Mechanisms Controlling Parallel Flows in the SOL A.V.Chankin Max Planck Institute for Plasma Physics, EURATOM Association, Garching, Germany A.V.Chankin, Zinnowitz Theory Meeting, 17.11.2003 – 20.11.2003, Zinnowitz, Germany

Outline mechanisms determining parallel flow pattern in the SOL Experimental evidence and theoretical explanation of the main mechanisms determining parallel flow pattern in the SOL JET data on flows, impurity behaviour, location of surface layers; code modelling results, unresolved issues Summary A.V.Chankin, Zinnowitz Theory Meeting, 17.11.2003 – 20.11.2003, Zinnowitz, Germany

Impurity Control Limiter Experiment in DITE (Matthews et al.,1988) As part of this experiment: set of Langmuir probes on both ion (i) and electron (e) drift sides of limiter parallel plasma flows can be measured: (use either ; or for the Mach number M of parallel ion flow) Independent reversal of Bt, Ip (4 combinations in total) to study the nature of plasma flows Bt Interpretation of Asymmetries in the SOL of DITE through combination of ion Pfirsch-Schlüter flow and asymmetric (outboard enhanced) perpendicular turbulent transport (“ballooning”) (Hugill,1992) A.V.Chankin, Zinnowitz Theory Meeting, 17.11.2003 – 20.11.2003, Zinnowitz, Germany

Ion Pfirsch-Schlüter Flow in the SOL Perpendicular Ion Drift Flow: Ion Pfirsh-Schlüter Flow (reverses with Bt reversal) R EB drift ion diamagnetic flux In the SOL, due to Debye sheath drop: (plasma – target  3Te /e) eEr  -3dTe /dr > 0  poloidal EB drift adds to ion diamagnetic flux  Finite (non-zero) ion poloidal flow High recycling tends to reduce parallel ion flows in the divertor Divergence of this flow in toroidal geometry (B-var., R-var.) generates: A.V.Chankin, Zinnowitz Theory Meeting, 17.11.2003 – 20.11.2003, Zinnowitz, Germany

||  Poloidally Asymmetric Turbulent Transport (“Ballooning”) R Outboard enhanced perpendicular transport is closed (partly) by parallel Ion flows away from the midplane position  R A.V.Chankin, Zinnowitz Theory Meeting, 17.11.2003 – 20.11.2003, Zinnowitz, Germany

Poloidal SOL Asymmetries and Toroidal Flow in DITE (Pitts et al.,1990) a number of Langmuir probes imbedded into both ion (i) and electron (e) drift sides of poloidal limiters provided good poloidal coverage of the SOL Interpretation of Asymmetries in the SOL of DITE through combination of net toroidal rotation of the edge plasma in the direction of the main plasma current and - enhanced particle diffusion at the outside midplane (“Ballooning”, same as Hugill’s) P-S flow A.V.Chankin, Zinnowitz Theory Meeting, 17.11.2003 – 20.11.2003, Zinnowitz, Germany

Rotation + Ballooning in the SOL T-10 (Vershkov et al.,1989) Effects of Rotation & Ballooning also observed in many other experiments Qualitatively, the same conclusion as in Pitts’ work, has been obtained on T-10 (Pfirsch - Schlüter flows don’t contribute at the top and bottom of the magnetic configuration, but large parallel/toroidal ion flows were routinely measured there) P-S flow Toroidal rotation present in the output from EDGE2D and B2.5 codes A.V.Chankin, Zinnowitz Theory Meeting, 17.11.2003 – 20.11.2003, Zinnowitz, Germany

Toroidal Rotation in the SOL Driven by Poloidal EB Drift First related to baffling of poloidal EB flow by target plates in (Tendler & Rozhansky, 1990) R EB flow High recycling EB flow R “baffle” “baffle” High recycling Poloidal EB Drift + Parallel Flow = Toroidal Flow (along Ip, as in exper.) A.V.Chankin, Zinnowitz Theory Meeting, 17.11.2003 – 20.11.2003, Zinnowitz, Germany

Toroidal Rotation in the SOL Driven by Poloidal EB drift (cont.) The same conclusion for EB flow in the divertor: compensating parallel flows should result in toroidal rotation A.V.Chankin, Zinnowitz Theory Meeting, 17.11.2003 – 20.11.2003, Zinnowitz, Germany

Parallel Flux Pattern in the SOL (so far …) Parallel Ion Pfirsh-Schlüter Flow (reverses with Bt reversal) R “uniform” Flow due to “Ballooning” (Does not ! reverse with Can be modelled by 2D codes without additional assumptions Requires assumptions about perp. transport A.V.Chankin, Zinnowitz Theory Meeting, 17.11.2003 – 20.11.2003, Zinnowitz, Germany

Toroidal Rotation vs. Pfirsch-Schlüter Flow (only exists in toroidal geometry, ~ r/R) (present in cylindrical geometry)  (rough theoretical estimate) Toroidal Rotation R Pfirsch- Schlüter However, owing to the factor 2 in and the sum of dpi/dr and Er terms, and can be very close near the midplane The two mechanisms can be easily confused when measuring ion velocity only near outer midplane (accuracy in experimental evaluation of Er is poor) A.V.Chankin, Zinnowitz Theory Meeting, 17.11.2003 – 20.11.2003, Zinnowitz, Germany

Toroidal Rotation vs. Pfirsch-Schlüter Flow (cont.) JT-60U: Reversal of ion parallel flow near outer midplane with Bt reversal (Asakura et al., 2000) Ion B “Natural A.V.Chankin, Zinnowitz Theory Meeting, 17.11.2003 – 20.11.2003, Zinnowitz, Germany

Toroidal Rotation vs. Pfirsch-Schlüter Flow (cont.) A.V.Chankin, Zinnowitz Theory Meeting, 17.11.2003 – 20.11.2003, Zinnowitz, Germany

Toroidal Rotation vs. Pfirsch-Schlüter Flow (cont.) Alcator C-Mod with “Toroidal rotation as an explanation for plasma flow observations in the Alcator C-Mod scrape-off layer” (LaBombard et al., 2003) A.V.Chankin, Zinnowitz Theory Meeting, 17.11.2003 – 20.11.2003, Zinnowitz, Germany

Toroidal Rotation vs. Pfirsch-Schlüter Flow (cont.) Competition between the two flows is expected on the inboard side, where they have different directions In JT-60U, an Inner Mach Probe was recently installed to establish the flow pattern in the SOL (Asakura et al., 2002) A.V.Chankin, Zinnowitz Theory Meeting, 17.11.2003 – 20.11.2003, Zinnowitz, Germany

Toroidal Rotation vs. Pfirsch-Schlüter Flow (cont.) Expected behaviour of the old probes, but: In normal field plasmas, the Inner Mach Probe showed the ion flow towards divertor, opposite to the Pfirsch-Schlüter Flow across most of the SOL (could also be affected, apart from the Er/B toroidal rotation, by the sink to the target due to its proximity to the divertor ?) Near the separatrix, however, the flow reversed, and was in the direction of the Pfirsch-Schlüter Flow. This could be due to: a) its “competitor” has zeroed (Er=0 at the separatrix), or b) “ionization driven flow reversal” (2D effect) A.V.Chankin, Zinnowitz Theory Meeting, 17.11.2003 – 20.11.2003, Zinnowitz, Germany

Experiments on JET Erents et al., 2000 Normal Bt Rev. Bt P-S flow Normal Bt Rotation Flow towards inner divertor Ballo-oning Rev. Bt Flow towards outer divertor Large parallel velocity toward inner divertor in normal Bt plasmas Reversal of the flow direction in reversed Bt plasma, but the flow is much smaller Deposition of impurities (with formation of loosely bound surface layers) on the inner target and large build up of carbon (and trapped tritium) at the inner divertor louvers (Coad et al., 2001) A.V.Chankin, Zinnowitz Theory Meeting, 17.11.2003 – 20.11.2003, Zinnowitz, Germany

Is There a Link Between the Flow and Impurity Deposition ? Operation mostly in Normal field configuration (with large parallel flow) Outer target – clean Can large parallel ion flow scoop impurities towards the inner target by parallel friction force ? Pe=Pi=2.5MW Prad=0.77MW ns=1.11019m-3 Midplane coeff. (constant in flux space): D=0.1 m2s-1 e=i=1 m2s Vpinch=3 ms-1 Input parameters and computational grid for the modelled EDGE2D case  Match with experimental target ne and Te profiles achieved All drift flows included Modelling with EDGE2D code A.V.Chankin, Zinnowitz Theory Meeting, 17.11.2003 – 20.11.2003, Zinnowitz, Germany

