Newsgroups: sci.physics.plasma
From IRBY@CMOD2.PFC.MIT.EDU Tue Apr 9 17:02:17 1996
From: IRBY@CMOD2.PFC.MIT.EDU
Organization: MIT
Subject: Alcator C-MOD Weekly Highlights

Alcator C-MOD Weekly Highlights
April 9, 1996


In-vessel work continues. A careful vacuuming of the vessel
was done to collect dust as small as 2u in diameter. This is part
of an ITER support activity in which the size distribution and makeup of
dust produced during operation is quantified. Several outer divertor cover
plates have been removed and sent out for analysis along with wipes of the
inner and outer wall. This analysis should measure the amount of boron
hydrides still remaining relative to the breakdown products (mainly
boric acid). A final cleanup of the vessel will now be done with
distilled water wipes of the vessel walls. We would like to acknowledge
all the help we received over the past month in dealing with the
boron hydride deposits, particularly from Frederick Hawthorne (UCLA),
Gary Jackson (GA), Joerg Winter and Guenther Esser (Juelich), and
Richard Pitts (Lausanne).
In preparation for installation of the prototype divertor cryopump
on the vacuum test stand, instrumentation is being debugged and interfaced to
the data acquisition system, and the pump is being degreased and cleaned.
The vacuum test stand will be used for leak, cooldown, cycle, and pumping
speed tests of the cryopump. All vacuum hardware needed to install and
instrument the pump is now in house and ready for installation.
The motor starters for the #3 and #4 RF transmitter water cooling
units are being installed. Design of the control cables out to the
Hi-Yard is complete, and installation is about to begin. PLC control
cables are being fabricated. Work continues on the crowbar cabinets with
the fabrication of ignitron support hardware.
The alternator inspection is now complete and we are awaiting
a complete report from GE. Work has already begun on design changes
to the lower flywheel bearing support.
The bus tunnel is now being opened up so that the LN2 manifold
can be modified and the new valves needed to improve TF cooling installed.
This change will reduce between shot cooldown time considerably.
Work on the diagnostic stand extension continues. Most major
support hardware is now in place on the west wall, and the decking is
being prepared for installation. Air and water service at some locations
has been relocated to accommodate the extension.
As part of the analysis of results from the last campaign, we have been
studying the local conditions near the plasma edge at the L-H transition.
A dedicated run in January, in which power and density were systematically
scanned, showed that the transition always occurred at about the same edge Te,
as measured by ECE. Data from several other diagnostics are being examined to
try to understand the physics behind the observed low density limit for H-mode
on Alcator C-Mod. Some results from this analysis were presented at the
most recent TTF meeting, and an abstract has been submitted for the
IAEA meeting.
Up until June of 1995 the compression of gas into the divertor
(P_div/P_midplane) in Alcator C-Mod was typically in the range 50-70 with
values as high as 150. It is desired to maximize this ratio in order to
minimize the midplane pressure (possibly better for core confinement) and
also maximize the divertor pressure for pumping purposes. The thesis work
of Artur Niemczewski modelling neutral transport in the divertor region in
the presence of a plasma showed that there were gaps around the edges of
the outer divertor which, if plugged, could result in higher compression
ratios. This gap was plugged before the winter run period with the
result that compression ratios are now typically in the range 100-200 with
values often over 300. This increase is mainly due to an increase in the
divertor pressure for a given line-averaged density with a small decrease
in the midplane pressure.
We are investigating the effect of this change on other divertor
and core parameters as well. At the moment there does not appear to be any
change in core confinement. Screening of recycling gases (Ar, Ne) from the
core appears to be better. This may also be true for non-recycling gases.
Mike Mauel (Columbia) visited last week and gave a seminar on a
reactor concept using a levitated dipole to produce the confinement field.