Alcator C-Mod Weekly Highlights
				Sep. 3, 1996

Operation continued on Alcator C-Mod last week. A total of four run-days were
scheduled and completed.  Two boronizations were carried out, on Monday and
Thursday night, with a total of about 2300 A being deposited. Experiments
included H-mode confinement scaling and a study of RF edge effects. Up to
2.5MW of RF power was launched through two antennas.

RF edge experiments to study parametric decay instabilities and energetic
particle generation were carried out using the charge-exchange neutral
particle analyzer and a new fast-scanning RF probe. The purpose was to study
the poloidal and pitch angle structure of the RF edge heating observed with 
the CX analyzer, and explore the spatial structure of the attendant RF
waves in the scrape-off layer. These experiments were in support of two
graduate student theses. A series of identical shots were carried out at
a field of 4.72T, with scans of the sightline for the neutral particle
analyzer and the RF probes; the RF power was stepped so data could be obtained
at 8 different power levels. A second sequence of shots was then carried out
with the field ramped from 4.8 to 3.8T at constant RF powers of 100kW and
250kW, with the charge-exchange sightlines again being varied
shot-to-shot. All of the RF probes were successfully scanned, including the
new fast-scanning probe, and the A-top and J-side probe sets. Finally, a
series of lower density shots were run, with ramped and fixed TF. 

In addition to the RF edge studies, this run was also used to continue
conditioning of the RF systems. In order to accommodate multiple experiments,
and to increase the data available from each shot, the pulse length was
increased to 2 seconds. On the final sequence, h-modes were obtained at the
lowest density so far in C-Mod, about 6.5e19/m3; the threshold was observed to
be about a factor of three higher than normal. 

H-mode confinement scaling studies were carried out on Friday, following an
overnight boronization. Both ELM-free and ELMy h-modes were obtained. In
addition to energy confinement, the impurity particle confinement times were
also determined using laser blow-off technique, with Nb, Sc, and Zr
injected. Data was obtained at different powers, up to about 3.5MW (total
power) at a current of 1MA. H-factors were found to be around 1.6, relative to
ITER89-P, which is intermediate between our previous unboronized and
fully boronized results.  Additional boronization is scheduled before the
first run next week.

A new RGA (residual gas analyzer) was brought online last week.  It is
designed to allow measurement of neutral impurity species levels in the
divertor as a function of time during shots.  With this measurement we are
able to characterize the compression of impurities into the divertor.  This is
of interest because it determines the divertor pumping capability for those
species, the efficacy of the divertor in collecting those species, and is also
relevant to the screening of impurities from the core plasma.  Each mass
sample time is approximately 17.5 ms, which allows us to monitor up to 4
masses with an overall sample time of 70 ms.  The time delay due to
conductance from the divertor to the RGA is of order 100-150 ms.

A preliminary assessment of the data from the new in-vessel strain gauges,
which were installed prior to the current campaign, has been completed. There
are two of these solid-state gauges separated vertically by about 5 cm, at one
toroidal location, behind the inner divertor hardware, oriented to measure
toroidal strains in the inner wall arising from halo currents during
disruptions. Data from approximately 100 disruptions during the past month
were found to exhibit an approximately linear correlation between the peak
measured strain and the product of the total halo current and toroidal
field. Typical peak strains are in the range of a few times 10^-4, which is
consistent with calculations carried out previously. A few outliers, with
higher strains than might be expected from the linear trend, are found to
correspond to cases with high toroidal peaking factors in the halo current.

Gary Hallock (FRC) and his student, Ashley Shugart, visited to work with
PFC personnel to complete design of an optical system for Phase Contrast
Interferometry and to begin system installation.