Alcator C-MOD Weekly Highlights 
			Dec 11, 1995

Operations continued on Alcator C-MOD last week, with four run days scheduled
and completed. A total of 125 plasma shots were obtained.

A new circuit was used to control the outer (plasma to antenna) gap to
maintain antenna loading during ICRF operation and avoid RF trips due to
changing loading, particularly at H-mode transitions. This technique was quite
successful, producing some of the best sustained H-modes observed on C-MOD to
date. While constant loading was not achieved, the feedback reduced the
decrease sufficiently to avoid RF trips during H-mode. Steady-state H-modes
lasting through the entire rf pulse with very low level (grassy?) elms were
produced in this manner.

Further exploration of ICRF H-mode performance was carried out on Wednesday.
The position of the x-point and target strike point were varied. The outer gap
was varied somewhat by programming a higher target value for loading on
position feedback. A plasma current scan from 0.6 to 1.05MA was carried out.
Quasi-steady ELMing H-modes with densities up to nebar~4e20/m3 were produced. 

Thursday's run was devoted to ohmic confinement studies, with the aim of
completing the low-density dataset. Scans of density from 0.4 to 1.5e20 at
fixed plasma current for two values of elongation (1.66 and 1.3) were carried
out, followed by a scan of kappa and Ip at constant q95 and density. All of
these discharges were limited on the inner wall. Analysis of this data is in
progress. 

A comparison of screening for recycling (argon) and non-recycling (nitrogen)
impurities in limiter discharges was carried out on Friday. The
same discharges were used to study SOL parameters (ne, Te, and flow profiles)
in limited plasmas. Impurity puffing from poloidal positions, (inner wall,
lower divertor area, outer midplane) was done to determine relative screening
efficiency dependence on source location. Screening was found to be a factor
of two better for impurities injected at the outside and bottom than
for inner wall source. Comparison with standard divertor discharges indicated
impurity screening was better by a factor of eight to ten in the divertor case
than for the limiter configuration. The fast scanning probe was used to obtain
profile of SOL parameters in these discharges. There was an interesting
contrast between the limiter and divertor SOL profiles.  T_e in the SOL was
~30 eV in limiter discharges, compared to ~50eV in diverted discharges.  n_e
was also lower in limited discharges.  These differences would account, at
least in part, for the lower screening in limiter discharges.

The concrete pad for the switchgear associated with the new RF systems has
been poured. Installation of this equipment will take place next week. 

All extra sensors and interlocks required for unattended discharge cleaning
have been installed and tested. Auto-dialout alarm notification to alert
off-site personnel of interruptions in this operation has also been
implemented. Unattended ECDC will now be routinely carried out during
off-hours. 

Prof. Peter Stangeby and PhD graduate student Steve Lisgo from the University
of Toronto  met with the C-Mod edge group for two days last week to discuss
DIVIMP modelling of impurity  transport in C-Mod plasmas. Steve Lisgo will be
basing much of his thesis work on impurity  modelling in C-Mod, including the
modelling of impurity emission 'plumes' from capillary gas injection
experiments. His results on impurity 'plumes' with DIVIMP will be directly
compared to those obtained from a fluid model developed at MIT by Dave
Jablonski (Alcator PhD student).