Total Flow Vector in the SOL N. Smick, B. LaBombard MIT Plasma Science and Fusion Center APS-DPP Annual Meeting Atlanta, GA November 3, 2009
Motivation and Goals Measurements have revealed high parallel flow velocities in the High Field Side () SOL (approaching Mach 1) 1 that appear to be driven by a strong ballooning-like transport asymmetry. Goals of current investigation: Determine whether the measured parallel flows drive a net poloidal particle flux or are just part of toroidal rotation. Separate out transport and driftdriven components of poloidal flows. Identify mechanism for returning poloidal particle flux to the core. LSN Near Sonic Parallel Flows Observed Poloidal projection of parallel flows driven by ballooning transport asymmetry in upper and lower single null USN Detailed measurements of the total flow vector are needed to address these goals 1. LaBombard, Brian, Nucl. Fusion 44 (2004)1047-1066
Plasma Flow Diagnostics in the SOL Midplane Scanning Probe is equipped with two pneumatic scanning probes on the low-field side (): a midplane probe and a vertical probe near the lower divertor. A key diagnostic is a new scanning probe located near the midplane. Midplane Scanning Probe All scanning probes can measure the total flow vector: v, v and v ~~, ExB r, neθ (from fluctuation-induced particle flux). Vertical Scanning Probe
Poloidal Flow Components The poloidal flow pattern is composed of: A transport-driven part that Depends on magnetic topology Has finite divergence v transp
Poloidal Flow Components The poloidal flow pattern is composed of: A transport-driven part that Depends on magnetic topology Has finite divergence A drift-driven part that Depends on field direction Bx B Is divergence-free v drift
Poloidal Flow Components The poloidal flow pattern is composed of: A transport-driven part that Depends on magnetic topology Has finite divergence A drift-driven part that Depends on field direction Is divergence-free We measure: = + Bx B Bx B A favorable drift direction flow pattern (Bx B towards x-point) : v fav = v transp + v drift
Poloidal Flow Components The poloidal flow pattern is composed of: A transport-driven part that Depends on magnetic topology Has finite divergence A drift-driven part that = + Bx B Bx B Depends on field direction Is divergence-free = - Bx B Γ Bx B r We measure: A favorable drift direction flow pattern (Bx B towards x-point): v fav = v transp + v drift An unfavorable drift direction flow pattern (Bx B away from x-point): v unfav = v transp v drift
Flow Components on and midplanes Transport-Driven Component Bx B Drift-Driven Component Poloidal Flow [km/s] Towards Inner Divertor Poloidal Flow [km/s] Electron DiamagneticDirection Transport-Driven Part Drift-Driven Part Flow components have been extracted from probe data at the and midplanes. Parallel, transport-driven flow is dominant on. Drift-driven flows are dominant on.
Drift-Driven Flow Component: / comparison indicates divergence-free flow Total drift-driven flow is consistent with divergence-free flow pattern as expected from theory: similar profiles of nv θ /B θ to. Bx B Drift-Driven Component nv θ /B θ (Elec Dia) [10 23 Tm -2 s -1 ]
Drift-Driven Flow Component: / comparison indicates divergence-free flow Total drift-driven flow is consistent with divergence-free flow pattern as expected from theory: similar profiles of nv θ /B θ to. Bx B This calculation uses total fluid motion, not guiding center only; includes Parallel, ExB and Diamagnetic flow. Drift-Driven Component Poloidal Flow [km/s] Electron DiamagneticDirection Drift-Driven nv θ /B θ (Elec Dia) [10 23 Tm -2 s -1 ]
Drift-Driven Flow Component: Toroidal rotation and Pfirsch-Schlüter contributions V, ExB (Inferred from Φ) V, Diamagnetic (Inferred from p) Net poloidal flow, vθ Vφ B θ V rot, V Pfirsch-Schlüter Measured v Net Fluid Flow φ Bx B Drift-Driven Component We can now unambiguously decompose the drift-driven parallel flow into Pfirsch- Schlüter and toroidal rotation components.
