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ELM filamentary heat load in ASDEX Upgrade
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ELM filamentary heat load in ASDEX Upgrade
• Motivation• Decay lengths• Filament dynamics
A . Herrmann, A. Schmid, A. Kallenbach, ASDEX Upgrade team
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6.-10. January 2008 ITPA wall&divertor, A. Kallenbach, A. Herrmann, A. Schmid et al. 2
ELM heat deposition – simplified picture
• Radial movement of the filament – Decelerated, accelerated, size and
density dependent – See A. Schmid, PhD work
• Energy loss by ion convection.
• q|| small compared to SP values.
• Do not penetrate deep into a limiter shadow.
• Filaments are starting with pedestal (separatrix) values of ne, Te, Ti
• Filament in contact to target plates (wall) looses a significant fraction of the energy on short time scales (10 μs) (near to the separatrix)
ToDo:• Follow individual filaments.• Measure the radial dynamics.• Statistics.• Model validation/falsification.
Inne
r w
all
ELM
Inter ELM
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6.-10. January 2008 ITPA wall&divertor, A. Kallenbach, A. Herrmann, A. Schmid et al. 3
Measure the filament dynamics in the far SOL
• Local measurement
– Probe heads
• Langmuir probes
• Limiter like probes
– Filament probe
– Thermography
• 2D Filament observation by cameras (No information on radial movement – constant shear)
• Thomson scattering
• Magnetic probes
• SXR pedestal channel
Filament probe, Reciprocating probe (LPs and thermographic heat load measurement)
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6.-10. January 2008 ITPA wall&divertor, A. Kallenbach, A. Herrmann, A. Schmid et al. 4
Far SOL decay lengths – LPs and thermography
• Comparable decay lengths for particle (Isat) and heat flux (q||).
• No clear dependence on global/pedestal parameters such as Wmhd and density.
• λ increases with q95
• Large (factor 5) scatter of the filament intensity.
• We do not follow individual filaments!
• E0 or λ variation?γTe = 100 – for this plotPosition of the Filament probe: Sep_dist + 1 cm
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6.-10. January 2008 ITPA wall&divertor, A. Kallenbach, A. Herrmann, A. Schmid et al. 5
Local measurement of filament behavior at the LFS
• Magnetically driven probe (in front of the limiter), Tungsten covered, 9 LPs,
• pins 6-9 radially separated for measurements of the radial propagation velocity
• Pins 1-6 to measure the poloidal/toroidal velocity
A. Schmid et al.,RSI 78(5), 2007
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6.-10. January 2008 ITPA wall&divertor, A. Kallenbach, A. Herrmann, A. Schmid et al. 6
Method for vrad measurement
Trace filaments over several pins
-> straight line for constant/accelerated filaments
-> Slope gives vrad,shape gives size (fitted, Gaussian with linear background)
Zoom -in
A. Schmid et al.,submitted to PPCF
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6.-10. January 2008 ITPA wall&divertor, A. Kallenbach, A. Herrmann, A. Schmid et al. 7
Filament data
• Series of type-I ELMy H-mode discharges• Probe @ different separatrix positions
-> changes the time of flight (the time the filament takes to reach the probe)
• Manually analyzed 466 filaments
Parameters:
• Vrad
• temporal peak width -> radial extent, Δrad
• ion saturation current -> density, nfil (denotes the maximum)
(using Ti=30-60eV, Te=5eV from IR/Langmuir comparison)
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6.-10. January 2008 ITPA wall&divertor, A. Kallenbach, A. Herrmann, A. Schmid et al. 8
velocity vs. density
vrad ~√nfil
(averaged values)
Lower limit on radial velocity, i.e.velocity increases with density
(more dense filaments move faster)
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6.-10. January 2008 ITPA wall&divertor, A. Kallenbach, A. Herrmann, A. Schmid et al. 9
velocity vs. radial extent
vrad~√Δrad
(averaged values)
detection limit due to finite sampling rate
Lower limit on radial velocity, i.e.velocity increases with radial extent
(bigger filaments move faster)
(radial size)
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6.-10. January 2008 ITPA wall&divertor, A. Kallenbach, A. Herrmann, A. Schmid et al. 10
Velocity vs. distance from separatrix
mean values, do not show a constant acceleration
(as has been observed on MAST.)
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6.-10. January 2008 ITPA wall&divertor, A. Kallenbach, A. Herrmann, A. Schmid et al. 11
Probability distribution functions
vrad: 1.1km/s Δrad: 2.7mm (FWHM)
nfil: 2.6x1018/m3
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6.-10. January 2008 ITPA wall&divertor, A. Kallenbach, A. Herrmann, A. Schmid et al. 12
Summary
• Typical decay length (heat, particle) are about 2-3 cm in the far SOL of AUG.
• Statistics for local measurement of filament dynamics (466 filaments)
• Data seems to favor the Garcia model, i.e. bigger filaments move faster.
• Large scatter probably due to hidden parameters, e.g. poloidal size.
• Upper limits on radial extent, line integrated density, and density gradient.
• Most probable values from PDFs:
vrad=1.1km/s, Δrad=2.7mm (FWHM), nfil=2.6x1018 /m3 (Δsep= 5 cm)
• Radial evolution:
Filament density decreases, filaments broaden with time
Total particle content decreases (due to parallel losses).
Values are in agreement with free parallel flow.
• No constant acceleration has been found.