kG pulse is the design driver, not only because the forces are 4x
higher than at 3kG, but also because of the short duration of the 6kG
pulse, where minimal time is available for heat diffusion. This is the
most severe condition in terms of peak local temperature rise at the
joint. Figure 6 shows the result of a finite element analysis which
shows how the temperature distribution tends to peak at the corner of
the joint, where the current bunches up. Simulations have shown that
this current bunching effect is due primarily to resistive effects, and
that inductive effects are not significant.

![](media/image8.png){width="5.425in" height="4.047916666666667in"}

***Figure 6 -- Temperature Distribution***

The TF system is driven by a 1kV power supply, so the voltage is of
order 30V/turn. However, the TF conductor faces structure which can be
biased to 1kV during Coaxial Helicity Injection (CHI) operations, so the
groundwall insulation needs to anticipate 2kV. Thus the hipot
requirement is 2E+1=5kV.

# OPERATIONAL HISTORY

Approximately 7200 shots were executed with the original TF Inner Leg
Assembly prior to failure. A spectrum is shown if Figure 7. The test
shots, performed at 50% and 100% of the operating envelope in effect,
are evident, along with the plasma shots which comprise the vast
majority.

![](media/image9.png){width="6.336111111111111in" height="4.5375in"}

Figure 7 -- TF Operational History

It is noted that the early shots were mainly at 3kG, and that most of
the machine operations have been conducted at 4.5kG. The number of 6kG
shots was quite limited. In this context it is noted that the forces
present during the 3kG and 4.5kG shots are 25% and 50%, respectively, of
those at 6kG.

# FAILURE OF ORIGINAL TF ASSEMBLY

On February 14, 2003, following our morning test shots, the first plasma
attempt of the day resulted in a loud bang (heard on thecontrol room
audio monitors) accompanied by a plume of smoke (visible on the control
room video monitors). Waveforms of the prior 100% test shot and the
failed shot are shown superimposed in Figure 8.

![](media/image10.pct){width="5.999305555555556in"
height="4.170833333333333in"}

Figure 8 -- TF Operational History

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:

-   *TF power supply fault detector section overcurrent*

-   *TF Analog Coil Protection (ACP) overcurrent*

-   *TF Rochester Instrument System (RIS) overcurrent*

-   *TF ground fault current relay*

Initial inspection revealed that one of the TF "flags" had been
displaced radially by about 1 inch as shown in Figure 9. Subsequent
inspection revealed the extent of the damage as shown in Figure 10.

![](media/image11.pct){width="5.059027777777778in"
height="3.7534722222222223in"}

Figure 9 -- Radially Displaced Flag

![](media/image12.pct){width="6.055555555555555in"
height="3.9930555555555554in"} ***Figure 10 -- Peak Damage Area***

Analysis of the electrical signals led to the following postulated fault
scenario:

-   *An open circuit fault (the flag joint opening up) led to
    turn-to-turn (inner and outer legs) and turn-to-ground faults (to
    the hub and umbrella assemblies*

-   *This fault condition led to a spike of fault current from the power
    supply which shunted the coil inner/outer leg assembly.*

-   *Once the power supply tripped, the current spike decayed.*

-   *This was followed by an L/R decay which can be modeled by an arc
    with V=125V and R=500µΩ.*

-   *The energy dissipated in the arc was of order 1.4MJ.*

A combination of factors are thought to have led to the failure as
follows.

[Design Factors]{.underline}

> *- hub stiffness not adequate to react moment*

*- communication of load from flag to hub uncertain with G10 shims*

> *- bolt thread and shoulder engagement too small*
>
> *- bolts necked down too far at threads, not enough on shaft*
>
> *- dual shear/preload function of bolts*
>
> *- torque collar connection to hub too stiff for vertical and radial
> displacement*
>
> *- lack of feature to facilitate joint resistance measurement without
> disassembly*

[Quality Factors]{.underline}

> *- frequent manual reworking of contact surfaces*
>
> *- non-planar flag surfaces*
>
> *- shoulder bolt concentricity*

[Operational Factors]{.underline}

-   *monitoring of joint integrity too infrequent, too imprecise (1µΩ
    resolution)*

All of the above factors have been addressed in the new design.

# KEY DESIGN DRIVERS

Electrical contact resistance vs. pressure behavior is a key design
driver. The contact pressure requirement drives the structural design,
and the contact resistance determines heating and limits flat top time.
The project has made precision measurements of contact resistivity vs.
pressure for silver plated copper joints as shown in figure 11.

![](media/image13.pct){width="5.997916666666667in"
height="5.664583333333334in"}

Figure11 -- Contact Resistance Data

As shown in Figure 12 the contact resistance and temperature allowable
will essentially dictate the realizable system performance in terms of
flat top time.

![](media/image14.pct){width="6.001388888888889in"
height="4.110416666666667in"}

##### Figure 12 --Effect of Contact Resistance on Temperature

In actuality, since contact pressure and resistance vary across the
joint, the situation is more complex. However, this result suggests the
general trend.

The following design guidelines have been established:

-   *Structural systems shall maintain 1ksi minimum contact pressure
    across joint*

-   *Silver plated joint with R ≤ 2μΩ-in^2^ @ P ≥ 1ksi*

*- Peak local temperatures to be limited to 120^o^C*

Future measurements (prototype joints and actual installation) will
provide more data and ultimately establish the operating envelope.

#  

# SUMMARY OF FORCES

The in-plane EM forces on the flags and links (contribution out to
half-way through flex link) at 6kG (71.2kA/turn) are given in the
following table.

**Summary of In-Plane Forces and Moments at 6kG**

+-----------------------+----------+-----------------+-----------------+
|                       | F~v      | dR from         | Moment          |
|                       | ertical~ | effective point |                 |
|                       |          | of application  | (in-lbs)        |
|                       | (lbs)    | to flag inboard |                 |
|                       |          | edge            |                 |
|                       |          |                 |                 |
|                       |          | (in)            |                 |
+-----------------------+----------+-----------------+-----------------+
| Outer Layer, Inner    | 7380     | 4.3             | 31719           |
| Tier Flag             |          |                 |                 |
+-----------------------+----------+-----------------+-----------------+
| Outer Layer Inner     | 1855     | 13.8            | 25652           |
| Tier Link             |          |                 |                 |
+-----------------------+----------+-----------------+-----------------+
| Total Per Inner Tier  | 9235     | 6.2             | 57371           |
| Flag/Link             |          |                 |                 |
+-----------------------+----------+-----------------+-----------------+
| Inner Layer, Outer    | 1845     | 4.3             | 7930            |
| Tier Flag             |          |                 |                 |
+-----------------------+----------+-----------------+-----------------+
| Inner Layer, Outer    | 464      | 13.8            | 6413            |
| Tier Link             |          |                 |                 |
+-----------------------+----------+-----------------+-----------------+
| Total Per Outer Tier  | 2309     | 6.2             | 14343           |
| Flag/Link             |          |                 |                 |
+-----------------------+----------+-----------------+-----------------+
| Total from All Turns  | 249325   |                 |                 |
+-----------------------+----------+-----------------+-----------------+

Clearly the forces on the inner tier (associated with the 24 turn outer
layer) are much higher than the outer tier.

The out-of-plane EM forces on the flags and links (contribution out to
half-way through flex link) at 6kG (71.2kA/turn), with OH and all PF
coils at maximum current, are given in the following table. Note that a
significant fraction of these forces are due to the OH which is
bi-directional.

**Summary of Out-of-Plane Forces and Moments at 6kG**

+-----------------------+----------+-----------------+-----------------+
|                       | F~       | dR from         | Moment          |
|                       | lateral~ | effective point |                 |
|                       |          | of application  | (in-lbs)        |
|                       | (lbs)    | to flag inboard |                 |
|                       |          | edge            |                 |
|                       |          |                 |                 |
|                       |          | (in)            |                 |
+-----------------------+----------+-----------------+-----------------+
| Outer Layer, Inner    | 3174     | 8.9             | 28286           |
| Tier Flag             |          |                 |                 |
+-----------------------+----------+-----------------+-----------------+
| Outer Layer Inner     | 1523     | 17.9            | 27323           |
| Tier Link             |          |                 |                 |
+-----------------------+----------+-----------------+-----------------+
| Total Per Inner Tier  | 4698     | 11.8            | 55609           |
| Flag/Link             |          |                 |                 |
+-----------------------+----------+-----------------+-----------------+
| Inner Layer, Outer    | 1979     | 9.1             | 18061           |
| Tier Flag             |          |                 |                 |
+-----------------------+----------+-----------------+-----------------+
| Inner Layer, Outer    | 1060     | 18.2            | 19113           |
| Tier Link             |          |                 |                 |
+-----------------------+----------+-----------------+-----------------+
| Total Per Outer Tier  | 3039     | 12.2            | 37174           |
| Flag/Link             |          |                 |                 |
+-----------------------+----------+-----------------+-----------------+
| Total from All Turns  | 149198   | 11.9            | 1.78e6          |
|                       |          |                 |                 |
|                       |          |                 | (148392 ft-lbs) |
+-----------------------+----------+-----------------+-----------------+

The out-of-plane EM forces on the inner leg bundle at 6kG (71.2kA/turn),
with OH and all PF coils at maximum current result in a net torsional
moment of 40870 ft-lbs. Note that a significant fraction of this force
is due to the OH which is bi-directional.

It is noted that thermal displacements potentially result in additional
forces and stresses. The significant effects are as follows:

-   *Temperature of inner leg conductor rises from 12^o^C to 29^o^C
    SOFT, 66^o^C EOFT, 95^o^C EOP (worst case)*

-   *Vertical length of inner leg bundle from bottom to top increases by
    approximately 0.35"due to inner leg temperature rise during a pulse*

-   *Vertical length of inner leg from torque collar to top of bundle
    increases due to inner leg temperature rise, whereas flag and hub
    remain relatively cool*

-   *Radius of inner leg bundle increases bundle increases by
    approximately 0.006" during a pulse*

-   Flag heats modestly during a pulse (∆T \~ 5^o^C) but can ratchet by
    as much as ∆T ≤25^o^C with rated I^2^T pulsing at 300 second rep
    rate, corresponding to an increase in length of approximately
    0.005".

# LOAD PATHS

A simplified diagram of the in-plane force application and the elements
available to react the load is given in Figure 13.

![](media/image15.pct){width="5.0569444444444445in"
height="3.817361111111111in"}

***Figure 13 -- Load Paths***

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
bolted joint, contact pressure results in friction which takes part of
the shear load. In addition, shear keys or the bolts themselves can be
used to take part of the load. At the torque collar, a maximum torsional
load of 22480 lbf will add to any additional vertical load from the hub.

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.

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.

# DESCRIPTION OF NEW DESIGN

The ideal design would have the following attributes:

-   *Perfect communication of flag load through insulation/potting to
    hub*

-   *Infinitely stiff hub*

-   *Full vertical load shear capability at the inner leg/flat
    interface*

-   *All inner leg torsion shunted away from flag via torque collar*

-   *Contact pressure everywhere along the joint ≥ minimum allowable
    over the full temperature range*

These design goals guide in the selection of the various design features
and dimensions.

Several views of the new design are given in Figures 14-16. The
following are some highlights of the design features:

-   *Solid flags (no split) are insulated with two half-lapped layers of
    Kapton, then glass tape wrapped and potted (Hysol RE2039 & HD3561)
    in 304SS boxes. Communication of in-plane and out-of-plane forces
    from flag to hub is very efficient.*

-   *Boxes are bolted to the hub disks using 5/8" diameter hardware,
    forming a very stiff "I-beam" type hub structure.*

-   *Mold release agent is used so that potting is not bonded to the
    box, thereby allowing for thermal displacement of the flag.*

-   *Potting is accomplished "in place" with boxes flags already
    installed and tightened so as to ensure best possible contact
    surface fit. However, potted flag-boxes can be removed as necessary
    when sliding TF Inner Leg Assembly into OH Coil Tension Tube*

-   *Flags are attached to inner legs using 3/8" dia. Inconel
    studs(Alloy 718, 5% Cold Worked, Aged,UTS 210ksi, Yield 185ksi) in
    threaded inserts and preloaded to 5000lbf each. Washer plate is used
    under tee at end of flag. Belleville washer and nut then complete
    the attachment. The total force of 20000lbf provides for high
    contact pressure leading to excellent electrical conductivity and
    frictional load bearing capability.*

-   *Flag fasteners are necked down over their length to enhance
    elasticity. In this regard the Belleville washers are a redundant
    feature to ensure that fasteners remain tight after load cycling.*

-   *Shear shoe located on outer edge of flags is bolted to ends of
    inner leg conductor using A286 SS bolts, one oriented directly for
    the vertical load and the other angled for the moment reaction. The
    shoe provide a load path for the vertical EM force and also provides
    a means of registering the vertical position of the flags during
    assembly.*

-   *Torque collar consists of three segments attached with two 1/2"
    diameter bolts w/Belleville washers at each joint, attached to
    Scotchply groundwall using 0.180" thick wet lay-up glass epoxy
    (Hysol E-120HP) wrap. Collar is sized to provide modest compression
    to enhance shear capability of wet lay-up while still accommodating
    radial thermal expansion of bundle.*

-   *Collar transmits torque to hub structure at three anchor points
    which are designed to slide in vertical and radial directions to
    accommodate anticipated displacements (0.020" and 0.006",
    respectively). Low friction coatings (e.g. Magnaplate HMF) are under
    evaluation to minimize wear and friction.*

-   *Redundant voltage probes are located on each side of the flag in
    grooves half way up. These spring loaded probes provide a very
    effective means for monitoring joint resistance during maintenance
    periods (200A injected) and voltage drop during actual machine
    operations.*

![](media/image16.pct){width="5.988888888888889in" height="3.5625in"}

***Figure 14 -- Cross Section of New Design (Top of Machine)***

![](media/image17.jpeg){width="4.850694444444445in"
height="3.7583333333333333in"}

##### 

##### Figure 15 -- Torque Collar

![](media/image18.jpeg){width="3.2131944444444445in"
height="4.877777777777778in"}

***Figure 16 -- Isometric View of New Design***

![](media/image19.jpeg){width="3.3305555555555557in"
height="2.5965277777777778in"}

***Figure 17 -- Isometric View of Inner Conductor,***

***Flag, Shoe, and Accessories***

# COMPARISON OF OLD AND NEW DESIGNS

Features of the old and new designs are compared in the following table.

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

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

  **Bolts/Studs**   Shoulder engagement was too     Shear Shoe using two 3/8"
                    small (4 bolts x 0.1 depth = .4 dia bolts
                    @ 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 too   Neck down to root dia of
                    large, bolt was not compliant   3/8\" bolt = 0.314\"
                    for thermal cycling             

  ** **             No back-up to bolt compliance   Belleville washer,
                    to accomodate creep             initially flattened

  ** **             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 applied (4\*5000=20000lbf)
                    vs. 9230lbs, requires mu \~1.0) 

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

  **Inserts**       Keensert type, thread           Taplok type, thread
                    engagement = 0.113\"            engagement \> 0.5\"

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

  **Out-of-Plane    Wedged G10 blocks with pusher   Flags potted in boxes,
  Load Path**       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" thick Hysol E-120HP
                    adhesion.                       (improved adhesive
                                                    strength). Serrations in
                                                    collar to enhance
                                                    adhesion.

