Alcator C-Mod Weekly Highlights
				August 27, 1996

Plasma operations continued on Alcator C-Mod last week. Four runs were
scheduled, but a vacuum accident forced the cancellation of one run day and
reduced the utility of two others. Start-up reliability, which had seemed to
be improving by the end of the previous week, was still not satisfactory.  
Nevertheless, some progress was made. Experiments carried out were concerned
with conditioning of the RF systems, commissioning the divertor cryopump,
developing low triangularity discharges, and measurements of impurity
screening. 

Prior to Tuesday's run, we carried out the first overnight boronization of the
current campaign. A boron layer with an average thickness of 600 Angstroms was
laid down (Last week's report gave an erroneous value for this thickness). RF
conditioning was carried out on Tuesday, with up to 1.5 MW obtained from each
antenna.  A brief H-mode was obtained on at least one shot. 

A vacuum accident before the start of the run on Wednesday resulted in an
up-to-air. After repairing the problem, a broken feedthrough on an ion gauge,
the machine was pumped back down and overnight discharge cleaning started,
with a mild (60 deg. C) bake. The boronization layer was presumably poisoned
by the large amount of H20 vapor admitted to the vessel during the up-to-air. 

Thursday's run was expected to be primarily a clean-up session, but after a
difficult beginning, with very dirty plasmas, we actually made some progress
on a number of topics. Breakdown and successful current rise were established
at our standard operating field of 5.3 Tesla. RF conditioning continued,
although heating was not good, possibly due to the high H/D ratio in the
plasma. Hydrogen fractions up to 50% were measured, using spectroscopy and
neutral particle analysis, as the walls were still somewhat contaminated from
the vacuum accident.  Toroidal field ramps up to 6.5T were used to look for
mode conversion heating with the high minority fraction, but only a small
amount of electron heating was observed; conditions were clearly far from
ideal for this experiment.  The divertor cryopump prototype was operated with
plasmas for the first time. Clear pumping of deuterium and hydrogen were
observed; the results during plasma are being analyzed. Finally, we began
development of low triangularity equilibria, which are required for
dimensionlessly identical comparisons with JET, which are planned for later
in this campaign. 

The first half of Friday's run was devoted to a study of impurity screening
and divertor compression (part of MP#167). The goal of the run was to puff
trace amounts of Ar and Ne into ohmic plasmas under attached and detached
divertor conditions. The screening of these recycling impurities is measured,
and spectroscopic diagnostics, viewing the X-point/divertor region measure the
puffed-impurity ionization states. Central impurity confinement times were
measured using laser blow-off technique. The Chromex spectrometer, viewing the
x-point and divertor surfaces, measured lines from ArI, ArII, NeI, and
observed a clear recombination signature. The McPherson VUV spectrometer,
viewing the x-point, measured lines from higher ionization states of Ar (ArIII
through ArIX) and Neon (NeIII - NeVIII). 

The second half of the run on Friday was a continuation of development of low
triangularity equilibria, which are required for two experiments planned for
this campaign, a study of the effects of shaping on ELM behavior and
confinement in H-mode and a dimensionless parameter comparison with JET. The
JET group has obtained results at 0.8MA and 0.8T which have the same
non-dimensional parameters as C-Mod operating at 5.3T and 1.2MA; however, JET
cannot access triangularities above about .4, while the typical C-Mod
equilibrium has a triangularity of about .55. On Friday, we were able to
produce a plasma with a shape similar to the target JET shot, with upper and
lower triangularity of 0.38 and 0.41 respectively, although more work remains
to get a completely satisfactory match. 

The new tangential interferometer prototype is now operational on the bench. 
Signals at both 0.532 and 1.064 um have been obtained. As expected, the signal
phases track together with vibrations and show differential phase shifts when
transparent materials are introduced in one beam path and the index of
refraction is modified (plasma simulation). The limiting amplifiers and
counters for the detector electronics are also operational and work is
proceeding on the phase demodulator.  This work has been moving forward in
collaboration with Horacio Lamela and Pablo Acedo from Carlos III University.

The U. Texas DNB hardware is in the custody of the shipper and is expected
to arrive at MIT on September 9.