Alcator C-Mod Weekly Highlights Sep 27, 1999 Plasma operations continued at Alcator C-Mod last week. A total of 68 plasma shots were obtained with a start-up reliability of 92% over 3 1/2 days. Two mini-proposals were completed, and progress made on a third. Monday's run was dedicated to completion of a study of the scaling of the rotation velocity with plasma current, for otherwise constant H-mode target plasmas. The range of currents achieved was from 1.36 down to .42 MA, at nearly constant target electron density, and at 2 MW ICRF power. There is a very clear trend of rotation velocity normalized to the plasma stored energy decreasing with increasing plasma current. There were some CaF2 injections as well, with some shell formation seen on the top x-ray array, with very good time resolution. Some of the discharges had all four ICRF transmitters operational, with a total of 4.3 MW achieved on one shot. Tuesday's run was in support of MP#230, "Effect of Divertor Baffling on H-mode Power Threshold". Standard 0.8 MA, 5.4 T discharges with a variety of target plasma densities (nl04 = 0.7 to 1.4) were used to investigate the H-mode power threshold. RF heating was programmed to give slow linear ramps. Identical discharges were run, first with flaps closed for the entire discharge and then a comparison discharge with flaps open. Trace amounts of argon and helium were used to investigate divertor screening. Results indicate that the flappers appear to have no measurable effect on the H-mode threshold (within an error of around 10%). As usual, with flapper open at moderate to high density, the divertor neutral pressure is reduced by a factor of ~ 2, with no measurable effect on the midplane pressure. Wednesday's run was dedicated to MP#232, "Investigation of the EDA/ELM-free operational boundary". The main focus of this run was the effect of shaping. The average triangularity was scanned from 0.6 to 0.28 at several values of q95 (Ip). Good pedestal data was taken with the new diagnostics and fluctuation measurements were obtained during the transition time. The pedestal widths from several diagnostics (edge soft x-rays, edge Thomson, VB) varied directly with triangularity as did Halpha. The run was hampered by lack of steady ICRF power at the desired 2-2.5MW level. It was possible to confirm that low triangularity leads to ELMfree plasmas even at q95 high enough to be EDA with our normal shape. Thursday's run was cancelled in order to allow for repair of the arc in the D-port transmission line that was responsible for some of the RF problems encountered on Wednesday. This was accomplished. The purpose of Friday's run was to determine the profiles of the impurity diffusion coefficient and convection velocity during EDA H-mode (MP#218). A satisfactory discharge was established, but the run was stopped early due to an indication, based on resistance measurements, that one arm of the TF was not cooling properly. This effect is being investigated. This week is a maintenance week; no plasma runs are scheduled. ICRF System ------------ A record amount of ICRF power,4.3 MW total, was coupled into C-Mod. This was distributed among the three antennas roughly as D-Port = 1.3 MW, E-Port = 1.4 MW, and the new PPPL antenna J-Port = 1.6 MW. In general, J-port antenna conditioning is continuing while physics experiments emphasizing moderate power from D- and E-Ports are conducted. For a density scan in Ip=800 kA plasmas, J-port was run in piggy back and had greater success in coupling to the lower density target plasmas than target densities greater than nl~1.1e20 m^-2. Piggy back opportunities for J-port on other experimental days were limited. D and E-port antennas were run into plasmas with various plasma shapes. For high triangularities, the match between the plasma and antenna shape was poor, with 2 cm gaps at the antenna extremeties and 0.2-0.8 cm gaps on the midplane. The E-port antenna was limited to <800 kW because of faults. At powers >800 kW, the E-port antenna faulted often. The D-port antenna ran at roughly 1 MW before faulting on HV limits. After repairing an arc in the transmission line from FMIT#1, the ICRF was operated into Ip~800 kA, nl~1e20 m^-2 plasmas. Maximum injected power was 2.8 MW. D-port diagnostics indicated that for this plasma the 1.4 MW injected was about maximum because of the antenna voltage limit. E-port however was capable of higher power because the system was only ~75% of maximum. Travel and Visitors ------------------- Yijun Lin visited PPPL last week Monday and Tuesday. The main purpose is to evaluate two channels (128-136 GHz and 136-144 GHz) of the frequency-swept TFTR reflectometer. These two channels are proposed to be installed in C-Mod in near future. The second purpose was to discuss with Dr. Raffi Nazikian on the development of the code simulating reflectometry fluctuations measurement. D. Winslow (U. Texas) completed a two week visit to begin driven tile probe experiments. R. Chatterjee (U. Texas) completed a two week visit to re-install a section of the ECE radiometer and get first results from the ECE correlation system.