  **Joint           10A Biddle measurement via      200A precision measurement
  Resistance        connection to two half flags on using voltage taps in situ
  Measurement**     disassembled joint              
  ----------------- ------------------------------- --------------------------

# ANALYSIS

The primary means of analyzing joint behavior and guiding the design is
a Finite Element Analysis (FEA) using the NASTRAN package which includes
suitable features for non-linear behavior such as the friction
interfaces. In-plane and out-of-plane forces and SOFT, EOFT, and EOP
temperatures were calculated separately using ANSYS and input to the
NASTRAN analysis.

![](media/image20.jpeg){width="5.426388888888889in"
height="4.386805555555555in"}

***Figure 17 -- NASTRAN Finite Element Model***

The model consists of a 1/24^th^ segment of the joint and represents all
significant features including both tiers of flags, out-of-plane load
path through the spline, as well as an axisymmetric equivalent model of
the torque collar with torsion-only connection to the hub. The model
neglects load transfer through the flex links to the outer legs
(conservative for the joint).

A large number of cases were run to test the model in stages as features
were added. Simplified spreadsheet calculations were performed when
possible as a parallel check. The model was used as a design tool and
influenced the final selection of features and dimensions.

Cases were run with friction coefficient equal to 0.2 and 0.4, and with
temperatures corresponding to SOFT, EOFT, and EOP.

Key outputs from the model include the following:

-   contact pressure distribution over joint

-   tension in flag studs and shear shoe bolts

-   shear stress in wet lay-up

-   displacements

Additional analysis was performed on the box bolts, collar bolts, flag
fasteners, etc.

# TESTING

Test results are summarized in the following table.

<table>
<colgroup>
<col style="width: 9%" />
<col style="width: 12%" />
<col style="width: 9%" />
<col style="width: 9%" />
<col style="width: 8%" />
<col style="width: 8%" />
<col style="width: 5%" />
<col style="width: 8%" />
<col style="width: 9%" />
<col style="width: 18%" />
</colgroup>
<tbody>
<tr class="odd">
<td>Type</td>
<td>Load</td>
<td>Cycles</td>
<td>Temp</td>
<td>Samples</td>
<td>Avg</td>
<td>3σ</td>
<td>Avg-3σ</td>
<td><p>Yield/</p>
<p>Failure</p></td>
<td>Comment</td>
</tr>
<tr class="even">
<td>Insert Pull-out in Copper</td>
<td>To Failure</td>
<td>1</td>
<td>RT</td>
<td>7</td>
<td>12714</td>
<td>2898</td>
<td>9817</td>
<td>0.84</td>
<td> </td>
</tr>
<tr class="odd">
<td> </td>
<td><p>5000-</p>
<p>6000lbf</p></td>
<td>&gt; 50000</td>
<td>100C</td>
<td>4</td>
<td>No Failures</td>
<td> </td>
<td> </td>
<td> </td>
<td> </td>
</tr>
<tr class="even">
<td> </td>
<td>To Failure</td>
<td>1</td>
<td>RT</td>
<td>4</td>
<td>13003</td>
<td>2156</td>
<td>10847</td>
<td> </td>
<td>After Cycling</td>
</tr>
<tr class="odd">
<td>Bolt Pull-out in Copper</td>
<td>To Failure</td>
<td>1</td>
<td>RT</td>
<td>8</td>
<td>12658</td>
<td>2641</td>
<td>10016</td>
<td>0.82</td>
<td> </td>
</tr>
<tr class="even">
<td> </td>
<td><p>5000-</p>
<p>6000lbf</p></td>
<td>&gt; 50000</td>
<td>RT</td>
<td>5</td>
<td>No Failures</td>
<td> </td>
<td> </td>
<td> </td>
<td> </td>
</tr>
<tr class="odd">
<td> </td>
<td>To Failure</td>
<td>1</td>
<td>RT</td>
<td>5</td>
<td>12828</td>
<td>2734</td>
<td>10094</td>
<td> </td>
<td>After Cycling</td>
</tr>
<tr class="even">
<td>Wet Lay-Up Shear</td>
<td>To Failure</td>
<td>1</td>
<td>RT</td>
<td>9</td>
<td>2617</td>
<td>746</td>
<td>1871</td>
<td> </td>
<td>Normalized to Zero Compression, w/o peaking</td>
</tr>
<tr class="odd">
<td> </td>
<td>0 to 1800psi, up to1000psi compression</td>
<td>&gt; 50000</td>
<td>RT</td>
<td>6</td>
<td>No Failures</td>
<td> </td>
<td> </td>
<td> </td>
<td>Various load and compression combinations</td>
</tr>
<tr class="even">
<td>Friction</td>
<td>Up to 7560lbf</td>
<td>1</td>
<td>RT</td>
<td>5</td>
<td>0.41</td>
<td>0.04</td>
<td>0.36</td>
<td> </td>
<td>Coefficient of Friction</td>
</tr>
</tbody>
</table>

In regard to the wet lay-up shear tests, a rectangular sample
configuration was used. Stress analysis was performed using ANSYS to
determine the peak-to-average stress level for the test sample
configuration, as shown in figures 18 and 19.

![](media/image21.pct){width="4.905555555555556in"
height="3.7118055555555554in"}

***Figure 18 -- Shear Test Sample Configuration***

![](media/image22.png){width="4.5256944444444445in" height="3.4in"}

***Figure 19 -- Distribution of Shear Stress (xz):max= 45.4MPa
(6.6ksi)***

This analysis indicates a peak-to-average stress ratio of 1.65 for the
test samples. An additional factor to be considered in interpretation of
the shear data is the level of normal compression, which was varied from
test to test. To account for compression, the following model was
used...

$$s_{\text{failure}} = s_{0} + k_{N}s_{N}$$

where:

σ~failure~ = shear stress to failure

σ~0~=shear stress to failure with zero compression

k ~N~ =coefficient applied to normal stress

σ~N~=normal stress

In interpreting the raw test data, values of σ~0~ and k ~N~ were first
determined using a least squares curve fit. Then, based on this k ~N~, a
σ~0~ value was determined for each data point, and a average and
standard deviation was taken of these values. Using these results a peak
stress failure model is....

$${\hat{s}}_{\text{failure}} = k_{\text{PA}}\left\lbrack {\overline{s}}_{0} - 3s + k_{N}s_{N} \right\rbrack$$

where:

${\hat{s}}_{\text{failure}}$ = peak shear stress to failure

${\overline{s}}_{0}$ = average shear stress to failure with zero
compression = 2617psi

σ = standard deviation of ${\overline{s}}_{0}$= 746psi

k ~N~ =coefficient applied to normal stress = 1.27

σ~N~ =normal stress

k ~PA~ =peak/average ratio for test sample configuration = 1.65

These results are shown in figure 20.

![](media/image23.pct){width="4.552777777777778in"
height="3.236111111111111in"}

***Figure 20-- Interpretation of Shear Test Data***

It is important to note that the wet lay-up does not experience elevated
temperature coincident with high shear loads. This justifies testing at
room temperature.

To demonstrate this, an analysis was performed which simulated radial
thermal diffusion from the inner leg copper, through the CTD turn
insulation, Scotchply ground insulation, Hysol wet lay-up, and stainless
steel torque collar. As shown in figure 21, the temperature rise of the
Hysol is minimal during the pulse.

![](media/image24.jpeg){width="4.442361111111111in"
height="3.245138888888889in"}

***Figure 21 - Temperatures vs. Time***

*Copper (Cu), Turn Insu. (CTD), Ground Insul. (SP), Wet Lay-up (HS) and
Collar (SS)*

# SUMMARY OF DESIGN MARGINS

Design margins are summarized in the following table. In those cases
where the margins are derived from test data...

-   the average --3σ results were used

-   for the wet lay-up, 1000psi compression is assumed

-   for the yield loads, test data for yield/failure was used

In the case of loads derived from the FEA, the worst of the four
"nominal" cases (friction coefficient =0.2, 0.4 and temperature at SOFT,
EOFT) was used.

