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. 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 20072 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 20073 of 14

B

BxB

ErxB, pxB

Ballooning

Pfirsch-SchlüterDivertor sink

ExB

Determine transport of impurities from source to destination in a tokamak – material migration – T-retention

FWD-B

Components of SOL ion flows

B

BxBREV-B

Poloidal

Parallel

Paper O4.007, R. A. Pitts et al., 34th EPS Conference: 5 July 20074 of 14

Magnetic configurations

#26092 #33345 #33517

Mach

Ohmic L-mode diverted plasmas

Ip = 260 kAB = 1.43 T

FWD and REV-B(Ip, B always reversed together)

Density scans from 2.5x1019 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 20075 of 14

wall

R. A. Pitts et al., J. Nucl. Mater 363-365 (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|| ~ 0.05-0.1

Paper O4.007, R. A. Pitts et al., 34th EPS Conference: 5 July 20076 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 20077 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 20078 of 14

wall

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

Paper O4.007, R. A. Pitts et al., 34th EPS Conference: 5 July 20079 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

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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

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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

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Comparing with theory: field dependence

2

cos2

B

B

en

pE

c

qM

er

s

PS||

Simple expression for return parallel Pfirsch-Schlüter ion flows compensating non-divergence free parts of p and ErB 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 200713 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., 363-365 (2007) 575, IAEA 2006

Simple ansatz to estimate time averaged M|| due to flow generated by blobs: M|| ~ 0.5fp > p with fp > p fraction of time over which significant parallel pressure gradient exists ( = enhancement over time averaged pressure, <p>)

W. Fundamenski et al., Nucl. Fus. 47 (2007) 417R. A. Pitts et al., J. Nucl. Mater 363-365 (2007) 738

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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

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Reserve slides

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12

10 mm

12

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 ne, Te, Vp

Mach No. defined in the usual way:M||=v||/cs 0.4ln(Isat,1/Isat,2)

+ve flow defined UPWARDSAll data mapped to outer midplane

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Edge profiles, SNU FWD & REV-B

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Edge profiles, SNL, z = 0, FWD & REV-B

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Comparing with theory: field dependence

2cos2

B

B

en

pEq

er

PS||

v

Simple expression for return parallel Pfirsch-Schlüter ion flows compensating non-divergence free part of p and ErB 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 200720 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 363-365 (2007) 738

Measurements below the midplane

• Clear, field independent negative offset, M|| ~ 0.05-0.1