Title: Disruptions and Alcator CMods new divertor
1Disruptions and Alcator C-Mods new divertor
- R. Granetz and
- Alcator C-Mod group
Columbia Plasma Physics Colloquium 22 November
2002
2Whats 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
3H? image of C-Mod plasma
Inboard wall
Inboard divertor
Outboard divertor
4Disruption 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
5C-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
6Previous inboard divertor
Protruding nose (very closed divertor)
R
7New inboard divertor
Flatter profile (and strengthen inboard wall)
8Previous 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
9Disruption 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
10Previous halo current characteristics
- Halo current asymmetry usually rotated at ?1 kHz
for 1-2 ms - Total halo current was uni-directional (i.e.
unipolar)
11Outer divertors are also instrumented
- About 1/2 of the outboard instrumentation remains
operational
12Previous halo current scaling
- Peak Ihalo ? 0.63 Ip/q95 (or equivalently 1.08
10-6 Ip2/B?)
13Disruption instrumentation onnew inboard divertor
Halo Rogowskis
Retro-reflectors
Tiles (a few)
Eddy Rogowskis
14Halo current instrumentation onnew inboard
divertor
New halo Rogowski coils (10 toroidal sectors)
15Typical 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 !
16FFT of n1 component does show rotation
(same direction as previously)
17Halo currents are lower with new divertor
Previous scaling
18VDE disruptions with new divertor
Halo current flowing
- Much of scrape-off layer misses new inboard
divertor
19Typical halo currents for midplane-quench
disruptions with new divertor
- Little or no n1 asymmetry
- Total halo current is nearly unipolar
20FFT shows very little toroidal asymmetry
21Midplane-quench disruptionswith new divertor
- Plasma stays diverted until almost gone ? halo
current cant short from inner to outer divertors
22Rotating halo currents with new divertor
- Rotation is no longer the norm
- Rotation, when it occurs, is less pronounced
23Summary of differences
Different halo current characteristics with new
inboard divertor
Comments/Explanations
- 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
24Conclusion
- The shape of the divertor strongly affects halo
current characteristics!
25Relevance to reactor design (ITER)
Halo currents in this high-stress region could be
reduced by pulling the blanket module further
away from the plasma.
26Midplane vs VDE disruption
- Thermal quench occurs before vertical motion and
halo current
- Vertical motion occurs before thermal quench and
halo current