Alcator C-Mod Weekly Highlights June 25, 2012 FY2012 weeks of research operations Target: 18 weeks Completed: 8.5 weeks Plasma Shots: 1330 Operations ----------- Plasma operations continued at Alcator C-Mod last week. Four run days were scheduled and 3.5 were completed. A total of 81 plasma discharges were produced with a reliability of 77%. The runs supported experiments in the MHD and Transport topical areas; all of the experiments last week contributed to student thesis research. A fresh boronization was carried out over Wednesday night. Plasma operations are planned to continue this week. Physics ------- The paper "Lower hybrid current drive at high density in the multi-pass regime", by G.M. Wallace, et al., has been published online on 22 June, 2012 in Physics of Plasmas (Vol.19, Issue 6). This paper is based on the invited talk given by Dr. Wallace at the APS DPP meeting in Salt Lake City last November. Parametric Decay Instabilities (PDI) have been observed during C-Mod Lower Hybrid experiments above and below the LHCD density limit. PDI activity detected by the inner wall probes increases sharply with the line averaged density, resulting in pump depletion and spectral broadening. When the line averaged density was raised above 1e20 m^-3, the spectral amplitude of the pump (4.6 GHz) decreased, while that of the lower sideband increased to a level similar to that of the pump. The frequency difference of the sideband indicates that PDIs occur on the inboard side. These observations were reported at the weekly C-Mod Science meeting by an MIT graduate student. Initial observations of temperature fluctuations in ohmic plasmas using the C-Mod Correlation Electron Cyclotron Emission (CECE) diagnostic were described at the weekly Science Meeting by an MIT graduate student. The fluctuations were observed in linear ohmic confinement (LOC) regime discharges, and because of the high optical depth (>2), they could be definitively identified as electron temperature fluctuations. The fluctuations were observed only in the LOC regime, and not in Saturated Ohmic Confinement (SOC) conditions in the same series of discharges. This result may be indicative of a change in turbulence characteristics in the SOC regime. Operation Details ----------------- Run time on Tuesday and on Wednesday morning was devoted to MP#697, "First operation of Shoelace Antenna: Survey of boundary plasma response". This experiment comprises part of the thesis research of an MIT graduate student. The primary goal of the MP is to survey the L-mode edge plasma response to the application of magnetic perturbations from C-Mod's new shoelace antenna system. The antenna was driven in the 40-300kHz band with antenna currents of up to 80 Amps. In addition to the standard Mirnov coils, the A-port Scanning probe (ASP) with a magnetic sensor head and the gas-puff imaging (GPI) system were employed to observe the plasma response. Scans of q95, density, and outer gap were carried out. Data are being analyzed. Wednesday afternoon was devoted to MP#702, "Cold pulse modulation experiments for the study of non-local heat transport and correlation with rotation reversals". This experiment comprises part of the thesis research of an MIT graduate student. Cold pulses were generated by the Laser Blow-Off technique, using CaF2 as the injected impurity. A density scan including the LOC and SOC regimes was carried out at 1.1MA, and data were obtained under SOC conditions at 0.55MA. Preliminary analysis shows evidence for a heat pinch in the LOC regime, but further analysis is required, with particular attention to the effect of sawtooth perturbations on the FFT amplitude analysis. Thursday's run was dedicated to MP#648,"Direct boron density asymmetry measurements". This experiment comprises part of the thesis research of an MIT graduate student. Collisionality and pedestal parameters were varied in EDA, ELM-free, and ELMy H-modes to study the effect on in-out boron impurity asymmetries, and to measure the total velocity vector in the pedestal region. These data are being analyzed. Friday's experiment was devoted to MP#675,"Initial disruption mitigation experiments with two gas jets". In addition to comprising part of the thesis research of an MIT graduate student, this experiment contributes to critical design issues for the ITER disruption mitigation system. Following a series of shots to re-commission the gas jets following repair of a power supply, and a sequence of single jet disruptions to characterize each jet individually, we performed a scan of gas jet stagger times to test the effects of one vs two gas jets on mitigated disruption figures of merit. We also carried out two forced, unmitigated VDE disruptions for comparison with the mitigated cases. In all phases, the heat load deposited in the divertor was monitored using IR thermography, to determine the effectiveness of disruption mitigation on reducing divertor heat loads. The divertor Langmuir probes were used to study halo currents during unmitigated and mitigated disruptions. The experiments were carried out in 1MA, 5.6T lower single null L-mode discharges with nebar~1.6e20/m^3 and 1MW of ICRF heating. ICRF Systems ------------ ICRF Operation supported physics experiments on Thursday and Friday last week. The Field Aligned antenna was operated into ELM free and ELMing discharges for the first time. The ELM free H-mode target decreases the loading significantly and results in higher voltages for given coupled power. This result is similar to the behavior of the other antennas; the pedestal height appears to dominate the loading variation over evanescence length. The FA-antenna also appears less sensitive to transient loading variations, including ELMs; the reason for the behavior is under investigation. Lower Hybrid System ------------------- The LH transmitter protection system (TPS) firmware was upgraded to version 12 last week. This firmware upgrade addresses issues with the asymptotic resolution of the collector over temperature system (COTS) calculation. The TPS boards were removed, reprogrammed, tested, and reinstalled in the cell. The LH system klystrons were operated into dummy loads and proper operation of the new firmware was verified. Travel and Visitors -------------------- Glen Wurden, LANL, visited MIT on Thursday & Friday (June 20-21), to work with Jim Terry to plan upgrades for the C-Mod IR camera system. He also discussed possible international collaborations with other PSFC staff. A new FLIR Systems SC8303 digital mid-wavelength IR camera is being procured with FES director's reserve funds, for use on Alcator C-Mod, and later the W7-X stellarator in Germany. The new camera will have 1344x784 pixel resolution, and transfer 14-bit full-frame data at 130 Hz. Sub-windowing allows much faster frame rates, albeit with restricted image sizes. Ahmed Diallo visited C-Mod all week to discuss machine requirements and diagnostic availability for an experimental proposal (MP709) on the pedestal characterization in both ELMy and EDA H-mode. He also met with A. Dominguez and S-G. Baek to get an update on the reflectometer system and to discuss fluctuation analysis in the pedestal region in preparation for the pedestal experiment. Dennis Whyte and Graham Wright presented talks on "Overview of Plasma-Surface Interaction Science Center Research" and "Update on C-Mod PSI research" respectively at the US Plasma-Facing Components meeting held at PPPL June 20-22. Dennis Whyte of the PSI Science Center, along with the other center PIs (R. Doerner UCSD, B. Wirth, U. Tenn, D. Buchenauer, Sandia) met with Peter Pappano of DOE June 19 to discuss PSI research and connections to confinement devices like Alcator. Nathan Howard and Anne White attended the GYROKINETIC THEORY WORKING GROUP MEETING, in Madrid, Spain at EURATOM-CIEMAT Association, partnered with E.T.S.I. Aeronauticos, UPM last week. The two week workshop is organized by Ivan Calvo (CIEMAT) and Felix Parra (MIT). Anne White (MIT) and Greg Hammett (PPPL) were responsible for organizing sessions on the connection between experiments and simulations, and Michael Barnes (MIT) and Peter Catto (MIT) organized the sessions on transport. During the first week, Felix Parra gave an invited seminar at the University of Madrid on gyrokinetic theory, Anne White gave an invited review talk at the workshop on validation of gyrokinetic simulations via comparisons with experiments, and Nathan Howard (MIT) gave an invited talk at the workshop on comparisons of impurity transport measurements on Alcator C-Mod and nonlinear gyrokinetic simulations. Other PSFC scientists and students are giving talks during the second week of the workshop, including Justin Ball, Michael Barnes, Jungpyo Lee, and Matt Landreman. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly