Alcator C-Mod Weekly Highlights September 4, 2012 FY2012 weeks of research operations Target: 18 weeks Completed: 15.5 weeks Plasma Shots: 2224 Operations ----------- Plasma operations continued at Alcator C-Mod last week. Four run days were scheduled and completed. A total of 115 plasma discharges were produced with a reliability of 93%. The runs supported research in the Pedestal Physics, Transport Physics, and Advanced Scenarios topical areas. Tuesday's run began one hour early (8:00 AM) to allow additional time for calibration of a critical diagnostic. A fresh boronization was carried out over Tuesday night. Two hours of run time were lost on Wednesday morning in order to investigate a problem with an ICRF transmitter. Plasma operations are planned to continue this week. Operation Details ----------------- Following an hour of calibration and alignment verification discharges for the benefit of the Hirex_SR spatially resolved high resolution X-ray spectrometer diagnostic, the main part of Tuesday's run was devoted to MP#707 "Pedestal and heatflux-width scaling studies in I-mode plasmas". This experiment comprises part of the thesis research of an MIT graduate student. Tuesday's experiment was designed to examine the scaling of the pedestal and scrape-off layer heat flux with density, plasma current, and ICRF heating power, continuing the series of scans that were begun on Friday, Aug 24. Data were obtained at values of n/nG < 0.3 with Ip = 1, 1.15, and 1.3MA for I-mode and mixed H- and I-mode discharges. The 1MA discharges also included CXRS data with D2 and He puffing and a dynamic inner gap scan. We also obtained impurity transport data with multiple CaF2 LBO injections, measured by the HIREX_SR diagnostic. The run on Wednesday was dedicated to MP#712 "Density range and dependence of I-mode in reverse B_T LSN plasmas". The object of this 2-day MP is to determine the achievable density range in I-mode, and document the minimum and maximum powers at each density. For the main part of the run, we conducted a density scan to progressively lower density. The highest density of the scan was at an nebar of 1.5e20/m^3, the only condition at which we were able to also get I-H transitions with the power available. The lowest density with clear I-modes had an nebar 0.87e20/m^3. We varied ICRF power steps shot to shot, to get good L-I threshold data, and also obtained good pedestal and fluctuation data. Pedestals temperatures were typically T_e > 800 eV. Thursday's run was completed in support of MP692 "The influence of ion temperature profiles and toroidal rotation profiles on the formation of ITBs in I-mode plasmas". The purpose of this experiment is to determine whether ITBs, that typically form in C-Mod off-axis heated EDA H-mode discharges, can also be formed in I-mode target plasmas. Examination of the role of the ion temperature profile and plasma rotation profile on the transport barrier production is a key goal in this effort. Exploration of the effects of the edge barrier on formation of the internal barrier is also important. We were successful at making I-modes with off-axis ICRF resonance on both the low and high field side of the plasma. No ITB formation was observed in these I-mode discharges. The low field side operation resulted in heating of limiters and the shoelace antenna, and in impurity influxes, so we did not fully explore the parameter space on the low field side. The plasma rotation was quite high in the co-current direction in the high field side I-modes. These cases will be ideal for stability analysis. These results will contribute to the understanding of the transport physics associated with ITB formation. In addition, useful data on I-mode thresholds were obtained. Two experiments were carried out on Friday, both in support of thesis research of MIT graduate students. The first half of the day was devoted to MP#648 "Direct boron density asymmetry measurements" . The purpose of these experiments was to run a number of I-mode shots with inner gap scans to improve profile coverage for HFS CXRS measurements. Data were obtained at 1.1 and 1.3MA, with RF power stepped from 3MW to 4MW during the discharges. The second half-day on Friday was dedicated to MP#719, "Shoelace antenna operation in EDA H-mode plasmas". We attempted to develop an ohmic, upper-single-null, reversed field EDA H-mode at low field (B < 4T) and low q95 ~3.1, to serve as a target for active MHD spectroscopy using the "shoelace" antenna to probe the