Transport phenomena in the edge of Alcator C-Mod plasmas

Abstract

Two aspects of edge turbulence and transport in Alcator C-Mod are explored. The quasi-coherent mode (QCM), an edge fluctuation present in Enhanced Dα H-mode plasmas, is examined with regard to its role in the enhanced particle transport found in these plasmas, its in/out asymmetry, its poloidal wave number and its radial width and location. It is shown to play a dominant role in the perpendicular particle transport. The QCM is not observed at the inboard midplane, indicating that its amplitude there is significantly smaller than on the outboard side. The peak amplitude of the QCM is found just inside the separatrix, with a radial width 5 mm, leading to a non-zero amplitude outside the separatrix and qualitatively consistent with its transport enhancement. Also examined are the characteristics of the intermittent convective transport, associated with the larger scale turbulent structures, also called blobs, and typically occurring in the scrape-off-layer (SOL). These turbulent structures are qualitatively similar in L-and H-mode. When their perpendicular extent, occurrence frequencies and magnitudes are compared, it is found that their size is somewhat smaller in ELMfree H-Mode, while their frequency is similar. A clear difference is seen in the magnitude of these turbulent fluctuations in the far SOL, with ELMfree H-mode showing a smaller perturbation there than L-mode. As the Greenwald density limit is approached (n/n GW 0.7), blobs are seen inside the separatrix consistent with the observation that the high cross-field transport region, normally found in the far SOL, penetrates the closed flux surfaces at high n/n GW .

Full text

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INSTITUTE OF PHYSICS PUBLISHING and INTERNATIONAL ATOMIC ENERGY AGENCY
NUCLEAR FUSION
Nucl. Fusion 45 (2005) 1321–1327doi:10.1088/0029-5515/45/11/013
Transport phenomena in the edge of
Alcator C-Mod plasmas
J.L. Terry1, N.P. Basse1, I. Cziegler1, M. Greenwald1, O. Grulke2,
B. LaBombard1, S.J. Zweben3, E.M. Edlund1, J.W. Hughes1,
L. Lin1, Y. Lin1, M. Porkolab1, M. Sampsell4, B. Veto1and
S.J. Wukitch1
1Plasma Science and Fusion Center, MIT, Cambridge, MA 02139-4307, USA
2MPI for Plasma Physics, EURATOM Association and Ernst-Moritz-Arndt University,
Greifswald, Germany
3Princeton Plasma Physics Laboratory, Princeton, NJ 08543, USA
4Fusion Research Center, The University of Texas at Austin, Austin, TX 78712, USA
E-mail: terry@psfc.mit.edu
Received 13 December 2004, accepted for publication 5 August 2005
Published 24 October 2005
Online at stacks.iop.org/NF/45/1321
Abstract
Two aspects of edge turbulence and transport in Alcator C-Mod are explored. The quasi-coherent mode (QCM),
an edge fluctuation present in Enhanced Dα H-mode plasmas, is examined with regard to its role in the enhanced
particle transport found in these plasmas, its in/out asymmetry, its poloidal wave number and its radial width and
location. It is shown to play a dominant role in the perpendicular particle transport. The QCM is not observed
at the inboard midplane, indicating that its amplitude there is significantly smaller than on the outboard side. The
peak amplitude of the QCM is found just inside the separatrix, with a radial width ?5mm, leading to a non-zero
amplitude outside the separatrix and qualitatively consistent with its transport enhancement. Also examined are
the characteristics of the intermittent convective transport, associated with the larger scale turbulent structures, also
calledblobs,andtypicallyoccurringinthescrape-off-layer(SOL).Theseturbulentstructuresarequalitativelysimilar
in L- and H-mode. When their perpendicular extent, occurrence frequencies and magnitudes are compared, it is
found that their size is somewhat smaller in ELMfree H-Mode, while their frequency is similar. A clear difference
is seen in the magnitude of these turbulent fluctuations in the far SOL, with ELMfree H-mode showing a smaller
perturbationtherethanL-mode. AstheGreenwalddensitylimitisapproached(n/nGW? 0.7), blobsareseeninside
the separatrix consistent with the observation that the high cross-field transport region, normally found in the far
SOL, penetrates the closed flux surfaces at high n/nGW.
