Alcator C-Mod Weekly Highlights
				August 6, 1996

Alcator C-Mod has completed a scheduled five-month maintenance and upgrade
period and is now beginning a short operating campaign. Experiments over the
next two months will concentrate on issues of dissipative divertor operation
and H-mode confinement and threshold conditions. New capabilities installed
during the upgrade include a prototype divertor cryopump and divertor Thomson
scattering (PPPL collaboration). Most of the activities last week were devoted
to final preparations for operation, scheduled to begin August 5.  

In-vessel tests of the new divertor cryopump were carried out. The pump was
cooled to liquid helium temperature, and its diagnostic thermocouples checked
out. Hydrogen pumping was observed with the main torus pumping valved off. 

A new pulsed gas driver circuit was installed and tested. The new circuit,
which employs pulse-width modulation, should give a more linear response in
terms of gas throughput vs. demand signal. 

Power tests  were carried out on all PF and TF supplies. Several minor
glitches were identified and fixed. All supplies were operated under
programmed control using the hybrid control computer. A faulty output from the
hybrid was identified and fixed. Operation with the EF4 coils connected in
parallel, rather than in series, was tested. Current sharing between the two
coils was found to be good. Operation in the parallel configuration is planned
for the new campaign in order to increase the slew rate and provide better
control of the plasma shape in the early flattop, and to permit greater
shaping flexibility. 

Baking and ECDC (electron cyclotron discharge cleaning) were continued. A
fault in the low current DC supply used for toroidal field during ECDC limited
the total discharge cleaning time this week. The supply has now been repaired. 

Boris Grek and Joe Bartolick from PPPL were at MIT getting the new divertor
Thomson scattering system ready for operation. The diagnostic was successfully
interfaced to the C-Mod timing system, and the alignment feedback system was
checked out. 

Plasma operation began on schedule on Monday, 8/5. Plasma currents up to 600kA
were obtained. The plasmas were quite resistive, and more machine cleanup is
indicated, both by ECDC and additional tokamak discharges. Operation will
continue this week. The planned C-Mod run schedule is now available through
a link on the C-Mod home page (http://cmod2.pfc.mit.edu/cmod/home.html). 

Progress continues in our collaboration with the University of Texas Fusion
research Center (FRC). Don Patterson (FRC) and Tom Fredian (PFC) completed
installation and test of software which allows access to the C-Mod archive
from IDL running on FRC VAX's, from user programs in C or Fortran also running
on FRC VAX's and from DWSCOPE running on FRC Unix machines.
Gary Hallock (FRC) visited MIT to deliver a phase plate for the C-Mod PCI
diagnostic and to present his initial work on an imaging analysis of the
PCI system.

Our collaborator from Los Alamos National Lab, Glen Wurden,
visited C-Mod to re-install the fast (1000 frames/s), gated TV camera, which
had been in use at TFTR. The system is now operational at C-Mod. Also
discussed during his visit were the details of the Los Alamos design for an
infared periscope  view of the C-Mod divertor.

During the last campaign we monitored the molybdenum source  rates at the
divertor and inner wall. A recent calculation of the penetration factor  (PF)
for the divertor, defined as the number of impurity ions in the confined plasma 
normalized to the influx, yielded values ( ~ 0.001 sec) similar to that found 
for nitrogen. Calculation of the PF for the inner wall during the current rise
(before the plasma gets diverted) yielded values which were typically an order
of magnitude higher. In addition, the spectroscopically measured source rate
from the outer  divertor (for the period following boronization) is in
reasonable agreement with the one calculated based on outer divertor probe
data and sputtering rates, assuming a combination of deuterium and boron (BIV) 
ions incident on the  divertor.