Alcator C-Mod Weekly Highlights Oct 14, 2002 Physics operations continued at Alcator C-Mod last week. Four run days were scheduled and completed. Experiments were carried out in support of the Transport and Advanced Tokamak efforts. Progress also continued in the Lower Hybrid Fabrication project. Plasma operations are scheduled to continue this week. Operations ----------- A fresh boronization of the C-Mod vessel was carried out Monday night, and plasma runs were conducted on Tuesday through Friday, A total of 78 plasma discharges were produced, with a startup reliability of about 75%. Following a series of boronization recovery shots on Tuesday morning, the runs on Tuesday and Wednesday were devoted to a continuation of MP#304, "H-mode regimes", The focus of this set of experiments was the dependence of the EDA regime, specifically the appearance of the quasi-coherent (QC) mode, on triangularity and safety factor. For a series of shots employing central ICRF minorty heating, at different values of delta, the plasma current was ramped during the H-mode phase, while the QC mode was observed using the PCI and Reflectometer diagnostics. At the higher values of triangularity (delta-av = .65) the QC mode persists even at low q values, while at low triangularity (delta-av=.35) the mode disappears as qpsi is reduced. Detailed analysis of these results is in progress. The runs on Thursday and Friday were devoted to studies of internal transport barrier formation with off-axis ICRF heating. A continuation of MP#301 on Wednesday showed no evidence of a dependence of the ITB foot location on plasma current over the range 600kA < Ip < 1.1MA, with the ICRF resonance on the high-field side (80MHz at 4.5T); previous experiments had seemed to indicate a posssible narrowing of the barrier region at low currents, but these results were not confirmed. On Friday, experiments in support of MP#317 concentrated on ITB's formed with the minority resonance on the low-field side of the magnetic axis (70MHz at 5.5T). The existence of ITB's induced with low-field side resonance was demonstrated in the 2001 campaign, but has received less study than the high-field side case. In these experiments, parameter scans indicated that the operational range for formation of the ITB is quite narrow, requiring BT between 5.5 and 5.7T; the high field limit may be imposed by proximity of the resonant surface to the launcher. Within this range, ITB's were produced with input power as low as 1.5MW, just over the H-mode threshold. Near-axis heating, inside the ITB, was also carried out, using one 80MHz source. The density profiles indicated that the barrier foot for these low-field side cases is closer to the axis than in the case of high-field side resonance. The primary emphasis in this run was documentation of the plasma temperature, density and rotation thresholds for detailed comparison with theoretical analysis. On Friday a preliminary review of the j-port tile failure was held, and suggestions for changes to the tiles made. A comprehensive testing program was also outlined to validate the new design. ICRF Systems ------------- ICRF operation supported the planned experiments for the past week. The E-port antenna was found to have ~ 25% higher voltages when D-port antenna was on. This suggests D-port is modifying the SOL density in the vicinity of E-port resulting in lower loading. This effect may explain why E-port can operate at higher power levels when D-port is off. Several hours were lost during Tuesday's run due to a failed cable in the motor drive of the E-port stub tuner. The cable was replaced and operation resumed. Transmitter #4, which drives half of the J-port four-strap antenna, developed a problem in the screen supply late in Friday's run. This fault is under investigation. DNB Systems ------------ The beam was operated throughout all of the experimental days this week. The automatic LN2 fill system has continued to work reliably. An assessment of overall computer software and hardware upgrades to the DNB control system was begun. The goal here is to simplify and modernize the control systems. The BES spectrometer setup was modified to improve rejection of ambient D-alpha light. As this modification was being made, some improvements in the optics were identified and will be undertaken. We continued to acquire CXRS data under a variety of density conditions and at various times during the discharge to find the range of conditions under which measurements can be made and to determine the difficulty of making measurements in the presence of interfering spectra. Following last week's MSE calibation shots (beam into gas-filled torus with various toroidal and poloidal fields) the C-Mod beam was injected into the higher-density ITB discharges under investigation this week. In an attempt to look at the diagnostic's behavior at lower densities, where signal from the core is expected, and which are anticipated for the LHCD experiments, the beam was also injected into the early part of discharges, when the density was still evolving. MSE data were acquired in the vicinity of sawtooth onset. Analysis is in progress. Lower Hybrid MIE Project ------------------------ Design work is underway for the superstructure to support the circulators and dummy loads. We have been notified by the vendor that the crowbar circuitry for the High Voltage Power Supply (HVPS) has been shipped. Delivery is expected in a few days. Travel and Visitors ------------------- Bruce Lipschultz was at JET Sept 25 - Oct 3 to participate in experiments to better understand SOL transport. Most of the time was spent becoming familiar with the data /data system and preparing for the run (Oct 1.). Dr. Lipschultz gave a presentation on two subjects - experiments done on C-Mod and DIII-D to compare transport (and the results of this analysis) and the development of a measurement of SOL ionization source rate in JET based on their wide-angle camera. The JET experiment was aimed at extending the DIII-D/C-Mod comparison to an even larger experiment. The experiment was moderately successful. Data were obtained at two currents and fields in ohmic plasmas. The H-mode data had some problems in that there was apparent interference in the Langmuir probe data. Analysis will be carried out over the next several months. Gerrit Kramer (PPPL) was at MIT 10/9-10 to reinstall the repaired Gunn diodes for the microwave reflectometer diagnostic. Reflection off the vacuum vessel inner wall was detected in off-line checkout, but faulty CAMAC triggering precluded operation with plasma. This will be tracked down and corrected as soon as possible.