Alcator C-Mod Weekly Highlights November 29, 2010 FY2011 weeks of research operations Target: 15 weeks Completed: 4.1 weeks Plasma Shots: 494 Operations ----------- Plasma operations continued last week at Alcator C-Mod. One and a half run days were scheduled and completed. A total of 40 plasma discharges were produced with a reliability of 89%. The experiments supported research in the MHD and Transport areas, as well as Diagnostic Development. Plasma operations are planned to continue this week. Program Planning ---------------- The C-Mod Experimental Program Committee met on Monday to review Miniproposals for upcoming runs and to discuss the campaign schedule and priorities for the Fall-Winter campaign. Four MP's were reviewed and approved, subject to minor revisions and clarifications. Operations Details ------------------ The first half of Tuesday's run was devoted to MP#616 "Is H-mode threshold power reduced near double null and why?". The session was used to explore the apparent reduction in H-mode power threshold in nearly double null (DN) plasma discharges. Such a reduction is found to be large (approximately a factor of two) in NSTX and MAST, but was observed to be more modest (~30%) on ASDEX Upgrade, which has a conventional machine aspect ratio. To examine this phenomenon on C-Mod, we operated in near DN configurations and programmed slow ramps to the plasma vertical position in order to shift magnetic balance from lower single null (LSN) to upper single null (USN) and vice versa. Input power was kept fairly steady throughout the balance scans, at levels found to be below the threshold power in LSN plasmas. Preliminary results indicate that the reduction in power threshold at DN is about 30%, as suggested by ASDEX Upgrade results. Indications are that this reduction occurs over a narrow window (~1mm) in SSEP, the midplane distance between the primary and secondary separatrices as reconstructed from magnetics. L-H transition data are being analyzed, particularly with regard to edge conditions. This experiment supports ITPA joint experiments PEP-6, "Pedestal structure and ELM stability in DN" and PEP-26, "Critical edge parameters for achieving L-H transition". The runs on Tuesday afternoon and Wednesday morning were dedicated to MP#620, "Driving Alfvén Eigenmodes with Amplitude-Modulated ICRF Waves". The primary goals of this experiment were to test our ability to couple to MHD modes from amplitude modulated ICRF waves and to determine the physics of the observed modes, particularly regarding their drive mechanism and structure. The AM technique was successfully applied to FMIT#1 (D-port antenna, center frequency 80.5MHz) up to a 12.5% modulation amplitude for both fixed and swept modulation frequencies in the range of the observed unstable modes (450-700kHz). The unstable modes were observed at low target density, nebar<9e19/m^3, both with and without additional unmodulated ICRF heating from the J-port antenna. At higher density, nebar>1.1e20/m^3, the unstable modes were suppressed. The data are being analyzed to determine the coupling of the AM ICRF wave to the MHD modes in both unstable and stable regimes. ICRF Systems ------------ Fabrication of the 4-strap rotated ICRF antenna continued. A blank flange has been sent to the vendor for machining. The bellows tube fabrication drawings were completed and are under review. Two straps have been cut from the inconel block, and the remaining two are in process. All bids for boron carbide coating of the Faraday rods have been received, and one sample for testing has arrived. Modulation of D antenna was performed in support of MP620. Some plasma conditions specified in the miniproposal resulted in RF faults when modulation depths greater than 12.5% were requested. The faults seem to have been caused by an increase in reflected power along with a decrease in forward power. More tests of the modulator during plasma discharges will be needed to understand and ameliorate these effects. Travel and Visitors ------------------- Hank Strauss (NYU) and Linda Sugiyama (MIT) visited the C-Mod Control Room on Tuesday to discuss C-Mod disruptions with Robert Granetz. Dr. Strauss is particularly interested in the causes of disruptions, as well as learning some details of our disruption mitigation studies. Three graduate students from EPFL in Lausanne visited MIT last week to present seminars and hold discussions with MIT scientists. Francesco Piras presented a talk on "Snowflake Divertor Experiments on TCV", Christian Theiler gave a seminar on "Turbulence and Transport in Simple Magnetized Toroidal Plasmas", and Theodoros Panis gave a seminar on Alfven Wave studies in JET. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly