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Disruptions and Alcator CMods new divertor

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Disruption halo currents increase stress on structures ... transient voltages drive halo' current through plasma SOL. Halo current completes circuit through ... – PowerPoint PPT presentation

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Title: Disruptions and Alcator CMods new divertor


1
Disruptions and Alcator C-Mods new divertor
  • R. Granetz and
  • Alcator C-Mod group

Columbia Plasma Physics Colloquium 22 November
2002
2
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

3
H? image of C-Mod plasma
Inboard wall
Inboard divertor
Outboard divertor
4
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

5
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

6
Previous inboard divertor
Protruding nose (very closed divertor)
R
7
New inboard divertor
Flatter profile (and strengthen inboard wall)
8
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

9
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

10
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)

11
Outer divertors are also instrumented
  • About 1/2 of the outboard instrumentation remains
    operational

12
Previous halo current scaling
  • Peak Ihalo ? 0.63 Ip/q95 (or equivalently 1.08
    10-6 Ip2/B?)

13
Disruption instrumentation onnew inboard divertor
Halo Rogowskis
Retro-reflectors
Tiles (a few)
Eddy Rogowskis
14
Halo current instrumentation onnew inboard
divertor
New halo Rogowski coils (10 toroidal sectors)
15
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 !

16
FFT of n1 component does show rotation
(same direction as previously)
17
Halo currents are lower with new divertor
Previous scaling
18
VDE disruptions with new divertor
Halo current flowing
  • Much of scrape-off layer misses new inboard
    divertor

19
Typical halo currents for midplane-quench
disruptions with new divertor
  • Little or no n1 asymmetry
  • Total halo current is nearly unipolar

20
FFT shows very little toroidal asymmetry
21
Midplane-quench disruptionswith new divertor
  • Plasma stays diverted until almost gone ? halo
    current cant short from inner to outer divertors

22
Rotating halo currents with new divertor
  • Rotation is no longer the norm
  • Rotation, when it occurs, is less pronounced

23
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

24
Conclusion
  • The shape of the divertor strongly affects halo
    current characteristics!

25
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.
26
Midplane vs VDE disruption
  • Thermal quench occurs before vertical motion and
    halo current
  • Vertical motion occurs before thermal quench and
    halo current
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