From:
WOLFE@PSFC.MIT.EDU
Reply-To: WOLFE@PSFC.MIT.EDU
Subject: Alcator
C-Mod Weekly Highlights
Newsgroups: sci.physics.plasma
Organization:
MIT
Alcator
C-Mod Weekly Highlights
Sept.
13, 1999
Plasma operations continued at Alcator C-Mod last week. Three
run-days were
scheduled and completed. A total of 52 plasma shots were
produced. This week's
runs were in support of the continuing ICRF
commissioning activity.
Physics and Analysis
---------------------
A
quasi-coherent mode in the frequency range from 50 kHz up to 300 kHz is
often
observed by both reflectometry and phase contrast imaging (PCI) during
EDA
H-modes. This mode was believed to be located on the EDA H-mode pedestal;
however,
no direct experimental evidence had been reported due to the
limitation of
both diagnostics. After configuring the reflectometer to take
fluctuation
data in four channels, (the channel with highest frequency, 110
GHz, has
the critical density of 1,5e20 m^-3), the location of the mode has
been
roughly identified. Evidence shows that in some shots with low H-mode
pedestal
height, the mode seen from the 110 GHz channel can appear or
disappear
dependent on whether the critical density is on or inside the
pedestal. The density profile used for the comparison
is from the visible
bremsstrahlung measurement. Comparison among different
reflectometry channels
also indicates that the mode starts to appear at
the top of the pedestal and
soon fills the whole barrier region.
Analyses
of measurements from recent experiments with high density ohmic
plasmas
have shown that the divertor bypass flaps do not directly affect the
threshold
for divertor detachment. Open bypass
flaps provide for a faster
density increase (for the same gas puff) than
closed flaps. Thus, while the
density at which detachment is observed
(ne_bar~2.2e20 m^-3) does not depend
on the state of the bypass flaps,
that density is reached earlier when the
flaps are open. In addition
nitrogen impurity puffing into these higher
density discharges showed that
the detachment threshold and depth of
detachment were modulated by the
amount of nitrogen puffed. This is not
a new
result. However, new images of the detachment in D_alpha and D_gamma
light
show the detachment modulation clearly. A region of recombination is observed
to move from the outer
strike point up to the x-point along the outer leg as
the detachment
deepens. After the nitrogen puff, the
recombination region
retreats back to near the outer strike point. In the
cases of deepest
detachment the D_gamma emission reaches closed field
lines inside the
separatrix.
Observation of ohmic H-modes with
a large electron density gradient in the
core of the plasma was reported.
The plasma core and edge density profiles are
measured using Thomson
scattering diagnostics. In case of
"normal" H-modes
(both ohmic and RF induced) the profile is
characterized by steep edge density
gradients and more or less flat core
profiles. The long H-modes
observed
during two recent ohmic runs (05/27/99 and 08/31/99) developed a
sharp core
density gradient in addition to the edge pedestal. This core
density gradient
develops gradually during the H-mode and is not a
continuation of the edge
pedestal but starts well inside the separatrix at
r_mid of about 0.87 m while
the edge pedestal is located around the
separatrix at r_mid of about .89
m. The resulting core density profile can
be strongly peaked, with the central
density as high as 6e20 m^-3 and the
pedestal height around 2e20 m^-3, and is
similar to the profiles observed
when internal transport barriers are formed
in the plasma. Further
investigations of these phenomena are under way.
Prior observations
with the BES viewing optics had not seen any evidence of
poloidal
propagation of D-alpha fluctuations, which we attributed to our views
(almost
strictly toroidal) not being parallel to the field lines in the edge.
The
chord-integrated signals were therefore averaged over many flux tubes.
The
fiber holder was rotated in a direction which effectively increased
the
view angle, after which we saw dramatically increased phase shifts
between
nearby channels. The phase
shifts were largest during Ohmic plasmas and
dramatically decreased during
RF. This implies the poloidal ExB
velocity
dramatically increases during RF. We are presently working on quantifying
these
observations.We had not seen any evidence of poloidal propagation of
D-alpha
fluctuations, which we attributed to our views (almost strictly
toroidal)
not being parallel to the field lines in the edge. The
chord-integrated signals were therefore averaged over
many flux tubes. Matt
rotated the
fiber holder in a direction which effectively increased the view
angle,
after which we saw dramatically increased phase shifts between nearby
channels. The phase shifts were largest during Ohmic
plasmas and dramatically
decreased during RF. This implies the poloidal ExB velocity dramatically
increases
during RF. We are presently working on
quantifying these
observations.
ICRF System
------------
Three
experimental runs were dedicated to RF conditioning. The focus was
continuing the conditioning for J-port, and
bringing up D and E-port antennas.
The J-port antenna continued to show
improvement: heating was observed with
various phases and 1.5 MW injected
power was obtained routinely. The
relative
strap current phase (varies the antenna wave spectrum) was
scanned on J-port.
Intial analysis suggests that both the heating and
impurity influx vary with
phase.
For added experimental
flexibility with the J-port antenna, a design for the
external antenna
connection loops giving an additional 180 degree current
strap phase shift
has been started. This will permit
operation with the four
straps phased 0-pi-0-pi, which is not accessible
with the present connections.
ICRF power up to 1 MW from the D-port
antenna (FMIT#1) has also been added,
both in parallel with the J-port
power and separately. EDA H-mode
conditions
were obtained with D-port operated alone at the 1MW power
level.
An arc in the transmission line to E-port (FMIT#2) was
located and repaired.
We anticipate operation of this antenna into plasma
this week.
Work on eliminating a high frequency oscillation in
FMIT#1 and #2 has centered
upon changing the FPA screen voltage to -1500 V
from -960 V and the FPA bias
voltage to -600 V from -500 V. This has resulted in a higher gain
final
stage. The solution to the
high frequency oscillation suggests that the cause
was high harmonic
generation from a clipping drive component; this would
result from the
power feedback loop attempting to overcome loss in output
stage gain due
to excessive screen supply droop.
Advice and circuit drawings
on adding screen supply regulation have
been obtained from the PPPL RF
engineering group.
The temporary
high voltage modifications to FMIT#1 and #2 were insufficient in
extremely
humid weather as experienced last Friday.
The high voltage
connection in the output cavity was arcing to such
an extent that these two
transmitters could not be operated for part of
the run. All crowbar protection
circuitry operated normally, and there was
no damage to any of the
hardware. Purging of the cavity with dry N2 gas
improved the situation. Design
modifications based on FMIT#3 and #4 are
being pursued to improve high voltage
operation.
Travel and
Visitors
-------------------
Miklos Porkolab attended the FESAC
meeting last week on Wednesday and Thursday
in Gaithersburg, Md.
Gary
Taylor returned from PPPL and worked on the GPC2 ECE instrument. He made
good progress in reducing the
pickup which has been experienced during
operation of the new J-port RF
antenna.
Matt Sampsell (U.Texas) visited C-Mod last week to make
some adjustments to
the BES viewing geometry, as detailed above. David
Winslow (U. Texas) began a
visit to CMOD to start driven divertor probe
experiments and explore divertor
fluctuation measurements. R. Chatterjee
(U. Texas) began a visit to continue
work on temperature fluctuations with
the ECE radiometer.
Jesus Ramos attended the US-Japan workshop on
high-beta systems at GA.
He gave a talk on "MHD-stability and current-drive
studies for advanced
tokamaks".