Modelling with EDGE2D (drifts included) Experiment Code Modelling Normal Bt Rev. Bt Reversal of the flow direction with Bt reversal can be modelled (attributed to both toroidal rotation and Pfirsch-Schlüter flows) More symmetric flow reversal than in experiment: Ballooning transport was not incorporated into the code Modelled Mach numbers are much smaller than in experiment. This includes the difference in M’s between normal and reversed field directions A.V.Chankin, Zinnowitz Theory Meeting, 17.11.2003 – 20.11.2003, Zinnowitz, Germany

Modelling with EDGE2D (cont.) EDGE2D so far can not reproduce large Mach numbers of parallel ion flow velocity measured in JET and other experiments (the same is true for JT-60U and UEDGE code results (Asakura et al., 2002)): Neither the “ballooning” introduced into the code later, No drift effects result in sufficiently high flow velocities More code work is required A.V.Chankin, Zinnowitz Theory Meeting, 17.11.2003 – 20.11.2003, Zinnowitz, Germany

||  More theoretical work is required Even if all radial particle flux goes throught outer side, Mach numbers will be rather low due to low sink to divertor (“baffle” on the figure) How can high M|| due to “ballooning” transport be generated ? A.V.Chankin, Zinnowitz Theory Meeting, 17.11.2003 – 20.11.2003, Zinnowitz, Germany

||  More theoretical work is required (cont.) M|| due to “ballooning” can be much higher if the flux on the inside is negative, directed inside the plasma A.V.Chankin, Zinnowitz Theory Meeting, 17.11.2003 – 20.11.2003, Zinnowitz, Germany

More theoretical work is required (cont.) Scenario that could generate negative radial particle flux on the inboard side: blobs originating on the outboard side but propagating along field lines to the inboard side EB drift EB drift A.V.Chankin, Zinnowitz Theory Meeting, 17.11.2003 – 20.11.2003, Zinnowitz, Germany

Does this agree with experiment Impact of the Flows on the Impurity Migration It is clear that: Neither the Rotation (which is toroidal) No the Pfirsch-Schlüter Flow, which connects top and bottom of the flux surface (along the field lines) Can be responsible for impurity migration from outer to inner divertor (if there is one) R Parallel Ion Flow due to “Ballooning” (Does not ! reverse with Bt reversal) Only the flow caused by “ballooning” has the potential of dragging impurities from the outside to inside This flow has the same direction for normal and reversed Bt. Hence, IF only the above 3 mechanisms for plasma flows are considered AND IF we relate target impurity deposition with SOL plasma flows THEN we must expect a similar in-out deposition asymmetry in reversed as in normal field configuration Does this agree with experiment A.V.Chankin, Zinnowitz Theory Meeting, 17.11.2003 – 20.11.2003, Zinnowitz, Germany

New JET Data (field reversal experiments) (Pitts et al., 2003) Norm-Rev. field averaged flow: from outside to inside as before ( ballooning ?) The difference (M_nor.Bt - M_rev.Bt) is the same as in earlier experiments Shape of the M-number profile has been established; Field reversal (drift?) effects small near the separatrix. Can be understood by assuming : Er  0 at sep. (reducing rotation with Er/B), Pfirsch-Schlüter Flow  0 at sep. (no P-S flow in the core + effect of viscosity) A.V.Chankin, Zinnowitz Theory Meeting, 17.11.2003 – 20.11.2003, Zinnowitz, Germany

New JET Data (cont.) In reversed Bt plasmas, deposition of impurities (creating surface layers) is found on the outer target ! While the inner target is clean How important are SOL flows for impurity behaviour in the divertor? Can it be that in-out asymmetries in the divertor itself play a crucial role? Survace layers tend to be formed in low Te plasmas Divertor target can be cleaned of impurities in high Te plasmas (by sputtering) A.V.Chankin, Zinnowitz Theory Meeting, 17.11.2003 – 20.11.2003, Zinnowitz, Germany