Quasi-Coherent Mode (QCM). While we obtained short ELM-free H-modes, we were unable to obtain ohmic EDA conditions. This may have been due to the lack of a recent boronization, as the radiated power fraction was relatively high. On two discharges, we observed weak cross-coherence between the shoelace signal and the PCI diagnostic in the QCM band, between 100 and 150kHz. These data are being analyzed. ICRF Systems ------------ The RF systems were operated in support of experiments this week. On Wednesday, FMIT#2 driver failed during turn on - the grid bias failed to apply as normal. Upon inspection, the driver tube was found to have shorted from the bias grid to the cathode. This resulted in damage to the metering and caused the grid current shunt to fail. A new replacement tube was ordered and received, and has been installed. FMIT#2 was operated to full power on Saturday and should be ready for operations this week. The first of two ARRA-funded FFTs had their mounting brackets attached and the FFTs should be ready for initial laboratory testing this week. Lower Hybrid System ------------------- Attempts at coupling LH waves to I-mode target discharges were unsuccessful this week. The observed density at the launcher was extremely low as measured by the LH Langmuir probes and the x-mode SOL reflectometer. The densities measured were approximately 5x10^16/m^3, which is at least a factor of 10 too low for efficient coupling. Future experiments will attempt to use local gas puffing to increase the density near the LH launcher for coupling of LH waves to I-mode target discharges. Faulting in the klystron 1 waveguide at moderate power levels (80-100 kW) is being investigated. The load and switch will be visually inspected for damage and replaced with spares as necessary before the next LH run. Diagnostic Neutral Beam Systems ------------------------------- Installation of a backup cathode emitter assembly did not result in improved beam performance. The original emitter was then re-installed, following repair and refurbishment. The beam is presently operating with ion current up to 5.8 amps at 43 kV. Diagnostics ----------- The AGNOSTIC beamline was separated from the C-Mod vacuum system early last week. Beam instrumentation (Faraday cup and in-beamline Pearson coil for current measurements, digital camera for beam imaging) has been installed and was used during Thursday night's 6pm-midnight AGNOSTIC run to diagnose RFQ accelerator beam current and beam alignment. Initial analysis appears to show that the beam diverges by a few mm from the ideal beamline axis near the exit of the RFQ cavity, which would cause high beam losses in the beamline and prevent much of the beam from entering the C-Mod vacuum vessel. This finding is consistent with the low count rates observed on several gamma detectors deployed to measure gammas produced when the beam impinges upon C-Mod PFC tiles on the inner wall. Analysis of Thursday's data continues and new methods to ensure precise beam alignment are being developed. Travel and Visitors -------------------- Jeffrey Doody, Lihua Zhou, Soren Harrison (PPPL), and Han Zhang (PPPL) were in Nashville, Tennessee last week to participate in the 20th ANS Topical Meeting on the Technology of Fusion Energy (TOFE 2012). They presented three posters and two oral presentations: Jeffrey Doody, poster presentation, "Ansys Model to Predict Magnetic Fields near new Outer Divertor in Alcator C-Mod during a Disruption" Lihua Zhou, poster presentation, "Heat Transfer Simulation of A-Frame Assembly to Support Alcator C-Mod Hot Divertor Upgrade" Soren Harrison, oral presentation, "Requirements, Criteria, R&D, and Design for the C-Mod Outer Divertor Upgrade" Soren Harrison, poster presentation, "Motivation for the C-Mod Outer Divertor Upgrade" Han Zhang, oral presentation, "Thermal analysis to calculate vessel temperature and stress in Alcator C-Mod" Copies of these presentations are available at http://www.psfc.mit.edu/research/alcator/pubs/index.htm Martin Greenwald traveled to Washington with Bill Tang and Mike Zarnstorff (PPPL) to brief OFES staff on the Fusion Simulation Project. The Consul General of Canada to New England, Patrick Binns, and the Canadian attaché for Academic and Economic Affairs, Marc Jacques, visited the PSFC on August 30. The Consul General met with several Canadian nationals working at the PSFC and discussed the goals of the PSFC's fusion energy research. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly