Alcator C-Mod Weekly Highlights August 6, 2012 FY2012 weeks of research operations Target: 18 weeks Completed: 12.6 weeks Plasma Shots: 1828 Operations ----------- Plasma operations continued at Alcator C-Mod last week. Four run days were scheduled and three and a half were completed. A total of 84 plasma discharges were produced with a reliability of 84%. The runs supported research in the MHD, Pedestal Physics, and H-mode Scenarios topical areas. A Pedestal Physics experiment scheduled for Tuesday had to be deferred due to unsatisfactory machine conditions. Over four hours of operating time were lost on Friday due to a computer network problem. Fresh boronizations were carried out over Tuesday and Thursday nights in preparation for experiments. Plasma operations are planned to continue this week. Program Planning ---------------- An EPC meeting was held to discuss status and near term plans for the campaign. The switch to reversed field will take place in mid-August on a precise date depending on the number of high-priority runs remaining requiring forward field. Four experimental proposals were reviewed and approved. Physics ------- At the C-Mod Weekly Science meeting, Jim Terry presented observations of a decrease in edge and SOL turbulence during the application of LH power to ICRF-heated EDA H-mode plasmas. As this decrease occurs, the stored energy increases by up to ~30%, during the LH pulse (P_LH ~650 kW). The turbulence measurements are from the outboard-midplane Gas-Puff-Imaging diagnostics. Both the signals from the 2D diode array and the 2D camera images show a fast and significant decrease in the broadband fluctuation power across the radial field-of-view, ~-1.5 cm < rho < 2 cm. 'Blobs' no longer appear in the far SOL during the LH application. Average emission levels in the far-SOL are also significantly increased when the LH is applied. The frequency of the Quasi-Coherent Mode decreases, indicating a change in plasma rotation in the pedestal. The density at the top of the pedestal in these EDA H-mode discharges is well above the cut-off density for LH wave penetration. Operation Details ----------------- The experiment on Tuesday was scheduled for MP#709, "Pedestal profile and fluctuation evolution in ELMy H-Mode". The run began with several disruptions, which were determined to have been caused by a faulty magnetics sensor and an unrelated fault in a poloidal field power supply. Once these problems had been repaired, it was determined that the hydrogen fraction, H/(H+D) was over 30%, too high for efficient minority heating. The remainder of Tuesday's run was then given over to conditioning shots with ICRF, which succeeded in reducing the hydrogen fraction below 10% by the end of the day. After additional overnight ECDC, H/(H+D) was under 5% during Wednesday's experiment, and remained low throughout the rest of the week. The run on Wednesday was devoted to MP#577, "ITER-like Discharge Rampup Studies". The goals of this experiment were to revisit previous experiments focusing on the ramp-up phase with density variations and various heating sources, including ohmic, ICRF, and LHRF, with improvements to the early divertor formation to minimize wall contact after the initial X-point formation. The improvements in gap control proved difficult to achieve, largely due to power supply limitations, and this aspect of the experiment was not fully successful. Density scans were obtained at the nominal ramp rate, reaching flattop at 0.5sec, corresponding to about 80-100 sec current rise phase in ITER, with ohmic heating alone and with 1MW of ICRF injected early in the current ramp. Initial observations do not indicate significant variation in volt-second consumption, in contrast to earlier experiments with similar parameters. Attempts to increase the ramp rate to correspond to a 50sec ITER current ramp, as proposed in recent scenarios developed by the IO, were not successful due to power supply voltage limitations. Thursday's run was devoted to MP#714,"MHD effects on massive gas injection disruption mitigation". Target plasma elongation (kappa) was scanned in inner wall limited discharges at constant safety factor and plasma current, with varying Bt, in order to determine the effect of kappa on important disruption parameters. The safety factor q* was then varied at constant elongation in LSN equilibria, again by varying Bt, in order to determine the effect of q on toroidal radiation asymmetry in mitigated disruptions. The data set included plasmas with kappa=1.6, q*=3.2, Bt=4.0, Ip=750kA in both limited and LSN configuration in order to determine whether the magnetic configuration has an effect on important disruption parameters. Each configuration was repeated two or three times to improve statistics and determine the extent of shot-to-shot variations in observed disruption characteristics. A single gas jet was employed for most of the day's experiment. Additional data were obtained firing both gas jets into limited target plasmas at low, medium, and high elongation. One gas jet disruption was produced for a target with an intentionally induced locked mode, to determine the effect of a pre-existing 2/1 mode on the radiative asymmetry in mitigated disruptions. These data are being analyzed, and will contribute to the thesis research of an MIT graduate student. Friday's run was devoted to MP#648, "Direct boron density asymmetry measurements". The goal of this experiment, which supports the thesis research of an MIT graduate student, was to vary the collisionality and gradient scale lengths to vary the Boron density asymmetry, which is observed using edge CXRS measurements at the inboard and outboard midplane. The plan for the day was to focus on EDA and ELM-free H-mode target conditions with varying current and RF power. The start of the run was delayed due to a computer network problem, and only a portion of the run plan could be completed. Data were obtained in EDA H-modes at 0.6, 0.8, and 1.0MA, with two levels of RF power at each condition. ELM-free conditions were also obtained at 1MA, which will contribute to the dataset in that configuration. The remaining of this experiment will be rescheduled. ICRF Systems ------------ The ICRF system was run in support of experiments this week. FMIT#3 grid bias regulator would not reset during the run on Tuesday after a tube arc. The supply and circuit were tested and placed back into service. The system operated without incident thereafter. The first two new ARRA-funded FFTs arrived in Boston and should be delivered in the coming week. Lower Hybrid System ------------------- The LH system was operated into plasma on Tuesday. A high voltage power supply (HVPS) module which had been replaced during the previous week was repaired and successfully returned to service. Work on the LH3 control interface board design and schematic continued. Diagnostics ----------- The data acquisition system for the accelerator-based first wall diagnostic system (AGNOSTIC) has been installed in the C-Mod cell and has been successfully operated remotely from the C-Mod control room. The accelerator and supporting racks were successfully installed in the C-Mod cell on Saturday. The AGNOSTIC diagnostic server and crate were installed in racks and electrical service and communications provided. The diagnostic probe was also installed, and data were taken with this system. Travel and Visitors -------------------- Martin Greenwald, Greg Wallace, and Steve Scott (PPPL) were in College Park, Md to participate in a meeting of the APS-DPP program committee. Ahmed Diallo (PPPL) visited MIT last week, and served as Session Leader for Tuesday's planned experiment; however, the observed hydrogen fraction was too high for the goals of this experiment to be met, and the experiment will be rescheduled. Chuck Kessel and Francesca Poli (PPPL) visited MIT on Wednesday to participate in the H-mode Scenarios experiment. Chuck served as Session Leader for the experiment. Thomas Puetterich (ASDEX-Upgrade) is visiting MIT for 10 days to collaborate primarily on the gas-puff based charge exchange measurements in the pedestal region. This work is a continuation of the shared development of this diagnostic technique as well as investigations into pedestal physics, primarily on impurity transport. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly