Alcator C-Mod Weekly Highlights August 23, 1999 Plasma operations continued on Alcator C-Mod last week. Five run days were scheduled. The run on Thursday was terminated after one plasma shot due to malfunction of a 13.8kV breaker; this was repaired and normal operation resumed on Friday morning. The machine operated well, with 72 plasma shots produced with typical currents of 1MA and toroidal fields of 5.1 to 5.4T. The main focus of operations continued to be commissioning of the J-port ICRF antenna. In addition, a series of ohmic H-mode shots were run in support of several experiments, including Edge Fluctuation imaging, Puffing probe plumes, and Omegatron; on these shots, the RF was pulsed at the end of the current flattop. Physics and Analysis -------------------- On Wednesday's run, the F-port and A-port scanning Mach probes were configured to record cross-field profiles of floating potential and ion saturation current fluctuations during L and H-mode phases of ohmic H-mode discharges. In previous attempts to monitor ion saturation currents with these probes, the Isat probes were held at a constant bias. This caused a runaway heating and arcing condition. For this run, a voltage sweep was maintained on the "Isat" probes. During data analysis, the Isat portion of the sweep will be extracted for fluctuation analysis. Preliminary results suggest that the 6 mm poloidal spacing between the "north" and "south" probes is too large to reliably record poloidal E-field fluctuations. This is necessary to allow an estimate of the local cross-field fluctuation-induced fluxes. However, good correlation is seen in floating potential fluctuations between the "North" and "East (or "West") probes which have a 3 mm poloidal separation. A redesign of the high power-handling Mach probe head with closer probe spacing is being considered. Such a design would allow both fluctuation-induced particle fluxes and parallel plasma flows to be recorded up to the separatrix. The Beam Emission Spectroscopy receiver was used to look at fluctuations in D-alpha emission. The views were essentially toroidal. We hoped to see correlations and phase shifts between poloidally separated views, with the goal of determining the radial electric field. Instead, we saw highly correlated fluctuations (for frequencies below ~50 kHz) between *radially* separated views across almost the entire plasma. In contrast, poloidal correlations rapidly dropped off with separation of the views. In both cases there was little (poloidal) or no (radial) indication of propagation. We interpret these results as due to contamination by D-alpha emission from the near- and far-fields of the views. We will next try a couple of modifications of the system: i) reduction of the collection angle of the optical system to minimize D-alpha collected in the near and far field (by stopping down the light cone exiting the optical fibers), ii) detection of bremsstrahlung emission, which peaks closer to the foci of the views. ICRF System ------------ We continued J-port antenna conditioning and testing into 5.2 T plasmas (H resonance is on-axis at 78 MHz), and were successful in demonstrating heating with the J-port antenna. Scanning different relative antenna strap current phases, the best heating was observed with [0,pi,pi,0]. Other phases did not result in H-mode and had signficantly more impurity influx, particularly Ti, Mo, and C. The H/D ratio was increasing with RF power and throughout the day. The hydrogen source may be the TiCN coated Faraday screen. We found that full current disruptions could be avoided by lowering the RF power. A small density scan suggested that the impurity injections were insensitive to density. With an outer gap of 8-10mm the antenna loading was high (above 10 Ohms). The antenna operated with lower loading (more like normal D and E-port loading values) with an outer gap at 1.2 cm. Operation this week will determine if more conditioning of the antenna can reduce the impurity source. In FMIT#3 and #4, the low power RF amplifiers and control system have been further investigated. An upgrade to power feedback circuit has been implemented and tested into dummy load, vacuum, and plasma. It functions better than its predecessor and will undergo further refinement. Although the reason is unclear, a 56 kHz amplitude modulation in the 78 MHz signal was found to be related to the presence of a splitter between the preamp (10 W) and the Kalmus amplifier (<500 W). A new set of splitters was installed and the amplitude modulation was eliminated. The Eimac 2274 tube from GA arrived, and, following inspection (including photographs), cleanup, and hi-potting, was installed in the FMIT#2 transmitter. FMIT# 2 will be tuned and tested into dummy load at power levels up to 2 MW. The cavity high voltage connection was upgraded as well, and was high potted to 30 kV in situ. We anticipate operation with this tube into plasma (E-port antenna) this week. Travel and Visitors -------------------- Miklos Porkolab attended the FESAC Panel meeting in Knoxville, Tenn, from Aug. 18 through Aug. 21. Ron Bravenec (U. Texas) was at C-Mod last week looking at D-alpha fluctuations with the BES system optics. Perry Phillips continued his stay working on the UT ECE system. Software was developed to look for changes in local Te gradients, the slow ECE channels were reconfigured to improve signals, and a new temperature fluctuation IF system was added.