Edge plasma electrostatic turbulence in tokamaks

1
Edge plasma electrostatic turbulence in tokamaks
Jan Horacek at CRPP PhD-students Forum

• Outline
• Edge turbulence description
• Turbulence diagnostics
• How to suppress edge turbulence?
• Edge turbulence at TCV study of ELMs and density
  limit

• Turbulence plays important role in plasma
  confinement major part of radial particle
  transport in the edge is driven by electrostatic
  turbulence gt anomalous diffusion.

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Edge turbulence description

• Universality turbulence character is independent
  of device size, edge magnetic configuration
  tokamaks, stellarators, limiter/divertor
• Electrostatic (fluctuations in ne, Fpl, Te) ExB
  drift is dominant for turbulent transport
  (magnetic play minor role), strongly correlated
  with ?rne. Transport is non-diffusive
• In modern self-organized critical state driving
  avalanches, 1/f power spectrum, scale-invariant
  (fractal)
• ?rpe is the source of free energy for turbulence
• Drift-interchange instability mechanism
• Character changes during L/H-mode, ELMs, density
  limit
• M. Endler JNM 1999 p.84

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Visualization of turbulence

• Density fluctuations in poloidal cross-section
  of CASTOR tokamak
• Solution of 2D Hasegawa-Wakatani equations by K.
  Dyabilin (Moscow)
• Slowed down 10ms gt1s
• Perfect correspondence with experimental
  Langmuir probes measurement
• in poloidal plane size 1-4cm, life-time 10-50
  ms, vkm/s
• In toroidal direction found 90 correlation at
  12m

limiter
wall
Plasma centre not in simulation
http//crppwww.epfl.ch/horacek/turbmovies/ http/
/crppwww.epfl.ch/horacek/turbmovies/movies/movie_
flux.avi
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Turbulence diagnostics

• Density fluctuations
• Langmuir probe (ne,fpl, G, Te)
• Ha, Da spectroscopy
• Collective scattering
• Line radiation of fast lithium beam
• Microwave reflectometry
• Magnetic field fluctuations
• Mirnov coil
• Rogowski coil

Signal dB/dt
B
Ip
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Langmuir probe edge diagnostic
Langmuir probe is a simple electric pin inserted
into plasma

• highly localized in space (lt1mm) and time
  (dtltlt100ns), however, influencing and dangerous
• turbulence diagnostic at Isat or Vfl regime or
  fast sweeping for dTe

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Turbulence suppression by electrode biasing

• Artificial LgtH transition by
• radial electric field gradient ?rEr
• velocity shear ?rvpol gt
• poloidal turbulent structures decorrelation
  C(dn,dEpol)0
• radial flux GrltdndEpolgt/B drops gt
• particle energy confinement increases as
  desired ?

limiter
Er
Show movie again!
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Biasing works!
L-mode Biasing

• At CASTOR we found strong radial turbulent flux
  suppression ?
• But, electrode cannot be used ? in large machines
  like JET or ITER. So can one do it by another
  way?

decorrelation
Electrode position
Limiter 85mm
centre
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Crazy idea biasing by NBI

• Neutral beam ionized in the edge creates a
  2rL2-10cm region of Er by charge exchange
• How to ionize it there?
• By laser not possible (too low efficiency) ?
• By another NBI 0.2 (100) efficiency for 1MW
  (400MW) ?
• Another idea??

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Edge turbulence at TCV

• Using the divertor Langmuir probes the
  reciprocating probe SRAP, up to 10MHz sampling
• Investigations of turbulence in Te, Eq, ne, Vfl,
  vpol, Gr in L/H-mode, ELMs, density limit
• Bursty transport for both directions of Bq (is
  EpxBq still outwards?)

Head of Sonde Langmuir RAPide
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A method for plasma velocity measurement (I)

• Cross-correlation between 2 pins separated by 4mm
  results in phase plasma velocity, time space
  dimensions of turbulent structures

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A method of plasma velocity measurement (II)
Experiments at CASTOR proves that velocities
measured by ExB drift, cross-correlation and Mach
probe correspond well ? gt SOL fluctuations are
frozen to the plasma medium (they carry mass),
i.e. they are not waves
VpolErxBtor drift Mach probe (flux) Cross-correla
tion
limiter
Radius mm
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Why ELM studies on TCV?

• Fast Te during an ELM is rarely known
• ELMy SOL radial power flux is too large for ITER
  first wall (not designed for high power flux)
  type I ELMs destroy ITER target during 1 shot!
  Its necessary to increase SOL ELM radial flux or
  increase ELM duration or frequency
• ELMs help to take off the helium ash from the
  edge lets take out particles but not energy!

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ELM turbulence phenomena's
Between ELMs acceleration from 4 to 8 km/s
upwards
Similar measurement on ASDEX, M. Endler but no
acceleration observation.
During ELM vpolgt10km/s downwards
1ms
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ELM turbulence phenomena's
After ELM whistle of 600kHz sound ELMs 23755
Sawtooth-like structure (30kHz) with a whistle
(600kHz) following an ELM
1ms
100ms
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Density limit

• Density limit is generally max. operational
  density in a plasma device its exceeding leads
  to disruption. Its desired to increase it since
  Pfusion?n2
• Recent idea DL caused by edge turbulence? M.
  Greenwald PPCF 2002 R27
• Recent observation the classical explanation of
  radiation limit Prad Pin is not necessarily
  true gt its just a coincidence, not the cause
• Does turbulence increase in some critical way as
  the density raises? SOL Te, ne profile determined
  by two processes conductive ? convective
• Observation on C-Mode approaching the DL the
  convective part broadens and as it crosses the
  LCFS the edge cools down, Ip shrinks gt disruption

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Density limit experiments on TCV
1st DL experiments on 19/02/2003, perfect data
not yet analyzed, unfortunately
SOL of C-Mod
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Summary

• Turbulence is intrinsic to plasma and poorly
  understood
• Its suppression leads to better confinement,
  however, in SOL it helps distribute ELM power
  over larger surface on the 1st wall
• It might be responsible for density limit
• This talk is at http//crppwww.epfl.ch/horacek/fo
  rum/
• Thank you for your attention ?