Newsgroups: sci.physics.plasma
From WOLFE@CMOD2.PFC.MIT.EDU Mon Jan 30 18:02:25 1995
From: WOLFE@CMOD2.PFC.MIT.EDU
Organization: MIT
Subject: Alcator C-MOD Weekly Highlights

Alcator C-MOD Weekly Highlights
Jan. 30, 1995

Alcator C-MOD operation continued this week. Four run days were carried out as
scheduled. The principal experiments included ICRF heating, high elongation
equilibria, and divertor detachment studies with ICRF and different
geometries.

The effects of increasing RF power on divertor parameters, including detachment
characteristics, were studied in a series of discharges on Wednesday. The
standard lower SN configuration with the strikepoints on the inclined
divertor plates was used. RF power was varied during the pulse at constant
core density and plasma current; several target densities were explored.
Increases in divertor density and temperature, as well as divertor radiation,
were observed, with the divertor density being affected more than Te. The
divertor radiation shifted from near or above the x-point down to the vicinity
of the strikepoint as the power was increased. For discharges which were
initially completely detached, a low initial RF power level was sufficient to
reattach most of the common flux region; however, the separatrix and private
flux regions remained detached up to the highest powers used.

Experiments to optimize the gain settings on the vertical position control for
high elongation equilibria were carried out on Thursday. A systematic mapping of
the proportional and derivative gain space was performed for a constant
equilibrium configuration with elongation around 1.75, with amplitude and
frequency of the observed vertical oscillations being tabulated, as well as
disruptivity. These results are being compared with modeling predictions based
on the perturbational equilibrium approach. Using optimized gain settings
based on these experiments, we then increased the elongation up to 1.85, and
maintained this configuration for the entire flattop without disruption. This
marked the first time we had exceeded our nominal design elongation of 1.8 in
a sustained manner.

Experiments on the effect of divertor geometry on detachment were continued on
Friday. The "flat-plate" geometry, with the strikepoints located on the top of
the divertor structures, proved to be significantly harder to detach than
either the standard or "slot" configurations. The line-averaged density was
raised to 3.4e20 in 800kA ohmic discharges with the "flat-plate" geometry, but
complete detachment was not obtained except with impurity injection. The
density at which local detachment was obtained (2.9e20) was clearly higher
than that for comparable inclined plate (2.1e20) and slot (1.7e20)
configurations.

The multi-pulse NdYAG core Thomson Scattering system observed scattered
photons for the first time this week.

Earl Marmar participated in a meeting of the DIII-D program advisory committee
in San Diego. While there, he also presented a talk to the DoE-JAERI Technical
Planning Meeting, summarizing the C-Mod program (results and plans), and one
to the TPX physics meeting, concentrating on C-Mod divertor results.
Prof. Ian Hutchinson presented a seminar on recent C-MOD results at PPPL.