Alcator C-MOD Weekly Highlights
					Nov 2, 1993

The run on Wednesday, October 27, was devoted to ICRF heating experiments. The
hydrogen minority fraction was varied to try to observe the production of high
energy tail. ICRF power was in the range of 0.5 to .7 MW .Lithium pellet
injection was carried out in conjunction with the ICRF to evaluate the effect
of "lithiumization" as a wall conditioning technique in our metal-wall
machine. Use of lithium pellets is known to have beneficial effects in carbon
machines such as TFTR. Minority ion tails and increases in neutron rate were
observed during the ICRF heating pulses. 

In several shots during this run, transitions to ohmic H-modes were obtained.
The H-modes appeared typically later in the pulse, when the ICRF was off.
These H-modes, at densities around 10^20 m^-3 and toroidal fields of 5.3
Tesla, were unlike those obtained last week at lower field(3.1 Tesla)in that
the 5 Tesla cases exhibited frequent ELMs and smaller enhancements in energy
confinement. The power/unit surface area required to access the ohmic-H
regime at 5.3 Tesla was similar to the 0.15 Mw/m^2 found at 3 Tesla,
corresponding to a much lower coefficient in terms of the parameter nB. 

Following the Wednesday run, in the course of preparing the Thomson scattering
diagnostic for operation, an alignment target became stuck in the inserted
position, extending over half-way across the plasma chamber. Since it would
have been impossible to operate with the target in the chamber, we decided on
Thursday morning to attempt a clean vent to retract it. With the vessel warmed
such that the coldest point was above 5 degrees C, the machine was backfilled
with high-purity helium, and the conflat seal at the target mechanism was
opened and the target pulled back by hand. The opening lasted approximately two
hours.  After pumping down and discharge cleaning for two hours, we attempted
to run beginning at 4PM. 

The first two plasma attempts were successful, but the run was plagued by
disruptions and fizzles, presumably because of poorer wall conditions.