Transcript Overview of TCV Results

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
Outline
 Brief introduction
• Contributions to parallel flow
 Experiment
• Isolating the flow components
 Comparison with theory
• Pfirsch-Schlüter, ballooning+turbulence
 Conclusions
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Paper O4.007, R. A. Pitts et al., 34th EPS Conference: 5 July 2007
Components of SOL ion flows
 Determine transport of impurities from source to destination in a
tokamak – material migration – T-retention
ErxB, pxB
EqxB
Poloidal
PfirschSchlüter
Divertor
sink
Bj
Bj
Ballooning
Parallel
FWD-Bj BxB
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REV-Bj BxB
Paper O4.007, R. A. Pitts et al., 34th EPS Conference: 5 July 2007
Magnetic configurations
 Ohmic L-mode
diverted plasmas
 Ip = 260 kA
Bj = 1.43 T
Mach
 FWD and REV-Bj
(Ip, Bj always
reversed together)
 Density scans
from 2.5x1019 m-3
to density limit
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
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Paper O4.007, R. A. Pitts et al., 34th EPS Conference: 5 July 2007
Measurements below the midplane
 Strong field direction and
density dependence
• Flows always co-current
• Directions consistent with
Pfirsch-Schlüter flow
Mach
wall
Probe
R. A. Pitts et al., J. Nucl. Mater 363-365 (2007) 738
• Clear, field independent negative offset, M|| ~ 0.05-0.1
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Paper O4.007, R. A. Pitts et al., 34th EPS Conference: 5 July 2007
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
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Paper O4.007, R. A. Pitts et al., 34th EPS Conference: 5 July 2007
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
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Paper O4.007, R. A. Pitts et al., 34th EPS Conference: 5 July 2007
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
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Paper O4.007, R. A. Pitts et al., 34th EPS Conference: 5 July 2007
wall
“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
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Paper O4.007, R. A. Pitts et al., 34th EPS Conference: 5 July 2007
Mach
Probe
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
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Paper O4.007, R. A. Pitts et al., 34th EPS Conference: 5 July 2007
wall
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
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Paper O4.007, R. A. Pitts et al., 34th EPS Conference: 5 July 2007
wall
Comparing with theory: field dependence
 Simple expression for
return parallel PfirschSchlüter ion flows
compensating nondivergence free parts of
p and ErB poloidal
drifts:
M||PS


2q cosq 
p  B
 Er 
 2

cs 
ene  B
 Take region
6 < (r - rsep) < 12 mm
 p, Er estimated from
probe profiles of Te and Vf
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 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
Comparing with theory: flow offset
 Simple ansatz to estimate time
 2D electrostatic fluid
averaged M|| due to flow generated
turbulence simulations of TCV
by blobs: M|| ~ 0.5fp > ap with fp > ap
midplane SOL plasma (ESEL
fraction of time over which significant
code, Risø) – successfully
parallel pressure gradient exists (a =
benchmarked against
enhancement over time averaged
turbulence measurements
pressure, <p>)
O. E. Garcia et al., PPCF 48 (2006) L1, J. Nucl.
Mater., 363-365 (2007) 575, IAEA 2006
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W. Fundamenski et al., Nucl. Fus. 47 (2007) 417
R. 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
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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Paper O4.007, R. A. Pitts et al., 34th EPS Conference: 5 July 2007
Reserve slides
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Paper O4.007, R. A. Pitts et al., 34th EPS Conference: 5 July 2007
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
 +ve flow defined UPWARDS
All data mapped to outer
midplane
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1
Paper O4.007, R. A. Pitts et al., 34th EPS Conference: 5 July 2007
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1
Edge profiles, SNU FWD & REV-B
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Paper O4.007, R. A. Pitts et al., 34th EPS Conference: 5 July 2007
Edge profiles, SNL, z = 0, FWD & REV-B
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Paper O4.007, R. A. Pitts et al., 34th EPS Conference: 5 July 2007
Comparing with theory: field dependence
 Simple expression for
return parallel PfirschSchlüter ion flows
compensating nondivergence free part of
p and ErB poloidal
drifts:
v||PS



p  B
 2
 2q cosq  Er 
ene  B

 Take region
6 < (r - rsep) < 12 mm
 p, Er estimated from
probe profiles of Te and Vf
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 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
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
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Paper O4.007, R. A. Pitts et al., 34th EPS Conference: 5 July 2007