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
Oct 23, 2000

Plasma operations continued at Alcator C-Mod last week. Three run days were
scheduled and completed. A total of 52 plasma shots were obtained, with a
start-up reliability of over 80%. Experiments included testing of the J-port
antenna in the four-strap configuration, high performance operation with all
three ICRF antennas operational, and SOL transport measurements in ohmic
L-mode plasmas.

Fresh boronizations were carried out on Monday, and again on Thursday
night. Average boron layer thicknesses of 1460 and 2050A were deposited in the
two sessions. The second round of boronization was done in an attempt to
further reduce the H/D ratio, which was still in the .05 to .1 range on
Wednesday. However, H/D was not much reduced on Friday, indicating the source
of the hydrogen is not being effectively covered by the boron layer. The
hydrogen levels have been somewhat elevated since the partial up-to-air
incident in late September. Startup reliability was relatively poor (under
60%) on the first run following the first boronization, which was followed by
extensive ECDC in Helium, but startup on Friday, with only 90 minutes of
post-boronization ECDC, exhibited high reliability.

No plasma operations are scheduled for this week, as most of the physics staff
is participating in the APS Division of Plasma Physics Meeting in
Quebec. Operations are planned to continue next week.

Physics
-------

One run day was devoted to completion of MP#267, to explore the dependence of
SOL fluctuation characteristics and attendant cross-field particle transport
on the collisionality of the SOL. This run consisted of a partial repetition
of an earlier experiment, during which the Lyman-alpha array was not
operating. The goal was to complete a density, current and B-field scan with
ASP and FSP probes reaching the separatrix and with the Lyman-alpha array
working. The run was quite successful, completing four density scans with four
combinations of current/field: (Ip=1.04, Bt=5.3), (Ip=0.53, Bt=5.3), (Ip=0.8,
Bt=4) and (Ip=0.8, Bt=6). In addition, CII and CIII plume images from the
'burping' FSP probe were obtained. From these results, we hope to infer the
magnitude of Er and perhaps its variation across the SOL under a variety of
collisionalities. Analysis of data from this set of discharges should allow
us to compute Deff profiles across the SOL for a variety of collisionalities
and to relate changes in particle transport to fluctuation characteristics.

One run day this week was devoted to exploitation of the J-port ICRF antenna
in the four-strap configuration, in combination with the D- and E-port dipole
antennas, to produce high-performance EDA H-mode discharges. Maximum RF power
was ~4.5 MW, but there were problems with keeping all the transmitters on for
the entire flattop. Density, triangularity, RF power, and
plasma current were varied.

ICRF Systems:
-------------

A separate day was devoted to investigating the power limits of the 4-strap
antenna. The antenna coupled power was brought up to 2.8 MW; however, the
upper corner adjacent to K-port developed a glow and injected impurities into
the plasma. Typically, the plasma would transition into H-mode, some time
later injections would begin, and the plasma would then revert to L-mode, with
most shots ending in an early disruption. The threshold for this phenomenon
was about 2 MW early in the run. In an attempt to eliminate these injections,
variations in outer gap (from 0.5 to 1.5 cm), a change in plasma current, and
a change in target plasma density were made. None of these changes eliminated
the glow or injections, and it appears that the effect is not strongly
dependent on the target plasma parameters. The j-port external feedline and
resonant loop has now been disassembled and arc damage and tracking has been
found in an elbow in the decoupler section. An arc at this location could
affect the relative phases of the straps and conceivably result in the effects
seen.


Review
--------

The Fall Alcator C-Mod Quarterly Review was held via video-conference on
Wednesday, October 18. The main topics presented were Machine Status and
plans; Engineering Activities; ICRF Results, focussing on the J-port antenna;
Lower Hybrid Project Status; and the DNB Status and plans. Participants
included:

>From DoE OFES site: R. Dagazian and C. Finfgeld

>From MIT PSFC site: S. Wolfe, J. Irby, S. Wukitch, R. Parker, E. Eisner,
M. Porkolab, and I. Hutchinson;

>From Univ. of Texas FRC site: R. Bravenec and W. Rowan

>From PPPL site: N. Bretz and N. Sauthoff