Newsgroups: sci.physics.plasma
From WOLFE@CMOD2.PFC.MIT.EDU Tue May 23 08:20:40 1995
From: WOLFE@CMOD2.PFC.MIT.EDU
Organization: MIT
Subject: Alcator C-MOD Weekly Highlights

Alcator C-MOD Weekly Highlights
May 22, 1995

Plasma operations continued on Alcator C-MOD last week. Four run days were
scheduled, but only two were successfully completed, as detailed below.
Principal experiments were concerned with current profile diagnostics,
employing two different techniques, and impurity screening. The two
experiments which were not completed, on kappa-dependence of confinement and
q-dependence of SOL characteristics, will be re-scheduled.

Early in the run on Tuesday, it was found that a piece of the Thomson
scattering viewing dump had become dislocated from the inner wall at G-port,
and was lying across the TCI interferometer beam at H-port. The run was
halted, and a clean vent (helium backfill) was performed to remove the piece.
The system was back under vacuum by 15:30, and discharge cleaning (ECDC) was
performed overnight. Plasma operation resumed on Wednesday, and the wall
conditioning did not seem to be significantly degraded; in fact, several ohmic
H-mode transitions were observed, indicating a reasonably clean machine. The
Thomson scattering diagnostic was back in operation, with some enhancement in
stray light levels on one channel of one spectrometer view. However, not much
of the run originally scheduled for Wednesday was in fact accomplished, owing
to instrumentation problems.

Normal operation resumed on Thursday, with a run devoted to testing of the
q-profile diagnostic using Zeeman polarimetry in conjunction with the
Li-pellet injector. This technique, pioneered on Alcator C, and employed on
TFTR in collaboration with an MIT team, has some advantages with respect to
the ablation cloud imaging method employed earlier. In these experiments, both
methods were used on the same shots. Data was obtained at several currents,
and several shots employed two pellets timed close together to ascertain the
effect of the pellet itself on the current density profile.

On Friday, half the run was devoted to the first live test of the ORNL Faraday
rotation diagnostic, installed by C.H. Ma, who served as co-Session Leader for
this portion of the run. For these tests, channel 7 of the existing ten-
channel TCI interferometer system was used. The rotation is measured by
modulating the CO2 probe beam polarization at 70kHz by a few degrees and then
measuring the phase of the detected signal with a lock-in amp. The predicted
rotation for the experimental conditions was of the order of 0.1 degree.
Significant rotation signals of this order were observed, with good signal to
noise at the higher density, but the scaling with current and density were not
quite as expected. Possible instrumentation problems are being investigated.

The other half of Friday's run was concerned with a comparison of impurity
screening in limiter and divertor discharges. These experiments represented
the completion of MP 091, begun earlier in this run period. Argon screening
was studied at high density. In addition, an outer gap scan and divertor
strikepoint scan was carried out for medium density plasmas. Preliminary
analysis indicates that for high density, nebar=2.8e20/m3, the percentage of
argon penetrating to the core for a limited plasma is roughly 1.5 times that
for a diverted plasma. Argon screening shows no dependance on outer gap or
strikepoint location.

Dr Charles Skinner visited the PFC on 15 - 17 May to install a Fabry Perot
spectrometer on C-Mod as part of our collaborative agreement with PPPL. The
objective of this diagnostic is to measure optical line widths with high
resolution and thus determine velocity distributions and ion temperatures.
The installation was successful and preliminary data were obtained. The
measured Halpha linewidth (for chords viewing down from the top of vacuum
vessel)indicate a mean atom velocity of 1.4e4 m/s. Optimization of the
system will be continued for the rest of the present campaign and a more
comprehensive system will be set up in the fall.

Ian Hutchinson and Brian LaBombard participated in the IEA Workshop on Edge
Plasma and Divertor Physics at Garching this week. Results from C-MOD
experiments in these areas were presented.

Miklos Porkolab, Yuichi Takase, Steve Golovato, and Paul Bonoli presented
papers on C-MOD RF results at the Topical Conference on RF Power in Plasmas at
Palm Springs.