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Turbulence Imaging in

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in measured turbulence characteristics. between L- and H-mode confinement ... in turbulence. characteristics of ... edge turbulence and associated transport ... – PowerPoint PPT presentation

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Title: Turbulence Imaging in


1
Turbulence Imaging in the SOL of Alcator C-Mod
by J.L. Terry1, S.J. Zweben2, M. Greenwald1, B.
LaBombard1, R.J.Maqueda3, M.W. Muterspaugh1, C.S.
Pitcher1, and B. Rogers4
1 Plasma Science and Fusion Center, M.I.T. 2
Princeton Plasma Physics Lab., Princeton Univ. 3
Los Alamos National Laboratory 4 University of
Maryland
2
Observations of Filaments with long longitudinal
wavelengths
approx. view
10 ms snapshot
filaments
RF antenna
central column
3
Diagnostics for Studying Edge Turbulence
4
Diagnostics for Studying Edge Turbulence
gas puff nozzle
telescope
5
Diagnostics for Studying Edge Turbulence
gas puff nozzle
telescope
6
The Far-SOL is Turbulent - probe observations
7
The Far-SOL is Turbulent - gas puff emission
time series of emission from gas puff (r10
mm) IRMS/ Iave 0.35
1 ms blowup
8
Typical Fast-camera Image
Spatial resolution of system 1 mm
Exposure time typically 2 µsec / frame Signal
/ background 5-10 with D gas
separatrix
puff nozzle
Framing rate is much too slow to follow
turbulent structures
plasma center
9
Fast-camera Images for 6 T L-mode shot (Ip1 MA)
puff nozzle (D2)
10
Imaging plasma turbulence, NOT gas turbulence
The gas-puff-emission fluctuation Spectrum is
similar to both the probes Isat and Vf spectra
Reynolds 20 ltvatomgt 1000 m/s or 1 cm
in 10 ms
still not clear whether emission
fluctuations result from are Te, ne, or both


11
however, emission response to density
fluctuations is illustrated below
simulated density image
CONCLUSIONS - relatively narrow region where
emission image reproduces density image - still
some diagnotic issues
atomic physics gas puff filter
simulated emission image
increasing ne, Te
12
Results Frequency Spectra from Fast Diode Signals
- Freq. Spectrum is broadband with most of
the power at lt 20 kHz
L-mode - remission11.5 mm
- In Far-SOL no obvious difference in
measured turbulence characteristics between
L- and H-mode confinement
--- EDA H-mode remission11.5 mm
13
Results Probability Distribution Functions
Auto-correlation Times from Fast
Diode Signals
14
Results Size-scale of turbulence
radial correlation lengths from fast diode
signals are 0.7-1.1 cm
poloidal and radial correlation lengths from
images are 0.8 and 1.2 cm respectively
o during ELM-free H-mode x L-mode before
H-mode L-mode after H-mode
Poloidal Correlation Length (mm)
In the data set of images analyzed so far
(8/29/00), there is no significant difference
between the correlation lengths during
ELM-free H-modes and the L-mode periods
before and after
Radial Correlation Length (mm)
15
Results Nothing obvious yet in turbulence
characteristics of images
distinguishing L- and H-mode
in H-mode ave. emission is typically reduced
and further in radially
H-mode
L-mode
16
Results Cross-phase measurements between
fast diode signals show radial phase
velocities 102 - 103 m/s
outward
When the emissions from radially separated views
are correlated, we some-times observe phase
differences in the signals, from which
Vf(w)DR/(Df/w) is calculated. In those cases
ltVf gt is such that the features appear to move
out radially with Vf102-103 m/s
17
Comparisons With Theory
Philosophy quantitative comparison between
measurement and theory /computational
simulations needed to understand edge
turbulence and associated transport
Implications 1) Ask theory/computational
simulations to
produce 2D images of ne and Te fluctuations.
2) Apply the same analytic
techniques (what are
they?) to both the experimental images/time
series and the simulations.
Preliminary comparison with non-linear fluid
codes of Rogers/Drake (U.Md.) - DBM
(Drift Ballooning Mode) Code (with
resistive ballooning modes collisional drift
waves)
18
simulation
experiment
DBM 123
1 cm
Preliminary Comparisons With Theory (contd)
1 cm
features in exp. bigger by x2-3
1001109013 F52-54
density contours
19
Summary
Filaments/striations with long parallel
wavelengths observed in SOL Large
fluctuations levels in emission (15-80) seen
in edge (from separatrix to r3 cm) Wide
variety of shapes in 2-D structure, but
often looks like isolated blobs Typical size
scale of turbulent structures 1 cm
poloidally and radially (ltkgt rs 0.02 _at_ Te
25 eV) Structures appear to propagate
radially outward at 102-103 m/s - no
information yet on poloidal propagation
Skewness of Probability Distribution Functions of
Emission signals increases with distance into
SOL
20
Summary (contd)
Typical auto-correlation times for the
far-SOL emission are 5-30 ms. For the far-SOL
Isat they are 10 ms. No obvious
differences in SOL emission turbulence
characteristics between L- and H-mode.
Comparisons between measurements and
theory/simulations are necessary and have begun.
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