Newsgroups: sci.physics.plasma
From WOLFE@CMOD2.PFC.MIT.EDU Tue Jan 3 17:29:59 1995
From: WOLFE@CMOD2.PFC.MIT.EDU
Organization: MIT
Subject: Alcator C-MOD Weekly Highlights

Alcator C-MOD Weekly Highlights
January 3, 1995

This report covers the weeks of December 19, 1994 and December 26, 1994.
Four runs, covering four experiments (Miniproposals), were scheduled for the
week of Dec. 19, including one experiment which had been postponed from the
previous week. All of these were successfully carried out. The week of
December 26 was a scheduled maintenance week. C-MOD operations will resume
this week.

One run was devoted to a study of the effect of the outer gap on RF loading
and divertor characteristics, with the goal of establishing the optimum
configuration for ICRF-heated divertor experiments. This experiment, which
included a scan of outer gap at fixed density, a power scan at fixed gap, and
an RF power scan at the same gap, was successfully completed. The loading
resistance varied as predicted, increasing from about 5 to 10 ohms as the gap
was scanned from 2.1cm to 0.5cm. Gross parameters in the outer divertor leg
were relatively insensitive to the gap, while some variation was noted on the
inner divertor. This data is being analyzed in more detail. Divertor
detachment was obtained for high density ( >2.e20/m3).

Development of a slot divertor equilibrium for ITER-related divertor studies
was accomplished. The outer divertor leg was moved to miss the outer plate and
terminate on the floor tiles during much of the flattop portion of the shot.
At higher densities there were indications of divertor detachment in this
configuration. The method for achieving this configuration was to feed back on
the strikepoint, allowing the x-point location to move somewhat.

An exploration of the ohmic H-mode threshold behavior was carried out. Both
high and low density thresholds with Ip=0.8MA and Bt=5.3T were determined;
these turn out to be around nebar = 1.2e20/m3 and 8e19/m3 respectively. The
upper threshold is consistent with a power threshold of P/S = 0.2 nebar Bt,
similar to what we had reported previously and about half the ASDEX-UG/DIII-D
value. The low density limit appears to be a real effect, and the value was
quite reproducible; the reason for this effect is unclear. Operation at lower
current (500 - 650kA) at the low density also showed no evidence of H-mode.
The addition of lithium pellets did not appear to significantly affect the
H-mode threshold or quality at low, intermediate, or high density.

The last run of the week (and of the calendar year) was dedicated to
conditioning and tuning of the E-port ICRF antenna. Good progress was made,
with the power coupled to the plasma from this antenna being raised to over
1MW. The D-port antenna, which was conditioned earlier, is routinely running
at 1.7-1.8MW. The total net power from the two antennas has been increased to
2.7 MW into the plasma.

The week of Dec 26 was scheduled as a maintenance week. A short vacuum break
was carried out on Tuesday, during which the cryo-pump was re-installed with
a new gate valve. In addition, a new periscope, x-ray window, and capacitance
pressure gauge were installed. The break was completed in one day, and bakeout
and discharge cleaning (ECDC) were carried out over the remainder of the week.

An upgrade to the EF4 power supply has been completed, doubling the available
voltage on this coil set during the plasma phase. This should permit higher
EF4 currents early in the flattop, which in turn will provide additional
flexibility in the discharge shaping.

An Argon purge system for the secondary seals on the horizontal ports was
installed and is now operational.

A new stairway from the loading platform to the diagnostic stand was
installed.

A new wide-angle plasma TV view has been installed in A-port.