1. Recycling and impurity retention in high-density,
improved-confinement plasmas in W7-AS
Y. Feng, F.Sardei, P.Grigull, F. Wagner, K.McCormick,
J.Kisslinger, D.Reiter
Max-Planck-Institut für Plasmaphysik, Greifswald/Garching
and
Institut für Plasmaphysik, Forschungszentrum Jülich
Outline
Recycling, neutral screening & core refueling
Impurity retention in the edge islands

2. Motivation Experimental observations at NC-HDH transition in W7-AS:
• Strong gas-puff needed to access HDH-plasmas
Abrupt increase of island density (upstream density)
• Shift of radiation peak from core to edge
Two SOL quantities affect core plasma:
• Gcore from plasma recycling, affects core plasma density
• nIs from impurity release, affects core impurity density
they are largely independent of core transport!
Aim of this study: check SOL-transport for consistency with
the experimental observations
Y. Feng, Zinnowitz Theory Meeting, November 2005

3. Recycling & neutral screening

4. Warm, dense islands stop recycling neutrals
cm X
Core Island SOL
Low n
transparent
efficient
island screening
Low T
transparent
100
temperature effect
Neutral penetration length:
Neutral penetration length / cm 10
density effect
Island width 1
nd / cm-3
Y. Feng, Zinnowitz Theory Meeting, November 2005

5. Island neutral screening effect
EMC3/EIRENE: PSOL=1.2 MW, 2% carbon sputtering
cm X
Core Island SOL
Grecy
Gcore
Gas-puff Gpuff
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
Y. Feng, Zinnowitz Theory Meeting, November 2005

6. Island screening effect seen in experiments
to detach.
HDH
~20% NC
~ factor of 3
large change of
SOL plasma state
Strong gas-puffing necessary to overcome the island screening effect
in order to access the high density region
Y. Feng, Zinnowitz Theory Meeting, November 2005

7. under NC and HDH conditions
Impurity retention
under NC and HDH conditions

8. Impurity transport in the islands
EMC3 includes self-consistently a trace impurity transport model:
continuity
force balance
with
and
Y. Feng, Zinnowitz Theory Meeting, November 2005

9. Impurity retention: simple analysis
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!
Y. Feng, Zinnowitz Theory Meeting, November 2005

10. Simple analysis contd. Condition for V*zII>0 : (Yu. Igitkhanov,
Contrib. Plasma Phys. 1988)
qII q qi
cm X
Core Island SOL
Because of the small Q (~0.001) in W7-AS,
the parallel conductive heat flux can be significantly
reduced by the perpendicular one:
The cross-field heat conduction dominates under high-n, low-T condition:
It can be shown that this condition can be established
for HDH-plasmas in W7-AS
Y. Feng, Zinnowitz Theory Meeting, November 2005

11. From II to -dominated transport for Ti
EMC3/EIRENE
(PSOL=1.2 MW , 2% carbon sputtering, e= i =3D, D=0.5m2/s)
II > 
II <  Ti
high Ti
nes=41013 cm-3
nes=11013 cm-3
low
Y. Feng, Zinnowitz Theory Meeting, November 2005

12. Experimental indication of >II even for Te
Strong drop of Teu
already in attached case
Condition of >II for e:
for ion:
detached
This condition more
easily reachable for ions
than for electrons
Y. Feng, Zinnowitz Theory Meeting, November 2005

13. + = + = Impurity flow reversal thermal force > friction inward flow
EMC3/EIRENE
thermal force > friction inward flow =
low edge density
thermal force < friction outward flow =
high edge density
Y. Feng, Zinnowitz Theory Meeting, November 2005

14. Strong reduction of C density at separatrix
Y. Feng, Zinnowitz Theory Meeting, November 2005
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

15. 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
Y. Feng, Zinnowitz Theory Meeting, November 2005

16. Summary
• SOL impurity transport investigated with EMC3-EIRENE for attached plasmas
from low-density NC to high-density HDH plasma conditions
• Island screening of recycling neutrals explains abrupt increase of nes at NC-HDH
transition. Island screening has to be overcome by cooling down the islands
through large SOL fueling
• At high densities, // heat conduction is strongly reduced by ^ heat conduction
due to small internal pitch of W7-AS islands 
• This leads to sharp transition of impurity transport from thermal-force dominated
to friction dominated
• Friction force prevails throughout the islands at high densities, dragging
impurities back to targets
(Carbon density at separatrix drops by one order of magnitude)
Y. Feng, Zinnowitz Theory Meeting, November 2005