Drift-Driven Flow Component: Toroidal rotation and Pfirsch-Schlüter contributions V, ExB (Inferred from Φ) V, Diamagnetic (Inferred from p) Net poloidal flow, vθ Vφ V rot, V Pfirsch-Schlüter Measured v Net Fluid Flow B θ φ Bx B Drift-Driven Component Drift-Driven We can now unambiguously decompose the drift-driven parallel flow into Pfirsch- Schlüter and toroidal rotation components. Data confirm expectation that these components should cancel on closed field lines. Parallel Flow [km/s] Co-Current Direction
Transport-Driven Flow Component: Should Agree with Measured Transport S nv θ /B θ Toward Inner Divertor S = 1 S = 0 Transport-Driven Component Model transport-driven poloidal particle flux as sin(θ) on S: Normalized poloidal distance from outer to inner divertor
Transport-Driven Flow Component: Should Agree with Measured Transport S nv θ /B θ Toward Inner Divertor S = 1 S = 0 Transport-Driven Component Model transport-driven poloidal particle flux as sin(θ) on Constrain with measured value of poloidal flux on S: Normalized poloidal distance from outer to inner divertor nv θ [10 22 m -2 s -1 ] nv θ
Transport-Driven Flow Component: Should Agree with Measured Transport S nv r nv θ /B θ Toward Inner Divertor S = 1 S = 0 Transport-Driven Component Model transport-driven poloidal particle flux as sin(θ) on Constrain with measured value of poloidal flux on Calculate radial particle flux implied by poloidal flux nv r [10 20 m -2 s -1 ] S: Normalized poloidal distance from outer to inner divertor nv r Limiter nv θ [10 22 m -2 s -1 ] nv θ
Transport-Driven Flow Component: Consistent with fluctuation-induced radial flux data S nv r nv θ /B θ Toward Inner Divertor S = 1 S = 0 Transport-Driven Component Model transport-driven poloidal particle flux as sin(θ) on Constrain with measured value of poloidal flux on Calculate radial particle flux implied by poloidal flux Result shows agreement with measurements of fluctuation-induced particle flux on the through n ~ E ~ θ nv r [10 20 m -2 s -1 ] S: Normalized poloidal distance from outer to inner divertor nv r Fluctuationinduced nv r Limiter nv θ [10 22 m -2 s -1 ] nv θ
Sink for Transport-Driven Flow There must be an efficient particle sink on the to sustain pressure gradients driving near-sonic transportdriven parallel flows. Could particles be returning to the core via a convective pinch? This is a solution commonly employed by 2-D edge codes to explain flows. 2,3,4 -? 2 A.Y. Pigarov et al. Contrib. Plasma Phys. 48 (2008) 82-88 3 J.D. Elder et al. J. Nucl. Mater. 390-391 (2009) 376-379 4 G.S. Kirnev et al. J. Nucl. Mater. 337 (2005) 271-275 Ballooning Particle Pinch?
Sink for Transport-Driven Flow There must be an efficient particle sink on the to sustain pressure gradients driving near-sonic transportdriven parallel flows. Could particles be returning to the core via a convective pinch? This is a solution commonly employed by 2-D edge codes to explain flows. 2,3,4 Could a volume recombination zone be present in the far SOL of the inner divertor leg, returning particles to the core as neutrals? Γ n 2 A.Y. Pigarov et al. Contrib. Plasma Phys. 48 (2008) 82-88 3 J.D. Elder et al. J. Nucl. Mater. 390-391 (2009) 376-379 4 G.S. Kirnev et al. J. Nucl. Mater. 337 (2005) 271-275 Volume Recombination Zone?
Measurement of Pinch Velocity DIV /n e [m/s] radial particle flux is robustly zero. -? DIV Hint of pinch near from 2007 data was shown by improved statistics to be anomalous. Ballooning Particle Pinch?
Measurement of Pinch Velocity DIV /n e [m/s] X - radial particle flux is robustly zero. DIV Hint of pinch near from 2007 data was shown by improved statistics to be anomalous. Ballooning Particle Pinch?
Evidence that Volume Recombination Plays a Key Role in Closing Flow Loop USN, No MARFE Probe Location 60 n e T e [10 20 evm -3 ] Electron Pressure 0 0 [mm] into SOL 10 1.0-1.0 Parallel Mach # Strong flow to partially-detached inner divertor 0 To Divertor 0 [mm] into SOL 10
Evidence that Volume Recombination Plays a Key Role in Closing Flow Loop USN, No MARFE USN, MARFE in Lower Chamber Probe Location MARFE 60 n e T e [10 20 evm -3 ] Electron Pressure 0 0 [mm] into SOL 10 1.0-1.0 Parallel Mach # Strong flow to partially-detached inner divertor. 0 0 [mm] into SOL 10 Electron Pressure To Divertor 60 n e T e [10 20 evm -3 ] 0 0 [mm] into SOL 10 1.0-1.0 Parallel Mach # ~ no plasma makes it around MARFE to populate. Residual flow is towards MARFE (recombination zone). 0 To MARFE 0 [mm] into SOL 10
Evidence that Volume Recombination Plays a Key Role in Closing Flow Loop USN, No MARFE USN, MARFE in Lower Chamber Probe Location MARFE 60 n e T e [10 20 evm -3 ] Electron Pressure 0 0 [mm] into SOL 10 1.0 0-1.0 Parallel Mach # 0 [mm] into SOL 10 60 Electron Pressure To Divertor n e T e [10 20 evm -3 ] 0 0 [mm] into SOL 10 MARFE closes the flow loop. Calls for careful modeling of inner divertor recycling / recombination effects. 1.0 0-1.0 Parallel Mach # To MARFE 0 [mm] into SOL 10
Conclusions ExB flow data show that transport-driven parallel flows are responsible for net poloidal motion of plasma toward active x-point. Measured drift-driven flows are divergence-free. Transport-driven poloidal particle flux is consistent with measured fluctuation-induced radial particle flux. particle pinch is zero; this is not the mechanism that closes the mass flow loop on the. Recycling / recombination physics in inner divertor leg is a strong candidate to close mass flow loop.