Observation of Turbulence in Wendelstein 7-AS
description
Transcript of Observation of Turbulence in Wendelstein 7-AS
M. Endler and M. Hirsch
Max-Planck-Institut für Plasmaphysik, EURATOM Association, D-17491 Greifswald, Germany
1. General considerations
2. Confinement region
3. Scrape-off layer (SOL)
Observation of Turbulence in Wendelstein 7-AS
experiment
theory
theory with additional effects
ion heat transport electron heat transport
From: M. Kick et al., IAEA 1996 (Montreal), vol. II, 27
1. General Considerations
Radial heat transport in W7-AS – comparison between neoclassical theory and observation:
Reason for interest in plasma turbulence: Turbulent transport
Turbulence and Transport
T1
T0
T1 > T0
pot of boiling water fusion plasma
n, T
core edge
n1, T1 n0, T0
Twofold Motivation for Observing Plasma Turbulence
• Directly measuring the turbulent transport ( synchronised observation of ≥ 2 quantities required)
• Comparison with turbulence models, simulations, theory (aim: understanding parameters controlling turbulence; influencing turbulence)
6 cm–1
1 cm
Which structure sizes can be observed?
Doppler reflectometrymicrowave scattering
Measurement in k space:spatial band pass CO2 laser scattering
ks = 1 for 300 eV
dissipation
k [cm ]–10.1 1 10 100
[cm]60 6 0.6 0.06
Measurement with limited resolution:spatial lowpass
Mirnov probes,SX
ECEBESreflectometry
Langmuir probes (multi-tip)
drivinginstabilities
a=17cm
kin
etic
en
erg
y
Raw data and statistical analysis
raw data
Example: Langmuir probe data from the W7-AS SOL (Isat)
probability distribution function
(auto)correlation function
Topics:
• Te fluctuations
• Doppler reflectometry
• Transient events
• Turbulence and transport
• Transition edge/SOL
Topics:
• Te fluctuations
• Doppler reflectometry
• Transient events
• Turbulence and transport
• Transition edge/SOL
2. Turbulence in the W7-AS confinement region
Te fluctuations in the plasma core by ECE
decorrelate thermal fluctuations
without decorrelating Te fluctuations
Challenge: < 1 % Te/Te fluctuations are masked by
thermal fluctuations of the radiation field
~ –
by observing the same volume from two positions under sufficiently large angle (first demonstration on W7-AS)
by observing at slightly different frequencies = shifted volume (first demonstration on TEXT)^
Decorrelation of thermal fluctuations
Demonstration of the principle using an artificial source for “temperature fluctuations” but true thermal fluctuations
lines of sight of observation below/above decorrelation angle
From: S. Sattler and H.-J. Hartfuß, PPCF 35 (1993) 1285, figs. 9&10
Different features in Te fluctuations
1. broadband fluctuations (bandwidth ~ 100 kHz)2. low-frequency fluctuations (< 5 kHz)3. quasicoherent modes
From: S. Sattler et al., PRL 72 (1994) 653, fig. 2
norm
aliz
ed c
ross
-cor
rela
tion
Broadband fluctuations disappear for Te = 0
From: H.-J. Hartfuß et al., PPCF 38 (1996) A227, figs. 9&10
In a region with Te = 0, only the low-frequency feature remains
From: M. Häse et al., RSI 70 (1999) 1014, fig. 5
Correlation between n and Te
~~
Topics:
• Te fluctuations
• Doppler reflectometry
• Transient events
• Turbulence and transport
• Transition edge/SOL
2. Turbulence in the W7-AS confinement region
Doppler reflectometry – using turbulence as a tracer for poloidal rotation
corrugated and fluctuating reflecting layer
antenna
microwave signal
“ordinary” reflectometry: use of 0th diffraction order of reflected signal
From: M. Hirsch et al., PPCF 43 (2001) 1614, fig. 1
Doppler reflectometry: use of (–1)st diffraction order of reflected signal
Comparison of poloidal velocity from Doppler reflectometry and from
spectroscopic data
poloidal velocity of fluctuations
≈ poloidal velocity of impurities
≈ vEB
From: M. Hirsch et al., PPCF 43 (2001) 1614, fig. 7
Time resolution of Doppler reflectometry
From: M. Hirsch et al., PPCF 48 (2006) S155, fig. 6
4 µs resolution reveals strong and fast changes in poloidal velocity and scattered power at HL backtransition
Topics:
• Te fluctuations
• Doppler reflectometry
• Transient events
• Turbulence and transport
• Transition edge/SOL
2. Turbulence in the W7-AS confinement region
ELM-like transient transport events
• profile flattening in ECE Te signals
• in region of strong pressure gradient• causing cold pulses propagating inward on diffusive
time scale• simultaneously bursts in broadband Mirnov activity
and small-scale density fluctuations
From: S. Zoletnik et al., 32nd EPS (Tarragona, 2005) P-5.023, fig. 1
Correlation analysis:
From: M. Hirsch et al., 25th EPS (Prague, 1998) 2322, fig. 1a
Transient magnetic activity – poloidal mode structure
Magnetic activity:
• poloidal mode number related to edge rotational transform
• bursts of ~ 100 µs
See: M. Anton et al., J. Plasma Fusion Res. SERIES 1 (1998) 259
Arrangement of Mirnov coils in poloidal cross section:
From: S. Zoletnnik et al., PPCF 44 (2002) 1581, fig. 24
Correlation between magnetic and density fluctuations
Complement poloidal resolution of Mirnov coils with radial resolution of Li beam
See: S. Zoletnik et al., PoP 6 (1999) 4239, fig. 5
= r
adia
l^
correlation of Mirnov signal with various BES channels along the Li beam
Tentative model for transient transport events
After: S. Zoletnik et al., 32nd EPS (Tarragona, 2005) P-5.023
• poloidally localised event (associated with broadband
turbulence) causes radial transport of hot, dense plasma
• flattening of pressure (temperature, density) gradient
• initial poloidal gradient causes MHD oscillations with m =
1/until gradients on flux surface are balanced (after a few
ion transit times ~ 100 µs)
Topics:
• Te fluctuations
• Doppler reflectometry
• Transient events
• Turbulence and transport
• Transition edge/SOL
2. Turbulence in the W7-AS confinement region
• Is fluctuation amplitude related to transport?
• Turbulent transport cannot be measured directly in the confinement region
Turbulence and transport in the confinement region
We may still be lacking important diagnostic information (phase between quantities? small scales, e. g., ETG turbulence?)
- Sometimes, yes: ne, Te amplitude is correlated with heat
diffusivity for density variation (at fixed heating power)
~~
- Sometimes, not in the expected way: ne, Te amplitude is
anti-correlated with heat diffusivity for heating power variation (similar: for variation)
~~
Topics:
• Te fluctuations
• Doppler reflectometry
• Transient events
• Turbulence and transport
• Transition edge/SOL
2. Turbulence in the W7-AS confinement region
Density fluctuations inside and outside the last closed magnetic surface (LCMS)
• no significant radial correlation across the LCMS
• different character of density fluctuations in edge and SOLSee: S. Zoletnik et al., PoP 6 (1999) 4239, fig. 4
(from fast Li beam diagnostic)
Transport in the scrape-off layer
Definition of last closed magnetic surface (LCMS):
by a limiter by a magnetic separatrix
scrape-off layer (SOL)
B
radial transport B
transport B to the target plates
confinement region
3. Turbulence in the W7-AS scrape-off layer
Topics:
• Spatial structure of turbulence
• Phase between fluctuating quantities
• Transport
Topics:
• Spatial structure of turbulence
• Phase between fluctuating quantities
• Transport
Langmuir probe heads:
1 cm
Diagnostics – Langmuir probes
Positions of Langmuir probes in W7-AS:
Diagnostics – H fluctuation diagnostic
emissivity: ne n0 f (Te)
only weaktemperature dependence
pla
sma
lens
vacuum vesselwindow
glass fiber16
gas valve(H2/D2)
photomultipliers
filters
Raw data from the H fluctuation diagnostic (density fluctuations)
Individual “fluctuation events” are propagating in poloidal direction
lifetime: several 10 µspoloidal correlation length: 1–5 cmpoloidal velocity: O(100)–O(1000) m/s
time
poloidal position
500 µs
9 c
m
Frequency spectrum
(floating potential data)
Auto power density spectrum
arb.
uni
ts
103
104
105
106
107
0 200 400 600 800f [kHz]
10 100f [kHz]
same, double logarithmic
From: J. Bleuel et al., NJoP 4 (2002) 38, fig. 5
Poloidal-temporal correlation function
d [
cm]
3
2
1
0
1
2
320 10 0 10 20
[µs]
1.0
0.8
0.6
0.4
0.2
0.0
0.2
grey
sca
le
(floating potential data)
Correlation/coherency || B
fl data from SOL, 6.3 m probe tip separation || B, torus outboard side
cross correlation cross coherency
From: J. Bleuel et al., NJoP 4 (2002) 38, figs. 20&22
Correlation || B in W7-AS – comparison of poloidal-temporal correlation functions
Correlation function between single probe tip and the tips of the poloidal array displaced by 6 m || B (at radial position of maximum correlation)
Correlation function between the tips of the poloidal array
From: J. Bleuel et al., NJoP 4 (2002) 38, fig. 21
Radial-poloidal correlation function – obtained from the angular array
(floating potential data)
5 different time lags:
radial separation dr [cm]
From: J. Bleuel et al., NJoP 4 (2002) 38, fig. 11
polo
idal
sep
arat
ion
d [
cm]
Comparison with spatial structure from model calculations
3D simulation of ITG/drift wave turbulence, Te fluctuations
at fixed time:
From: B. Scott, Phys. Plasmas 7 (2000) 1845–1856
Topics:
• Spatial structure of turbulence
• Phase between fluctuating quantities
• Transport
3. Turbulence in the W7-AS scrape-off layer
Particles:
Energy (for each species):
Transport due to “electrostatic” turbulence
Correlation and phase between different fluctuating quantities
Correlation between floating potential and ion saturation current:
Typically, a phase of /2.../3 between these quantities is observed, maximising transport, if fl fluctuations are considered equivalent to pl fluctuations
From: J. Bleuel et al., NJoP 4 (2002) 38, fig. 8
p0
Phases between n and in interchange instability
r
B
p
Target plate
j ≠ 0 due to curvature
j||
E vExB
fl
Isat
fl
Phase between n, Te and pl fluctuations
n - Te
n - pl
Te - pl
From: M. Schubert, PhD thesis, Greifswald (2005), figs. 5.24&25 (accessible through http://edoc.mpg.de/)
Isat - fl
Modelling of SOL turbulence
• The observed phases are consistent with a drift-interchange type of turbulence
• The impact of the target plate boundary conditions has not yet been fully explored
• The changes of the phases in radial direction are not yet understood in detail
Topics:
• Spatial structure of turbulence
• Phase between fluctuating quantities
• Transport
3. Turbulence in the W7-AS scrape-off layer
Fluctuation-induced radial energy transport
From: M. Schubert, PhD thesis, Greifswald (2005), fig. 5.33 (accessible through http://edoc.mpg.de/)
Observed: (6.6 ± 1.5) kW/m2
Expected from global energy balance: 24 kW/m2
(assuming homogeneous transport across LCMS, taking into account local flux expansion)
Summary
• improving knowledge about relations between different quantities
• capability to observe directly the turblence-induced transport
• qualitative agreement with transport to be expected from global confinement
Confinement region:
• Progress to be expected from improvement of diagnostic capabilities
SOL:
• detailed knowledge of spatial structure of turbulence
Tentative outlook
Confinement region:
• high temporal & spatial resolution required problem of intensity – could progress in lasers help?
• combine several methods to obtain information on different quantities, or complementary information on one quantity
SOL:
• improve advanced methods (fast sweeping of electrostatic probes?) and perform parameter studies
• continue detailed comparison with theory and modelling
• “turbulence engineering” by suitable shaping of targets or by active methods?