Alcator C-Mod Weekly Highlights
			June 30, 1997

Plasma operations continued on Alcator C-Mod last week. Four runs were
scheduled, but only two and a half were completed, because of an alternator
fault on Thursday afternoon. This week begins a scheduled two-week maintenance
period. 

The first boronization of the present campaign was carried out on Monday
night. An electron cyclotron discharge run in a He-B2D6 mixture was used to
deposit a 1000 Angstrom coating on the vessel surface (assuming uniform
coating). Several hours of ECDC in D2 were carried out following completion of
the boronization.

Tuesday's run was used to continue debugging and conditioning of the RF, and
to evaluate plasma conditions following boronization.  ELM-free H-modes were
obtained on most discharges with at least 1MW of RF power, indicating good
wall conditions. In addition, the deuterium pellet injector was run for the
first time during this campaign; two pellets were injected on each of three
shots, late in the discharge. "Snake" phenomena were observed on several
diagnostics, including the ECE, following several of the pellets. A decrease
in the toroidal rotation rate, as measured by high-resolution xray
spectroscopy (HIREX), was also observed following pellet injection.  The
H/(H+D) ratio was determined to be in an acceptable range for minority heating
(less than or of order 5%) throughout the day; the boronization apparently has
helped in reducing the hydrogen levels. Startup reliability was poor on
Tuesday. This may have been due to insufficient D2 ECDC following the
boronization; the succeeding runs, after overnight ECDC and using identical
startup programming, had good startup reliability.

Wednesday's run was devoted to testing of fast disruption termination using
massive D2 pellet injection (MP158). This experiment is based on a request
from ITER.  Theoretical work by Marshall Rosenbluth predicts that injection of
high-Z killer pellets (such as our Ag-doped lithium pellets) will result in
dangerous high energy runaway electron populations via an avalanche
mechanism. Furthermore, Rosenbluth predicts that this mechanism produces much
less of an effect for low-Z injection. ITER is so concerned about this
prediction that they have proposed using fast injection of massive amounts of
deuterium (or hydrogen or helium) for fast and/or preemptive termination of
the discharge. The present experiment was designed to test whether this
technique actually speeds up the current quench. All twenty barrels of the
deuterium pellet injector were fired into the plasma simultaneously to
initiate the disruption. The results conclusively showed that massive D2
pellet injection does NOT speed up the current quench at all, nor does it
reduce the halo current.  The pellets typically raised the plasma density
momentarily to nebar=1.6e21 m-3 (about 3 times the Greenwald limit), and
lowered Te to 100-200 eV in the outer portion of the plasma. However, the
current quench timescale was about 5 ms, which is the typical value.  The same
result was obtained when we tried to kill a VDE.  Simultaneous injection of D2
pellets and tungsten (from the laser blowoff) did not do any better.  We were
able to kill Ohmic, L-mode, ELMy H-mode, and ELM-free H-modes, with identical
results.

Thursday's run was scheduled to address optimization of D(He3) minority
heating at a field of 8 tesla. Following a successful 1.2MA, 8T shot, the
alternator, which provides AC power for the experiment, experienced an
electrical fault indication. The protection and safety circuits functioned as
designed, and the system shut down and was secured without incident. The run
was terminated and an investigation of this fault was begun. Measurements
confirmed a ground fault condition on the rotor. General Electric, which
manufactured the alternator, and carried out an extensive inspection and
refurbishing of the alternator last year, has dispatched a team of engineers
to conduct testing and evaluate the status of the system.

Work on DNB power and control wiring in the power area continued as allowed
by machine operation.  Installation of the low capacitance transmission
line for the DNB began with installation of the first connector at the
arc/filament/snubber supply.  Other connectors are in various stages of
design or fabrication.  Other aspects of transmission line design,
fabrication and installation await the results of the current week's
consultation on mechanical supports.  Assembly of the beamline vacuum
system continued but with final assembly awaiting fabrication of additional
support structures.  Design and construction of PLC interface circuitry
continued.

Roger Bengtson of UT-FRC was on-site to continue installation of the UT-FRC
turbulence probe.

Gary Taylor of PPPL visited on Monday to discuss possibilities for
collaboration in ECE diagnostics.  Two proposals which were outlined and found
to be of mututal interest are a heterodyne radiometer optimized for the
pedestal region and a 20-channel grating polychromator.

Ambrosio Fasoli, from JET, also visited the Center, Alcator and the Physics
Department last week. He presented a talk on TAE modes to a joint PSFC/Physics
Department Seminar. 

Steve Lisgo and Professor Peter Stangeby visited from the University of
Toronto on Wednesday and Thursday.  We had a report from them on current
ststus of modelling of C-Mod impurity transport with DIVIMP.  At the moment
they are concentrating on developing a proper connection to the EIRENNE
neutrals code to get a better handle on flows derived from ionization in the
SOL.  Another aspect they are working on is modelling of the plumes from the
'burping' probe.  It was a very useful visit with an extensive exchange of
information in all areas.

Earl Marmar was in Korea last week as a member of the committee for the KSTAR
physics validation review. The all superconducting magnet tokamak is proposed
to be built at the Korea Basic Science Institute, Taejon, Korea.

Francesca Bombarda spent two weeks at Frascati, from Jun 9 to 20, for a
periodic exchange of information between C-Mod and FTU.