Alcator C-Mod Weekly Highlights May 30, 2006 FY2006 weeks of research operations: Planned: 14 weeks Completed: 12 weeks Operations ---------- Plasma operations continued at Alcator C-Mod last week. Four run days were scheduled and two and a half days completed. Difficulties with power room systems and the MIT alternator resulted in delays or outages during the runs on Tuesday through Thursday. A total of 62 plasma discharges were produced with a reliability of over 80%. Experiments were carried out in support of Transport and MHD topical science areas. One overnight boronization was performed on Tuesday night. C-Mod operation is planned to continue this week. The toroidal field and plasma current remain in the reversed configuration for this series of runs. Run summaries ------------- The abbreviated run on Tuesday and half of the run day on Friday were dedicated to MP#415 "Rotation and H-mode Power Threshold for LSN and USN at Different IP and density". The reverse field portion of this MiniProposal calls for investigation of the H-mode power threshold in matched Upper and Lower single null discharges, to augment previous studies with standard field and current directions. Data were obtained in both x-point configurations with Ip=800kA and nebar~1.5e20/m^3. The run on Wednesday was devoted to MP#452 "Reverse Field Pedestal Studies". The goal for this experiment was to acquire and characterize H-modes and pedestals with the direction of field and current reversed, in order to make a comparison with similar discharges in the normal configuration. Only a half-day was available for this experiment, owing to an alternator repair and a gas-valve malfunction. The target equilibrium was established and the H-mode threshold for this configuration determined to be in the range of Prf~3MW. These reversed field lower null discharges were found to be susceptible to marfe activity before the application of RF power, which complicated control of the target density. Additional time has been scheduled for completion of these experiments. Thursday's half-day run addressed MP#444a, "Alfven cascade studies during current ramp with ICRF heating". The purpose of these experiments is to expand the database of Alfven cascades, specifically looking at the dependence of the initial frequency on temperature. Excitation of the cascade modes is induced by resonant interaction with the RF generated ion tail. The cascades are observed during the reverse shear part of the current rise phase of the discharge. A number of discharges with cascades were produced, using both on- and off-axis ICRF heating (at fields of 5.4 and 5.9T). In addition to the H-mode threshold experiment mentioned above, Friday's run was partially devoted to MP#404a, on Alfven mode damping measurements using the C-Mod Active MHD Spectroscopy antenna. Experiments in reverse field were motivated by results from JET which reported increased damping for n=1 Alfven modes when the ion grad-B drift was opposite the active X-point. Damping experiments were conducted during the upper and lower null discharges produced in support of the MP#415. In the afternoon, dedicated TAE damping experiments were carried out in limiter equilibria with varying ICRF power levels, to invesitgate the dependence of the damping on the minority tail. Over the range 0 < P_rf < 2.5MW the observed damping was non-monotonic with a maximum damping of about 14% with P_rf~2MW, and values as low as 3-4% at higher and lower power. The CNPA (Compact Neutral Particle Analyzer) diagnostic was used to obtain minority tail temperatures for these experiments. Diagnostics ----------- The tuning control for the Active MHD Spectroscopy Amplifier has been modified. In the original design, tuning was strictly a function of input frequency and the tuning capacitors might switch several times during a frequency ramp, resulting in lost data during the 20 ms required for the capacitors to switch. The new design allow the Digital Plasma Control System (DPCS) to tune the amplifier for the middle of the ramp, where the Alven resonances are most likely to occur. The new design was used during the experiments on Friday, and demonstrated improved tuning performance. DNB Systems ----------- The DNB was successfully operated at full parameters into C-Mod plasmas this week with near 100% reliability. The beam was used in conjunction with the CNPA diagnostic to obtain minority tail measurements during Friday's Alfven wave damping experiment. The arc source conditioning has greatly improved, and the arc current rise time last week was typically 100-150 msec, with a total beam-on time of 1.2sec. Lower Hybrid System ------------------- Several PSFC personnel (Beals, Beck, Zacks, MacGibbon) visited the vendor to inspect progress on the manufacture of five new klystron cathode ceramic assemblies for the rebuilding of derated 250 kW klystrons, which will be spares for the current lower hybrid system. Brazing of one end of two assemblies has been completed,and the overall quality of the braze looked very good. The first ceramic assembly is expected to be shipped next week. There are currently two klystrons at the rebuilder awaiting new ceramic assemblies. Work on the coupler protection system continued last week. Calibrations and trip level adjustments were done and tests were performed during plasma operation. The noise levels on the jungle gym coupler signals are much lower with the added high pass filters. However, during plasma operation unexplained trips occurred on several channels. Work will continue this week on these issues. Travel and Visitors ------------------- Paul Bonoli attended the yearly advisory committee meeting to the US SCiDAC Initiatives that was held at PPPL on May 24-25, 2006. He gave a talk to the Advisory Committee reporting on scientific progress in the SciDAC Center for Simulation of Wave - Particle Interactions. Brian Labombard, Bruce Lipschultz, Yijun Lin, Jim Terry, Dennis Whyte (Univ. of Wisc.), and Steve Wukitch presented results of C-Mod research at the Plasma Surface Interactions Conference in Hefei, China. Lin and Wukitch also visited the ASIPP ICRF group and discussed current and future collaborations. A workshop for the US Burning Plasma Organization (BPO) CODAC (Control, Data Acquisition and Communication) Task Group was held at MIT on May 22-23. Jo Lister, who has been contracted by the ITER team to coordinate the preparation of a conceptual CODAC design, led discussions. US members "signed up" to work with their international partners in areas of particular interest. Task group members from MIT include Tom Fredian, Josh Stillerman, Steve Wolfe and Martin Greenwald, who also serves on an advisory committee for the overall international effort. Ian Hutchinson also participated in the workshop discussions. On May 26, Martin Greenwald traveled to Dartmouth College to give a colloquium for the engineering school "Fusion Energy: Promise and Prospects". He also continued collaborative work on edge transport and the density limit with Prof. Barrett Rogers. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly