Title: Turbulence Imaging in
1Turbulence 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
2Observations of Filaments with long longitudinal
wavelengths
approx. view
10 ms snapshot
filaments
RF antenna
central column
3Diagnostics for Studying Edge Turbulence
4Diagnostics for Studying Edge Turbulence
gas puff nozzle
telescope
5Diagnostics for Studying Edge Turbulence
gas puff nozzle
telescope
6The Far-SOL is Turbulent - probe observations
7The Far-SOL is Turbulent - gas puff emission
time series of emission from gas puff (r10
mm) IRMS/ Iave 0.35
1 ms blowup
8Typical 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
9Fast-camera Images for 6 T L-mode shot (Ip1 MA)
puff nozzle (D2)
10Imaging 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
11however, 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
12Results 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
13Results Probability Distribution Functions
Auto-correlation Times from Fast
Diode Signals
14Results 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)
15Results 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
16Results 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
17Comparisons 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)
18simulation
experiment
DBM 123
1 cm
Preliminary Comparisons With Theory (contd)
1 cm
features in exp. bigger by x2-3
1001109013 F52-54
density contours
19Summary
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
20Summary (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.