Transcript Interaction of ELMs and fast particles with in vessel

EURATOM - IPP Association,
Garching, Germany
Filaments in the SOL and their impact to
the first wall
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
A. Herrmann
ITPA - Toronto - 2006
1/19
Wall and divertor heat load
Plasma
ELM heat load to
outboard limiter
Sepparatrix
mapped to midplane
a few mm
(R) 7 mm
a few cm
Rule of thumb:
The wall heat load is comparable to the heat flux in the wing of the divertor profile.
A. Herrmann
ITPA - Toronto - 2006
2/19
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
A. Herrmann
ITPA - Toronto - 2006
3/19
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
• …
A. Herrmann
ITPA - Toronto - 2006
4/19
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
A. Herrmann
ITPA - Toronto - 2006
5/19
Diagnostics
• Combined measurements
• Langmuir probes
• Reciprocating
• Filament probe
• Thermography
• Magnetic pick up coils
• Probes are toroidal connected
•
•
A. Herrmann
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).
ITPA - Toronto - 2006
6/19
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
•
•
•
•
•
•
A. Herrmann
ITPA - Toronto - 2006
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
7/19
Magnetic configuration
• Field line connection to the
•
•
•
A. Herrmann
divertor entrance.
No effectd from the 2nd X-point
Inner divertor -> heat shield
But, large gap.
ITPA - Toronto - 2006
8/19
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:
A. Herrmann
ITPA - Toronto - 2006
 Te  100 200
9/19
Heat load to the probe head is non-uniform
6 cm
Leading edge
texposure = 2 μs
Tframe = 100 μs
A. Herrmann
• Rotation in co-current direction
• ‘Sharp’ edge in the limiter shadow
ITPA - Toronto - 2006
10/19
Radial decay in the far SOL
• Decay of maximum values.
• Langmuir probes and heat flux
•
j sat
q||   Te
e
A. Herrmann
have the same e-folding lengths!
Filament probe is about one
radial e-folding length behind the
reciprocating probe.
For this plot:
ITPA - Toronto - 2006
 Te  100
11/19
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)
A. Herrmann
ITPA - Toronto - 2006
12/19
Heat flux and ELM energy balance
 ~ ne cs ~ ne Te
q ~  ( Te ) ~ neTe3/ 2
• 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.
A. Herrmann
ITPA - Toronto - 2006
13/19
Heat loss channels
Heat conduction
(Kaufmann S 112, Stangeby S 394)
q   T
• n = 2e19m-3
• Te = 0.1 Ti
electrons
||e  2000Te5 / 2
ITER
Ions (D)
 ||i  60Ti 5 / 2
~n
q|| ~ Te7 / 2
Heat convection (ions)
W 
i
3/ 2
qconv
 2   3neTi
m 
W 
i
15
qconv
 2   4.8 10 
m 
1
ne (m 3 ) Ti 3 / 2 (eV )
A
A. Herrmann
ITPA - Toronto - 2006
14/19
Collisional SOL
* 
; electron collision time
(Wesson 2.15.3)
L||
 vth
Te1.5
1.091016
 ie 
(s, eV )
3.16105 ni Z 2 ln 
; ion collison time
 ii   ie M
mp
me
; energy exchange time
 ex   ie M
mp
2 me
• Collisional edge
• No significant heat exchange between electron and ions
A. Herrmann
ITPA - Toronto - 2006
15/19
The ion temperature is below 100 eV
q ~  ( Te ) ~ neTe3/ 2
 ~ ne cs ~ ne Te
• This is consistent with
• Te < Ti : Heat load is dominated by ions:

2T
2 me  Ti 
2
1
1  
 i  0.5 ln(
)
2
1   e Te
mi  Te  1   e 
q||   Ti
  3
Ti
Te
jsat
,  3
e
• Experimentally:
q||  (100  200 )
A. Herrmann
jsat
 Ti  (30  60) eV
e
ITPA - Toronto - 2006
16/19
Radial blob velocity
S.I. Krasheninikov, PL A 283 (2001) 368
• Filament in contact with the wall
•
– sheath resistivity
Far from the X-point.
 i  lb nb
vb  cs  
   R nt
2
ITPA - Toronto - 2006
Blob / background density
A. Herrmann
Ion gyro ratio/ poloidal size
Blob velocity
• Larger filaments are slower.
• Faster with increasing density.
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Radial blob velocity
• From experiment:
• Poloidal size of 1-2 cm
• Ion temperature <100 eV
• Qualitative agreement with
•
prediction.
But:
• Size dependence?
A. Herrmann
ITPA - Toronto - 2006
18/19
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. Herrmann
ITPA - Toronto - 2006
19/19