Alcator C-Mod Weekly Highlights
			Sept. 10, 1996

Three runs were scheduled on Alcator C-Mod last week, but only one was
completed. C-Mod operations are presently on hold pending resolution of
questions arising from the off-normal behavior described below. 

Following a fresh boronization Tuesday night, Wednesday's run was dedicated to
dimensionless identity experiments between C-Mod and JET. The JET team had
previously produced h-mode discharges with 0.83MA, 0.83T, nebar=2e19, and
Ptot=1.2MW. A C-Mod equilibrium was produced to match the JET shape, with
upper triangularity 0.43, lower triangularity 0.40, Ip=1.2MA, Bt=5.4T
(q95=3.2), nearly double null.  Input power was limited because of a fault in
the high-voltage breaker for one transmitter that occurred during the run, so
we were unable to reach the properly scaled power of 3.5 MW. At the accessible
power, we did not obtain h-mode in these plasmas. The edge temperature was
observed to be low on these discharges, confirming the previous results
(Hubbard, et al.) regarding the edge Te threshold for h-mode
transitions. Later in the run, we increased the current to 1.5MA,
corresponding to q95=2.7.

Thursday's run, the first of the current campaign to produce plasmas at 8
Tesla, was terminated after 3 shots when an automatic monitor indicated that
the resistance of a critical joint in the OH2U bus had exceeded its allowable
value. This joint, which has a normal static no-load resistance of less than
0.5 uOhm, and has been operating near the high end of its allowed range of 1-2
uOhm under load, had registered over the 3 uOhm administrative limit for the
first time since its installation two years ago. Subsequent tests confirmed
that the joint resistance had indeed increased and, in accordance with
established procedure, the run was aborted. Thermal cycling of the oh coil and
bus from LN2 operating temperature to near 0C and back again restored the
joint to its normal resistance of 0.4 uOhm, demonstrating that no damage or
degradation in the joint itself has occurred. However, the behavior may be
consistent with a loosening of the connection.  In accordance with procedures
established two years ago, a review of the situation is being undertaken to
determine whether to continue the present campaign, which is scheduled to run
for three more weeks, or to begin the maintenance and inspection period
scheduled to start in October, which would enable us to examine and, if
necessary, re-tighten or replace this joint.  This review includes analysis of
all of the existing data on this connection, and the other two similar
connections on the other oh coils, and additional tests on a mock-up joint and
coil set to try to reproduce the observed anomalous behavior.

As part of the analysis of the mode conversion ICRF heating experiments on 
C-Mod, the plasma repsonse to the puffing of He-3 on a number of plasma
runs has been studied. A distinct electron density rise follows the He-3 puff, 
and the incremental number of electrons is proportional to the duration of the 
full-on valve pulse. The assumption that these electrons are due to helium is 
supported by corresponding increase in Zeff on shots where sufficient helium is 
puffed to see the response. The location of the mode conversion heating in 
D(He3) plasmas, which is sensitive to the helium ion fraction in the plasma 
core, is also consistent with this assumption. Gas calibration shots taken last 
week have allowed absolute calibration of the He-3 puffing system. This allows 
an estimate of the helium screening [the ratio of the number of helium ions 
which make it into the plasma to the total number of helium atoms puffed into 
the machine] to be made at 40% +/- 15%. This number is considerably higher than 
previous estimates made with smaller He-4 puffs more than two years ago. 

Toroidal rotation of centrally located Ar17+ ions has been measured with a
tangentially viewing x-ray spectrometer. During RF heating experiments, rotation
velocities up to 1 e7 cm/sec, in the co-current direction have been observed.
The rotation increases with RF power at constant electron density. During
H modes, the rotation velocity increases with increasing stored energy.

Ashley Shugart (U. Texas FRC) continued his visit to the PFC.  He is assisting
PFC personnel with installation of a Phase Contrast Interferometry system.
The hardware for the ECE fluctuation experiment, including the data
acquisition hardware, was tested at FRC and shipped to Cambridge. The first
Technical Progress Report for the FRC side of the MIT-Texas collaboration is
now available (on request from the FRC). The Texas group is on still on
schedule in preparing for experiments on C-Mod.