Alcator C-Mod Weekly Highlights May 1, 2000 Operations ---------- Plasma operations continued last week at Alcator C-Mod. A total of 33 plasmas were obtained during two run days. Substantial power was coupled to plasma through each of the ICRF antennas, the lithium pellet injector was operated successfully into plasma, and the diagnostic neutral beam was successfully operated into a C-Mod plasma for the first time. Wall and antenna conditioning are now satisfactory for us to carry out the first boronization of the campaign. Plasma operations are scheduled to continue this week. Physics ------- Lithium pellets were injected during both runs this week. Long-lived P-modes were obtained, with sawtooth suppression lasting more than 150 ms and neutron rate of 1e12/s. The Los Alamos camera very nicely showed the bremsstrahlung torus that forms after the density peaks on each shot. Impurity (Argon) peaking during the P-modes was studied using the HIREX x-ray spectrometer. ECE Signal changes associated with Type III ELMs have been explained in terms of refraction effects; the ELM perturbations are modeled as a poloidally elongated density loss located just inside the LCFS. Based on the ECE observations, the inferred geometrical dimensions were as follows: width of the affected region ~2cm, minimum density 0.5 e20 m^-3, poloidal elongation ~1.5. ICRF Systems ------------ All four ICRF transmitters have been brought back into operation. Each antenna has been vacuum conditioned to at least 20 kV, and conditioning in plasma has started. The D-Port antenna is up to 1.25 MW, E-Port is up to 1.25 MW, and J-Port is up to 1.2 MW. Total power into the plasma has reached 2.2 MW so far. At net powers above 2MW, ELM-free H-modes were obtained. At the highest powers obtained from J-port, RF pickup effects were observed on several systems, notably in the fault-sensing circuitry of the EF4 power supply. The source of the pickup is being investigated, and improved noise rejection will be implemented in the supply. DNB Systems ------------ The DNB was fired into C-Mod plasmas on Friday using deuterium as both the beam fuel and the neutralization gas. Two 26kV, 100ms pulses were fired, each drawing roughly 1A of ion current, and two 21kV, 50ms pulses were fired. The effects on C-Mod of opening the DNB gate valve, both with and without source gas puffing, were assessed. Analysis of the data from these pulses is continuing. During C-Mod pulses, the arc current is significantly lower than the arc current from an equivalent conditioning pulse. Plans for determining the cause of the deficit were developed. Waveform generator #1 was repaired and debugging of the new, isolated current measurement system continued. A problem with suppressor timing which causes the suppressor to fire after the arc starts was traced to overheating. Travel and Visitors ------------------- Rejean Boivin, Catherine Fiore, Amanda Hubbard, Dmitri Mossessian, Ian Hutchinson, Ron Bravenec (UTx), Joe Snipes, Martin Greenwald, and Bill Rowan (UTx) participated in the Transport Task Force (TTF) meeting in Burlington, VT, last week. Rejean Boivin also attended the APS Division of Plasma Physics ExComm meeting in Long Beach, CA. Randy Wilson (PPPL) came to MIT for the week to participate in the ICRF startup experiments. Romik Chatterjee of UT-FRC visited C-Mod for installation of the heterodyne ECE diagnostic. The installation is complete and shakedown will start this week.