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
April 12, 1999

Maintenance continued at Alcator C-Mod last week, with the primary effort
being on the ICRF system. No plasma runs were carried out.

Physics and Analysis:
-----------------------

The absolute density calibration has been applied to the core Thomson
scattering measurements and core plasma density profiles have been obtained
for L-mode, various types of H-modes, L-H and H-L transitions. Hollow density
profiles were observed at L-H transition with edge density growing faster than
the core density, and sharp peaking of the profile was observed at the
collapse of the H-mode. The results from the edge TS measurements are also
being analyzed. The edge electron temperature profiles in the vicinity of the
separatrix were obtained with reasonably small error bars in L-mode. However,
in H-mode, due to the sharp increase in temperature above the range for which
the spectrometer was optimized, the recorded temperature profiles are noisy
and only qualitative comparison is possible with the results of the ECE
measurements. The density profiles are less noisy in all cases, and the
formation of a sharp density pedestal is clearly observed at H-mode.

Analysis has begun of the data collected from the Omegatron probe. The
Omegatron probe combines a gridded energy analyzer and an ion mass
spectrometer. Configuring the probe to operate as a gridded energy analyzer,
upper divertor bulk ion temperatures of approximately 6 eV were measured.
Configuring the probe as an ion mass spectrometer, resonant currents were
collected using M/Z=2 (deuterium majority species) to benchmark. Attempts to
measure impurity spectra will be made during the next run. A technique is
under development to measure the majority ion species temperature by
configuring the probe as a hybrid gridded energy analyzer and ion mass
spectrometer, using the ratio of resonant and non-resonant ion currents; the
technique would require fewer data points to determine a temperature than does
the gridded energy analyzer method.


ICRF Systems:
-------------

During the last run an arc occurred in the transmission line of the
FMIT#2/E-port antenna system. Since shutting down to repair this arc, a
separate problem has arisen in the FMIT#1/D-port antenna system. Testing
FMIT#1 into dummy load or into vacuum resulted in an arc detection fault.
This problem was originally noticed only into the dummy load with powers greater
than 1.5 MW, but the power level at which the faults occurs deteriorated to
about 5 kW. Upon disassembly, no evidence of damage was found in the
input or output cavities and the tube high potted satisfactorily. Disassembly
of the driver output cavity and coax between driver output and FPA input
revealed an arc track on the coax teflon insulator.

In the course of diagnosing FMIT#1, an additional problem was found
with the instrumentation on all four transmitters. The time response on some
of the camac instrumentation (including FPA and Driver anode, grid, and screen
voltages & currents and IPA anode voltage and current) was affected by the
inductance introduced by a set of meters used for manual tuning and redundant
monitoring. Equivalent resistors have been installed to remove the
meters from the circuit, and an extensive testing and recalibration process
has begun.

The effort on J-port last week was limited to low power measurements and
tests. About 400 W was injected into the antenna under vacuum conditions to
check diagnostic channels. A problem with the time base between transmitter
and antenna diagnostics was noted. Measurements indicate that the decoupling
stub length may be incorrect. Further analysis and testing will be done to
investigate decoupling.

Travel and Visitors
-----------------

David Winslow of UT-FRC visited for the week to work with Brian Labombard.
David set up and tested fast sampling data acquisition for the tile probes.
The goal is to make plasma turbulence measurements.

Nobuyuki Asakura (JAERI) visited us last week, for the whole week. He gave a
talk on "Core confinement and W-shaped divertor experiments on JT60-U".
Dr. Asakura spent most of the week in discussions with Dr. LaBombard, and
spent considerable time reviewing the C-Mod results with the divertor bypass
valve and impurity injection experiments. Our experience with impurity
injection during H-modes is different than his: in C-Mod, confinement
decreases with impurity injection, while in JT60-U a a slight increase in
confinement was observed with injection of Ar near the Greenwald density
limit. We discussed the effect on pedestal profiles for the two experiments.

Dr. Ricky Maqueda, our collaborator from LANL, was on-site the week of March
29 - April 2. He finished bringing two diagnostics fully on-line. One
diagnostic was the fast-framing visible camera (frame rate=1 khz, gateable to
10 ns). This will be used to look at "snapshots" of striations (filaments) in
visible light. The other diagnostic is the IR camera, imaging the inside of
the vessel from above in 4.24-4.42 micron light. The view is made somewhat
tangential by using a mirror mounted within the vacuum vessel. As such, the
outer strike point (beneath the nose of the outer divertor) is imaged. The
observed image is close to what was expected.