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

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

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

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

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

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

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

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

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

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

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

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

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.

Impurity retention under high-density condition force balance 1D radial continuity: (Z-independent) 0 4cm X Core Island SOL Solution for target-released impurities: A positive (outwards), large V*ZII reduces the impurity density at separatrix!

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

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

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

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

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 0.5 1 2 3 4 5 EMC3/EIRENE with PSOL = 1 MW Intrinsic carbon Abrupt detachment transition observed in experiments under conditions: In good agreement with the code prediction

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

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

(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

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)

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

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.

Principle: Divertor vs limiter q 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.