Disruptions and Alcator CMods new divertor

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Disruptions and Alcator C-Mods new divertor

• R. Granetz and
• Alcator C-Mod group

Columbia Plasma Physics Colloquium 22 November
2002
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Whats a disruption?

• Sudden (usually unexpected) termination of plasma
• Bane of the tokamak

• Fast Ip transient causes large induced voltages,
  currents, forces
• Rapid thermal losses cause surface damage

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H? image of C-Mod plasma
Inboard wall
Inboard divertor
Outboard divertor
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Disruption halo currents increase stress on
structures

• High transient voltages drive halo current
  through plasma SOL
• Halo current completes circuit through
  conducting structure
• Jhalo ? B? generates extra forces on vessel wall,
  particularly inboard wall
• Toroidal asymmetry of Jhalo increases peak loading

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C-Mod inboard divertor has changed

• Shape has changed (nose job)
• Structurally strengthen inner vessel wall
  (girdle)
• New disruption instrumentation added
• Some previous disruption instrumentation removed

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Previous inboard divertor
Protruding nose (very closed divertor)
R
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New inboard divertor
Flatter profile (and strengthen inboard wall)
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Previous halo current instrumentation

• Full halo Rogowskis measured total poloidal halo
  current in vessel wall at top and bottom.
• Bottom coil damaged during upgrade (will be
  replaced during next manned access)

• Partial halo Rogowskis measured toroidal
  variation of poloidal halo currents in vessel
  wall.
• Removed during upgrade

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Disruption geometry with previous divertor
Halo current flowing

• Most of SOL contacted both inboard and outboard
  divertors
• Therefore most of the halo current current flowed
  from inboard to outboard through vacuum vessel

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Previous halo current characteristics

• Halo current asymmetry usually rotated at ?1 kHz
  for 1-2 ms
• Total halo current was uni-directional (i.e.
  unipolar)

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Outer divertors are also instrumented

• About 1/2 of the outboard instrumentation remains
  operational

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Previous halo current scaling

• Peak Ihalo ? 0.63 Ip/q95 (or equivalently 1.08
  10-6 Ip2/B?)

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Disruption instrumentation onnew inboard divertor
Halo Rogowskis
Retro-reflectors
Tiles (a few)
Eddy Rogowskis
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Halo current instrumentation onnew inboard
divertor
New halo Rogowski coils (10 toroidal sectors)
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Typical halo currents for VDE disruptions with
new divertor

• Lower magnitude
• n1 asymmetry (as before), but not usually
  rotating rotation is not as obvious
• Total halo current changes sign during quench !

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FFT of n1 component does show rotation
(same direction as previously)
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Halo currents are lower with new divertor
Previous scaling
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VDE disruptions with new divertor
Halo current flowing

• Much of scrape-off layer misses new inboard
  divertor

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Typical halo currents for midplane-quench
disruptions with new divertor

• Little or no n1 asymmetry
• Total halo current is nearly unipolar

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FFT shows very little toroidal asymmetry
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Midplane-quench disruptionswith new divertor

• Plasma stays diverted until almost gone ? halo
  current cant short from inner to outer divertors

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Rotating halo currents with new divertor

• Rotation is no longer the norm
• Rotation, when it occurs, is less pronounced

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Summary of differences
Different halo current characteristics with new
inboard divertor
Comments/Explanations

• Lower magnitude

• Smaller nose descending plasma remains farther
  away

• Total halo current changes polarity during VDE
  quenches

• Current may enter wall above inboard divertor and
  exit through face of inboard divertor

• Mid-plane disruptions and VDEs differ in
  toroidal asymmetry

• ?

• ?

• VDEs limit on outboard divertor Midplane
  disruptions remain diverted

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Conclusion

• The shape of the divertor strongly affects halo
  current characteristics!

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Relevance to reactor design (ITER)
Halo currents in this high-stress region could be
reduced by pulling the blanket module further
away from the plasma.
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Midplane vs VDE disruption

• Thermal quench occurs before vertical motion and
  halo current

• Vertical motion occurs before thermal quench and
  halo current