1. Features of Divertor Plasmas in W7-AS
Y. Feng, P.Grigull, F.Sardei, K.McCormick,
J.Kisslinger
Island divertor vs tokamak divertor
-basic differences and similarities
Transport features of attached plasmas
Recycling, neutral screening & core-fueling
Impurity screening of the edge islands
 Detachment and detachment stability

2. Island divertor concept for low-shear stellarators
W7-X
W7-AS
 An intermediate low-order island chain between confinement core and
plasma-surface Interaction region, screening the core from direct
penetration of recycling neutrals and sputtered Impurities
Island divertor experiments in W7-AS from 2000 to 2002

3. Divertor vs limiter in W7-AS
Easier density control even in the presence of strong NBI-sources
- Significant improvement of recycling condition and
particle pumping
 High Density H-mode (HDH) maintainable quasi-stationary
- concomitant dense, cold plasma in the edge islands
Strong reduction of diverted energy flux onto targets via impurity line radiation
- Existence of stable partial detachment in certain
geometry and plasma parameter ranges
- Intensive radiation outside confinement region, no serious
degradation of global energy content

4. Island divertor vs tokamak divertor
Tokamak Stellarator
Poloidal-field divertor island divertor
single-null double-null from single- to multi-null
W7-AS: 8,9,10
Standard=9
W7-X: 4,5,6
Standard=5

5. 1D SOL transport model core -V X-P. +V y
target
X-P. +V -V x y
(Q introduced only for simple analysis)
Stellarator specific

6. Flow damping & momentum loss
nV (EMC3) + -
V =0,stagnation
divertor plate
shadow
Schematic expression
V =0
V =0
V =0 -V +V
radial x
poloidal y
target
shadow
target
radial Q
to upstream
toroidal

7. An extended 2P-model with -transport
(no CX-momentum loss, no volume energy losses)
Large Q -> standard 2-point model

8. Extended-2P-model results
-transport strongly damps downstream evolution

9. No high-recycling regime
EMC3/EIRENE Langmuir probes on targets

10. Island neutral screening & particle re-fueling
EMC3/EIRENE
Experiment
HDH-plasmas
Island screening
Low n
low T
Recycling provides the main fueling source for the core
Island screening -> flattening of fueling rate
In the flattening range, gas-puff increases only the SOL density, rather than the core density
High density core correlates with dense, cold islands

11. Dense, cold islands shift CX-neutrals
to a low energy band
Energy spectrum of CX-neutrals
hitting the Fe-wall
EMC3/EIRENE
nes=5×1019 m-3 CX
Fe-wall
1×1019
H-atom / eV-1
E0 /eV

12. In/out asymmetry in reducing high-energetic
CX-neutral on wall
(E0 > threshold energy for Fe-sputtering)
nes=1×1019 m-3
nes=5×1019 m-3
nes=1×1019 m-3
D=2 m2/s
peaked ne-profile
nes=5×1019 m-3
D=0.5 m2/s
flat ne-profile
Wall distribution of neutral flux with energy > 180 eV for a low- and high-nes case

13. Sensitivity of Fe-yield to nes, ne(r) and D
EMC3-EIRENE
→A high density in the edge islands is the most efficient parameter for
reducing the wall-sputtering yield.

14. Impurity retention under high-density condition
force balance
1D radial continuity:
(Z-independent)
cm X
Core Island SOL
Solution for target-released impurities:
A positive (outwards), large V*ZII reduces the impurity density at separatrix!

15. Simple analysis contd. Condition for V*zII>0 : (KRASHENINNIKOV,
Nucl. Fusion 1991)
1D energy transport for ion:
Because of the small Q (~0.001) in W7-AS, the parallel heat conduction
can be significantly reduced by the perpendicular one. The latter becomes
even dominating under the condition:
=> high-n, low-T SOL plasma favorable for SOL impurity retention

16. + = + = Impurity flow reversal
EMC3/EIRENE
thermal force > friction inwards flow =
low edge density
thermal force < friction outwards flow =
high edge density

17. Strong reduction of C density at separatrix
(normalized to total carbon yield in order to isolate transport from production)
thermal-forces draw carbon
to separatrix
frictional plasma flow flushes
carbon back to targets
EMC3/EIRENE
low nes high nes
low nes
high nes

18. Sharp transition from thermal-force dominated
to friction-dominated transport
EMC3/EIRENE
Friction
dominates
Thermal force
dominates
sharp transition because of high sensitivity of classical heat conductivity to Ti

19. High density for detachment transition
Td=10 eV
Extended 2P-model
absence of a high-recycling regime
shift of detachment transition
to a high nes
EMC3/EIRENE with PSOL = 1 MW
Intrinsic carbon
Abrupt detachment transition observed in experiments under conditions:
In good agreement with the code prediction

20. Detachment stability depends on island geometry
Experiments
change of Dx
stable
E /kJ
P/MW
time (density ramp)
PNBI
Prad
#56846
#56848
change of Lc
#56843
#56847
time
Experimental results
Lc para. targ-X-p. dist. /m
DX radial targ-X-p. dist. /cm

21. Stable partial detachment
Experimental finding: a) stable detachment requires large islands with large Q
b) stable detachment always partial
Power load on target
EMC3/EIRENE
EMC3/EIRENE
C-radiation H-ionization
(inboard side) (divertor region)
Thermography
power supply
hot spot

22. (independent of island geometry)
Marfe-like phenomena
small islands or
field-line pitch
~ density limit
(independent of island geometry)
EMC3/
EIRENE
Weak neutral screening
Unstable (exp.)
Complete detachment
Unstable
Strong degradation of te
Unstable, intensive radiation zone appearing at the inboard midplane
observed by a CCD camera when plasma approaches density limit.
Increase
density

23. Impact of radiation location on neutral screening
sensitivity of neutral screening to configuration, nes and Psol
EMC3/EIRENE
divertor radiation
Psol=1 MW
G / G
recyc
core
NBI
0.8 MW
inboard side radiation
nes 1013 cm-3
Divertor radiation  cold recycling zone  less efficient for neutral screening
‘less efficient’ means: 1) higher Grecyc into core (smaller DX)
2) more sensitive to change of nes or Psol (radiation location)

24. Summary
Plasma:
The -to-‖ transport ratio can be changed from <1 to >1,
depending on divertor configurations and plasma parameters.
Flow-damping -> no high recycling regime.
Weak neutral screening -> strong edge-core coupling
- recycling neutrals are the main fueling source for the core
- recycling and refueling process nonlinear -> instabilities
For example, the abrupt change of edge plasma state observed in density-ramp
experiments and the geometry-related detachment instability.
Recycling neutrals:
Impurity:
Dense, cold islands – favorable for reducing influx of intrinsic
impurities
- Reduction of impurity sputtering yield from CX-neutrals
- Frictional plasma flow flushes Impurities

25. Summary contd. Detachment: Detachment transition:
High densities needed for detachment
Abrupt change in radiation level and location
Stability (depending on island geometry)
stable detachment is always partial and needs sufficiently-
large islands
-inboard-side radiation -> warm recycling region -> good
neutral screening
small islands -> divertor radiation -> loss of neutral screening
-> unstable
Marfes (unstable)
appear always on the inboard side, inside LCFS whenever a plasma approaches density limit, independent of configuration.

26. Principle: Divertor vs limiterq G
core
Limiter
target
impurity
radial
separatrix q G
Divertor
core
impurity
 An intermediate SOL exists between confinement core and plasma-surface
Interaction region, screening the core from direct penetration of
recycling neutrals and sputtered Impurities.