2nd thesis report

1
2nd thesis report
Candidate Jan Horacek Supervisor R.A. Pitts

• Outline
• SRAP the edge diagnostic, confirmed by Thomson
  scattering
• Statistical analysis of density fluctuations
• Radial particle flux
• Study of ELMs
• Parallel flows
• Turbulent structure characterization

2
Hardware development

• 3 different heads, D-tacq acquisition (6MHz, 8
  chans.)
• Pins measure
• Te (flt3kHz)
• floating potential Vflf-3Te (plasma potential
  f, temperature Te)
• ion saturation current IsatneTe1/2 (density ne)

Head of Sonde Langmuir RAPide
Grand merci a P.Conti, P.Gorgerat, X. Llobet
Aligned to magnetic surface, 2002
Original head, DIII-D
Mach number, radial gradients, 2003
Magnetic field
4mm
3
Major plasma parameters

• ohmic L/H-mode, ELMs
• density ramp gt limit
• change in Btor direction
• Ip260-400kA

Wall shadow
SOL
4
DC edge profiles

• 2003 Edge Thomson scattering, confirmed SRAP
  profiles of Te, ne.
• Excellent match of absolute values and space
  position

5
2nd thesis report
Candidate Jan Horacek Supervisor R.A. Pitts

• Outline
• SRAP the edge diagnostic, confirmed by Thomson
  scattering
• Statistical analysis of density fluctuations
• Radial particle flux
• Study of ELMs
• Parallel flows
• Turbulent structure characterization

6
SOL plasma is bursty
Plasma in wall shadow SOL LCFS Confined
plasma

• Positive density bursts become dominant far away
  from LCFS.

7
Density fluctuation analysis

• Autocorrelation time ltn(t)n(ttAC)gtt 1/e
  characterizes time scale of turbulent structures
• inside LCFS the PDF of density is Gaussian,
  however, nearby wall not at all.

8
Stochastic modeling of density fluctuations

• In progress with J. Graves gt publish ASAP as J.
  Horacek, PPCF 2004.
• Statistical approach based on Gamma process in
  analogy with the sandpile model
• Time evolution and PDF of skewed
  Gamma-distribution PGm,s(ne) of no free parameter
  fits well neSOL of any TCV Ohmic discharge

9
Model predictions
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2nd thesis report
Candidate Jan Horacek Supervisor R.A. Pitts

• Outline
• SRAP the edge diagnostic, confirmed by Thomson
  scattering
• Statistical analysis of density fluctuations
• Radial particle flux
• Study of ELMs
• Parallel flows
• Turbulent structure characterization

11
N.B. nearby LCFS lpol small gt Gr is wrong
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ltGrgt versus tp

• Assuming the particle flux is constant everywhere
  on LCFS ne2pRpa2ltGrgttp2pR2pa gt ltGrgtane/2tp
• tp1-3tE15-60ms (Weisen, Nucl. Fusion 37 12,
  1997), a22cm
• ltGrSRAPgtt is 3-25x larger than ltGrgtLCFS lt likely
  since its at LFS

13
ELM detail structure
Da

• Detail time evolution of Isat during an ELM shows
  a complex structure
• High quality data but no systematic analysis yet
• Correlating with Da gt chaotic vrad, why?
• The new LFS LPs improve a lot

Isat (mA)
dmid
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Post-ELM oscillation discovery
Link to sawtooth and MHD
SRAP floating potential fluctuations
Frequency (kHz)
Time-correlation with 37kHz oscillation on HFS
MHD probe
Help from A.Scarabosio, D.Raju, F.M.Poli
Time (s) during SRAP reciprocation
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2nd thesis report
Candidate Jan Horacek Supervisor R.A. Pitts

• Outline
• SRAP the edge diagnostic, confirmed by Thomson
  scattering
• Statistical analysis of density fluctuations
• Radial particle flux
• Study of ELMs
• Parallel flows
• Turbulent structure characterization

16
New Mach probe

• 2003 installed on TCV
• parallel plasma flow Mach number
  Mv/csClog(Isat,5/Isat,2)
• Radial density (potential) gradients
  (Isat3-Isat5)/dr and velocities

Magnetic field
Isat2
Isat5
Isat3
radial
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Parallel flow
Density increase

• Classical drifts M decreases with density
• Strong outside midplane wall sink action
• Too high for classical flow mechanisms gt
  ballooning instability plays role?
• Prediction of important wall sputtered material
  migration for ITER

ELMy H-mode
Density increase
To present at PSI04 and publish as J. Horacek,
JNM 2005
18
Parallel flows during ELMs

• ELMs do not perturb M in SOL, why?

SRAP close to LCFS at wall
at wall shadow
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2nd thesis report
Candidate Jan Horacek Supervisor R.A. Pitts

• Outline
• SRAP the edge diagnostic, confirmed by Thomson
  scattering
• Statistical analysis of density fluctuations
• Radial particle flux
• Study of ELMs
• Parallel flows
• Turbulent structure characterization

20
Poloidal velocity and turbulence dimension on TCV

• Cross-correlation of Isat and Vfl results in the
  same vpol ?
• VSL position confirms well the LCFS (Liuque) wrt.
  absolute SRAP position ?
• Direction change in reversed B-field but not in
  the whole SOL!

Vfl1
B-field
Vfl4
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Radial velocity results from oblique turbulent
structures
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Impossible to derive radial velocity from
cross-correlation

• Radial velocity shear stretches out turbulent
  structures gt oblique gt virtual radial
  propagation
• vvirtualradvpol/(dvpol/drtlife), dominant if
  lpolgtdpol
• 4 space points allows to reconstruct a blob
  described by vpol,vrad, lpol, lrad but head
  geometry modification required

23
4-parameter cross-correlation fit
Allows finally structure size l, life time tlife
and velocity v proper calculation

• Assuming correlation in form
• Cezcos(z), where
• z-(t/tlife)2d-vt/l)2H
• tAC-2 tlife-2 (l/v) -2

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• Applying new technique on experimental data in
  reasonable agreement with the old technique
• Obviously potential for improvement

25
Turbulence during an ELM

• The new technique shows its not vpol which
  changes but l
• To be confirmed!

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Probable 2D flow

• In total a 2D flow is present, together with
  vcsM
• Nearby LCFS close to DL vpol Bz/Btor csM gt
  rather parallel flow
• Outside VSL vpol ? -vpol gt close to toroidal
  direction
• This behavior is observed on C-mod (LaBombard),
  too

B
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Poloidal velocities
Poloidal components of the parallel Mach flow
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• Summary
• 2003 1st systematic edge turbulence
  investigation on TCV
• Evaluation of Eq, ne, f, M, vpol, Gr
  during L/H-mode, ELMs, density limit,
  deuterium/helium, change in Btor direction, very
  high time-resolution
• Intermittent bursts dominate density far away
  from LCFS. Successful statistical description gt
  prediction.
• SOL profile flattening of ne, M when close to
  DL. Anomalously high M
• Discovery of post-ELM-oscillations, its link to
  MHD and saw tooth
• Small effect of ELMs on M
• Outlook
• Discussion on Post-ELM-oscillation. Study the
  complex ELM structure
• Compute radial propagation of ELMs, IPOs, link
  between M and vrad
• Continue Ne and start Gr stochastic modeling with
  J. Graves, apply on other tokamaks
• Experiments in Btorlt0 on M at another poloidal
  positions
• This talk is at http//crppwww.epfl.ch/horacek/pu
  blications/.
• Thank you for your attention ?