Alcator C-MOD   
			    Weekly Highlight Report  
				September 23, 1993


This week plasma operation is continuing. Pulse lengths over one second, with
flattop times over 0.6 sec, are now becoming standard, as we continue to
exploit the four-quadrant capabilities of the OH and EF1 supplies. 

Exploration of higher elongations and axisymmetric stability have been carried
out, with single null plasmas with elongations up to about 1.6. Passive growth
rates of the axisymmetric mode were measured by turning off the feedback and
observing the evolution of vertical position for 10 msec before restoring
feedback control. The closed loop characteristics are also determined from the
recovery after feedback is restored. The results are being compared to code
predictions from TEQ and from our own code. These studies will be used to help
optimize our control laws. 

Some modifications to the Hybrid Control Computer configuration and to the PCS
operator interface have been made to facilitate more complicated control
schemes, including feedback control of x-point position and/or strike point
location for diverted discharges. Initial experiments on controlling the
x-point height have been successful. 

Unfortunately, the modifications to permit control of more plasma parameters
during the main part of the discharge have necessitated changes in the way the
startup phase is programmed; robust techniques using the new system are not
yet established, and we have experienced some difficulty with discharge
reliability.  Previous experience has indicated that wall-conditioning is also
important for reliable startup, and we have instituted a program of routine
overnight discharge cleaning (ECDC) in addition to the usual two hours of
conditioning before each run. 

The McPherson grazing incidence VUV spectrograph and the Johns Hopkins "Moly
Monitor" diagnostic, which were installed at the last break, are both now 
operational and producing data. 

Halo currents to the outer divertor modules have been measured during vertical
disruptions (VDE's). When the plasma disrupts downward, over 10kA is observed
in a shunt connecting one (out of ten) of the lower divertor modules to the 
vacuum vessel. Due to instrumentation problems, the currents in the other nine 
shunts were not measured, but if the halo current is axisymmetric we infer that 
of order one-third of the plasma current appears to flow to the divertor during
the VDE. When the plasma motion is upward, less than 1kA is observed in the
shunt, and during normal, well-positioned, operation a current of order 
50 Amps is measured. 

The ICRF Group has measured the antenna loading as a function of plasma position 
relative to the antenna and edge density.