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
May
15, 2000
Operations
-----------
Plasma Operations
continued at Alcator C-Mod last week. Four run days were
scheduled and
completed. A total of 79 plasma shots were produced, with a
startup
reliability of 90%.
Last week was the first official week of the
physics phase of the current
campaign. Runs were dedicated to experiments
in Core Transport, Impurity
Control, and Neutral Effects. One run was also
devoted to continuing
development and commissioning of the ICRF system.
A fresh boronization is being performed on Monday. Physics operations are
scheduled to
continue this week.
Physics and Diagnostics
-----------------------
One
run day last week was dedicated to a determination of the profiles of the
impurity
diffusion coefficient and convection velocity during P-mode, induced
by
lithium pellet injection (MP#256). Complete radial brightness profiles
for
He-like argon lines were obtained. Density and temperature profiles
were
obtained from Thomson scattering.
The radial scan was done at 1 MA plasma
current, with the prospect
of moving the location of the P-mode peaking out in
radius. However, the
peaking occured inside of 6.5 cm in minor radius, the
same location that
was seen for 0.8 MA previously. Most
shots had 2 pellets,
for a comparison of Ohmic P-mode with ICRF PEP
mode. This should allow for a
complete
characterization of the impurity transport coefficient profiles in
conjunction
with modeling from MIST.
The effects of main chamber recycling on
H-mode threshold and confinement were
investigated (MP #262) in a
systematic scan of the inner gap, with the outer
gap held constant at a
relatively large value of approximately 20mm. The gap
to the inner wall
was varied from 0 (marginally limited) to over 25mm. Plasma
current,
target density, and overall plasma shape were held constant. The
dominant
x-point was at the bottom, and the upper, secondary x-point was
outside
the vessel. During the scan from 26mm to 3mm we observed no change in
divertor
gas pressure, outboard midplane gas pressure, divertor radiation, or
Zeff.
However, the gas pressure in the upper "divertor" region changed by
a
factor of about 5 and the H-mode power threshold increased by ~40%; no
H-mode
was obtained for the marginally-limited cases. Detailed analysis of
these data
is in progress.
The purpose of Friday's run (MP#225)
was to isolate the relative contribution
of Mo sources from the inner
wall, outer limiter/antennas, and divertor to the
core Mo levels. A new
feature of these experiments was the use of emissive
probes to directly
measure the plasma potential during RF operation. the
potentials measured
by the probes (one on each side of the AB limiter) were
found to correlate
with operation of the antennas connected to them by field
lines. In the
case of the A side of the limiter this means the J4 antenna, for
the
B-side of the limiter this means the D-antenna. Potentials in the range
of
50-200V were observed whan the corresponding antenna was energized.
The
measured potentials verify that sheath rectification does occur, and
that the
potentials are substantial.
The results on the comparison of antenna sources
indicate that the
Mo source rate and effect on the core by the J antennas are
considerably
reduced from last run period. However, the source rate from the J
antennas
and the effect on the core (n_mo) were typically a factor of 2-3
larger than
with the D-antenna for equivalent launched power.
Results from the
optical diagnostic investigating fluctuations in the C-Mod
SOL show no
obvious differences in the SOL fluctuation characteristics during
EDA
H-mode and L-mode plasmas. Frequency spectra at radial locations rho=4,
8,
and 11 mm are similar, as are the autocorrelation functions of the
intensity
fluctuations. In addition, radial profiles of intensity
fluctuations have been
measured using this diagnostic by scanning the
separatrix dynamically during
ohmic, L-mode discharges. Inside of rho=20
mm the fluctuation level is 10 to
20%, rising slightly at the smaller
rho's. Outside of rho=20 mm, it increases
rapidly to ~60%. Since the
dependence of the line intensity has a
less-than-linear dependence upon
n_e, this intensity fluctuation profile
implies that the normalized
density fluctuation profile decreases (with
increasing rho) from 40-80% at
rho's < 10 mm, is ~20-30% for 10<rho<20 mm, and
then increases
sharply beyond rho>20 mm. These
observations show an opposite
trend from the results in which an
"effective diffusion coeficient" is derived
from determinations
of the SOL particle fluxes, gradients, and source
functions. The trend in
D_eff is one that decreases to quite low values as rho
decreases from 10
mm to 0mm (the separatrix).
ICRF Systems
------------
The
ICRF system was run in support of physics experiments this week, as noted
above.
D-port and J-port antennas were used to provide up to 1.5MW and 2.5MW,
respectively.
An arc in an elbow located below the phase shifter in the
E-port external
transmission line prevented use of the E-port antenna this
week. This
component has now been replaced.
DNB Systems
------------
The
DNB operated into C-Mod plasmas 4 days last week, although two days were
cut
short due to HV overheating problems. A
perveance scan at 39kV was
completed, voltage regulation of filament
supplies was tested with and without
CMOD shots, and a boron CXRS line at
7619A was checked. The BES signals
were
compared for low voltage (34kV & 32kV), close-to-perveance beams
and
high-voltage (45kV), far-from-perveance but higher current beams. Data
analysis is ongoing.
Instrumentation
was set up to monitor substation voltage at the alternator
exciter supply
input. A survey of available power
sources for the new DNB
service was made and plans for both boosting the
filament voltage and adding
different service are being made.
Travel,
Visitors, etc.
------------------------
The regular Quarterly
Review of the Alcator C-Mod Program was held on 11 May,
by video
conferencing. Bill Rowan (U. Texas) presented a Status Report on the
Diagnostic
Neutral Beam; Jim Irby reported on other aspects of the Machine and
Facility
Status; Ron Parker and Montgomery Grimes presented a Lower Hybrid
Project
Status Report; Amanda Hubbard and Catherine Fiore summarized some
Recent
Physics Results; and Steve Wolfe presented plans for the Physics
Schedule
for the current campaign. Rostom
Dagazian, Warren Marton, Curt
Bolton, and Erol Oktay participated from the
Office of Fusion Energy Sciences.
Martin Greenwald spent two days at
ORNL participating in a review of the
fueling and heating technology
programs.
Ron Bravenec (UTx) was at C-Mod last week primarily to
learn how to operate
the DNB. This
is in keeping with the commmittment of the Texas group to take
on primary
responsibility for routine operation of the beam. BES signals from
the beam emission were also measured and
found to be small. This result is
believed
to be due to two principal reasons: i) the deuterium beam does not
provide
enough Doppler shift to separate the beam emission from the background
D-alpha
emission, and ii) the beam is presently off perveance and therefore
not
well focussed.
David Winslow (UTx) installed the probe head on the
UT Turbulence Probe and
started baking the line. A small leak was discovered in a vent valve which
will be
repaired.