Divertor/Target Asymmetries Expected even without drifts Due to a number of factors: (geometrical: larger outer surface, Shafranov shift, larger turbulence at low field side (ballooning) more power goes to the SOL through outer surface, leading to Te,inner < Te,outer Pressure conservation along the field lines: pe,inner = pe,outer  ne,inner > ne,outer Particle flux to the target target~ne(Te)1/2~pe(Te)-1/2 inner > outer Inner target must be denser and cooler  more prone to impurity deposition A.V.Chankin, Zinnowitz Theory Meeting, 17.11.2003 – 20.11.2003, Zinnowitz, Germany

ne Influence of Bt Direction on Divertor Asymmetries – Major Trends: Agreement between Experiment and Modelling with Drifts Included ne distribution (from EDGE2D code) normal Bt reversed Bt ne ne,inner > ne,outer Te,i,inner < Te,i,outer More symmetric distribution of ne and Te Drift effects (mainly EB dirft: (Stangeby et al, 1996) ) result in: - further cooling of the inner divertor in normal Bt plasmas - cooling of the outer divertor in reversed Bt plasmas more symmetric distribution A.V.Chankin, Zinnowitz Theory Meeting, 17.11.2003 – 20.11.2003, Zinnowitz, Germany

Expected Formation of Surface Layers ne normal Bt: on the inner target ne reversed Bt: not clear… Reversal of asymmetries reported (JT-60U (Itami et al., 1992) ), but in JET, generally, they do not reverse A.V.Chankin, Zinnowitz Theory Meeting, 17.11.2003 – 20.11.2003, Zinnowitz, Germany

Helium Enrichment Studies in JET (Lehnen, Pitts et al., 2003) Result of puffing He Normal Bt: more He reaches INNER divertor Reversed Bt: more He reaches OUTER Normal Bt data Not surprising: Denser plasma at INNER target, with higher recycling Reversed Bt data: why more He at OUTER target ? He subdivertor pressure measured A.V.Chankin, Zinnowitz Theory Meeting, 17.11.2003 – 20.11.2003, Zinnowitz, Germany

Helium Enrichment Studies in JET (cont.) Is there higher particle flux/ neutral recycling at OUTER target in reversed Bt ? (Pitts et al., 2003) Fairly symmetric recycling, if not larger at the INNER target So what is pushing He towards the OUTER target ? A.V.Chankin, Zinnowitz Theory Meeting, 17.11.2003 – 20.11.2003, Zinnowitz, Germany

Summary SOL flow pattern is well established and the driving mechanisms are reasonably well understood (at least, qualitatively) More efforts are required to correctly model SOL flows with 2D codes (both on the computational side and understanding underlying physics, including turbulence) No direct or indirect evidence exists that SOL flows are primarely responsible for asymmetries in impurity target deposition / surface layer formation, between INNER and OUTER targets Results of reversed field experiments in JET with surface layer formation and preferential He accummulation at the OUTER divertor target are not understood. Better understanding of divertor asymmetries and related effects may provide a clue A.V.Chankin, Zinnowitz Theory Meeting, 17.11.2003 – 20.11.2003, Zinnowitz, Germany

On the Results of Reversed Field Experiments There is no obvious explanation why either impurity accumulation or surface layers formation should occur at the OUTER target in reversed field configurations, when the INNER target remains clean Asymmetries between inner and outer divertor plasma parameters in these particular experiments should be studied in more detail to see if the OUTER target was denser and/or cooler than the inner one A.V.Chankin, Zinnowitz Theory Meeting, 17.11.2003 – 20.11.2003, Zinnowitz, Germany

SOL Flows Caused by Divertor Asymmetries Higher recycling in the INNER divertor in normal Bt plasmas  neutrals flow from inner to outer divertor (Kukushkin et al., 2001) Mass conservation results in the SOL plasma flow towards INNER and away from OUTER divertor  extra drag force on impurities towards inner divertor. The SOL flow can be increased by pumping (Asakura et al., 2002: in JT-60 detached plasmas with pumping, M|| at the entrance to inner divertor reached 0.7) Can this flow reverse in reversed Bt plasmas ? / Can the asymmetry of neutrals recycling reverse ? Neutral flux is not too large (only ~ 0.06 of recycled neutrals) It can be blocked by a septum, and conflicting evidence exists on the role of septum in JET: a) Maggi et al., 1999: septum can influence divertor asymmetry - EDGE2D modelling b) Rapp et al., 2003: septum doesn’t affect plasma detachment at targets - experiment A.V.Chankin, Zinnowitz Theory Meeting, 17.11.2003 – 20.11.2003, Zinnowitz, Germany

Toroidal Rotation in the SOL Driven by Poloidal EB drift (cont.) The same conclusion for EB flow in the divertor: compensating parallel flows should result in toroidal rotation Diamagnetic flows don’t contribute to net toroidal rotation of the SOL Larmor gyration Vertical B and Centrifugal drifts A.V.Chankin, Zinnowitz Theory Meeting, 17.11.2003 – 20.11.2003, Zinnowitz, Germany