  ------------ ---------- -------- -------- -------- --------- --------- ---------
  Component    Material   Max Load Max      Yield    Safety    Failure   Safety
                          (lbf)    Stress   Stress   Factor on Stress    Factor on
                                   (psi)    (psi)    Yield     (psi)     Failure

  Flag Studs   Inconel    6181     79771    185000   2.3       210000    2.6
               718                                                       

  Insert       Copper     6181     14579    19412    1.3       23152     1.6
  pullout                                                                

  Shear Shoe   A286       5279     68125    102000   1.5       146000    2.1
  Bolt                                                                   

  Shear Shoe   Copper     5279     11971    18701    1.6       22713     1.9
  Bolt pullout                                                           

  Wet Lay-up   Hysol      123848   1532                        5197      3.4
  Peak Shear                                                             
  (Bond Plane)                                                           

  Wet Lay-up   Hysol      n.a.     3000                        5197      1.7
  Peak Shear                                                             
  (Combined)                                                             

  Collar Bolts A286       9084     64018    102000   1.6       146000    2.3

  Box Bolts    304SS      5068     19800    31200    1.6       73200     4.4

  Box Bolt     304SS      5068     7303     15600    2.1       36600     5.0
  Pullout                                                                

  Box Friction 304/304    4700                                           2.5
  ------------ ---------- -------- -------- -------- --------- --------- ---------

In addition to the above, it is noted that the flag threaded inserts ,
the shear shoe bolts, and the wet lay-up samples have been exposed to
50,000 cycles at or above their design loads, without failure.

Finally, mechanical and electrical prototype tests will provide further
design verification prior to commencing machine operation.

# SUMMARY OF PERFORMANCE

FEA runs using ANSYS, with contact pressure distribution from NASTRAN
runs, were made for the SOFT and EOFT conditions. The results indicate
that the variation in contact pressure does not result in significant
deviations from earlier runs which assume constant pressure distribution
and joint resistivity of 1 micro-ohm-in^2^. This is due in part to the
fact that the pressure is maintained at high levels across the joint,
plus the fact that the current takes the path of least resistance, so
that the phenomenon is somewhat self-correcting. See following figures.

  ---------- ------- ------------- ------- ------- ---------- ------- ------------- -------
  SOFT               SOFT                          EOFT               EOFT          
  Pressure           Resistivity                   Pressure           Resistivity   
  (psi)              (μΩ-in^2^)                    (psi)              (μΩ-in^2^)    

  14727      11877   0.336         0.366           15662      12639   0.328         0.357

  10092      7835    0.390         0.431           10152      7808    0.389         0.431

  9022       6719    0.408         0.458           8707       6344    0.413         0.468

  7752       5380    0.433         0.499           7137       4736    0.447         0.525

  6903       4513    0.453         0.535           6083       3732    0.476         0.576

  6683       4272    0.459         0.547           5774       3453    0.486         0.594

  6793       4408    0.456         0.540           5869       3617    0.483         0.583

  6478       4068    0.464         0.557           5534       3297    0.494         0.605

  5696       3215    0.488         0.611           4689       2393    0.527         0.686

  5054       2552    0.512         0.669           3928       1631    0.565         0.797

  4858       2329    0.520         0.693           3624       1313    0.583         1.228

  5033       2519    0.512         0.672           3803       1512    0.572         0.821

  4840       2295    0.520         0.697           3836       1528    0.570         0.818

  4207       1587    0.550         0.806           3681       1286    0.579         1.285

  3840       1202    0.570         1.458           4011       1554    0.560         0.812

  3985       1312    0.562         1.230           4995       2505    0.514         0.674

  4316       1633    0.544         0.797           6219       3719    0.472         0.577

  4060       1503    0.558         0.823           6923       4417    0.452         0.539

  3005       917     0.627         2.049           6364       3904    0.467         0.566

  2087       306     0.724         3.318           6681       3013    0.459         0.627
  ---------- ------- ------------- ------- ------- ---------- ------- ------------- -------

##### Figure 22 --FEA Element Pressures and Resistivities

![](media/image25.jpeg){width="5.983333333333333in"
height="4.351388888888889in"}

Figure 23 --Temperature Simulation

It is noted that the above simulation is for a 0.7 second flat top (vs.
0.6 second requirement), but does not attempt to model the effect of the
bolt holes. From prior runs using constant pressure and resistivity,
this effect is noted to add approximately 10^o^C to the peak
temperature. In any case it is clear that the new design meets the
criteria of 0.6 second flat top at 6kG with peak temperature below 120
^o^C.

# 

# 

# INSTRUMENTATION

Joint voltage drop measurements are considered to be the top priority.
They provide the best "end result" indication as to whether all of the
various elements of the system are working properly. Voltage drop
measurements during maintenance with a low current (200A) injected
continuously will provide an excellent and precise means of assessing
the condition of the joints, albeit without large EM loads applied.
Voltage drop measurements made during actual pulsing will provide a
real-time indication that the structural support systems are working
properly, maintaining adequate contact pressure on the joints. Although
non-trivial due to 5kV voltage isolation requirement, low signal level,
and high noise environment, prior experience on MAST and C-MOD indicate
that these measurements can be made successfully. NSTX envisions the
trending of the real-time voltage drop signals, at least from the daily
test shots (where noise from RF, NBI, and plasma is absent), as a means
of monitoring joint integrity between maintenance intervals.

As shown in figure 24, voltage probes will be used to measure joint
resistance. These are type IDI100526 spring loaded probes which are
manufactured in large quantities for the semiconductor test industry.
(ref. [www.idinet.com](http://www.idinet.com/)) Although only one is
shown in the figure, two will be implemented per flag, one on each side
of the joint.

![](media/image26.jpeg){width="5.165972222222222in"
height="3.7979166666666666in"}

***Figure 24 --Voltage Probe Configuration***

Sensitivity to locally high contact resistance was examined using ANSYS
(ref. NSTX-CALC-13-21, "TF Joint Voltage vs. Contact Resistivity").
Results are given in figures 25 and 26, which show the difference in
probe measurement as a function of nominal contact resistivity for
between a joint with constant contact resistivity and one where the
corner region of the joint is at 1.1x and 2x nominal resistivity, for
various lengths extending beyond the corner (0 length corresponding to
no abnormal region at all).

![](media/image27.pct){width="4.913194444444445in"
height="3.4923611111111112in"}

***Figure 25 -- Sensitivity for 1.1x High Resistivity***

![](media/image28.pct){width="4.93125in" height="2.9791666666666665in"}

Figure 26 -- Sensitivity for 2x High Resistivity

Tests are underway to quantify the resolution the joint resistance
measurement system which is being designed to measure joint voltage drop
while 200A is injected through the TF coil system during maintenance.
This measurement is challenging, due to the low voltage and various
effects such as thermoelectric potentials in the wiring, bulk resistance
changes in copper resistivity vs. temperature, etc.. We expect to
experience nominal joint contact resistivity on the order of 2
micro-ohm-in^2^ or less, in which case the typical joint voltage drop at
200A should be of order 100 microvolt. Based on preliminary results,
however, it is expected that the system will be able to resolve down to
5 microvolt or better. On this basis, from the above two figures, 110%
resistivity regions in the corner of the joint will be marginally
detectable, but 200% abnormalities will be easily detected.

A fiber optic measurement system, which can in principle measure
temperature, strain, and displacement in real time, is under evaluation.
If implemented, a few (e.g. 4) channels would be procured but
temperature, strain, and displacement transducers (e.g. 4 of each, 12
total) would be installed, positioned and plugged into the channels as
required. If the fiber optic system is not procured and implemented,
then some type of measurement scheme will be adopted for the spline
axial and angular displacement. It would probably consist of two linear
potentiometers with voltage source and buffered output.

#########