PRELIMINARY INFORMATION\
PACKAGE

FOR

NSTX TF FLAG JOINT

#### DESIGN REVIEW

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

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

B. Paul, H. Schneider, I. Zatz

4/3/3

[Table of Contents]{.underline}

INTRODUCTION [3](#introduction)

PERFORMANCE REQUIREMENTS [7](#performance-requirements)

OPERATIONAL HISTORY [9](#operational-history)

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

Design Factors [9](#design-factors)

Quality Factors [10](#quality-factors)

Operational Factors [10](#operational-factors)

KEY DESIGN DRIVERS [10](#key-design-drivers)

SUMMARY OF FORCES [12](#summary-of-forces)

LOAD PATHS [12](#load-paths)

DESCRIPTION OF SELECTED DESIGN [13](#description-of-new-design)

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

Deficiencies of Original Design [**Error! Bookmark not
defined.**](#__RefHeading___Toc447879227)

Corrective Features of New Design [**Error! Bookmark not
defined.**](#__RefHeading___Toc447879228)

#   {#section .unnumbered}

# INTRODUCTION

The TF Inner Leg Assembly failed on 2/14/03 at the Inner Leg-to-Flag
joint. The NSTX Project is now mobilizing to fabricate a new TF Inner
Leg Assembly. The need for a new flag joint design is the schedule
driver at present. A spare set of extruded conductor was purchased some
time ago. However, the Project cannot proceed with the machining of the
new inner leg conductors until an approved design is ready. Therefore
the design of the new Inner Leg-To-Flag joint is the pacing item.

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*

The purpose of the design review is to approve the design of the new
joint. Design shortcomings needing attention need to be highlighted by
chits. However, an important objective is to obtain approval to
*[release inner leg conductors for machining.]{.underline}*

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.)*

-   *Hub Assembly/Torque Collar*

Other improvements may be discussed but are out of scope.

![](media/image1.pct){width="5.992361111111111in"
height="3.848611111111111in"}

Figure 1 -- TF Coil System

![](media/image2.pct){width="5.992361111111111in"
height="3.5902777777777777in"}

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

![](media/image3.pct){width="5.997222222222222in"
height="2.5965277777777778in"}

Figure 3 -- Joint Fasteners

As shown in Figure 4, recovery activites have led to the development of
a plan which consists of three main thrusts:

-   *Design (drawing) development*

-   *Detailed Finite Element Analysis*

-   *Testing*

![](media/image4.pct){width="6.004166666666666in"
height="3.953472222222222in"}

***Figure 4 -- Joint Development Plan***

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 the schedule constraint this will not be possible.

Component testing consists of:

-   *Pull-out tests on threaded inserts (one time and 50,000 cycle)*

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

-   *Shear tests on simulated torque collar attachment*

-   *Tensile tests on e-beam welded joints*

Original NSTX R&D provides other essential information such as contact
resistance vs. pressure.

Prototype testing consists of:

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

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

Since some of the 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.
The exact form has not been decided upon yet, but it could be, e.g., 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/image5.pct){width="5.570138888888889in"
height="4.283333333333333in"}

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 (of the
original joint design) 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/image6.pct){width="4.61875in" height="4.628472222222222in"}

***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/image7.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/image8.pct){width="5.999305555555556in"
height="4.170833333333333in"}

Figure 7 -- 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 8. Subsequent
inspection revealed the extent of the damage as shown in Figure 9.

![](media/image9.pct){width="5.059027777777778in"
height="3.7534722222222223in"}

Figure 8 -- Radially Displaced Flag

![](media/image10.pct){width="6.055555555555555in"
height="3.9930555555555554in"} ***Figure 9 -- 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) at or near the high-voltage
    terminals of the TF circuit.

-   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 of the coil current as the coil
    released its stored energy. The L/R decay 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

> *- hub stiffness not adequate to react moment*
>
> *- imprecise shimming, poor communication of load from flag to hub*
>
> *- bolt thread and shoulder engagement too small*
>
> *- bolts necked down too far at threads, not enough on shaft*
>
> *- dual shear/preload function of bolts*
>
> *- split flag aggravating current density peaking*
>
> *- lack of feature to facilitate joint resistance measurement without
> disassembly*

## 

## Quality Factors

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

## Operational Factors

*- monitoring of joint integrity too infrequent, too imprecise*

# 

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

# KEY DESIGN DRIVERS

Electrical contact resistance vs. pressure behavior is the key design
driver. The contact pressure requirement drives the structural design,
and the contact resistance determines heating and limits flat top time.
The performance range is bracketed by two extremes, namely 1) an
ordinary Cu-Cu joint, and 2) a high quality silver plated joint (as
tested during NSTX R&D). Prior in-situ measurements hint at what is
achievable, and future measurements will ultimately establish the
operating envelope.

The design basis information is summarized in Figure 8.

