Alcator C-Mod Weekly Highlights January 10, 2011 FY2011 weeks of research operations Target: 15 weeks Completed: 8.2 weeks Plasma Shots: 985 Operations ----------- Plasma operations continued at Alcator C-Mod last week, after a brief maintenance interval. Four run days were scheduled and completed. The experiments supported five miniproposals in the H-mode Scenarios, Transport, and Advanced Scenarios tasks. A total of 117 plasma discharges were produced with a reliability of 95%. Operations spanned a range of toroidal field from 2.7 to 8.0 tesla. Overnight boronizations were carried out prior to the runs on Tuesday and Friday. A faulty regulator in the diborane cabinet was identified and replaced during the week. Plasma operations are planned to resume this week following the C-Mod Ideas Forum, which is scheduled for Monday, Jan. 10, through Wednesday, Jan. 12. Operation Details ----------------- The run on Tuesday was devoted to MP#574 "ITER-like Discharge Studies at Reduced Bt". The goal was to obtain ITER-like non-dimensional parameters q95~3, n/nG > 0.7, beta_N~1.8 in steady H-mode (EDA) plasmas at Bt=2.7T, using ICRF heating in the second harmonic proton minority scenario. These parameters were largely achieved. H98y2 values above 0.9 were obtained in plasmas with beta_N up to 2.0. The highest beta discharges exhibited interesting low frequency (f < 20kHz) MHD activity in addition to the usual EDA signature quasi-coherent mode, as well as high frequency (f>500kHz) activity in the Alfven mode range. Wednesday morning's experiment supported MP#477, "Formation of Ohmic H-Mode ITB with Improved Diagnostic Set". The purpose of this experiment was to study the conditions which lead to the spontaneous formation of Ohmic H-mode ITBs with an extended diagnostic set, including CXRS and soft X-ray measurements of ion temperature and rotation profiles, MSE measurements of current profile evolution, and reflectometry and PCI fluctuation diagnostics. The target discharge was an 800kA Ohmic H-mode formed by ramping the toroidal field initially from 5.4 T to 3.2 T, and then back up to 4.5T. We were successful in obtaining the desired ITB conditions and documenting the phenomenon with a full complement of diagnostics. It was found that the ITB formation was sensitive to the level of impurity radiation, and care had to be exercised in the amount of argon introduced for the X-ray spectroscopy measurements. The role of impurity radiation in the ITB formation dynamics, as evidenced by this dataset, is also being analyzed. The experiment on Wednesday afternoon was the first devoted to MP#632 "q Dependence of Rotation Reversal Density". This MP is designed to explore the abrupt rotation reversals observed in TCV and C-Mod Ohmic plasmas which occur with small changes in electron density, plasma current, and plasma shape, with no change in overall plasma performance. The density at which the reversal occurs has been observed to increase with plasma current and decrease with magnetic field. The reversal mechanism is unknown, but the phenomenon is suggestive of a bifurcation. The present experiments are designed to further elucidate the dependence of the reversal on current and field. Dynamic ramps of the density were performed in USN discharges to identify the reversal density in a series of discharges at B=5.0 T, with Ip=0.5, 0.75, and 1.05MA, and at B=7.0T with Ip=0.75 and 1.05MA. The reversal density scaled linearly with plasma current, but the toroidal field dependence was weaker than 1/q95, with an overall scaling closer to Ip/sqrt(B). Additional scans, including a set at lower toroidal field, are planned. On Thursday, we carried out the second part of MP#613 "LHCD effectiveness in high Te H-modes". The first order aim of the run was to measure lower hybrid current drive and non-thermal electron generation as a function of density in RF-heated EDA H-modes, for comparison with the trend in L-modes. In addition, we wanted to obtain edge and SOL data suitable for subsequent modeling and analysis. We were successful in obtaining good datasets in a series of 600kA H-mode discharges, including USN, LSN, and DN equilibria. SOL data included scanning probe results in addition to SOL reflectometry. In the lower null discharges, with nebar~1.8e20/m^3 application of LH resulted in modest density reduction, to 1.7e20/m^3, and increases in core and pedestal temperatures, but little indication of non-thermal electrons. In the upper null configuration, lower H-mode density was obtained, nebar~1.55e20/m^3, and non-thermal signatures were observed on ECE and HXR. High performance H-modes (H98~1.2, betap~1, Te0~4.7keV) were obtained with P_icrf=3MW, P_lh=0.8MW. Neon seeding was successfully employed for some of the higher P_icrf cases. Double null cases had non-thermal signatures similar to the USN, but suffered from increased vertical instability and were therefore more problematic for analysis. Based on these results, it appears that LH is a useful control tool for H-mode particle transport, helping keep density and impurities low. CD efficiency in H-mode requires further analysis. An initial assessment indicates it is non-negligible, possibly greater than would be obtained in L-mode discharges with comparable density. Friday's experiment was devoted to the high-field portion of MP#636, "Study of ELMy H-mode: Toroidal field scan", which contributes to the 2011 OFES Joint Research Target on pedestal physics. Phil Snyder (GA) participated as co-session leader for this experiment using remote collaboration tools. The goal of this run was to extend the data set in ELMy H-mode up to 8T, providing a new set of conditions under which to test the EPED model for the pedestal height. Relatively early on we demonstrated that we could access ELMy H-modes with high Te pedestals with D(He3) heating at 8T. ELMy 8 tesla H-modes were obtained on five discharges, and one discharge produced an I-mode. However, progress on the run plan after that was stymied by a number of operational difficulties. These included an interlock failure in a poloidal field power supply, which was resolved after a ~90 minute delay, and a leak which developed in one of the piezoelectric valves used for impurity seeding, which had to be taken out of service and will be replaced this week. Therefore, this piece of the ELMy H-mode study remains open. ICRF Systems ------------ The ICRF system was employed in support of experiments on Tuesday, Thursday, and Friday. There was a brief delay on Tuesday morning due to trips (crowbars) in the FMIT#3 FPA system, but the system was reset successfully and all systems performed without incident for the remainder of the week. We completed the first 4 bridge connections, for current straps #2 and #3, for the ARRA-funded rotated antenna, through conventional machining. The next set of four are also underway via conventional machining. EDM machining, required to complete the bridge connections for straps #1 and #4, awaits completion of repairs to our EDM machine. The current strap detail work (gasket, tapped holes, and bridge braze surface) is complete, and the strap will be ready for brazing once the restraining connection and final radius are cut on the central short via EDM. The plating tank setup at the vendor is nearing completion and we plan to run a test piece to qualify the tank once it is operational. We will also plate a stripped inconel drop to verify a replating process in case this might be needed. The horizontal port vacuum flange has been received, and requires polishing before welding the six small flanges onto it. The top/side/bottom plate design has been completed and we are moving forward with sending drawings out for quote. The purchase order for the Faraday screen rods has been amended, and our vendor resumed bending the rods as specified. The boron carbide washers are in production, and shipment of the washers is expected in about two weeks. Preparations for 70 MHz operation were completed, and the transmitter and loop configuration switch will be completed this week. Lower Hybrid System -------------------- The LH system was used in support of Thursday's run. These experiments required simultaneous operation of the LHCD and ICRF systems in H-mode discharges. Anomalies in the power output of klystron 7 required adjustment of drive levels throughout the day. Otherwise, the LH system performed nominally. Progress was made on new software for an improved LH phase and amplitude calibration method. This method will calibrate the vector-modulators separately from the klystrons on a finer I-Q raster grid to allow for more precise phase and amplitude control. Design and procurement of components for the 4th cart and control system continue. Diagnostics ----------------- Fast differential phase measurements are being added to the Two-Color-Interferometer diagnostic. The new system will measure the phase between adjacent CO2 chords. Vibrational effects are mostly subtracted out using this technique, allowing high resolution phase measurements of fluctuation levels to be made. A prototype monitoring one pair of chords has already identified interesting features during ELMs. Travel and Visitors ------------------- Chuck Kessel (PPPL) traveled to MIT on Tuesday to serve as Session Leader for the experiment devoted to his Miniproposal MP#574 "ITER-like Discharge Studies at Reduced Bt". _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly