Alcator C-Mod Weekly Highlights
				Sept. 17, 1996

Alcator C-Mod is now entering a maintenance and inspection period. There will
be no further plasma operation scheduled this year. Calibration activities for
various diagnostics and other subsystems are continuing while the machine
warms up.

Analyses and testing related to the elevated resistance readings observed on
the oh2u connection continued this week. In addition to tests carried out on a
dummy coil, the tokamak was thermally cycled to room temperature and back to
LN2 operating temperature. It was found that the resistance of the joint is
affected by the thermal cycling, as well as by Lorentz forces. The joint
resistance varied between 0.6uOhm and 1.4uOhm during these tests; these values
are within the allowable range, but the observed variations were unexpected.
Several scenarios which seem consistent with the data have been identified. An
in situ inspection will be required to arrive at a final determination.

ECDC discharge cleaning was carried out to provide a light source for
calibration of the plasma tv systems, to be used in the analysis of images
obtained during the last campaign.

The fast valve normally used for the injection of argon has been 
re-calibrated.  Since an extensive analysis of the screening properties of 
the plasma has been done relying on the absolute calibration of this valve, 
done back in 1994, it was important to check its reliability after an 
extensive period of operation.  The valve flow has been found to be very 
close to the previous value, with some uncertainty still left as to the 
minimum opening time, which was more difficult to measure on the large 
volume of the tokamak. 

Disruptions during the 960904 run were observed to have unusual halo current
behavior.  Typical downward-going disruptions generate finite halo current in
the bottom portion of the vessel, but negligible halo current in the top (and
vice versa for upward-going disruptions).  However, during the 960904 run,
halo currents were observed simultaneously in both the top and the bottom of
the machine during disruptions.  Magnetic reconstructions of these shots show
that the post- thermal quench equilibria are more highly elongated than
normal, resulting in a large outboard gap and open flux contours which stretch
all the way from the outboard divertors to the ceiling of the vacuum vessel.
This provides a halo current path through the SOL from the bottom of the
machine to the top, and subsequent flow down the entire length of the vessel
inboard wall to complete the poloidal circuit.  Speculation on the reason for
the different post-quench plasma shape on this day centered on the low
triangularities that were being run as part of similarity studies with JET,
but no definite conclusions have been drawn.

The Diagnostic Neutral Beam hardware, including the beamline, neutralizer,
associated power supplies, vacuum and cryogenic system, and beam modulator,
arrived at MIT from Texas. This equipment has been temporarily moved into the
C-Mod power room, pending relocation to the setup lab.  At FRC (U. Texas),
progress continued on diagnostic system design including BES and CHERS.
Dr. G. Cima from U. Texas, and Drs. R. Gandy and C. Watts from Auburn, are
visiting Alcator this week to discuss plans for their new ECE diagnostics. An
O-mode radiometer for temperature and fluctuation measurements, will be
installed in the existing beamline during the current maintenance interval.
In the longer term, an X-mode radiometer will be added, either in the present
ECE lab or at F-port.

The ESnet Steering Committee has approved a proposal to upgrade the link to
MIT.  The connection, which serves the PFC and MIT's Lab for Nuclear
Science (LNS), will be upgraded from T1 at 1.5 Mbps to T3 at 45 Mbps.
This will make a substantial improvement in our abilities to support remote
collaborators.

Joseph Snipes has carried out regressions of the ITER H-mode threshold
database including both global parameters (density, toroidal field, elongation
and major radius) and local parameters (Te95, ne95) across all 10 tokamaks in
the database.  The global free fits were surprisingly close to the constrained
fits that keep the result dimensionally correct, suggesting that despite large
variations in the data across machines, the statistics manages to keep the
result almost dimensionally correct by itself.  The free fit to
Pthresh = C * nebar^Xn * BT^XB * R^XR * kappa^XK led to coefficients of
C = 0.32, Xn = 0.67, XB = 0.99, XR = 1.99, and XK = 1.12, which would predict 
a threshold power for ITER of about 133 MW at a density of 5 x 10^19 m^-3.
When the radiated power is subtracted out of the input power, the fits change
considerably.  Taking the "ITER appropriate" dataset as defined at the Moscow
database meeting in April, and leaving out the kappa dependence because it is
constrained to be within a small range, the coefficients become, C = 0.22,
Xn = 0.74, XB = 1.28, and XR = 2.66, which would predict a power threshold in
ITER of more than 300 MW.