![](media/image11.pct){width="5.998611111111111in"
height="3.795138888888889in"}

Figure 8 -- Contact Resistance Data

As shown in Figure 9 (from original design, assuming constant contact
resistance across the joint) the contact resistance and temperature
allowable will essentially dictate the realizable system performance in
terms of flat top time.

![](media/image12.pct){width="5.903472222222222in"
height="3.638888888888889in"}

##### Figure 9 --Effect of Contact Resistance on Temperature

The following design guidelines have been established:

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

-   *Performance range is assumed bracketed by two extremes\...*

> *• ordinary Cu-Cu joint: R=10000\*P^1.04^*
>
> *(4μΩ/in^2^ @ 2ksi, 7.5μΩ/in^2^ @ 1ksi)*
>
> *• high quality silver plated joints: 1μΩ/in^2^ @ P ≥ 1ksi*

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

Future measurements will provide more data and ultimately establish the
operating envelope. Improved procedures (surface prep, silver plating),
quality control and maintenance can improve situation.

# 

# 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 Forces and Moments at 6kG**

+-----------------------+----------+-----------------+-----------------+
|                       | F~v      | dR from point   | Moment          |
|                       | ertical~ | of application  |                 |
|                       |          | to flag inboard | w.r.t. point X  |
|                       | (lbs)    | edge            |                 |
|                       |          |                 | (in-lbs)        |
|                       |          | (in)            |                 |
+-----------------------+----------+-----------------+-----------------+
| Inner Tier Flag       | 7380     | 4.3             | 31719           |
+-----------------------+----------+-----------------+-----------------+
| Inner Tier Link       | 1855     | 13.8            | 25652           |
+-----------------------+----------+-----------------+-----------------+
| Outer Tier Flag       | 1845     | 4.3             | 7930            |
+-----------------------+----------+-----------------+-----------------+
| Outer Tier Link       | 464      | 13.8            | 6413            |
+-----------------------+----------+-----------------+-----------------+
| Total from All Turns  | 249324   |                 |                 |
+-----------------------+----------+-----------------+-----------------+

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

# LOAD PATHS

A simplified diagram of the in-plane force application and the elements
available to react the load is given in Figure 10.

![](media/image13.jpeg){width="5.424305555555556in"
height="3.216666666666667in"}

***Figure 10 -- Load Paths***

The three primary load paths are 1) the hub, 2) the bolted joint, and 3)
the torque collar attachment. 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.

*Sharing of the vertical forces and the moments amongst these three
paths, and establishing their ability to react same while maintaining
adequate contact pressure along the joint, is the essential design
problem.*

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

-   *Vertical length of inner leg from torque collar to top of flag
    increases due to inner leg temperature rise (∆T ≤80^o^C) during a
    pulse, whereas flag and hub remain relatively cool*

-   *Radial length of flag increases 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.*

# DESCRIPTION OF NEW DESIGN

The ideal design would have the following attributes:

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

-   *Infinitely stiff hub*

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

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

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

Several views of the new design are given in Figures 11-15. 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 SS boxes which are bolted to the hub disks. Communication of
    in-plane and out-of-plane forces from flag to hub is very
    efficient.*

-   *Boxes bolted to hub disks form 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. A286 SS studs
    screwed into threaded inserts. Washer plate is used under tee at end
    of flag. Belleville washer and nut then complete the attachment.*

-   *Studs necked down over their length to enhance elasticity.*

-   *Upper 24 outer layer, lower tier flags are e-beam welded, joint at
    end of flag. Number of high performance joints is reduced from 72 to
    48.*

-   *Shear shoe/key located on outer edge of flags (except e-beam welded
    flags).*

-   *Voltage taps positioned on flag, routed across joint then twisted
    around water tube, allowing in-situ joint resistance measurements
    without breaking outer flag joints.*

![](media/image14.pct){width="4.801388888888889in"
height="4.053472222222222in"}

***Figure 11 -- Unfolded View of New Design***

![](media/image15.pct){width="3.08125in" height="4.5in"}

***Figure 12 -- Isometric View of New Design***

![](media/image16.pct){width="3.7083333333333335in" height="3.1in"}

***Figure 13 -- Section View of New Design***

![](media/image17.pct){width="6.0375in" height="7.240277777777778in"}

##### Figure 14 -- Details of New Design, Top End of Machine

![](media/image18.pct){width="5.988194444444445in"
height="4.700694444444444in"}

***Figure 15 -- Details of New Design, Bottom End of Machine***

# 

# 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 pin (0.688\" @ 1/2\"
                    small (4 bolts x 0.1 depth = .4 dia, Area = 0.344 in\^2)
                    @ 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                      place of long bolts

  ** **             Dual purpose bolts, combined    Loose fitting clearance
                    tension and shear functions,    holes for studs, separate
                    tolerance issues,               shear keys
                    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                     under 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
  ----------------- ------------------------------- --------------------------