Newsgroups: sci.physics.plasma
From WOLFE@CMOD2.PFC.MIT.EDU Tue May 30 18:46:51 1995
From: WOLFE@CMOD2.PFC.MIT.EDU
Organization: MIT
Subject: Alcator C-MOD Weekly Highlights

Alcator C-MOD Weekly Highlights
May 30, 1995


Plasma operations continued on Alcator C-Mod last week. Four runs were
scheduled and completed. ICRF and edge plasma studies were the principal
topics. Over 120 plasma shots were obtained during the four run days.

H-mode operation with ICRF heating was explored on Tuesday. The outer gap was
adjusted dynamically to maintain loading during the L-H transition. Elongation
and triangularity were scanned to investigate the effect on H-mode
confinement. No improvement was observed with increasing triangularity, up to
about 0.6.

Wednesday and Thursday were devoted to alternate heating scenarios, using our
fixed frequency 80MHz transmitters. Second harmonic heating of He3 (in D) was
attempted, with the toroidal field between 3.6 and 4.0 Tesla. This scenario
was unsuccessful. The plasmas were apparently dirty, and had high radiated
power during the RF. This may have been associated with low single pass
absorption, and the presence of the hydrogen resonance near the plasma edge.
H fundamental minority heating in a He3 majority plasma was also attempted,
with fields between 5 and 5.8 Tesla; heating was observed, but there was still
substantial D in the plasma, so the experiment was not conclusive.

Successful second harmonic H minority heating (in D+ majority) was obtained at
2.6 Tesla. This is a standard heating scenario in JT-60U. The lower field was
obtained by ramping the field down following breakdown at 5 Tesla, so the
power deposition was moving during the heating pulse.

The objective of Friday's run was to inject impurities (CH4) at the inner wall
and at the divertor with the NINJA (capillary) system and follow the spatial
spreading with the CCD cameras and filters from B-side and B-top. Upper and
lower single null and inner wall limited discharges were compared. In
addition, impurity screening was measured in each condition. In the lower null
configuration, CII and CIII were observed with the B-side CCD
camera in alternate discharges: in both cases the flow was clearly seen going
towards the divertor. Density was scanned from 1.5 t0 3e20/m3. A similar
density range was studied with an upper null geometry (ion grad-B drift away
from the x-point). Again the plume directions indicated flow toward the
x-point. Clear evidence of toroidal flow was observed in positions which, with
the upper X-point , should be close to the stagnation point. Good fast
scanning probe (FSP) data were obtained for these discharges, which should
allow us to compare the flow estimated from the plumes with that estimated from
the ratio of the parallel and antiparallel Isat on the FSP. Next we studied
the injection of CH4 into inner-wall, limited discharges. As was to be
expected the impurities were ionized rapidly and the CII distribution was a
small (10-20 mm) diameter symmetric blob. The CIII appeared to spread all
round the inner column but was also roughly symmetric.

Dr. Kimura of the JT-60U ICRF Group was visiting C-MOD all week, and
participated in the ICRF runs.

Ian Hutchinson visited the ITER San Diego Co-center last week, and presented a
talk on recent C-MOD results of particular interest to ITER. He also visited
DIII-D.

Upcoming events: Operation will continue next week. Plans include first
operation at increased toroidal field (7.9 Tesla).