From: WOLFE@PSFC.MIT.EDU
Reply-To: WOLFE@PSFC.MIT.EDU
Subject: Alcator C-Mod Weekly Highlights
Newsgroups: sci.physics.plasma
Organization: MIT


Alcator C-Mod Weekly Highlights
Nov. 8, 1999

Plasma operations continued at Alcator C-Mod last week. Four run-days were
scheduled, with both toroidal field and plasma current reversed from their
normal directions. A fresh boronization was carried out on Thursday night,
with an average boron deposition layer of 2000A. A total of 37 plasma shots
were produced this week; startup reliability was only ~40%, due to difficulty
in tuning the null with reversed field early in the week and post-boronization
wall conditioning on Friday.

Tuesday's run was devoted to power system checkouts and establishing
reverse-field and current operation. An attempt to study H-mode pedestals and
impurity transport with the ion grad-B drift away from the divertor was
unsuccessful, due to a fault in one of the ICRF transmitters. The remainder of
Wednesday's run was turned over to SOL and edge flow studies. CIII plumes
indicated that the ExB flows in the scrape-off layer reversed, as
expected. Similar to the FSP probe, the ASP recorded parallel flows in the
opposite direction to that obtained in normal Bt discharges. SOL particle
transport analysis and fluctuation analysis needs to be done on these data.
We also did some basic flapper tests during Wednesday's run. Individual flaps
were activated while H-alpha increases were monitored on a reticon array and
on the C-side H-alpha survey view. These data are being used to estimate the
flow rate through the flapper.

On Thursday, we ran upper single null plasmas with reverse-field, so that the
ion-grad-B drift was again toward the x-point. H-modes were obtained with
about 1.3MW of ICRF power. Triangularity scans of the upper x-point from 0.6
to 0.36 were performed. The H-modes were impurity dominated, and ELM-free
with rapid accumulation. It is not clear why, although the machine had not
been boronized for a while. There are signs that the trends on the x-ray
pedestal widths, wider at higher triangularity, are confirmed in the absence
of significant changes in the divertor baffling (because of being diverted at
the top). This will need further analysis, but indications are that we will
find that it really is plasma shape, not divertor effects, which influences
the pedestal width. Plasma rotation measurements verify that the directions
of the flows have all reversed. Useful data was also obtained on the top x-ray
array.

Friday's run was intended to continue H-mode studies with unfavorable grad-B
drift direction, following boronization. However, startup reliability was
poor, possibly because of inadequate helium ECDC following the boronization,
and no H-mode discharges were obtained.

On Saturday the C-Mod bus was reconfigured for standard field operation.
Operation on Monday, 11/8, will begin with power tests in the morning,
immediately followed by plasma operation with the j-port antenna.

Physics and Analysis
-------------------

Based on analysis of a systematic comparison of the 4 different antennas,
D-port, E-port, J3 and J4, it has been found that much more impurities enter
the plasma when J3 and J4 antennas are energized than for the same power from
the D or E antennas. The difference is roughly a factor of 10 for Mo. In
addition, there is a large amount of titanium coming from the J antenna,
correlating with the Mo. Since the tiles on those antennas are TiC coated Mo,
this may indicate that the tiles are the source of impurities, as opposed to
the Faraday screen.

ICRF System
------------

An arc in the input cavity of the high power stage of ICRF transmitter #2
(E-port) reduced the available rf power last week. Repair of this transmitter
will take several days, and has been postponed until after the end of this
campaign. This week's runs will concentrate on further conditioning and
studies of the J-port ICRF antenna.

Travel and Visitors
-------------------

Gary Taylor visited last week from PPPL, and worked on TORIC simulations
and analysis of RF heating.

Prof. Ken Gentle (U Tx) was at MIT to present a seminar on "Cold Pulses and
Other Constraints on Critical-Gradient Transport Models", which included
results from his experiments on C-Mod last summer. He also consulted with John
Rice and Catherine Fiore about possible future C-Mod experiments on this
topic.

Ron Bravenec was onsite Tues.-Thurs. to retune the BES spectrometer to
D-alpha (it had been tuned to bremsstrahlung) and work on the BES views. We
are continuing to look at edge D-alpha fluctuations.