Newsgroups: sci.physics.plasma
From WOLFE@CMOD2.PFC.MIT.EDU Tue Mar 14 11:07:15 1995
From: WOLFE@CMOD2.PFC.MIT.EDU
Organization: MIT
Subject: Alcator C-MOD Weekly Highlights

Alcator C-MOD Weekly Highlights
March 13,1995

Physics operation is continuing on Alcator C-MOD. Three runs were
scheduled and completed last week. The principal experiments were concerned
with measurements of 2D neutral density profiles, characterization of the SOL,
and fueling characteristics with different gas puff locations. A total of
nearly 100 successful plasmas were produced in the three days of operation.

Neutral density profiles were measured by the HIREX diagnostic using the
charge exchange recombination technique onto hydrogen-like argon (MP#049A).
At a line-density of 5e19m-2, a dramatic up-down asymmetry was observed. X-ray
emission from near the X-point was observed, from charge-exchange populated
levels, and from radiative recombination continuum. The data were of high
quality, and detailed profiles should be forthcoming once the analysis is
complete. The successful completion of this experiment is particularly
gratifying, as previous attempts had been plagued by a series of extraneous
difficulties, including power outages, vacuum problems, etc. By contrast, this
week's run went smoothly, with 36 consecutive good shots, establishing a new
record for one-day consistency.

The ongoing program in systematic characterization of scrape-off-layer
conditions (MP084) was continued on Wednesday. Data were obtained for
single-null
equilibria at two currents (0.8MA and 1.0MA) with density programmed to ramp
up during the shot. The fast scanning probe was inserted at three times during
the shot to get data at different densities. The program on SOL
characterizations for SNB ohmic discharges is now about half-completed.

The NINJA system was used to study the effects of localization of the fueling
on fueling efficiency and on divertor behavior (MP071B). Preliminary analysis
of the data from this run indicates:
(a) There is little or no difference in the effect of puffing in different
poloidal and toroidal locations on central plasma parameters or edge density
and temperature profiles.
(b) There is little or no difference in the effect of puffing in different
poloidal locations on edge neutral densities, but there is a large temporal
difference in toroidally specific neutral densities for different toroidal
puffing locations. (i.e. There is a much different time history of a neutral
gauge at B-port for a puff at B-port compared to a puff at F-port.)
(c) Impurity puffing (in the case of this run, of methane) shows up as highly
directional plumes emanating from the puffing locations, indicative of strong
flows at the edge. D2 fueling also usually gives rise to highly directional
plumes, implying that CX or elastic collisions may be causing significant
atomic flows of the injected deuterium.

During operation the week of February 27, the D-port icrf system arced on one
of the four feed lines in the 4" coax section. These arcs went undetected
because they did not show up as significantly increased reflected power in the
matched line between the transmitters and the antenna. We are developing an
arc detector based on the balance between the voltage probes in the two feeds
to each strap since the reflected power back in the matched line was not
affected much by these arcs and did not trip the transmitter. We are also
implementing an optical arc detection system which will look through one of
the 4" 45 degree bends and view both the one 4" Teflon insulator and the
vacuum feedthrough in each line. After cleanup of the arc residue, we will
evaluate the power handling capability this week.

Martin Greenwald attended the Transport Task Force Meeting in Los Angeles last
week, and this week will be participating in the ITER Expert Group meetings on
Confinement and Confinement Databases at the Joint Work Site in San Diego.