Alcator C-MOD Weekly Highlight Report June 17, 1993 Our efforts to improve the reliability of discharge initiation are beginning to pay off. Wednesday's run consisted of 28 consecutive "good" shots, all over 200kA and lasting at least 200 msec. In addition to maintaining consistent field structure and evolution at breakdown and during the early current rise, control of the pre-fill pressure and wall conditioning contribute significantly to reproducible startup. ECDC discharge cleaning before operation and between shots has been used to help condition the walls. Initial attempts to elongate the plasma have resulted in a marginally diverted, nearly symmetric (double null) plasma with a current close to 300kA. Further work on developing diverted discharges is planned. The first attempts to position the plasma with respect to the RF launcher have been carried out, using the vertical position control derived from magnetics signals to place the plasma at a height such that probe signals on the outboard RF limiter were balanced. The RF antenna remained safely withdrawn to a position well behind the limiter for these experiments. Occasional runaway discharges are replacing "fizzles" as our main operational bugaboo. We have recently experienced a number of such non-thermal discharges, with currents in the 200-300 kA range lasting for of order 300 msec, which lead to hot spots and some Mo injections from the outboard limiter, as well as substantial hard x-ray fluxes. Incorrect fill pressure and bad wall conditioning are the principal suspects in causing these events, although there is some evidence that plasma position in the early stages of current channel formation may also play a role. Diagnostics continue to come on-line. The Michelson ECE system has been operated successfully, obtaining cyclotron emission spectra in both thermal and non-thermal discharges. Data reduction giving electron temperature profiles is pending. The HIREX high-resolution x-ray spectrometer has successfully observed H-like and He-like spectra of injected Argon, suitable for measurement of ion temperatures. The RF group has installed the tuners and resonant loop in the cell, and are preparing for low power loading measurements on the monopole antenna. In the lab, a fit-up has been done of the two-strap antennas. The power systems have continued to perform well; some remaining imbalances in regulator response, particularly during hard inversion, were addressed this week. Some operating time was devoted to investigations aimed at allowing operation with higher currents in the OH stack. We are also investigating the possibility of using only the inversion capability of the OH1 and OH2 power supplies to provide the voltage for breakdown, eliminating some of our commutation (interrupter) circuits. The interaction of the cryo and vacuum systems has continued to cause some difficulties, with leaks opening up during cooldown having been responsible for some loss of plasma operating time again this week.