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Grasping Plasma Turbulence Fundamentals: Where do we …Kolmogorov’s theory from 1941 K41 is based...
Transcript of Grasping Plasma Turbulence Fundamentals: Where do we …Kolmogorov’s theory from 1941 K41 is based...
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Frank Jenkowww.ipp.mpg.de/~fsj/gene
Max-Planck-Institut für Plasmaphysik, GarchingUniversität Ulm
Kinetic-scale turbulence in laboratory and space plasmasCambridge University, 20 July 2010
GraspingPlasma Turbulence
Fundamentals:Where do we stand?
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The plasma turbulence challenge
The ultimate answer to Life,the Universe and Everything is...
42
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How about our neighbors?The fluid turbulence challenge
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Turbulence as a local cascade in wave number space…
The Navier-Stokes equation in action
kE
ηk k
energy flux
fk
driverange
dissipationrangeinertial
range
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Spectral energy balance
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E = v2 d3x = E(k) dk⌠⌡
∞12V
⌠⌡0
Kolmogorov’s theory from 1941
K41 is based merely on intuition and dimensional analysis –it is not derived rigorously from the Navier-Stokes equation
Key assumptions:
• Scale invariance – like, e.g., in critical phenomena• Central quantity: energy flux ε
E(k) = C ε2/3 k-5/3
This is the most famous turbulence result: the “-5/3” law.However, K41 is fundamentally flawed: Scale invariance is broken!
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Key open issues: Inertial range• Is the inertial range physics universal (for Re → ∞)?
• If so, can one derive a rigorous IR theory from the NSE?
• How should one, in general, handle the interplay betweenrandomness and coherence?
Example: Trapping of tracers in vortex filaments
Note:
The observed deviations from self-similaritycan be reproduced qualitatively by relativelysimple vortex models.
See, e.g., Wilczek, Jenko & Friedrichs, PRE 2008Bife
rale
et a
l. 20
05
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Key open issues: Drive range• Often, one is interested mainly in the large scales. Here,
one encounters an interesting interplay between linear(drive) and nonlinear (damping) physics. – Is it possibleto remove the small scales?
• Candidates: LES, Dynamical systems approach etc.
Ore
llano
and
Wen
gle
2001
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How about us?Fundamental open issues
in plasma turbulence research
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Appropriate field equations
X = gyrocenter positionV� = parallel velocityµ = magnetic moment
Advection/Conservation equation
The gyrokinetic equations in action
The kinetic version of reduced MHD…
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Microinstabilities driving plasma turbulence
Doyle et al.
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Some fundamental open issues
• How do the various microinstabilities saturate?
• How is the (free) energy distributed and dissipated?
• How useful is the concept of an inertial range?
• What is the role of sub-ion-gyroradius scales?
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What do we really knowabout nonlinear saturation?
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The drift wave / zonal flow paradigm
Many years of studying ITG turbulence(mostly using adiabatic electrons)
led us to think thatthe physics of nonlinear saturation is
synonymous with zonal flow shearing.
Is this view really correct?
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Historical parallels
Gaul is entirely occupiedby the Romans.
Well, not entirely...
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Trapped electron mode turbulence
Pure TEM turbulence simulations [Dannert & Jenko, PoP 2005]:
- In the drive range, nonlinear and linear frequencies are identical
- In the drive range, there is no significant shift of cross phases w.r.t. linear ones
– No dependence of transport level on presence or absence of zonal flows
Zonal flows suppressed
linear
nonlinear
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ZF / Non-ZF regimes
Ernst et al., PoP 2009
ExB shearing rates exceed thegrowth rate only for ηe < 1
For mainly temperature gradientdriven TEM turbulence, ZFs(and GAMs) are relatively weak
Thus, in a wide region of parameterspace, the standard drift-wave / ZFparadigm does not hold
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� Both weak and strong turbulence theories suggest that the ExB nonlinearity can be represented by a coherent part and a random noise part
� and are fluctuating quantities; minimizing the model error , we obtain
Theory-motivated statistical analysis
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Low-ky drive range: large transport contributions, but small random noise; here, one finds:
Saturation of TEMs: “eddy damping”
~ky2
Merz & Jenko, PRL 2008
This is in line with various theories, including Resonance Broadening Theory (Dupree), MSR formalism (Krommes), Dressed Test Mode Approach (Itoh).
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Implications for transport modeling
NL frequency spectra
Results motivate use of thefollowing quasilinear model: GENE sim’s
QL model
In situations where ITG modesand TEMs compete, they cancoexist, and there can be non-trivial nonlinear interactions
Merz & Jenko, NF 2010
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Dissipation & cascadesin plasma turbulence
D.R. Hatch, P.W. Terry, W.M. Nevins, F. Merz & FJ
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Turbulence in fluids and plasmas –Three basic scenarios
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Saturation via damped eigenmodes
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Excitation of damped eigenmodes
Using GENE as a linear eigenvalue solver to analyzenonlinear ITG runs via projection methods, one finds…
unstableeigenmode
least damped eigenmode
ky=0.3drive range
strongly dampedeigenmodes
(fine-scale structurein v║ and z)
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Energetics in wavenumber space
Damped eigenmodes are responsible forsignificant dissipation in the drive range.
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Some energy escapes to high k
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Resulting spectrum decays exponentially @lo k, asymptotes to power law @hi k
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Multiscale wavenumber spectra
Poloidal wavenumber spectraof density fluctuations for pureTEM / ITG / ETG turbulence
Poloidal wavenumber spectraof density fluctuations for mixed
TEM / ITG – ETG turbulence
NL (!) ETG drive
L ETG drive
Universality? T. Görler & FJ, PoP 15, 102508 (2008)
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Poloidal wavenumber spectraof density fluctuations for pureTEM / ITG / ETG turbulence
Poloidal wavenumber spectraof density fluctuations for mixed
TEM / ITG – ETG turbulence
T. Görler & FJ, PoP 15, 102508 (2008)
ky spectra at kx=0 are steeper
kx spectra differ
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What is the role ofsub-ion-gyroradius scales?
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High-k turbulence simulationsETG turbulence can induce significant electron heat transport:
is possible (Jenko et al., PoP 2000;Dorland et al., PRL 2000)
For comparison: (Cyclone base case)
Confirmed, e.g., by (Idomura et al., NF 2005) and (Nevins et al., PoP 2006).
Latter paper: A prefactor of ≤10 is sufficient to explain certain experiments.
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Coexistence of ITG and ETG modes
ITG/TEM/ETG turbulence: Large fraction ofelectron heat transport is carried by electron scales.
[Görler & Jenko, PRL 2008]
Ion-scale transport muchlarger than in experiments.
box size: ~64 ρi resolution: ~2ρe
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Physics of H-mode barriers
• Strong ExB shear flows thought to suppress long-wavelength turbulence
• Ion heat transport close to neoclassical, but other transport channels
remain anomalous
• What sets the residual electron heat transport?
Some candidates for setting the residual electron heat transport
• Paleoclassical transport (theoretical foundations are disputed)
• Residual long-wavelength turbulence (not ITG)
• High-wavenumber turbulence (e.g., ETG)
This possibility will be investigated by means of gyrokinetic simulations…
Special case: The H-mode edge
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ASDEX Upgrade #20431 (ρpol = 0.98)
separatrix
ρpol = 0.98
Edge transport barrier region:
• kyρs < 0.1 → ITG mode
• kyρs ~ 0.15 → microtearing mode
• kyρs > 0.2 → ETG mode
Told et al., PoP 2008
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Linear stability of edge ETG modes
In the core, linear growth rates tend to peak at kx = 0;here, they peak at large kx values.
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Transport spectrum from GENE
In contrast to the linear growth rate spectrum,the transport spectrum peaks at low kx values.
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Electron heat transport in edge barriers
ETG turbulence is able to explain the residualelectron heat transport in H-mode edge plasmas.
AUGJenko et al., P
oP16, 055901 (2009)
More on this topic:Next week
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Summary and outlook
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• Recent surprises and advances concerning thenonlinear saturation, dissipation, and multi-scaleproperties of plasma turbulence(see also the posters on nonlocal effects by S. Brunner & T. Görler)
• More investigations targeted at improving ourunderstanding of fundamental issues in plasmaturbulence are certainly called for
• In the end, all of this is bound to have importantpractical consequences concerning the efficienttransport modeling of ITER plasmas