PACS numbers: 52.35.R, 52.70, 52.55, 52.40.H
(Some figures in this article are in colour only in the electronic version)
1. Introduction
Transport in the edge and scrape-off-layer (SOL) can play
a crucial role in overall plasma confinement, for example,
through the formation and character of the H-mode edge
transport barrier or as a key aspect of the density limit [1].
The research described here concentrates on two phenomena
observed in the edge and SOL of Alcator C-Mod plasmas,
both involved with cross-field particle transport.
phenomenon is the quasi-coherent mode (QCM) fluctuation
that is observed in Enhanced Dα (EDA) H-mode plasmas [2],
while the second is the intermittent convective turbulence [3],
often identified as blobs, typically associated with the far SOL
The first
and enhanced main chamber recycling. In the case of the
QCM, after showing evidence that it is largely responsible
for the enhanced particle transport in EDA H-mode, we will
address the inboard–outboard asymmetry of its magnitude, its
radial width and location at the outboard midplane and its
poloidal wave number. The importance of the intermittent
blob transport derives from the fact that the cross-field particle
fluxes it drives can be greater than the parallel fluxes [4],
with significant implications for recycling and divertor design.
It has also been implicated as playing an important role in the
density limit [1]. We will discuss the in/out asymmetry of
the broadband fluctuations dominated by the blobs, as well
as characterize the similarities and differences of the blob
0029-5515/05/111321+07$30.00© 2005 IAEA, ViennaPrinted in the UK
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J.L. Terry et al
turbulence in C-Mod’s L- and H-mode plasmas.
we will present observations of blobs being generated inside
the separatrix as the density limit is approached, thereby
supportingthehypothesisthatthepenetrationofblobtransport
intotheclosedfluxsurfacesmayberelatedtothedensitylimit.
Finally,
2. Experimental diagnostics
The primary diagnostics used to characterize the two
edge phenomena of interest here are the optical ‘gas-puff-
imaging’ (GPI) diagnostics, the phase-contrast-imaging (PCI)
diagnostic, and scanning Langmuir probes. While the QCM
is readily observable by a number of other diagnostics, e.g.
reflectometry, BES and magnetic pick-up coils mounted in
a scanning probe head, we discuss characteristics of the
mode as measured by the GPI optical diagnostics and PCI.
In determining characteristics of the blob turbulence, we
use measurements from the GPI and the scanning Langmuir
probes.
GPI [5–7] was developed in order to study edge
turbulence. With this technique, emission is localized in the
toroidaldimension,therebyovercomingadisadvantageofline-
of-sight-integrating, passive optical diagnostics. With GPI,
emission (Dα or He I) from a localized gas-puff is made to
be much greater (typically ?5×) than any intrinsic emission
along the line-of-sight. Since the gas-puff emission is viewed
along the sight lines that are configured to cross it, spatial
localizationisprovided. InC-Modgas-puffbarrelsarelocated
on both inboard and outboard sides, near the plasma midplane
and typically only 1–3cm from the separatrix. The outboard
gas-puff is viewed by two arrays—one made up of discrete
fibres whose focal spots are arranged in a radial array in front
ofthebarrel. Thefocalspotsaretypically4–5mmindiameter
and together span the plasma edge. The fibres transmit light to
photodiodes filtered for Dαand with a flat frequency response
for frequencies ?250kHz. A coherent fibre bundle is also
employedtoimagetheemissionintwo-dimensionalinfrontof
the outboard barrel. Since its view is parallel to the magnetic
field at the gas-puff location, it images the structure of the
emission perpendicular to the field.
image is transmitted to a 300 frame movie camera with a
maximum frame rate of 250kHz. The camera’s 64×64 pixel
array, in combination with the imaging optics, yields ∼3mm
spatial resolution in the plasma [8]. Both these GPI systems
detect the QCM and the blobs. The inboard gas-puff is viewed
by another radially-resolving array of fibres. These array-
views span the inboard separatrix and are used for comparing
with observations from the outboard side. It is important to
remember that the GPI technique measures the effects of the
underlying density and temperature turbulence on emission,
since it is sensitive to both [9]. Nonetheless, neither density
nor temperature fluctuations are being measured directly.
The two-dimensional
3. Characteristics of the QCM
EDA H-mode confinement in C-Mod is distinguished by good
energy confinement, but with enough particle transport that
the density does not increase monotonically and impurities
do not accumulate, as occurs in ELMfree H-mode.
enhanced particle transport appears to be provided by the
The
QC mode Amplitude (1016/m2)
Deff (at separatrix)
(m2/sec)
0 1 2 3 4
0.04
0.03
0.02
0.01
0
Figure 1. Increase of Deffwith the increase of the QCM amplitude.
Here Deff≡ −?perp/∇ne, where ∇neis determined from probe
measurements and ?perpis inferred from spatially-resolved
measurements of the ionization source. The QCM amplitude
is the line integral of the density fluctuation within the QCM
frequency feature.
QCM, a fluctuation that is localized to the edge region. The
QCM is a fluctuation of density, potential and magnetic field
[10], with a frequency spectrum peaked typically between 90
and 200kHz. The evidence that the QCM is responsible for
the enhanced transport is (1) the magnitude of the oscillation
is observed to increase the time-averaged ‘effective particle
diffusioncoefficient’,Deff(definedbelow),and(2)theabsence
of the fluctuation in ELMfree H-modes, in which both density
and impurities accumulate. The first statement is illustrated
in figure 1, where Deff ≡ −?perp/∇ne, is plotted versus the
QCMamplitude. Deffisdeterminedfromprobemeasurements
of ∇ne, with ?perp inferred [11] from spatially-resolved
measurements of the ionization source, including the effects
of parallel plasma flows towards the divertor. (Note that the
use of Deffis not meant to imply that the transport is wholly
or even primarily diffusive.) The QCM amplitude is measured
by the PCI diagnostic which measures line integrated density
fluctuationsalong12verticalchordsthattogetherspana∼4cm
section of radial width, crossing the midplane near the plasma
centre [10]. Only the observed trend of increasing Deffwith
QCM density fluctuation amplitude is significant, primarily
because of the relatively few data points and the uncertainties
in the Deffdetermination.
Because of the primary role of the QCM in the particle
transport out of the pedestal, and because the QCM is still not
understood theoretically, we have investigated a number of its
characteristics. The first is its poloidal character. It is known
that the QCM is localized to the edge region, at least on the
outboard side, as documented by probes [12,13], BES [14]
and reflectometry [2].In addition, the PCI measurements
of the mode are consistent with its existence at the top and
bottom edge of the plasma at major radii near the plasma
centre but still outboard of the x-point. The outboard GPI
arrays also detect the QCM since its density fluctuation is
manifested as an emission fluctuation. An example of the
frequency spectrum of GPI emission from 50 to 250kHz is
shown in figure 2(a). The emission is detected by one of
the radial array views, viewing the puff at ρ ∼ −7mm, i.e.
∼7mm inside the separatrix. (ρ is the radial distance beyond
the separatrix of a given point when flux-surface-mapped to
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Transport phenomena in the edge of Alcator C-Mod plasmas
1040122029 - FDA view at R=88.4 cm, t=0.9-0.93 s)
1040122029 - FDA view at R=45.8 cm, t=0.9-0.93 s)
outboard
midplane
ρ=-4.1 to -9.1 mm
inboard midplane
ρ=-3.5 to -6.5 mm
0.08
0.06
0.04
0.02
0.0
0
Frequency (kHz)
50100150200
0.0
0.1
0.08
0.06
0.04
0.02
QC feature
Fourier amplitude (arb)
(a)
(b)
0.1
Frequency (kHz)
250050 100150200250
Figure 2. (a) QCM feature as observed on a single outboard view; such a feature is seen on views spanning −13 < ρ < 3mm (spot sizes
∼5mm). (b) Same spectral region (at the same time) for fluctuations from a single inboard midplane view. No feature is seen for
−6.5 < ρ < 5mm.
the outside midplane.) In contrast, there is no evidence of
the QCM feature at the inboard midplane, even at inboard
locationsthatareonfluxsurfaceswhichexhibitthefluctuation
on the outboard side. The absence of an inboard QCM feature
is evident in figure 2(b), where a spectrum from one of the
inboardviewsisshown. ThusweconcludethattheQCMmode
amplitude is much reduced at the inboard midplane, implying
that curvature is involved in the drive for this mode.
Using the two-dimensional view of the GPI camera, we
can examine the radial structure and poloidal wave number
of the mode at the outboard midplane. With the camera’s
250kHz frame rate, frequencies up to 125kHz are measurable
without aliasing. The camera’s view is such that it is
sensitive to fluctuations with poloidal wave number in the
range 0.4cm−1< |kθ| < 10cm−1(limited by the ∼8cm
maximum vertical extent of the view and the ∼0.3cm camera
resolution). The QCM is much more difficult to detect with
the camera than it is by using the outboard diode array (which
samples at 1MHz for ∼125ms). In fact, the frequency spectra
from individual camera pixels (with relatively noisy 300-point
time-series signals) show no clear QCM feature above the
noise. In order to overcome this limitation we utilized the
two-dimensional, many-pixel features of the camera images
and the fact that the mode is field-aligned [10]. By averaging
the(complex)frequencyFouriercoefficientsfromthosepixels
viewing the same flux surface and by performing that average
at different kθ values, the QCM amplitude on the specific
flux surface emerges from the noise. Spectra as functions of
kθ and f that have been determined in this way are shown
in figure 3(a). In this case, the fluctuation is seen to have
a peak amplitude at a frequency of ∼100kHz and a kθ of
∼1.0cm−1.
electron diamagnetic direction (up in the camera’s view).
The frequency of the feature is the same as that measured
simultaneouslybythePCI(figure3(b))andtheoutboarddiode
array. Figure 3(b) shows the (kR,f ) spectrum1of density
kθ > 0 indicates a wave propagating in the
1Note that the PCI measures kR, and |kR| = |kθ|/sinζ, where ζ is the angle
between the separatrix and the vertical at the locations where the PCI chords
cross the plasma edge.
Frequency (kHz)
100
80
60
40
20
-2 -1 0 1
kθ (cm-1)
QCM
kR (cm-1)
-5 0 5
(a) GPI camera
(b) PCI
100
80
60
40
20
QCM
(at top)
QCM
(at bottom)
Figure 3. (a) QCM feature in GPI camera’s (kθ,f) spectrum
measured along that fraction of the flux surface in the field-of-view
having ρ ∼ −10mm. (b) QCM feature in (kR,f) space as detected
by the PCI at the same time. The colour scales are linear in Fourier
amplitude.
fluctuations as measured by the PCI. The two features at
kR∼ −4 and ∼5.5cm−1are evidence of the QCM fluctuation
present in the PCI laser beam at the top edge (thus generating
a kR < 0) and bottom edge (with kR > 0) of the plasma.
This measurement also shows it propagating in the electron
diamagnetic direction. (For a schematic of the PCI chords and
QCM relative to the plasma see figure 2 in [10].) The three
measurements of wave number are approximately consistent
with a field-aligned mode, k · B = 0, in which case kθvaries
onafluxsurfaceaskθ(θ1) = kθ(θ2)[(Bθθ2)/(Bθθ1)](R2/R1)2.
Valuesof−3.7and4.3cm−1arepredictedforkRatthetopand
bottom PCI locations for a midplane kθ = 1cm−1, the value
measured by the GPI. Measurements of kθby a probe (located
∼10cm above the midplane) [12, 13] and measurements of
the poloidal variation of kθ using BES and PCI [14] have
been done previously. They also show that kθtypically varies
between 0.8 and 2cm−1near the outboard midplane and that
the poloidal variation is approximately consistent with a field-
aligned mode.
