a. herrmannitpa - toronto - 20061/19 filaments in the sol and their impact to the first wall euratom...

A. Herrmann ITPA - Toronto - 2006 1/19

Filaments in the SOL and their impact to the first wall

EURATOM - IPP Association, Garching, Germany

A . Herrmann, A. Kirk, A. Schmid, B. Koch, M. Laux, M. Maraschek, H.W. Mueller, J. Neuhauser, V. Rohde, M. Tsalas

E. Wolfrum, ASDEX Upgrade team


Wall and divertor heat load

mapped to midplane

Pla

sma

(R) 7 mm

a few mm

a few cm

ELM heat load to outboard limiter

Sep

para

trix

Rule of thumb:The wall heat load is comparable to the heat flux in the wing of the divertor profile.


Filamentary heat load

• Filaments in the far SOL are a small contribution to the ELM energy balance.

• They are no problem at the divertor target.

• But the parallel heat flow is up to 100 MW/m2 in AUG.

• Requires tilted structures at the inner wall.

• Extrapolation to ITER.

Eich, T., et al., Physical Review Letters, 2003. 91(19).

Eich, T., et al., Plasma Physics Controlled Fusion, 2005. 47


3 ELM phases - diagnostics

A. Kirk et al, PPCF 47 (2005) 315–333

Filament evolution in the pedestal region

Hot filament near to the separatrix.

Radial travveling into the far SOL, attached to the divertor.

• Thomson scattering• Magnetic probes• Langmuir

probesThermography• Li-beam• …


Outline

• Combined measurement of heat and particle flux in the mid-plane• ELM structure and correlations

• Wall impact – e-folding lengths• Particle flux and heat load

• Qualitative explanation

• Filament expansion – Prediction and experiment

• Summary


Diagnostics

• Combined measurements• Langmuir probes

• Reciprocating

• Filament probe

• Thermography

• Magnetic pick up coils

• Probes are toroidal connected along field lines.

• Outside the shadow of the protection limiter.

• RP 5 mm in front of the ICRH limiter (connection length into the divertor about 5 m).


Discharge scenario for radial SOL scan

• Move the probes in front of the limiter.

• Move the plasma away from the Limiter.

• Radial scan 3.5 -12 cm

• Discharge parameters

• Ip= 0.8; 1.0 MA

• Bt = -2; -3 T

• n/ngw = 0.6

• Wmhd = const (500 kJ)

• Pheat = 5; 6.6 MW NI

• q95 = 3.5-6.5


Magnetic configuration

• Field line connection to the divertor entrance.

• No effectd from the 2nd X-point

• Inner divertor -> heat shield

• But, large gap.


Correlation between signals

• Filaments are seen on all probes (Langmuir pins, heat flux, magnetic)

• Magnetic activity strongest at the beginning of an ELM.

• jsat signals are correlated on a short

spatial scale (Mach probe).

• Parallel mass flow towards the outer lower divertor (M ab. 0.1).

• Single filaments are detected as heat load: 200100 eT


Heat load to the probe head is non-uniform

6 cm

texposure = 2 μsTframe = 100 μs

Leading edge

• Rotation in co-current direction• ‘Sharp’ edge in the limiter shadow


Radial decay in the far SOL

• Decay of maximum values.

• Langmuir probes and heat flux have the same e-folding lengths!

• Filament probe is about one radial e-folding length behind the reciprocating probe.

e

jTq sate|| 100eTFor this plot:


Radial decay is independent on the strength of the filament

• The radial decay is independent on the strength of the filament. (Statistics, we do not follow a single filament)• Scatter due to different source strength or different radial velocity (less time for

parallel convection)

• Both Langmuir probes have comparable decay lengths.

• Larger scatter for heat flux decay.

• Heat flux decay is comparable (or larger) than the particle flux decay (jsat)


Heat flux and ELM energy balance

• The e-folding length is dominated by the density decay (Te, Ti = const)

Qualitative explanation

• We are measuring in the far SOL (away from the steep gradient near to the separatrix)

• The electrons have lost their energy (modeling, experiment).

• Loosing particles (and energy) without altering the temperature.Convective losses but collisional far SOL.

eese Tncn ~~ 2/3~)(~ eee TnTq


Heat loss channels

• n = 2e19m-3

• Te = 0.1 Ti

~n

ITER

Heat conduction (Kaufmann S 112, Stangeby S 394)

Tq

2/5|| 2000 ee T

2/5|| 60 ii T

Heat convection (ions)

)()(1

108.4

2/33

152

eVTmnA

m

Wq

ie

iconv

Ions (D)

electrons

2/7|| ~ eTq

2/32

3 ieiconv Tn

m

Wq


Collisional SOL

• Collisional edge

• No significant heat exchange between electron and ions

),(ln1016.3

1009.12

5.1

5

16

eVsZn

T

i

eie

; electron collision time (Wesson 2.15.3)

e

pieii m

mM

; ion collison time

e

pieex m

mM2

; energy exchange time

thv

L

||*


The ion temperature is below 100 eV

• This is consistent with

• Te < Ti : Heat load is dominated by ions:

• Experimentally:

3,|| e

jTq sati

eVTe

jq i

sat )6030()200100(||

eese Tncn ~~ 2/3~)(~ eee TnTq

)

1

11

2ln(5.0

2

1

22

ee

i

i

e

e

i

e T

T

m

m

T

T

e

i

T

T3


Radial blob velocity

• Filament in contact with the wall – sheath resistivity

• Far from the X-point.

t

bbisb n

n

R

lcv

2

S.I. Krasheninikov, PL A 283 (2001) 368

Ion gyro ratio/ poloidal size

Blob

/ background de

nsity

• Larger filaments are slower.

• Faster with increasing density.

Blob

velocity


Radial blob velocity

• From experiment:• Poloidal size of 1-2 cm

• Ion temperature <100 eV

• Qualitative agreement with prediction.

• But:• Size dependence?


Summary and conclusions

• The heat and particle decay length is a few centimeters in the far SOL

• Particle and heat flux decay length are comparable.• The decay is dominated by ion-convection (energy and particles).

• With a low Mach number (midplane, flow towards the lower divertor).

• The ion temperature in the filament is below 100 eV.

• The radial velocity from experiment and model is in agreement.

• The fraction of ELM energy to the wall decreases with ELM size

a. herrmannitpa - toronto - 20061/19 filaments in the sol and their impact to the first wall euratom...

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