Alcator C-Mod Weekly Highlights Sept 25, 2000 Plasma operations continued last week. Four run days were scheduled, but only two were accomplished, due to a loss of vacuum accident late Wednesday afternoon. The experiments carried out included an investigation of the dependence of pedestal widths and the EDA/ELM-free boundary on current ramp rate; and a study of recombination and ionization in the divertor region. Late in the run on Wednesday, C-Mod experienced a sudden loss of vaccuum which was quickly traced to a hole in a bellows in the beamline of the Diagnostic Neutral Beam. INdications are that the damage was a result of a portion of the beam impinging on the bellows. The port was blanked off and the machine pumped back down successfully, and the vessel was baked to 100C for two days, followed by discharge cleaning in deuterium over the weekend. Plasma operation will resume this week; five run days have been scheduled, to make up for the time lost. Physics -------- An experiment was carried out to quantify the dependence of H-mode pedestal parameters on dI/dt, which is presumed to influence primarily the edge current density. Previous experiments had indicated that the width of the soft xray pedestal was influenced by current ramps. A second objective of these experiments was to determine if the transition from ELM-free H-modes to the EDA regime was sensitive to the current ramp rate or edge currents. Experiments were carried out at 0.8 and 1MA, with positive and negative ramp rates up to 1.25MA/s. In these experiments, no clear dependence on dI/dt was observed in the edge Thomson scattering profiles; a barely discernible effect was noted in the xray pedestal widths in the 1MA case, but it was smaller than expected and nearly masked by the well-established dependence on total current. However, the visible continuum profile (representative of ne) showed a strong variation with dIp/dt. No consistent dependence of the EDA/ELM-free boundary was observed. The goals of Wednesday's experiment, which will contribute to a doctoral thesis, were to characterize the magnitude, location, and evolution of the volume recombination and ionization in ohmic detached plasmas and EDA H-mode plasmas; and to obtain spatial temperature and density profiles using D_gamma to continuum profile ratios. Spectrally filtered TV images of the divertor region were obtained in D-alpha, D-delta, D-gamma, and continuum emission. The initial observations from this run include: 1) Detachment on the outer leg was observed by the cameras as a region of emission that extends up the outer leg. 2) There were no obvious differences in the locations of the D_alpha, D_gamma, and "continuum" emission regions during any conditions. (2-D inversions are being generated) 3) During EDA H-modes the emission (D_alpha, D_gamma, and "continuum") changes from being located at the outer strike-point and weakly on the inner nose to no observed emission on the outer leg or strike-point and strong emission on the inner nose. 4) Absolutely calibrated 2-D profiles are being generated for all shots in this run. ICRF Systems ------------ Further analysis of the J-port antenna results by the ICRF and Engineering Groups has led to new insights into the performance of this system. A subtle error in the antenna transmission line model had led to a phase error of 180 degrees. Based on an assessment of the antenna waveforms of the previous week, and the subsequent model revisions, a half-wavelength segment of transmission line was removed from one of the two active current straps. ICRF heating was attempted during Tuesday's run with the antenna in the two strap configuration, and heating efficiency equal to that from the D- and E-port dipoles was observed. Overlap of all three antennas brought the total power up to 4MW. The antenna was then reconfigured to a four-strap configuration, and heating experiments were continued in piggyback mode on Wednesday. The two antenna halves were initially brought up to the 0.5MW level individually, for conditioning and phase checkout. These experiments were intended to test the modified model and preliminary analysis indicates good agreement between the model and experiment. Additional experiments will be performed this week in an effort to develop a proper transmission line configuration for heating experiments. Diagnostic Neutral Beam Systems ------------------------------- The BES diagnostic is being used for measurements of the beam width by grouping its channels poloidally. The results of the analysis show a clear reduction in beam width from ~13 cm to ~ 9 cm as the beam current is increased from about 3.6 A to 5.1 A at a fixed accelerator voltage of ~44 kV. This is a very positive result which indicates that the beam is focussing properly and will assist with development of the diagnostics. In particular, it will be used to benchmark the beam's internal thermocouple measurements. This work needs to be repeated at intervals to assure reproducibility and to assist in diagnostic development. The spectra that will be used by MSE and BES were observed with good spectral resolution. The predicted spectrum agrees with that measured. The MSE polarizer was positioned to transmit both pi and sigma components, but because of the low TF (3.5 T), the sigma and pi components were merged. With the experience gained during this work and with the predictions developed in the course of it, we will be able to easily repeat the measurements with the polarizer set to transmit only pi and then only sigma components. Continued analysis of existing data at higher TF may provide additional information as well. Both the perveance measurements and these spectral measurements benefited from firing the beam into neutral gas. As mentioned, The bellows in the DNB duct developed a leak and is in the process of being replaced. The replacement bellows will be provided with protection from internal beam interaction and from external abuse as well. Travel and Visitors -------------------- Norton Bretz and Garrit Kramer came to MIT 9/18-22 for further work on the MSE diagnostic.