Using the better spatial resolution of the camera (∼3mm,
compared with ∼5mm of the outboard diode array), we can
characterize the radial structure of the Dαemission response
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J.L. Terry et al
Major radius of flux surface (cm) (at z=-2.54 cm)
87 88 89 90 91 92
6
5
4
3
2
1
0
Spectral Integral of QCM Fourier ampl.
separatrix
-3
-2
-1
0
-1
-2
-3
phase from diode-array
phase from camera
relative phase (rad)
ampl. from probe
Figure 4. Radial profile of the amplitude of the GPI Dαemission at
the frequency and kpolof the QCM (solid dark line). This is
compared with the amplitude of the QCM feature in Isatas measured
by the probe during a scan at the same time (thick short dashes).
The two probe traces have been shifted out by 1mm and are from
the inward and outward plunges. Also shown is the relative phase as
determined by the GPI camera (solid grey) and by the radial diode
array (grey long dashes), shifted out by 3mm.
to QCM. By generating (kθ,f) spectra (as in figure 3(a))
for different flux surfaces within the view, we calculate the
maximum Fourier amplitude of the QCM feature around
f ∼ 100kHz and kθ∼ 1cm−1versus a radial coordinate that
identifies the radial location of the flux surface. Such a profile
is shown in figure 4. In a similar way the radial profile of the
relative phase is determined. Since the phase measurement
from the camera is relatively uncertain (due to noisier data),
we plot it together with that determined using diode array,
whosesignal-to-noiseisgreaterbutwhoseresolutionispoorer.
As shown, agreement is good. The observed radial width is
significantly larger than the resolution of the camera. There
appear to be two features: a larger peak of ∼0.5cm width
(FWHM) which is radially inside a smaller peak of similar
width. We note that the outboard diode array yields a radial
profile for the mode amplitude which is consistent with that
from the camera (figure 4), albeit with poorer resolution. Also
showninfigure4isthelocationoftheseparatrix,illustratedasa
stripeasaresultofuncertaintiesduetotheEFITreconstruction
(∼2mm),aswellasuncertaintiesintheregistrationofthefield-
of-view (∼2mm). We have compared these measurements
with those of the amplitude of the QCM feature in the ion
saturationcurrentasmeasuredbytheprobeduringaradialscan
made at the same time. This is also plotted in figure 4 as the
thick dashed curve. Over that portion of the profile common
to both measurements, the agreement is good. Nonetheless,
the fact that no probe scan has ever shown a double-peaked
feature like that measured by GPI and the fact that the GPI-
measured profile implies existence of the mode inboard of the
top of the H-mode pedestal (whose density width is typically
only 2–6mm [15]) motivated us to model the response of the
Dαemission to a QCM-like perturbation. This was done using
the one-dimensional steady-state fluid neutrals code, KN1D
[16], for which density and temperature profiles are specified
and neutral profiles (and emission) are calculated. Assuming
that the processes in the ‘steady-state’ simulation respond
fast enough to model the 100kHz QCM density oscillation,
we find that an additional oscillation in the emission occurs
just inboard of the emission oscillation at the radius of the
2 cm
123
456
1040115013
Figure 5. Movie frames of edge turbulence at n/nGW= 0.8. The
ovals locate the ‘birth’ and motion of a blob. The separatrix is also
shown. The emission is He I, and the time between frames is 4µs.
modelledQCM-likedensityperturbation. Thetwooscillations
in the simulation are predicted to be π radians out of phase.
