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
Oct.
2, 2000
Plasma operations continued on Alcator C-Mod last week. Five
run days were
scheduled and completed. A total of 84 plasma shots were
obtained, with a
startup reliability of approximately 80%. Experiments
carried out included
an investigation of main chamber recycling,
completion of the recombination
experiment using the divertor TV views,
scanning probe studies of the
quasi-coherent mode in low power (ohmic) EDA
H-modes, and further work on the
four-strap ICRF antenna.
After
the previous week's vacuum incident, a 110C bake followed by electron
cyclotron
discharge cleaning (ECDC) in D2 were carried out. Machine conditions
were
evaluated during the runs Monday and Tuesday; aside from a relatively
high
H/D ratio, initially over 20%, the vacuum conditions were
satisfactory. On
Tuesday evening a fresh boronization was carried out; about a
1600
Angstrom thick layer was deposited. H/D ratios on Wednesday were below
5%,
in the range for efficient minority heating, and EDA H-modes were readily
obtained
with both ohmic and ICRF heating.
Physics
------------
Experiments were
run to get additional data on edge fluctuations and profiles
in EDA
H-mode. The EDAs were more clear than in some previous OH runs with
Quasi-Coherent
fluctuations clearly seen on the PCI, reflectometer, and
electrostatic
probes. We have some good data for analysis of fluctuation
driven
flux. We should also have Ly_alpha data
to look at the ionization
source, neutral density and Deff. Some progress
was made on localizing the QC
mode.
As before, it is clearly seen in the steep density gradient
region.
Images of carbon plumes from the gas puffer in FSP showed a flow
reversal just
outside the region where the QC mode began. Since the flow reversal implies a
change
of sign of Er, this observation might be interpreted to localize the
mode
relative to the Er well. On several
shots it appeared that the north
floating probe of the ASP traversed the
entire mode. If so, the structure
would
be not much more than 2mm in width.
Plume studies were continued to
investigate the variation of plume structure
with injection depth during
EDA H-mode. Plumes are generated via gas-injection
of deuterated ethylene
(C2D4) through the F-bottom scanning probe. The plumes
are imaged from two
near-perpendicular locations (F-side, F-top) with coherent
fiber bundles,
which are optically coupled to gated, intensified CCD cameras
through
beam-splitter optics and bandpass interference filters. The optical
system
allows for imaging of C^+1 and C^+2 emission simultaneously at each
view
location. During the run, the injection depth was varied over a fine
scale
ranging from right at the separatrix to just a few mm outside. Some
interesting
results were obtained, including: (1) Plumes generated at the
separatrix
exhibit cross-field dispersion indicative of a radial electric
field
pointed inwards. Similar results were obtained for plumes generated at
this
depth during ELM-free H-mode; (2) Plumes generated ~ 2.5-3.5 mm outside
the
separatrix exhibit a shearing in cross-field dispersion (as seen from the
F-side
view), in which sections of the plume cross-field dispersion indicate a
radially
inward Er and other sections indicate a radially outward Er; (3)
Plumes
generated ~ 4 mm or beyond the separatrix exhibit cross-field
dispersion
consistent with Er directed radially outward.
These results
suggest a shear layer which exists for a thickness of
~ 4 mm. Measurements
from the FSP of the QC fluctuation associated with
EDA indicate that the mode
exists only within the region of negative
(radially-inward) Er. Analysis is
on-going.
ICRF
Systems
------------
Following reconfiguration of the feeds to
the J-port four-strap antenna, based
on an improved transmission line
model, the antenna performance was markedly
improved. For an upper x-point
L-mode target plasma, J-port heated as
effectively as the D and E-port
dipoles with no major impurity
injections. Comparison shots were taken at
a power level of 1.5MW. Additional
experiments to evaluate the maximum
power capability of the antenna in the
present configuration, and the
heating efficiency into H-mode targets, are
planned for this week.
DNB
Systems
-----------
We continued our investigation of the DNB
drift duct failure. Beam
reionization
calcuations for the full range of conditions in C-Mod, including
disruptive
discharges and fizzles, were completed.
Measurements of the
magnetic field in the vicinity of the damaged
area were completed.
Measurements of the beam profile were correlated with
the background gas
pressure in C-Mod.
The implications of failure of the diverting magnet to
turn on
during the pulse were investigated. The results of these
investigations
will be discussed in a meeting on Tuesday with the expectation
that a plan
can be finalized for repair and modification to prevent a
recurrence of
the failure.
Travel and Visitors
-------------------
Jim
Irby attended the first meeting of the Fusion Facilities Operations
Committee
at PPPL last week. He discussed machine
design, machine operations,
and safety issues with representative from
PPPL, GA, and DoE. A list of
experts
from each facility is being developed to help all sites with technical
problems.