Newsgroups: sci.physics.plasma
From WOLFE@CMOD2.PFC.MIT.EDU Wed Jan 31 06:43:15 1996
From: WOLFE@CMOD2.PFC.MIT.EDU
Organization: MIT
Subject: Alcator C-MOD Weekly Highlights

Alcator C-MOD Weekly Highlights
Jan 29, 1996

Plasma operations continued on Alcator C-MOD last week. Boronization was
carried out for eight hours on Monday. Three run days were scheduled and
completed.

Exploration of non-standard ICRF heating schemes continued on Wednesday, with
studies of second harmonic He3 minority heating (3.4-4Tesla) and H-He3 mode
conversion heating (6.5-7.5Tesla). Previous attempts at second harmonic He3
heating had been unsuccessful, the problem being attributed to the presence of
the hydrogen fundamental resonance in the high-field side edge region. This
week's results were more positive. As a result of exploration with different He3
gas puffing and TF ramps, we found heating and obtained H-modes in the second
harmonic He3 regime. With boronization, the fundamental H resonance near the
edge does not appear to cause problems; rather, it appears to result in edge
heating and helps attain H-mode. Heating got progressively better at higher
fields (4->4.8T), presumably because the hydrogen resonance moves towards the
center. It was thought that removing the edge H fundamental resonance by
reducing the field (BT<3.4T) might result in better heating, but the 2nd
harmonic H resonance comes in on the low field side and caused problems
(increased impurity and reduced heating). Density and He3 concentration were
varied at a stationary BT of 4.2T. Lower density and lower He3 concentration
seemed to work better, somewhat contrary to expectations. This scenario will
probably work much better in combination with 40MHz (fundamental He3 minority
heating), which can greatly enhance 2nd harmonic He3 absorption at 80MHz.

Thursday's run continued our studies of H-modes with boronized walls using
H-minority heating at 5.3 Tesla. A density scan was carried out at a plasma
current of 1.1MA, corresponding to edge q just over 3. During this scan, the
fast scanning probe was used to observe the SOL. The best shots were
under similar conditions to the best shots of last week, with similar but
slightly reduced performance.

H-mode studies continued on Friday with a study of threshold conditions. This
run plan called for a controlled scan of density and current, with RF powers
stepped up gradually so as to find the global and local H-mode thresholds. The
focus was on obtaining well diagnosed conditions at the threshold, not
necessarily long or record-breaking H-modes. A density scan was performed at a
fixed 800kA equilibrium. A quick analysis confirms that Te at the 95% surface
is always close to 200 eV at the L-H transition, with density varying over a
factor of two. The behavior at the H-L threshold is less clear. An
interesting aspect of this scan was the behavior near the low-density limit.
As the limit is approached the power threshold climbs steeply. Shortly before
the L-H transition, the radiated power was observed to drop, and the edge
temperature rose to the typical threshold value. Additional cases at 1.1MA
indicated that the edge Te at the L-H transition was higher at the higher Ip.
At 800kA the Fast Scanning probe was used to obtain SOL profiles both in
H-mode and just before the L-H transition.

Additional boronization is scheduled for Monday night, and a full four-day run
scheduled is planned for this week.