Paper O4.007, R. A. Pitts et al., 34th EPS Conference: 5 July 2007 Neoclassical and transport driven...
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Transcript of Paper O4.007, R. A. Pitts et al., 34th EPS Conference: 5 July 2007 Neoclassical and transport driven...
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
Paper O4.007, R. A. Pitts et al., 34th EPS Conference: 5 July 200710 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 200711 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 200712 of 14
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
Paper O4.007, R. A. Pitts et al., 34th EPS Conference: 5 July 200714 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 200716 of 14
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
Paper O4.007, R. A. Pitts et al., 34th EPS Conference: 5 July 200717 of 14
Edge profiles, SNU FWD & REV-B
Paper O4.007, R. A. Pitts et al., 34th EPS Conference: 5 July 200718 of 14
Edge profiles, SNL, z = 0, FWD & REV-B
Paper O4.007, R. A. Pitts et al., 34th EPS Conference: 5 July 200719 of 14
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