Alcator C-Mod Weekly Highlights April 5, 2004 Research operations continued at Alcator C-Mod last week. We have now completed 56 research run days (14 weeks) out of a target of 72 days (18 weeks) for the FY04 fiscal year. During the week, C-Mod operated at a toroidal field of 8 tesla for the first time during the current campaign. Progress on the Lower Hybrid project also continued. Plasma operations are scheduled to continue this week. Operations ---------- C-Mod operated for two and a half days last week. A total of 44 plasma discharges were produced with a startup reliability of 80%. Loss of 13.8 kV power from the local utility necessitated cancellation of planned operation on Thursday afternoon and Friday. Experimental power has been restored, and the planned experiments have been rescheduled. On Tuesday, we revisited MP#380, to investigate the role of fast particle pressure and ion cyclotron current drive in modifying the sawtooth period. The experiment focussed on toroidal field scans at 1 MA to investigate the impact of antenna phase on sawtooth period. There was not a strong impact on the sawtooth period for the B-field sweep performed for any antenna phase at 1 MA. The sawtooth period was increased over L-mode and of similar period and amplitude for central deposition for all antenna phases. This behavior is in contrast to the 0.8 MA results where the sawtooth period is sensitive to antenna phase. In addition a set of modes were observed on PCI and magnetics. The modes observed by the PCI were 700-750 kHz, appeared during the flat top of each ST and were insensitive to antenna phase. The modes vanished for off-axis deposition and 0.8 MA discharges. These modes may be a core localized Alfven mode (energetic particle or resonant toroidal Alfven eigenmode). On Wednesday, we resumed 8 tesla operation for the first time during the present campaign, in preparation for several experiments which call for high field. Following a thorough checkout of the TF instrumentation and inspection and retorquing of the TF bus, a standard sequence increasing the toroidal field by ~0.5T per shot was carried out, leading to successful plasma operation at 8.1T, 1.05MA. Initial D(He3), 8 T experiments were then carried out to identify the proper He3 gas fueling required to obtain effective heating; 5 keV electron temperatures were obtained in L-mode discharges at currents up to 1.2MA. A fresh boronization was carried out over Wednesday night, in preparation for additional high field experiments. Recovery from boronization, including reconditioning of the ICRF antennas, took up most of the morning on Thursday. Contrary to the experience on Wednesday, startup at 8T proved problematic, likely as a consequence of the boronization. At the time of the utility outage, no progress had been made on the scheduled MP#371, which calls for operation at 2MA plasma current. This experiment has been rescheduled. On Friday a series of beam-into-gas shots was carried out in support of calibration of the MSE system. This activity did not involve operation of the tokamak fields, and so could be done during the 13.8kV outage. Physics --------- High frequency modes in the range of 500 - 600 kHz have been observed bursting on and off in the middle of Enhanced D-alpha H-modes during the 1040318 run. They are correlated with increased edge soft x ray emission as well as increased neutron rate. While this correlation was first noticed during EDA H-modes with internal transport barriers, it was found that there is no correlation with profile peaking. The modes were observed up to line averaged densities of 3.5 x 10^20 m^-3 with only 2.5 MW of ICRF power from the D and E antennas. Stored energy increases up to 15 kJ and drops in the loop voltage by up to 0.2 V were observed during these modes. The modes appear to rotate in the electron diamagnetic drift direction and have toroidal mode numbers of 8 - 9. While in some cases, the mode frequency agrees well with the calculated TAE frequency at the q=1 surface, the agreement is not so close at the highest densities. So, while the modes have a number of characteristics of core fast particle driven modes, they also have characteristics of steep edge gradient driven modes. Alan Lynn (U. Texas) distributed his thesis to his committee. The thesis is devoted to ECE measurments in C-Mod for both fluctuations and for electron temperature profiles. He plans his defense for late April. ICRF System ----------- The J antenna and associated transmitters were reconfigured for 78 MHz operation. Additional fine tuning was performed on FMIT#1 (which has the lower power tube in its final stage) to lower the FPA anode dissipation for 0.75-1.4 MW operation into dummy load. This allowed reliable 1 MW operation into plasma. The first new DC break based on the PPPL design is nearing completion. We intend to test several insulators (teflon, kapton tape, kapton wrap, Cu clad kapton, and a high dielectric strength coating) and plan to use either teflon or kapton to have the breaks ready for April 16th. Lower Hybrid System ------------------- We continued preparations for brazing the lower hybrid couplers last week. Twenty-three of the 24 windows in the 24-window prototype were leak tight. Inspection of the leaking window indicated that approximately 1/2 the required braze material had been used (one braze wire had been missed). We are improving our procedures to insure that this error does not recur. We have now successfully tested eight out of twelve circulators to full power. This activity was slowed somewhat by a questionable power level reading from a directional coupler. The coupler has been replaced and will be returned to the vendor. The forward waveguide has been pumped down and leak checked successfully prior to a bake. The bake and high sensitivity leak check activities will begin this week. Tests of the heaters installed in the fwg indicate some of the heaters were not useable. Personnel from PPPL and MIT will meet this week at MIT to resolve this issue. A complete feedback loop consisting of phase and amplitude control components and a serial fiber optic link is now operational and performing well. Travel and Visitors -------------------- T.V. George (DoE), Rich Callis (GA), and Joe Tucker (GA) visited to discuss a variety of techincal and programmatic issues including the status of the fast ferrite tuner experiments, ICRF antenna design, and the lower hybrid system.