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
Mar
29, 1999
The scheduled maintenance period on Alcator C-Mod continued
last week. No
plasma runs were scheduled.
Physics/Analysis:
-----------------
At
the weekly staff meeting, first results from the newly rebuilt A-port
horizontal
scanning probe were presented. The new
probe is virtually
identical in size and geometry to the F-port vertical
scanning probe. Both
probes are Langmuir-Mach probes, having four
eletrical elements arranged
around a pyramid structure. These probes can
be used to monitor parallel flows
as well as density and temperature
profiles up to the separatrix.
Initial analysis has focussed on
comparing the denstiy and temperature
profiles bwtween the F-port and
A-port probes. Only ohmic, L-mode discharges
has been considered so far.
The density and temperature profiles were found to
match very well. However,
there appears to be a flux surface mapping and/or
probe position error:
The density and temperature profiles overlay when the
F-port data is
shifted outwards in flux surface mapping, corresponding to 6 mm
in major
radius at the midplane. Possible reasons
for this discrepancy are
being investigated.
A new CCD array
detector has been installed on the tokamak to measure visible
continuum
emissivity profiles with sub-mm spatial resolution. In the spectral
region
covered by the transmission filters, the emission is dominated by
free-free
bremsstrahlung. The camera views the plasma tangentially, in the
midplane,
with major radius coverage 0.60m<R<0.93m. The camera has a 2048x1
pixel
detector, and laboratory calibrations of the optical system show that
chordal
resolution of 1 mm or better should be achieved along the entire length
of
chords passing tangentially near to the location of the steep density
gradient
seen in H-Mode. Depending on signal
strength, the camera is capable
of time resolution between 0.25 and 3.3
ms. The system is ready to take data
when tokamak operation resumes.
ICRF
Systems:
------------
The RF group in collaboration with PPPL
continued to work on the j-port
antenna transmission line and new
transmitters. The transmission line
is
installed from the transmitters in the power room to the antenna. We are
currently leakchecking the
line. Once it is leak tight, we will
pump it down
and outgas it for approximately 24 hours, at which point it
will be ready to
be pressurized and brought into operation. One of the phaseshifters is
undergoing
a repair after it was discovered that it did not respond to tuning
commands. This repair should be a one to two day
operation. Both j-port
transmitters
have been modified with new current limiting resistors on the
screen
supply and have been tested at over 2 MW into a dummy load for 1
second. We have also continued to upgrade both the
#1 and #2 transmitters
that feed the D and E-port antennas. A new feedback board has been
successfully
tested in the #1 transmitter and installation in #2 is
proceeding. We expect this new board to improve the
response of the RF system
to changes in plasma loading. Current limiting resistors are also
being
installed in the #1 and #2 screen supply. The new resistors should provide
higher immunity against
screen trips during operation.
Diagnostic Neutral Beam:
------------------------
Progress
on DNB systems remains satisfactory, with present status
approximately one
week behind baseline (2/11) schedule. All high voltage
systems up to the
modulator/regulator tube cage have been operated to full
performance. The
supressor grid supply has been operated and the
modulator/regulator for
the supressor grid is being tested at the present
time.
Visits
and Travel:
------------------
Joe Snipes, Paul Bonoli, Antonio
Bruno, Xavier Bonnin, Miklos Porkolab, Eric
Nelson-Melby, and Sergei
Krasheninnikov attended the APS Centennial Meeting
and the associated
Sherwood Theory Meeting, in Atlanta last week.
Ron Bravenec,
UT-FRC, began a one-week visit to install the BES fiber
optics and to
calibrate the BES system in preparation for test of his BES
system using
ambient plasma light and for benchmarking of his estimates of
the
background plasma light.