failure of original tf inner leg assembly c neumeyer 4 10 3 n.
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Failure of Original TF Inner Leg Assembly C Neumeyer 4/10/3. February 14, 2003: Following our morning “test shots”, the first plasma attempt of the day resulted in a loud bang (heard on the control room audio monitors) accompanied by a plume of smoke (visible on the control room video monitors).

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Presentation Transcript
slide2

February 14, 2003: Following our morning “test shots”, the firstplasma 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).

Test Shot

Failed Shot

slide3

Target level was 53.4kA which produces Bt=4.5kG

  • Fault occurred just prior to flat top as the current passed 50kA
  • Several protective devices tripped within milliseconds....

- TF power supply fault detector section overcurrent,

    • - TF Analog Coil Protection (ACP) overcurrent,
    • - TF Rochester Instrument System (RIS) overcurrent,
    • - TF ground fault current relay.
slide4

Initial inspection revealed that one of the TF “flags” on the bottom end of the machine was displaced radially by about 1 inch

TF Flags

peak damage area
PEAK DAMAGE AREA

Start & Finish Leads

(DV = 1kV)

Peak Damage

Area

fault scenario
Fault Scenario
  • An open circuit fault (the flag joint opening up) led to multiple turn-to-turn and turn-to-ground faults (to the hub and umbrella assemblies) at or near the high-voltage terminals of the TF circuit.
  • Spike of fault current from the power supply 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 fault with V=125V and R=500.
  • The energy dissipated in the arc was of order 1.4MJ.

Spike Current

Coil Current

current decay waveshapes
Current Decay Waveshapes

Normal vs. Fault L/R Decay

Curve Fit to Fault Model

Varc=125V

Rfault=500

Other “glitches” in current decay are explainableby mutual coupling to

OH power supply circuit along power cable run

mechanical electrical failure of joint
MECHANICAL/ELECTRICAL FAILURE OF JOINT

Hub Assembly

TF

Bundle

TF Flag

Preload

(bolts)

Hub Assembly

Shim

EM

Load

“Keensert[tm]”

history of tf operations
HISTORY OF TF OPERATIONS
  • Approx. 7200 Shots mainly at 3kG, 4.5kG
  • Limited number at 6kG

6.0kG

4.5kG

3kG

FAULT

tf pulse spectrum
TF Pulse Spectrum

3kG

4.5kG

6.0kG

Max I2T = 5.46x109 A2-sec, T = 65oC

Rated I2T = 6.5 x109 A2-sec, T = 80oC

precursors
PRECURSORS

Evidence of a problem surfaced after ‘02 run period

Also, loose bolts and broken inserts were discovered

response to precursors
RESPONSE TO PRECURSORS
  • Corrected Various Defects
    • Resurfaced non-planar flag faces and chamfered bolt holes (bottom)
    • Improved bolt washers and bolt retention (top and bottom),
    • Retorqued bolts (top and bottom) and replaced 4 “keenserts” with “tap-lok” inserts (bottom),
    • Replaced G-10 flag shims with inflatable epoxy shim design (top and bottom)
  • Initiated more detailed FEA
  • Returned to 4.5kG limit
  • Initiated more regular inspections

Shim

Too Little, Too Late

factors leading to failure
FACTORS LEADING TO FAILURE
  • Design Factors
    • 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
    • lack of feature to facilitate joint resistance measurement w/o 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 THESE FACTORS HAVE BEENADDRESSED IN THE NEW DESIGN