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

Paper O4.007, R. A. Pitts et al., 34th EPS Conference: 5 July of 14 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 of 14 BB Bx  B E r xB,  pxB Ballooning Pfirsch- Schlüter Divertor sink ExBExB  Determine transport of impurities from source to destination in a tokamak – material migration – T-retention FWD-B  Components of SOL ion flows BB Bx  B REV-B  Poloidal Parallel

Paper O4.007, R. A. Pitts et al., 34th EPS Conference: 5 July of 14 Magnetic configurations #26092#33345#33517 Mach  Ohmic L-mode diverted plasmas  I p = 260 kA B  = 1.43 T  FWD and REV-B  (I p, B  always reversed together)  Density scans from 2.5x10 19 m -3 to density limit  AIM: use toroidal field reversal, density scans and plasma geometry to isolate neoclassical and perpendicular transport driven contributions to parallel SOL flow Probe

Paper O4.007, R. A. Pitts et al., 34th EPS Conference: 5 July of 14 wall R. A. Pitts et al., J. Nucl. Mater (2007) 738 Measurements below the midplane Mach Probe  Strong field direction and density dependence Flows always co-current Directions consistent with Pfirsch-Schlüter flow Clear, field independent negative offset, M || ~

Paper O4.007, R. A. Pitts et al., 34th EPS Conference: 5 July of 14 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 of 14 wall 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

Paper O4.007, R. A. Pitts et al., 34th EPS Conference: 5 July of 14 wall Flow offset preserved in SNU  Mean M || up to 0.2  Unlike SNL case, strong peak in offset flow at r – r sep ~ 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

Paper O4.007, R. A. Pitts et al., 34th EPS Conference: 5 July of 14 “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 of 14 wall  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 Measurements on the midplane

Paper O4.007, R. A. Pitts et al., 34th EPS Conference: 5 July of 14 wall  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 No flow offset on the midplane

Paper O4.007, R. A. Pitts et al., 34th EPS Conference: 5 July of 14 Comparing with theory: field dependence  Simple expression for return parallel Pfirsch- Schlüter ion flows compensating non- divergence free parts of  p and E r  B poloidal drifts:  Take region 6 < (r - r sep ) < 12 mm   p, E r estimated from probe profiles of T e and V f  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 of 14 Comparing with theory: flow offset  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., (2007) 575, IAEA 2006  Simple ansatz to estimate time averaged M || due to flow generated by blobs: M || ~ 0.5f p >  p  with f p >  p  fraction of time over which significant parallel pressure gradient exists (  = enhancement over time averaged pressure, ) W. Fundamenski et al., Nucl. Fus. 47 (2007) 417 R. A. Pitts et al., J. Nucl. Mater (2007) 738

Paper O4.007, R. A. Pitts et al., 34th EPS Conference: 5 July of 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 of 14 Reserve slides

Paper O4.007, R. A. Pitts et al., 34th EPS Conference: 5 July of mm 1 2  Fast reciprocating probe with Mach probe head mounted on the machine midplane How the flows are measured  Two separate heads used to account for varying poloidal plasma contour  Non-Mach pins used to measure profiles of n e, T e, V p  Mach No. defined in the usual way: M || =v || /c s  0.4ln(I sat,1 /I sat,2 )  +ve flow defined UPWARDS All data mapped to outer midplane

Paper O4.007, R. A. Pitts et al., 34th EPS Conference: 5 July of 14 Edge profiles, SNU FWD & REV-B

Paper O4.007, R. A. Pitts et al., 34th EPS Conference: 5 July of 14 Edge profiles, SNL, z = 0, FWD & REV-B

Paper O4.007, R. A. Pitts et al., 34th EPS Conference: 5 July of 14 Comparing with theory: field dependence  Simple expression for return parallel Pfirsch- Schlüter ion flows compensating non- divergence free part of  p and E r  B poloidal drifts:  Take region 6 < (r - r sep ) < 12 mm   p, E r estimated from probe profiles of T e and V f  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 of 14 wall  Strong field direction and density dependence Flows always co-current Directions consistent with Pfirsch-Schlüter flow R. A. Pitts et al., J. Nucl. Mater (2007) 738 Measurements below the midplane Clear, field independent negative offset, M || ~