This is similar to what is measured (figure 4). We interpret
this to be the result of ionization ‘shadowing’ by the density
perturbation. BecausetheQCMfeatureislocatedwellinboard
of the peak in the Dαemission profile (both in the experiment
and in the simulation), changes in the density profile outside
of a region where the Dαemission is relatively weak can affect
the emission inboard of the change. Thus we hypothesize that
thesecond,inboardpeakintheamplitudeoftheemissionatthe
QCM frequency and kpolmay be due to this effect, while the
outboardonereflectstheactualQCMperturbation. Finallywe
pointoutthatsimulationswiththeBOUTturbulencecode[10]
yield a single radial peak with a width of ∼5mm.
These measurements make it very likely that the QCM
spans the separatrix. Thus we see that the extension of the
QCM onto open field lines is qualitatively consistent with the
observation that the QCM strongly affects particle transport
(figure 1).
4. Comparisons of SOL ‘blob’ turbulence in L- and
H-mode plasmas
We now consider a different aspect of turbulent transport in
the edge. This is the intermittent, cross-field transport that is
associatedwiththeradialandpoloidalpropagationofturbulent
structures and that has been shown to dominate the total cross-
field transport in the SOL [17].
using probes and GPI. Although the turbulent structures are
actually filaments, aligned with the local field, having small
k||, they have the appearance of ‘blobs’ when viewed along
the field. As an example of the birth and propagation of a
blob, we show six consecutive frames from the GPI camera
in figure 5. Although the birth location of the prominent blob
identified in these frames is atypical (and will be discussed in
the next section), the blob’s size and its outward propagation
are typical. The blobs are detected as large amplitude events
on the emission signals of the diode array views and as
large amplitude ion-saturation-current and floating-potential
events by probes. An example of the GPI emission time
history from one of the outboard views is shown in figure 6.
In the far SOL the ‘event’ probability distribution function
On C-Mod it is studied
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Transport phenomena in the edge of Alcator C-Mod plasmas
ELMfree H-Mode
L-Mode
1031121030
H-to-L
trans-
ition
time (s)
ρ=0.6 cm
intensity
2.5
2.0
1.5
1.0
0.5
0
1.368 1.369 1.370
1.384 1.385 1.386
1.2 x mean
Figure 6. Time history of GPI emission from an outboard view at ρ = 0.6 cm (far SOL) during an H-mode period well after the L-to-H
transition (left) and during the L-mode period (right) following the H-to-L back transition.
is typically strongly skewed towards larger amplitude events;
both skewness and kurtosis are >0. It is important to note
that both the GPI diagnostics and the probes show that this
turbulence is reduced by a factor of 5 or more on the inboard
side compared with the outboard side [18], evidence of its
probable ballooning character.
We now turn to a comparison of the SOL blob turbulence
in L- and H-modes. Similar studies using probes have been
reported in [1, 17, 19]. Figure 6 shows the local emission
signal first during an ELMfree period and then during a
following L-mode period. The time histories indicate that
the nature of the blob turbulence is qualitatively similar in
L- and H-modes. Using the outboard GPI diagnostics we will
compare quantitatively the spatial size of the blob structures,
and the frequency and magnitude of the blob events, as well
asdiscussobservationsabouttheirradialpropagationvelocity.
InthesecomparisonsweexaminetheH-modeafterthepedestal
has developed and the core density has increased significantly,
i.e. not immediately after an L-to-H transition when the GPI
diagnosticsindicatethattheSOLismorequiescent. Ameasure
of the spatial size of the blobs is found from the poloidal
correlation lengths of emission in the GPI camera images.
Poloidal and radial correlation lengths are similar [7]. The
comparison is shown in figure 7(a). The shaded areas are
the ranges of mean values determined from each sequence of
images; approximately 10 sequences, each from a different
discharge, were analysed for each confinement mode. Also
shown are two characteristic error bars. The larger (dashed) is
representativeofthestandarddeviationforasinglesequenceof
images. The smaller error bar (solid) assumes that correlation
length at each ρ is the same for the given confinement mode
and represents the standard deviation using all of the images
analysed for that confinement mode. As shown, correlation
lengths of the blobs in the far SOL are somewhat smaller in
ELMfree H-mode compared with L-mode. The frequency of
the blob events has been determined using the signals from the
outboard diode array. In the analysis it is assumed that the
blobs sweep past the views and are thus detected as emission
spikes above what would be expected from random Gaussian-
distributed fluctuations.The frequency of distinguishable
events above a threshold is calculated (with the threshold
set relative to the mean value of the signal). As illustrated
in figure 6 by the tick marks at the top of the figure, this
amounts to counting the number of time-series local maxima
that are above the threshold. In the case where the threshold
is 1.2 times the mean, the frequencies are plotted versus ρ
in figure 7(b) and are seen to be similar for L-mode and
ELMfree H-mode.This result depends somewhat on the
freq. of events with
amp. > 1.2xmean (kHz)
ρ (cm)
0 1 2
1.2
1.0
0.8
0.6
0.4
0.2
0-1 0 1 2
ρ (cm)
50
Pol. correlation length (cm)
(a)
(b)
L-mode
EDA
ELMfree
40
30
20
10
ELMfree
L-mode
sep
Rmajor(cm)
90 91 92
1.5
1.0
0.5
0
Magnitude relative
to L-mode
L-mode
(c)
sep
ELMfree
EDA
Figure 7. Comparisons of (a) blob size (i.e. poloidal correlation
length), (b) blob frequency and (c) blob magnitude versus radius for
the different confinement modes. In (b) data from single ELMfree
H-mode periods are also plotted (as connected line segments).
These show some outliers from the range typical for ELMfree
H-modes (the light shaded region). In (c) abssisa values are the
major radii for the views and the dashed lines indicate the range
of separatrix locations during the measurement times.
chosen threshold, since the mean value in H-mode is smaller
than in L-mode. We conclude, however, that the frequency
of blob events with magnitudes greater than this threshold
is similar in the two cases. The magnitudes of blob events
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J.L. Terry et al
are compared by examining the integral under those events
whose maxima are greater than the threshold, i.e. the integral
of the intensity above the threshold. Since in this case we
are comparing intensities (and the diode array views are not
absolutely calibrated), we compare the magnitudes relative to
L-mode for each view separately. This comparison is shown
in figure 7(c), for which the threshold is also 1.2 times the
mean.Here we see a distinct separation between L- and
H-mode, with significantly smaller blob magnitudes in the
far SOL during ELMfree H-mode as compared with L-mode.
The EDA/L-mode comparison shows an intermediate effect.
Decreased ‘event’ magnitudes in H-mode relative to L-mode
were also seen in DIII-D [20].
The fluctuation phase velocities can also be compared
usingatime-delaycross-correlationanalysisoftheGPIcamera
images [21].The velocity fields, which are calculated as
time averages over the 1.2ms of camera data, typically show
a mixture of radially outward and poloidal motion outside
the separatrix. In the SOL the fluctuations are dominated
by blobs, so we assume that the fluctuation phase velocity
fields reflect the actual motion of the blobs.
single null discharges the dominant poloidal direction in
the SOL is downward towards the divertor (ion diamagnetic
drift direction). The magnitudes of the radial and poloidal
componentsvarybetween∼0and1000ms−1. Todatewehave
not found a systematic difference in velocity fields between
L- and H-mode. This is primarily because of a wide variation
in SOL velocity fields observed for L-mode discharges.
In lower
5. ‘Blob’ turbulence near the density limit
Of particular interest is the generation of the blobs in plasmas
near the Greenwald density limit in L-mode discharges. For
discharges well below the limit, the typical event distributions
around the separatrix and in the near SOL (both from the
probe measurements and the optical measurements) are more
Gaussian, less skewed towards larger events (i.e. fewer blobs)
than is the case in the far SOL [1]. The two-dimensional
camera images, which typically show blob generation only
outside the separatrix for discharges with n/nGW < 0.6,
are consistent with this.However, as the density limit is
approached, the region of intermittent convective transport
expands inwards radially, eventually crossing into the closed
flux surfaces. Not coincidently, when that occurs, the levels
of cross-field convected power are seen to be larger than the
power conducted to the divertor, separatrix temperatures and
temperature gradients are reduced [1], typically a MARFE
appears, the plasma column shrinks and the plasma is abruptly
terminated. The observations of blob generation inside the
separatrix at densities approaching the limit, as illustrated
in figure 5, add further support to this picture. Although it
is hard to quantify the spatial distribution of the blob birth
locations, we do observe, as is pointed out in [1], the inward
expansion of the region of higher fluctuation levels as the
limit is approached. Similarly, the region where the skewness
values in distributions of emission intensities are large also
shifts inwards. The radial profile of normalized GPI emission
fluctuations and the associated Te profiles at two values of
normalized density, n/nGW = 0.33 and 0.8, are shown in
figure 8. For the case near the density limit, fluctuation levels
0
-1 0 1 2 3
20
40
60
Te (eV)
ρ (cm)
ASP(PSI) - 1040115009(015) - t=.95s (0.8s) & 1040115013 - t=1.38s
n/nGW=0.8
n/nGW=0.33
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
Irms/<I>
&
&
Figure 8. Radial profiles of Teand normalized emission
fluctuations, IRMS/Iave, at two values of n/nGW, 0.33 (black) and 0.8
(grey). The emission fluctuations are in He I. Near the density limit,
blobs are seen inside the separatrix (figure 5).
in the −1 < ρ < 2cm region have increased significantly,
while Teand the temperature gradient at the separatrix have
decreased. A larger temperature gradient must exist further
inside of ρ = −1cm in order to connect to core Teprofiles.
Whereas blobs are typically seen only outside the separatrix,
near the density limit, under these conditions, blob generation
inside the separatrix is observed (figure 5). This is indicative
that blob generation in C-Mod is associated with gradients
rather than with the transition from closed to open field lines.
6. Discussion
A first principles’ theoretical description for the QCM does
not exist. Experimentally it has been shown to be important
in providing the beneficial particle transport that flushes
impuritiesandinprovidingtheopportunityfordensitycontrol.
Since other transport-driving coherent fluctuations that are
qualitatively similar to the QCM have been identified on
other devices in other transport regimes (e.g. type II ELMy
H-mode in Asdex Upgrade [22,23] and the ‘edge harmonic
oscillation’ in quiescent H-modes [24, 25]), it is important
to investigate the common features and the drive for these
phenomena. In the QCM case the observed in/out asymmetry
in its amplitude provides a clue to its drive. The observation
that it extends across the separatrix qualitatively supports its
effect on transport. Although the QCM has been shown to
exist at the edge of the plasma, there are still differences in
what is measured for its typical radial width. The optical
diagnostics (GPI and BES) measure a larger width than do
the probe and reflectometer, although GPI-probe comparisons
made at the same time show good agreement over that part
of the profile measured by both. One-dimensional modelling
of Dα response to a QCM-like density perturbation yields
an amplitude/phase relationship similar to the observation,
with the implication that the profile measured by the emission
is broadened as a result of ionization ‘shadowing’ by the
perturbation.
Blob generation and transport dynamics [26] are also
not fully understood. The in/out asymmetry is indicative
of a ballooning-like drive [18].
characteristics in L- and H-mode, both here and elsewhere
[17,19], indicate that the blobs are not directly involved with
The comparisons of blob
1326

Page 7

Transport phenomena in the edge of Alcator C-Mod plasmas
the physics of the L–H transition, the pedestal formation or its
sustainment. Whilethistypeofintermittenttransportoccursin
most tokamaks [27] and stellarators [28], we note differences
in some of the details of the blob characteristics. For example,
onDIII-Dtheinferredblobradialvelocitiesaretypicallyinthe
1–3kms−1range [17], not the <1kms−1range observed on
C-Mod. On the other hand the blob generation and transport
do appear to play a role in density limit, with blob generation
extending into closed flux surfaces as the limit is approached.
Acknowledgments
Work supported at MIT by DoE Coop. Agreement DE-FC02-
99ER54512, Contract No DE-AC02-76CHO3073 and Grant
DE-FG03-96ER54373.
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