Alcator C-Mod Weekly Progress Report
                        June 2, 1997

Startup plasma operations continued on C-Mod last week. A total of 57
physics-quality plasmas were produced during two run days. No runs were
carried out on Thursday or Friday to allow for repair of two vacuum
leaks which developed during operation.

Machine conditioning is proceeding well, based on the increased startup
reliability obtained last week, and also on the level of carbon emission
(CII) observed during the breakdown phase.  Startup reliability on Wednesday
reached nearly 70%.  As noted last week, almost all of the successful startups
evolve into full current well-controlled plasmas that last into the ramp-down
phase. The hydrogen fraction, H/(H+D), rose during the course of each
run, reaching levels around 20%, which is too high for efficient ICRF
minority heating. Additional cleanup is required to reduce the hydrogen level and
to further increase the startup reliability.

Rf conditioning into plasma made good progress last week. Transmitter #3
was used to launch over 1MW of RF power into the plasma. Hybrid control of
the toroidal field was used to ramp the field down to 4T and then to 2.8T;
the latter is appropriate for D(H) heating with the 40MHz system. A good
ELM-free RF H-mode was obtained at the lower field with only 0.4MW of RF power,
early in Wednesday's run when the hydrogen fraction was sufficiently low for
efficient minority heating. The new 1mm spatial resolution soft xray array was
able to clearly resolve the steep gradient in the barrier region.

Diagnostic systems continue to be brought into operation and calibrated.
A cross-calibration of the multi-channel high resolution x-ray
spectroscopy system, used for measurements of ion temperature and plasma rotation,
was completed. A new calibration for the ECE electron temperature diagnostic
was applied.

The Prisma divertor Residual Gas Analyzer came 'online' last week. The
divertor RGA will be utilized to determine the neutral density of different
impurity species in the divertor.  For the purposes of maximizing divertor
radiation and minimizing core impurity levels one wants to maximize the level of
impurities in the divertor.  This will give us a measure of this parameter and
we can determine how different operating regimes affect it.

During Wednesday's run, a progressively worsening leak was noted on the
residual gas analyzer. Leak checking on Thursday revealed three new 
leaks, two of which were sufficiently serious to warrant an up-to-air for repair.
While the system was open, a defective bolometer was replaced and a jammed
shutter on the tangential interferometer was serviced. A new pre-ionization
assistance device was also installed for testing. The machine was pumped down again
on Saturday, and baking and discharge cleaning carried out. Plasma
operation will resume this week.

One of the Uptegraff high-voltage supplies used for the 80MHz transmitters has been
moved to replace the failed unit on the 40MHz transmitter #4. Transmitter
#4 is now operational. The dielectric dummy load formerly used for the
80MHz system has been refurbished and is being used to tune the 40MHz
transmitters. Unit #3 is now producing 1.5MW into this load. Unit#4 is still being
tuned, and as of Sunday was producing 0.3MW for 0.5sec.  The position control
feedback circuit used to maintain RF loading during H-mode operation has been
debugged. Antenna arc detection circuits are being checked out.

We have submitted 127 time-slices of 1996 C-Mod data to the new ITER
pedestal database. This submission is in support of work on the ITER Urgent Task
on characterization of the H-mode pedestal.

The conceptual design for the DNB high voltage transmission line between power
supplies in the power area and the beamline in either its test lab
location or in the cell has been completed.  It will employ the technology used on
TEXT but is modified to allow simple switching between the test lab and the
cell. The design was submitted for implementation.  We began design of the
interface circuitry for the PLC.

Pepi Cima, UT-FRC, was on-site to work with John Heard, Auburn, to test
the UT O-mode heterodyne ECE system and prepare it for experiments. The
diagnostic is in the final software debugging stages. The heterodyne system is
presently set up for 8 channels of ECE. The frequency range of the diagnostic is 112
GHz- 127 GHz with corresponding B field intensities of 4 to 4.5 T.  For a B0=
5.3 T, this will be from the edge to r/a ~ 0.5.