Neoclassical and transport driven parallel SOL flows on TCV

1. Neoclassical and transport driven parallel SOL flows on TCV R. A. Pitts, J. Horacek1 and TCV Team École Polytechnique Fédérale de Lausanne (EPFL), Centre de Recherches en Physique des Plasmas, Association Euratom – Confédération Suisse, 1015 Lausanne, Switzerland 1Association EURATOM–Institute of Plasma Physics, Prague, Czech Republic Paper O4.007, R. A. Pitts et al., 34th EPS Conference: 5 July 2007

2. Outline • Brief introduction • Contributions to parallel flow • Experiment • Isolating the flow components • Comparison with theory • Pfirsch-Schlüter, ballooning+turbulence • Conclusions Paper O4.007, R. A. Pitts et al., 34th EPS Conference: 5 July 2007

3. ErxB, pxB EqxB Ballooning Pfirsch-Schlüter Bj Divertor sink BxB BxB REV-Bj Components of SOL ion flows • Determine transport of impurities from source to destination in a tokamak – material migration – T-retention Poloidal Bj Parallel FWD-Bj Paper O4.007, R. A. Pitts et al., 34th EPS Conference: 5 July 2007

4. Magnetic configurations • Ohmic L-mode diverted plasmas • Ip = 260 kABj = 1.43 T • FWD and REV-Bj (Ip, Bj always reversed together) • Density scans from 2.5x1019 m-3 to density limit Mach Probe #26092 #33345 #33517 • AIM: use toroidal field reversal, density scans and plasma geometry to isolate neoclassical and perpendicular transport driven contributions to parallel SOL flow Paper O4.007, R. A. Pitts et al., 34th EPS Conference: 5 July 2007

5. Mach Probe Measurements below the midplane • Strong field direction and density dependence • Flows always co-current • Directions consistent with Pfirsch-Schlüter flow wall R. A. Pitts et al., J. Nucl. Mater 363-365 (2007) 738 • Clear, field independent negative offset, M|| ~ 0.05-0.1 Paper O4.007, R. A. Pitts et al., 34th EPS Conference: 5 July 2007

6. Origin of flow components • Main, field dependent component identified as Pfirsch-Schlüter (see later) • Pushes flow up (FWD-B) or down (REV-B) in the SOL  no direct contribution to impurity migration • Field independent flow offset due to “ballooning” transport on LFS? • Can contribute to impurity migration  important • BUT, on TCV open divertor geometry means that cannot rule out in this single experiment a flow offset generated by outer divertor target sink  change configuration to eliminate this possibility Paper O4.007, R. A. Pitts et al., 34th EPS Conference: 5 July 2007

7. Measurements above the midplane • Use SNU to put probe ABOVE midplane and reduce strength of outer target sink • Same directions of FWD and REV-B flows as for SNL • Similar absolute magnitudes of M|| • Clear negative offset at given density wall Paper O4.007, R. A. Pitts et al., 34th EPS Conference: 5 July 2007

8. Flow offset preserved in SNU • Mean M|| up to 0.2 • Unlike SNL case, strong peak in offset flow at r – rsep ~ 4 mm • Some evidence now for an increase in offset with increase in density • Can now rule out strong outer divertor sink as origin of offset wall Paper O4.007, R. A. Pitts et al., 34th EPS Conference: 5 July 2007

9. “Neutral” point of ballooning comp. • Picture is therefore of parallel flow generation due to enhanced outboard radial transport causing local “overpressure” which dissipates along the field • If this enhancement peaks at the outboard midplane, a flow measurement there should not detect the offset  change configuration again to investigate this possibility Mach Probe Paper O4.007, R. A. Pitts et al., 34th EPS Conference: 5 July 2007

10. Measurements on the midplane • Similar field and density dependence as for measurement in SNL below midplane AND • Similar absolute magnitudes of M|| BUT • FWD and REV-B flows more symmetric around M||= 0 wall Paper O4.007, R. A. Pitts et al., 34th EPS Conference: 5 July 2007

11. No flow offset on the midplane • In “connected” SOL, mean M|| ~ 0 for all densities • Confirms that outboard midplane is null point for offset flow  transport (turbulence) drive peaks in this region wall Paper O4.007, R. A. Pitts et al., 34th EPS Conference: 5 July 2007

12. Comparing with theory: field dependence • Simple expression for return parallel Pfirsch-Schlüter ion flows compensating non-divergence free parts of p and ErB poloidal drifts: • Take region 6 < (r - rsep) < 12 mm • p, Er estimated from probe profiles of Te and Vf • Good agreement  field dependent flow component well described by neoclassical drift physics Paper O4.007, R. A. Pitts et al., 34th EPS Conference: 5 July 2007

13. Comparing with theory: flow offset • Simple ansatz to estimate time averaged M|| due to flow generated by blobs: M|| ~ 0.5fp > apwithfp > ap fraction of time over which significant parallel pressure gradient exists (a = enhancement over time averaged pressure, <p>) • 2D electrostatic fluid turbulence simulations of TCV midplane SOL plasma (ESEL code, Risø) – successfully benchmarked against turbulence measurements O. E. Garcia et al., PPCF 48 (2006) L1, J. Nucl. Mater., 363-365 (2007) 575, IAEA 2006 W. Fundamenski et al., Nucl. Fus. 47 (2007) 417R. A. Pitts et al., J. Nucl. Mater 363-365 (2007) 738 Paper O4.007, R. A. Pitts et al., 34th EPS Conference: 5 July 2007

14. Conclusions • Reasonably complete understanding of parallel SOL flows in the TCV outboard midplane vicinity • Field direction dependent, co-current flows, dominant at low to medium plasma density and consistent with neoclassical Pfirsch-Schlüter return flows • Field direction independent “offset” flow, relatively independent of density, comparable with neoclassical flows at high density and consistent with “overpressure” due to enhanced radial “blobby” (or filamentary) transport on LFS Paper O4.007, R. A. Pitts et al., 34th EPS Conference: 5 July 2007

15. Reserve slides Paper O4.007, R. A. Pitts et al., 34th EPS Conference: 5 July 2007

16. 10 mm How the flows are measured • Fast reciprocating probe with Mach probe head mounted on the machine midplane • Two separate heads used to account for varying poloidal plasma contour • Mach No. defined in the usual way:M||=v||/cs 0.4ln(Isat,1/Isat,2) • Non-Mach pins used to measure profiles of ne, Te, Vp 1 1 2 2 • +ve flow defined UPWARDSAll data mapped to outer midplane Paper O4.007, R. A. Pitts et al., 34th EPS Conference: 5 July 2007

17. Edge profiles, SNU FWD & REV-B Paper O4.007, R. A. Pitts et al., 34th EPS Conference: 5 July 2007

18. Edge profiles, SNL, z = 0, FWD & REV-B Paper O4.007, R. A. Pitts et al., 34th EPS Conference: 5 July 2007

19. Comparing with theory: field dependence • Simple expression for return parallel Pfirsch-Schlüter ion flows compensating non-divergence free part of p and ErB poloidal drifts: • Take region 6 < (r - rsep) < 12 mm • p, Er estimated from probe profiles of Te and Vf • Good agreement  field dependent flow component well described by neoclassical drift physics Paper O4.007, R. A. Pitts et al., 34th EPS Conference: 5 July 2007

20. Measurements below the midplane • Strong field direction and density dependence • Flows always co-current • Directions consistent with Pfirsch-Schlüter flow wall R. A. Pitts et al., J. Nucl. Mater 363-365 (2007) 738 • Clear, field independent negative offset, M|| ~ 0.05-0.1 Paper O4.007, R. A. Pitts et al., 34th EPS Conference: 5 July 2007