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). Test Shot Failed Shot

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.

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

Subsequent inspection revealed the extent of the damage Peak Damage RegionMating Flags

PEAK DAMAGE AREA Peak Damage Area Start & Finish Leads (  V = 1kV)

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 V arc =125V R fault =500  Normal vs. Fault L/R DecayCurve Fit to Fault Model Other “glitches” in current decay are explainableby mutual coupling to OH power supply circuit along power cable run

MECHANICAL/ELECTRICAL FAILURE OF JOINT TF Flag Hub Assembly TF Bundle “Keensert[tm]” Shim Preload (bolts) EM Load

HISTORY OF TF OPERATIONS Approx Shots mainly at 3kG, 4.5kG Limited number at 6kG 3kG 4.5kG 6.0kG FAULT

TF Pulse Spectrum Max I 2 T = 5.46x10 9 A 2 -sec,  T = 65 o C Rated I 2 T = 6.5 x10 9 A 2 -sec,  T = 80 o C 3kG 4.5kG 6.0kG

PRECURSORS Evidence of a problem surfaced after ‘02 run period Also, loose bolts and broken inserts were discovered

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 Too Little, Too Late Shim

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 BEEN ADDRESSED IN THE NEW DESIGN