Alcator C-Mod Weekly Highlights
			June 23, 1997

Physics Operation continued on Alcator C-Mod last week. Four runs were
scheduled and completed. In addition, a short run was conducted on Monday,
June 17, in support of conditioning of the 80MHz RF system.

The divertor cryopump was employed in Tuesday's run, which was in support of
Mini-proposals #160 (Cryo-pump commissioning) and #177 (Exploration of the
Prototype Cryopump on Impurity Transport). Raw pumping speeds in the range of
1000-1500 l/sec were determined. In nitrogen injection experiments, pump
operation reduced nitrogen concentrations in the core and in the divertor by
about a factor of two. The pump was also observed to affect the core density,
with slightly larger puff rates required to obtain the same density during the
flattop portion of the discharge, and density pump-out observed in the
ramp-down phase of high density shots. It is worth noting that these results
were obtained with a single prototype pump, representing about 10% of the
ultimate pumping speed, once the full cryopump system is installed.

Wednesday's run was the first attempt at "Deep Tracer Gas Experiments",
MP#175. These experiments use a plenum and inertial valve carried on the Fast
Scanning Probe assembly to inject trace impurity gases into the SOL, close to
the last closed flux surface. Using N2 as the trace gas, plumes in NII and
NIII emission were observed using the F-top camera. Analysis of these data are
underway. The local plasma conditions at the injection point were
significantly perturbed by the plume, even for the relatively small amount of
N2 injected in these shots. This may cloud the interpretation.  Larger amounts
of N2 were injected into low-density targets (nebar ~ 1e20/m3) using the probe
valve, to investigate screening in the SOL. For injection essentially at the
LCFS, and into the SOL at 4mm outside the separatrix, the intensity and
time-history of the Li-like N signature, which corresponds to an electron
temperature of around 50eV, were the same. Some Doppler shift data indicative
of flows may be available from these larger N2 puffs, but these injections
were highly perturbing, so the data may be difficult to interpret.

The purpose of Thursday's run (MP#167) was to determine where injected
impurities which do not go into the core plasma end up. The new Divertor RGA
'Prisma' was used to measure the neutral density in the divertor.  The core
levels were determined from McPherson and HIREX measurements.  The injected
quantities are determined from a calibration of the piezo valves used.  This
run dealt with ohmic plasmas. Argon was the main injected impurity used in
this run; future runs will involve other gases, including N2 which behaves as
a non-recycling impurity, and will include H-mode targets.  Analysis of the
RGA data using the calibration numbers from the offline vacuum test stand work
indicates high Ar neutral densities in the divertor (of order .1 mTorr Ar)
with nAr/n_neutral there ~ .3 percent.  The compression of Ar in the divertor
compared to the core, n_Ar(div)/n_Ar(core) ~ 150. Based on these numbers we
can account for all of the injected Ar. However, a final online calibration
must be performed on the tokamak to confirm these rather high values. For
typical injections the core Ar level as determined from Li-like Ar emission
stayed constant after the puff.  However, for one shot the time behavior was
different.  At the partial detachment from the outer divertor the Ar level in
the core rose by a factor of 2, and later there was another jump in the
brightness due to the 'shark fin' or poloidal detachment.  This may be
attributed to a rise in density of Ar and/or D2 in the core.

Friday's run was planned to investigate access to enhanced reverse shear
operation (MP#149A) in C-Mod. While reverse shear was apparently obtained with
one ICRF transmitter operating with up to 1.5MW as early as 100msec into the
discharge, no evidence of enhanced confinement was observed. The electron
temperature exceeded 3keV before sawtoothing commenced around 230msec. On
several shots MHD activity was observed on the core ECE channels and on the
magnetics loops during the current rise. This run was compromised in that
the two ICRF transmitters would not operate simultaneously (see below) and by
an H/(H+D) ratio that increased during the day. As a result, the planned
parameter scans to optimize the shear reversal were not accomplished.
Additional runs on this topic are planned.

Testing and conditioning the 80 MHz system was begun last week and was fairly
successful.  FMIT#1 came on line without any problems, but FMIT#2 continued to
have to have difficulties.  Although we successfully injected up to 1.5 MW
from both transmitter #1 and #2 into 5.4 T D(H) plasmas separately,
transmitter #1 was causing #2 to fault.  FMIT#2 also had difficulty running
through a discharge without faulting when running alone.  The problem may
be a malfunctioning feedback circuit in transmitter #2.  Earlier in the week,
matching calibration numbers were determined and a malfunctioning demodulator
was found in E-port's measurement system.

Fabrication of the low capacitance transmission line for the DNB began:
Some parts were completed; bid review for the sheet metal work is in
progress, and arrangements were made for consulting on mechanical supports.
Work on DNB power and control wiring in the power area continued as allowed
by machine operation.  Assembly of the beamline vacuum system continued as
did design and construction of PLC interface circuitry.  Some additional
DNB control racks used in the original installation at UT-FRC arrived at
MIT in response to a recent request.

Gary Hallock of UT-FRC was on-site to assist MIT personnel with the
installation and development of the phase contrast interferometry system.

Yuichi Takase attended the ITER Heating and Current Drive Expert Group
Workshop in Garching on June 16-17, 1997.  The primary objective of this
workshop was to provide input to the Physics Basis Document.

Dr. J. Snipes attended the EPS meeting in Berchtesgaden during the week
of 9 - 13 June then went on to visit JET for a High Performance Workshop 
organized by Gormezano and Parail of JET from 16 - 19 June.  At the EPS meeting,
Dr. Snipes presented two papers on Enhanced D alpha H-modes in C-Mod and on
An Analysis of the H-mode Threshold in ITER using the ITER H-mode threshold
database.  At the High Performance Workshop, he presented work by Paul Bonoli on
calculations of possible high performance RF scenarios in C-Mod.  In addition,
after the High Performance Workshop, he analyzed JET data with Dr. Righi of 
JET to compare the ICRF H-modes observed in JET with those found in C-Mod.
Further discussions were also held regarding the upcoming ITER H-mode threshold
paper that Dr. Righi will present at the H-mode Workshop in Germany in 
September.

Last week we were visited by Doug McCune and Jim Hirsch of PPPL.  The visit
was to discuss MDSplus (the C-Mod data system software) and it's proposed
implementation on NSTX.  In particular, McCune was interested in the
TRANSP/MDSplus interface and the possible use of MDSplus for DIII-D.

Ben Carreras and Larry Owen of ORNL visited last week to begin a
collaboration on analysis of neutral effects on the L-H transition. 
This work will be an extension of similar work carried out earlier on
DIII-D, taking advantage of the higher plasma density regimes available on
Alcator C-Mod. Three C-Mod discharges from the 1996 campaign were selected for
the initial analysis, which is proposed to be completed in August or
September of this year. 

Boronization is being carried out overnight Monday, June 23; this is the first
boronization of the current operating campaign. Tuesday's run will be used to
complete debugging and conditioning of the RF, and to evaluate plasma
conditions following boronization. The remainder of the week is scheduled for
physics runs.