Alcator C-Mod Weekly Highlights
			August 13, 1996

Plasma operations have resumed on Alcator C-Mod. Four runs were scheduled and
carried out last week. The object of the scheduled experiments was to
re-establish routine operation and to develop a new startup scenario designed
to allow access to higher plasma current and improve shaping flexibility. 

Plasmas were obtained with 25% higher pre-charge (initial flux) and with the
EF4 coils configured in parallel rather than in series. The increased flux
swing afforded by higher pre-charge will permit operation at higher plasma
current as well as increased pulse length. Power system performance with the
higher pre-charge currents was nominal.  Operation with the EF4 coils in
parallel, which will improve shape control, was also successful. During these
experiments we experienced several firings of the toroidal field crowbar
circuit,  which appeared to be associated with the new EF4 configuration.
These crowbars, which are believed to be due to spurious signals in the TF bus
instrumentation, are being investigated.  Although plasma currents up to 800kA
were produced with the new startup arrangement, the breakdown and current rise
reproducibility was poor, and the tokamak is not yet ready for physics
operation. More commissioning runs are scheduled for this week. 

ECDC (Electron Cyclotron Discharge Cleaning) continued overnight and during
maintenance periods, as we continue to clean up the machine. The glow
discharge electrodes were operated during ECDC at low current to check system
continuity and the new power supply.  The current to the electrodes went up as
the ECDC resonance passed through the electrode location, as expected.

Recent re-analysis of data from previous campaigns has altered our
interpretation of the impurity transport in L-mode plasmas.  Careful
comparison of the measured and calculated brightness time histories of
lithium-like scandium following laser blow-off injection has led to a change
in the inferred profiles of the impurity diffusion coefficient and convection
velocity. The data are found to be consistent with  V=0 everywhere and
anomalously large D from the center out to r/a=.9, with smaller D near the
edge. These new coefficients, when incorporated into MIST calculations of
total impurity density, lead to somewhat different values of deduced impurity
screening.

Fueling and H-mode behavior were compared for a pair of runs last February, 
before and after overnight He ECDC.  A fiducial shot and a high power shot
were taken at the end of the 960221 run.  Overnight He ECDC was performed
after the run. The following day's run began with a fiducial shot and a high
power shot. Gas puffing required for the post-He ECDC fiducial is estimated to
be a factor of two more than for the pre-He ECDC fiducial. The neutral
pressures in the main chamber and the divertor chamber just prior to the
H-mode transition were lower by 20% and 30%  respectively for the post He ECDC
fiducial. The pre He ECDC fiducial  exhibited an "enhanced recycling" type of
H-mode whereas the post He ECDC  fiducial exhibited an ELM-free H-mode.  The
H-mode in the pre-He ECDC high-power shot also had a large D-alpha enhancement
with a low H-factor (1.4), in contrast to the post-He ECDC shot, which was
ELM-free with a high H-factor (2.2).

The collaboration with the University of Texas FRC Group is progressing well. 
In Texas, FRC personnel and V. Bertolino (PFC) continued documentation,
disassembly, and packing of Diagnostic Neutral Beam components.  All remaining
DNB hardware with the exception of the beamline itself was documented and
moved into staging areas in preparation for shipment.

Roger Bengtson of the FRC is visiting MIT for 10 days to observe operation of
C-Mod and to begin development of edge experiments on turbulence and
transport.

Alex Mazurenko (PFC) visited the FRC Aug. 8-9 to study the TEXT-U
Phase-Contrast Interferometry system and to work with Gary Hallock on the
implementation of a C-Mod PCI system.

Horacio Lamela, from Carlos III University, Madrid, is visiting Alcator to
discuss interferometry and help in the design of the new frequency doubled
tangential interferometer prototype.  

Rudolf Neu from Asdex Upgrade is visiting Alcator for four months, in order to
study machine performance with metallic plasma facing components, before and
after boronization.