Alcator C-Mod Weekly Highlights July, 10, 2006 FY2006 weeks of research operations: JOULE target: 14 weeks Completed: 15.0 weeks Operations ---------- Plasma operations resumed on Wednesday last week. Three run days were scheduled and 2.5 completed. A total of 43 plasma discharges were produced with an overall reliability of 66%. Experiments were carried out in support of research in the MHD and Transport topical science areas and one day was devoted to a continuing IEA/ITPA Joint Experiment (MDC-6). The restart of Research Operations was delayed on Wednesday morning due to difficulties with the EF3 Poloidal Field Supply, which had undergone preventive maintenence during the previous week's outage. The supply was successfully returned to service, and all systems performed nominally during the remaining 2.5 days. Operation at 7.8T and 1.6MA in support of the experiment on Thursday was successfully completed. An overnight boronization was carried out on Thursday night, to prepare the machine for the ITB Physics experiment on Friday. This week will be a scheduled maintenance week, with no plasma operations planned. Research operations are planned to resume during the following week. Run Details ----------- A half-day run was conducted on Wednesday in support of MP#444b "Alfven Cascades during current ramp with ICRF heating". The primary goal of these experiments is to test the theoretical scaling of the "base frequency" of the cascade modes, particularly with temperature. Excitation of the cascade modes is induced by resonant interaction with the RF generated ion tail. Previous thresholds for activity were about 2.5 MW. A scan of RF power at 1 MW steps was used to vary the bulk temperature. Shots were obtained with multiple base frequencies which are observed to increase with time as Te increased. Density was found to be a key parameter in the development of cascades, with the strongest modes observed for line-averaged density of order 1e20/m^3 or lower. Thursday's run was devoted to MP#398 "Complete identity experiments with JET on error field locked mode thresholds", which comprises the C-Mod portion of ITPA/IEA Joint Experiment MDC-6. The main focus of the experiment was to reconcile an apparent discrepancy in the observed scaling of the non-dimensional locked mode threshold. Previous experiments at 6.3T and 4.1T resulted in a much stronger than expected scaling with field, which disagreed both with independent experiments on C-Mod and on other devices. The experiment on Thursday extended the C-Mod dataset to 7.8T, maintaing q95=3.25 and a JET-like equilibrium. Preliminary analysis indicates that the locked mode threshold is the same as that found at 6.3T, though significantly less that that at 4.1T. This near-constant threshold behavior is consistent with previous experiments, and appears to confirm the non-dimensional identity with respect to matched shots on JET obtained earlier. No such matched shot on JET is presently available for the C-Mod 4.1T case, and the discrepancy observed in these experiments may be indicative of more complicated physics governing the threshold. The results are important because the projection to ITER depends very sensitively on the observed dependence of the threshold on Bt. Friday's run was devoted to MP#445 "L_Ti Measurements in ITB plasmas". This experiment comprises part of the thesis research of an MIT graduate student, and is aimed at improving the understanding of the triggering mechanism for producing Internal Transport Barriers with off-axis ICRF heating. The question is whether ITB formation can be explained in the framework of suppressing ITG turbulence by increasing ion temperature scale length to the critical scale length. This hypothesis is tested by varying Bt and thus shifting the RF resonance location on shot-to-shot basis. Experiments were carried out with ICRF heating at 80MHz and at 70MHz, in each case scanning the resonance location across the low-field side (to 6.3T for 80MHz and 5.5T for 70MHz). The data are being analyzed. ICRF System ------------ The J-port antenna/transmission line and FMIT#3 and #4 were successfully changed over from 50MHz to 70MHz operation during the previous maintenance week. This system successfully operated as requested during experiments on Wednesday and Friday. A low-power pre-amplifier in the FMIT#1 (D-port) driver section was found to be overheating on Wednesday. A fan was replaced and the unit was returned to service. A low current discharge and tracking along the insulator was observed in the FMIT#2 crowbar cabinet last week. This unit was opened up, repaired and tested, and returned to service prior to Friday's run. Lower Hybrid System -------------------- Conditioning of klystrons to higher voltage continued last week. The #6 klystron (A2V2), which had been exhibiting excessive body current, was replaced with a spare. Following adjustment of the beam focusing, the replacement tube is operating satisfactorily. High power drive leg calibrations are planned for this week, in preparation for additional LH physics experiments. Travel and Visitors ------------------- On June 29-30, Ron Parker attended a meeting of the IPP Fachbeirat, or Advisory Committee, in Garching, Germany. Ron serves as chair of the committee and is responsible for preparing the report, which is transmitted to the President of the Max-Planck-Society. Some time was spent on reviewing the situation with the ASDEX motor generator that failed last April. Prof. Kaufmann has asked MIT to provide an expert to participate in a formal review of the accident and its causes, and recommended course of action for repair. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly