ELM filamentary heat load in ASDEX Upgrade

• Motivation• Decay lengths• Filament dynamics

A . Herrmann, A. Schmid, A. Kallenbach, ASDEX Upgrade team

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

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)

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

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

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

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)

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)

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)

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.)

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

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.