Disruption halo currents with C-Mod

1
Disruption halo currents withC-Mods new divertor

• R. Granetz and C-Mod group

APS-DPP 11-15 November 2002
2
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

3
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

4
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

5
Halo current rotation was ubiquitous

• Halo current asymmetry usually rotated at ?1 kHz
  for 1-2 ms
• Total halo current was uni-directional (i.e.
  unipolar)
• No difference between midplane- quench
  disruptions and VDE disruptions

6
Previous halo current scaling

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

7
Halo current instrumentation onnew inboard
divertor
New halo Rogowski coils (10 toroidal sectors)
8
Typical halo currents for VDE disruptions with
new divertor

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

9
Halo currents are lower with new divertor
Previous scaling
10
VDE disruptions with new divertor
Halo current flowing

• Much of scrape-off layer misses new inboard
  divertor

11
Typical halo currents for midplane-quench
disruptions with new divertor

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

12
Midplane-quench disruptionswith new divertor

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

13
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

• VDEs have n1 asymmetry, but dont usually rotate

• ? (Dont even know why they used to rotate)

• Mid-plane disruptions and VDEs differ in
  toroidal asymmetry

• VDEs limit on outboard divertor, Mid-plane
  disruptions remain diverted.

14
Conclusion

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