Alcator C-Mod Weekly Highlights
			Sept 8, 1997

The maintenance period continued at Alcator C-Mod last week. No runs were
scheduled. Physics activities concentrated on diagnostic preparation and data
analysis. The alternator repair is progressing. 

Manufacture of the new pole-to-pole jumper connections and new main leads for
the alternator field winding was completed. Fabrication of temporary wooden
blocking to be used during re-installation of the field coils was
completed. Cutting and preparation of the slot armor for assembly is in
process. Installation of the field coils on the rotor has begun.

Calibration of the core Thomson scattering system using Rayleigh scattering
from Argon was carried out during the week. This completes the scattering
calibration work for the core system. 

Based on J. Terry's derivation of the recombinations/photon we are now
reviewing older data with spectrometer views of the outer divertor for
information about the role of recombination there.  We have found that a
significant amount is occurring before detachment.  At the onset of detachment
we observe pressure peaking at the divertor plate that moves away from the
separatrix into the common flux region, leaving low plasma pressure regions
behind (detachment).  The recombination increases during this period and
follows the detachment up the plate.  The amount of recomnbination (integrated
over the plate) is less that the integrated current loss during detachment,
indicating that an appreciable fraction of the current loss is due to a loss
in ionization.

Preliminary results using the new "helium probe" on C-Mod have now been
obtained.  The helium probe follows the technqiue developed at TEXTOR, where
local values of electron density and temperature can be derived from the ratio
of three near-IR lines of HeI.  In the case of C-Mod, a crude "beam" of helium
gas is formed using a capilliary that injects through the AB limiter at the
outside mid-plane.  The capilliary nozzle is at the leading edge of the
limiter, typically within 10 mm of the separatrix.  The helium atoms cross the
SOL, penetrating several mm's inside of the separatrix.  The helium emissions
are observed with a tangentially-viewing fiber array.  Initial results
indicate a relatively robust signal level, with low background inteference
(1%).  Local conditions at the limiter location as deduced with the technique
are Te = 10 eV to 20 eV and ne = 2 to 8 e19 m-3 in Ohmic discharges.  A
thorough comparison with the fast scanning probe has not yet been made, but
these values are consistent with earlier probe measurements.  These initial
results are being presented this week at the ITER Diagnostic workshop in
Varenna, Italy.

The power systems group installed two new shunts in the EF4 bus. These
provide independent measurement of the current in each of the two EF4 coils.

Maintenance on the 80 MHz ICRF transmitters FMIT#1 and #2 is progressing.
FMIT#2 testing into the detuned dummy load has been completed.  An MIT arc
detector was found to be producing false arc indications and was repaired.
The E-port (FMIT#2) Princeton arc detector was also found to be
malfunctioning.  These faulty arc detectors are the most likely source of the
problems running FMIT#2 in the previous campaign.  Testing of the phase
balance arc detectors will be started this week.  Work on FMIT#3's and #4's
PLC wiring progresses.  The H-probe on the E-port external resonant loop was
pressure tested.  The repaired H-probe does not leak, but another on the same
loop does.  Work is under way to repair this probe as well.

On last Thursday (09/04), the Final Design Review for the new Princeton four
strap, ICRF antenna array was held at MIT.  The Princeton collaborators were
represented by the cognizant engineer R. Ellis and physicists J.R. Wilson and
J. Hosea.  In addition to the C-Mod ICRF physicists, several other PSFC staff
members attended.  R. Ellis presented the design and thermal, electromagnetic
and stress analysis of the antenna.

The design appears to be solid.  Issues raised in the Preliminary Design
Review were shown to have satisfactory solutions.  For example, samples of the
TZM rods for the Faraday shield were provided to show that the manufacturing
of these rods as specified could be done.  The design meets the thermal,
electromagnetic and stress specifications.  The thermal analysis indicates
that a maximum of 9-10 4 MW 10 second pulses at 20 minute intervals is
possible.  The electromagnetic analysis indicates the insulating Faraday
shield rod end connectors are critical to the success of the design.  If the
connectors short out, repairs to the Faraday shield would be necessary.  The
insulating connector design is the same which has been successfully used at
C-Mod previously.  For the stress analysis, a full finite element analysis was
performed.  The results indicate there were no excessive stresses and the
location of maximum stress was where the current feed connect to the antenna
straps.  A few issues remain including relocation of the in-vessel mounting
hardware and design of the side plate.  These issues have straightforward
solutions which were discussed.

The antenna fabrication process is beginning.  Contracts for parts which
require longer lead time have been awarded and the C-Mod port mockup at PPPL
has been built.  The plan is to have the antenna built and tested in the
mockup at PPPL in January 1998.  It will then be disassembled and shipped to
MIT in February and ready for installation in March.

The associated coax system was also discussed.  A scale drawing of the C-Mod
diagnostic stand and cell were provided to PPPL.  The tight physical space and
two resonant frequencies are the primary factors influencing the design.

Reconstruction of the suppresser switch for the diagnostic neutral beam has
begun.  On completion, all high voltage components, including input and
output snubbers,  will be fully enclosed within an interlocked cabinet.
The CAMAC system for acquisition of the DNB thermocouple/secondary emission
data was received after having been assembled and tested at UT-FRC.  The
system will accommodate 28 channels of thermocouple data and 19 channels of
faster secondary electron emission data.  These will be the principal
diagnostics for beam operation and development.  The beamline cryopumps and
associated equipment were assembled and located in preparation for the
routine maintenance that is required before regular operation can begin.
With completion of required maintenance, the turbomolecular pumping system
is now ready for long term operation.

Earl Marmar attended the International Workshop on Diagnostics for
Experimental Fusion Reactors in Varenna, Italy. He presented an invited
paper discussing spectroscopic and other diagnostic techniques.