Alcator C-Mod Weekly Highlights July 31, 2012 FY2012 weeks of research operations Target: 18 weeks Completed: 12.0 weeks Plasma Shots: 1744 Operations ----------- Plasma operations continued at Alcator C-Mod last week. Four run days were scheduled and completed. A total of 98 plasma discharges were produced with a reliability of 88%. The runs supported research in the Pedestal, Edge, and Lower Hybrid Physics topical areas. A fresh boronization was carried out over Tuesday night in preparation for the experiment on Wednesday. Plasma operations are planned to continue this week. Program Planning ---------------- A meeting of the Experimental Program Committee was held on Monday 7/23. At that time we reviewed machine status and plans for the week. We are still on schedule for reversing the field at the middle of August. Deadlines are set for new MPs for any run time this year that require reversed field - MP's must be in for EPC review on 7/30. Six new experimental proposals were reviewed and approved. As the end of the campaign approaches, supervisors and students were asked to review the status of thesis work and prospects of gathering essential data. Operation Details ----------------- Two experiments from the Lower Hybrid Physics topic were carried out on Tuesday. The morning was devoted to MP#665, "Field, Current and q-dependence of the LH Density Limit". The goal of this run is to expand our database to better characterize the conditions in which LH current drive efficiency degrades. The experiment on Tuesday emphasized low-field operation, and obtained data over a range of densities with Bt=3T, Ip=600kA. A single density case was also documented at B=4.2T. The second experiment on Tuesday was devoted to MP#663,"Using alternate limiting surfaces with LHCD". Target plasmas were produced alternately using the upper flat plate "shelf" and the inner-wall "nose" as limiter surfaces, to evaluate LHCD efficiency as a function of density for these configurations. Dynamic scans from an upper null configuration to the upper plate limiter configuration, and from LSN divertor to nose-limited, were also performed. A "conventional" USN shot with a density ramp was taken for comparison with the other shots of the day. The initial assessment is that neither the nose-limited nor the upper shelf-limited equilibria exhibited a significant improvement over diverted discharges, in contrast to the inner-wall limited cases studied previously. Further analysis of these data is in progress. On Wednesday we carried out MP#701, "Impact of low-Z impurity concentration on core and pedestal transport in EDA H-modes". The purpose of this experiment is to study the core and pedestal transport in more detail in plasmas similar to those run under MP#564, in which impurity seeding helped to enable high performance, H98 > 1.0, and high fusion reactivity. Neon seeding was employed for target plasmas with Ip=800kA and ICRF power up to 5MW. The level of neon seeding was scanned over a factor of two, spanning the range of low-Z (Ne) dominated to high-Z (Mo) dominated radiated power. In addition to the seeding, the synergistic effects of lower hybrid on the pedestal and core confinement behavior, observed in previous experiments, were also studied. The density drop and temperature rise with application of LH to EDA H-modes was clearly demonstrated, and increased with P_LH in the range of 300-600kW. Edge turbulence was observed to decrease promptly during application of the LH. Extensive data were obtained from multiple profile and fluctuation diagnostics. Analysis of these data is underway. Thursday's experiment was devoted to MP#667, "Commissioning of new SOL probes". The purpose of this run day was to acquire ion temperature profiles in the SOL with the Ion Sensitive Probe (ISP) over a range of densities. This experiment supports the thesis research of an MIT graduate student. Good data were obtained with the ISP system. A density scan from sheath-limited to detached divertor conditions was performed in ohmic discharges with Bt0=5.4T, Ip=800kA. The SOL temperature profiles measured with the ISP indicate Ti~3*Te, as shown previously and expected from simple transport arguments. Initial inspection of the data reveals that Ti appears to decrease in the SOL as the density is raised, as is expected due to increased collisionality, but does not fully equilibrate with Te. A density scan was also performed at lower current, Ip=580kA. The lower current allowed us to scan the edge density regimes, in to, or slightly beyond the LCFS, without exceeding current limits on the probe power supplies. Ti was lower at this current, and some probe scans were deep enough to observe Ti~Te. Comparison data were also obtained by the CXRS diagnostic. The data are being analyzed to explore the relative levels of heat transport by ions and electrons in the SOL. The run on Friday was devoted to MP#699, "Explore LHCD efficiency at high density with H(He-3) plasma". The goal of this MP is to reproduce previous He3 minority MC heated plasma, and apply LHCD to see the benefit of higher Te target plasma on current drive efficiency. The hydrogen majority, He3 mode conversion ICRF heating scenario was employed for this experiment, with Bt0~6.3T. Comparison LH target discharges at Te0~2keV with ohmic heating, and up to 5keV with mode conversion heating, were produced. In the MC case, the PCI diagnostic observed that the MC layer was located close to the plasma center. We observed good LHCD at nebar~1.1e20/m^3, even in the case with only ohmic heating. Initial comparisons indicate that the LHCD could generate similar populations of fast electrons at this density, independent of the ICRF heating. This result contrasts with our prior experience in deuterium majority plasmas, and the data are being analyzed. ICRF Systems ------------ ICRF was operated in support of physics runs this week. One of the experiments successfully utilized H majority and He3 minority/mode conversion at 6.3 T. We noted some interesting rotation associated with this mode conversion scenario, as we have seen in He3 mode conversion in deuterium majority plasmas. The first pair of ARRA-funded fast ferrite tuners passed their final inspection at the vendor's facility, and are to be shipped by July 31st. Lower Hybrid System ------------------- The LH system was used to support C-Mod runs on Tuesday, Wednesday, and Friday. Joint operation with the D- and E-port ICRF antennas on Wednesday's run showed minimal deleterious effects on LH coupling, while joint operation with the E-port antenna on Friday did show a significant increase in reflection coefficients relative to Ohmic plasmas. The 8-way splitter assembly for the LH3 launcher was plated with copper and is ready for grit blasting of the welding surface. Welding of the cover plate can begin after the copper plating is removed from the welding surface. Diagnostics ----------- Fabrication has begun on machined components for a single-channel, prototype MSE background subtraction polychromator. This system will simultaneously measure the beam-induced MSE signal for two wavelength pass-bands (corresponding to the sigma-0 and pi-3 lines of the MSE spectrum) as well as the polarized background signal in two wavelength pass-bands. This will enable the MSE system to measure the polarized background intensity in real time even when the beam is firing, and should lead to reduced uncertainties in measured polarization angle that arise from uncertainties in the background light. Fabrication, assembly and alignment are expected to be completed before the end of August. The system will be tested with plasmas during September. Travel and Visitors -------------------- David Mikkelsen (PPPL) visited MIT last week, where he worked with other members of the C-Mod group on turbulence simulations, and searched for well diagnosed shots to use in studies of density peaking in low-density H-mode and I-mode plasmas. Dennis Whyte was at DIII-D the week of July 23rd to lead an I-mode experiment : MP #2012-93-04 "Exploration of I-mode operating space". The half day experiment was run over two days on July 25 and 26, and Amanda Hubbard and Jerry Hughes participated remotely during the runs as assistant session leaders. Anne White was on also site at DIII-D to support the I-mode experiment. The DIII-D I-mode experiment consisted of a wide parameter scan in current, density,and heating power to determine the conditions most favorable for I-mode. Higher than expected H-mode power thresholds were observed over a wide range of densities ( ~ 2.5-4.0 x 10^19 m^-3 ). A shape scan also provided new information that increasing triangularity (from 0.52 to 0.71) at does not favor I-mode formation in DIII-D, an interesting result, given the C-Mod USN results indicating that triangularity of about ~ 0.7 is favorable for I-mode [Whyte NF 2010]. Several very promising I-mode candidates were obtained during the DIII-D experiment, with plasmas exhibiting clear formations of electron temperature pedestals (with no density pedestal) and increasing confinement as heating power increased. These transitions to higher confinement were not accompanied by drops in edge D-alpha light or by increases in density or radiated power, further evidence that these were not H-modes. Analysis of fluctuation data is ongoing, in order to determine if changes in edge and core turbulence measurements in these plasmas of interest are consistent with the C-Mod I-mode phenomenology. While at DIII-D the week of July 23rd, Anne White also worked to prepare for an upcoming JRT 2012 experiment -- planned for the coming week -- to explore the transport shortfall seen in gyro modeling of transport and turbulence in L-mode plasmas. The simulations underpredict the transport and fluctuation levels, compared to the experimental values. Anne met with Terry Rhodes (UCLA) and the transport group to finalize the shot plan for the shortfall experiment. Nathan Howard participated in the experimental planning remotely from MIT. One goal of the experiment is to test a prediction that the transport shortfall should become larger at higher q, and should also extend deeper into the core [J. Kinsey TTF 2012]. The L-mode shortfall JRT 2012 experiment is planned for August 2nd, and the half-day run plan includes an engineering q-scan, in conditions that very closely match the plasma shape and q95 values from past C-Mod experiments [N. T. Howard, POP 2012]. This new experiment at DIII-D will allow the C-Mod and DIII-D teams to study the shortfall in detail for the first time, using targeted data sets from two tokamaks. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly