Alcator C-Mod Weekly Highlights August 13, 2012 FY2012 weeks of research operations Target: 18 weeks Completed: 13.5 weeks Plasma Shots: 1940 Operations ----------- Plasma operations continued at Alcator C-Mod last week. Four run days were scheduled and completed. A total of 112 plasma discharges were produced with a reliability of 87%. The runs supported research in the Pedestal, Edge, Transport and Lower Hybrid Physics and H-mode Scenarios topical areas. Fresh boronizations were carried out over Sunday, Tuesday and Thursday nights in preparation for experiments. About one hour of scheduled run-time was lost on Friday due to a malfunction of a high voltage AC breaker. Plasma operations are planned to continue this week. Physics ------- On Friday, August 3, Arturo Dominguez, graduate student in the MIT Physics Department, successfully defended his PhD thesis, entitled "Study of Density Fluctuations and Particle Transport at the Edge of I-Mode Plasmas". The main result of the work is the experimental investigation of the role of the weakly coherent mode in regulating particle transport across the I-mode temperature pedestal. Operation Details ----------------- Tuesday morning was devoted to MP#645a "Scoping study of N2 seeding in high current ITER-like EDA H-modes". The goal of this experiment was to develop nitrogen seeding suitable for use with high power ITER-like discharges at 1.3MA, 5.4 T, at maximum available ICRF heating power. Seeding was accomplished using the divertor-localized piping behind the B-port divertor module. Using N2 flow rates of 9 to 13 Torr-l/sec during the RF pulse, we successfully produced quasi-stationary EDA H-mode discharges with up to 4.5MW of ICRF power in ITER-relevant equilibria with q95~3.1 and kappa~1.75, and n/nG~0.65 . The plasma stored energy reached 220kJ, with volume averaged pressure exceeding 1.5 atmospheres. Core molybdenum radiation was reduced with N2 puffing, and divertor radiation was observed to increase. The power reaching the outer divertor appeared to be lower with seeding, but the strike-point was not detached. Detailed analysis, including IR camera data and core and divertor spectroscopy, is proceeding. Runtime on Tuesday and Friday afternoons was devoted to MP#715, "Do current ramps modify H-mode access?". This experiment supports ITPA Joint Experiment PED-33, and supports ITER scenario development in which access to H-mode is desired both during current ramp-up and in the early stages of ramp-down. Pre- and post-flattop access to H-mode will impact both volt-second consumption and control capability, and is considered an urgent ITER R&D requirement. The experiment was carried out by determining the L-H threshold power in the flattop phase of a discharge and then imposing current ramps (both up and down) with |Ipdot|~3MA/sec, and observing the effect on the L-H and H-L thresholds by varying the RF input power on a shot-to-shot basis. Experiments were carried out at two values of target density, nebar=1.4e20 and 2.1e20/m^3. At the lower density, the static L-H threshold was close to the ohmic input power, while at the higher density substantial RF power was required. In both cases, we found that H-mode is more easily accessed at a given input power during an Ip ramp-down, and that access is not significantly enhanced, and may be inhibited, during ramp-up. Quantitative analysis of these data is in progress. Wednesday's run was dedicated to MP#713, "Impurity transport in H-modes of varying collisionality". This experiment comprises part of the thesis research of an MIT graduate student. The objective was to explore the transport of injected CaF2 in a variety of EDA H-mode conditions. H-mode plasmas were obtained with 430 < Ip < 1MA, and with RF power between 1.5 and 4MW. In addition to the current and power scans, neon puffing was employed on a few discharges in order to vary collisionality through Z_eff. These data are being analyzed. The experiment on Thursday morning was devoted to MP#667,"Commissioning of new SOL probes", which supports the thesis research of an MIT graduate student. The purpose of this run was to operate the new A-port scanning probe (ASP) retarding field analyzer (RFA) to measure ion and electron temperatures in the SOL and in to the separatrix. These data will be used for validation of ion temperature data obtained with the ion sensitive probe (ISP). The ISP data covered a density scans at both 0.58 and 0.80 MA. Density scan data with the RFA were also obtained at 0.58 and 0.8MA, as well as low density points at 1.2 and 1.1MA. These electron and ion temperature data will be compared with the results using the ISP. Thursday afternoon was devoted to MP#665 "Field, Current and q-dependence of the LH Density Limit". Data were obtained in deuterium majority discharges at 3.8, 6.3, and 7 tesla in USN equilibria. Preliminary analysis indicates significant current drive was obtained with nebar~1.1 to 1.3e20 at the higher fields, and with Ip=800kA. These results are comparable to recent results at 6.3T and 800kA in hydrogen majority discharges. The parametric decay instability (PDI) signature appears to be stronger in the D plasmas, but the magnitude of current drive is very similar. Friday morning was primarily devoted to MP#718 "Effect of poloidally-localized sources on in-out asymmetries"; the run also addressed parts of MP#648 "Direct Boron density asymmetry measurements". These experiments comprise part of the thesis research of an MIT graduate student. Four sets of experimental conditions were attempted during this 5 hour run. In support of MP#718, dynamic scans from LSN to DN to USN configurations were performed to determine the effect of magnetic topology on impurity density. There were difficulties in maintaining steady H-modes after the DN to USN transition, but the LSN to DN comparison should provide useful data. Scans of the inner (HFS) gap from 14mm to 2mm were successfully performed during a LSN EDA H-mode. In support of MP#648, current and RF power scans were performed in ELM-free H-modes, with dynamic sweeps of the inner gap. These data complement EDA H-mode data obtained in an earlier experiment. Finally, N2 injections were performed from the LFS and HFS capillaries into ohmic L-mode plasmas. These data are being analyzed. ICRF Systems ------------ The ICRF system was run successfully in support of experiments this week. The first two new ARRA-funded Fast Ferrite Tunerss were delivered and have passed incoming inspection. These components will replace the current pair on the E-port antenna, which will then be reconditioned. Lower Hybrid System ------------------- The LH system was used in support of experiments on Thursday. Work continued on development of an FFT tuner system for the lower hybrid launcher. Operation of the tuner is currently limited to about 120 kW with a goal of 150 kW (~15 kW/cm^2). Changes to the positioning and shape of the ferrite material will be made for the next test. Diagnostic Neutral Beam Systems ------------------------------- The arc control circuit of the Diagnostic Neutral Beam has been repaired, and the arc is now operating normally. High voltage breakdown in part of the beam circuitry is temporarily restricting operation to about 45 kV. Diagnostics ----------- Installation of the accelerator-based first wall diagnostic system (AGNOSTIC) is proceeding. The RFQ beamline, including focusing quadrupoles, has been installed and the RFQ and beamline have been aligned. The RFQ vacuum system has been aligned, leak-checked, and pumped down. First measurements in C-Mod using the AGNOSTIC system are planned for early this week. Travel and Visitors -------------------- Jerry Hughes visited GA from Aug 6--10 to work with Rich Groebner and other GA scientists on coordination of presentation of joint research in pedestal physics at the fall meetings. Rich and Jerry will give major talks on the FY11 JRT at the IAEA FEC and APS-DPP, respectively, discussing collaborative efforts among C-Mod, DIII-D, NSTX and US theory and modeling groups. They developed tentative outlines for presentations and papers, and made a workplan for proceeding forward with the assistance of the other JRT participants. While there, Jerry began some follow-up analysis on an H-mode pedestal experiment he previously ran at DIII-D as a dimensionless match to C-Mod ELMy H-mode. In addition, while at GA, Jerry also made use of remote collaboration tools to serve as Session Leader for experiments at C-Mod on Tuesday and Friday. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly