Alcator C-Mod Weekly Highlights December 20, 2010 FY2011 weeks of research operations Target: 15 weeks Completed: 6.74 weeks Plasma Shots: 810 Operations ----------- Plasma operations continued last week at Alcator C-Mod. Three and three-quarter run days were scheduled and completed. A total of 114 plasma discharges were produced with a reliability of 96%. Experiments supported research in the Transport Physics, ICRF Physics, and H-mode Scenarios topics. Boronizations were carried out on Sunday and on Tuesday night. Friday's experiments were carried out at Bt=8.0 tesla. Plasma operations are planned to continue this week. Operations Details ------------------ Tuesday's run was devoted to MP#578 "Experimental tests of EPED1 pedestal prediction in ELMy H-Mode", which comprises part of the effort in support of the 2011 OFES Joint Research Target on pedestal physics. Phil Snyder (GA) again participated remotely as co-Session Leader for this experiment. The goal for this day's run was to extend the range of shape parameters over which ELMy H-mode data is available in C-Mod. The elongation was scanned from kappa~1.45 to kappa~1.6. With variations in density and ICRF power programming, we were able to obtain ELMs with kappa up to about 1.58. ELMy access was greatly enhanced at kappa of 1.55 and less. A broad target density scan was carried out at kappa=1.54, allowing us to document the empirical boundary between ELMy H-mode and EDA H-mode in this condition. (The QCM is favored by nu*,95>1). We also compared shots at kappa~1.54 with varied upper triangularity. When delta_u was ~0.2, ELMs appeared early in the H-mode and the rise in radiated power was quite tolerable. For delta_u~0.4, and with other plasma parameters nearly identical, the H-mode was ELM-free, and the radiated power rose uncontrollably. An interpretation is that the very weak shaping in the former case destabilized the ELMs. These will be very interesting data points to compare using detailed studies of peeling-ballooning stability with ELITE. On Wednesday and Thursday morning we carried out MP#630 "Expanding the operating space of high performance impurity seeded EDA H-modes". The goal of this MP is to extend the favorable results of low-Z impurity seeding obtained in moderate q95, Ip=800kA, kappa=1.5 EDA H-mode discharges to higher current and elongation cases. On the first day we successfully applied neon seeding to achieve steady EDA H-modes at currents up to 1MA, q95~3.5, with over 5MW of ICRF heating power. These discharges exhibited high performance (H98~1, betaN>1.5) and low divertor heat loads. Seeding of higher current (1.1MA) discharges with neon and with N2, resulting in low divertor heat loads and high performance, was also demonstrated, although steady EDA conditions were not obtained in these cases. On Wednesday morning the experiments continued using N2 seeding. Steady EDA H-modes at high power were obtained at Ip=700kA and Prf>5MW. While the divertor heat loads remained low in these discharges, heating of the outboard limiters was observed at the lower current. Wednesday afternoon was devoted to MP#477 "Formation of Ohmic H-Mode ITB with Improved Diagnostic Set". The goal was to obtain improved measurements of ion temperature and rotation profiles and density fluctuations in an ohmic plasma with an Internal Transport Barrier. The experiment was hampered by difficulties in obtaining satisfactory ITB conditions. Data from the successful ITB discharges that were produced are being analyzed. Friday's run was devoted to MP#624 "Measurements of mode converted waves in D(He3) plasmas with PCI", which comprises part of the thesis research of an MIT graduate student. This portion of the experiment was carried out at Bt=8.0T with the ICRF transmitters operating at 78(J- and D-port), and 80 MHz (E-port); the D-port antenna (FMIT#1), which normally operates at a frequency of 80.5MHz was re-tuned to 78 MHz during the run, to allow comparison of the observed MC waves from antennas at different toroidal distances from the PCI diagnostic, which is located at E-port. The He3 fraction was optimized at 9 to 11%. The results are being analyzed and will be be compared to full wave simulations. ICRF Systems ------------ The new (ARRA-funded) ICRF Final Power Amplifier tetrode was successfully high potted to 60 kV. For the D(3-He) mode conversion experiment on Friday, the FMIT#1 was re-tuned from 80.5 MHz down to 78 MHz so that the D antenna would have the same frequency as the J antenna. This allowed direct comparison of the measured density fluctuation profile for waves launched toroidally offset by ~36deg and ~150deg from the PCI location. The flexibility to switch frequencies was facilitated by the recently installed solid state IPA stage, which has a wider bandwidth than the previous, tube-based, IPA. Fabrication of the ARRA-funded rotated antenna continued, with most work centered on the current strap fabrication and preparing for fabrication of the strip lines and shields. All of the straps are now cut from the inconel block with enough material remaining to cut one more full strap. The straps have had a radius cut on all sides and we are preparing to machine the bolt pattern at the ground location. Thermal analysis has been performed on the Faraday rod design including the boron carbide washer. With the inclusion of radiation the Faraday screen elements remain ~200 deg C for 5 sec pulses with 20 minutes between pulses. Thermal expansion of the rods is being considered, and is not presently expected to be an issue for the design. Lower Hybrid System -------------------- Testing of the final two new ARRA-funded klystrons was completed this week, well ahead of schedule. Pat MacGibbon traveled to the vendor's facility to witness the tests and packing of the klystrons for shipping. Delivery of the klystrons is expected on Dec 20. Progress was also made on the transmitter protection system upgrade and in troubleshooting the klystron serial fiber-optic links. Travel and Visitors ------------------- Bruce Lipschultz traveled to Cadarache to attend two international meetings. The first, held Monday, Dec. 13, was the ITPA coordinating committee meeting, where he represented the ITPA SOL/Divertor group in discussions of ITER high priorities. On the second and third day he attended the IEA/ITPA joint planning meeting, both as the representative of the SOL/Div group and also as the facility representative from the C-Mod group. On Tuesday he gave a talk outlining results from the various IEA/ITPA DSOL collaborations as well as the new proposals. On Wednesday he presented the C-Mod proposals for work in support of ITER in 4 areas "Tungsten vs carbon", "What to do about ELMs", "What to do about disruptions", and "heating systems". Martin Greenwald was at PPPL 12/13-16 for a meeting of the FSP management team. Sean Fu, a University of Texas graduate student, has joined the UT C-Mod collaboration. Sean visited MIT last week and worked with Igor Bespamyatnov to improve the datasets for several TRANSP runs by addition of CXRS-derived ion temperature and B density data. John Sarff (U. Wisconsin) visited the MIT PSFC on Friday. He met with several C-Mod scientists, and presented a seminar entitled "Ion heating and confinement in MST". Eric Edlund, PPPL, visited the PSFC on Dec 16,17 to collaborate with Miklos Porkolab on writing a paper on the nonlinear mode coupling of Alfven Wave Cascades which was observed using the PCI diagnostic during Eric's thesis work on Alcator C-Mod. They also discussed the possibility of looking for kinetic effects on Alfven wave propagation using the PCI diagnostic on C-Mod. This subject was discussed recently by Raffi Nazikian in his APS Meeting poster. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly